Advances in High-Performance Ceramic Materials for Aerospace and Defence Applications: A State-of-the-Art Review

Abstract

Ceramic materials are indispensable to aerospace and defence technologies, where structural and functional components are required to withstand extreme thermal, mechanical, and chemically aggressive environments. Traditionally valued for their exceptional thermal stability, oxidation resistance, and corrosion resistance, ceramics have nonetheless been constrained by their inherent brittleness, which has limited their widespread adoption in load-bearing structural applications. This review surveys the principal tough ceramic systems currently employed in aerospace and defence, including SiC, Al₂O₃, ZrO₂, Si₃N₄, SiC/SiC composites, and ultra-high-temperature ceramics (UHTCs) such as ZrB₂ and HfB₂. In parallel, it outlines advanced processing and manufacturing routes that enable enhanced microstructural control, improved reliability, and scalability for industrial deployment. Special attention is devoted to thermal and environmental barrier coatings (TBCs and EBCs), which provide critical protection against oxidation, corrosion, and severe thermal cycling in propulsion, power-generation, and hypersonic systems. Finally, the review highlights key material selection criteria for aerospace and defence platforms and discusses emerging trends that integrate tough ceramics with next-generation manufacturing technologies, underscoring their pivotal role in enabling high-performance, durable, and resilient systems for future extreme-environment applications.

1. Introduction

Ceramic materials, composed of inorganic, non-metallic elements, have long been essential in engineering due to their strength, hardness, and resistance to heat and corrosion [1,2,3]. Their use in aerospace, automotive, electronics, and medical sectors is constrained by high manufacturing costs and the scarcity of key raw materials such as alumina [4,5], as well as the need for specialized design expertise to meet demanding performance requirements [6]. Nevertheless, ceramics remain indispensable in extreme-environment applications because of their durability and reliability [7]. As technologies operating under harsh conditions, hypersonics, gas-turbine engines, nuclear reactors, and military armour, continue to advance, demand grows for materials that can withstand extreme temperatures, mechanical loads, and radiation. Although refractory metal alloys have historically fulfilled these needs, ceramics are increasingly preferred for their higher melting points and superior behaviour under thermal stress, despite the manufacturing challenges associated with tailoring them for such environments.

Within this class, ultra-high temperature ceramics (UHTCs) have gained particular prominence [8]. Stable above 2000 °C in reactive atmospheres such as atomic oxygen, UHTCs include transition-metal borides, carbides, and nitrides [9,10,11,12,13]. With melting points above 3000 °C and outstanding oxidation and ablation resistance, they outperform metal alloys (<1200 °C), C/SiC composites (≤1650 °C), and C/C composites (>500 °C without coating), maintaining integrity in oxidizing environments above 1800 °C [1,2,3]. Transition-metal diborides combine tuneable densities (4.5–12.5 g·cm⁻³), moderate coefficient of thermal expansion (CTE) (6.3 × 10⁻⁶–8.6 × 10⁻⁶ K⁻¹), high thermal conductivity (60–120 W·m⁻¹·K⁻¹), and low electrical resistivity (10–30 μΩ·cm), whereas carbides such as HfC show higher resistivity (109 μΩ·cm) and poorer oxidation resistance, and nitrides typically exhibit lower strength, higher CTE (7 × 10⁻⁶–10 × 10⁻⁶ K⁻¹), and reduced conductivity (19–23 W·m⁻¹·K⁻¹). Research on UHTCs began in the 1950s with studies on HfB₂ and ZrB₂ for nuclear reactor applications due to their corrosion resistance at high temperatures [14,15]. Hypersonic-flight programmes from the 1960s to 1980s renewed interest, supported by the U.S. Air Force’s Man Labs initiative focused on oxidation and ablation resistance [16,17]. In the 1990s, NASA’s Ames and Glenn Research Centers advanced composite systems such as HfB₂–SiC and ZrB₂–SiC [18,19]. Later testing by Sandia National Laboratories and the Air Force Space Command validated their extreme-temperature performance but revealed a persistent limitation: intrinsic brittleness [20,21], prompting recent reinforcement strategies using particulates, whiskers, platelets, and fibres.

Hypersonic aerospace vehicles impose exceptionally severe thermal protection system (TPS) requirements, as components like nose cones and SCRAMJET inlets experience surface temperatures above 2000 °C due to aerodynamic heating and reactive, low-oxygen chemistries [22,23,24,25,26]. Current TPS materials, including SiC and coated C/C composites, show limitations: SiC loses oxidation resistance above 1600 °C [27], and C/C composites undergo ablation during prolonged exposure [28]. Although incorporating ceramic phases has improved performance, protective oxide layers often fail under dynamic thermal loading [29]. The conflict between brittleness and thermal resistance is a major constraint in the design of materials for extreme environments such as aerospace, nuclear energy, automotive, and advanced manufacturing. Toughness, common in ceramics and glasses, limits deformation before fracture, whereas thermal resistance describes the ability to withstand high temperatures without degradation [30]. These properties often oppose each other, making their balance crucial for material development [2,8,19]. Brittle materials, despite their high hardness, fracture readily under stress concentrations or thermal fluctuations, while materials with high thermal resistance, such as refractory ceramics and UHTCs, typically have high melting points and oxidation resistance but suffer from low fracture toughness (KIC) and susceptibility to thermal shock [20,22,31].

Ceramics like alumina (Al₂O₃) [32] are attractive for hypersonic and re-entry applications due to their oxidation resistance and mechanical strength [33,34,35], yet their brittleness and low ductility produce poor thermal shock resistance [36,37]. Foundational studies by Kingery [38] and Hasselman [39] identified mechanisms of thermal stress and crack propagation, though experimental quenching conditions do not represent the complex thermal gradients and constraints present in real aerospace environments [40]. Brittleness continues to limit ceramic use, contributing to failures in turbine rotors, bearings, metalworking tools, and biomedical implants [41]. Crack formation under transient or steady-state thermal loads remains common [38,42,43], motivating decades of research on improving thermal shock resistance through modelling, microstructural refinement, reinforcement, and defect control [44,45,46,47]. UHTCs remain central to TPS due to their high-temperature capability [23,24], but brittleness-driven thermal shock failure persists as a key disadvantage [48]. Reinforcement strategies have been studied, although few works consider external constraints inherent to real TPS assemblies, limiting prediction accuracy and underscoring the need for system-level evaluation under operational conditions [44]. Similarly, high-temperature electromagnetic-wave-transparent materials, used in radomes and antenna windows, must combine thermal stability, low dielectric constants, and reduced brittleness. Silicon nitride (Si₃N₄) and silica (SiO₂) meet many requirements [49], but brittleness restricts their deployment [50,51]. Emerging nanowire-based composite aerogels improve insulation, stability, and wave transparency while reducing brittleness [52,53]. Overall, extreme-temperature materials often lack the mechanical robustness needed for thermal shock and external stresses. Composite systems and engineered architectures such as ceramic matrix composites (CMCs), aerogels, and multilayers offer promising pathways, but achieving an optimal balance requires integrated consideration of intrinsic properties, environmental loads, and structural constraints for next-generation high-performance materials.

1.1. Justification for Focusing on Tough Ceramics in High-Temperature Applications

Ceramics are widely valued for their thermal stability, corrosion resistance, and low thermal conductivity, yet their inherent brittleness limits their use in demanding environments. The development of tough ceramics, engineered to resist crack propagation, has expanded their applicability, particularly in high-temperature systems. UHTCs, especially transition-metal carbides and borides, have become critical for extreme aerospace environments such as hypersonic flight and atmospheric re-entry, where localized heating can exceed 2400 °C [25,54,55]. Their thermal resistance is attributed to oxide by-products with melting points near 3000 °C [56,57] and to their ability to retain mechanical integrity above 2000 °C, including hardness values over 20 GPa and flexural strength near 200 MPa at 1800 °C. Fine-grained UHTCs exhibit superior properties, and SiC-reinforced diborides show up to 20% higher fracture toughness due to enhanced densification and grain refinement.

Despite these strengths, monolithic UHTCs suffer from brittleness and low fracture toughness, especially under thermal shock and fluctuating loads in oxidative or corrosive environments, posing challenges for applications in aerospace and combustion systems [58,59]. As a result, research increasingly targets tougher UHTC composites that improve mechanical performance without compromising thermal stability. Recent work shows that such composites surpass monolithic UHTCs in tensile strength, Young’s modulus, fracture toughness, and hardness at high temperatures, aided by mechanisms such as high-temperature sintering, microstructural homogeneity, distributed solute phases, and solid-solution strengthening, all contributing to enhanced hardness and stability [60]. Processing advancements, particularly in grain-size control and reinforcement with phases such as SiC, remain essential for improving resilience. Full densification around 2000 °C is required for optimal performance, yet often causes grain coarsening, which improves creep resistance but reduces fracture toughness and structural stability at lower temperatures [54]. Complementary progress in hard ceramic coatings, including carbides, borides, nitrides, and oxides, especially in multilayer and nanocomposite forms, has demonstrated improved hardness, wear resistance, and thermal stability [21,22,57,58], highlighting the broader relevance of tough ceramic systems.

An Ashby-inspired chart (Figure S1) correlating fracture toughness (K_IC) and maximum service temperature is included to compare advanced ceramics, CMCs, and metals. The figure highlights the intrinsic trade-off between toughness and thermal capability, illustrating that even the toughest ceramics remain significantly less tough than metallic materials, thereby clarifying material selection considerations for aerospace and defence systems.

High-temperature thermodynamic stability further underscores ceramics’ importance. High-entropy ceramics (HECs) exhibit lower Gibbs free energy than their single-component or low-entropy counterparts, enhancing phase stability under service conditions [61,62,63]. Additionally, flexible ceramic-based nanofibres, with low density, low thermal conductivity, and chemical resistance (Figure 1), offer new opportunities for high-temperature insulation, filtration, and catalysis [64]. Ongoing advances in ceramic aerogels, fibres, and processing innovation are expected to further expand performance and application range [2]. Ultimately, although UHTCs deliver unmatched thermal performance, their widespread adoption depends on overcoming brittleness and low toughness. Toughened UHTC composites and engineered ceramic architectures represent essential progress toward balancing toughness with high-temperature capability. This review therefore examines recent advances in monolithic ceramics, ceramic matrix composites, and nanostructured systems, with particular emphasis on their performance in high-temperature and high-stress environments relevant to aerospace and defence applications.

1.2. Fundamentals of Tough Ceramics

Advanced ceramics are critical for aerospace, defence, energy, and industrial systems because of their high thermal stability, chemical inertness, low density, and oxidation resistance, enabling operation above 2000 °C [45,52,56]. However, their broader structural application is limited by intrinsic brittleness, low fracture toughness (typically 1–5 MPa·m^1/2), and sensitivity to thermal shock. Recent advances in toughened ceramics and engineered architectures have improved crack resistance while maintaining high-temperature capability [19,22,25,34]. Fracture toughness (K_IC) controls crack growth resistance, and unlike metals, ceramics rely primarily on extrinsic toughening mechanisms acting in the crack wake. Microstructural strategies such as grain refinement, crack deflection and bridging, transformation toughening, and the incorporation of reinforcing phases enhance damage tolerance. CMCs further mitigate brittleness through engineered fibre-matrix architectures that promote controlled interfacial debonding and progressive, non-catastrophic failure. Their multiscale damage mechanisms, enabled by tailored grain size, secondary phases, fibres, whiskers, and interfaces, activate crack deflection, branching, bridging, and pull-out. A comprehensive description of the associated failure mechanisms is provided in the Supplementary Materials. These effects are particularly effective in anisotropic or laminated designs, making CMCs highly suitable for demanding aerospace, defence, nuclear, and high-temperature propulsion environments under complex loading condition

1.3. Desired Properties for Aerospace and Defence Use

Ceramics are essential in aerospace and defence due to their unique physical, thermal, and chemical properties, enabling components to withstand extreme temperatures, mechanical loads, and harsh environments while maintaining structural integrity. Their use in engines, exhaust systems, thermal protection shields, and high-speed vehicles, including missiles and hypersonic aircraft, relies on oxide, carbide, nitride, glass-ceramic, and CMC systems that operate effectively up to ~2200 °C and resist ablation and thermal shock [25,65,66,67]. Applications span turbine parts, re-entry heat shields, brakes, bearings, seals, insulation, armour, radiators, and optical systems, with increasingly complex components benefiting from advanced manufacturing, such as 3D printing. Overall, ceramic selection is driven by the need for reliable performance in conditions where metals or polymers fail, with key properties summarized in

Table 1. Key properties required of ceramic materials for use in aeronautics and defence applications.

Property	Importance in Aerospace and Defence
High hardness	Resists abrasion and penetration, critical for armour and high-speed components.
Low density	Reduces weight in aircraft, spacecraft, and armour systems.
High melting point	Withstands extreme temperatures, such as those in re-entry or engine components.
Thermal shock resistance	Handles rapid temperature changes without cracking—important in high-speed flight.
Chemical inertness	Resists corrosion and oxidation in harsh environments.
High compressive strength	Withstands high loads, important in structural and armour applications.
Low thermal conductivity	Acts as an insulator, protecting sensitive equipment from heat.
Electrical insulation	Prevents electrical interference and shorts in avionics and sensors.
Fracture toughness	Helps resist crack propagation, improving durability under stress.
Radar transparency (for some applications)	Allows radar waves to pass through, used in radomes and stealth tech.
Creep resistance	Maintains shape under prolonged high-temperature stress, useful in engines.

. To address brittleness limitations, CMCs offer improved toughness, hardness, radiation resistance, and thermal insulation through short or continuous fibre reinforcements, including glass and carbon [68,69]. Short fibres enhance crack-propagation resistance but may still permit catastrophic failure, whereas continuous monofilaments provide superior strengthening and load-bearing capacity [65,70]. As a result, CMCs have become increasingly important in modern aerospace systems, particularly when integrated with carbon fibre composites [66,68,71].

Among structural ceramics used for protection, SiC and boron carbide (B₄C) are preferred for their light weight, high hardness, and multi-hit performance, making them effective in body and vehicle armour systems [72]. These materials are deployed in platforms such as C-130 and C-17 aircraft, helicopter armour, and soldier body armour, with B₄C suited for small-calibre threats and SiC for larger projectiles [73]. Additional armour ceramics include Al₂O₃, TiB₂, AlN, and Syndite (Synthetic diamond composite), selected for durability in combat environments [72]. For radomes, ceramics must combine mechanical robustness with electromagnetic transparency, requiring low dielectric constant and loss tangent, high thermal and structural resistance, and strong environmental durability [74,75,76]. Suitable candidates include α-Al₂O₃, which offers thermal stability but is difficult to machine [77,78]; pyroceram, valued for thermal shock resistance though sensitive to stress corrosion; and Si₃N₄, which provides high strength up to 1300 °C, wear resistance, and low density for advanced radomes and engine components [74,79].

Historically, ceramic armour has been driven by military needs, with major advances promoted by the Defence Advanced Research Projects Agency (DARPA), Department of Defence (DoD), and national laboratories since the Vietnam War, particularly in boron carbide systems, and continues evolving in response to modern conflicts [80].

In gas turbine engines, attempts to replace nickel-based superalloys with monolithic SiC and Si₃N₄ ceramics were limited by water-vapour-induced recession [81,82]. CMCs have since emerged as superior alternatives, providing high thrust-to-weight ratios, reduced degradation, and improved thermal efficiency. Oxide CMCs (e.g., Al₂O₃, ZrO₂, mullite) offer oxidation resistance and lower cost but limited high-temperature strength, whereas non-oxide CMCs (e.g., SiC, carbon) deliver higher strength, lower thermal expansion, and excellent thermal stability, making them suitable for hot-section engine components. These trade-offs, including cost, strength, and temperature capability, are captured in

Table 2. Oxide vs. non-oxide CMCs: a comparison of key characteristics and uses [81]. Copyright: © 2025 The Author(s). Advanced Engineering Materials published by Wiley-VCH GmbH. Under the terms of the Creative Commons Attribution-Non-Commercial Licence, Licence CC BY-NC 4.0.

Features	Non-Oxide CMCs	Oxide CMCs
Matrix composition	Non-oxides like SiC, B4C, or C	Oxide ceramics like Al2O3, ZrO2, 3Al2O3–2SiO2
Fibre reinforcement	Carbon fibres, SiC fibres, and boron fibres	Al2O3 and SiO2
Temperature stability	Highly stable (>1200 °C)	Low temperature stability (<1200 °C)
Mechanical properties	High strength, stiffness, and fracture toughness, especially at high temperatures	Good thermal shock resistance, but may have lower strength and toughness at high temperatures
Oxidation resistance	Lower oxidation resistance unless protected	Excellent oxidation resistance
Cost	Generally, more expensive due to the complexity in processing	Usually less expensive
Environmental resistance	Poor resistance in wet environments (unless treated)	Good resistance to moisture and certain corrosive environments
Applications	Aerospace, automotive (brake components, engine parts) and energy (nuclear reactor components)	High-temperature applications, including aerospace (thermal protection systems) and furnace linings

[81]. Modern CMCs are further enhanced through engineered fibre coatings that create weak interfaces for crack deflection and improved load transfer, using commodity-grade oxide and SiC fibres optimized for performance and manufacturability [25,81,83].

Despite their outstanding performance, ceramics require accurate predictive models, linking composition and processing to elasticity and fracture behaviour, to enable broader structural use in aerospace. Such models help determine where ceramics can effectively replace heavier materials, improving efficiency and performance. The drive for weight reduction and thermal efficiency in the $2 trillion aerospace turbine market (2017–2031) continues to accelerate innovation in ceramics and CMCs [81,84]. Their integration into advanced alloys and composite architectures, supported by processing techniques such as forging and casting, further enhances performance, blast resistance, and formability, solidifying the role of ceramics in next-generation aerospace and defence engineering [85].

2. Ceramic Systems in Current Use

Advanced ceramics have become increasingly important in aerospace and defence due to their high efficiency, durability, and resilience under demanding conditions. Modern tough ceramics, such as silicon carbide and alumina, now integrate microstructural enhancements and reinforcement techniques that significantly improve impact resistance, abrasion performance, high-temperature stability, and overall fracture toughness [86]. Ceramic composites reinforced with carbon fibres or nanostructures further provide low weight, high strength, and corrosion resistance, supporting applications ranging from protective equipment to structural aircraft components and military vehicle systems. Nanotechnology continues to elevate the composites’ energy absorption and impact tolerance, while their integration with smart materials and high-temperature alloys strengthens next-generation platforms by improving mobility, payload efficiency, and operational survivability without compromising structural integrity [87].

Despite their strategic importance, the development of advanced ceramics remains costly, complex, and slow to scale, requiring substantial investment, specialized infrastructure, and multidisciplinary collaboration across academia, industry, and government. Innovation within individual companies is often restricted by high barriers to entry, and limited cross-industry technology transfer slows the spread of advances and best practices. Overcoming these challenges is essential to fully exploit the potential of advanced ceramics and expand their adoption across high-value manufacturing sectors.

Table 3. Material properties and strategic applications of advanced ceramics in defence.

Material	Key Properties	Typical Applications
Silicon Carbide (SiC)	- High thermal conductivity - Thermal shock resistance - Very hard, lightweight	- Heat shields - Engine components - Telescope mirrors - Armour
Alumina (Al2O3)	- Excellent electrical insulator - High hardness - Cost-effective	- Radomes - Electrical insulators - Substrates - Wear parts
Zirconia (ZrO2)	- High fracture toughness - Thermal insulation - Corrosion resistance	- Thermal barrier coatings - Oxygen sensors - Missile parts
Boron Carbide (B4C)	- Ultra-hard - Lightweight - Neutron absorber	- Body and vehicle armour - Blast shields - Neutron shielding
Silicon Nitride (Si3N4)	- Strong under high stress - Wear & thermal shock resistance	- Bearings - Turbine blades - Engine parts
UHTCs (e.g., HfC, ZrC, TaC)	- Extremely high melting points (>3000 °C) - Chemically stable	- Leading edges of hypersonic vehicles - Re-entry thermal protection systems
Transparent Ceramics (e.g., AlON)	- Optical transparency - High hardness - Lightweight	- Transparent armour - Sensor windows - Optical domes
CMCs (e.g., SiC/SiC)	- High strength-to-weight ratio - Crack resistance - Tougher than monolithic ones	- Jet engine components - Spacecraft structures - Thermal shields

summarizes the ceramic materials most frequently used in aerospace and defence based on their key properties and applications.

2.1. Silicon Carbide (SiC)

SiC, or carborundum, first synthesized by Edward G. Acheson in 1891, was initially commercialized for its hardness and thermal stability. Mid-20th-century technological developments expanded its applications from abrasives to structural ceramics and later to high-performance semiconductors for high-power and high-temperature electronics. As a highly stable IV–IV covalent compound with multiple polytypes, most notably β-SiC and α-SiC, it is widely employed in radio-frequency and power electronic devices [88,89]. Today, SiC is indispensable in aerospace and defence systems requiring thermal and radiation resistance, as well as in energy technologies and electric vehicles, with ongoing research aimed at improving both performance and cost efficiency.

As noted above, silicon carbide ceramics are extensively used across multiple industries [90] due to their high strength, low thermal expansion, and strong resistance to wear and corrosion [91,92], as illustrated in Figure 2 [93]. Their applications range from space mirrors [94,95] to chemical reactors, pipelines [96,97], and wear-resistant components such as bearings and grinding wheels [98]. Although SiC was once limited mainly to refractory uses, its exceptional thermal, mechanical, and electrical properties [99] have established it as a critical engineering material. Because sintering strongly determines the resulting microstructure and performance, the fabrication of SiC ceramics remains challenging: the material’s strong covalent Si–C bonding and low self-diffusion require extreme sintering conditions (∼2500 °C, 50 MPa), often making sintering aids necessary to achieve adequate densification.

As shown in Figure 3 [93], common methods for the densification of SiC ceramics, include reaction-bonded sintering (RBS) [100], pressureless sintering (PLS) [101], hot-pressed sintering (HPS) [102], hot isostatic pressing (HIP) [103], recrystallized sintering [104], and spark plasma sintering (SPS) [105]. The various sintering methods used for densifying silicon carbide ceramics result in different material properties.

Selecting an appropriate processing method is crucial for achieving the desired properties of SiC components for specific applications.

Table 4. Comparison of the physical properties of SiC ceramics produced using various sintering techniques [84,88,89,91,93,94,101,106,107].

Property	Pressureless Sintered SiC (SSiC)	Hot-Pressed SiC (HPSiC)	Hot Isostatic Pressed SiC (HIPSiC)	Reaction-Bonded SiC (RB-SiC)	Spark Plasma Sintered SiC (SPS-SiC)
Density (g cm−3)	3.10–3.15	3.15–3.20	3.21	3.00–3.05	3.15–3.21
Porosity (%)	<1	<0.5	<0.25	5–15	<1
Flexural Strength (MPa)	350–550	450–600	640	250–400	500–750
Fracture Toughness (MPa·m1/2)	3–4	3.5–4.5	3.8	2–3	4–5.5
Hardness (Vickers GPa)	22–26	24–27	18–20.5	20–24	25–30
Thermal Conductivity (W·m−1 K−1)	120–200	150–200	220	16–120	140–220
Electrical Resistivity (Ω·cm)	>106	>106	-	10–103	>106
Grain Size (µm)	1–5	1–10	0.2–5.0	5–15	0.2–1.5
Sintering Temperature (°C)	2000–2200	1900–2100	1100–2000	~1400	1700–2000
Sintered Atmosphere	Argon/Nitrogen	Vacuum/Inert gas	Argon	Silicon vapour/N2	Vacuum/Inert gas

compares the resulting physical characteristics across methods. For complex-shaped SiC parts used in aerospace and defence, RBS-SiC [100] and HIP [103], often combined with PLS, remain among the most widely applied techniques. Additive manufacturing followed by sintering is an emerging option for producing highly precise and intricate geometries. Choosing SiC for aerospace and defence systems requires assessing material properties that match the extreme environments and functional demands of these sectors, namely exceptional thermal stability, high specific strength, strong chemical and radiation resistance, and multifunctionality spanning structural, electronic, and optical performance.

2.2. Alumina (Al₂O₃)

Al₂O₃ is one of the earliest and most extensively studied advanced ceramics, valued for its low cost, ease of processing, and favourable mechanical, electrical, thermal, and chemical properties [108]. With a high modulus of elasticity, refractoriness, hardness, and strong commercial viability, it provides an excellent cost–benefit ratio among engineering ceramics. These characteristics have established alumina as a key material in aerospace and defence, where its mechanical strength, thermal stability, and electrical insulation are essential. In aerospace applications, alumina is widely used in thermal barrier coatings, electrical insulators, sensor housings, and satellite structural components, ensuring reliability under extreme temperatures, vibrations, and high-voltage environments. In defence systems, its hardness and durability support applications in body and vehicle armour, missile guidance systems, radar equipment, and fusing mechanisms, where it withstands severe mechanical, thermal, and electromagnetic stresses.

The performance of Al₂O₃ ceramics is highly sensitive to processing parameters such as sintering temperature, atmosphere, impurity content, and grain size. Although multilayer shields using alumina offer good ballistic performance at reasonable cost, their relatively high density (≈3.9 g·cm⁻³) compared with SiC (≈3.2 g·cm⁻³) and B₄C (≈2.6 g·cm⁻³) can increase armour weight and reduce mobility [109], while their low fracture toughness limits protection against higher-calibre threats. To address these drawbacks, small additions of oxides such as ZrO₂, MgO, Y₂O₃, and Nb₂O₅ are used to enhance mechanical properties [110,111], reinforcing alumina’s role in advanced aerospace and defence technologies. For specialized applications such as radomes, alumina remains critical due to its thermal stability, chemical resistance, hardness, and low thermal expansion, outperforming organic materials at high temperatures and enabling use in missile systems operating above Mach 2 [112,113]. High-purity alumina (≥97%) withstands temperatures above 1760 °C, while Lucalox (99.9% Al₂O₃), stabilized with magnesium oxide, offers even greater density and thermal stability [114].

Table 5. Essential physical and electrical characteristics of alumina for use in advanced radome structures [74,79,112].

Property	Alumina (Purity 99%)
Density (g/cm3)	3.9
Dielectric constant (10 GHz) 25 °C 1000 °C	9.6 11.4
Loss tangent (10 GHz) 25 °C 1000 °C	0.0001 0.0014
Flexural strength (MPa) 25 °C 1000 °C	270 220
Coefficient of thermal expansion (CTE) (10−6/°C)	5–9
Thermal shock	Fair
Water absorption (%)	0
Rain erosion	Excellent

[74,79,112] summarizes the essential physical and electrical properties of alumina grades commercially employed in advanced radome structures. However, machining complex-shaped alumina components with diamond tools remains costly, up to 80% of total manufacturing expenses. Advanced fabrication methods such as digital light processing (DLP) printing with photosensitive resins present a promising alternative for producing intricate alumina components for defence and other high-performance applications.

Al₂O₃ ceramics offer excellent dielectric performance for electromagnetic transmission, though their processing-induced contraction depends on raw material selection [115]. Recent advances include alumina–graphene nanoplatelet composites, which show higher density and fracture toughness and can be rapidly densified by high-frequency induction heat sintering [116,117]. These composites also exhibit reliable high-temperature behaviour and strong wear resistance under vacuum, making them suitable for next-generation aerospace components, as demonstrated by Gao et al. [118]. Space missions require thermal insulation materials with low density, low thermal conductivity, and high thermal stability [119,120], especially for cryogenic fuel tanks (−253 °C for liquid hydrogen, −183 °C for liquid oxygen) and spacecraft exposed to thermal cycles between −180 °C and +180 °C or propulsion environments up to 1000 °C [121]. Alumina fibres meet these requirements due to their stability, low conductivity, corrosion resistance, and strength [122,123], and low-cost Al₂O₃ fibres developed by Sinkó et al. [124] advance scalable TPS solutions. Oxide materials such as alumina also enhance turbine hot-section durability under extreme conditions [125,126,127]. Alumina performance depends on purity, microstructure, and processing: high-purity grades (99.8–99.9%) excel in dielectric behaviour and stability for radar insulators, TPS elements, and hypersonic components; ~99.5% alumina offers strong mechanical and thermal performance for engine and control-system parts, and technical grades (85–95%), sometimes reinforced, are used in armour, refractory linings, and thermal-shock-resistant structures where cost efficiency is key. Selecting the appropriate grade, balancing purity, grain size, reinforcement, and mission conditions, is critical for ensuring reliability, longevity, and optimal performance in aerospace and defence systems.

2.3. Zirconia (ZrO₂)

ZrO₂ is a high-performance ceramic widely used in aerospace and defence due to its mechanical strength, thermal stability, and chemical resistance. Its unusually high fracture toughness results from stress-induced tetragonal-to-monoclinic transformation toughening (TT), which absorbs energy and suppresses crack growth [128]. This makes zirconia ideal for severe-load applications such as TBCs in jet engines and hypersonic vehicles, as well as for lightweight armour and high-reliability electronic components [129]. YSZ is the leading TBC material because of its low thermal conductivity and high melting point (~2700 °C), with modern TBCs typically using 6–12 wt.% Y₂O₃. However, adhesion issues during thermal cycling continue to cause cracking and spallation [130]. Zirconia enables next-generation military and aerospace engines by tolerating temperatures up to 1600 °C (with cooling), reducing component weight by as much as 60%, lowering centrifugal loads, and improving efficiency through simplified cooling designs [131]. Since turbine inlet temperature governs thrust-to-weight and fuel consumption, zirconia ceramics and zirconia-based CMCs are critical for engines operating above 1500 °C, well beyond the <1200 °C limits of alloys and intermetallic [122,125,126].

Ceramics also improve fuel efficiency, reducing diesel consumption by over 30% by insulating combustion chambers and enhancing exhaust-energy recovery, while their durability supports long-life automotive sensors exposed to extreme thermal and chemical environments [132]. Thin-film zirconia coatings, adapted from aerospace, reduce heat dissipation in pistons and cylinder liners to produce lighter, more efficient engines [132,133]. In the transition toward sustainable propulsion, TBCs remain essential for thermal management, protecting components, raising combustion temperatures, and reducing emissions. Typical TBCs incorporate a metallic substrate, a bond coat, and a ceramic topcoat [134], often using ZrO₂, mullite, or Al₂O₃ for their thermal stability and insulation performance [130,134]. Comparative effectiveness of these materials is presented in

Table 6. Comparative performance of thermal barrier coating materials. From Ref. [134]. Copyright: © The Author(s) 2025. B. S. Babu et al. (Eds.), Proceedings of International Conference on Advanced Materials, Manufacturing and Sustainable Development (ICAMMSD-2024), Advances in Engineering Research 257. Under the terms of the Creative Commons Attribution-Non-Commercial 4.0 International Licence (http://creativecommons.org/licenses/by-nc/4.0/).

Data	Al Alloy	ZrO2	TiO2
Brake Thermal Efficiency	10.329%	12.223%	11.137%
Indicated Thermal Efficiency	24.604%	29.136%	26.278%
Mechanical Efficiency	41.980%	41.953%	42.381%
Volumetric Efficiency	47.663%	51.816%	49.783%
Air–Fuel Ratio	10.575	13.264	11.611

[134].

Composite ceramic TBCs remain essential in aero-engines for enhancing thermal insulation and extending hot-section component life, with YSZ being the most widely used option [135]. As reported by Patdure et al. [127] and Darolia [136], TBCs can reduce surface temperatures by ~170 °C, lower cooling-air demand and fuel consumption, and substantially increase blade durability. Among candidate materials, 6–8 wt.% YSZ continues to dominate due to its low density, low thermal conductivity from high point-defect concentration, compatible CTE, and strong strain tolerance [137,138]; although, it is limited to <1200 °C, vulnerable to hot corrosion and environmental deposits, and prone to accelerated thermally grown oxide (TGO) because of high oxygen-ion diffusivity in ZrO₂-based ceramics [139]. No superior alternative has yet outperformed YSZ.

To further raise turbine inlet temperatures, modern aero-engines rely on TBCs on metallic components and the use of SiC/SiC CMCs protected by EBCs [83,140,141]. EBCs are required because SiC forms volatile Si(OH)₄ when its SiO₂ layer reacts with water vapour in combustion gases [142]. Although wrought, cast, and single-crystal superalloys have improved temperature capability [143], metals alone remain thermally constrained, making TBCs and advanced cooling strategies indispensable [144]. A typical TBC consists of a ceramic topcoat, metallic bond coat, and metal substrate, with a TGO that forms between them during service; the bond coat reduces thermal-expansion mismatch and limits oxidation, producing a protective α-Al₂O₃ TGO. A schematic of this multilayer structure is shown in Figure 4 [145].

TBC development began in the 1950s, with the first applications thereof in the 1960s [146]. The introduction of YSZ marked a major advancement due to its low thermal conductivity, high thermal expansion, toughness, phase stability, TGO compatibility, and slow sintering rate [137,140]. TBCs are primarily applied by thermal spraying or electron beam physical vapour deposition (EB-PVD). Thermal-spray methods, atmospheric plasma spray (APS), plasma spray physical vapour deposition (PS-PVD), and high-velocity oxy-fuel (HVOF), melt the feedstock and accelerate it toward the substrate using plasma or high-velocity gas jets [145,146]. Coating performance depends strongly on deposition method and adhesion; Almeida et al. [147] demonstrated that EB-PVD provides superior adhesion and columnar microstructures that accommodate thermal expansion. Ongoing global research continues to seek oxides with improved high-temperature capability and lower thermal conductivity to potentially replace 7YSZ, given the critical importance of fracture toughness for resisting impact, erosion, and spallation.

TBCs protect turbine and engine components from extreme temperatures, while TPSs shield entire aerospace vehicles, including spacecraft and missiles, from severe aerothermal loads encountered in high-speed flight and atmospheric re-entry [66]. As highlighted by Zarko [67] and Venkatapathy et al. [148], TPSs function as single-point-of-failure subsystems that must provide reliable thermal protection without sacrificing structural integrity or adding excessive mass. Next-generation TPSs require materials with high melting points, thermal shock resistance, oxidation resistance, and controlled thermal conductivity [149]. Oxide ceramics (alumina, zirconia, mullite) offer high-temperature stability and low cost, but their limited mechanical performance reduces structural reliability at elevated temperatures [149]. Non-oxide ceramics exhibit higher strength and creep resistance but are oxidation-sensitive [123,140,142], motivating the adoption of CMCs such as SiC/SiC, which provide improved damage tolerance and are integrated with EBCs and TBCs in advanced propulsion systems.

Zirconia-based materials continue to play a central role in heat-resistant aerospace systems, though challenges in high-temperature stability, service lifetime, and energy efficiency still constrain progress in hypersonic and next-generation vehicles. Research efforts now target improved thermal protection materials (TPMs) and the development of new systems, with aerogels gaining rising interest for exceptional thermal insulation. Despite limited comprehensive reviews, ZrO₂ aerogels, first synthesized by Teichner in 1976 [150], show high-temperature resistance, wear and corrosion resistance, chemical stability, and Mohs hardness above 7, making them promising TPS candidates. Their main limitation is poor structural stability at high temperatures due to phase transformations that cause volumetric change and pore collapse; therefore, improving the high-temperature robustness and structural integrity of ZrO₂ aerogels is a major goal for next-generation TPSs. TBC degradation arises from diverse mechanisms, including thermal shock, thermal gradients, sintering, phase transformations, oxidation, impacts, calcium–magnesium–aluminosilicate (CMAS) infiltration, corrosion, and environmental erosion, with thermal fatigue, corrosion, and erosion identified as the dominant failure modes [151]. Thermal fatigue results from cyclic thermal stresses; corrosion involves high-temperature oxidation or attack by aggressive species; and erosion is driven by high-velocity particle impacts, ranging from gradual wear by fine particles to severe foreign object damage (FOD) from larger ones. A schematic of these interacting mechanical, thermal, and chemical degradation mechanisms is shown in Figure 5 [66].

Advances in zirconia-based ceramics and processing technologies have enabled the broad application of TBCs on critical air-cooled turbine engine components, including combustors and high-pressure turbine vanes and blades. Since their commercial adoption in the 1980s [67], TBCs have outperformed other materials, such as nickel-based superalloys, by lowering metal temperatures in high-pressure turbines by up to 100 °C, with future improvements potentially exceeding 200 °C through advanced low-thermal-conductivity coatings [67,151].

2.4. SiC/SiC Ceramic Matrix Composites (CMCs)

SiC/SiC CMCs, composed of fine polycrystalline SiC fibres (10–15 μm), have become leading structural materials for extreme aerospace and defence environments due to their high strength, low density (2.5–3.0 g/cm³), thermal shock resistance, and chemical and oxidation stability [142,146,149,152]. Their ability to operate reliably above 1300 °C, unlike conventional superalloys, enables higher engine efficiency, reduced cooling demands, and weight savings, making them essential for next-generation gas turbines, hypersonic platforms, and advanced aerospace structures [66,153,154]. A critical limitation of SiC/SiC CMCs arises in high-temperature, moisture-rich environments, where the protective SiO₂ layer volatilizes into Si(OH)₄, leading to surface recession and structural degradation [66]. To mitigate this, EBCs are required. These multilayer systems typically include a silicon bond coat, an intermediate layer such as mullite (3Al₂O₃·2SiO₂), and a top barrier made of barium–strontium–aluminosilicate (BSAS) or rare-earth silicates like Yb₂Si₂O₇, which provide chemical resistance and high-temperature stability [155,156]. As illustrated in Figure 6 [142], current Generation II CMCs operate up to ~1316 °C, with future systems projected to reach 1482 °C, supported by advanced SiC fibres, improved matrices, and EBCs with strong adhesion, low thermal conductivity, and thermal-cycling resistance.

The first generation of EBCs, developed in the 1990s for CMC combustor liners in solar turbine engines, employed a BSAS topcoat over a silicon bond layer and a mullite interlayer, demonstrating ~14,000 h of reliable operation and validating EBCs for long-duration use [157,158,159]. However, BSAS showed performance limits above 1350 °C, including phase instability and volatilization. To address these issues, second-generation EBCs emerged in the early 2000s under NASA’s UEET programme, incorporating rare-earth silicates such as ytterbium and gadolinium silicates, which offered higher melting points, improved thermal and chemical stability, and enhanced compatibility with SiC substrates under thermal cycling [156]. A notable architecture developed in NASA’s HSCT program with GE and United Technologies featured a three-layer plasma-sprayed system consisting of a silicon bond coat, a graded mullite–BSAS interlayer, and a BSAS topcoat in the Celsian phase (Ba_0.5Sr_0.5Al₂Si₂O₈), the latter requiring post-deposition heat treatment to ensure full crystallization and optimal resistance to steam-induced corrosion [160,161]. Figure 7 schematically depicts this multilayer EBC architecture designed to protect SiC/SiC CMCs from oxidation and volatilization in high-temperature, water-vapour-rich environments, thereby ensuring long-term structural and thermal stability in advanced aerospace propulsion systems.

EBC development continues to advance rapidly, with current research exploring alternative rare-earth systems, improved bond coats, and optimized deposition methods to enhance durability and operating temperature capability. Progress in EBCs is closely linked to improvements in SiC fibre quality, matrix fabrication, and interface engineering, which collectively determine the thermal–mechanical performance of SiC/SiC CMCs. Because EBCs prevent the volatilization of the protective silica layer in steam-containing atmospheres, they are essential for the reliable use of SiC-based composites in propulsion, TPSs, and defence applications [160]. As metal alloys reach their thermal limits, SiC/SiC CMCs, capable of operating near 1400 °C with densities roughly one-third that of nickel-based superalloys, are emerging as leading candidates for next-generation turbine components, offering improved thrust-to-weight ratios and fuel efficiency, as noted by Spitsberg and Steibel [162]. Rare-earth (RE) silicate-based EBCs have become particularly important; Eaton et al. [158] report that their low thermal expansion coefficients and stability above 1480 °C enable thousands of hours of operation.

Despite these material advances, EBC performance strongly depends on deposition technique. Basu et al. [163] achieved dense crystalline mullite coatings via chemical vapour deposition (CVD), though this method remains limited by slow deposition rates and hazardous precursors. Xu and Yan [164] produced thin but uniform ytterbium disilicate layers using plasma spraying and sol–gel approaches, yet their coatings were insufficiently thick for turbine environments. Richards et al. [165] found that APS introduced microcracks and gaps, compromising durability, while slurry dip coating, though economical, often produced drying and sintering cracks leading to spallation [166]. Consequently, integrated TBC/EBC systems have become essential. In this multilayer architecture, the TBC reduces gas temperatures from ~1700 °C to ~1480 °C, creating conditions in which the EBC can function effectively under oxidizing, moisture-rich, high-velocity flows [167]. At these temperatures, the EBC must provide thermal stability, chemical resistance, low oxygen permeability, thermal expansion compatibility, and protection against moisture-driven silica volatilization. The combined TBC–EBC system is thus fundamental to enabling SiC/SiC CMCs as durable hot-section materials for advanced turbines, supporting lighter, more efficient, and thermally robust propulsion systems [167].

Considering these developments,

Table 7. Summary of the key requirements for a successful EBC [155,157,168,169,170].

Requirement	Description	Relevant Materials/Notes	References
Environmental Stability	Coating must resist degradation in high-temperature, steam-rich, oxidizing environments.	Rare-earth silicates are preferred for their stability.	[168]
Thermal Expansion Compatibility	Coefficient of thermal expansion (CTE) must closely match the CMC substrate to reduce thermal stresses and prevent cracking during thermal cycling.	Mullite, BSAS, and rare-earth silicates offer a good CTE match with SiC/Si3N4.	[168]
Water Vapour Stability	Low volatility in water vapour is critical to prevent material loss and coating recession.	Volatility ranking: SiC > Mullite > BSAS > RE disilicate > RE monosilicate.	[168]
Adherence	Oxides and silicates require bond coats to adhere well to CMCs; poor adhesion can lead to crack formation and spallation under thermal cycling.	Silicon is the most effective bond coat due to oxidation resistance and CTE match.	[155,168,169,170]
Chemical Compatibility	Coating layers must be chemically stable with each other to avoid reactive low-viscosity interfacial zones that promote spallation under shear stress.	Careful material selection to prevent reactions at oxide–silicate interfaces.	[168]

summarizes the key material and performance criteria governing EBC design for SiC/SiC CMC protection in high-temperature turbine environments. These include environmental stability in oxidizing and steam-rich atmospheres, thermal expansion matching, resistance to water vapour-induced volatilization, strong adhesion via silicon bond coats, and chemical compatibility across the multilayer architecture, requirements essential for long-term coating integrity and resistance to spallation under severe thermal and mechanical stresses.

2.5. Silicon Nitride (Si₃N₄)

Si₃N₄ is a high-performance ceramic widely used in aerospace and defence due to its high strength, fracture toughness, thermal-shock resistance, and fatigue resistance under extreme conditions [52,111,117]. These properties support its use in turbine rotors, combustor liners, high-speed bearings, seals, and lightweight structural components. In defence applications, Si₃N₄ offers high hardness, corrosion resistance, thermal stability, and favourable dielectric characteristics for armour, missile components, and low-observable technologies [171]. Advances in manufacturing, including additive methods such as lithography-based ceramic manufacturing (LCM), now enable complex Si₃N₄ components such as aerospike nozzles with precise flow and cooling features [172,173]. A major milestone was JAXA’s in-orbit demonstration of a fully Si₃N₄ ceramic engine during the Akatsuki mission, confirming global leadership in ceramic propulsion technologies [174].

In high-speed aerospace systems (Mach 3–5), Si₃N₄’s ability to sustain extreme thermal–mechanical loads makes it indispensable. Si₃N₄ bearing balls surpass steel in weight, hardness, stiffness, smoothness, and thermal stability, improving efficiency and service life; NASA’s use of Si₃N₄ bearings in RS-25 Space Shuttle turbopumps highlights this mission-critical role [175]. Once applied mainly in semiconductor systems, Si₃N₄ now performs in high-temperature aerospace environments exceeding 300 °C during heat soak-back [176]. Its strength retention up to ~1400 °C and low density support its integration into turbine blades, rings, and air bearings, with CMCs mitigating brittleness [177]. Si₃N₄ has also gained strategic value in modern defence systems amid rising global conflict risks [178,179]. Its lightweight, high-performance ballistic resistance enhances unmanned aerial vehicles (UAVs), helicopters, armoured personnel carriers (APCs), and body armour [178]. In advanced energy systems, elongated-grain Si₃N₄ is being evaluated in the U.S. Department of Energy (DOE) Microturbine Programme to increase efficiency (>40%), reduce emissions, and improve reliability, emphasizing probabilistic design based on real component geometries [180,181]. Reduced FOD sensitivity in microturbines further promotes ceramic adoption [182].

However, FOD remains a significant limitation, as brittle ceramics are sensitive to crack initiation from debris impacts [183]. Studies on various projectile types assess critical velocities, residual strength, and damage mechanisms to support life-prediction models [184,185]. Mitigation strategies include grain-toughened Si₃N₄, optimized designs, and protective coatings. Reinforcing Si₃N₄ with CNTs improves fracture toughness and impact resistance via crack bridging, pull-out, and deflection mechanisms [186]. CNT/Si₃N₄ composites show promise for aero-engine rotors, hypersonic vehicles, and defence components, though challenges remain with CNT dispersion, interfacial bonding, and oxidation above 500–600 °C. Advances in powder processing, functionalization, and sintering are enabling high-performance CNT/Si₃N₄ composites, while protective measures are required to prevent CNT degradation in oxidative high-temperature environments [187,188].

Table 8. Summary of strategies for protecting CNTs in CNT-reinforced Si₃N₄ composites under high-temperature conditions. The table outlines key approaches aimed at mitigating CNT degradation, including oxidation and volatilization, during processing and service.

Strategy	Protection Mechanism	Effective Temperature Range	Notes
Ceramic Matrix Encapsulation	Limits oxygen exposure	Up to ~1000 °C	Most common baseline
CNT Surface Coating	Creates local oxidation barrier	Up to ~1000–1200 °C	Requires uniformity
Inert/Reducing Processing	Prevents degradation during fabrication	N/A (processing only)	Not sufficient alone
Environmental Barrier Coating (EBC)	Surface-level protection from hot gases	Up to ~1500 °C	Under active research
Functionalization/Hybridization	Enhances bonding and thermal stability	Depends on system	Still experimental

summarizes approaches for extending CNT usability under extreme aerospace conditions.

Although CNTs offer promising toughening mechanisms for Si₃N₄, their use does not always produce consistent improvements. Reported fracture toughness values for CNT-reinforced Si₃N₄ show significant scatter, even at similar CNT concentrations, due to differences in processing methods (HPS, SPS, and PLS), raw material quality, and microstructural features including CNT dispersion, grain size, and porosity. Variability in measurement techniques, such as single-edge notched beam (SENB), indentation fracture (IF), and chevron-notched methods, further contributes to inconsistencies. Nonetheless, a general trend indicates that low CNT contents (~1 wt.%) typically enhance fracture toughness through mechanisms like crack bridging, pull-out, and crack deflection [189,190]. Higher CNT loadings often fail to improve, and may degrade, mechanical properties due to agglomeration, weak interfacial bonding, or increased porosity. Thus, the optimization of processing parameters, dispersion, and CNT content is essential to achieve reliable performance. CNT-based ceramic nanocomposites hold substantial potential for aerospace systems; NASA’s Ames and Johnson Space Centers have explored their use in thermal radiation shielding and impact protection [191,192]. However, knowledge gaps remain regarding specific aerospace applications where CNT-reinforced ceramics yield maximal benefit. Current research aims to clarify their contributions to structural performance and operational efficiency, positioning CNT–ceramic composites as promising next-generation aerospace materials pending further validation.

In aerospace and defence technologies, optimizing Si₃N₄ processing, sintering route, atmosphere, additives, and temperature profile are crucial for achieving the required mechanical strength, toughness, oxidation resistance, and thermal performance. The microstructure–processing relationship ultimately determines Si₃N₄’s suitability for propulsion systems, thermal protection, and ballistic-resistant structures.

Table 9. Typical physical property data for Si₃N₄ obtained via different manufacturing processes. RBSN: Reaction-bonded silicon nitride; HPSN: hot-pressing silicon nitride; SRBSN: sintered reaction-bonded silicon nitride; SSN: sintered silicon nitride. Adapted from Ref. [193].

Property	RBSN	HPSN	SRBSN	SSN
3-point RT Modulus of Rupture (MPa)	200	700	700	850
RT Young’s Modulus of Elasticity (GPa)	175	300	300	300
RT Hardness Vickers Hv0.3 (kg/mm2)	800	1650	1450	1450
Fracture Toughness K1C (MPa·m1/2)	2.5	4.5	6.0	7.5
Density (g/cc)	2.3	3.2	3.3	3.24
Porosity (%)	30	0	5	0
Thermal Expansion Coeff. (0–1200 °C) (10−6/K−1)	3.2	3.2	3.1	3.1
RT Thermal Conductivity (W/m/K)	10	26	25	22
Thermal Shock Resistance (ΔT °C)	400	700	700	800
RT Electrical Resistivity (ohm·m)	1010	1010	1010	1010

summarizes how different fabrication methods affect key physical properties of Si₃N₄ ceramics, highlighting the processing–property link essential for advanced applications [193].

The α-Si₃N₄ and β-Si₃N₄ polymorphs play a decisive role in defining the mechanical, thermal, and chemical behaviour of silicon nitride ceramics, particularly in high-temperature aerospace and defence applications. β-Si₃N₄ is preferred for structural use due to its superior toughness, thermal stability, and creep resistance, making control of the α → β transformation during sintering essential for achieving high-performance microstructures. Optimizing β-phase content and grain morphology is therefore critical for components exposed to extreme thermal and mechanical conditions [194]. Advancing Si₃N₄ technologies for aerospace also requires addressing integration challenges, as joining Si₃N₄ with metals, ceramics, and composites is essential for complex engine assemblies and multifunctional systems where dissimilar interfaces must retain mechanical integrity under harsh environments.

Ongoing research has examined the joining behaviour of Si₃N₄ using active braze interlayers with liquidus temperatures from 750 °C to 1240 °C. This effort evaluates the self-joining of commercial Si₃N₄ grades (Kyocera and St. Gobain) using a ductile Cu-based active braze alloy containing titanium (Cu-ABA). The resulting joint microstructures, compositions, hardness, and strengths underscore the importance of Si₃N₄ integration technologies for aerospace systems that require robust hybrid structures capable of enduring severe service conditions [194,195]. Overall, Si₃N₄ is positioned to play a pivotal role in future aerospace and defence applications, as advanced propulsion and hypersonic technologies demand lightweight, durable materials; Si₃N₄, especially when combined with reinforcing systems, offers a strong pathway for high-temperature turbine components, armour solutions, and structural elements [194].

2.6. Zirconium Diboride (ZrB₂) and Hafnium Diboride (HfB₂)

UHTCs such as ZrB₂ and HfB₂ are critical TPS materials in aerospace and defence due to their extreme thermomechanical performance, including very high melting points (~3250 °C and ~3400 °C), oxidation resistance (enhanced with SiC), hardness, and thermal conductivity [17,20,196,197]. They enable components exposed to >2000 °C in hypersonic flight and re-entry, nose cones, leading edges, SCRAMJET combustors, and nozzles [198,199]. Although research originated in the late 1800s [90], major development occurred during the Cold War for heat shields, rocket motors, and spacecraft structures [200,201], including foundational U.S. work at Manlabs by Clougherty and Kaufman using HPS and floating-zone processing to obtain high-purity ZrB₂/HfB₂ [202,203]. Interest resurged in the 1980s–1990s with hypersonic systems and SCRAMJETs, supported by NASA Ames, Sandia, and the U.S. Air Force [203].

ZrB₂ and HfB₂ exhibit high thermal conductivity (60–120 W·m⁻¹·K⁻¹) [92], low CTE (~5.9 × 10⁻⁶ K⁻¹ for ZrB₂) [204,205], and better processability than nitrides [20], though oxidation resistance declines above 1500 °C. Composite approaches, e.g., 20 vol% SiC or silica-forming additives, improve protection [20,206]. Hypersonic vehicles face surface temperatures of 1200–2300 K or higher [207], beyond the limits of C/C composites, SiC, or Si₃N₄. UHTCs like ZrC and HfC offer superior stability [55,202,208,209], but monolithic UHTCs are brittle, leading to the development of ultra-high temperature ceramic matrix composites (UHTCMCs), which enhance toughness through fibres, whiskers, and compliant phases [22,210].

Research on ZrB₂- and HfB₂-based UHTCMCs has grown across the U.S., Europe, China, and Japan [211,212], focusing on sharp leading edges, propulsion components, air intakes, and vertical stabilizers [213,214]. Their stable oxides, especially with continuous C-fibre or SiC reinforcement, maintain structural integrity above 2500 °C [215], and these materials are also promising for nuclear applications. Persistent challenges include oxidation resistance, toughness, and thermal-shock performance above 2000 °C [216]. Processing remains difficult because these non-oxide powders are oxygen-sensitive and exhibit low self-diffusivity, which complicates densification [20,210], prompting efforts to optimize powder synthesis, consolidation, coatings, and additive strategies.

Table 10. Comparative analysis of monolithic and additive-assisted ZrB₂ and HfB₂ ceramics.

Property	Monolithic ZrB2	Monolithic HfB2	ZrB2 with Additives (e.g., SiC, MoSi2, B4C)	HfB2 with Additives (e.g., SiC, B4C, WC)	References
Melting Point (°C)	~3250	~3400	Slightly reduced due to eutectic interactions	Slightly reduced due to eutectic interactions	[20,217]
Density (g/cm3)	~6.08	~10.5	~6.1–6.3 (depends on additive and porosity)	~10.5–10.8	[17,218,219]
Thermal Conductivity (W/m·K)	60–120	70–115	40–90 (varies with additive and porosity)	50–95	[219,220]
Coefficient of Thermal Expansion (10−6 K−1)	5.9–6.3	6.4–6.8	6.0–6.5	6.5–7.0	[219,220]
Hardness (GPa)	~20–22	~21–23	~18–22	~20–23	[20,220]
Fracture Toughness (MPa·m1/2)	2–3	2–3.5	3–5 (with SiC or fibres)	3.5–5.2	[17,20,218]
Sintering Temperature (°C)	>2000 (pressureless)	>2100 (pressureless)	~1650–1900 (with sintering aids)	~1700–1950 (with sintering aids)	[17,217,218]
Typical Additives	—	—	SiC, B4C, MoSi2, WC	SiC, B4C, WC, Ni	[17,217,218]
Sintering Method	Hot-pressing or SPS required	Hot-pressing or SPS required	Pressureless sintering possible with aids	Pressureless sintering possible with aids	[17,20,217,218]

compares monolithic and modified ZrB₂/HfB₂ systems. Hypersonic programmes such as DARPA’s Falcon HTV-2 have renewed the demand for TPS capable of withstanding low-pressure, radical-rich flows and curved shock interactions [217,218,219,220,221,222]. ZrB₂/HfB₂ UHTCs show strong performance in sharp leading edges under shock loading [223,224,225], functioning like passive heat pipes that dissipate heat efficiently. Advanced processing routes, including reactive melt infiltration (RMI), precursor infiltration and pyrolysis (PIP), CVD, and additive manufacturing (AM), are enabling more cost-effective and high-performance UHTCs and UHTCMCs [13,226,227,228]. Remaining barriers include oxide-volatility under high-velocity flow, oxygen ingress after removal of molten layers, and large inconsistencies in reported melting points (up to 500 °C) [229,230].

UHTCs also play an important role in the defence sector, supporting the development of advanced ballistic missile nozzles, high-temperature armour, and aero-thermal structures requiring extreme thermal survivability [231]. Their integration into critical components ensures structural integrity and performance during high-speed atmospheric flight and precision strike operations. Overall, ZrB₂- and HfB₂-based UHTCs are strategic materials that enable high-speed flight, extended mission duration, and enhanced operational capability under severe thermal and mechanical conditions [210]. A recurring issue in the ceramic-materials literature is the interchangeable use of terms such as Ultra-High Temperature Materials (UHTMs) and UHTCs, which can lead to confusion in the material selection for aerospace, defence, and energy applications. UHTCs represent a specific subset of ceramic compounds, mainly transition-metal diborides, carbides, and nitrides, capable of service above 2000 °C, whereas UHTMs encompass a much broader range of materials, including refractory metals, composites, and other high-temperature systems. Overlap in applications such as TPS and hypersonic structures contributes to this ambiguity, but while UHTMs cover a wide material spectrum, UHTCs are uniquely valued for their extreme thermal stability and structural reliability in the harshest environments, including aerospace TPS and advanced nuclear systems.

The outlook for ZrB₂ and HfB₂ in aerospace and defence remains highly promising despite inherent challenges, such as poor sinterability and oxidation sensitivity at intermediate temperatures. Continued advances in processing, e.g., SPS, AM, along with the use of additives and composite reinforcement, are progressively mitigating these limitations. As manufacturing technologies mature, these ceramics are expected to become increasingly central to next-generation aerospace and defence systems. In 2025, the aerospace and defence sectors underwent a major transformation driven by artificial intelligence (AI), 3D printing, robotics, and blockchain, with an emphasis on innovation and sustainability. These technologies support decarbonization, next-generation defence platforms, and improved supply-chain transparency, accelerating the transition toward efficient, secure, and environmentally responsible operations [232].

3. Processing and Manufacturing

The processing and manufacturing of tough ceramics for aerospace and defence applications have evolved into critical and rapidly advancing fields, driven by the stringent performance demands of next-generation propulsion, protection, and hypersonic systems. Modern aerospace components must endure extreme environments characterized by ultra-high temperatures (>1500–2000 °C), rapid thermal cycling, high mechanical stresses, corrosive atmospheres, and intense particle erosion. Under these harsh conditions, traditional ceramics, despite their high melting points and chemical stability, are limited by their intrinsic brittleness and low tolerance to flaws. Consequently, the development and extreme manufacturing of tough ceramics, such as SiC, ZrO₂, Si₃N₄, ZrB₂, HfB₂, UHTC composites, and CMCs, has become a foundational enabler for advanced aerospace and defence systems [66,67,91,160,161,162].

These tough ceramics offer superior fracture toughness, enhanced resistance to crack initiation and propagation, and improved reliability when subjected to severe thermal–mechanical loads. Their ability to maintain structural integrity during conditions such as hypersonic flight (>Mach 5), turbine inlet temperatures exceeding 1500 °C, or ballistic impacts is directly tied to innovations in manufacturing that allow precise tailoring of microstructure, phase composition, and grain morphology. To achieve the required performance, extreme manufacturing approaches go far beyond conventional sintering or powder compaction. Techniques such as HP, HIP, SPS, reaction sintering, and advanced AM enable unprecedented levels of control over densification, grain growth, and defect distribution. HP and HIP remain essential for producing dense structural ceramics with minimal porosity, ensuring high strength and thermal conductivity stability. SPS, with its rapid heating rates and pulsed electric current, allows for fine-grained microstructures and tailored interfaces that enhance toughness through mechanisms such as crack deflection, bridging, and grain pull-out [100,101,102,103,104,105].

Meanwhile, AM has transformed the design possibilities for tough ceramics. Methods such as stereolithography (SLA), LCM, directed energy deposition (DED), binder jetting (BJ), and robocasting now allow for the fabrication of complex, lightweight components with integrated cooling channels, graded architectures, and novel geometries previously impossible with traditional machining. These capabilities are particularly important for aerospike nozzles, turbine rotors, lattice-reinforced armour tiles, hypersonic leading edges, and multi-functional TPS. In UHTCs like ZrB₂ and HfB₂, extreme manufacturing also includes reactive SPS, field-assisted sintering, and nanocomposite engineering, which refine microstructures to withstand oxidation, ablation, and thermal gradients during hypersonic re-entry [223,224,225,226,227,228]. For CMCs such as SiC/SiC, multi-step processing, chemical vapour infiltration (CVI), PIP, slurry infiltration, and melt infiltration (MI), is used to achieve a balance of toughness, stiffness, and oxidation resistance, enabling their deployment in turbine hot sections and missile propulsion systems [84,88,91,92,210].

Ultimately, the transition from brittle to tough ceramic systems in aerospace and defence is inseparable from advances in extreme manufacturing. By enabling microstructural precision, defect minimization, multimaterial integration, and high-performance geometries, these processing innovations unlock the full potential of tough ceramics for mission-critical applications. As hypersonics, high-efficiency propulsion, and next-generation protection systems continue to evolve, extreme manufacturing methods will remain central to producing ceramics capable of surviving and thriving in the world’s most demanding engineered environments. Given the extensive scientific literature available on advanced ceramic manufacturing, only a brief description of the fundamental methods will be provided here, focusing on those techniques that are most critical for achieving the toughness, reliability, and performance required in aerospace and defence applications.

3.1. Advanced Ceramic Forming Methods

The development of high-performance ceramic components for aerospace and defence increasingly relies on advanced manufacturing techniques that go beyond conventional forming methods. These processes are specifically designed to overcome the inherent brittleness and processing limitations of ceramics, enabling the fabrication of complex shapes with superior mechanical strength, thermal resistance, and oxidation stability, which are essential for UHTC like ZrB₂ and HfB₂ used in hypersonic vehicles and re-entry systems [13,233].

3.1.1. Additive Manufacturing Technology (AM)

AM has emerged as a powerful approach for producing complex ceramic geometries with internal channels or graded compositions, offering greater design flexibility and reduced waste while enabling multifunctional aerospace and defence components [6,172]. When combined with sintering additives and nano-reinforcements, AM enhances crack resistance and thermal shock performance. Ceramic oxides such as Al₂O₃ and ZrO₂ remain widely used due to their excellent properties [234,235,236], supporting applications in turbine blades, nozzles, and combustion liners where they improve thermal efficiency and reduce emissions [127,237]. Traditional methods like extrusion moulding, slip casting, and sol–gel processing face limitations in mould fabrication, geometric complexity, and reliance on solid-state sintering, leading to long processing times, shrinkage, and porosity [238,239]. AM overcomes these barriers through layer-by-layer fabrication, eliminating moulds, reducing post-processing, and enabling shorter production cycles and greater design freedom [240,241,242]. Consequently, AM enhances the feasibility of producing high-performance, complex ceramic oxide components for demanding aerospace and defence applications [236].

3.1.2. Robocasting

Robocasting, also known as direct ink writing (DIW) [243], is an extrusion-based additive manufacturing method that enables the layer-by-layer fabrication of complex ceramic structures, including lattices, graded porosity, and internal channels that are difficult to achieve with traditional forming techniques [244]. It is particularly suited for producing green bodies of tough ceramics such as SiC, ZrO₂, and Al₂O₃ composites used in aerospace and defence applications. Robocasting inks require high solid loading, shear-thinning behaviour, and carefully optimized additives to ensure printability, stability, and minimal shrinkage [243,244,245,246]. After printing, parts undergo drying, debinding, and sintering or pressure-assisted densification to achieve high mechanical and thermal performance. A comprehensive mapping of key robocasting variables, encompassing both the properties of the initial ceramic paste and post-printing parameters, and their influence on the final structural and functional properties of printed ceramic components are shown in Figure 8. Originally developed at Sandia National Laboratories [247] and patented in 2000 [248], robocasting continues to advance, offering tailored porosity, functionally graded structures, and digital manufacturing advantages that support next-generation aerospace and defence components [245].

Robocasting has proven effective for producing CMCs reinforced with short carbon fibres in ceramic matrices such as Al₂O₃, SiC with Al₂O₃–Y₂O₃, and SiC–B₄C systems [6,236]. Feilden et al. [249] demonstrated that HfB₂ components with complex geometries, uniform microstructures, and high strength, ~350 MPa at room temperature and ~200 MPa up to 1950 °C, can be fabricated by robocasting, comparable to conventionally processed materials. Ongoing advances in ink formulation, process monitoring, AI-driven optimization, and hybrid post-processing are expected to enhance reliability and broaden the technique’s applicability, positioning robocasting as a key technology for next-generation aerospace and defence ceramics.

3.1.3. Selective Laser Sintering (SLS)/Selective Laser Melting/(SLM)

SLS and SLM are key AM methods for processing UHTCs such as in ZrB₂, HfB₂, and SiC systems [6,250]. While SLM fully melts powders and SLS relies on localized sintering, both face challenges due to the extreme melting points, low optical absorption, and brittleness of UHTCs. Strategies including preheating, fine powders, and sintering aids or secondary phases (e.g., SiC, MoSi₂) improve densification, with SLS often supplemented by post-sintering or HIP [251,252]. Successful demonstrations include SiC and ZrB₂–ZrC–B₄C coatings by King et al. [253] and near-theoretical-density HfB₂ with strength >200 MPa up to 2000 °C. However, SLS ceramics still suffer from high shrinkage and porosity, as seen in Al₂O₃ composite microspheres with <50% density [254,255], necessitating post-processing such as infiltration or isostatic pressing [256]. Process-optimization routes to reduce porosity and enhance mechanical performance are summarized in Figure 9 [239]. Persistent challenges remain, particularly sintering temperature and final density; optimal laser energy, which is dependent on material thermal properties and powder packing, is crucial for successful SLS, as noted by Muthukutti et al. [254].

SLS sinters ceramic powders below their melting point, yielding partially fused, porous parts, whereas SLM fully melts the powder to produce dense components with superior mechanical properties, making it more suitable for high-strength structural applications [254]. Emerging hybrid strategies, such as combining SLM with reactive sintering, laser-assisted deposition, or infiltration, aim to overcome current limitations. Continued advances in processing and materials are positioning both SLS and SLM as key technologies for manufacturing UHTC components in aerospace propulsion, thermal protection, and hypersonic systems [251,257,258].

3.1.4. Binder Jetting (BJ)

BJ, first developed at MIT in the late 1980s [259,260] and later standardized by ASTM as a binder-based powder-joining process [261], enables flexible and scalable fabrication of ceramic parts without the high thermal gradients of laser-based methods. In this technique, a liquid binder selectively bonds powder particles to form a green body, which is later strengthened through debinding, sintering, or infiltration [262,263]. As illustrated in Figure 10, the process involves powder deposition, spreading, optional compaction, and binder application, with various powder-dispensing approaches available [264,265] and compaction not included in all systems [266]. After layer-by-layer printing and drying, the part is removed, cured, and post-processed to achieve the required density and mechanical performance [263].

BJ offers key advantages such as low cost, no thermal stresses, high production rates, and broad material compatibility [267,268,269], making it suitable for ceramics like SiC used in turbines, gears, and heat exchangers [270]. However, low density and strength remain major challenges, as green density strongly affects shrinkage and defects [263]. Critical parameters influencing density include layer thickness [271,272], compaction thickness [263], roller speed [273], binder saturation [274], and ultrasonic intensity [271]. Strategies to improve density fall into materials preparation (e.g., particle coating [275], granulation [276], mixed particle sizes [277], and slurry feedstocks [278]), process optimization [272,279], and post-processing, such as infiltration [280] and isostatic pressing [281]. Studies on Al₂O₃ show that additives like hydrophilic fumed silica improve flowability and green strength [282], while sol concentration and coating enhance shell properties [283]. Sintering behaviour indicates that additives promote low-temperature densification, though BJ-printed Al₂O₃ often remains below 80% relative density. Examples of complex BJ parts are shown in Figure 11a–f [284]. Overall, BJ is emerging as a versatile AM technology, with ongoing advances in resolution, material capability, and post-processing expanding its potential across ceramics, metals, and other powders.

BJ is not yet a complete solution for aerospace-grade ceramics, but it shows strong potential for components requiring complex geometries, thermal insulation, and weight reduction. With continued advances in material science and post-processing, it may become a key technology for next-generation ceramic parts in hypersonics, propulsion, and TPS. Recent developments highlight active efforts by NASA to refine BJ for high-performance ceramic components that are difficult to produce through conventional manufacturing.

3.1.5. Laminated Object Manufacturing (LOM)

LOM (Figure 12) builds 3D parts by bonding and cutting stacked material layers [285,286]. Recent advances allow LOM to process technical ceramics using laminated green tapes, which are later debound and sintered for densification [284]. LOM enables the fabrication of large, complex ceramic components with low residual stresses, since it avoids high-energy melting, making it suitable for aerospace structures exposed to thermal and mechanical cycling [284]. Its high deposition rates also support the production of large ceramic tiles and armour systems for defence applications.

LOM reduces tooling costs and manufacturing time but suffers from poor surface quality, lower dimensional accuracy, and slow removal of excess material, making it unsuitable for highly complex geometries [287]. Ceramic materials commonly used in LOM include Al₂O₃, SiC, Si₃N₄, and ZrO₂, chosen for their performance and process compatibility [288]. Despite moderate porosity, LOM-fabricated parts can achieve competitive flexural strength, such as Si₃N₄ components reaching 475 ± 34 MPa [289], comparable to PLS results [290] and far exceeding gel casting (54.5 ± 2.3 MPa) [291]. Limitations, however, arise from interlayer defects such as delamination and shrinkage mismatch [288]. A DARPA-funded curved-layer LOM advancement improved fibre continuity, reduced waste, and enhanced surface finish, expanding CMC applicability and strengthening LOM’s potential for aerospace and defence structures [292].

Table 11. Pros, cons, and aerospace/defence applications of ceramic AM methods [6,236,243,250,260,262,286,288].

Method	Advantages	Disadvantages	Aerospace & Defence Applications
SLS (Selective Laser Sintering)	High precision and good mechanical properties. No need for support structures. Suitable for complex geometries.	High thermal stresses. Limited to specific ceramic powders. Expensive equipment.	Turbine blades, thermal protection systems. Lightweight brackets and housings for satellites. High-temp ceramic parts for hypersonic vehicles.
Robocasting (Direct Ink Writing)	Excellent control over porosity and architecture. Room temperature processing. Can print highly viscous ceramic slurries.	Low printing speed. Limited to extrusion-compatible materials. Post-sintering shrinkage.	Porous ceramic insulation for re-entry vehicles. Custom-shaped dielectric ceramics for radar systems. Bio-ceramic implants for military med-tech.
LOM (Laminated Object Manufacturing)	Low material waste. Good for large parts. No need for powder handling. Can preserve fibre alignment.	Limited resolution. Mechanical properties depend on interlayer bonding. Limited commercial use.	Large CMC structural components (e.g., engine ducts, control surfaces). Prototyping of aircraft heat shields and armoured panels.
Directed Energy Deposition (DED)	Repair and remanufacturing capability. Large-scale fabrication. Functionally graded materials (FGMs). High deposition rates. Multi-material processing. Reduced material waste.	Lower dimensional accuracy. Microstructural anisotropy. Residual stresses & distortion. Process complexity. Porosity and defect risk.	Turbine engine components. Structural airframe components. Rocket & space systems. Tooling & moulds. Armoured vehicle components. Naval systems. Military aircraft maintenance. Weapon systems.
Binder Jetting (BJ)	Fast printing speed. No thermal distortion. Wide material compatibility. Cost-effective for large batches.	Requires extensive post-processing. Lower as-printed density. Fragile green parts.	Nozzle throats, ceramic cores for investment casting of turbine parts. Lightweight components for UAVs (Unmanned Aerial Vehicles). Complex ceramic parts for missile systems.

summarizes the advantages, limitations, and applications of major ceramic AM methods.

In aerospace, AM contributes to weight reduction, lowering fuel consumption and CO₂ emissions, while also enabling innovative geometries that enhance performance. In defence, its ability to produce custom, mission-critical components on demand, even in remote or supply-limited environments, enhances operational readiness and reduces dependence on traditional supply chains. Beyond large corporations, AM’s growing accessibility has democratized innovation, allowing participants from small enterprises to established research centres [293]. However, it is in the high-performance demands of aerospace and defence where additive manufacturing proves its highest value, pushing design limits, shrinking development timelines, and delivering agile, cost-effective solutions where conventional manufacturing falls short [294].

Among additive manufacturing technologies, only SLM and DED have demonstrated single-step shaping and sintering of advanced ceramics. DED, in particular, plays a strategic role in aerospace and defence applications, where it enables repair, refurbishment, and near-net-shape fabrication of high-performance metallic and ceramic components. Its applications span turbine engine repair, titanium airframe structures, space propulsion systems, and in situ manufacturing for long-duration missions. In defence contexts, DED enhances operational readiness through rapid field repair, reinforcement of armoured and naval systems, and corrosion-resistant overlay deposition.

3.2. Thermal Barrier Coatings (TBCs) and Environmental Barrier Coatings (EBCs)

EBCs and TBCs protect CMCs and metallic substrates from severe thermal, oxidative, and corrosive conditions [106,130]. These multilayer ceramic systems, typically based on YSZ, rare-earth silicates, and Al₂O₃, are produced using specialized deposition methods selected according to thermal-gradient resistance, oxidation protection, cycling performance, and substrate compatibility [134,140,144,151]. APS and EB-PVD remain standard for TBCs, while chemical vapour deposition/chemical vapour infiltration (CVD/CVI) is widely used for EBCs. Emerging AM and hybrid approaches aim to enable more complex and durable coating architectures.

3.2.1. Atmospheric Plasma Spray (APS)

APS is a widely used process for depositing TBC and EBC layers, producing thick (100–500 µm), strain-tolerant coatings through the melting and propulsion of ceramic powders in a plasma jet, with applications across aerospace, automotive, and energy industries [295]. It offers high deposition rates, versatility across materials and geometries, and enhances resistance to corrosion, wear, oxidation, and heat [296,297]. Advancements in such coatings have supported major technological progress, including gas turbine temperature increases from 400 °C in 1933 to 1600 °C in 2011, with goals of 1700 °C [298]. A schematic of the APS system is shown in Figure 13 [299], illustrating the formation of pores influenced by spray conditions [300], while bond coats improve adhesion and reduce thermal stresses. A cross-sectional TBC view is presented in Figure 14 [299], showing the topcoat, bond coat, substrate, and TGO.

Figure 14b shows an enlarged view of the coating microstructure, where high-velocity plasma deposition produces a layered arrangement with two main pore types: small circular pores (<2 µm²) from partially melted particles, and horizontally aligned lamellar pores formed by the stratified deposition process.

Thermal conductivity in APS-deposited TBCs is mainly governed by lamellar pores, while circular pores have a lesser effect. Their formation can be reduced by optimizing spray distance, particle temperature, powder feed rate, substrate preparation, and torch speed. As TBC requirements evolve alongside advances in hot-section materials, deposition methods have progressed from APS to EB-PVD and, more recently, to PS-PVD [301]. APS TBCs could not endure the high rotational speeds (>20,000 rpm) and thermal cycling of turbine blades in the 1980s [302,303], leading to the adoption of EB-PVD, which provides excellent cycling performance due to its strain-tolerant columnar structure despite its higher thermal conductivity [304,305]. However, EB-PVD’s thermal conductivity (1.6–2.3 W/m·K) remains a limitation [306]. PS-PVD, introduced in the 2000s, offers a third-generation solution with multiphase deposition and higher vapour-phase concentration, enabling dendritic, feather-like 7YSZ coatings as shown in Figure 15a–c [301], and positioning it as a strong candidate for next-generation TBCs [307,308].

The main challenges of PS-PVD, a third-generation TBC deposition method, involve its high process complexity, requiring precise control of plasma conditions, temperature, pressure, and feed rate, and the significant cost of advanced plasma equipment. Although potentially cheaper than EB-PVD in some aspects, PS-PVD still requires expensive systems and faces additional limitations such as restricted material compatibility in its multiphase environment, challenges in porosity control, and scalability issues for uniformly coating large or complex components.

3.2.2. Electron Beam Physical Vapour Deposition (EB-PVD)

Several methods exist for producing TBCs [301], but EB-PVD remains the most widely used for coating high-pressure/high-temperature (HPHT) turbine blades operating at temperatures above the alloy melting point [309]. Its main advantage is the formation of columnar microstructures that provide excellent strain tolerance during thermal cycling. However, EB-PVD faces challenges when evaporating novel or complex materials. In general, physical vapour deposition (PVD) is a vacuum-based technique in which material is vaporized from a target and transported through a low-pressure chamber before condensing on the substrate to form thin films ranging from nanometres to micrometres [310,311]. The three main stages of the PVD process are shown in Figure 16 [310].

In EB-PVD, a focused electron beam serves as the energy source, enabling much thicker coatings, often hundreds of micrometres, due to higher deposition rates compared to other PVD methods such as sputtering. The specialized equipment used for EB-PVD is shown in Figure 17 [310], and a detailed description of the technique is provided in reference [312].

In EB-PVD, gas-phase species accumulate on the substrate to form a columnar microstructure, but the lack of transverse discontinuities reduces its thermal insulation capability [313,314]. Research has shown that thermal conductivity increases with coating thickness [315], while variations in deposition parameters can adjust coating morphology, with intra- and inter-columnar porosity strongly influencing thermal conductivity [316]. A typical EB-PVD partially Y₂O₃-stabilized ZrO₂ (PYSZ) coating with uniform, independently grown columns and horizontal discontinuities is illustrated in Figure 18 [314].

The columnar microstructure of EB-PVD coatings strongly influences TBC thermal performance, with higher porosity near the bond coat reducing conductivity and lower porosity near the surface increasing it. Designing layered coatings to enhance porosity can further decrease effective conductivity, while Y₂O₃ doping increases phonon scattering in ZrO₂, lowering conductivity through reduced phonon mean free path [313,314]. These combined microstructural and compositional strategies improve thermal insulation, ensuring that EB-PVD remains essential for aerospace and defence applications requiring extreme thermal protection and superior thermal cycling resistance.

3.2.3. Chemical Vapour Deposition (CVD)

APS and EB-PVD remain standard TBC deposition methods for aircraft engines and industrial gas turbines [316], but both are limited by their “line-of-sight” nature, making uniform coating of complex geometries difficult without advanced robotics. EB-PVD adoption is further constrained by high equipment cost, prompting interest in alternative technologies offering comparable coating quality without geometric or economic limitations [316]. Current TBC/EBC methods include APS, EB-PVD, sputtering, slurry deposition, PS-PVD, and CVD, with APS most common for EBCs [317,318]. EB-PVD also faces challenges in achieving proper stoichiometry, particularly due to the low vapour pressure of silica, increasing interest in CVD [319] and emerging sol–gel/slurry approaches [320].

CVD enables the deposition of dense, uniform TBCs and EBCs with excellent control of thickness, composition, and microstructure, producing materials such as YSZ for TBCs [316,321] and rare-earth silicates for EBCs [322]. Its ability to conformally coat complex geometries, including internal surfaces, is a major advantage [323], and the process can yield columnar microstructures under suitable conditions [324]. Effective bonding requires proper surface preparation. Multiple CVD variants exist, including low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), metal–organic CVD (MOCVD), atomic layer deposition (ALD), Hot-Wall, and Cold-Wall systems, each tailored to specific applications [325,326]. Figure 19 illustrates the transport and reaction steps of typical CVD processes [326], with a more detailed description available in Ref. [324].

The deposition technique has a strong impact on coating microstructure and thus on properties such as thermal conductivity, strain tolerance, and durability. CVD provides several major advantages for TBCs and EBCs in advanced turbine engines and extreme environments [321,322,323], including excellent conformality on complex or internal geometries, strong coating–substrate adhesion for improved thermal cycling life, precise control of thickness and composition for graded or multilayer architectures, and compatibility with high-temperature ceramic materials such as SiC, Si₃N₄, SiO₂, Al₂O₃, Y₂O₃, and ZrO₂ [322,326]. Although CVD is generally more costly and slower than plasma spray methods, its superior coating quality and lower deposition temperatures make it highly valuable for mission-critical aerospace applications requiring long service life and reliability.

3.2.4. Sol–Gel Processing

The sol–gel process is a versatile soft-chemical technique for producing advanced inorganic materials, especially ceramic coatings such as TBCs and EBCs. It converts a precursor “sol” into a solid “gel” through hydrolysis and condensation, followed by deposition and heat treatment to create dense or porous ceramic layers [327,328]. The method provides precise control of composition and microstructure, enabling tailored thermal, mechanical, and corrosion-resistant properties. For TBCs, sol–gel processing allows deposition of materials like YSZ onto metallic turbine components. The fundamental steps of the process are shown in Figure 20 [327].

The sol–gel method enables the low-temperature deposition of compositionally tuneable coatings with controlled porosity and uniform coverage, including on complex internal geometries, using dip, spray, or hybrid deposition strategies [329,330]. At the industrial scale, EB-PVD [309,313] and APS [295,299] dominate TBC fabrication, producing columnar and lamellar microstructures, respectively, while the sol–gel approach offers a low-cost alternative with equiaxed morphologies, adaptable porosity, and strong substrate coverage [321]. Advances such as precursor-powder dispersion and multimodal mixtures have yielded homogeneous YSZ coatings with excellent durability, surviving >55 cycles at 1100 °C and retaining performance up to 1150 °C [327,329,331]. Studies at Cirimat [332] showed sol–gel YSZ TBCs enduring >1000 cycles at 1100 °C, aided by stabilization of the metastable t′-phase and optional fibre reinforcement [333,334]. Surface cracking remains a challenge, but optimized sintering and multi-pass deposition significantly improve life, enabling >1500 oxidation cycles and revealing distinct failure modes compared with EB-PVD [333]. Sol–gel processing is also advancing EBC development using rare-earth silicates for CMC protection. Recent tests demonstrated sol–gel TBCs withstanding 1485 cycles (1 h at 1100 °C) without major degradation [335]. CMAS infiltration remains a critical threat to TBCs [336], with APS showing greater resistance than EB-PVD [151,337,338]. Sol–gel coatings offer a tuneable, low-temperature route to CMAS-resistant layers [336]. CMAS-induced thermomechanical and thermochemical attacks have caused severe turbine damage, with a notable case reported by Song and Guo [339]. Awareness of volcanic ash hazards increased dramatically after the 2010 Eyjafjallajökull eruption (see Figure 21).

Coatings doped with ytterbium (Yb) and dysprosium (Dy) enhance CMAS resistance in TBCs, as 6 at % additions promote the formation of a new surface phase, potentially apatite, that creates a sealing layer on top of TBC columns without damaging the underlying structure [336]. Although mechanical durability and thermal-cycling stability remain concerns, sol–gel coatings continue to offer a promising route for CMAS mitigation. Ongoing research on rare-earth-doped sol–gel layers is expected to support next-generation TBCs capable of operating under harsher conditions and higher TET levels. Overall, the sol–gel process shows strong potential for future TBC/EBC manufacturing and CMAS resistance due to its chemical flexibility, cost-effectiveness, compatibility with current systems, and ability to address environment-specific threats.

The degradation of thermal barrier coatings under CMAS exposure cannot be interpreted as a simple surface contamination phenomenon; rather, it represents a multi-scale instability involving thermochemical reactivity, phase destabilization, microstructural densification, and evolving mechanical incompatibilities.

CMAS attack reveals intrinsic vulnerabilities in conventional YSZ-based systems that stem directly from the very microstructural features that make them effective thermal insulators. At the microscale, capillary-driven infiltration transforms a compliant, strain-tolerant porous architecture into a stiffened, glass-infiltrated composite. At the crystallographic level, yttria depletion and dissolution–reprecipitation processes compromise the metastable tetragonal phase, reducing transformation toughening capability and promoting monoclinic formation with associated volumetric expansion. At the mesoscale, these mechanisms generate stiffness gradients and residual stress concentrations that intensify during thermal cycling.

A critical insight emerging from this analysis is that CMAS attack and sintering are not independent degradation pathways but mutually reinforcing processes. Intrinsic high-temperature sintering already reduces porosity and increases elastic modulus over service time; CMAS infiltration accelerates and localizes this densification. The resulting loss of compliance undermines the fundamental design principle of TBCs: strain accommodation under severe thermal gradients. Consequently, spallation becomes a mechanically inevitable outcome once a critical stiffness threshold and interfacial stress state are reached. Furthermore, phase instability introduces a second-order reliability concern. The t → m transformation not only induces microcracking but also progressively diminishes the toughening mechanisms that initially delay catastrophic failure. In this context, the degradation is cumulative and self-amplifying: chemical destabilization enhances mechanical fragility, which in turn accelerates crack propagation under cyclic loading. On the other hand, from a materials design standpoint, these findings underscore several strategic imperatives: (i) the reduction in microstructural permeability without excessively compromising strain tolerance, (ii) improved thermochemical compatibility through compositions resistant to silicate dissolution, (iii) enhanced phase stability at ultra-high temperatures to prevent yttria depletion and destabilization, and (iv) sintering-resistant architectures, particularly for long-duration high-temperature exposure [336,339].

Importantly, mitigation strategies must balance chemical resistance with mechanical compliance. Dense or highly CMAS-resistant coatings may reduce infiltration but risk premature failure due to thermal mismatch stresses. Conversely, highly porous systems improve strain tolerance but remain vulnerable to capillary attack. The future of advanced TBC systems, therefore, lies in multi-layered designs that decouple permeability, compliance, and chemical resistance functions. Therefore, CMAS attack should be conceptualized as a coupled thermochemical–thermomechanical instability that progressively erodes microstructural integrity, phase stability, and mechanical reliability [336]. The durability of next-generation ceramic coatings will depend not only on compositional innovation but on integrated control of processing, architecture, and high-temperature evolution mechanisms.

4. Sintering of UHTC Coatings

Bulk UHTCs are typically densified using SPS, HP, and MWS to achieve near-theoretical densities, refined microstructures, and high-temperature mechanical performance, making them suitable for leading edges and nose caps, though brittleness and oxidation susceptibility require protective coatings [340]. UHTCs are increasingly used in TBCs and EBCs, where systems such as ZrB₂–SiC and HfB₂–SiC improve oxidation resistance through silica-scale formation, while carbides and nitrides provide ablation resistance; coating methods include plasma spraying, CVD, slurry deposition with sintering, and laser-assisted processes [106,313]. Sintering provides structural integrity in bulk UHTCs and consolidation/adhesion control in coatings, but for TBCs, the key factor is the deposition method, since excessive sintering during service degrades performance. Porosity is beneficial in TBCs, while EBCs require high density, and additives such as SiC, rare-earth oxides, and graded layers improve thermal compatibility and environmental stability [313]. Coating performance depends on the sintering-driven control of adhesion, grain size, porosity, and formation of protective secondary phases such as SiO₂ from SiC-containing UHTCs [83,140,141]. Strategies to overcome poor UHTC sinterability include adding SiC/Si₃N₄, silicides (MoSi₂, WSi₂), rare-earth oxides, and nanopowders [208,341]. As shown in Figure 22 [342], UHTC development for hypersonic applications proceeds from pellet processing and sintering optimization to full property characterization, qualification testing in plasma arc jets [343,344], plasma wind tunnels [345], and oxyacetylene rigs [346], supported by computational modelling. Final component fabrication remains challenged by the difficulty of achieving full densification without grain coarsening and by microstructural degradation from non-uniform sintering temperatures [20].

To address the challenges associated with dimensional scalability and the fabrication of large-scale ceramic components, critical aspects of the UHTC development cycle, Mukherjee and Basu [342] provide, in Figure 23, a comprehensive overview of the UHTC development pathway and its application in hypersonic technology.

The precise optimization of sintering parameters, processing routes, and additive compositions is essential to achieving the desired microstructure–property balance in UHTCs, which, after aerothermal performance evaluation and TPS design, are scaled up, machined into complex geometries, and ultimately validated through flight testing. A key distinction exists between traditional UHTC coatings and those produced via sol–gel methods: sol–gel-derived coatings are intentionally sintered as part of their consolidation process [327,328], whereas many conventional coatings remain partially porous. Sol–gel routes have been extended to UHTC precursors such as ZrB₂, HfB₂, SiC, and ZrC using organometallic or polymeric sources [347], in which precursor layers are deposited and pyrolyzed at 1200–1600 °C to yield dense nanoscale-sintered coatings, though thickness and precursor stability limit their applicability [348,349]. Porous UHTC sol–gel processing generally follows templating, foaming, or solvent-evaporation approaches, as depicted in Figure 24I–III [348].

The templating method infiltrates a porous template with a pre-ceramic sol, followed by drying and heat treatment up to 1600 °C to form porous UHTCs via carbothermal reduction. The final ceramics replicate the template morphology with some shrinkage, with UHTC phases forming around 1200 °C and fully developing between 1300 °C and 1500 °C, accompanied by weight loss. This approach enables varied porosities and pore sizes in systems such as ZrC, ZrC/SiC, ZrB₂, and ZrB₂/SiC [341,348,349,350].

For hypersonic flight (>Mach 5, 1715 m·s⁻¹), UHTCs like ZrB₂ and HfB₂ are key TPS candidates, though their oxide by-products (ZrO₂, HfO₂) undergo phase transformations that cause cracking. Dopants are used to stabilize these oxides and improve oxidation resistance. Zheng [347] produced submicron HfB₂ powder (~200 nm) using a modified sol–gel method with carbothermal reduction, mitigating cracking and enhancing high-temperature oxidation resistance. Figure 25 shows the synthesis route for submicron HfB₂ powder.

The sol–gel-derived HfB₂ powders produced via carbothermal reduction (Figure 26) exhibited a mean particle size of 0.19 μm and a surface area of 2.98 m²·g⁻¹, markedly finer than commercial Tribacher HfB₂ (1.90 μm, 1.92 m²·g⁻¹). Their ultrafine nature is linked to the small HfO₂ nuclei in the precursors, and purity was enhanced by adding excess boron to offset losses during reduction. Despite varying the nominal Hf:B:C ratios of 1:2:5 to 1:3:5 and 1:4:5 (Figure 26a–c), particle size remained largely unchanged, with all powders remaining significantly finer than commercial material, as shown in Figure 26d.

The particle size of HfB₂ is crucial for hypersonic ceramics. Fine particles enhance sinterability, densification, and oxidation resistance, leading to stronger, more durable microstructures. In contrast, coarse particles reduce densification, increase defects, and weaken performance under hypersonic conditions. In the near future, the sol–gel method will evolve from a specialized domain synthesis route into a strategic platform for engineering UHTCs with customized morphologies and multifunctional properties, particularly for lightweight, high-performance TPS and aerospace components exposed to extreme thermo-oxidative environments. Its compatibility with low-temperature processing, nanoscale design, and advanced manufacturing techniques positions sol–gel as a key enabler of next-generation ceramic solutions in aerospace technology.

5. Materials Selection for Aerospace and Defence Systems

The integration of aerospace and military technologies has strengthened operational capabilities, with 2025 global defence spending reaching $1.8 trillion and over 60% of defence systems relying on aerospace innovations. Unmanned aerial vehicle (UAV) deployment has increased by more than 50% since 2023, while hybrid propulsion systems can cut fuel use by up to 30%. Companies in both sectors show 10% annual growth, and the U.S. Department of Defence plans a 25% rise in prototyping, supported by dual-use projects such as NASA’s Artemis programme [351]. Aerospace-derived materials significantly enhance protective equipment: carbon-fibre composites reduce weight, multi-layer ceramics improve impact resistance by up to 30%, AM is projected to grow 20.9% to $35.6 billion by 2027, smart materials may cut armour weight by up to 50%, and thermal barrier coatings boost durability by 40% [351]. Ceramics remain vital for aerospace due to their thermal resistance and low density [170,352,353], enabling lighter, more efficient components [170,353]. They are essential in turbine blades [353,354], re-entry tiles [352], and sensors, with recent advances such as Y-Al-Si-O glass-coated oxide–oxide ceramic matrix composites (OCMCs) for thrust chambers [355], despite challenges of brittleness and processing [165]. Current research targets tougher CMCs [65,356], AM-based fabrication, and novel SiCN–YSZ–CNT multilayer composites that offer 22.5% lower thermal conductance and electromagnetic interference (EMI) shielding [357].

Advances in nanotechnology and hybrid materials are driving the development of ceramics with higher fracture toughness, lower weight, and improved thermal resistance for more efficient engines and robust spacecraft [352]. Weight reduction remains central to aerospace design, relying on dual alloys and ceramic, metallic, and polymer composites, along with thin-walled, buckling-resistant structures. Designers must balance functional demands with constraints to achieve optimal material and design combinations through an iterative process, supported by early physical understanding to reduce lifecycle costs [358]. Material selection evolves from broad, approximate comparisons during concept development to precise data and internal testing for critical components. As indicated in Figure 27, polymers and light alloys (Zn, Mg, and Al) lose effectiveness above 300 °C, titanium alloys and high-strength steels function up to 600 °C, superalloys (Ni, Fe, and Co) are required beyond this range, and extreme conditions demand refractory metals (e.g., tungsten) or ceramics (SiC, Al₂O₃). The proper selection of advanced ceramics, especially UHTCs, TBCs, and EBCs, is essential for aerospace and defence systems operating in severe environments such as hypersonic flight, propulsion, and turbine components.

Each candidate ceramic material must undergo rigorous qualification protocols, thermal cycling, oxidation and ablation resistance tests, creep and fracture-toughness evaluations, and microstructural stability assessments, to define its true performance envelope and match it to aerospace or defence requirements. Material selection is therefore a systematic, multidisciplinary process, as an incorrect choice can compromise efficiency, durability, or safety, while the correct one ensures structural integrity, thermal protection, and long-term reliability. A five-step framework (Figure 28) guides this selection by integrating technical requirements, operational constraints, and organizational factors to optimize component performance.

The deliberate selection and validation of advanced ceramics enable higher operating temperatures, improved fuel efficiency, reduced weight, and greater mission resilience, highlighting their essential role in advancing aerospace and defence technologies through the synergy of materials science, engineering design, and operational needs.

6. Innovative Trends in Ceramics for Aerospace and Defence Applications

The aerospace and defence ceramics market is expanding rapidly, driven by demand for materials that withstand extreme temperatures, investments in propulsion technologies, and advances in ceramic composites. UHTCs, projected to grow 8–10% annually, are essential for hypersonic vehicles, space propulsion, and TPS, with progress in ZrB₂ and HfB₂ improving thermal stability and oxidation resistance while R&D addresses brittleness and manufacturing challenges [359,360,361]. Sustainability, nanotechnology, and AM further accelerate adoption, and North America and Asia-Pacific lead market growth. Key materials include SiC, B₄C, ZrB₂, HfB₂, and TiC. TBCs and EBCs remain critical for extreme-condition aerospace applications, supporting higher engine temperatures beyond the ~1100 °C limit of alloys [127,362,363]. Emerging ferroelastic rare-earth tantalates show promise for next-generation TBCs with operation at ≥1600 °C [363]. Rising service temperatures, ~1700 °C in the F119, nearly 2000 °C in the F135-PW-100, and >2000 °C in future engines [127,358,364,365], along with >2000 °C re-entry heating on the X-37B [366,367,368], demand superior coatings. Lunar missions also require erosion-resistant ceramics due to highly abrasive lunar dust, as highlighted by Wiesner et al. [369] and addressed through NASA’s Artemis and LO-DuSST initiatives.

Entropy-stabilized ceramics containing five or more principal elements have accelerated the development of HECs across nitrides, oxides, carbides, borides, and related chemistries [370,371]. Their highly disordered lattices provide low thermal conductivity, high phase stability, and strong resistance to oxidation and sintering, enabling next-generation TBCs capable of operating above 1200 °C in turbines, hypersonic vehicles, and rockets [372,373,374]. Techniques such as plasma spraying, EB-PVD, and SPS demonstrate their manufacturability, while ongoing research focuses on optimizing composition, phase stability, and structural design. Deploying HECs in extreme aerospace and defence environments requires evaluating thermophysical, mechanical, chemical, and microstructural properties, as well as processing and integration challenges. Emerging strategies, including entropy-based design, multilayer architectures, and functionally graded materials, enhance performance, and Figure 29 summarizes the key factors governing HEC applications [375].

This emerging material system has drawn strong research interest because of its unique structure and exceptional performance. As outlined by Wang et al. [375], a comprehensive framework, encompassing fundamental concepts, synthesis methods, material properties, structural characteristics, and application prospects, enables clearer pathways for optimizing HECs and unlocking their potential for next-generation aerospace, defence, and ultra-high-temperature energy systems.

7. Conclusions

According to Padture [25], aerospace ambitions are imposing unprecedented demands on propulsion systems, positioning advanced structural ceramics as essential materials for meeting extreme thermal, mechanical, and environmental requirements. Ceramics are central to technologies such as TBCs and EBCs, where materials like YSZ dominate, and SiC- and oxide-based CMCs are increasingly used in hot-section and propulsion components. For the harshest conditions, UHTCs are indispensable, though their inherent brittleness necessitates reinforcement; continuous fibre-reinforced UHTCMCs, supported by engineered interphases and optimized architectures, provide the most significant gains in toughness and fracture resistance.

Emerging processing approaches, particularly AM, are reshaping ceramic manufacturing by enabling lightweight, complex geometries with high precision. Although challenges remain, fatigue behaviour, surface finish, certification, AM offers major advantages for critical aerospace parts and even supports in situ resource utilization in space. Concurrently, research is advancing hybrid processing routes to overcome the inefficiencies of traditional CVI and PIP while exploring matrix chemistries beyond ZrB₂–SiC.

Overall, continued investment in advanced ceramics is strategically vital as aerospace and defence move toward hypersonic systems, reusable spacecraft, and next-generation UAVs. These materials not only address current engineering limitations but also enable future high-performance platforms. Their adoption also aligns with global sustainability goals: reduced weight and improved thermal management decrease fuel consumption and emissions. As nations seek lighter, stronger, and more resilient defence systems, ceramics play a key role in enhancing mobility, efficiency, and survivability. Future progress will depend on integrating manufacturing innovations with advances in damage modelling, non-destructive evaluation, and high-temperature testing. Together, these efforts confirm advanced ceramics and ceramic composites as indispensable enablers of next-generation aerospace and defence technologies.