Mechanical Behavior and Reliability of Engineering Ceramics

1. Introduction

Engineering ceramics are successfully used as structural or functional materials in a wide range of technical and biomedical applications [1,2,3,4,5] thanks to advantages such as durability, chemical stability and temperature resistance. However, they are prone to brittle fracture under mechanical or thermal stresses due to crack propagation from pre-existing flaws introduced during processing or surface machining, which serve as stress concentrators. Mechanical properties such as hardness, strength, elastic modulus and fracture toughness are incredibly important for engineering ceramics, depending on their applications. Wear and creep behavior are also key factors affecting material durability, as well as fatigue under static or cyclic stresses. Statistical analysis is necessary to estimate material reliability, often evaluated by the Weibull modulus that characterizes the flaw distribution [6,7]. Material properties depend strongly on the composition and microstructure, particularly the porosity and grain size [8,9], and this relationship must be well understood to optimize the performance of the materials.

Manufacturing processes play a key role in mechanical properties and behavior. For example, it has been shown that colloidal processing can eliminate agglomerates and organic inclusions [10], resulting in improved fracture strength. The development of new ceramic manufacturing processes, such as spark plasma sintering (SPS) [11,12], has significantly enhanced the strength of ceramic materials by increasing densification and reducing grain size. Recent advances in additive manufacturing and 3D printing processes of ceramic materials [13,14,15] has enabled complex shapes to be obtained that could not be produced by other technologies. Another aspect that influences the mechanical properties of engineering ceramics is surface preparation, such as surface texturing, which contributes to improving tribological performance [16,17].

Over recent decades, research has focused on material design by tailoring microstructures to obtain flaw-tolerant ceramics with enhanced fracture toughness. Different strategies have been applied based on crack shielding–toughening mechanisms such as crack bridging [18,19,20] and zirconia transformation toughening [21,22,23,24]. In particular, transformation toughening has been employed to increase the fracture toughness of a variety of zirconia-based ceramics and composites, including zirconia-toughened alumina (ZTA) [5] and tetragonal zirconia polycrystals (Y-TZPs), in which the metastable tetragonal phase is retained at room temperature by adding yttria (Y₂O₃), typically with a content of 3 mol% (3Y-TZPs) [25]. In these materials, stress-induced tetragonal-to-monoclinic (t-m) phase transformation, accompanied by volume expansion [26,27], may occur around a propagating crack tip. This leads to compressive stresses that prevent crack propagation and thus increase the material toughness [24,28,29]. However, the mechanical strength of zirconia ceramics is often limited by the phase transformation [30,31] and recent studies attempted to meet the challenge of obtaining zirconia ceramics that combined both high toughness and high strength, using different strategies to optimize the rate of the t-m phase transformation [32,33,34].

Among the ceramic materials recently developed, high-entropy ceramics (HECs), which are potentially useful as functional materials for thermoelectric, catalytic, dielectric and magnetic applications, are rapidly growing due to their wide range of properties and behaviors [35,36,37,38,39]. These materials, including carbides, borides and oxides, typically contain five or more elements and form a single-phase solid-solution crystal structure. The concept of HECs was applied to ultra-high-temperature ceramics (UHTCs), known for their high melting points (>3000 °C), to form multiple-component systems [40,41] with high mechanical performance, particularly in a high-temperature environment. The mechanical behavior of these materials, which has been scarcely studied, need to be understood to ensure their reliability.

Engineered cementitious composites (ECCs) are fiber-reinforced materials that can provide enhanced tensile strength, ultra-high ductility and crack resistance without steel reinforcement [42]. This is due to crack bridging by high-strength fibers which dissipate the fracture energy and prevent sudden brittle failure under tensile loads. The application of 3D printing technology to these materials has received great attention [43,44] as it reduces the construction time and cost. The challenges in the development of 3D-printed ECC materials include tailoring them to balance printability, buildability, mechanical strength, and durability.

The aim of this Special Issue is to bring together papers that advance recent understanding of the mechanical behavior and reliability of engineering ceramics, in relation to their microstructure and manufacturing processes.

2. Overview of the Contributions

This collection brings together 1 comprehensive review and 13 research papers that address different types of engineering ceramic materials, including zirconia-toughened ceramics and composites, high-entropy ceramics, and engineered cementitious composites. Their important mechanical properties or reliability are discussed in terms of composition, microstructure and manufacturing process. Here we present a brief overview of selected contributions that cover the key topics of this Special Issue. Five focused on zirconia-based ceramics and it was shown that optimizing the stress-induced t-m phase transformation rate increases their mechanical performance. In the case of Y-TZP ceramics, it is known that 3Y-TZPs may have high strength (>1 GPa) but only a moderate fracture toughness (4–6 MPa^1/2) [45,46]. In their contributions, Kern and Osswald [47] and Imariouane et al. [48] showed that reducing the Y₂O₃ content in new alumina-doped Y-TZPs to 2 and 1.5 mol%, respectively, increased the t-m phase transformation rate significantly, which enabled the combination of high strength and relatively high toughness, which reached 8.5 MPa^1/2 for 1.5Y-TZPs. In [48], it was also reported that the t-m phase transformation is beneficial for the impact resistance of zirconia-based ceramics. By comparing the impact behavior of 3Y-TZPs, 1.5Y-TZPs and a Ce-TZP composite, the authors showed that the higher the transformability, the greater the ability to absorb energy upon impact and the higher the impact resistance. The Ce-TZP composite exhibited the highest impact resistance due to a high phase transformation rate, with ductile behavior. In [49], the relationship between the composition, microstructure and mechanical properties of zirconia–magnesia–alumina composites (ZrO₂–MgO–Al₂O₃) was investigated. It was shown that adjusting the material composition leads to the formation of metastable tetragonal and cubic zirconia phases and significantly improves the biaxial bending strength. In [50], the authors reported beneficial effects of the t-m phase transformation on the reliability of a zirconia–ceramic composite, stabilized with yttria and ceria and reinforced with alumina platelets (ZrO₂CeYAl₂O₃), which exhibited a high Weibull modulus (16.8) and fatigue resistance. In [51], a finite element analysis was used to explore the influence of surface texturing on wear and friction in zirconia ceramic-on-ceramic hip implants, which are important for their durability. It was shown that using textured surfaces with dimples induced a significant wear reduction (17%), compared to smooth surfaces. This is particularly linked to the fact that dimples act as lubricant reservoirs and trap wear debris.

In the field of high-entropy ceramics, the creep behavior of (Co, Cu, Mg, Ni, Zn)O transition metal high-entropy oxide ceramics (TM-HEO) was reported in [52]. Samples with grain sizes ranging from coarse to nano-scale were deformed under compressive stresses (5 to 31 MPa) at temperatures between 600 and 850 °C. Shear-thickening superplastic transitions were identified and the results enabled the construction of mechanism maps that could predict the deformation behavior of TM-HEO for a wide range of deformation conditions. In [53], it was demonstrated that the combined action of adding Ni and SiC whiskers improved the fracture toughness of boride HECs, due to crack bridging. Moreover, adjusting the sintering temperature to promote densification increased their fracture strength. In [54], it was shown that the wear resistance of carbide UHTCs synthesized by SPS was improved by adding components to form a multiple-component system. The wear resistance of ternary (Ta, Nb, Hf)C and quaternary (Ta, Nb, Hf, Ti)C increased respectively by 29% and 49% compared to binary (Ta, Nb)C. This was attributed to high densification, high hardness and complete solid-solution formation in the quaternary system.

Tulliani’s review paper [55] was dedicated to recent advancements in 3D-printed engineered cementitious composites (3DP-ECCs). The author analyzes, in particular, the potential of 3D printing technologies and the mechanical properties and durability of the materials. They showed that 3D printing enables the design of complex structures, like Bouligand, knitted or tilted structures, that may improve fracture performance for specific applications. 3DP-ECCs exhibit anisotropic properties due to fiber alignment in the printing direction. In particular, the compressive strength and stress–strain relationships are different in the parallel and perpendicular directions to the printing direction. Among the parameters influencing mechanical properties of 3DP-ECC, the composition, the fiber-reinforcing index and the loading direction play a crucial role. Internal curing may also improve the mechanical strength of the materials.

3. Conclusions

The contributions compiled in this Special Issue provide recent research developments on mechanical behavior and properties of a variety of engineering ceramics including zirconia-toughened ceramics and composites, high-entropy ceramics, and engineered cementitious composites. They show that tailoring the microstructures and compositions of materials or using advanced manufacturing techniques as SPS or 3D printing may enhance the mechanical performance of materials depending on their applications.