Editorial for Special Issue: “Research Progress and Prospect of Functional Thin Films & Hard Protective Coatings”

As guest editor of Coatings, it is my great pleasure to introduce this Special Issue focused on “Research Progress and Prospect of Functional Thin Films & Hard Protective Coatings”. Functional thin films and hard protective coatings are indispensable enabling materials that boost performance upgrades across aerospace [1], electronics [2], energy [3,4], environmental governance [5,6,7], biomedical engineering [8] and advanced manufacturing [9]. They endow common substrates with targeted functions such as photoelectric response [10], wear resistance [11], corrosion protection [12], thermal control [13], self-healing [14,15,16] and photocatalysis [17]; break through the performance bottlenecks of conventional bulk materials; and provide core support for the development of high-end equipment and emerging technologies.

This Special Issue was launched to display state-of-the-art advances, summarize cutting-edge progress, and clarify existing challenges and prospect future directions in the field of functional thin films and hard protective coatings. It aims to connect fundamental material design, preparation technology, characterization methodology and industrial application, covering both basic mechanism research and innovative engineering practice. The scope includes advanced deposition technologies, optoelectronic thin films, transparent conductive oxides, high-entropy alloy coatings, hard protective coatings, diamond and carbon-based films, aerospace functional coatings, surface modification, nanostructured composite films and computational modeling for coating design.

After rigorous peer review, we are delighted to present 12 high-quality papers in this Special Issue, including 2 comprehensive review articles and 10 original research contributions. These works fully cover the core topics of the Special Issue, reflect the latest research trends, and involve multi-system materials, diversified preparation processes and multi-scenario applications, forming a complete research system from material synthesis to performance evaluation.

The two review articles provide systematic and critical summaries for key subfields, offering valuable guidance for researchers. Dr. K. Bertuol et al. from Concordia University, Canada, deliver a systematic overview of metallic abradable coatings for gas turbine engines, focusing on material fundamentals, fabrication, microstructural design, performance trade-offs and reliability evaluation [18]. Abradable coatings act as sacrificial sealing layers to minimize blade wear and maintain tight blade-clearance, thus boosting engine efficiency and safety. These coatings are classified into polymeric, metallic and ceramic systems by operating temperature; metallic coatings excel in the 250–750 °C range, striking a favorable balance between mechanical stability and controllable wear.

Typical metallic systems adopt AlSi, Ni-based, Co-based or Cu-alloy matrices, combined with solid lubricants (hBN, graphite, MoS₂) and tailored porosity to optimize abradability. Atmospheric plasma spray (APS) is the mainstream thermal spray technique for coating deposition, as it enables flexible tuning of microstructure, thickness and interfacial bonding. A core design dilemma lies in balancing abradability (requiring low hardness and high porosity) and erosion–corrosion resistance (needing high structural integrity). Conventional tribological tests cannot reproduce engine-realistic high-speed rubbing and thermal conditions, so specialized high-speed abradable test rigs are indispensable for assessing reaction forces, wear mechanisms, blade damage and coating degradation. This review unifies material design, thermal spray processing and performance validation, highlights unresolved challenges and proposes future directions including functionally graded coatings and computational modeling, to support the development of next-generation high-performance metallic abradable coatings.

In the other review, Dr. Guo et al. systematically elucidate the doping control technologies for SnSe thin films, a rising star in thermoelectric and optoelectronic fields. Aiming at the inherent bottlenecks of intrinsic SnSe thin films such as high defect density, low carrier concentration, and high carrier recombination rate, this review summarizes three core regulation mechanisms including band engineering control, defect control, and carrier concentration control, classifies doping strategies into isovalent doping, aliovalent doping, and chalcogen doping, and analyzes the influence of doping atomic fraction on optical and electrical properties. It also compares the advantages and limitations of thermal evaporation, magnetron sputtering, pulsed laser deposition (PLD), and electrochemical deposition in SnSe thin film preparation, and proposes a predictive design route for SnSe thin film doping. This review points out that low-concentration doping optimizes band structure and light absorption, while high-concentration doping leads to carrier scattering and phase separation, and prospects the development directions of precise atomic-level doping, multi-element co-doping, and large-scale fabrication, providing a solid theoretical guidance for the application of SnSe thin films in photovoltaics, near-infrared photodetection, thermoelectric devices, and flexible optoelectronics [19].

The 10 original research papers address scientific and technical challenges from multiple perspectives, delivering concrete and impactful results. In transparent conductive oxides, Dr. Wang et al. reported that Al–Sb co-doped SnO₂ thin films fabricated via sol–gel spin-coating achieve precise control of conduction type switching from n-type to p-type and back to n-type. The optimal 15 at% Al-doped sample yields the lowest resistivity of 1.058 × 10⁻⁴ Ω·cm and carrier concentration up to 5.855 × 10²⁰ cm⁻³, while visible transmittance exceeds 80%, showing high promise for transparent optoelectronic devices [20]. For wide-bandgap semiconductors, Tan et al. present high-performance solar-blind deep-ultraviolet photodetectors based on Mg-doped β-Ga₂O₃ thin films with AlN passivation layers, fabricated via radio-frequency magnetron sputtering into a metal–insulator–semiconductor–insulator-metal (MISIM) structure. AlN passivation layers of 3 nm, 5 nm, and 10 nm are deposited to regulate the optoelectronic characteristics. The 5 nm AlN layer delivers the optimal device performance. Under 10 V bias, the detector achieves a responsivity of 2.17 A/W, an external quantum efficiency of 1100%, a specific detectivity of 1.09 × 10¹² Jones, and a photo-to-dark current ratio of 2200. The AlN layer effectively suppresses dark current from 0.16 nA to 0.09 nA and accelerates the response speed. Performance improvement stems from three mechanisms: AlN passivation reduces surface defect states and carrier trapping; a built-in electric field at the AlN/β-Ga₂O₃ interface enhances carrier separation and transport; and the AlN film inhibits oxygen adsorption/desorption, retaining more photogenerated carriers. This work validates that AlN passivation integrated with Mg doping greatly improves the sensitivity and stability of β-Ga₂O₃-based solar-blind photodetectors [21].

In hard protective and corrosion-resistant coatings, significant advances are reported. As a promising alternative to conventional plasma electrolytic oxidation, plasma electrolytic fluorination (PEF) enables the formation of dense, thermodynamically stable magnesium fluoride coatings, yet its industrial adaptation is hindered by overly strict anhydrous processing conditions. Yang et al. systematically investigated the influence of water content in ethylene glycol-ammonium fluoride electrolytes on PEF coating formation, microstructure, and protective performance [22]. By varying the glycol-to-water ratio from 10:0 to 0:10, the authors reveal that controlled water addition enhances the ionization of ammonium fluoride and boosts electrolyte conductivity, thereby lowering the working voltage while preserving coating thickness. Notably, all resultant coatings remain dominated by MgF₂ owing to the strong thermodynamic preference for fluorination over oxidation, with oxygen exerting negligible competitive effects. Increasing water content refines the coating microstructure by reducing microvoids and improving structural integrity, gradually transitioning the morphology toward that of conventional PEO coatings. Electrochemical evaluations confirm that corrosion resistance first increases and then decreases with rising water content. The optimal glycol-to-water ratio of 6:4 yields the best protective performance, featuring a remarkably low corrosion current density and high breakdown voltage. This research establishes a feasible, cost-effective pathway to stabilize PEF electrolytes and improve coating quality by introducing moderate water content. Dr. Zhang et al. also reported that electrodeposited Ni coatings on brass substrates achieve superior corrosion resistance and thermal emissivity tuning by optimizing current density and electrode configuration, satisfying strict requirements for aerospace thermal management components [23]. These aforementioned findings not only deepen the mechanistic understanding of plasma-assisted surface fluorination but also provide practical guidance for developing high-performance protective coatings for lightweight structural alloys, aligning closely with the core themes of advanced functional thin films and hard protective coatings.

For biomedical applications, Flores et al. reported that TiO₂ nanotube (TNT) arrays prepared via anodic oxidation on Ti–6Al–4V dental implants exhibit uniform morphology with average inner diameter of 64.88 nm and length of 5.34 μm. In vitro tests confirm excellent cytocompatibility with human osteoblasts (hFOB 1.19), maintaining over 95% cell viability at 24 and 48 h, strongly supporting Osseo integration and clinical translation [24]. For environmental functional coatings, M. C. Grijalva-Castillo et al. showed that SnO₂ and WO₃-based photocatalytic films doped with Ag, Cu₂O, and ferrite nanoparticles demonstrate over 86% antifungal efficiency against Candida albicans under UVC illumination, and nearly 100% inhibition under combined UVC and low-voltage electric field, offering a safe, chemical-free strategy for hospital surface disinfection [25].

Advanced deposition and surface modification technologies are validated with tangible improvements. Sol–gel, magnetron sputtering, electroplating, and plasma-assisted processes achieve controllable thickness, crystallinity, and uniformity at low cost. Tan et al. adopted direct-current magnetron sputtering to systematically examine the influence of N₂/Ar flow ratios on the microstructure and electrochemical capacitive performance of TiN thin-film electrodes for micro-supercapacitors. Phase composition transitions from a TiN/Ti₂O₃ mixture to pure TiN as the nitrogen proportion increases. The optimal N₂/Ar ratio of 5:15 yields the highest oxygen vacancy concentration, the lowest electrical resistivity, and the maximum surface roughness (RMS = 15.9 nm). These features contribute to an outstanding areal specific capacitance of 3.29 mF/cm² at 5 mV/s in 1 M KCl electrolyte. However, the metastable Ti₂O₃ phase leads to noticeable capacitance degradation after 500 charge–discharge cycles, while pure TiN films exhibit better cycling stability. This study establishes a clear processing–structure–performance relationship for TiN micro-supercapacitor electrodes and highlights the role of oxygen vacancies and phase composition in regulating capacitive behavior [26].

The work by Leal et al. targets the sustainable recycling of coated WC–Co hardmetals by exploring nanosecond Nd:YAG laser ablation for selective removal of ~18 μm AlTiN-based multilayer coatings. Real-time LIBS monitoring is integrated to precisely track ablation progress via distinguishing coating (Ti, Al, O) and substrate (W, Co) elemental signals. Systematic optimization of laser parameters (fluence: 0.1–11.7 J/cm², pulse delay: 20–180 μs, pulse number: 1–300) determines the optimal window: 60–80 μs delay and 5–10 pulses (4.8–7.1 J/cm²), which achieves full coating stripping with minimal substrate damage. Excessive fluence or pulses induce melting, material redeposition and surface roughening. SEM, EDS and optical profilometry validate selective ablation and preserved substrate integrity. This laser–LIBS synergistic approach provides an eco-friendly, efficient alternative to toxic chemical stripping, supporting the refurbishment and reuse of WC–Co cutting tools and promoting circular manufacturing in advanced machining [27].

Multilayer and nanostructured composite coatings show synergistic enhancement. Dr. Ma et al. explored light-induced interfacial charge transport in In₂O₃/reduced graphene oxide (rGO) nanocomposites modified with non-conjugated polymers across a wide spectral range [28]. By introducing polyvinyl alcohol (PVA) and amine-terminated dendrimers, the composite’s interfacial contact and defect passivation are significantly improved. The non-conjugated polymers fill grain boundaries and facilitate photogenerated carrier extraction via quantum tunneling, enabling stable and sensitive photocurrent responses from visible light to near-infrared regions (405–1064 nm). When coated on silk fibers, the hybrid shows reversible positive–negative photoconductivity conversion and strain-sensitive electrical switching. Notably, the aged nanocomposite (over four years) still generates detectable zero-bias photocurrent, revealing a robust built-in electric field at the inorganic–carbon interface that suppresses carrier recombination. This work provides a low-cost and eco-friendly strategy for designing broadband photoresponsive hybrid films with flexible and multi-functional potential.

Computational contributions include molecular dynamics and first-principles calculations that reveal formation energy, bandgap, atomic growth and defect formation in metal and oxide material systems, enabling predictive design of optoelectronic performance without excessive experimental trial-and-error. Dr. Cheng et al. employed molecular dynamics (MD) simulations to reveal the atomic-scale growth mechanism and mechanical properties of Ni-W alloy films deposited on Al(001) substrates [29]. Tungsten incorporation lowers the Ehrlich–Schwoebel barrier for Ni adatoms, promoting interlayer diffusion and suppressing the Volmer–Weber island growth mode, thus producing smoother and more uniform films. W atoms segregate at grain boundaries, inducing lattice distortion and grain refinement; a nanocrystal-to-amorphous transition occurs when W content exceeds 15 at%. Nanoindentation simulations confirm that film hardness continuously increases with W concentration. The strengthening mechanism is composition-dependent: dislocation pinning dominates at low W content (≤5 at%), whereas the amorphous structure becomes the primary hardening factor at high W content (≥15 at%). This atomic-level insight offers theoretical guidance for designing Ni-W coatings with tailored morphology and hardness.

This Special Issue builds an interdisciplinary communication platform for materials science, surface engineering, optoelectronic technology, aerospace engineering and biomedical engineering. The published works reflect the current research hotspots: high-performance transparent conductive oxide films, ultraviolet photoelectric detection materials, high-strength wear-resistant and corrosion-resistant coatings, environment-friendly surface modification technology, and computational-assisted intelligent material design. These studies not only enrich the theoretical system of functional thin films and hard protective coatings, but also provide practical technical solutions for aerospace, electronic devices, medical implants and environmental protection.

Looking forward, functional thin films and hard protective coatings will develop towards multi-functional integration, precise nanostructure control, intelligence, green sustainability and high-temperature stability. Advanced deposition technologies with atomic-level precision, multi-response composite coatings, self-healing intelligent coatings, high-entropy alloy coatings and diamond-based ultra-hard coatings will become key research directions. The combination of artificial intelligence and computational modeling will realize high-throughput design and performance prediction, accelerating the transformation from basic research to industrial application.

We sincerely thank all authors for their high-quality contributions, anonymous reviewers for their rigorous and constructive comments, and the editorial office of Coatings for their professional and efficient support. We hope this Special Issue can inspire new research ideas, promote academic exchanges and industrial cooperation, and drive the continuous innovation and application expansion of functional thin films and hard protective coatings.