With the continuous improvement of aero-engine thrust-to-weight ratio and turbine inlet temperature, the performance limitations of conventional superalloys become increasingly prominent. SiC_f/SiC ceramic matrix composites (CMCs) can be core candidate materials for hot-section components due to their excellent high-temperature mechanical properties and low density. As a key technology to realize the engineering application of CMCs, compatible abradable/environmental barrier coatings (A/EBCs) that can simultaneously achieve gas path sealing, high-temperature protection and abradable performance become a research focus in the field of advanced aero-engine sealing technology. This review represents the research progress of such coatings from three dimensions, i.e., material design, microstructural regulation, and performance evaluation, while analyzing key technical challenges and development trends. In terms of material system design, conventional yttria-stabilized zirconia (YSZ) abradable coatings suffer from thermal expansion mismatch with SiC_f/SiC CMCs, which are prone to failure, while conventional solid lubricants undergo oxidative degradation at > 1200 ℃. It is urgent to develop new matrix materials with a high thermal stability, a water vapor-oxygen corrosion resistance and a thermal expansion compatibility. Multi-layer structure is the main design to realize functional synergy, and the interface matching and thermal expansion adaptability between layers are a key to the service durability. The introduction of negative thermal expansion materials provides an idea to solve the mismatch problem. In addition, the construction of material system matching for multi-layer coatings and the compatibility analysis of interlayer interfaces/multiphase interfaces also become important aspects in the design of abradable/environmental barrier coating systems.

In the aspect of microstructural regulation, improving porosity is a main way to obtain excellent abradability, but there is a prominent contradiction among abradability, erosion resistance, corrosion resistance and thermal stability. Excessive or uneven porosity, as well as high-temperature sintering and closure, will lead to the performance degradation and early failure. The core challenge is to realize the precise regulation of multi-scale pore structure and the multi-performance synergy balance. In terms of performance evaluation, the existing test devices have high cost and poor universality, and it is difficult to simulate the real multi-field coupling service environment. The lack of perfect preparation and evaluation standards restricts the engineering and standardized development of CMC-compatible coatings. Finally, the development trends of A/EBCs are prospected, providing a reference for the research and development of high-temperature sealing technology and coating system for advanced aero-engines.

In summary, with the increasing service temperature of aero-engines, the abradable/environmental barrier coatings (A/EBCs) that match SiC_f/SiC ceramic matrix composites (CMCs) become a key research direction. This review represents the research progress of A/EBCs in material design, microstructural regulation and performance evaluation, and points out that the current challenges mainly include thermal expansion mismatch between conventional coating materials and CMC substrate, poor high-temperature stability of lubricants, difficult balance between multi-scale pore structure and multi-performance, and lack of standardized evaluation systems and test standards suitable for multi-field coupling service environment. In the future, the research and development of A/EBCs should focus on the multi-objective synergistic design of material composition, multi-scale microstructure and performance evaluation system. It is necessary to strengthen the analysis of failure mechanism under multi-physical field coupling environment, develop new high-temperature stable matrix and lubricant materials, realize the precise regulation of multi-scale pore and interface structure, and establish a standardized preparation and performance evaluation system. Through the breakthrough of the above key technologies, the comprehensive performance and service durability of A/EBCs will be effectively improved, so as to promote the leapfrog development of high-temperature sealing technology and provide an important support for the performance improvement of next-generation aero-engines.

Ti-6Al-4V (TC4) titanium alloy is widely used in industrial and biomedical fields due to its excellent mechanical properties and biocompatibility. However, its inherently poor wear resistance significantly limits its further application. Plasma electrolytic oxidation (PEO) as a green and efficient method for in-situ fabrication of ceramic coatings with strong adhesion to the substrate, offering an excellent solution for surface protection of titanium alloys. However, conventional PEO coatings exhibit a porous outer layer composed mainly of high friction TiO₂, resulting in insufficient wear and friction reduction performance. To overcome this limitation, incorporating MoS₂ (a solid lubricant with a layered structure) can be introduced to the PEO coating to form a composite coating, which has been demonstrated as an effective approach to enhance its tribological properties. Although previous studies have confirmed the potential value of TiO₂/MoS₂ composite coatings in antifriction, most studies rely on high concentrations of MoS₂ additives or prolonged treatment times, which often lead to particle agglomeration and high energy consumption. Even at lower concentrations, the friction coefficient remains high, and systematic studies on the influence of key process parameters, such as applied voltage are still lacking. Therefore, this study aims to systematically investigate the effects of different PEO voltages on the microstructure, chemical composition, and tribological properties of TiO₂/MoS₂ composite coatings under low MoS₂ concentration and short processing time, so as to provide theoretical and practical guidance for the design and fabrication of high-performance wear resistant and antifriction coatings.

In this study, Ti-6Al-4V alloy was employed as the substrate. TiO₂/MoS₂ composite coatings were fabricated in a single-step process via PEO with incorporation of MoS₂ nanoparticles. The applied voltage was varied at 400, 500 V, and 600 V. The influence of voltage on the coating surface morphology was characterized using scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). Localized chemical analysis of different regions on the coating surface was performed by energy dispersive spectroscopy (EDS) attached to the SEM. Phase composition was further determined by X-ray diffraction (XRD). Finally, the tribological properties of the coatings were evaluated using a ball-on-disk friction and wear tester.

The test results demonstrate that as the PEO applied voltage increases, the pore size, coating thickness, surface roughness, and deposited MoS₂ content of the coatings all increase. Tribological tests on samples prepared at different voltages demonstrated that the coating produced at 500 V exhibited the lowest friction coefficient of approximately 0.2, representing a 69.2% reduction compared to the substrate, indicating excellent antifriction performance. In contrast, coatings prepared at 400 V and 600 V exbibited significantly higher friction coefficients of 0.75 and 0.82, respectively, and suffered from severe adhesive and abrasive wear. The primary reasons for this behavior are as follows: At the lower voltage (400 V), the coating thickness is thin and the MoS₂ content is insufficient to provide effective lubrication. During the running-in stage, the coating lacks adequate capacity to accommodate wear debris, leading to inadequate debris removal. This results in pronounced abrasive wear, rapid penetration of the coating, and direct interaction between the substrate and the counterface, thereby increasing the friction coefficient. At the higher voltage of 600 V, although the coating thickness and MoS₂ content increase substantially, the surface roughness rises significantly ((4.7 ± 0.3) μm). This leads to the generation of large, coarse debris during the initial running-in stage. Before the coating can effectively accommodate these debris particles, they cause rapid spallation of the coating, generating even more debris and accelerating wear. Under these conditions, the MoS₂ particles embedded in the outer layer fail to provide any meaningful lubrication, and the coating is quickly worn through, resulting in a sharp increase in the friction coefficient. At the applied voltage of 500 V, the coating exhibits moderate thickness and surface roughness, maintaining adequate coating thickness and MoS₂ content without excessive surface roughness. The friction and wear mechanism of the TiO₂/MoS₂ composite coating under this voltage can be elucidated as follows: The inherent porous outer layer of the PEO coating undergoes initial smoothing of surface asperities during friction, generating wear debris containing embedded MoS₂ particles that fill surface depressions and inherent pores. With continued sliding, the MoS₂ particles within the debris gradually spread across the contact interface. Owing to their unique two-dimensional layered structure, these particles undergo interlayer sliding under shear stress, forming a continuous surface film with excellent lubricating properties. Furthermore, the porous structure of the coating not only accommodates wear debris but also functions as a reservoir for MoS₂ particles, enabling continuous replenishment of lubricant to areas where the surface lubricating film becomes locally depleted, thereby achieving remarkable self-lubricating performance.

At the optimized voltage of 500 V, the TiO₂/MoS₂ composite coating exhibits a moderate thickness ((28.0 ± 0.6) μm) and surface roughness ((3.0 ± 0.2) μm), along with a relatively high MoS₂ content. This combination results in the lowest friction coefficient of 0.2, representing a 69.2% reduction compared to the uncoated Ti-6Al-4V substrate; During the friction process, MoS₂ particles embedded in the TiO₂/MoS₂ composite coating are progressively exposed under shear stress and undergo interlayer sliding. This leads to the formation of a lubricating film on the wear track, which provides effective self-lubrication and significantly enhances the tribological performance of the Ti-6Al-4V alloy.