Y₂O₃ is regarded as one of the potential environmental barrier coating (EBC) materials for Al₂O_3f/Al₂O₃ ceramic matrix composites owing to its high melting point and close thermal expansion coefficient to Al₂O₃. However, the relatively high thermal conductivity and unsatisfactory calcium-magnesium-aluminosilicate (CMAS) resistance are the main obstacles for the practical application of Y₂O₃. In order to reduce the thermal conductivity and increase the CMAS resistance, four cubic bixbyite structured high-entropy oxides RE₂O₃, including (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃, (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃, (Sm_0.2Eu_0.2Er_0.2Y_0.2Yb_0.2)₂O₃, and (Sm_0.2Eu_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ were designed and synthesized, among which (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ bulks were prepared by spark plasma sintering (SPS) to investigate their mechanical and thermal properties as well as CMAS resistance. The mechanical properties of (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ are close to those of Y₂O₃ but become more brittle than Y₂O₃. The thermal conductivities of (Eu_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ and (Sm_0.2Er_0.2Lu_0.2Y_0.2Yb_0.2)₂O₃ (5.1 and 4.6 W·m⁻¹·K⁻¹) are only 23.8% and 21.5% respectively of that of Y₂O₃ (21.4 W·m⁻¹·K⁻¹), while their thermal expansion coefficients are close to those of Y₂O₃ and Al₂O₃. Most importantly, HE RE₂O₃ ceramics exhibit good CMAS resistance. After being attacked by CMAS at 1350 °C for 4 h, the HE RE₂O₃ ceramics maintain their original morphologies without forming pores or cracks, making them promising as EBC materials for Al₂O_3f/Al₂O₃ composites.

Titanium alloys are widely used in various structural materials due to their lightweight properties. However, the low wear resistance causes significant economic losses every year. Therefore, it is necessary to implement wear-resistant protection on the surface of titanium alloys. In this study, four types of in situ composite ceramic coatings with two-layer gradient structures were prepared on a Ti-6Al-4V (TC4) substrate using laser cladding. In order to reduce the dilution rate, a transition layer (Ti-40SiC (vol.%)) was first prepared on TC4 alloy. Then, a high-volume-fraction in situ composite ceramic working layer (Ti-xFe-80SiC (vol.%)) with different contents of Fe-based alloy powder (x = 0, 5, 10 and 15 vol.%) was prepared. The working surface of Ti-40SiC (TL) exhibited a typical XRD pattern of Ti, TiC, Ti₅Si₃, and Ti₃SiC₂. In comparison, both Ti-80SiC (WL-F0) and Ti-5Fe-80SiC (WL-F5) exhibited similar phase compositions to the TL coating, with no new phase identified in the coatings. However, the TiFeSi₂ and SiC phases were presented in Ti-10Fe-80SiC (WL-F10) and Ti-15Fe-80SiC (WL-F15). It is proven that the addition of the Fe element could regulate the in situ reaction in the original Ti-Si-C ternary system to form the new phases with high hardness and good wear resistance. The hardness of the WL-F15 (1842.9 HV₁) is five times higher than that of the matrix (350 HV₁). Due to the existence of self-lubricating phases such as Ti₅Si₃ and Ti₃SiC₂, a lubricating film was presented in the WL-F0 and WL-F5 coatings, which could block the further damage of the friction pair and enhance the wear resistance. Furthermore, a wear-transition phenomenon was observed in the WL-F10 and WL-F15 coatings, which was similar to the friction behavior of structural ceramics. Under the load of 10 N and 20 N, the wear volume of WL-F15 coating is 5.2% and 63.7% of that in the substrate, and the depth of friction of WL-15 coating is only 14.4% and 80% of that in the substrate. The transition of wear volume and depth can be attributed to the wear mechanism changing from oxidation wear to adhesive wear.

Ameliorating the high-temperature performance of cast Al-Si alloys used as engine components is essential. The effects of different T6 heat-treatment processes on the microstructure and mechanical properties of cast Al-Si-Cu-Mg-Ni-Cr alloys were investigated in the present study. The results demonstrate that, under the optimal solution treatment conditions of 500 °C for 2 h and 540 °C for 4 h, the T-Al₉FeNi phase was present in the alloy, and the roundness of primary Si and the aspect ratio of eutectic Si in the alloy reached valley values of 1.46 and 2.56, respectively. With increasing ageing time at 180 °C, the tensile strength significantly improved, while the microhardness first increased and then decreased. When the ageing time was 4 h, microhardness reached a peak value of 155.82 HV. The fracture characteristics changed from quasi-cleavage to the coexistence of quasi-cleavage and dimples. After heat treatment, the high-temperature tensile properties of the alloy improved, which is a significant advantage compared to the as-cast alloy. The stable Al₃Ni and Al₉FeNi phases inhibited the cracking of the alloy at 350 °C.

To improve the corrosion inhibition and wear resistance of materials, the pack cementation method was used to prepare chromized coatings on the surfaces of high-carbon T9 steel and low-carbon SPCC steel. The results showed the formation of a uniform and dense double-layer structure with a thickness of ~10 μm on the surfaces of two different types of steel. The coating layer for T9 steel was mainly composed of Cr₂₃C₆ and Cr₇C₃, while that for SPCC steel was mainly composed of Cr₂₃C₆ and Fe–Cr solid solution. Additionally, both of the steels showed different hardness distributions. The hardness measurements of the outer layers of the T9 steel and SPCC steel were ~1737.72 HV and 1771.91 HV, while the hardness values of the secondary layers were 1378.31 HV and 448.52 HV, respectively. The polarization curves in 3.5 wt.% NaCl solution demonstrated the better corrosion resistance of the chromized coating. Chromizing increased the corrosion potential by ~0.2 V and reduced the corrosion current density by one order of magnitude. Under the presence of an 8 N load, the friction factor before and after the chromizing of T9 steel was about 0.69, and the mass wears were 2 mg and 0.6 mg, respectively. Meanwhile, the friction factor of the SPCC steel before and after chromizing was about 0.73, with respective mass wears of 2 mg and 2.9 mg. The wear resistance of T9 steel after chromizing was superior, but it became worse after chromizing for the SPCC steel.

To improve the wear resistance of high-strength and high-conductivity Cu–Cr–Zr alloys in high-speed and heavy load friction environments, coatings including Ni–Cu, Ni–Cu-10(W,Si), Ni–Cu–10(Mo,W,Si), and Ni–Cu–15(Mo,W,Si) (with an atomic ratio of Mo,W to Si of 1:2) were prepared using coaxial powder-feeding laser cladding technology. The microstructure and wear performance of coatings were chiefly investigated. The results revealed that (Mo,W)Si₂ and MoNiSi phases are found in the Ni–Cu–10(Mo,W,Si) and Ni–Cu–15(Mo,W,Si) coating. WSi₂ phases are found in the Ni–Cu–10(W,Si) coating. The degree of grain refinement in Ni–Cu–10(Mo,W,Si) was greater than that of the Ni–Cu–10(W,Si) coating after the effect of Mo. The excellent wear resistance and micro-hardness of the Ni–Cu–15(Mo,W,Si) coating were attributed to the increase in its dispersion phase, which were approximately 34.72 mg/km and 428 HV, 27.1% and 590% higher than the Cu–Cr–Zr substrate, respectively. The existence of silicide plays an important role in grain refinement due to the promotion of nucleation and the inhibition of grain growth. In addition, the wear mechanism transformed from adhesive wear in the Ni–Cu coating with no silicides to abrasive wear in the Ni–Cu–15(Mo,W,Si) coating with high levels of silicides.

To solve the problem of silicide coatings on tantalum substrates failing due to elemental diffusion under high-temperature oxidation environments and to find diffusion barrier materials with excellent effects of impeding Si elemental spreading, TaB₂ and TaC coatings were prepared on tantalum substrates by the encapsulation and infiltration methods, respectively. Through orthogonal experimental analysis of the raw material powder ratio and pack cementation temperature, the best experimental parameters for the preparation of TaB₂ coatings were selected: powder ratio (NaF:B:Al₂O₃ = 2.5:1:96.5 (wt.%)) and pack cementation temperature (1050 °C). After diffusion treatment at 1200 °C for 2 h, the thickness change rate of the Si diffusion layer prepared using this process was 30.48%, which is lower than that of non-diffusion coating (36.39%). In addition, the physical and tissue morphological changes of TaC and TaB₂ coatings after siliconizing treatment and thermal diffusion treatment were compared. The results prove that TaB₂ is a more suitable candidate material for the diffusion barrier layer of silicide coatings on tantalum substrates.