Calcium–magnesium–alumina–silicate (CMAS) corrosion has attracted special attention in the thermal barrier coating (TBC) field. At high temperatures, when CMAS melts, it adheres to the coating surface and penetrates the interior, severely destroying the TBC. In this study, a promising CMAS-phobic and infiltration-inhibiting material, GdPO₄, on which molten CMAS is difficult to wet and penetrate, was proposed. These desirable attributes are explained by analyzing the material characteristics of GdPO₄ and its interfacial reaction with CMAS. GdPO₄ is demonstrated to have low surface energy, making it difficult for molten CMAS to wet and adhere to the surface. When in contact with molten CMAS, a double-layer structured reaction layer consisting of an acicular upper sublayer and a compact lower sublayer is formed on the GdPO₄ surface, which can effectively impede molten CMAS spreading and penetration. First-principles calculation results revealed that the reaction layer has low surface energy and low adhesion to CMAS, which are favorable for molten CMAS phobicity. Additionally, the formation of the reaction layer increases the viscosity of the molten CMAS, which can increase melt wetting and penetration. Hence, GdPO₄, which exhibits excellent CMAS-phobicity and infiltration-inhibiting ability, is a promising protective layer material for TBCs against CMAS adhesion and attack.

Environmental sediments mainly consisting of CaO–MgO–Al₂O₃–SiO₂ (CMAS) corrosion are a serious threat to thermal barrier coatings (TBCs), in which Fe element is usually ignored. Gd₂Zr₂O₇ TBCs are famous for their excellent CMAS resistance. In this study, the characteristics of Fe-containing environmental sediments (CMAS-Fe) and their corrosiveness to Gd₂Zr₂O₇ coatings were investigated. Four types of CMAS-Fe glass with different Fe contents were fabricated. Their melting points were measured to be 1322–1344 ℃, and the high-temperature viscosity showed a decreasing trend with increasing Fe contents. The corrosion behavior of four types of CMAS-Fe to Gd₂Zr₂O₇ coatings at 1350 ℃ was investigated. At the initial corrosion stage (0.1 h), anorthite was precipitated in CMAS-Fe with a high Ca : Si ratio, while Fe-garnet was formed in the melt with the highest Fe content. Prolonging the corrosion time resulted in the formation of a reaction layer, which exhibited an interpenetrating network composed of Gd-oxyapatite, ZrO₂, and residual CMAS-Fe. Some spinel was precipitated within the reaction layer. After 1 h or even longer time, the reaction layers tended to be stable and compact, which had comparable hardness and fracture toughness to those of Gd₂Zr₂O₇ coatings. Under the cyclic CMAS-Fe attack, the residual CMAS-Fe in the interpenetrating network provided a pathway for the redeposited CMAS-Fe infiltration, resulting in the continuous growth of the reaction layer. As a result, the Gd₂Zr₂O₇ coatings had a large consumption in the thickness, degrading the coating performance. Therefore, the Gd₂Zr₂O₇ coatings exhibit unsatisfactory corrosion resistance to CMAS-Fe attack.

Ti₂AlC has been demonstrated as the promising protective layer material for thermal barrier coatings (TBCs) against calcium–magnesium–alumina–silicate (CMAS) attack. In this study, the reliability of Ti₂AlC coatings against the CMAS corrosion was explored, and new Ti₂AlC/YSZ TBCs more efficiently resistant to CMAS were designed. The fabricated Ti₂AlC coatings inevitably contain some impurity phases (TiC and Al₂Ti₃), the contents of which were minimized by optimizing the spraying distance. Corrosion tests revealed that Ti₂AlC/YSZ TBCs yielded higher resistance to the CMAS attack than YSZ TBCs, but with long-term exposure to CMAS, the Ti₂AlC protective coating exhibited microstructure degradation due to the presence of the impurity phases, which caused the formation of a layer mixed with Al₂O₃ and TiO₂ rather than a continuous compact Al₂O₃ layer on the surface. Pre-oxidation schemes were designed in air or with a controlled oxygen partial pressure, which revealed that the pre-oxidation at an oxygen partial pressure of ~630 Pa could promote a continuous Al₂O₃ layer formed on the Ti₂AlC protective coating surface. Furthermore, a vacuum heat treatment at 867 ℃ for 10 h before pre-oxidation was beneficial for the formation of the compact Al₂O₃ layer. Through the above scheme design, new Ti₂AlC/YSZ TBCs were obtained, which had reduced impurity phase contents and a pre-oxide layer with an ideal structure on the surface. New TBCs exhibit higher microstructure stability exposed to CMAS and more efficient CMAS resistance.

Calcium-magnesium-alumina-silicate (CMAS) corrosion is a serious threat to thermal barrier coatings (TBCs). Ti₂AlC has been proven to be a potential protection layer material for TBCs to resist CMAS corrosion. In this study, the effects of the pellet surface roughness and temperature on the microstructure of the pre-oxidation layer and CMAS corrosion behavior of Ti₂AlC were investigated. The results revealed that pre-oxidation produced inner Al₂O₃ layer and outer TiO₂ clusters on the pellet surfaces. The content of TiO₂ decreased with decreasing pellet surface roughness and increased along with the pre-oxidation temperature. The thickness of Al₂O₃ layer is also positively related to the pre-oxidation temperature. The Ti₂AlC pellets pre-oxidized at 1050 ℃ could effectively resist CMAS corrosion by promoting the crystallization of anorthite (CaAl₂Si₂O₈) from the CMAS melt rapidly, and the resistance effectiveness increased with the pellet surface roughness. Additionally, the CMAS layer mainly spalled off at the interface of CaAl₂Si₂O₈/Al₂O₃ layer after thermal cycling tests coupled with CMAS corrosion. The Al₂O₃ layer grown on the rough interface could combine with the pellets tightly during thermal cycling tests, which was attributed to obstruction of the rough interface to crack propagation.

Sc was doped into Gd₂Zr₂O₇ for expanding the potential for thermal barrier coating (TBC) applications. The solid solution mechanism of Sc in the Gd₂Zr₂O₇ lattice, and the mechanical and thermophysical properties of the doped Gd₂Zr₂O₇ were systematically studied by the first-principles method, based on which the Sc doping content was optimized. Additionally, Sc-doped Gd₂Zr₂O₇ TBCs with the optimized composition were prepared by air plasma spraying using YSZ as a bottom ceramic coating (Gd-Sc/YSZ TBCs), and their sintering behavior and thermal cycling performance were examined. Results revealed that at low Sc doping levels, Sc has a large tendency to occupy the lattice interstitial sites, and when the doping content is above 11.11 at%, Sc substituting for Gd in the lattice becomes dominant. Among the doped Gd₂Zr₂O₇, the composition with 16.67 at% Sc content has the lowest Pugh’s indicator (G/B) and the highest Poisson ratio (σ) indicative of the highest toughness, and the decreasing trends of Debye temperature and thermal conductivity slow down at this composition. By considering the mechanical and thermophysical properties comprehensively, the Sc doping content was optimized to be 16.67 at%. The fabricated Gd-Sc coatings remain phase and structural stability after sintering at 1400 ℃ for 100 h. Gd-Sc/YSZ TBCs exhibit excellent thermal shock resistance, which is related to the good thermal match between Gd-Sc and YSZ coatings, and the buffering effect of the YSZ coating during thermal cycling. These results revealed that Sc-doped Gd₂Zr₂O₇ has a high potential for TBC applications, especially for the composition with 16.67 at% Sc content.

Calcium–magnesium–alumina–silicate (CMAS) corrosion is an important cause for thermal barrier coating (TBC) failure, which has attracted increased attentions. In this study, some thermal barrier coating (TBC) materials including YSZ (yttria partially stabilized zirconia), GdPO₄, and LaPO₄ were prepared into bulks, and the effects of their surface roughness on wettability and spreading characteristics of molten CMAS were investigated. As-fabricated and polished bulks with different surface roughness were exposed to CMAS corrosion at 1250 ℃ for 1 and 4 h, following by macro and micro observations. Results revealed that compared with the as-fabricated bulks, molten CMAS on the polished samples had lower wettability and a smaller spreading area, mainly attributable to the reduced capillary force to drive the melt spreading. Meanwhile, GdPO₄ and LaPO₄ bulks exhibited lower CMAS wettability than YSZ bulk. It is thus considered that reducing the surface roughness is beneficial to CMAS corrosion resistance of TBCs.

Y₂O₃ stabilized ZrO₂ (YSZ) thermal barrier coatings (TBCs) are prone to hot corrosion by molten salts. In this study, the microstructure of atmospheric plasma spraying YSZ TBCs is modified by laser glazing in order to improve the corrosion resistance. By optimizing the laser parameters, a ~18 μm smooth glazed layer with some vertical cracks was produced on the coating surfaces. The as-sprayed and modified coatings were both exposed to hot corrosion tests at 700 and 1000 ℃ for 4 h in V₂O₅ molten salt, and the results revealed that the modified one had improved corrosion resistance. After hot corrosion, the glazed layer kept structural integrity, with little evidence of dissolution. However, the vertical cracks in the glazed layer acted as the paths for molten salt penetration, accelerating the corrosion of the non-modified coating. Further optimization of the glazed layer is needed in the future work.