Resistance to High Temperature Oxidation of Al2O3-SiC-AlPO4 Coatings on γ-TiAl Based Alloys

γ-TiAl based alloys are considered as promising high temperature light-weight structural materials due to their high specific strengths and elastic moduli. However, through analyzing the phase diagram of the Ti-Al-O system, it can be seen that the γ-TiAl based alloys can not form protective α-Al2O3 scale because of inadequate aluminum content in the alloy. Therefore, high temperature oxidation resistance coatings are still necessary to improve the anti-oxidation properties of γ-TiAl alloys at temperature above 800 ℃. Traditional coatings for high temperature oxidation, such as MCrAlY, suffer from severe inter-diffusion between coatings and the substrate, which will cause negative effect on alloy mechanical properties. Other types of coatings developed wildly, such as CrAlN ceramics, halogenation Si. Phosphate coatings are also ideal oxidation resistance coatings for their high temperature stability up to 1 400 ℃. In this work, Al2O3-SiC-AlPO4 composite phosphate coatings were designed and coated on γ-TiAl alloy substrate by conventional paint spraying method. The kinetic behavior of quasi-isotherm oxidation of γ-TiAl alloy and coating samples were studied at 900 ℃ under static air conditions. The physical phase composition, surface morphology and micro-zone composition of the coating samples before and after oxidation were characterized and analyzed by XRD and SEM/EDS, respectively. The element distribution of the coating samples were analyzed by electron probe (EPMA). The results of 900 ℃ anti-temperature oxidation experiments showed that the oxidation rate constant of γ-TiAl based alloy was 32.501×10-2 mg/(cm2·h1/2) at early oxidation stage, which was close to the oxidation rate constant of 28.113×10-2 mg/(cm2·h1/2) at late oxidation stage. This result indicated that the oxidation kinetics of the γ-TiAl based alloy followed straight line law, and the oxide scales of the bare alloy were not protective. The oxidation rate constant of Al2O3-SiC-AlPO4 composite coatings was 5.967×10-2 mg/(cm2·h1/2) at late oxidation stage, which was significantly lower than the initial oxidation rate constant 23.941×10-2 mg/(cm2·h1/2). The oxidation kinetics line of the coatings followed typical parabolic law, which indicated that protective oxide scales were formed on the coatings. Micro-structure analysis results showed that the composite coating and the γ-TiAl based alloys were combined well. The main phase of the coatings consisted of Al2O3, SiC and SiO2. AlPO4 existed in the coating in an amorphous state. After oxidation, AlPO4 crystallized and formed the main matrix of the coatings, forming a dense network structure of the coatings. Titanium elements of the γ-TiAl based alloy diffused into the network in the coatings and oxidized to TiO2 filling the holes, which led to a more dense coating. A continuous aluminum-rich inter-facial layer of about 2 μm between the coatings and the substrate was observed, which was the formation of a continuous and dense Al2O3 scale, preventing the oxygen from the air diffusing into the substrate. The Al2O3-SiC-AlPO4 composite coatings effectively prevent the diffusion of oxygen into the interior of the γ-TiAl alloy matrix at the temperature of 900 ℃, and show excellent anti-temperature oxidation properties. Due to the existence of the coatings, a 2 μm Al2O3 inter-layer is formed between the coating and the alloy after oxidation. This layer is condense without cracks, which may be the reason for the good adhesion and oxidation resistance property of the coating. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.