TiAl-based alloys are used in the aerospace and automotive industries because of their low density, reliable strength, and good oxidation resistance. However, the Al content of the TiAl-based alloys is approximately 50 at.%; thus, it cannot form a protective alumina scale when oxidized over 850 celcius, resulting in poor high-temperature oxidation resistance. Many coatings such as metallic, ceramic, aluminide or silicide diffusion, and glass coatings have been investigated to improve the oxidation resistance of TiAl alloys. Furthermore, the effects of alloying or halogens on the oxidation of TiAl alloys have been studied. Nitride coatings have unique advantages for improving the comprehensive properties of TiAl alloys owing to their wear and oxidation resistance. However, studies on the effect of nitride coatings on the oxidation resistance of TiAl-based alloys are limited. In this study, a multi-arc ion platingsystem was used to deposit AlCrN, AlCrSiN, CrAlN, and CrAlSiN coatings on a TiAl alloy. The influence of AlCr(Si)N and CrAl(Si)N coatings on the cyclic oxidation of the TiAl alloy was investigated at 900 celcius. Furthermore, the influence of the Al content on the coating structure and thermal cycle resistance was investigated. The as-deposited AlCr(Si)N and CrAl(Si)N coatings were homogeneous, compact, and well combined with the matrix alloy. Compared with the CrAl(Si)N coatings, the AlCr(Si)N coatings exhibited a deeper contrast in scanning electron micro scopes back scattered electron pattern, which might be related to the higher Al content. X-ray diffraction patterns showed that the AlCrN and AlCrSiN coatings exhibited an AlN structure, whereas the CrAlN and CrAlSiN coatings exhibited CrN structures. Si may have dissolved into the crystal lattice of the AlCrSiN and CrAlSiN coatings to form (Al, Cr, Si)N and (Cr, Al, Si)N solid solutions. During oxidation for 300 cycles at 900 celcius, oxide scales primarily composed of Al2O3 formed at the surface of AlCrN and AlCrSiN coatings, and numerous cracks formed in the coatings. Aluminum and titanium reacted with oxygen through cracks to form many ridged oxides on the surface of the coatings. However, the matrix was severely oxidized. Therefore, the AlCr(Si)N coatings exhibited poor thermal cycle resistance, which could not improve the cycle oxidation resistance of TiAl alloys. Therefore, research on the development of coatings should consider not only the improvement of oxidation resistance, but also other properties to adapt to complex environmental conditions in the actual service process. After oxidation, continuous and dense oxide scales without cracking or peeling formed on the surface of the CrAlN and CrAlSiN coatings primarily composed of Cr2O3. The weight gain of the CrAlN coating was higher than that of the CrAlSiN coating, which was attributed to the formation of an internal oxidation zone under the outer Cr2O3 layer. Therefore, the addition of Si efficiently inhibited the formation of an inner oxidation zone in the CrAlSiN coating. In addition, the CrAl(Si)N coating decomposed into CrN, Cr2N, Cr, and h-AlN after annealing. After oxidation, thick interdiffusion zones formed at both the CrAlN / TiAl and CrAlSiN / TiAl interfaces owing to the significant interdiffusion between the CrAlN or CrAlSiN coatings and the TiAl matrix. TiN and Ti2AlN beneath the CrAlSiN coating in the interdiffusion zone (IDZ) at the CrAlSiN / TiAl interfaces were primarily formed by the diffusion of N from CrAlSiN to TiAl. Moreover, the released Al diffused from the TiN and Ti2AlN layers to form TiAl2. In the discontinuous Laves phase, Ti5Si3 and Al3Nb were formed between the TiAl2 and nitride layers, and the diffusion barrier of Si addition for N to the TiAl alloy was not apparent, which might be due to the lack of formation of a continuous Ti5Si3 layer in the IDZ. It could be concluded that both the CrAlN and CrAlSiN coatings significantly improved the high-temperature oxidation resistance of the TiAl alloy, and the addition of Si to the coating further improved the oxidation resistance. These results provide prospects for the application of nitride coatings for high-temperature oxidation protection of TiAl-based alloys.
