Oxidation behavior of SiC-Al_2O_3-mullite multi-coating coated carbon/ carbon composites at high temperature

摘要

Carbon/Carbon composites are ideal candidates for structural materials in high thrust-weight ratio turbine engines [1,2] but their use is limited at high temperature because the composites oxidize above 450 °C. To reduce the oxidation, various coatings have been explored [3,4], and multi-layer coatings are considered as one of the best choices to protect them at high temperature [5-7]. Therefore, multi-later coatings such as SiC/MoSi_2, SiC-Al_2O_3-mullite, SiC-Al_2O_3-mullite-Al_4SiC_4, SiC/ SiO_2-ZrO_2 and SiC/yttrium silicate/glass have been explored [8-12]. A new SiC-Al_2O_3-mullite coating, which could effectively protect C/C composites at 1600 deg C for 45 h was developed using a two-step pack cementation technique [9]. We report here, the optimization of this coating by optimizing preparation conditions. The substrate material was a 2D-C/C with a density of 1.72g/cm~3 cut from aircraft brakes. The substrates were 10 mm x 10 mm x 10 mm in size. The SiC coating was prepared by a pack cementation process with Si, C, SiC and A1_2O_3 powders in an argon atmosphere at 1600 deg C for 2 h. The adding of A1_2O_3 was helpful for the penetration of Si into the C/C substrate, which will lead to good oxidation resistance of SiC coating and good bonding of SiC coating to C/C matrix. Then a second pack cementation was conducted using SiO_2, A1_2O_3, Si, C (graphite) to form the SiC-Al_2O_3-mullite multi-coating at 1800 deg C. Details were reported in [9]. The optimum multi-coating could be achieved with the optimized pack chemistry of SiO_2/Al_2O_3/Si/C (weight ratio) = 3/7/(0.5 - 3)7(0.5 - 3).