| Due to their excellent mechanical properties at high temperatures, Carbon-Carbon composites C/C are very attractive for aerospace applications such as integral throat entrance components, exit cones and nose cones for rocket motors; space plane thermal protection systems and advanced braking systems for aircrafts and Formula One racing cars. Without oxidation protection, however, C/C materials are susceptible to severe oxidation and volatilization problems above 500℃. In order to improve the oxidation resistance at high temperatures, suitable oxygen diffusion barriers have to be developed. To address this issue a dual layer coating comprising of bulk compositions of 72-84 wt.% Al2O3 of mullites chemically synthesized via sol-gel routes and SiC conversion coatings were deposited. The coatings were applied onto C/C composite substrates using a two step approach; involving pack cementation sintered at 1800℃and silica sol based slurry coating at 550℃. A release agent (cork) was applied to the C/C composite substrate prior to coating SiC with the pack cementation process. Commercially available fused-mullite was also used as a reference material. The performances of the coated samples were evaluated for their oxidation resistance during thermal cycling in a simulated combustion environment between 1500℃and room temperature (RT) up to 10 cycles. The characterizations of the samples were done using XRD and SEM (mounted with EDX). Research findings indicate that oxidation protective mullite-SiC dual layer coatings for C/C composite substrates could be formed with an inexpensive approach involving pack cementation and slurry coating process. The density of the substrates had marked effect on coatings. Dense and thick coatings were obtained in high density substrates. Oxidation of the low density C/C substrate coated with fused mullite was severe, leading to a weight loss after 106-h of isothermal oxidation exposure. In contrast, the high density substrate coated with fused mullite gained weight. Due partially to the SiC conversion gradation layer and the fused mullite-silica sol slurry coating that sealed the micro-cracks by forming a top coat low viscous silica layer that helped in the self-sealing process. The oxidation protection for commercially obtained fused mullite coating on SiC-coated C/C substrate of high density comparatively offered an effective protection than the low density substrate after 106-h of isothermal oxidation cycle at 1500℃. The high density sample had a weight gain of≈5.17% with a corresponding weight gain rate of 1.4 gcm-2h-1 while low density sample registered a weight loss of≈50.34% with a weight loss rate of 12.8 gcm-2h-1. However, synthesized mullite sol-gels of bulk compositions 84,79, 76 and 72 wt.% AI2O3 remained adherent toβ-SiC-C/C substrate layer and after 110-h of isothermal oxidation despite the thinβ-SiC layer coating thickness of≈13.2μm as compared to≈205.9μm in fused-mullite-SiC coating showed no sign of diffusing into the C/C substrates. Optimized oxidation protection for series of synthesized sol-gel mullite-SiC coating was achieved with higher AI2O3 content. The weight loss rate of sol-gel mullite with bulk composition 84 wt.% AI2O3(SG84)was 0.09 gcm-2h-1 as compared to bulk composition 72 wt.% AI2O3(SG72) which had a weight loss rate of 0.20gcm-2h-1.
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