Investigating Antioxidative Coatings on Carbon/Carbon Composites for High Temperature Applications

Carbon/carbon (C/C) composites have been extensively researched due to their increasing demand in the aerospace industry for applications such as rocket nozzles, heat shields of space vehicles, and disk brakes of airplanes. Advantages of C/C composites include their high tensile strength, high heat conductivity, low coefficient of thermal expansion (CTE), and ability to maintain these properties under high-temperature environments. A challenge inherent with C/C composites is their susceptibility to mass loss when exposed to oxygen in environments that exceed 500°C. To protect the C/C composites in these moderate to high-temperature environments, high-temperature ceramic coatings are used as anti-oxidative coatings onto the C/C composite.

Several variables can affect the efficiency of these coatings such as adhesion to the substrate, similar CTE to that of the substrate, high melting point, low reactivity with oxygen, and ability to prevent diffusion of oxygen. The formation of oxides can be beneficial by providing a thin layer that prevents the diffusion of oxidative gases and, in some cases, acts as a self-healing agent which fills the pores and eventual cracks. However, some coatings rely on oxides that can only form at high temperatures (around 1500°C), making them not as effective at lower temperatures. Eventual cracks in the coating can also be closed due to the expansion of the coating at high temperatures (> 1000°C), preventing the oxidative gases from reaching the C/C composite substrate. With a high melting temperature (2,730°C) and a CTE close to carbon, SiC (silicon carbide) has been widely used as HTC coating for C/C composites. In addition, the formation of a glassy SiO₂ film when SiC oxidizes can prevent the diffusion of oxidative species through the coating.

It is common for C/C composite coatings to be multilayered with a combination of different methods and chemicals to optimize performance. In this work, some commonly explored methods for obtaining SiC coatings namely pack cementation (PC) and slurry-dipping/CVD will be explored as the first layers of protection. To this effect, six C/C composite samples will be used in this study: uncoated, PC, PC with boron, slurry/CVD, PC + slurry/CVD, and PC with boron+ slurry/CVD. The hypothesis is that the first SiC buffer layer should be from PC as the conversion of the carbon from the C/C composite to SiC will allow high adhesion of the coating. Since the viscosity of SiO₂ glass is too high to flow into and seal the microcracks at intermediate temperatures, boron will be added as B₂O₃ has lower viscosity and thus can flow into and seal coating cracks. The second SiC layer will be applied through slurry coating with ZrB₂ and a polymeric SiC precursor (SMP-10, Starfire Systems) in an endeavor to improve the self-healing properties of this coating. To understand the effectiveness of preventing mass loss, each sample will be tested using thermal gravimetric analysis in an air environment at 850°C. The corresponding data will then be compared to understand the advantages of different methods, chemicals, and multilayered coatings.