Exploring the impact of carbon fiber surface chemistry on the structure of their alumina-silica protective coatings: A comparative study of high resistance, medium and high modulus fibers

Due to the ease of supply, it has become usual for investigations of carbon fiber coatings to be carried out on high resistance carbon fibers, for intended applications at high temperatures in air. Yet, medium modulus and high modulus carbon fibers, for which carbon fiber coatings serves further purpose or are initially designed for, present significant variation in surface roughness and graphite plane stacking from their HR counterparts. As such, this study focuses on the transposition of the deposition of an environmental barrier coating onto HR, MM and HM carbon fibers, and further compares the coated fibers following a thermal cycle. To do so, a 20 nm bilayer thin film of amorphous alumina/alumina-silica was deposited using both atomic layer deposition (ALD) and catalytic ALD, and further annealed in inert atmosphere at 1100 °C. It was demonstrated that the HR fibers required a prior annealing to outgas the fiber core, while residual elements in the surface region of HR and HM fibers impeded the integrity of the film. Furthermore, the density of grooves at the carbon fiber surface was shown to hold substantial impact on the kinetics of crystallisation of the thin films, being determinant to the fiber surface O/C atom ratio. • High resistance carbon fibers required pre-annealing to outgas the fiber core. • Residual elements in the fiber such as B impeded the integrity of the film after annealing. • Fiber surface groove width and density influenced kinetics of thin film crystallisation. • The impact of HM fibers on film microstructure was best mimicked by MM fibers.