Al2O3 atomic layer deposition on a porous matrix of carbon fibers (FiberForm) for oxidation resistance

Atomic layer deposition (ALD) was used to coat a porous matrix of carbon fibers known as FiberForm with Al2O3 to improve oxidation resistance. Static trimethylaluminum (TMA) and H2O exposures for Al2O3 ALD were used to obtain the uniform coating of this high porosity material. The carbon surfaces were initially functionalized for Al2O3 ALD by exposure to sequential exposures of nitrogen dioxide and TMA. A gravimetric model was developed to predict the mass gain per cycle under conditions when the ALD reactions reached saturation during each reactant exposure. The uniformity of the Al2O3 ALD coating on FiberForm was confirmed by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) analysis. The SEM, EDS, and gravimetric models were all consistent with a uniform Al2O3 ALD coating on the porous carbon fiber network when the ALD reactions reached saturation on the entire surface area. In contrast, the profile of the Al2O3 ALD coating on the FiberForm was also characterized using undersaturation conditions when the ALD reactions did not reach saturation throughout the FiberForm sample. Based on comparisons with results from models for ALD in porous substrates, these Al2O3 coverage profiles were consistent with diffusion-limited Al2O3 ALD. Oxidation of the FiberForm and the Al2O3 ALD-coated FiberForm was also investigated by thermogravimetric analysis (TGA). TGA revealed that a 50 nm thick Al2O3 coating deposited using 400 Al2O3 ALD cycles enhanced the oxidation resistance. The Al2O3 ALD coating increased the oxidation onset temperature by ∼200 °C from 500 to 700 °C. The oxidation of the FiberForm removed carbon and left the Al2O3 ALD coating behind as a white “skeleton” that preserved the shape of the original FiberForm sample. The Al2O3 ALD coating also decreased the oxidation rate of the FiberForm by ∼30%. The oxidation rate of the Al2O3 ALD-coated FiberForm samples was constant and independent of the thickness of the Al2O3 ALD coating. This behavior suggested that the oxidation is dependent on the competing O2 diffusion into the FiberForm and CO2 diffusion out of the FiberForm.