The effect of ultrathin ALD films on the oxidation kinetics of SiC in high-temperature steam

Abstract

Ultra-thin, oxidation-resistant, pinhole-free alumina coatings were deposited on silicon carbide (SiC) particles using atomic layer deposition (ALD), and investigated as environmental barrier coatings (EBCs) for high-temperature steam oxidation. The uncoated and alumina coated SiC particles were exposed to high-temperature steam for 20 h at temperatures ranging from 1050 °C to 1150 °C to assess oxidation rates. The kinetic triplet (activation energy, rate constant, and pre-exponential factor) for each sample was determined using the D4 kinetic model (representing 3-dimensional diffusion) which was demonstrated to be accurate via the isothermal isoconversional method. Activation energies for oxidation of 247.4 ± 0.1 kJ/mol, 250.6 ± 0.5 kJ/mol, and 253.0 ± 1.0 kJ/mol were calculated for uncoated SiC, SiC coated with a 5 nm alumina film, and SiC coated with a 10 nm alumina film, respectively. Reaction rate data for the oxidation reaction of uncoated SiC and SiC coated with a 10 nm film indicate that the ALD coating reduces the rate of oxidation of SiC by up to a factor of 5 for steam oxidation at temperatures between 1050 °C and 1150 °C. This is the first investigation of steam oxidation kinetics for SiC coated with an ALD deposited EBC. The results of this study indicate that ultra-thin alumina films deposited by ALD act as effective EBCs with oxidation resistance comparable to CVD films that are orders of magnitude thicker.