Process optimization for Hafnia-doped silicon bond coats fabricated by plasma spraying for SiC-CMC

An HfO2-doped Si bond layer is expected to work at higher temperature with better oxidation resistance of environmental barrier coatings for SiC-based ceramic matrix composites. Establishing the relationship between process parameters and powder feeders with the property of coatings could help to further optimizing their performance. Three sets of plasma spray processes and two kinds of HfO2 powders were used to fabricate Si-HfO2 bond coats with a different Si/HfO2 ratio. The particle size of HfO2 greatly affects the composition of coat, where smaller one induced an uncontrollable Si/HfO2 ratio inside coatings. Larger HfO2 powder greatly improved the composition uniformity with an Si/HfO2 ratio similar to the starting feeder. However, phase segregation occurred for both cases, indicating the limit of mechanical mixing method to make composite feedstock. More strategies should be explored for precise control of the phase and composition of composite coatings from spray technique.