Crystalline ytterbium disilicate environmental barrier coatings made by high velocity oxygen fuel spraying

Dense environmental barrier coatings (EBCs) are an essential prerequisite to exploit the advantages offered by SiC-based fiber reinforced ceramic matrix composites (CMCs) to increase efficiency in gas turbines. Today's state-of-the art materials for application as EBCs are rare-earth (RE) silicates which, however, form amorphous phases upon rapid quenching from the melt. This makes their processing by thermal spray a challenge. Recently, high velocity oxygen fuel (HVOF) spraying was proposed as potential solution since the melting degree of the feedstock can be controlled effectively. This work studies the deposition of ytterbium disilicate (YbDS) at short stand-off distances and variant total feed rates and oxy-fuel ratios of the working gas. It was found that the overall degree of crystallinity could be kept at high level above 90%. The kinetic energy transferred by impinging particles was found to be an effective parameter to control the densification of the coatings. Porosities well below 10% were achieved while fully dense coatings were impeded due to the progressive accumulation of stresses in the coatings.