Atmospheric plasma spraying of ytterbium disilicate for abradable and environmental barrier coatings: A story of processing-microstructure relationships

Environmental barrier coatings (EBCs) are required to protect SiC based composites in high temperature, steam containing combustion environments found in the latest generation of high efficiency gas turbine aeroengines. Ytterbium disilicate (YbDS) has shown promise as an environmental barrier coating, showing excellent phase stability at high temperatures and a coefficient of thermal expansion close to that of SiC however its performance is dependent on the conditions under which the coating was deposited. In this work, a parametric study was undertaken to demonstrate how processing parameters using a widely used Praxair SG-100 atmospheric plasma spraying (APS) torch affect the phase composition, microstructure and mechanical properties of ytterbium disilicate. Ytterbium disilicate coatings were deposited using 4 sets of spray parameters, varying the spray power from 12 to 24 kW. The phases present in these coatings were quantified using x-ray diffraction with Rietveld refinement, and the level of porosity was measured. Using this data, the relationship between processing parameters and phase composition and microstructure was examined. Using the optimum process parameter window determined in this work, abradable YbDS coatings were deposited using polyester feedstock additions as a pore forming phase. Two different polyester levels were added to create coatings with two different porosity levels.