Stresses in Ytterbium Silicate Multilayer Environmental Barrier Coatings

The internal stresses of plasma-sprayed multilayer ytterbium disilicate environmental barrier coatings were measured using microfocused high-energy X-rays in a transmission geometry. Stresses were measured for as-sprayed and ex-situ heat-treated ytterbium disilicate topcoats at room temperature and during in-situ heating and cooling experiments. In-situ loading experiments were also performed on the topcoat in order to establish its elastic constants. The ytterbium disilicate was found to have a relatively low coefficient of thermal expansion resulting in compressive stresses of approximately 100 MPa throughout the topcoat. In-situ heating experiments revealed a statistically significant stress relaxation in the ytterbium disilicate topcoat upon thermal cycling to temperatures above 1300°C, indicating the onset of stress relaxation but no cracks were observed in SEM micrographs. The stress states were also modeled using a numerical solution; measured stresses were found to be very close to the predicted stresses in ytterbium dilisicate topcoats, while the experimentally determined stresses in the intermediate layers were of much smaller magnitude than the calculated stresses.