In-situ Stress Analysis of Multilayer Environmental Barrier Coatings

The biaxial stress and thermal expansion of multilayer doped-aluminosilicate environmental barrier coatings were measured in-situ during cooling using microfocused high-energy X-rays in transmission. Coating stresses during cooling from 1000oC were measured for as-sprayed and thermally cycled samples. In the as-sprayed state, tensile stresses as high as 75 MPa were measured in the doped-aluminosilicate topcoat at 375oC, after which a drop in the stress occurred accompanied by through-thickness cracking of the two outermost layers. After thermally cycling the samples, the stress in the topcoat was reduced to approximately 50 MPa, and there was no drop in stress upon cooling. This stress reduction was attributed to a crystallographic phase transformation of the topcoat and the accompanying change in thermal expansion coefficient. The addition of a doped aluminosilicate to the mullite layer did not lower the stress in the topcoat, but may offer increased durability due to an increased compressive stress.