Strain and Phase Mapping of Ceramic Matrix Composites and Protective Coatings

Coatings are frequently required to provide oxidation protection for high temperature materials. Silicon carbide (SiC) coatings have been used to protect carbon-carbon composites on leading edges and zirconia coatings are used as thermal barriers on gas turbine aerofoils. The effectiveness and durability of these coatings is dependent on the residual strains created in these coatings during their formation or deposition and also during service. Tensile strains in the plane of the coating can lead to through thickness cracks that expose the substrate, while compressive strains can cause the coating to delaminate. This paper presents strain measurements of these high temperature material systems obtained with high energy X-ray diffraction. The diffraction also provided useful information on phase, crystallite size and texture as a function of depth. Tensile strains were found in the SiC coatings, and compressive strains were found in the zirconia coatings. Both these strains were parallel to their coatings’ surfaces. The differences in thermal expansion coefficients between the coatings and their substrates can account for both the compressive strain in the zirconia and the tensile strain in the SiC.