Dependence of splat remelt and stress evolution on surface roughness length scales in plasma sprayed thermal barrier coatings

Analysis of real surfaces in plasma sprayed thermal barrier coatings showed roughness features at different length scales. It was found that the basic physics of the underlying splat remelt process can be characterized using a single non-dimensional temperature. Significantly different levels of remelt were calculated for the different surface roughness wavelengths, highlighting the importance of multiple length scales. A fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification. It was found that only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentrations. It was found that the surface roughness features in real coatings are not sinusoidal. Instead, a more realistic ‘ideal’ surface roughness geometry is proposed, with periodic regions of stress concentration defined as small concave arcs with large curvatures surrounded by large convex arcs of smaller curvature on either side.