Effect of Surface Roughness on Remelting and Stresses During Splat Solidification

Abstract

This is a copy of the slides presented at the meeting but not formally written up for the volume. In order to understand the microstructural evolution in plasma sprayed coatings, the solidification process was modeled using a 2-D FEM model based on enthalpy formation. Studies of the surface of the coatings showed surface roughnesses across multiple length scales. The model was used to examine the effects of the substrate and splat temperatures and the surface roughness features on the onset of remelting of the underlying surface on which the splat solidifies. The surface roughness was found to promote remelting, indicating that it was an important parameter that determines splat solidification. The temperatures of the splat and substrate were consolidated into one non-dimensional parameter that captured the onset of remelting with a non-dimensional remelting point.A fully coupled thermo-mechanical finite element model was also run for a single splat case, to provide more insight stress buildup during solidification. An important result was that the relative size of the surface roughness features, as compared to the splat thickness, is very important. Very large wavelengths compared to splat thickness lead to smaller stresses, since the solidification and the interface are essentially 1-D. Very small wavelengths compared to splat thickness also leads to reduced stresses, since the solidification front quickly becomes 1-D. Only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration, which are locations of microcrack formation.