Modeling development of residual stresses in thermal spray coatings

The diameter, velocity and temperature of stainless steel and tungsten carbide cobalt particles applied onto stainless steel substrates using a high velocity oxy-fuel (HVOF) torch were measured. The microstructure of the coatings produced was examined using a scanning electron microscope and coating thickness, porosity and roughness measured. Using the experimental spray parameters as inputs to a 3-D stochastic model we simulated coating formation. Measured values of coating thickness and porosity agreed well with predicted values while calculated surface roughness was somewhat higher than that observed in experiments. An object oriented finite element code (OOF) developed at the National Institute of Standards and Technology was used to calculate residual stresses in the coating. The model uses an adaptive meshing technique to discretize the coating microstructure into a mesh suitable for finite element analysis. To define the coating geometry we used either micrographs of coating cross-sections or computer generated images of coatings. Similar values of residual stress are obtained in either case. High stresses are present at the interface between the coating and substrate. The magnitude of stresses increases significantly with coating thickness. Stresses are relieved by voids such as pores or cracks in the coating. Residual stresses increase with coating temperature and can be decreased by preheating the substrate.