Residual Stress Development during Thermal Spraying of WC-Co on Titanium

Tungsten carbide cobalt thermal spray coatings are used in the aircraft industry to reduce wear damage of lightweight metals such as titanium The performance and life of tungsten carbide (WC-Co) coated titanium materials depend on many factors. An important factor that has received increased attention in thermal spray research is the residual stresses in the coating and substrate. Residual stresses depend on the parameters of the application process. Parameters affecting residual stresses include the prespray treatment of the substrate material (grit blasting, shot peening) and the type of spray application process (HVOF, plasma arc) During the in-service life of a WC-Co coated material, residual stresses can change significantly. The goal of this work is to quantitatively evaluate the changes in residual stresses of the substrate and the WC-Co coating during various stages of processing. A destructive laboratory method, called the "Modified Layer Removal Method," was used to evaluate the through-thickness residual stresses of the WC-Co coating and the titanium substrate material. Residual stresses are determined for three conditions: (1) shot peened, (2) shot peened and grit blasted, and (3) shot-peened, grit blasted and thermal spray coated. The changes in the residual stresses are shown at selected stages during the processing history of the coated materials. Differences between residual stress levels at selected stages are identified and discussed. The effect of coating thickness and HVOF application process on the residual stress in the coating is also examined.