Effect of WC–10%Co–4%Cr coating on the Ti–6Al–4V alloy fatigue strength

Abstract

High strength/weight ratio and effective corrosion resistance are primary reasons to use titanium alloys replacing steel and aluminum in some aeronautical components. However, titanium alloys have poor tribological properties, which reduce devices performance under friction; making surface treatments a requirement to improve wear. Thermal spray coatings have attractive characteristics as high hardness and strong coating/substrate adhesion. Compared with thermal spray processes, the High Velocity Oxygen Fuel (HVOF) presents less porosity and oxide contents due to the lower flame temperature used in the process operation. Electroplated coatings used to improved abrasive wear and corrosion properties, affects negatively the fatigue strength, providing lower results than those for uncoated parts. To increase fatigue strength of coated materials, techniques as compressive residual stresses induced by shot peening are used. In this study the influence of WC–10%Co–4%Cr coating deposited by HVOF on the fatigue strength of Ti–6Al–4V alloy was evaluated. Comparison of fatigue strength of coated specimens and base material shows also a decrease when parts are coated. It was observed that the influence is more significant in high cycle fatigue tests. The shot peening prior to the thermal spray coating is an efficient surface treatment to improve fatigue resistance of coated Ti–6Al–4V. Scanning electron microscopy technique (SEM) was used to observe crack origin sites and thickness in all the coatings.