Effect of Heat Treatment on Wear Behaviour of DuplexCoatings on Ti6Al4 V

Abstract

Titanium and its alloys are extensively used in aerospace and mechanical applications because of their high specific strength and high fracture toughness. On the other hand, titanium alloys often show low hardness and poor resistance to sliding wear, so that surface properties improvement is in many cases recommended, often by thermal barrier coatings (TBCs). In this investigation, NiCrAlY as the intermediate coat and WC-Co as the top coat, deposited onto Ti6Al4V substrate with duplex thermal barrier coatings, consisting of a 200μm thick NiCrAlY interlayer deposited by High Velocity Oxygen Fuel (HVOF), followed by a 250μm, 350μm and 450μm WC-Co top coat deposited by detonation spray (DS). However, to gain further improvement in hardness, wear resistance and microstructural development, heat treatment is employed. Coatings have been heat treated at a range of temperatures between 600-1150°C. Inspections by SEM and phase analysis by XRD for top coat were characterised. The XRD results indicated that some brittle eta (η) phases (Co6W6C, Co3W3C and Co2W4C) were produced at high temperature heat treatments for the top coat. Tribological properties were studied under dry sliding condition by using pin on disc equipment and chrome steel as the counterface. The wear behaviour of the as-sprayed and heat treated coatings demonstrates that heat treatment improves the wear behaviour. Wear testing results indicate that heat treatment of the WC– 12Co coatings at 950°C resulted in the highest wear resistance among all the coatings due to the formation of the hard, unbrittle, crystalline Co6W6C phases in the top coat.