Microstructural Evolution of the WC-Co/NiCrAlY Duplex Coating System on Ti6Al4 V and its Influence on Mechanical Properties

Titanium and its alloys are extensively used in aerospace and mechanical application because of their high specific strength and high fracture toughness. On the other hand, titanium alloys often show poor resistance to sliding wear, so that surface property improvement is in many cases recommended, often by thermal spray coating processes. In this investigation, NiCrAlY as the intermediate coat and WC-Co as the top coat, deposited onto Ti6Al4 V substrate with duplex thermal barrier coatings, consisting of a 200 μm thick NiCrAlY inter-layer deposited by High Velocity Oxygen Fuel (HVOF), followed by a 250 μm, 350 μm and 450 μm WC-Co top coat deposited by detonation spray (DS). However, heat treatment is employed to obtain further improvement in microstructural development and mechanical properties. Coatings have been heat-treated at a range of temperatures between 600-1150 oC. Inspections by SEM and phase analysis by XRD indicated that some brittle eta (η) phases (Co3W3C and Co2W4C) and unbrittle Co6W6C phases were produced at high temperature heat treatments corresponding to the top coat. Strong Cr23C6 phase and aluminium carbides were generated near topcoat/bond coat interface. Also the phase composition near NiCrAlY/substrate produced γ-Ni, β-NiAl and α-Cr phases and after 800 oC heat treatment temperatures β-NiAl, γ’-Ni3Al Ti3Al, Ti2Ni and TiNi were produced. The generation of these phases improves the coating's mechanical properties (hardness, residual stresses and bond strength).