Integrity of detonation sprayed Cr _x C _y –NiCr coating under exposure to thermal cycling

CrxCy–NiCr coatings with thickness in the range 200–250 μm were deposited by detonation spray coating on a nickel base superalloy (IN718) substrate and subjected to thermal cycling. Each thermal cycle involved heating to 650°C, the creep temperature of the IN718 substrate, followed by a dwell and then natural cooling to ambient temperature. With thermal cycling, the bulk of the coatings experienced preferential oxidation along the splat boundaries, contributing to an increase in porosity and cracking. Despite the development of porosity and cracks, no spallation of coatings was observed even after nearly 600 thermal cycles. The microhardness of coatings did not undergo significant change, owing to oxide formation and retention of the total carbide content of Cr3C2, Cr7C3 and Cr23C6. The wear behaviour of the as sprayed and thermally cycled coated samples was evaluated in abrasive and erosive wear modes. Wear measurements on as sprayed and thermally cycled coatings showed virtually no deterioration in properties even after exposure to nearly 600 thermal cycles.