Microstructure, wear, and oxidation resistance of nanostructured carbide-strengthened cobalt-based composite coatings on Invar alloys by laser cladding

Abstract

To strengthen Invar alloy, nanostructured carbide-strengthened cobalt-based cladding layers were fabricated on an Invar alloy by laser cladding. The cladding layers contained cubic γ-(Fe, Ni) or Co-based solid solution and hard carbides such as W2C, Cr3C2/V8C7, CoCx, and niobium carbide at the grain boundaries. The cladding layers improved the wear and oxidation resistance of the Invar alloy without changing its coefficient of thermal expansion. The results indicated that the friction coefficient of composite coatings decreased by 28.6% compared with that of Invar substrate. The oxide layers of samples with cladding layers were 18.4 μm thick at 1073 K, which was thinner than that of Invar substrate (88.38 μm). Cr3C2 displayed higher oxidation resistance than that of VC and a mixture of Cr3C2 and VC; however, VC-doped cladding layers exhibited better wear resistance than that of cladding layers with Cr3C2 or VC and Cr3C2. Elemental diffusion, carbide strengthening, and interface strengthening through the dislocations and plastic deformability at the carbide/matrix interface were believed to be the surface strengthening mechanism of Invar alloys.