Effect of heat cycles on the delamination strength of WC film coated by high-velocity flame spraying
Kato Masahiko; Nakasa Keijiro; Egawa Fumiaki; Kamata Masanobu; Hara Nobuhiko
Abstract:Tungsten carbide cermet was coated on the smooth tensile specimens of annealed tool steel (JIS:SKD6) by high-velocity flame spraying. After the specimens were heat cycled between high temperatures and room temperature, the tensile tests of the specimens were carried out to examine the change of interfacial energy by heat-cycles, where the load was applied parallel to the film. For the specimen which is not heat-cycled, the film is divided by parallel and straight cracks repeatedly with an increase in load, and the film delamination occurs after the division is completed. For the heat- cycled specimens, three types of delamination patterns are observed :(1) The film is delaminated almost in the same way as the non heat cycled specimen, but the cracks are curved, (2) the small blocks of film are delaminated after the division finishes, and (3) the film is heavily damaged by oxidization and the delamination occurs in the film without the division. When the heating temperature T is 773 K or 873 K and the holding time at the temperature is short, the interfacial energy 2 g12 increases to reach a maximum with an increase in heat cycles and decreases with further heat cycles. When T are 973 K and 1073 K, 2 g12 decreases with small increase in heat cycles. The change in interfacial energy with heat cycles can be explained both by the strengthening due to the diffusion of Fe and Cr atoms to film and by the accumulation of fatigue damage due to the difference in thermal expansion coefficient between film and substrate. The crack interval just before delamination changes depending on the heating temperature and heat cycles, the decrease in which corresponds to the decrease in critical tensile strength of film and/or the increase in critical shear strength of interface. Key Words:WC film, high-velocity flame spraying, tool steel substrate, heat cycling, delamination strength