抄録
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 occures in the film without the division. When the heating temperature T is 773K or 873K and the holding time at the temperature is short, the interfacial energy 2γ12 increases to reach a maximum with an increase in heat cycles and decreases with further heat cycles. When T are 973K and 1073K, 2γ12 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.
