Effect of laser irradiation condition on bonding strength in laser plasma hybrid spraying

The effect of the laser irradiation condition on the bonding strength of a coating made by the laser-plasma-hybrid-spraying (LPHS) method was studied. LPHS is able to improve the bonding strength as the interfaces are melted and joined by the laser simultaneously with plasma spraying. However, the correlation between strength and interfacial properties has not been previously studied in detail. In this study, the relationship between bonding strength and laser irradiation intensity was evaluated using the shearing test for the NiTi-LPHS-coating made from a mixture powder of nickel and titanium. The main results are summarized as follows: The shearing strength was increased to 6 times that of the VPS-coating upon using LPHS. This is because diffusion between the coating and the substrate was promoted and a diffusion layer approximately 30-μm thick was formed by laser heating and melting. The shearing strength decreased as the power density increased over 4×108 W m−2 because an intermetallic compound which was brittle and had a low yielding point was formed at the interface. We proposed to spray an insert layer, such as chromium and molybdenum, on the substrate, to avoid the formation of brittle intermetallic compounds at the interface. By this method, both high bonding strength and high erosion resistance were achieved.