Improving the properties of magnetic bearing shaft material by HVOF coating of WC-metal powder and laser heat treatment

Micron-sized WC-metal (WC-0.6%C-21%Cr-6%Ni) powder was coated onto the substrate of magnetic bearing shaft material Inconel718 (substrate or In718) using JK3500 HVOF thermal sprayer for the improvement of the surface properties of the substrate. The optimal coating process for the highest surface hardness was obtained using the Taguchi experimental program. The coating was laser heat-treated (LIT) by CO2(g) laser for further improvement of the properties. During the thermal spraying, a small portion of metal carbides of powder decomposed to W2C, metals and free carbon. The free carbon reacted with excessively sprayed oxygen, and formed carbon oxide gases, forming porous coating. By laser-heating, porosity decreased and the porous strips at the interface of coating and substrate (coat/sub) compacted. At the interface, the precipitated graphite concentration decreased and the metal elements diffused from both the coating and substrate increased, enhancing the functions as buffer zone and increasing adhesion of coating. The surface hardness of substrate increased by coating and further increased by laser-heating from 410 +/- 30 Hy to 983 +/- 101 Hy and 1425 +/- 94 Hy respectively. Porosity of coating decreased by laser-heating from 2.6 +/- 0.4% to 0.35 +/- 0.06%, and coating thickness shrank from 280 mu m to 200 mu m. Friction coefficients of substrate decreased from 0.52 +/- 0.02 to 0.36 +/- 0.04 by coating, because the free carbon formed by decomposition of WC to W2C functioned as a solid lubricant. By increasing sliding surface temperature from 25 degrees C to 450 degrees C, the friction coefficients of substrate and coating were decreased from 0.52 +/- 0.02 to 0.31 +/- 0.02 and from 0.36 +/- 0.04 to 0.23 +/- 0.04 respectively, because of easy formation of free carbon and metal oxides which functioned as solid lubricants. Wear depth of surface was decreased by coating and by LH coating from 55 mu m to 32 mu m and to 12 mu m respectively. HVOF coating of WC-metal powder on In718 surface and laser heat-treatment of the coating are highly recommended for the improvement of the properties of magnetic bearing shaft.