Optimization of ceramic content in nickel-alumina composite coatings obtained by low pressure cold spraying

The paper describes the technology of nickel-alumina composite coatings obtained by low-pressure cold spray (LPCS). The content of alumina reinforcement was optimized by mixing the commercial K32, being a mechanical mixture of Al2O3 + 50 wt% Ni, with the Ni-cladded alumina powders developed in present study. The cladded powders were prepared by hydrogen reduction method. Two powders including Ni-films covering Al2O3 particles were developed: Al2O3 + 70 wt% Ni (powder # 1) and Al2O3 + 80 wt% Ni (powder # 2). The morphology of obtained cladded powders, analyzed using scanning electron microscope (SEM), showed a porous, cauliflowerlike morphology in cladded layer of powder # 1 and dense nickel layer in powder # 2. The measurements of granulometry enabled finding the powder size distribution of pure alumina powder having a mean value of about 50 um which increased by cladding to about 70 um in powder # 1 and to about 65 um in powder # 2. The cladded powders were mixed with K32 powder and sprayed using LCPS onto mild steel substrate. The microstructure of obtained coatings was characterized using SEM, energy dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD). The graphical analysis of coatings' cross-sections enabled determining content of alumina in sprayed deposits. This content increased from about 7% in K32 powder sprayed coatings to 9-13% in coatings obtained using its mixtures with powder # 2 and up to 20-30% for mixtures with powder # 1. The microhardness of coatings sprayed using the mixtures of powder # 1 with powder K32 as well as the coating sprayed only with the powder K32 were slightly greater than HV 0.1 > 150. The microhardnesses of about HV 0.1 N 200 was reached for the coatings sprayed using mixtures of powder # 2 with K32. Finally, the bond strength of the sprayed coatings, measured using test ISO 14916, was in the range from 1.7 to 5.9 MPa. The greatest bond strength was reached for the coatings sprayed using powder K32.