Comparison of Microstructure and Residual Stress of HVOF Double Carbides Coatings Deposited on Magnesium Substrate

Cermet double carbide coatings (WC-Cr₃C₂-Ni) were HVOF sprayed onto magnesium substrate. The variable parameter was spray distance (320, 360 and 400 mm). The microstructure of the coatings has been characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). Additional, porosity and residual stress have been estimated. Phase composition of WC-Cr₃C₂-Ni cermet coatings consists of hexagonal WC carbide, as well as the Cr₃C₂ and Cr₇C₃ carbides. For the longest spray distance, minor presence of WC₆O₆ was detected, most likely as an effect of higher spraying distance, leading to partially oxidation of WC at powders particles boundaries. Comparing lattice parameters with model data it should be noted that no significant contribution of stress is present, due to minor changes in WC lattice parameters in comparison to ICDD data. It also should be noted that Cr₇C₃ carbide in WC-Cr₃C₂-Ni coating has different lattice parameters than ICDD data what shows its reactive nature. In obtained results it is clearly seen, that residual stress have the lowest values for coating sprayed from the shorter distance. This tendency is visible for both, linear and shear stress. The crystallite sizes are also the smallest for the shorter spray distance. Such fine structure shows a tendency to good redistribute of the thermal stress in the sprayed coating and also on the coating-substrate interface.