Microstructure and properties of WC-10%Co-4% Cr spray powders and coatings:: Part 1.: Powder characterization

WC-10% Co-4% Cr-1 represents an important composition for thermally sprayed hardmetal-like coatings that are applied when simultaneous wear and corrosion resistance is required, In this paper, four commercially available spray powders obtained by various production techniques (sintering and crushing, agglomeration and plasma densification) were thoroughly characterized using a broad variety of physical and chemical methods, including scanning electron microscopy (SEM), energy-dispersive x-ray (EDX), x-ray diffraction, adsorption, mercury intrusion, and helium pycnometry. Special emphasis is given to the interdependence of the chemical and phase compositions. The cooling rate applied during preparation of the spray powders seems to be responsible for the appearance of equilibrium or nonequilibrium phases, as was established from the investigation of the spray powders after heat treatment at 1000 degreesC, The amount of Cr added to the composition, 4%, is higher than the solubility limit of chromium in the binder phase in the presence of WC; hence, a second carbide phase is formed. The carbon content determines which carbide phase is formed and how both cobalt and chromium are distributed between the hard and the binder phases. A substantial carbon deficiency leads to nearly complete bonding of both chromium and cobalt into carbide phases. As was shown by differential scanning calorimetry (DSC) experiments, such spray powders do not form a melt in the temperature range up to 1465 degreesC, while powders containing clearly detectable amounts of metallic cobalt form a melt above 1210 degreesC.