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 corros ion resistance is required, In this paper, four commercially available spra y powders obtained by various production techniques (sintering and crushing , agglomeration and plasma densification) were thoroughly characterized usi ng a broad variety of physical and chemical methods, including scanning ele ctron microscopy (SEM), energy-dispersive x-ray (EDX), x-ray diffraction, a dsorption, mercury intrusion, and helium pycnometry. Special emphasis is gi ven to the interdependence of the chemical and phase compositions. The cool ing rate applied during preparation of the spray powders seems to be respon sible for the appearance of equilibrium or nonequilibrium phases, as was es tablished 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 o f WC; hence, a second carbide phase is formed. The carbon content determine s which carbide phase is formed and how both cobalt and chromium are distri buted between the hard and the binder phases. A substantial carbon deficien cy leads to nearly complete bonding of both chromium and cobalt into carbid e phases. As was shown by differential scanning calorimetry (DSC) experimen ts, such spray powders do not form a melt in the temperature range up to 14 65 degreesC, while powders containing clearly detectable amounts of metalli c cobalt form a melt above 1210 degreesC.