Characterization of WC-VC-Co thermal spray powders and coatings

The characterization of WC-VC-Co and WC-Co thermal spray powders and coatings of equal Co mass fractions has led to a new quantitative insight into some of the processes occurring during coating deposition and to an explanation of the difference in abrasion resistance between WC-VC-Co and WC-Co coatings. In both materials, during coating deposition the WC grains acquire a less angular morphology and a finer mean size than in the powders and a significant fraction of WC grains is decarburized, with the formation of W2C and Co6W6C. Changes in the morphology of the (W,V)C grains could not be detected on account of their typical irregular shape, but their mean size also decreased. The outer surfaces of the carbides grains, therefore, dissolve in the Co at the high deposition temperatures and no re-precipitation on the un- dissolved grains occurs during cooling because of the very high cooling rate. As a result, a fraction of W, V and C remains in solution in the Co, although it is difficult to measure because the Co composition in the coatings varies from point to point. The higher abrasion resistance of the WC-VC-Co coatings relative to WC-Co coatings of equal Co mass fraction is attributed mainly to the finer WC grain size in the WCVC-Co coatings (even when starting from equal grain size in the powders), the lower Co volume fraction (due to the density of (W,V)C being lower than the density of WC) and the presence of the (W,V)C grains, which, on account of their hardness and fine size, are not only resistant to abrasion but also to fracture.