A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures

The microstructures of two tungsten carbide–cobalt (WC–Co) coatings, deposited using high velocity oxy-fuel (HVOF) thermal spraying method in different conditions, are studied. They are compared with that of the WC–Co powder grains injected in the flame, in an attempt to understand the transformations that occur during deposition. For this purpose, various imaging and analytical techniques in electron microscopy are used, in addition to global characterization methods such as X-ray diffraction and fluorescence. These methods reveal that the coatings are made of distinct islands, elongated along the substrate direction, which exhibit a nano-crystalline matrix containing tungsten, cobalt and carbon. The fraction of WC grains in the coating is smaller than that in the powder and fluctuates throughout the coating. A net loss in carbon is evidenced in the coatings as compared to the powder grains. New phases, W2C and W, appear in specific locations in the microstructure in relation with the local composition of the matrix. Very little metallic cobalt is retained. The extent of the transformation is related to the spraying conditions. Some processes that account for the change in microstructure and composition during spraying are proposed.