Deposition effects of WC particle size on cold sprayed WC–Co coatings

The WC particle size and its influence on the deposition of Co-based cermets are examined. Micron and nanostructured powders with similar Co content were employed. Varying the WC particle size influenced significantly the deposition efficiency of the coating process. Micrometer-structured WC–Co feedstocks did not permit coating build up when processed under comparable or elevated thermal spray parameters used for the nanostructured WC–Co feedstocks. In addition, micrometer-structured WC–Co coatings exhibited a conjoint erosion and deposition effect on the surface. Fine WC particles (< 1 μm) were observed near the substrate interface and larger WC particles (1–2 μm) in the vicinity of the coating surface. These observations indicate the existence of a critical WC particle size for deposition by the cold spray method and that the size criteria arises due to the formation and cohesion mechanisms within the coating layer.Nanostructured test specimens displayed (i) a dense microstructure with little presence of porosity, (ii) a crack free interface between the coating and substrate that indicated good adhesion, and (iii) no observable phase changes. The XRD patterns of each powder and their respective coatings did not have observable peak differences but the diffraction peak broadening of coatings indicated that there was grain refinement during the coating process. Furthermore, all nanostructured as-sprayed WC–Co coatings exhibited Vickers hardness values above HV1000. The nanostructured WC–Co coatings demonstrated adhesive strengths that exceeded the limits of the glue (60 MPa).

Research Highlights
► Micrometer- and nanostructured powders of similar cobalt content.
► Micrometer-structured coating did not build up and grain refinement occurs.
► Nanostructured coatings have (i) dense microstructure with no phase changes.
► The experiments demonstrate the existence of a critical WC particle size for deposition.