Influence of substrate hardness transition on built-up of nanostructured WC-12Co by cold spraying

The ability of cold spray process to retain the feedstock microstructure into coating makes it possible to deposit nanostructured WC-Co coatings. In the present study, the deposition behavior of nanostructured WC-12Co coating was examined through the surface morphology and cross-sectional structure of the deposited single WC-12Co particle impacting on the substrates with different hardness using a nanostructured WC-12Co powder. Substrates included stainless steel, nickel-based self-fluxing alloy coatings with different hardness. It was observed from the top surface and cross-section of individual WC-12Co particles that the penetration leading to particle deposition depends on substrate hardness. When the substrate surface is covered by WC-12Co particles, the hardness of the newly formed substrate, i.e. the coating, increases greatly. The significant increase of the surface hardness leads to the rebounding off of impacting particles and erosion of the deposited particles, which prohibits effective built-up of coating. However, it was found that with spray jet fixed, a deposit with a thickness up to over 700 μm can be built-up. A model involving in substrate hardness transition during deposition is proposed to explain such phenomenon, which can be employed to optimize the conditions to build up a uniform nanostructured WC-12Co coating.