The effects of fuel chemistry and feedstock powder structure on the mechanical and tribological properties of HVOF thermal-sprayed WC–Co coatings with very fine structures

Abstract

We have deposited a series of WC–Co coatings by the high velocity oxy fuel (HVOF) process, using three different powders, and different spray conditions. The powders are a nanocrystalline (Nanocarb), a near-nanostructured powder (Infralloy) containing a proprietary additive aimed at retarding grain growth, and multimodal (mixed micro and nano) powder (Nanomyte). HVOF spray conditions were stoichiometric, fuel rich and oxygen rich. ‘In-flight’ feedstock powder temperature and velocity were measured. The hardness and toughness of the coatings are found to depend on WC-binder adhesion and adhesion between splats. High flame temperatures increase WC-binder adhesion but increase decarburization. The latter is found to decrease adhesion between the splats. Decarburization is most pronounced for nanostructured powders because of their high specific surface. The additive in Infralloy decreases adhesion between WC grains and binder, but it also reduces decarburization. The wear resistance of the coatings increases with hardness and decreases with increasing decarburization. Sliding wear occurs by a attrition of the WC grains and the lifting of entire splats; abrasive wear occurs by ductile cutting, grain loss and lifting of splats; the low wear rate in sliding leads to splat-boundary weakening by fatigue. The effect of decarburization predominates in sliding wear and is less pronounced in abrasion; the high abrasive wear removes material before fatigue becomes important. The coating deposited at high temperature, from the multimodal powder Nanomyte, presents outstanding sliding and abrasive wear resistance but inflicts large wear on the opposing silicon nitride surface in sliding. Coatings deposited with the near nanostructured powder containing an additive present high sliding wear resistance, independent of the deposition parameters, and cause low wear of the opposing silicon nitride. Coatings deposited with spray-dried nanostructured powders offer comparatively low wear resistance, in agreement with previous reports.