Tribological Analysis and Characterization of HVOF Sprayed Tungsten Carbide-Cobalt Coatings

Abstract

Tungsten carbide cobalt coating has been extensively used for cutting and mining tools, aerospace, automotive and other wear resistance applications. These coatings not only have superior mechanical properties like high hardness, toughness and compressive strength but have also excellent controllable tribological properties. Nanocrystalline coatings can provide combination of high hardness and high toughness which otherwise are exclusive properties. But in the case of High Velocity Oxyfuel (HVOF) sprayed nanocrystalline WC-cobalt coatings, a higher degree of decarburization is reported in the published literature. This results in porosity and brittle phases formation in the coating structures. This in turn results in higher wear rate of nanocrystalline WC-cobalt coatings. In this study HVOF sprayed coatings made from duplex cobalt coated near-nanocrystalline WC-cobalt powders is used. The purpose of using these powders is to address the issue of decarburization and porosity formation in the coatings. Lower decarburization and hence porosity formation in these coatings along with the benefit of smaller WC grain sizes result in better tribological performance of these coatings. A comparison of wear behaviour is made between conventional microcrystalline and novel duplex cobalt coated near-nanocrystalline WC-17 wt% cobalt coatings. Coatings made from finer and coarser feedstock powders with same WC grain size are also compared. Optical and Scanning Electron Microscopic images show more porosity in the microcrystalline coatings than the duplex coated WC-cobalt coatings made from coarser feedstock powders while the highest porosity was observed when fine feedstock powder having agglomerate sizes 10-20 μm with 300-550nm WC grain sizes were used. Fracture toughness was 25.12 and 19.04 MPam 1/2 for near-nano and microcrystalline coatings respectively while it was only 3.01 MPam 1/2 for the coatings madeiv from fine feedstock powders. The higher toughness can be attributed to more frequent interruption in the crack propagation and only intergranular cracking in the near- nanocrystalline coatings in contrast to the microcrystalline coatings where both trans-granular and inter-granular crack propagation take place. Under similar test conditions, the wear rate was 4.0×10 -3 mm 3 /m for nearnanocrystalline and 6.6×10 -3 mm 3 /m for microcrystalline coatings. The lower wear rate in duplex cobalt coated near-nanocrystalline coatings can be attributed to its higher toughness and hardness values, the lower decarburization and porosity observed in it and the smaller fraction of brittle non-WC phases formation. In summary it is stated that the lowest porosity, W 2 C content and lower wear rate and highest hardness and fracture toughness was observed for the coatings made from duplex cobalt coated powders with near-nanocrystalline WC grains and with feedstock powder size of 45-60μm, which can be beneficial for high performance and longer tool life.