Effect of adding WC powder to the feedstock of WC-Co-Cr based HVOF coating and its impact on erosion and abrasion resistance

HVOF grade powders are now commercially available and being used in large scale for different components prone to abrasion/erosion. The literature on HVOF coatings based on WC-Co powder shows that there is a huge difference in hardness between the pure WC powder and WC-Co based HVOF coatings. The objective of this study was to improve the hardness of WC based HVOF coatings by adding pure WC powder to the commercially available powder. The hardness data shows that 20% addition of WC powder will improve the hardness of HVOF coating from 1106 to 1395 Hv_0.3. Hardness increase is due to the embedding of tungsten carbide hard metal matrix. This HVOF coated sample was tested for dry sand abrasion and slurry erosion as per ASTM standards. These tests show that abrasion and erosion resistance of HVOF coated samples goes down with the addition of tungsten carbide powder even though coating hardness has gone up. To understand the negative trend, porosity and SEM studies were carried out. SEM studies show that the porosity of the HVOF coating is higher than the conventional HVOF coating. With increase in WC content (30%), the porosity of the HVOF coating increased up to 10%. The higher porosity is believed to be the reason for poor abrasion and slurry erosion resistance.