Effects of the dispersion time on the microstructure and wear resistance of WC/Co-CNTs HVOF sprayed coatings

WC-Co is widely used as a tribological coating material providing a combination of high toughness, high hardness, and good strength. Herein, an attempt has been made to further enhance the mechanical and wear properties of tungsten carbide cobalt coatings by reinforcing them with multiwall carbon nanotubes (CNTs) using thermal spraying. In this work, 0.35 wt.% of CNTs were mixed by jar-milling in ethanol solution with WC-12%Co microcrystalline powders for different blending times. The mixture was thermally sprayed using the HVOF process onto a plain steel substrate. Also, coatings deposited with both WC-12%Co microcrystalline and nanostructured powders, using the same thermal spray process, were evaluated and compared with samples reinforced with CNTs. The microstructures of the coatings were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy and X-ray diffraction (XRD). The microhardness was measured by Vickers indentation and the abrasive wear resistance was evaluated using a dry sand rubber wheel technique according to ASTM G-65-00 standard. Effects of blending times of CNTs on the microstructure, abrasion wear and microhardness of the coatings were investigated. Experimental results have shown that 36 h of milling time was suitable to disperse CNTs with WC-Co feed powders since it produces an adequate relationship between CNTs' dispersion time and particle size which enhances the microhardness and porosity of the coatings. The C-36 coating showed an increase in wear resistance of almost 80% and 70% compared with conventional and nanostructured coatings, respectively. This was attributed to the CNTs acting as bridges, promoting the cohesion between lamellas and reducing the decarburization. All reinforced coatings showed a higher abrasive wear resistance than non-reinforced indicating that CNTs are a good alternative to improve abrasion wear resistance of WC-Co coatings.