Microstructural characterization of HVOF/plasma thermal spray of micro/nano WC–12%Co powders

Abstract

This research focuses on studying the powder reactions and coating microstructures produced by thermal spraying various particle sized (micro and nano-sized) tungsten carbide cobalt powder. High Velocity Oxy Fuel (HVOF) and Atmospheric Plasma thermal spray methods were used to perform a multi-layer coating applied onto carbon steel specimens (typical of those used in oil/gas industry). The thermally sprayed powders/coatings were analyzed using X-ray powder diffraction (XRD), Environmental scanning electron microscope (ESEM), and Energy Dispersive Spectrometry (EDS) to predict the metallic powder reactions. In addition, surface roughness measurement of the coating, hardness assessment, and evaluation of coating porosity and adhesion were conducted to determine the coating characterization. The comparative metallographic results of the various test coatings are discussed. Results show that higher phase transformation occurred in the plasma thermal spray and more cobalt evaporation occurred during the deposition of the nano-sized powder. The plasma spray of nano-sized powders yielded lower coating roughness (from 5.5μm to 4.6μm), whereas it increased the roughness when sprayed with the HVOF system (from 3.1μm to 4.3μm). Nano-sized powders improved porosity but not significantly (to 1.2%). However, the higher percentage of nano-sized particles in the starting powder increased the hardness in the coatings to 1367HV. Among the four produced coatings, the results concluded that the coating produced by HVOF using nanostructured WC–12Co powder was found to have the best coating microstructure due to its low porosity, high density, good adhesion, and fracture resistance.