Novel enhancement of HVOF thermal sprayed nanostructured WC-12Co / inconel-625 coatings for tribocorrosion applications

The HVOF thermal spray process is gradually becoming one of the leading coating techniques taking over traditional electrolytic chrome plating (EHC) due to EHCs harmful effects on the human body. Despite the high quality coatings produced by the HVOF thermal spray system, its role has yet to be validated in the replacement of other traditional coating techniques where specific surface properties are required in particular service operation. Obstacles associated with high-velocity oxy-fuel (HVOF) thermal spray significantly affect coating performance, especially in erosion and corrosion preventive applications. The coating layer therefore must be enhanced with a view to reducing microstructural defects and thereby prolonging the coating’s service life. This research is aimed at examining the effect of using a CO2 laser system as a post-heat treatment and enhancement procedure applied to two coatings types investigated in this research that were deposited by an HVOF thermally sprayed process onto carbon steel 4041 substrates: firstly, 100% tungsten carbide cobalt (nano-structured) WC-12Co (InfralloyTM S7412) and secondly, WC-12Co nanostructured powder mixed with a nickel chromium alloy (Diamalloy 1005- Inconel 625) at an optimised weight percentage composition of 75% and 25% respectively. The work was carried out through the introduction of experimentally based mathematical models developed by applying response surface methodology (RSM) through Box-Behnken design (BBD), based on three levels of each factor selected, namely laser power, scanning speed and focal position/beam size, using Design of Expert software related to the coating’s erosion resistance, melt-pool geometry, mechanical properties and operating cost of the laser treatment. Furthermore, the desirability optimisation approach, based on two criteria (quality and cost), was used for both coatings in conjunction with RSM to determine the optimal combination of the laser parameters to achieve the required laser-treated coating desirability. Different outcomes of surface properties were achieved by varying the laser-processing parameters. The results demonstrate that significant improvement in coating erosion wear (dry and slurry erosion) and mechanical properties (bending strength, surface roughness and microhardness), compared to as-sprayed coating, was achieved after laser treatment in both coating types, especially the singular nanostructured WC-12Co coating. The optimal laser settings found in the quality criteria are 350 W, 37.24 mm and 150.00 mm/min for laser power, focal position and scanning speed, respectively, for the monomial nWC-12Co coating and 350 W, 45 mm and 300 mm/min for the cost criteria. To the same extent, the optimal setting for the mixed coating for the quality criteria are 169 W, 35 mm and 257.4 mm/min and 250 W, 45 mm and 300 mm/min for the cost criterion. The optimal laser setting mentioned in the quality criteria for the erosive wear, for example, saw an approximately five- to seven-fold reduction in mass loss for dry and slurry erosion in comparison to the untreated monomial nWC-12Co coating. The latter setting created an approximately 7-fold reduction in mass losses for the dry erosion and a 27% reduction in mass losses for slurry erosion compared to their untreated counterparts. This can mainly be ascribed to the elimination of the discrete splat structure, porosity and microcrevice, as well as the enhanced homogeneity of the nano-scale WC hard ceramic distribution across the metal matrix. Less improvement was seen for the mixed coating as a result of high energy fluence (J/mm2); the coating surface became rough and gas pockets started forming within the melted zones, creating a porous coating layer that had a negative impact on coating bending strength and erosion performance. Moreover, the results indicate a strong correlation between irradiance and residence time of the laser processing, along with coating composition, with respect to the melt-pool dimensions. Finally, based on the enhancement achieved in the coating properties under the optimal laser settings for both coatings, compared to the untreated ones, the results prove that laser post processing is a cost-effective procedure (approximately 17% or less of that of HVOF) and therefore will markedly extend the service life of both coatings, saving a lot of money that would be wasted in the case of the untreated ones