Microstructure, Performance And Abrasive Wear Behaviour Of Metal Tungsten Carbide Coatings Deposited By HVOF And Oxygen-acetylene Flame Spray And Fuse

Electrolytic hard chrome (EHC) plating is an industrially widespread technology for their high wear and corrosion resistance, restrictive environmental norms which will increasingly limit the applicability of EHC plating because of its giving of a lot of Cr6+and SO42-ions which can cause great damage to human health. Therefore, the need to find technically and economically feasible alternatives arises.Metals and cermets deposited by HVOF-spraying and oxy-acetylene flame spraying and fusing is one of the most promising coatings for the replacement of EHC plating in those field which need high wear and corrosion resistance coatings.China is a country rich in tungsten, on the one hand, a lot of low-cost WC raw materials were exported, on the other hand a lot of spraying or spraying and fusing WC based powder with promising quality were imported with high price.In order to obtain high quality spraying and fusing WC based powder and high quality coatings which can replace of the importing materials, some new metals and WC were designed and corresponding process optimization were also carried.Microstructure, abrasive wear and corrosion resistance of these coatings deposited by new designing powders have been tested in order to obtain theoretical and practical basis for an alternative method for EHC process.In this paper, ultra-fine, submicron, micron and bimodal WC-12Co as well as micron and bimodal WC-10Co-4Cr HVOF spray powder were designed by means of changing the grain of WC,14kinds of spraying and fusing powders were also designed by means of changing the grain or proportion WC with NiCrBSi alloy.WC/Co(Cr) coatings were deposited by JP-8000HVOF spraying system using L9(34) orthogonal experiment, NiCrBSi/WC coatings were produced by promising oxy-acetylene flame spraying and fusing process.The abrasive wear resistances of all the coatings have been studied by wet sand rubber wheel abrasion tester. The phase compositions of the feedstock powders and coatings were analyzed by XRD. The characterizations of spraying feedstock powders, microstructure and surface micrographs of the prophase and anaphase attrition surfaces of these coatings were analyzed by SEM, the hardness, porosity and fracture toughness of these coatings have also been examined. The results indicated that: The spraying parameters had little effects on the phase composition but can brought great influence on the performance of sprayed coatings by using of JP-8000HVOF spraying system.The porosity, fracture toughness and the depositing efficient decreased while the hardness of all the coatings increase with the kerosene and oxygen flux, while all the coatings show reverse tendency when increase the feed rate and the spraying distance. The kerosene flux affects hardness mostly, and the feed rate is the most important parameters which affect the per pass thick of coating.According to the costing and the required performance of the coating, three group parameters could be obtained by optimizing spraying permanents. The economical spraying parameter is22.7L/h kerosene flux,55.2m3/h oxygen flux,75g/min powder feed rate and380mm spray distance, The moderate parameter is24.6L/h kerosene flux,55.2m3/h oxygen flux,75g/min powder feed rate and326mm spray distance, The highest spraying parameter is26.5L/h kerosene flux,59.5m3/h oxygen flux,75g/min powder feed rate and326mm spray distance,The hardness is inversely proportional to fracture toughness and proportional to the abrasive wear resistance, the abrasive wear resistance of coating can be increased in great degree by mean of increasing its hardness, when the coating service in the low attacking field.WC decarburizes in the model of direct and indirect during spraying HVOF process, the decarburizing degree of WC/Co(Cr) coating increases with the residual time in the flame, the temperature of the flame, distribution of bonding metal phase and decrease with the size of the WC grain and the density of sprayed powder. The inhomogeneity distribution of bonding metal phase increases the decomposition of WC both on the direct and indirect way. HVOF-sprayed WC-Co coatings still suffer from decomposition and decarburization during spraying process leading to generating of detrimental phases such as W2C, W and amorphous Co-W-C phase.The wear rate was primary affected by the relative hardness of abrasive compared with coating, when the hardness of abrasive is higher than WC/Co(Cr) coating such as SiC, the coating was severely cut by the SiC with high hardness and the wear rate of coating was high. When the hardness of abrasive is lower than WC/Co(Cr) coating such as SiO2, the wear rate of coating was low but increase with size of abrasive and the applied load. The mechanism of wear for WC-Co coatings was by selective removal of the binder caused probably by plastic deformation and fatigue due to the repeated action of the abrasive particles followed by the undermining of the carbide particles resulting in their eventual pull out.The grain size and distribution of WC affected the performance of HVOF WC/Co(Cr) coatings in great degree, the bimodal coating showed the highest hardness, fracture toughness and abrasive wear resistance in all the coatings, the smallest the mean free path of Co phase was responsible for the highest abrasive wear resistance.The small content of W2C phase in the WC/Co(Cr) coating showed no harmfulness on the abrasive resistance. In this case, the microstructure of coating was much more compact and the hardness of coating was higher which can lead to the increase of the abrasive and corrosion resistance of coating. As a result, the primary factors that affected the performance of coating are the distribution of Co bonding phase and the microstructure of the WC/Co(Cr) coating.The bimodal and conventional micron WC-12Co coating were heat treated at the temperature of550℃,750℃,950℃and1150℃in the protection of argon. The W2C and W phase disappear and the η phase such as Co3W3C, Co6W6C and Co2W4C gradually appear in the WC-12Co coating with the increase of heat treatment temperature. These η phase usually nucleate at the interface between W2C phase and bonding metal phase, because content of Co, C and W can easily attained by short distance diffusion of these elements. Both the hardness and fracture toughness of coating increased with heat treatment temperature first and then decrease after the950℃.The best abrasive wear resistance of the coating appeared at the temperature of950℃. The diffusion speed of element such as W, Fe and Co at the interface of coating and substrate also increase with the heat treatment temperature, an obvious diffusion bond at the width of10μm at the coating side and some Kirkendall voids with the size of1-10μm at the side of substrate appeared at the temperature of1150℃.The hardness of spray and fused Ni base WC composite coatings firstly increased and then decreased with content of an added WC, the WC content for the highest hardness of coating with the WC added in the way of mechanical blended is35wt.%, while the WC content for the highest hardness of coating with the WC added in the way of melt and casting was25wt.%.When the WC content were similar, the hardness of the coatings that the WC added in the way of mechanically blended were higher than those coatings that the WC added in the way of melt and casting. The highest abrasive wear resistance of coating in all the spray and fused Ni base WC composite coatings was the bimodal coating with the WC content of35wt.%, and the ratio of fine and coarse WC in the coating were3:7.The excellent abrasive wear resistance of bimodal Ni based WC spray and fused coating can be attributed to the homogeneous distribution of bonding metal phase and dense coating microstructure.The HVOF WC/Co(Cr) coatings and the Ni based with WC composite coatings deposited by these bimodal powders designed in this paper showed similar or even higher abrasive wear and corrosion resistance compared with those coatings produced by imported powder and showed similar corrosion resistance and10.6,9.2and2times abrasive wear resistance in comparison to the EHC coating with high thickness respectively. As a result, the new bimodal HVOF WC/Co(Cr) coatings and spraying and fused Ni based WC composited coatings deposited by optimal process and parameter can replace the corresponding coatings deposited by expensive imported powders and EHC that can bring great damage to human health.