Investigation On The Submicron Structural WC-12Co Coatings Deposited By HVOF

According to this thesis, high-performance submicron metal-ceramic coating was deposited by High Velocity Oxygen-Fuel (HVOF) technique with raw material of both sub-micron and micron WC-12Co powder. Influence of spray process parameters and the size of WC on the performance of the coatings (microstructure, phase structure, porosity, wear resistance) is studied.Based on index of coating hardness and porosity , influence of the change in propane flow rate, spray distance, barrel length, and cooling conditions parameters was studied. Preparation of submicron structure WC-12Co coating process parameters: oxygen pressure 0.7MPa, oxygen flow 166L/min, propane gas pressure 0.65MPa, propane gas flow rate 16.7L/min, spraying distance of 300mm.The porosity of submicron structure WC-12Co coating deposited in the optimum process parameters was 1.25%, lower than the micron coating, and the microhardness is 1110 HV0.1, 200 HV0.1 higher than micron coatings under the same conditions. Analysis of the coating microstructure showed a good combine between coating with the substrate, which combined with the main mode of mechanical binding. Pore distribution is uniform and fine in the coating. The sub-micron WC particles distributed uniformly in the Co binder phase.The experimental spray parameters (propane flow rate, nozzle length, cooling conditions) have little effect on phase structure of the coating. When the size of WC particles reduce from the micron level to sub-micron level, the oxidation and decomposition of WC particle is not as serious as the nano-particles, and will keep WC phase in the coating. Phase structure of the coating by axial powder feeding is more complex. There appered decomposition products of WC (W2C , W), and the W2C phase content a lot. The coating by radial powder feeding appeared only a small content of W2C. It can be seen that oxidation and decomposition of WC was controll low in a large extent.In the test parameters for 25kg/200r×min-1, the wear volume of 45 steel is 287 times than that of WC-12Co coating in the same condition after three hours. Wear mechanisms of WC-12Co coatings in sliding wear conditions are adhesion both the role of abrasive wear, surface fatigue, friction and oxidation.The coating by Radial powder feeding has better wear resistance than the coating by axial powder feeding. Wear volume of sub-micron structure WC-12Co coatings were less than conventional micron WC-12Co coatings in all wear conditions. Sub-micron coating manifest more advantageous in the long overload conditions.