| The mold is the core device in casting processing, known as the ―heart‖ of the casting,while copper plate as an important component of the mold, not only bear the heat load and hot corrosion of the molten stee l, but also withstand wear and stress of the casting movement. At the severe service conditions, copper plate frequently occur wear-out failure, hot corrosion and thermal crack, and shape deformation damage. Therefore, copper mold surface modification has been the research hotspot issue in the field of materials and metallurgy.Cr2O3-TiO2/CoNiCr AlY composite coatings were deposited onto the C uCr Zr copper substrate by high efficiency supersonic atmosphere plasma spray system in this paper. The phase composition and microstructure of the Cr2O3-TiO2 powder and composite of the coatings were characterized by XRD and SEM. The influence of different spraying process parameters on the temperature and velocity of in- flight particles was investigated with the help of Spray Watch 2i on- line monitoring system, and the flat particles morphology was observe. By orthogonal experiment was used to determine the optimum coating process parameters to reduce the porosity and improve the hardness of the coating. The bonding strength and thermal shock resistance of composite coating were evaluated by tensile test and thermal shock test, thermal conductivity and wear resistance properties of Cr2O3-TiO2 composite coating and N i-Co plating were measured by coefficient of thermal conductivity meter TC3000, and further analysis of the mechanisms of wear.The results are summarized as follows:(1) Flight speed and surface temperature of particles were at first increased and then decreased with the processing parameters of spraying power, spraying distance and powder feed rates in the supersonic plasma jet. Under the same spray conditions, when the spraying distance was 100 mm, the flat particle melt more fully, the spreading degree was greater and the particles presented as discoid morphology.(2) The influence sequence of supersonic plasma spraying process parameters on the order of the comprehensive performance of Cr2O3-TiO2 composite coating are as follows: the spraying power, primary gas flow rate, powder feed rate, spraying distance. The optimized process parameters for spraying were: spraying power 64 kW, the main gas flow rate 70 SLPM, powder feeding rate 35g·min-1, spraying distance 100 mm. Cr2O3-TiO2 composite coating had an average porosity of 1.36%, an average microhardness of 1573HV0.3 by the optimal spraying process parameters.(3) Cr2O3-TiO2 composite coating exhibited a bimodal microstructure composed of well- melted regions and partially- melted particles. The fracture morphology was lamellar structure. The cross section morphology was a typical wave- layered structure and existence of small spherical pore and tiny cracks in the middle layer of the coating.(4) The bonding of the Cr2O3-TiO2 composite coating was 31.1 MPa, mainly mechanical bond accompanied by a small amount of micro metallurgical bonding. Thermal conductivity of Cr2O3-TiO2 composite coating was 1.41W/m.K, which was less than the currently used N i-Co plating,but still matches the requirement.(5) Cr2O3-TiO2 composite coating exhibited excellent the thermal shock resistance. At 450 ℃, 600 ℃ and 750 ℃, the coating from cracks to failure respectively experienced 123, 57 and 27 cycles for bullet-type specimens, coin-type specimen experienced 133, 52 and 40 cycles.(6) Compared with the CuCr Zr substrate and N i-Co plating, Cr2O3-TiO2 coating exhibited excellent wear resistance; the wear scar width was 354.8μm, the wear volume lost was 21.75×10-9m3 and the friction coefficient was 0.40, which were the smallest among the three samples; the wear mechanism was abrasive wear and adhesive wear slightly. |
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