| In this thesis, the preparation of the nano-cxystalline WC powers by mechanical alloying and the electrodeposition technology of Ni-P/nano WC coating on hyper-eutectic Al-Si casting ahoy were studied. By XRD~ ThM~ SEM and so on, the morphology, structure, and properties of nano WC power and the Ni-P/ nano WC coating were analyzed. The composite electrodeposition mechanism was discussed tentatively. The prime contents and experimental results included: (1 )The nano-crystalline WC powder with average diameter 4Onm was prepared by mechanical alloying and then applied to electrodeposition of Ni-P/ nano WC coating. Tests show that parameters of ball milling influence markedly on shape and diameter of WC power, Under the conditions of 200rpm rotate speed and 20:1 of ball and material ratio, the WC power with below lOOnm can be obtained by over 24 hours milling. XRD analysis indicates that the structure of WC power remains in crystalline state, but its XIRD summit becomes spacious. (2)The pretreatment of hyper-eutectic Al-Si alloy and the electroplating technology of Ni-P/WC (nm) based on Watt and H3P03 system bath were studied systematically, the optimal electroplating conditions and the influences of technological parameters were acquired. (3)Adopting pretreatment and electrodeposition technologies developed in this paper, the Ni-PIWC (nm) composite coating with amorphous matrix on hyper-eutectic Al-Si - alloy was obtained successfully for the first time. It is found that the electrodeposition rate of coating is about 50jim/h, the content of WC in composite coating is higher up to 13.62%, and the adhesion of coating basic alloy is quite well. After suitable heat treatment, the microhardness of composite coating can reach up to 1456 LIV, and the surface hardness and the wear resistance of Al-Si alloy can be increased markedly. By means of XRD~ ThM~. SEM and optical metallogrophy , it has established Ni-P/WC(nm) composite coating was amorphous structure, nano-crysta]line III WC particles are distributed dispersedly over the matrix area, and the cysliform morphology with petaliod feature is appeared on the surface of coating. (4)Heat treatment tests of Ni-P/WC (nm) composite coating show that there is not precipitation of new phase from matrix during heat lreatment below 30(Pc, and its structure did not change; above 350c, the matrix Ni-P changed form amorphous to crystal, and Ni3P and other new phase are precipitated, microhardness of coating is increased, and its extent was higher as heat treatment temperature. At the same time, the petaliod feature of cystiform morphology is weaker and weaker ,and finally disappeared above 45O~ and the surface of coating becomes smooth. Distribution of WC(nm) in composite coating has not been influenced by heat treatment (5) It was compared with Ni-P~. Ni-P/WC(pm) and Ni-P/WC(nm) in structure and property of coating. It was shown that the matrix of coatings was amorphous Ni-P alloy, and that WC in coating existed by dispersed crystalline particles form; after heat treatment at suitable temperature, hardness of these three coating all was elevated, and the crystalline transformation and precipitation of new phases such as Ni3P ect. Take place. Under the same heat treatment conditions hardness of coating with Ni-P/WC (nm) was the highest~ hardness of coating with Ni-P/WC (pm) took place secondly, moreover hardness of c... |
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