Study On The Preparation And Properties Of Vacuum Fusion Sintered WC/Ni And WC/Co Composite Coatings

The development status of modem surface engineering technology and the research results of vacuum fusion sintering (VFS) technology were outlined in this paper. The advantages and disadvantages of VFS technology, the similarity and difference of VFS and vacuum sintering were analyzed and compared in detail. Both WC/Co composite coatings and WC/Ni composite coatings were made on 45 steel by VFS technology. The experimental and theoretic study on VFS technology were made in detail.Based on earlier liquid phase sintering theory and research results of VFS technology, the process and mechanism of VFS was summarized. The VFS process was firstly divided into four stages, which were formation of fluid phase and rearrangement of particles, dissolving and deposition of solid, forming of solid skeleton, diffusion of fluid alloy and solid substrate. The mechanism of VFS technology was to study the various possible migration way and speed of materials, which was all kinds of diffusion mechanisms.The molecular dynamics was firstly present in studying the VFS process and the atom's diffusion. The three-dimensional MD model was built, by which the atom's diffusion and migration contrail was simulated. The VFS process and the microcosmic bondage mechanism of the composite coating and substrate were analyzed. The formula of computing atom's diffusion space was also built. The SEM photograph of the VFS WC/Co and WC/Ni composite coatings and the spectrum analysis of electron probe showed that there were a large number of atoms diffusing nearby the interface, and that the results of MD simulation were right.As far as the steel substrate is concerned, VFS process is similar to a high temperature anneal treatment, and it's hardness is between HB160 and HB230. The heat treatment of VFS composite coatings was firstly present in improving the mechanical properties. The coatings were normalized and modified according to the 45 steel heat treatment technology. The constitution structure and composition of the composite coatings after different heat treatments were observed and analyzed by SEM and EDX, and the effect of heat treatment on the constitution structure was studied. Some phase in the coating were analyzed by X-ray diffraction meter. The effects of heat treatment on the hardness and microhardness of the VFS coatings were studied. The relation between the VFS temperature and the micro-hardness was analyzed, with the optimal VFS temperature of the WC/Co and WC/Ni composite coatings obtained.A new method of combining electron probe microanalysis and least square was present, by which the diffusion coefficient of every component was calculated which are greater than 10'6mm2/s. The effects of interfacial diffusion on the microstructure, micro-hardness and interfacial bonding strength of the composite coating materials were analyzed. The experimental results showed that interfacial diffuse which caused metallurgical bondage happened between the composite coating and steel substrate.The relationships of the wear resistance with the self-fluxing alloy, the WC percent, the applied load, the heat treatment and the VFS temperature were analyzed. The wear rate was inversely proportional to the hardness of the coatings, but it was nonlinear with the applied load. This nonlinear relation between the load and the wear rate can't be explained by Holm's wear theory. Based on Holm's wear theory, a new empirical wear formula was built in this thesis, in which the wear status observed in experiments can be explained well.The corrosion experiments of the composite coatings of different WC percent were made in acid solution, alkaline solution and brine. Corrosion initiated from the interface, and belonged to the intercrystalline corrosion.The effect of heat treatments on the fatigue strength of composite coatings was studied. The fatigue strength of the VFS coating without heat treatment was very low and it was improved greatly after heat treatment. The fatigue strength of the modified coatings was larger slightly than that of the normalized coating. The homogenization treatment of the VFS coating was recommended in application.