| Electroless Composite Plating is a new surface treatment technology on the basis of the ordinary electroless plating,which is carried out by adding different inert particles into electroless plating solutions, leading to the co-deposition of particles with the matrix alloy to obtain composite coatings with different functions. As an ideal high-hard and wear-resistant composite coating, the diamond composite coating has a wide range of potention applications in reducing equipment wear, prolonging service life and saving resources and energy.In this paper, the study was carried out on the preparation technique and related mechanism of high-performance diamond composite coatings on different substrate surface. SEM, TEM, Micro-3D Profiler (ADE), metallographic microscope, microhardness tester, wear tester, laser particle size analyzer, and so on, were employed to analyse the surface morphology, microstructure, micro-hardness and wear resistance of the composite coating and the dispersion characteristics of diamond particles in the bath and its distribution in the coating. The main work and results are as follows:1. The dispersion behavior of diamond particles in electroless nickel solution was studied. The effect of electroless solution content, temperature, ultrasonic treatment, surfactant type and its concentration, stirring speed and bath structure on the size distribution of diamond particles in aqueous water systems was studied. Results show that the dispersion of diamond particles in DI water is obviously superior to that in electroless nickel solution. Micron diamond particles agglomerate evidently when the concentration of electroless solution (Velect:VDI) in dispersion media exceeds 5‰. And the agglomerates are more and more seriously with the increase of the concentration of electroless solution. Applying ultrasonic treatment, adding surfactant, increasing stirring speed and adding baffles in the tank are helpful to improve the dispersion of diamond particles in the electroless nickel solution.2. The preparation technique of diamond composite coating was optimized. Based on the optimized bath structure and related depositing conditions, the effect of stirring speed and bath structure on the surface morphology of the composite coating, deposition rate, diamond content in coatings, micro-hardness, wear resistance were analysed in detail. Through Fluent simulation and testing, the characteristics of two-phase flow field in the bath, before and after optimization, were also investigated comparatively. The experimental and simulation results show that adding baffles in the bath and increasing stirring speed can improve the flow field characteristics of the bath, which is conducive to the full development of turbulence, increasing turbulence intensity and reynold stress, improving the uniformity of diamond particles distribution in the electroless nickel solution. It's also beneficial to the performance of diamond composite coatings improved greatly.3. The performance of different types of diamond composite coatings and their wear mechanism were researched. It focused on the effect of micro-diamond particle size on coating microstructure, coating of diamond content, micro-hardness, wear resistance, and compared with the nano-diamond composite coating and Ni-P coating. It also studied the deposition mechanism and wear mechanism of the micro-diamond composite coating. The results show that the content of diamond particles in coating and its wear resistance increased significantly with the micro-diamond particle size enlarged. Compared with Ni-P coating, for the difference of diamond form and its content in coatings, the hardness of micro-diamond composite coatings are obviously diverse at different temperatures. In the process of wearing, micro-diamond coating is mainly abrasive wear and adhesive wear, which are both related to the deposition of diamond particles in the coating and the self of particle characteristics.4. Technological processes of preparation high-performance diamond composite coating on the surface of aluminum and its alloys were developed. Based on the direct-plated design thinking, the pretreatment processes of aluminum substrate were researched and obtained nano-diamond composite coatings on aluminum surface. Both the effect of heat treatment on coating adhesion strength, micro-hardness, microstructure, diffusion of elements and the effect of nano-diamond content in electroless bath on the coating deposition rate, coating of diamond content were also studied. The results show that with the heat treatment temperature increased, for the mutual diffusion between coating elements and matrix elements, the adhesion strength of nano-diamond composite coating and aluminum substrate increase significantly. At the same time, the organizational form of composite coating is obvious change. Under the same mixing method and mixing intensity, with the increase of the nano-diamond concentration in the bath, the coating deposition rate decreases, while the diamond content in the coating first increases, reaches maximum, then decreases. When the concentration of nano-diamond in bath is 5g / L, the diamond content in the coating is largest.
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