Study On Preparation And Performance Of Cr-diamond Composite Coating

Chrome coating is one of three kinds of coating in contemporary electroplating industry, the composite coating with chrome as matrix metal has good wear-resistant performance, low cylinder liner wear-resistant performance, high heat load carrying capacity and strong ability to resist burning loss, etc., it is applied in the high power and low emissions modern engine very well, such as chrome base ceramic composite coating and chrome base diamond composite coating have become a high-end cutting-edge technology for surface treatment of piston ring. Trivalent chromium composite electroplating, namely is the technology to obtain composite coating by electrodeposition method adding suitable surface active agent and solid particles to trivalent chromium coating solution, the commonly used solid particles have SiC, Al2O3, WC, ZrO2, diamond, and so on. Diamond has high hardness, it is a kind of superhard materials, can be used to obtain the superhard materials composite coating.This paper obtained Cr-diamond composite coating using trivalent chromium coating solution with ultrafine diamond powder as the disperse particles based on the previous work of our research group. The experimental contents and results are as follows:(1) Modified the ultrafine diamond powder adopting the method of heating oxidation treatment using mixed acid. Confirmed sodium carboxymethyl cellulose as the optimum dispersant by settlement test. Investigated the effect of dispersant on steady dispersion of diamond in composite coating solution through settlement test, tested the surface morphology of Cr-diamond composite coating using scanning electron microscopy (SEM), results showed that the addition of dispersant in composite coating solution can contribute to the spread of diamond and the preparation of composite coating with uniform distribution diamond.(2) Investigated the effect of the components of the coating solution on Cr-diamond composite electrodeposition through the single factor experiment. Determined the best formula composite coating solution by orthogonal optimized experiment on this basis. Studied the effect of process conditions on Cr-diamond composite electrodeposition through the single factor experiment, determined the range of suitable technological conditions for electrodepositing Cr-diamond composite coating. Conducted repeated experiment under suitable technological conditions using the best coating solution formula, Cr-diamond composite coating is has smooth shiny appearance. Observed coating surface morphology with SEM, we can see diamond evenly distributed in the coating, we can know the composite coating contains a lot of elements C through the EDS testing, the results showed that diamond and Cr element realized codeposition.(3) Observed the coating morphology by SEM, we can find that the diamond quantity in the composite coating increase with the concentration of diamond in the coating solution. Analysed composition of the Cr-diamond composite coating by EDS, the results show that C element content in the composite coating was increased with diamond powder content in the coating solution, this verified SEM test results. Detected the organizational structure of Cr-diamond composite coating using XRD, the results show that the Cr-diamond composite coating have diffraction peak of diamond and is amorphous structure. Investigated the coating binding force using bending test and heating (quenching) test method, the results show that the binding force of Cr-diamond composite coating is weaker than that of pure chromium coating. By hardness tests, we can know that the hardness of Cr-diamond composite coating is much higher than that of pure Cr coating. Analysis the Tafel curve of coating in different corrosion medium, the results show that the corrosion resistance of Cr-diamond composite coating is superior to that of pure Cr coating in neutral and acid medium, in the alkaline medium, its corrosion resistance is inferior to that of pure Cr coating.(4) Discussed the mechanism of the effect of components of the coating solution on Cr-diamond composite electrodeposition using cathodic polarization curve method. The cathodic polarization curves showed that auxiliary complexing agent makes the work scope of current density amplify; wetting agent can make cathodic polarization decreased, inhibit hydrogen evolution reaction, improve the current efficiency, promote trivalent chromium electroplating; ammonium chloride and dispersing agent can increase the cathodic polarization of compound coating solution, promote to obtain more brightness and meticulous coating; diamond particles have played an role of seed crystal, promote the formation of chromium crystal, make the cathode electrodeposition reactions are more likely to happen.