Preparation And Tribological Properties Of Cr/Si₃N₄ Composite Coatings By Electrophoretic-Electrochemical Deposition

Chromium-based composite coatings can be obtained by using chromium metal and ceramic particles in combination to obtain their respective performance advantages for better mechanical properties and friction and wear resistance.In the current research on the preparation of chromium-based composite coatings,a certain concentration of ceramic particles is added to the plating solution for co-deposition with metallic chromium.However,the content of ceramic particles in the prepared composite coatings is very low,which is not sufficient to give full play to the performance advantages of ceramic particles,thus significantly limiting the performance of the composite coatings.The use of an electrophoresis-electrodeposition composite process can increase the ceramic particle content in the composite coating and thus improve the friction and wear resistance of the coating,which is of great scientific significance and engineering reference for further improving the performance of chromium-based coatings at present.In order to obtain Cr-based composite coatings with higher ceramic particle content and better friction wear properties,this paper selects Si₃N₄particles with higher hardness and certain lubricity,and tries to apply the combination of electrophoretic deposition and electrodeposition in the preparation of Cr-based composite coatings,and through the study and optimization of process parameters,focuses on the surface and cross-sectional microscopic morphology,formation mechanism,mechanical properties and friction wear properties of Cr/Si₃N₄composite coatings to carry out systematic research and analysis,the main conclusions are as follows.（1）A stably dispersed suspension of Si₃N₄particles was obtained by the combined action of stirring and sonication using ethanol and triethanolamine as the dispersion system,and a Si₃N₄electrophoretic coating was successfully prepared by this suspension.The deposition quality of the coating and its morphology were analysed,and the optimized electrophoretic process parameters were obtained:the concentration of Mg Cl₂-6H₂O in the electrophoretic solution was 0.5g/L,the concentration of Si₃N₄particles was 8g/L,the electrophoretic deposition voltage was 80V,and the electrophoretic time was 120s～180s.（2）Different Cr/Si₃N₄composite layers were obtained by changing the composition of the plating solution(Cr O₃concentration,Cr³⁺concentration,sodium dodecyl sulphate concentration),and the microscopic morphological observation of the intact layers revealed that:the thickness of the composite layer was only 1/3 of that of the pure Cr layer,there were no cracks on the surface,the Cr grains were unevenly distributed in size,and there were diffusely distributed Cr with particle size of about 20-40μm The surface is free of cracks,with a uniform distribution of Cr grain size and a diffuse distribution of Cr grain size of about 20-40μm.The formation mechanism of the electrophoretic-electrodeposited Cr/Si₃N₄composite layer was explained in conjunction with relevant theories.Under the electrodeposition process parameters of p H 1～2,current density of 50A/dm²,temperature of 50°C and time of 1h,Cr/Si₃N₄composite plating layers with different amounts of Cr-coated Si₃N₄particle agglomerates were obtained,and the optimized composition of the plating solution for the preparation of Cr/Si₃N₄composite plating layers by the two-step method was obtained as follows:Cr O₃concentration of 190g/L,Cr³⁺concentration of 5g/L,and 0.3g/L of sodium dodecyl sulfate.（3）The performance of Cr/Si₃N₄composite plating was found that the composite plating with different levels of Cr covered Si₃N₄particle agglomerates bonded well with the substrate and its microhardness increased with the number of Cr covered Si₃N₄particle agglomerates.Under dry friction conditions,the composite coating with a Si₃N₄particle concentration of 6g/L showed the best frictional wear performance with a minimum friction coefficient of 0.36 and a wear rate close to 0.