Study On Surface Properties And Bonding Behaviors Of Electroless Coatings Deposited On Micro-textured Substrate Surfaces Fabricated By Ultrasonic Vibration Turning

Electroless Ni-P coatings have drawn increasing attention for their advantages of low environmental pollution,stable performance,simple operation and uniform coating preparation.However,due to the different surface pretreatment methods,thermal expansion coefficient and wetting properties,between coating and substrate,Ni-P coating still have the problem of insufficient bonding strength and easy peeling off.Appropriate pretreatment of substrate surface,such as micro-textured the surface of the substrate,can change the bonding state of the coating-substrate interface and effectively increase the adhesion of the coating.The surface of 304 stainless steel was micro-textured by Radial Ultrasonic Vibration assisted Turning(RUVT)method,and the electroless coating was further deposited on the surface of micro-textured substrate.The microstructure,surface properties,and mechanical properties of the textured coating were systematically studied.The bonding strength mechanism of the coating was analyzed and discussed,and relevant research conclusions haven been drawn.The main research contents and conclusions include:(1)In order to improve the bonding performance between the electroless Ni-P coating and the substrate,the RUVT method was used to perform the micro-texturing pretreatment on the substrate surface.The surface morphology of micro-texture machined by RUVT was simulated based on the analysis of tool geometric kinematics,and the influence of the main processing parameters of RUVT on the micro-texture morphology was investigated.Five typical micro-textured substrate samples were machined by using "D" type carbide turning tool and selecting reasonable process parameters on the basis of computer simulation.The surface of the conventional turning substrate was used as the contrast sample,and the surface morphologies of the five micro-textured substrates were analyzed.After the radial ultrasonic vibration being applied,the motion trajectory of the turning tool have a significant change,forming a micro-pit textured surface that is completely different from the ordinary turning surface.With the decrease of the spindle speed,the pits become denser and the roughness(Sa)was significantly improved;with the increase of feed rate,the pit width becomes wider and the roughness(Sa)also increases;after applying ultrasonic vibration,substrate surface exhibited compressive residual stress,and the compressive residual stress increases with the decrease of spindle speed and increase of feed rate.(2)The Ni-P coating was deposited on the surface of the textured substrate by means of electroless plating.The material properties and surface states of Ni-P coatings,including surface morphology,thickness,hardness,bonding strength,specific surface area,wettability and surface energy,were systematically analyzed.The results showed that compared with the conventional turning substrate coating,the thickness and hardness of the five kinds of micro-texture substrate coating processed by RUVT are increased to varying degrees.The thickness of the coating increases by 166.80 % and the hardness increases by 72.42 %.At the same time,the bonding properties of five kinds of micro texture substrate coatings processed by RUVT also were enhanced with different degree.Compared with conventional turning substrate coatings,the micro-textured substrate coatings processed by RUVT have better wetting performance and surface energy,and the physical and chemical match of the coating to the substrate also become better.The interfacial bonding characteristics between the coating and the substrate were verified by nano-scratch test,three-point bending test and thermal shock test,respectively.And the denser the micro-texture,the wider size,and the higher roughness of pits lead to the more significant improvement of the coating adhesion.(3)According to the bonding mechanism of coating-substrate interface and coating adhesion,the improvement mechanism of micro-textured surface on coating bonding performance was analyzed from the perspective of physical bonding and chemical bonding.In terms of physical bonding,the micro-pits texture increases the surface roughness and specific surface area of the substrate,which helps to increase the thickness and hardness of the coating,so it can enhance the mechanical anchoring effect of the substrate-coating interface;while in terms of chemical bonding,the hydrophilicity of the substrate surface of the pit-type micro-texture was increases.At the same time,the improvement of the surface wettability is beneficial to increase the surface energy,so that the adsorption capacity of the surface atoms of the substrate to the metal ions in the plating solution increases,which is conducive to the deposition of the electroless plating solution on the surface.