| The SiCp/Al matrix composite metal mirror formed by selective laser melting(SLM)technology has unique advantages,such as light weight,good mechanical properties,structural integration and highly customized design,which has attracted great attention from scholars around the world in recent years.The surface relative density is a key factor to determine the reflectivity of the mirror,but the surface relative density of the Si Cp/Al matrix composite metal mirror fabricated by the current SLM technology cannot meet the requirements of the visible light band.Therefore,in order to improve the reflectivity of the mirror,the surface of the mirror is modified by electroless nickel-phosphorus coating.After the modification,the density,hardness and corrosion resistance of the mirror surface can be significantly improved.In this paper,the Si Cp/Al matrix composite fabricated by SLM technology is used as the matrix,and the surface of the matrix is modified by electroless plating,and the nickel-phosphorus coating is formed on the surface.The surface morphology,structure,hardness,bonding force,corrosion resistance,porosity and thermal expansion coefficient of the coating were characterized,and the influence of pretreatment process,bath composition and heat treatment conditions on the structure and properties of the coating were studied respectively.The main research conclusions are as follows:(1)The surface morphologies and structures of the Si Cp/Al matrix composites with different pretreatment processes during the electroless plating process were obtained.After alkaline unoil and pickling,uniform villous structure was produced on the surface of the substrate,and the mechanical bonding between the coating and the substrate was strengthened.A transition layer with high catalytic activity is formed on the surface of the substrate after the pretreatment process of twice dipping galvanizing,the twice dipping nickel plating and the twice flash plating,which can significantly improve the surface activity of the substrate.(2)The influence of pretreatment process on the surface morphology,deposition rate,adhesion,hardness and corrosion resistance of the coating were revealed,and the optimum pretreatment process for metal mirror was obtained.The twice dip galvanizing pretreatment has the fastest deposition rate,the best adhesion,good corrosion resistance and high hardness among the three pretreatment processes,and the coating surface is smooth and dense without void,which is the best pretreatment process for mirror surface.(3)The effects of bath composition on the thermal expansion coefficient,phase structure,hardness,surface morphology and phosphorus content of the coating were revealed.Among nickel sulfate,sodium hypophosphite and p H,sodium hypophosphite and p H have a greater influence than nickel sulfate on the thermal expansion coefficient of the coating.After annealing at 200?,the thermal expansion coefficient of the coating decreased with the increase of the phosphorus content of the coating.The coating with phosphorus content greater than 8% is an amorphous structure,and the hardness increased with phosphorus content of 9?10% as the node,first decreased and then increased.Good surface relative density can be obtained only when the p H is greater than 4.3,the thermal expansion coefficient can be adjusted in the range of p H 4.3(excluding)?5.2.(4)The effects of heat treatment conditions on the surface morphology,phase structure,hardness and thermal expansion coefficient of the coating were revealed.With the increase of heat treatment temperature,the surface flatness increases first and then decreases,and the surface flatness is the best at 300?.The coating transforms into a crystalline structure at 320°C.With the increase of temperature,the coating begins to transform from amorphous state to crystalline state at 300? and becomes a crystalline state completely at 400?.The hardness of the coating reaches the highest at 400?(HV1045).The thermal expansion coefficient of the coating decreases with the increase of the heat treatment temperature. |
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