Preparation And Properties Of Ni-P Coating On The Surface Of SiCp/Al Composite

The new SiC particle reinforced aluminum matrix(SiCp/Al)composite has the advantages of low coefficient of thermal expansion,light weight and high thermal conductivity,which is regarded as a promising space lightweight mirror material.However,due to the SiC particles,the internal structure of the material is inhomogeneity,which affects the corrosion resistance,polishing and wear resistance of the material.SLM is a new technology for manufacturing metal parts in recent years.However,SiCp/Al composites formed by SLM still have low hardness,poor wear resistance and corrosion resistance.In this work,Ni-P coating was deposited on the surface of SiCp/Al composite formed by SLM.The formula of Ni-P alloy coating was optimized by single factor method and orthogonal test.Through the methods of scanning SEM,XRD,TMA,Micro Vickers hardness,scratch test,electrochemical workstation and other characterization methods to study the effects of coating process and heat treatment temperature on the morphology,thickness,structure,hardness,bonding strength,thermal expansion coefficient and corrosion resistance of the coating.The main results are following:(1)The effects of pH,temperature and plating time on the surface morphology,deposition rate,phosphorus content and hardness of Ni-P coating were investigated.With the increase of pH and temperature,the electroless rate accelerates and the phosphorus content decreases.With the extension of plating time,the number of Ni-P particles on the surface of the coating gradually increase.When the substrate surface is completely covered,the particles begin to gather,grow and fuse,and finally show obvious cellular morphology.With the extension of plating time,the cellular structure gradually becomes larger and the coating gradually thickens,and the hardness increases firstly and then tends to be stable.(2)It was found that the amorphous high-phosphorus coating with dense and continuous surface can be obtained by using NiSO4·6H2O(25g/L),NaH2PO2(30g/L),C3H6O3(10g/L),C6H8O7(10g/L),NaAc(20g/L),NaF(lg/L),C4H2O3(1g/L)bath,and controlled the temperature at 88 ±1?,the pH value at 4.5± 0.1.Under these conditions,the thickness of the coating can reach 134?m.The bonding strength of the coating is 66.9N and the hardness(685HV)is three times that of SiCp/Al composites.(3)The variation of thermal expansion coefficient of Ni-P coating with heating temperature and heat treatment temperature were revealed.The thermal expansion coefficient of the coating is approximately a constant value in the range of room temperature to 100?.The thermal expansion coefficient of the coating is 14.3×10-6/?,13.5×10-6/?,12.4×10-6/? in the state of plating,200? and 400? heat treatment,respectively.When the temperature exceeds 100?,the thermal expansion coefficient of the coating gradually decreases with the increase of temperature.(4)The effects of heat treatment temperature on the surface morphology,structure,bonding strength,hardness,and corrosion resistance of the coating were clarified.When the temperature increases from 100? to 400?,the cellular substance on the surface of Ni-P coating decreases,the boundary is fuzzy and tends to be flat.With the increase of temperature,the coating becomes from amorphous to crystalline,and the coating completely changes to crystalline at 300?.Both the hardness and bonding strength increase with the increase of heat treatment temperature.At 400?,the bonding strength is 78.9N,and the hardness reaches the maximum value(1000HV).After heat treatment at 200?,the passivation range of the polarization curve of the coating is the widest,the AC impedance is the maximum,and the corrosion resistance is excellent.After heat treatment at 400?,the passivation range is the narrowest,the AC impedance is the minimum,and the corrosion resistance is the worst.