Preparation Of Electrodeposited And Electroless Ni-P Coatings On The Surface Of High Volume Fraction Si Cp/Al Composite

Nickl-phosphorous(Ni-P) alloys have been widely used in petrochemical industry, microelectronic, aerospace and auto filed, owing to their high hardness, good wear and corrosion resistance, good weldability and so on. The Si Cp/Al composites reinforced by high volume fraction of Si C particles have a poor weldability, because much of its surface is took up by Si C particles.Generally, the gold plating is applied to improve the weldability when the composites be used as baseplate of IGBT(Insulated Gate Bipolar Translator). The presence of interface, gas pores and other defects result in poor corrosion resistance of composites. In order to obtain lower coast Ni-P coatings with good solderability and corrosion resistance on the surface of Si Cp/Al composites reinforced by 60 volume fraction Si C particles, the process parameters for preparing Ni-P coatings were systematically researched.The optimum parameters for electro-depositing Ni-P coatings were obtained by orthogonal tests. The concentration of Ni SO4·6H2O is 135g/L,Ni Cl2·6H2O is 30g/L, Na H2PO2·H2O is 8g/L, H3BO3 is 20g/L, and p H is2.5,the current density is 3A/dm2, plating temperature is 75 ℃. The P content of Ni-P coatings prepared by optimal parameters is 9.38wt%,micro-hardness is 625.3HV and have a combination of crystalline and amorphous structure. It was found that the deposition rate of Ni-P alloys on the surface of Al was much higher than on the surface of Si C which resulting in Ni-P plating with an uneven micro-morphology. And the factors influencing the deposition rate, content of P and surface quality were studied in detail.The processes for activation and nickel pre-plating were fixed in order to prepare Ni-P coatings with a flat surface. Results show that deposited Ni-P alloys with smooth and uniform microcosmic surface could be obtained after pretreatment. Pretreatment can also improve the adhesive strength and corrosion resistance of Ni-P electro-deposites.The results show that the crystal structure and performance of Ni-P alloys are influenced mainly by the amount of P present in the alloy. The appearance of Ni-P alloys became more uniform and brighter, and structure of Ni-P alloys evolves from a crystalline to an amorphous one with increasing P content. And corrosion resistance be improved whileweld-ability get worse with increasing P content. Micro-hardness reaches maximum value when the content of P is 11.4wt%. Ni-P coatings that P content is 9.38wt% began to evolve from an amorphous to a crystalline one when heat treatment temperature at 350 ℃, with a small amount of Ni3 P precipitated out. Ni-P plating became a completely crystalline one after heat treatment at 500℃ for an hour. There are no meta-stable phases Ni12P5 and Ni5P2 precipitated out. Micro-hardness of Ni-P plating reaches maximum value while solderability and corrosion resistance decreased after heat treatment at 400 ℃ for one hour. It is concluded from test results that depositing Ni-P alloys with P content is 9.38wt% significantly improve the solderability, and also improve the corrosion resistance and strength of Si Cp/Al composites. And that has little impact on the heat conductivity of composites. As a consequence, depositing Ni-P plating on the surface of Si Cp/Al composites could satisfy the basic requirements of IGBT base-plate.