Ni-Co-P/Si_3N_4 electroless composite plating and its hardness

Ni-Co-P/Si3N4 composite coating on Al alloy is prepared by electroless composite plating in order to improve the minohardness of Al alloy.The influence of electroless plating process parameters on the micro-hardness of Ni-Co-P/Si3N4 coating on a hyper-eutectic Al-Si casting alloy is studied by an orthogonal experiment.The effects of process parameters,such as pH and mole ratio of Co 2+/(Co 2+ +Ni2+),Si3N4 concentration in electroless plating bath and plating temperature,on the micro-hardness of composite layer are obtained.It is indicated that the hardness of coatings can be increased by increasing pH,plating temperature,Si3N4 concentration and keeping mole ratio of Co2+/(Co 2+ +Ni2+)moderate.Moreover,the influence of heat treatment on the micro-hardness of coatings is also investigated.It is found that the hardness of films increases with the heat treatment temperature rising below 400 ℃ but decreases when temperature is over 400 ℃.