Study On Fabrication And Performance Of Ni-Co-P/Si₃N₄ Electroplated Composite Coating On Al Alloys

Along with the application of Aluminium alloy, its weakness, such as low hardness, poor wearing resistance and abrasion, is getting more and more obvious, so superficial treatment is needed.In this paper, the electro-deposition of Ni-Co-P/Si3N4 coating on a hyper-eutectic Al-Si casting alloy was studied. The weight, thickness, hardness, surface morphology, composition and wearing resistance of Ni-Co-P/Si3N4 coating have been measured by analytical balance, optical microscope, microhardness, SEM, EDS and abrasion testing machine. By an orthogonal experiment, the effect of process parameters ,such as electric current density, temperature, PH, plating time and component of electroplating bath on the performance of Ni-Co-P/Si3N4 electroplated coating, such as thickness, hardness , contents of Si3N4 in the coating and deposition speed,have been obtained. And the optimal technological parameters of composite plating were suggested. The pretreatment process of Aluminium alloy and the pretreatment of Si3N4 were studied. And the wearing resistance and adhesive properties of Ni-Co-P/Si3N4 coating were observed. By means of chemical titration, the ingredient of plating bath was analyzed. The main results are obtained as follows:The suitable electric current density range of Ni-Co-P coating is from 0.74A/dm2 to 3A/dm2, by experiment of Hall cell. The density and the adhesive force between the coating and the substrate was improved significantly by using proper technique of twice zincification. It indicates that the layer of Ni plated on the substrate by method of plating can protect the layer of Zinc, and improve the adhesive force between the coating and the substrate . The problem of peeling and burning was solved. Si3N4 are observed to exst in the composite deposite by SEM. And the surface morphology of Ni-Co-P/Si3N4 coating is cystiform. Ni3P alloy, which separate out after heat treatment, can strengthen the coating and improve the hardness and wearing resistance of the coating. It was found that the structure of coating was crystalline state after heat treatment(400℃,1h) by XRD.By an orthogonal experiment, the effect of process parameters on the performance of Ni-Co-P /Si3N4 electroplated layer have been obtained. It indicates that thickness of Ni-Co-P/Si3N4 electroplated layer increase with the increase of plating time, electric current density, temperature and PH, or decreasing contents of P in the electroplating bath. Deposition rate increase with the increase of electric current density, temperature and PH. With the increase of plating time, temperature ,PH and contents of Ni in the electroplating bath, the hardness increases. Decrasing electric current density and contents of Si3N4 in the electroplating bath, or incrasing temperature, contents of Ni in the coating increase. Contents of Co in the coating could be increased by increasing electric current density, contents of Co and Si3N4 in the electroplating bath,or decrasing temperature and contents of P in the electroplating bath.Along with the increase of temperature,or the decrease of plating time ,contents of Co and Ni in the electroplating bath and PH, contents of P in the coating could be increased. Contents of Si3N4 in the coating could be increased by increasing contents of Si3N4 in the electroplating bath,or decrasing PH.The wearing resistance of the coating is improved with the increase of both the contents of Si3N4 in the coating and hardness of the coating.After the study of film cohesive performance, it is obvious that the adhension of the coating is poor by increasing Contents of P and Co in the coating.At the same time, the coating is more hard ,friable, and easy to be broken.With the inctease of temperature, or the decrease of PH,the concentration of chloridion and Contents of Si3N4 in the coating.