A Study On Electroless Composite Plating Ni-P-SiC Coating On Magnesium Alloy (AZ91D) Substrate

As the lightest structural material and recycled easily, magnesium and its alloys are increasingly being utilized in many fields such as aerospace、computer industries. However, their applications are limited severely for their low friction/wear properies and corrosion -resistance. The effect of SiC concentration on the properties that included corrosion-resistance and wear-resistance of the Ni-P-SiC coatings which were made by the electroless composite plating had been researched in this paper. The obtained conclusions are summarized as follows:The nano particles accelerated the depositing rate,when the SiC concentration was 4g/L,the rate achieved the biggest. The surface of the coatings were impact. After heat treatment, the cellular crystals of coatings increases, also with the grain boundary increasing, the surface was not impact as that before heat treatment. The coating completely crystallized after 400℃heat-treatment, with the precipitation of the Ni₃P phase.The micro-hardness of the composite coating was significantly higher than that of Ni-P coating and after heat-treatment, its hardness achieved 2100HV.when the SiC concentration was 4g/L,the micro-hardness of the composite coating was 2129HV.The addition of SiC particles significantly reduced the porosity of the coating .It was found that the corrosion resistance of composite coatings was superior to the general coatings by salt spray test,after heat-treatment, the resistant time was significantly reduced .The corrosion potential （respectively,-0.59V and-0.58V） of Ni-P and Ni-P-SiC coating was significantly higher than the magnesium alloy, the corrosion potential （-1.42V）, the corrosion resistance increase obviously.After heat treatment the friction coefficient of composite coating increased significantly. The friction coefficient of composite coating was generally greater than the pure Ni-P coating, and with the SiC content increased,the friction coefficient first increased and then decreased. The abrasion of coating after 400℃heat-treatment was significantly less than that before heat-treatment,and when the SiC concentration was 6g/L, the abrasion was 0.2mg.The abrasion of pure Ni-P coating was more than that of the composite coating,indicating the wear resistance was less than the composite coating layer.The wear scar profile of coating was relatively stable, with little or sudden surges of low points, or surfaces. The structure of worn surface of Ni-P coating wasn’t impact,but the structureof worn surface of composite coating was impact. The presence of nano SiC reduced the shedding phenomenon, which also confirmed the wear resistance of composite coating was better than the single coating.The initial abrasion wear mechanism of Ni-P coating is mainly slight micro-cutting abrasive wear, post-performance was low-temperature adhesive wear. After heat treatment,the abrasion wear mechanism transfered to the grinding material adhesive wear , and there also was micro-machining abrasion. On the low load short-time conditions, the Ni-P-SiC coating surface was with slight abrasive wear, as the load increasing , longer wear time, there were adhesive wear and abrasive wear, both abrasiveon the soft metal matrix particles, also with SiC particles, when the coating after heat-treatment, there was still N13P in abrasive particles.The results of this study indicated that the electroless composite plating on the surface of magnesium alloy could get the ideal coating, the process was green. The obtained composite coatings had good corrosion resistance and wear resistance. For the composite coating in lubricated conditions, the wear mechanism needed further study. There was great significance for application of the Magnesium alloy into the scale of production .There are few literature relating to the present studies, the conclusions obtained in presentresearches provide a scientific basis for preparing high quality films and thick coating on Magnesium alloy substrate.