Research Of Mechanical Attrition Electroless Comopsite Coating On Zinc Alloy

The zinc alloy is applied in the trial manufacturing of punching, injectionmolding and blow molding and the small batch mold as an economical fast moldmaterial, and has achieved good economic and social benefits. However its intensityand hardness is low, wear-resisting and anticorrosion is poor, which is opposite to thesteel, the application has been limited. Electroless nickel plating can not onlyeffectively improve the surface performance of the material, and widely be used in thesurface modification of various materials, but also an important treatment craft ofmold surface. Electroless nickel plating for zinc alloy die can not only make full useof the zinc alloy advantages such as the perfect forming performance and low-cost,can also effectively improve the corrosion resistance, wear resistance and service lifeof zinc alloy die. The common electroless coating could not meet the need untilsubsequent heat treatment. But for zinc alloy, its heat treatment temperature is low asthe low melting point, the hardness is relatively low because the heat treatmenttemperature is low, and the coating may produce crack during the heat treatmentprocess, then comes the matrix exposing and the corrosion increasing. Gas protectionis required during the heat treatment for maintain the appearance of the coating, makethe coating’s subsequent treatment difficulty and cost more. Mold for some specialpurpose could not be take to have a heat treatment due to the precision requirementsare particularly high, which largely limits the good performance of the Ni-P coating.Obtaining an electroless coating with high hardness and high corrosion resistanceunder electroless condition, we can solve the shortage that the general electrolesscoating requires heat treatment before being use, while also addressing the coatingcorrosion resistance during the heat treatment, and simplify the coating post-treatmentprocesses, save the cost and broaden the use of electroless plating. In this paper,mechanical attrition（MA） media and nano Al₂O₃particles were incorporated into theconventional electroless（CE）-plating solution by electroless deposition to obtainmechanical attrition Ni-P coating,Ni-P-Al₂O₃composite coating and mechanicalattrition Ni-P-Al₂O₃composite coating. The influence of concentration of particles inbath, temperature, pH and other technique parameters on nano particles content incoatings and rate of deposition was investigated. Scanning Electron Microscope（SEM）and X-ray diffraction（XRD） were used to figure the surface and microstructure ofcoatings. The micro-hardness and corrosion resistance of coatings before and afterheat treatment were also studied. The preliminary analysis of crystallization process had been carried out by DSC.CE-plating solution is also applicable to mechanical attritionelectroless（MAE）-plating, process parameters for pH4.5, the temperature of85℃,grinding medium choice diameter of0.8¹.2mm glass ball.MAE-plated Ni-P coatingis very smooth, compact and has metal luster without small hole. The content ofphosphorus is7.62%of the coating. The hardness and corrosion resistance of thenovel coating are considerably improved compared with the CE-plated Ni-P coating.Nanoparticles of nano-composite coatings have the better dispersing performance,dispersed by the physical+chemical method. When the addition amount of thenano-Al₂O₃reachs12g/L, the surface of Ni-P-Al₂O₃composite coating is light andpale, the content of nano-Al₂O₃particle and phosphorus in deposits are7.25%and6.58%respectively. The micro-hardness of composite coating is increased， butcorrosion resistance was decreased. The coating has better corrosion resistance and itshardness can reach820.6HV.1, which combines the benefits of MA and nanocomposite plating technique.Three kinds of coating are amorphously plated and into crystal after heattreatment of350℃. According to the calculation find that the crystallizationactivation energy of the coatings is higher than the CE-plated Ni-P coating, it showsthat MA and nanoparticles can not only significantly enhance the mechanicalproperties of the coating and improve the stability of the coating structure.