Study On Direct-current Electro-deposition Ni-P-α-Al₂O₃Nano-composite Coatings

In this paper，Ni-P-α-Al₂O₃nano-composite coatings were electrodeposited byadd nano-α-Al₂O₃particles to Ni-P plating solution, the impact of certain factors onthe co-electrodepositing process are discussed， and the properties ofnano-composites as well as their applications are described. The suspending stabilityof dispersive system was investigated through sedimentation experiment. Themicro-hardness of the composite coatings was analyzed by HX-1000TM digitalmicro-hardness tester. The corrosion resistance were measured by the immersion testand the anodic polarization curves.Nano-α-Al₂O₃particles are easily aggregative in the production and storedprocedure due to its small size, large surface area and high surface free energy. Afterthe surface treatment of nano-α-Al₂O₃particles, the influences of ultrasonicdispersion time and the concentration （CTAB, LAS, PEG6000） of the surfactant onthe dispersion properties of the nano-α-Al₂O₃particles in aqueous solution weresystematic study. The results show that these anion, cation and non-ion surfactantscan effectively inhibit the aggregative of nano-α-Al₂O₃particles. The optimumdispersion parameters are that the concentration of LAS and ultrasonic dispersiontime are0.8g/L and35min respectively. The lowest sedimentation volume is5.4ml,and the highest micro-hardness of composite coatings up to1207HV.The effects of the changes in the micro-structure of electrodepositedNi-P-α-Al₂O₃nano-composite coatings before and after heat-treatment onmicro-hardness and corrosion resistance were investigated to discover thecorrelation between the microstructure and performance of the coating. The resultsindicated that the as-deposited Ni-P-α-Al₂O₃nano-composite coatings had theamorphous structure, and it starts to crystallize when being held at200℃, producemetastable phases Ni₍120P₅and Ni₅P₂. When the heat treatment at350℃, themetastable phases were transformed to stable phases Ni3P and Ni, and grain growth.Only Ni3P and Ni were formed after heat treatment at400℃. After heat treatment,composite coatings had lager hardness than the amorphous coatings, the coatingswhich were heat treatment at300℃partially nano-crystalline of crystallizedprecipitation dispersed in the amorphous phase, and its had the maximum hardness968HV. When the heat treatment temperature exceeds300℃， the corrosionresistance of composite coatings rapidly reduce. The optimal process of Ni-P-α-Al₂O₃nano-composite electrodeposition wasdetermined by orthogonal experiments in which micro-hardness of compositecoatings was used as evaluation criteria. The optimal process conditions were asfollows: temperature was35℃, pH value was3.5, current density was4A/dm2,agitation rate was320r/min, nano-powder content in plating solution was16g/L,plating time was70min. Micro-hardness value obtained under the optimumconditions can up to1332HV.