The Preparation And Properties Of Electroless Ni-P/SiC Gradient Coating On AZ91D Magnesium Alloy

Magnesium alloys have been widely used in the aerospace, automobile, electronics fields because of its low density, high strength-to-weight ratio and a relatively high stiffness. Among them, the die cast AZ91D alloy is undeniably widespread applied due to its relatively low cost and adequate mechanical properties and processing characteristics. In general, magnesium alloys exhibits low corrosion rates in alkaline, where the pH can increase, due to the formation of a partially protective Mg(OH)2layer, which is stable above pH11.0. However, in neutral sodium chloride aqueous solutions, chloride ions promote rapid attack, thus the insufficient corrosion resistance of this type alloy in neutral chloride environments is still a limitation for their more desirable applications. To enhance the corrosion resistance, the surface engineering techniques including chemical conversion, anodization, electroless plating and PVD are widely applied. Of such these techniques, electroless deposition technique of Ni-P alloy coating is considered as a very appropriate and cost-effective application technology due to outstanding physicochemical and mechanical properties of coatings, such as excellent corrosion resistance, unique wear and abrasion resistance, high hardness and acceptable ductility. Electroless composite plating technology was developed from electroless plating. It refers to co-depositing the solid particles together with the metal coatings to improve certain properties. However, delamination and spallation is easy to happen at the transition between different kinds of materials, such as from a metal to a ceramic layer, which affects the corrosion resistance of AZ91D magnesium alloy in the medium. In order to solve this problem, in recent years, researchers put forward to the concept of functionally graded materials. The concept of functional gradient materials is a composite of metal and ceramic, whose composition and structure change continuously by special means, so as to form a no interface, continuous change of physical parameters of composite materials during the transition from metal lateral to ceramic side. Based on this concept, in this paper, we prepared Ni-P/SiC gradient coating with excellent corrosion resistance on AZ91D magnesium by the electroless plating for the first time. The surface morphology, structure, thermal stability and corrosion resistance of gradient coating were detailed investigated, in order to provide theoretical and experimental basis for further expansion of the magnesium alloy surface treatment technology and applications. This thesis includes the following three parts: (1) Complexing agent is a key factor of maintaining relative stability of nickel ion in plating solution and a prerequisite of obtaining high quality nickel phosphorus alloy coatings. Based on the actuality that traditional electroless Ni-P formula can not meet the requirement of electroless Ni-P process on magnesium alloy, the plating solution formula containing the new ternary complexing agent was studied. The best solution compositions and plating conditions were determined by orthogonal test. Based on this, the effects of ternary ligands concentrations on deposition rate, structure, thermal stability and corrosion resistance of coatings were all-sided investigated.(2) Based on the research of the electroless Ni-P alloy plating solution on magnesium alloy surface, Ni-P-SiC composite plating composition of plating solution and plating conditions were determined. And on this basis, the effects of SiC particles on morphology, structure, phosphorus content and corrosion resistance of coatings were studied.(3) Based on the the research of the electroless Ni-P alloy and Ni-P-SiC composite plating, we prepared Ni-P/SiC gradient coatings and established dynamics equation of electroless Ni-P/SiC gradient coating on magnesium alloy surface. And on this basis, the surface morphology, structure, thermal stability and the structure after heat-treatment of Ni-P/SiC gradient coating on magnesium alloy were studied. The change of corrosion properties of Ni-P, Ni-P-SiC and Ni-P/SiC coatings and the effects of heat treatment of Ni-P/SiC gradient coating on corrosion properties of coating were contrast analyzed.