Study On Preparation And Properties Of Electroless Ni-P/Ni-W-P Duplex Coating On Aluminium Alloy

With the widespread application of the aluminum alloy,its shortcomings such as lowhardness,poor corrosion resistance become more clear,therefore superficial treatment isdistinctly important before using.As a new superficial treatment process,electroless plating onaluminum is a good surface modification technique to improve the performance.Ni-P/Ni-W-Palloy coating with high hardness and strong corrosion resistance has been attracted.In the article,two types of duplex coating were obtained through altering depositioncondition.The morphology,microstructure and components of the Ni-P、Ni-W-P、Ni-P/Ni-W-Pcoatings were characterized by SEM、XRD and EPMA.Microhardness tester was adopted tocharacterize mechanical properties,and the corrosion resistance of different coatings isresearched by the method of porosity and anodic polarization curve for the electrochemicalproperties.This thesis consists of three sections:1) the optimal formula and properties of Ni-Pand Ni-W-P coatings;2) the study on properties after electroless Ni-P/Ni-W-P duplex coatingwas prepared;3) the influence of heat treatment on Ni-P/Ni-W-P duplex coating.In order to get good behavior of Ni-P/Ni-W-P duplex coating in hardness and corrosionresistance,the approach of orthogonal experiment adopted the formula of L16(45) is used toprepare Ni-P and Ni-W-P coatings with various content and element on Aluminum alloy.Theoptimal formulas of Ni-P and Ni-W-P are firmed up through range analysis after investigatingthe effects of deposition condition (such as the various concentration of the bath componentsand technology conditions) on the coating character using material content and hardnessrespectively as an index.Then selected optimal process of Ni-P with different P content and high hardnessNi-W-P and got single coating with a compact and uniform composition character,and finallygot two types of Ni-P/Ni-W-P duplex coating called MP/HHV coating and HP/HHVcoating.Specimens were heat treated at temperatures ranging from350to550℃for1hourwithout gas protection and then cooled in room temperature.The effect of heat treatment onthe characteristic of surface morphology、organization structure、hardness and corrosionresistance of Ni-P/Ni-W-P duplex coating are studied by contrast with coating as-plated.All the coatings as-deposited have excellent adhesion to aluminum alloys.The surface morphology of Ni-P/Ni-W-P duplex coating is different with that of single Ni-W-P,but themicrostructure are the same: The HP coating is amorphous structure,as MP and HHV coatingsare mixed crystal. Compared with the Aluminum alloy, the hardness value of the as-depositedand heat treated Ni-P/Ni-W-P coatings were higher,and hardness of Ni-W-P coating is higherthan that of the corresponding Ni-P coating.The MP/HHV coating is also better thanHP/HHV coating.And the trend kept the same after heat treatment.The duplex coatingcontributed to decrease porosity than single Ni-P or Ni-W-P. After been heat treated, theporosity of both MP/HHV and HP/HHV coatings are increased firstly then decreased.Thecorrosion resistance of the coating decreased firstly and increased.