| In the formation process of zinc coating by mechanical plating, zinc powder remains solid state without obvious crystallization. It is difficult to explain zinc coating's formation either by traditional crystallization theory, or by foreign cold welding theory fundamentally. Especially in recent years, with the implement of ROHS order in European electronics industry, mechanical plating has undergone rapid development in surface technology field. However, the imperfect mechanism of coating formation has greatly restricted the growth and application of mechanical plating. Based on the technology of mechanical plating with little tin salt, which is widely used in China, the subject prepared samples of zinc coating, zinc-aluminum coating, zinc-RE coating, zinc-nickel coating by mechanical plating, did research on the bath environment, coating's structure, bonding interface and coating's physical and chemical properties. The thesis analyzed the adsorption and deposition of zinc powder in the formation process through testing methods like ICP-AES, XRD, and stereomicroscope, and studied the microstructure of zinc coating and the interface between the coating and steel substrate by OM, SEM, EDS, XRD and XPS. The physical and chemical properties of zinc coating were tested and analyzed by ferroxyl test, density calculation, tensile test, neutral salt spraying test(NSS), and electrochemical polarization. The results show that:In zinc coating's formation process, there is no electrocrystallization or liquid metal's crystallization under high temperature. Zinc coating is a multiphase composite, composed by zinc particles, tin, M metal(such as Mn, Fe, Cd) and interstices. Mechanical interlocking is the main adhesion mechanism of zinc coating, whose adhesion strength is 2-3 MPa. The reactions Zn+Sn2+→Zn2++Sn↓and Zn+Fe2+→Zn2++Fe↓are two main ones in zinc coating's formation. The former happens in seed coating building process, and the latter coating thickness increasing process. The energy consumption of zinc coating's formation in mechanical plating accounts for 1/10～1/20 of that in electroplating and 1/2～1/5 in hot-dip galvanizing. The densification process of zinc coating by mechanical plating mainly includes displacement and deformation of zinc particles and interstices. Displacement, rotation and deformation take place in densification process, which leads to the displacement of interstices, compact of volume, filling in interstices by small-sized zinc particles and the improvement of zinc coating's density. Without solid solutions or compounds, zinc coating's formation is a noncrystallization process under the existence of replacement deposition.Sn2+and Fe2+respectively play the role of "driving metal" in seed coating building process and coating thickness increasing process. The mechanisms of their actions reflect in the two displacement reactions Zn+Sn2+→Zn2++Sn↓and Zn+Fe2+→Zn2++Fe↓After the formation of zinc coating, the "driving metal" remains in interstices, or on the contact interface of zinc particles.In mechanical plating, the structures of zinc-based composite coatings are similar with that of zinc coating. There are no formation of new phases like solid solution and compounds caused by crystallization. But the alloy phase Ni3Sn4 exists in Zn-Ni composite coating. Zinc particles array in zinc coating, which is composed by lamellar zinc powder, interstices and inclusions. There is neither obvious plastic deformation of zinc particles found in coating formation process, nor new phases like solid solution or compounds.Sn and iron have on effects on the adhesion of zinc particles. The adhesion mechanism of zinc coating/steel substrates, and zinc particles cannot be influenced by the additions of non-zinc powder or RE. Moreover, neither the heating temperature 100℃-300℃nor the strengthening time 0 min—60 min can change the adhesion mechanism. There are no alloying phenomena like diffusion or metallurgy reaction in zinc coating and on the interface between zinc coating and the substrate caused by the extension of strengthening time. The tensile failure happens on the interface between zinc coating and the substrate. With the improvement of zinc coating's thickness, the adhesion strength reduces gradually. When the coating's thickness increases from 20μm to 60μm, the adhesion strength reduces from 2.68 MPa to 2.31 MPa.The density of zinc coating by mechanical plating is about 5.67 g/cm3. Its thickness has no obvious effect on the density. The decrease of zinc particle's diameter can cause the improvement of the coating's density. With the diameters of 800 mesh,1000 mesh and 1200 mesh, zinc coating's relative densities are 34.73%,60.08% and 77.85%. The extension of strengthening time improves the coating's density. When the strengthening time extends from 0 min to 60 min, the coating's relative density improves from 76% to 94.5%. No connected interspaces exist in coatings which are prepared with spherical and lamellar zinc powder. The additions of non-zinc powder or other metal salt to prepare zinc-based composite coatings(such as Zn-Al,Zn-Ni and Zn-RE) have no influence on the coating's porosity.Acting as the sacrificial anode that protects steel substrate, zinc coating by mechanical plating shows active dissolution property in NaCl solution with the concentration of 35 g/L. It can endure 600 hours in neutral spray corrosion test. The strengthening time(0 min—60 min) and heating temperature(100℃-300℃) have no effects on the coating's corrosion resistance performance. The electrochemical corrosion resistance of zinc coating prepared with lamellar zinc powder is slightly superior to that with spherical zinc powder. Compared with the latter, the former's free corrosion potential moves 2.9 mV to positive direction, and its corrosion current density is less than 1/3 of the latter. The additions of Al,Ni and RE can respectively improve the coating's salt spray corrosion resistance by making its free corrosion potential move to positive direction, increase the polarization resistance and reduce the corrosion current density.
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