Preparation And Characteristics Of Plasma Deposition Coatings On Metal Surface

Plasma electrolytic disposition (PED) is a kind of novel and effective surface engineering technique that can be used to prepare metallurgically bonded ceramic coatings on metals, so it possesses broad application prospects and has been given a wide attention by specialists in metal material researching field and enterprisers in metal product manufacturing field all over the world recent years, developing quickly. However, there are still plenty of projects to research no matter in theory and engineering technology. Main research aspects included in this dissertation are 1) Plasma electrolytic oxidation on aluminum surface; 2) Enhancement and brush plating of plasma electrolytic oxidation on aluminum surface; 3) Hybrid plasma electrolytic oxidation on Q235 steel surface; 4) Plasma electrolytic saturation on Q235 steel surface; 5) ESD-PED hybrid surface modification on W₉Mo₅Cr₄V₂ high speed steel.In plasma electrolytic oxidation on aluminum surface, the evolution law and mechanism of electric parameters, coating thickness, surface micro-morphology, coating composition were investigated in order to understand the growth characteristics of PEO ceramic coatings. Wear and corrosion resistances were also evaluated. The results show that, species from the electrolyte also take part in the PEO process and can be one of elements making up the ceramic coatings; The thickness of the ceramic coatings is mainly dependent on PEO current density and treatment time, there are different optimum treatment times at different densities, and the lower the current density, the shorter the optimum time; The size of the remaining discharge channels in the coatings increases and population of the channels decreases with treatment time; A physical model was set up according to the evolution law above, and a formulation was derived from the physical model.In the section on enhancement and brush plating of plasma electrolytic oxidation, effects of additives and ultrasonic wave on growth rate and properties of the ceramic coatings were investigated, and brush plating method of plasma electrolytic oxidation was proposed and electric isolation property of the ceramic coatings prepared with the new method was investigated. The results show that, density and growth rate of the ceramic coatings formed in the electrolyte with additive K₂Cr₂O₇ are improved very obviously, but Na₂WO₄, KF, Na₂B₄O₇, Al₂O₃ and some other additives did not play obvious positive roles, even played some negative roles, such as making discharge channels large; Ultrasonic wave has very obvious promotion influence on the plasma electrolytic oxidation process, so that the growth rate, hardness, wear resistance and corrosion resistance of the PEO coatings increase greatly under ultrasonic wave; The brush plating plasma electrolytic oxidation make the plasma electrolytic oxidation of large work-pieces not to be limited by size of the electrolyte tank and volume of the power supply; In addition, work-pieces can be treated partially with plasma electrolytic oxidation process; The breakdown voltage of the ceramic coatings prepared with brush plating plasma electrolytic oxidation increases with the thickness of the coatings, but breakdown strength of the coatings decreases with the thickness.In the section on hybrid electrolytic oxidation on Q235 steel, hot-dipped aluminum coatings and arc sprayed aluminum coatings after inductively heated were prepared as medium coatings of the plasma electrolytic oxidation process, and relative simulation, experiment and theory researches were done. The results show that the plasma electrolytic oxidation process can be applied to prepare oxide ceramic coatings metallurgically bonded on Q235 steel substrates by means of hot dipping aluminum or arc spraying aluminum induction-heated before the PEO process; The coatings can increase significantly corrosion and wear resistances of the steel substrate surface.In the section of plasma electrolytic saturation on Q235 steel, effects of some process factors such as electrolyte, voltage, treatment time, electrode distance on the process were investigated. The results show that, KCl-C₂H₅ONH₂ electrolyte is very effective to prepare ceramic coatings containing carbides and nitrides on Q235 steel with the plasma electrolytic saturation process, however, KCl-HCONH₂ and KCl-CO(NH₂)₂ electrolytes are not effective to do so. A series of excellent research results were obtained through some systematical investigations with KCl-C₂H₅ONH₂ electrolyte.In the section of ESD-PED hybrid surface modification on W₉Mo₅Cr₄V₂ high speed steel, the microstructure and properties of the ceramic coatings prepared on the W₉Mo₅Cr₄V₂ substrate with the YG8 electrode rod and the hybrid surface modification process were investigated. The results show that the new process is much better than the ESD one in growth rate, wear resistance of the coatings.Main new points in theory are as follows: 1) A physical model was set up and a formula was derived on temperature rising of discharge channels during plasma electrolytic oxidation process, and the instantaneous temperature of discharge channels in the plasma electrolytic oxidation coatings on aluminum was calculated; 2) Based on the previous induction re-melting theory, a temperature distribution formula was derived along the coating depth during the induction re-melting of the arc spraying aluminum coatings on steels; 3)A breakdown resistance concept and a theory on defect induction to promote energy saving growth and densification of plasma electrolytic oxidation coatings were proposed. Main new points in technology are as follows: 1) Hot dipping aluminum and arc spraying aluminum were used as pretreatment steps respectively and plasma electrolytic oxidation ceramic coatings were successfully formed on Q235 steel; 2)Plasma electrolytic oxidation process by relative movement between electrodes was proposed and some research work was carried out; 3)Ultrasonic wave vibration was adopted in the plasma electrolytic oxidation process, the process was driven notably; 4)K₂Cr₂O₇ added into the electrolyte played a rather great role in densification and growth rate of the plasma electrolytic oxidation coatings; 5)A ESD-PED process was proposed based on the idea for combining the advantages of both plasma electrolytic disposition (PED) and electric spark disposition (ESD), and a hard and wear resistant metal ceramic coating was formed on W₉Mo₅Cr₄V₂ high speed steel.