Preparation And Properties Of Metal,Ceramic And Metal Doped Al₂O₃ Coating By Cathodic Plasma Electrolysis

Cathodic plasma electrolysis（CPE）technology has been studied by researchers,and it has been used widely in the metal surface treatment field.Cathodic plasma electrolysis cleaning is possible to replace the pickling or caustic washing in the future because of its advantages of environmentally friendliness.There is also no need to add hazardous substance into solution preparing metal coatings using cathodic plasma electrolysis.Ceramic also can be coated on the metal substrate by cathodic plasma electrolysis and this technology has no limitation of substrate materials.However,the application of this technology has been greatly limited due to the lack of studies on the CPE formation process and mechanism of cleaning or deposition by cathode plasma electrolysis.In this thesis,the effect of cathodic electrolytic plasma on metal surface was systemically studied.The mechanism of plasma discharge was clarified by adjusting parameters and detecting light signals during the process of plasma electrolysis.The relationship between CPE cleaning and deposition was explained.According to deposition mechanism of metal and ceramic coating,a new type of metal doped Al₂O₃ coating was designed and prepared.The main conclusions and and innovations are listed as follows:1.The experimental results showed that increasing the concentration or temperature of electrolyte is favor of CPE process.The plasma electrolysis is due to the rapid increase of electric field intensity in bubbles if the cathode surface is covered by bubbles,which leads to gas breakdown and arcing in the bubbles.The plasma discharge in bubbles would melt the metal surface in micro-zone,and then strongly impact the metal surface in the process of bubble expansion and explosion.Continuous plasma discharge causes the special surface of low carbon steel with large amount of craters on it.This impact of bubbles explosion can be used to clean the oxide scale on the surface of low carbon steel.The coverage of the oxide scale can be reduced to less than 2%if the Q195 steel wire with a diameter of 2.5mm was treated by CPE cleaning technology for 1 min in NaHCO3 aqueous solution.The cleaning efficiency can be improved with the increase of voltage between the cathode and anode.All the residual oxides in the deep scratch of steel wire surface can be removed after CPE cleaning at voltage of 170V.Continuous cleaning of Q195 low carbon steel wire has been realized by developing equipment based on cathode plasma electrolysis.2.Zn coating can be deposited on the surface of low carbon steel wire by CPE in ZnSO4 solution under pulse power supply.The Zn coating obtained at the pulse power with 120V,80%duty cycle and 4000Hz frequency has the smallest porosity,and the coating completely covered the steel wire surface.By comparing the Zn coatings deposited with different parameters and detecting the light signals during the CPE deposition process,it was found that the interval time between pulses was the main factor affecting the deposition process,and the deposition of the coatings mainly occurred during the ascending stage of the pulse.The plasma discharge on the coating surface was to melt the coating surface in micro-zone and clean the loose part by the impact force of bubble explosion.The cold drawing can densify Zn coating on low carbon steel wire.Zn coating deposited by CPE technology can reduce the corrosion rate of low carbon steel wire significantly.3.Al₂O₃ coating can be prepared on the surface of Ti-45Al-8.5Nb alloy by cathodic CPE depositon in Al（NO3）3 solution.Both water and ethanol can be used as solvents of Al（NO3）3 solution.Al₂O₃ coating obtained in Al（NO3）3 aqueous solution has larger porosity.Compared with the coatings obtained under different parameters,the coating structure is greatly affected by the power supply parameters.The Al₂O₃ coating can improve the oxidation resistance of TiAl at 900℃.Al₂O₃ coating obtained in Al（NO3）3 ethanol solution has smaller porosity,smoother surface and better high temperature oxidation resistance.The main constituent of Al₂O₃ coatings obtained in both aqueous and ethanol solutions was γ-Al₂O₃.The y-Al₂O₃ would transformed to 8-Al₂O₃ if the coating is kept at 900 ℃.The surface pore size of Al₂O₃ coatings obtained in two solutions and surface tension of solutions were analyzed.The results showed that the main factor affecting the deposition process of the coatings is the surface tension of the solution.4.The feasibility of simultaneous deposition of metal and ceramic coatings was proved by analyzing the mechanism of metal and ceramic coatings deposited using CPE deposition,and a new type of metal doped Al₂O₃ ceramic coating was designed.Al₂O₃-Ni and Al₂O₃-Cr coatings were prepared by adding Ni（NO3）2 and Cr（NO3）3 into Al（NO3）3 ethanol solution,respectively.The pores on the surface of metal doped coatings were blocked,which reduces the chance of contacting O2 through the pore during oxidation at 900℃ of TiAl substrsate,and tt also reduced the potential crack source in the coatings.The high temperature oxidation resistance and scratch crack propagation resistance of the coating were improved significantly by doping metals into the coating.Ni existed in the Al₂O₃-Ni coating with NiAl2O4 and a small amount of metallic Ni.NiAl2O4 hindered the transformation of μ-Al₂O₃ in the coating to δ-Al₂O₃ at 900 ℃.In the Al₂O₃-Cr coating,Cr bonded with Al₂O₃ to form（Al0.9Cr0.1）2O3.After oxidation at 900 ℃,most of the y-Al₂O₃ in the coating was transformed into α-Al₂O₃.