Study On The Structure And Properties Of Cathode Electrolytic Plasma Deposited Ceramic Coatings And Its Electrical Breakdown Mechanism

As a direct and efficient coating preparation method,there are increasing reports on cathode plasma electrolytic deposition(CPED)technology at home and abroad.However,there are still some unsolved scientific problems on this technology,such as the lack of systematic research on the general correlation between the electrical properties of different conductive types of coatings and the coating structure,and further research on related electrical breakdown mechanisms is needed.The existence of these problems makes it difficult to control the structure of the CPED coating,thus it is impossible to obtain a better coating performance.Relevant research on these issues can enrich the basic theory of CPED technology,improve the reliability and stability of CPED technology,and further expand its application fields,which is of great significance to the development of CPED technology.In response to the above problems,the paper systematically studied the structural characteristics of ceramic coatings of different conductivity types,and established the correlation between the electrical characteristics of the coating and the coating structure based on the electromagnetic field theory.Base on the analysis of the correlation between the electrical characteristics and the structure of the coating,the coating is modified by introducing other components to improve the structure and performance,and the related mechanisms are studied.This paper firstly studies the typical structure of different ceramic coatings.The gas-solid double dielectric layer model is used to reveal the discharge mechanism in the CPED process.Based on the electromagnetic field theory,the correlation between the coating structure and the electrical properties of the coating is obtained:During the CPED deposition process,the electric field strength of the coating has a positive correlation with the conductivity ratio of gas film and coating(?gas/?coating).When the value is small(in the case of semiconductors and ion conductors),the electric field strength is small,and the main electrical breakdown occurs in the gas film,and the electrical breakdown in the coating is weak,which is beneficial to obtain a denser structure coating.Then,based on the analysis of the correlation between the electrical properties and the structure of the coating,C(glycerol as a carbon source)was introduced into the low-conductivity Al2O3 coating to improve the electrical properties of the coating and improve its structure and performance.The analysis shows that C is solid-dissolved into the Al2O3 crystal lattice,causing the shift of the XRD peak and the XPS peak.The first principle study of the C-modified Al2O3 band structure shows that the solid solution of C reduces the forbidden band width of Al2O3,thereby increasing its electrical conductivity,so the porosity of the coating decreases.The introduction of high content of carbon source will lead to organic residue in the coating,which is detrimental to its high-temperature oxidation resistance.Modification with lower C content is better.Porous ?-Al2O3 has a relatively high specific surface area and is widely used in the field of catalyst supports,but it is prone to phase change at high temperatures to transform into ?-Al2O3 with a relatively low specific surface area.The addition and modification of rare earth Ce can suppress its high-temperature phase transition,but at the same time it will also cause structural changes due to the changes in coating conductivity.In this paper,Ce is introduced into the porous Al2O3 coating to inhibit its phase change to ?-Al2O3,and the effect of Ce on the porous structure of the coating is discussed.Analysis show that Ce mainly exists in two forms in the coating:CeO2 and solid solution Ce.The introduced Ce can effectively inhibit the phase transition of activated alumina(?-Al2O3 and ?-Al2O3)to ?-Al2O3.But at the same time,the introduction of high content of Ce will increase the conductivity of the coating,resulting in a decrease in the porosity of the coating.In summary,a coating with a lower Ce content(1%Ce-Al2O3)is better,and it exhibits better catalytic support characteristics than an unmodified Al2O3 coating.Finally,this paper studies the changes in the characteristics of the gas film layer caused by electrolyte modification(addition of soluble starch)and its effect on the coating structure and performance,and perfects the CPED discharge mechanism based on the gas-solid dual dielectric model.Studies on the deposition of Al2O3 coatings on aluminum alloys and titanium alloys respectively show that the addition of soluble starch(5g/L)can achieve a denser coating structure and better corrosion protection for the substrate.After modification,the denser gas film layer and the smaller thickness of the gas film are the main reasons for obtaining better structure and performance.