Research progress of wear- and corrosion-resistant film layer on the surface of titanium alloy by micro-arc oxidation

Titanium alloy has many excellent properties, high specific strength, low density, biocompatibility and other characteristics, therefore, it is widely used in the military industry, people's livelihood supplies and the aerospace industry, however, due to the poor wear resistance of titanium alloy, the hardness is also lower, the contact with other metals corrosion hazards are relatively large, which have affected the scope of application of titanium alloy. In order to improve the physical defects of titanium alloys, the surface of titanium alloys has been treated to maximize its excellent performance, effectively expanding the scope of application of titanium alloys. Micro-arc oxidation technology greatly improves the wear and corrosion resistance of titanium alloys, by reducing the coefficient of friction, forming a layer of ceramic film on the surface, preventing contact corrosion.This paper briefly describes the micro-arc oxidation technology and advantages, and analyzes the research progress and application prospects.
1 Micro-arc oxidation technology and advantages
Micro-arc oxidation belongs to the application of titanium alloy new technology, it introduces the oxidation of Faraday work area to the high-pressure discharge area, resulting in the surface of its alloys as well as non-ferrous metals such as A,l T,i Mg placed in the treatment solution, spark discharge, micro-arc discharge, corona and other phenomena, in the electrochemical, thermo-chemical, plasma chemistry, a layer of oxidation of the ceramic film on the surface of the material in-situ growth, which in turn makes the material surface properties are strengthened. In general, the MAO process can be divided into four stages, before the spark stage for the first stage, the power supply is turned on, there are a large number of oxygen bubbles precipitation, the formation of an air seal, the air seal to oxygen-based, generating a layer of insulating passivation film on the surface of the specimen, the current density from zero very quickly rose to the peak, and then fell. Spark stage for the second stage, when the passivation film by the voltage continues to rise and breakdown, the current density stops falling and began to rise, the strong electric field between the electrodes to form a plasma and make the gas seal gas discharge, a large number of swimming in the surface of the specimen appeared in a small spark, this is because it is always in the region of the film layer is thin to break through the passivation film, and constantly change the breakdown site. Micro-arc stage for the third stage, with the continuation of MAO, dispersed micro-arc will gradually appear on the surface of the sample, and fast swimming, micro-arc intensity increased, the density gradually decreased, with the extension of time the current density tends to stabilize, the resistance and the thickness of the film layer gradually increased, the local arc stage for the fourth stage, the later stages of the MAO, the number of arc spots on the surface of the sample is gradually reduced, the arc spot obviously slower moving speed, and the current becomes lower density. Through the above four stages, the anode electrochemical reaction, plasma bombardment, melting, diffusion, sintering phase transition, solidification and other processes can be formed with the substrate combined with a strong and relatively thick ceramic film layer. Micro-arc oxidation technology has the following advantages: ① Because the ceramic film is grown in situ, so the combination is relatively high. ② Adjust the composition of the electrolyte and process conditions, so that the composition and performance of the film layer changes, the functional design of the film layer is effectively improved. ③ Instantaneous discharge temperature is relatively high, the temperature Krys-mann calculates that it can reach 8000K, Van believes that the temperature exceeds 2000℃, the oxide will melt in this region, the substrate temperature does not exceed 300℃, the performance of the substrate will not deteriorate. ④Even the complex shape of the workpiece can be formed on the inner and outer surfaces of the film; ⑤Simple operation, cost-effective, less pre-treatment process, do not need high temperature or vacuum conditions. ⑥ Combine the advantages of ceramics and metal, effectively improve the corrosion resistance and wear resistance of the metal surface.MAO film layer comprehensive mechanical and physicochemical properties are good, so it effectively improves its scope of application.With the continuous improvement of the technology as well as the deepening of the research, MAO technology will surely bring us better economic benefits.

3 Research Progress of MAO Film Layer in Titanium Alloy
From the 1980s, Russia began the study of titanium alloy MAO film layer, compared with other countries, not only started early, but also more thorough and in-depth research. The protection performance of the film layer, analysis of the chemical composition of the film layer and optimization of the electrolyte formulation are the important contents of its research. After Russia, the research work on titanium alloy MAO began to be carried out comprehensively. However, there are not many relevant reports on the treatment of corrosion and wear resistance of titanium alloy MAO at home and abroad.Xue et al. studied the organization and structure of the membrane layer of MAO made in sodium meta-aluminate system and silicate system, and also analyzed some mechanical properties including elastic abrasion, the hardness of the membrane layer, and the general distribution of the membrane layer. The utilization of bipolar pulsed power supply affects the electrical parameters in the MAO process of titanium alloys, which has also been carefully and deeply studied by Wu et al. As for the electrical parameters and the composition of the treatment solution on the wear resistance, like composition and growth rate of the titanium alloy MAO film layer, Wang et al. also made a relevant study. It is pointed out in relevant literature that in the phosphorus or calcium component of the treatment solution, a titanium alloy MAO film layer will be generated, which is not only corrosion and abrasion resistant, but also has the characteristics of biocompatibility, which is a very promising application of such a film layer in the bone transplantation in medicine.
4 Application prospects of titanium alloy MAO membrane layer
Analyzing its physical and chemical properties, the application prospect of MAO film layer is very wide, especially its corrosion and wear resistance. In the modern hull structure field of application, in the size difference, the complex shape of the parts, the formation of hard, dense, uniform MAO film layer, in steel and other piping components with contact and Cu alloy, alloy formation of TiO2 film layer, able to seawater corrosive properties are further improved. MAO film on titanium alloy applied in other industrial fields, with strong corrosion resistance properties and thermal barrier, especially for some key parts, such as the engine cover of the car, its protection is more significant, can be its heat insulation film layer easy to fall off the shortcomings of the effective way to overcome. Also due to its good abrasion resistance, in the textile industry also has a wide range of applications, especially for yarn cups and other key parts. In addition, in the high-speed heavy load, high pressure and high temperature and other stringent conditions, MAO film layer also has a good magnetic shielding ability, irradiation ability and resistance to high-energy rays, is widely used in electronic shielding board.
Titanium alloy has many excellent properties, high specific strength, low density, biocompatibility and other characteristics, therefore, it is widely used in the military, people's livelihood supplies and the aerospace industry, however, due to the poor wear resistance of titanium alloy, hardness is also lower, the corrosion hazards of contact with other metals is relatively large, these affect the scope of application of titanium alloy. MAO technology has an economic, environmentally friendly, simple and at the same time, in the treatment of the The process will not damage the body and many other advantages. At present, the research work on titanium alloy MAO is also being gradually launched and carried out, and has made progress in stages. Domestic research is still in its infancy. So we must expand the field of application, in-depth understanding and mastery of the technology. This paper briefly describes the micro-arc oxidation technology and advantages, and analyzes the corrosion-resistant film layer as well as the research progress of micro-arc oxidation wear-resistant in-situ generation on the surface of titanium alloy, and looks forward to the prospects for its application and development direction.