Titanium alloys have many advantages, such as high strength, good corrosion resistance, heat resistance, non-magnetic, etc. they have been widely used in aerospace, aviation, ship, chemical, biomedical and other fields. But titanium has small thermal conductivity and low surface hardness, and it is prone to adhesion wear. Titanium and its contact with the material will have a strong interaction, contact corrosion; in a special medium, pitting and crevice corrosion will occur. Therefore, the surface treatment of titanium alloy must be carried out so as to make the titanium alloy endure worse service environment and condition, so that the inherent excellent performance of titanium can be fully exploited. The surface treatment of titanium plate mainly has the following aspects.
1. Surface carburizing.
According to Japan reported that the use of Ar gas produced plasma cleaning titanium surface, at 1073 ~ 1323K temperature, with CH4 infiltration of C, you can get TiC as the main component of the hardness layer, thickness of several microns. In addition, the surface hardness of titanium material can be improved by electric discharge machining. The process is that the spark discharge between the tool electrode and the titanium part is generated in the insulating oil, and the thermal decomposition of the oil generates C, permeating the fused titanium surface and forming an infiltrating C layer. The composition of the hardening layer is TiC, the thickness is about 5 microns, and the hardness can reach 2.2 * 104MPa, which is 11 times of that of the matrix titanium.
2. Surface nitriding.
Nitriding method can form a layer of titanium nitride hardened layer on the surface of titanium, mainly including plasma nitriding, gas nitriding, high pressure gas nitriding and plasma nitriding. According to reports in Japan, the base vacuum is 5 * 10 3Pa and nitriding pressure is 0.13MPa under the condition of pure titanium nitride layer formation thickness of 20 microns at 850 DEG C, the nitride layer is the outermost layer of very thin TiN, the rest is N in the Ti alpha phase solid solution. In addition, high pressure nitriding has been reported to improve the surface hardness of titanium. The gas pressure in 0.5 ~ 10MPa, N2, NH3 or their mixture gas nitriding treatment on the titanium surface, then furnace cooling; when the pressure falls to 0.1MPa, and then evacuated to 1Pa, 300 to 400 DEG C tempering, thus obtained gold TiN layer thickness of 20 microns can be pure Ti surface, surface hardness of up to 10000MPa. However, these approaches require treatment at high temperature for a long time, will cause adverse effects on the mechanical properties of titanium, especially the fatigue performance; and the titanium hydrogen absorption occurs at high temperature, there is a risk of "hydrogen". Therefore, the focus of this study is how to achieve low temperature and hydrogen free treatment.
3. Oxidation treatment.
According to Poland reported that in the Ar+O2 atmosphere, glow discharge plasma can produce oxide and oxygen diffusion layer on the surface of Ti-6Al-4V at 1023K temperature. The corrosion resistance of the resultant material in NaCl solution is obviously improved. An oxide coating has been prepared on the surface of titanium by ultrasonic + thermal oxidation treatment, according to Russian reports. Because of the high energy density of ultrasound, the depth of oxygen diffusion increases and the grains become smaller. The surface hardness of titanium treated by ultrasonic + thermal oxidation is 6500 ~ 7200MPa, and the hardening depth is 10~15 microns. The samples treated by ultrasonic + thermal oxidation have better wear resistance than those of direct thermal oxidation treatment.
Surface alloying. According to Japanese reports, Mg alloying can be realized on the surface of Ti by vapor phase thermal diffusion. Research shows that the sample gas at 950 mg of 430H, formed on the surface of Ti-Mg alloy and its corrosion resistance is greatly improved, up to 80 times the best corrosion resistance of pure titanium.
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