Research Hydrogen-free Carbonizing And Hydrogen-free Carbonitriding On Titanium And Titanium Alloy Ti6A14V By Double Glow Plasma Discharge

The titanium and titanium alloys are widely used in aviation industry, medical science, chemical engineering, transportation, sports apparatus and electronics industry etc. for their high specific strength, high stability in low and medium temperature, excellent corrosion resistance and bio-compatibility etc. However, the titanium and titanium alloys have low hardness, poor wear resistance which limits their more applications .Conventional carburizing techniques, such as pack carburizing and gas carburizing, have been successfully used to improve the surface hardness and strength of iron and steels. However these methods are not applicable for titanium and its alloys due to the microcrack and embrittlement caused by the reactions between titanium and hydrogen in the carburizing media. This paper analyzes the properties of titanium and titanium alloys; evaluates various surface treatment methods; discuss hydrogen brittleness of titanium alloys. The double glow plasma hydrogen-free carburisation and double glow plasma hydrogen-free carbonitriding are adopted to enhance wear resistance of titanium and titanium alloys Ti6A14V , at the meantime avoid the hydrogen brittleness of titanium alloys and also keep their high corrosion resistance, high specific strength.The doubled glow plasma discharge hydrogen-free carburisation employs as grid shaped high purity solid graphite as the source electrode, the titanium or titanium alloy Ti6A14V as the cathode, argon as the working gas, the hollow cathode effect of the glow discharge is used to carburize the titanium alloys without hydrogen. Carburized layers with special physical and chemical properties are formed on the surface of the titanium or titanium alloys. The phase structure and the distribution of carbon are analyzed with the X-ray diffraction and the glow discharge optical spectrum; the friction and wear behavior and the electrochemical corrosion properties are examined. The results show that the high hardness phase (TiC) and the antifriction phase are formed in the compound carburized layer; the content of carbon is grade distributed; the surface hardness of carburized materials are increased significantly and the hardness profile from top to inside shows a gradient characteristic ; the antifriction and the abrasion resistance are greatly enhanced , the friction coefficient and the wear rate are reduced bymore than 50% and more than 3 orders of magnitude respectively. The corrosion resistance is increased by several times . Nick test shows that the materials' surface brittleness is slight, and the hydrogen brittleness is avoided. Compared with the ion nitrided phase, the layer is thick and also has better abradability and corrosion resistance.Using the solid graphite as the source electrode, the titanium and titanium alloys as the cathode , inputting a certain ratio gas of nitrogen and argon, depending on the glow discharge hollow cathode effct, we get a carbonitriding layer without hydrogen which has special physical and chemical properties. The phase structure and the distribution of carbon, nitrogen are analyzed with the X-ray diffraction and the glow discharge optical spectrum. The friction wear property and the electrochemical corrosion properties are checked. The results show that the high hardness phase (TiC,TiN) are formed in the compound carbonitriding layer; the content of carbon and nitrogen is graded reduced from top to inside; the surface hardness are increased a lot, the hardness is graded reduced from top to inside ; the antifriction and the abrasion resistance are enhanced a lot, the friction factor goes down by more than 40%, the wear rate goes down by more than 3 orders of magnitude, also the corrosion resistance goes up by several times . Nick test shows that the materials' surface fragility is gentle, and avoids the hydrogen brittleness as the existence of ion carburized phase. Compared with the ion nitrified phase, the layer is thick and also has nice abradability corrosion resistance.The results are analyzed in theoretically. The graded distribution of carbon (or carbon, nitrogen ) in the layer determines the graded distribution of TiC(or TiC, TiN) content, and further determines the graded distribution of hardness and the nonconstant variation rules of friction coefficient which varies from small to large gradually and finally tend to stabilization.Finally, the models of plasma transition in dynamic stable electric field are constructed; the differential pressure theory is put forward to with the statistic physics rules. Combined with the Fick's first law, the diffusion coefficient is calculated. The results show that the diffusion coefficient is not large during the carburization without hydrogen in the titanium alloys, and TiC prevents carbon from diffusing ,which is formed from reaction between carbon and titanium.In summary, this paper demonstrates that the double glow plasma hydrogen-free carburization and hydrogen-free carbonitriding technology are feasible way to enhance the wear resistance of titanium and titanium alloys Ti6A14V without bringing about hydrogen brittleness.