Diffusion Kinetics Of Mo-based Plasma Alloying On Ti6A14V And Its Wear Properties

Titanium alloys has wild application prospect in aerospace industry due to its features:low density, high specific strength, good corossion resistance. But it's disadvantage of low hardness, high coefficient of friction, poor wear-resistance and severe adhesive wear occurred easily which responds significant influence on structure safety, and goes against their further application. Surface alloying is an efficient way to improve their tribological property. Double glow plasma surface alloying technology gains more and more attention in academe and industry fields as its advantage of big choice range of alloying elements, simple and controlable process and rapidness of alloying.Research shows that molybdenum alloying on Ti6A14V could improve hardness and wear-resistance of the alloy. Mo-N and W-Mo alloying could modify its tribological properties further. In the plasma alloying process, the diffusing process of alloying elements determines the final structure and properties of modified layers. Temperature is one of the most important factors affecting the diffusion process strongly. This paper mainly studied the diffusion coefficient of Mo in double plasma alloying process at different temperatures to realize the diffusion behavior in molybdenum alloying layer on Ti6A14V, as well as the effect of the alloying elements, tungsten and nitrogen, to the diffuse behavior of Mo.Buehter automated image analyser, X-ray diffraction analyser and GDA-750 glow plasma discharge spectrum analyser were used to measure and analyze the struction morphologies, phase composition and the element distribution of different modified layers respectively. Results show that thick, well-proportioned modified layers were formed on the surface of Ti6A14V through molybdnizing at 800-1000℃and Mo, W-Mo and W-Mo-N alloying process at 850℃. The concentration of alloying elements in modified layers decreases gradually. The molybdnizing and W-Mo alloying layers consist of deposition and diffusion layers, while W-Mo-N modified layer contain compound layer. The hardness of these modified layers is reduced gradiently with the increase of distance from the surface, and molybdnizing and W-Mo-N modified layers reduce more rapidly than W-Mo film.We gain the diffusion coefficients of Mo under different conditions through calculating the monotone decreasing part of different Mo concentration gradient profiles. The results show that:the mean diffusion coefficients of Mo in Mo alloying layer on Ti6A14V is increasing by the growth of temperature in the range of 800 to 950℃, however it becomes abnormal at 1000℃which relates with the phase transition of Ti6A14V and anti-sputtering on the surface. In the whole diffusion process, Mo has the smallest diffusion coefficients in W-Mo-N alloying process. Under the same Mo concentration, because of different diffusion mechanism, adding of W reduces the diffusion coefficient of Mo obviously, while adding of N has little effect on it.Pin-on-Disc test shows that molybdnized layer has the lowest friction coefficient in a short slide distance at the beginning, while it gets the highest at the end of the test. W-Mo-N alloyed layer exhibits the lowest friction coefficient. Under the same condition, molybdnized layer exhibits the biggest wear rate, then the Mo-W alloyed layer, and W-Mo-N alloyed layer exhibits the smallest. It means the adding elements of tungsten and nitrogen could improve the tribology performance of titanium alloy.Plasma alloying technology has a defect that makes sample distortion easily because of the high treatment temperature and unstable Voltage. So in the paper, we quest for a combination of surface nanocrystallize and plasma alloying technique, molybdnizing on surface nanocrystallized Ti6A14V at low temperature (four temperatures from 460℃to 610℃). The experiment shows that the structure of surface nanocrystal could improve the mean diffusion coefficient of Mo atom and thickness of modified layer, especially at 560℃. The surface nanocrystallized and molybdnized Ti6A14V sample exhibits a lower wear rate than only nanocrystallized and only molybdnized sample. It means to combine the surface nanocrystallization with plasma alloying technique is propitious to improve the tribology properties of titanium alloy.