Nitriding of titanium in the glow discharge plasma with the hollow cathode

At the present time for increase in surface hardness, wear resistance and corrosion resistance of titanium alloys the method of diffusion saturation by nitrogen in plasma of gas discharges is used. In most cases nitriding is carried out at high temperatures (800-900)oC, pressure about of 10-100 Pa and at process duration about several tens hours. However, high-temperature nitriding of the titanium alongside with hardening of a thin surface layer can makes the sample bases softer. In experiments, the glow discharge with the hollow cathode was used for realization of process low-temperature nitriding and increasing of concentration of the atomic nitrogen formed at dissociation of molecular nitrogen. The high degree of ionization and dissociation nitrogen molecules in such discharge occurs due to oscillations of electrons, accelerated in cathode drop. The glow discharge with the hollow cathode was initiated and supported in high-current form due to plasma of an additional arc discharge plasma source. It allow to provide optimum of plasma density and intensity of dissociation nitrogen in cathode changing at wide range the discharge current, discharge voltage and pressure. The leak-in of technically pure nitrogen (N2) or a mix of nitrogen with argon (N2 + Ar) is produced through adjustments system of pressure of the gas discharge plasma source. Technically pure titanium samples in diameter of 26 mm and thickness of 5 mm were installed inside cylindrical hollow cathode in diameter of 80 mm and length 130 mm so that access of plasma to the back side of a sample has been excluded owing to small (<1 mm) gaps between a sample and a substrate. As a result of the researches the opportunity low-temperature (450-550oC) nitriding of technically pure titanium in glow discharge plasma with the hollow cathode has been shown. The feature of the process is that not only a surface contacting with plasma was nitrated, but also the back side of samples. Microhardness on both surfaces was approximately equal. At injection in discharge Ar there is some increase of microhardness. The following features and regularities of process based on the obtained experimental results and their analysis have been established: 1. nitriding technically pure titanium in the glow discharge plasma with the hollow cathode occurs to rather high efficiency at low pressure ((2-4) Pa) and rather low temperatures (<500oC).; 2. After 4.5 hours of nitriding there is a formation of the multilayered structure consisting of thin (∼ 1 μm) surface layer titanium nitride having high (∼ 14 GPa) microhardness, a cementation layer (∼ 30 μm) and, obviously, a layer of thermal influence (∼ 70 μm), smoothly passing in the basic volume material.; 3. Saturation by nitrogen of the titan is plasma chemical process in which the determining role is played with atomic nitrogen. Dissociations molecules of nitrogen are formed by electronic impact through excitation of various electronic states. (author)