Nitridation of Ti6Al4V by PBII: study of the nitrogen diffusion and of the nitride growth mechanism

To improve the wear properties of Ti6Al4V material, the growth of a hard and wear resistant nitride layer is proposed as an interesting low cost solution. Compared to conventional techniques of nitridation (with or without plasma), previous studies have shown the interest of PBII to obtain a nitride layer, even at low temperatures. Ti6Al4V pieces were here nitrided in the homemade PBII reactor, characterized by an independent control of the temperature, of the implantation and of the plasma parameters. The nitriding were performed with a 1 Pa, N2 (90%)–H2 (10%) R.F. plasma between 200 and 800 °C, for 2–6 h, with a 10–35 kV high-pulsed voltage, with a 200 Hz repetition rate and with a 10 μs pulse length. Samples were characterized by X-ray diffraction, XPS, GDOES, NRA and micro-indentation. At the highest temperature (800 °C), a 2-μm nitride layer is obtained, composed of a mixture of TiN and Ti2N, and is followed by a diffusion profile of 20 μm in α-Ti. Compared to conventional nitriding techniques, no improvement of the diffusion process is then induced by the PBII treatment as the nitridation is mainly governed by thermal activated long range diffusion. At temperatures of 500 and 200 °C, nitrogen is present only in the first hundred nanometers with an average concentration of 30 at%. The nitride is however nanocrystallized in TiN precipitate, preferentially to Ti2N, dispersed within an α-Ti matrix. The diffusion coefficient obtained by considering the diffusion beyond the implanted zone (Rp+ΔRp) are in the range 10−16–10−15 cm2/s at 500 °C. A slight increases can be observed with the ion energy, the ion dose or with the treatment duration. The high density of defects, the high nitrogen supply and the presence of much Ti2N phase may be responsible for such an increase.