Effect of continuous and cyclic Rf plasma processing time on titanium surface

A systematic study was undertaken with surface of titanium nitrided by Rf inductively coupled plasma. The continuous plasma processing time was changed from only 3 to 40 min in steps of 5 min or less. The other plasma parameters were fixed. The effect of cyclic plasma processing time on titanium sample was investigated too. The single period of plasma processing was adjusted to be 25 min and the nitriding process was repeated twice, triplet, and quadruple. X-ray diffraction (XRD), optical micrograph (OM), and Vickers microhardness (HV) were employed as analytical techniques. The results clearly show that the surface hardness and nitriding rate increase incessantly as the continuous plasma processing time increases to reach the maximum values of 2150 HV0.1 at 40 min and 3.38 μm2/s at 30 min, respectively. This behavior may be attributed to formation of new hard phases α-Ti(N), δ-TiN, and ε-Ti2N in the treated layer. In this interval, the active nitrogen species penetrate faster through surface grain boundaries and through the formed surface microcraks of titanium sample. For relatively long nitriding time more than 30 min of continuous plasma processing, the rate of nitriding decreases. The formed nitrided phases might block the previously formed microcracks in the treated layer. Therefore, the penetration of nitrogen species rate through these microcracks decreases and the rate of nitriding process decreases consequently. Using cyclic nitriding method yielded an increase in the microhardness of titanium sample to be 2650 HV0.1 with relatively high rate of nitriding. These investigations show that the short Rf plasma processing time treatment and cyclic nitriding method play an important role for producing hard surface titanium with high rate of nitriding.