CoCrMo alloy treated by floating potential plasma assisted nitriding and plasma based ion implantation: Influence of the hydrogen content and of the ion energy on the nitrogen incorporation

Abstract

Nitriding was performed on a medical grade CoCrMo alloy at 400°C in N 2 or N 2 -H 2 atmosphere at a working pressure of 0.84 Pa for 2 h. Various surface treatment techniques were used to incorporate nitrogen into the CoCrMo alloy: without any plasma assistance, by floating potential radio-frequency plasma assisted nitriding (FPPAN), by plasma based ion implantation (PBII) with several high voltage accelerations (up to 20 kV). Without plasma activation, no nitrogen is incorporated in the CoCrMo. On the contrary, all the plasma or PBII treated samples show the formation of a nitrogen-rich f.c.c. γ N phase. The layer nitrided over few microns has a nitrogen composition ranging from 30 at.% to about 20 at.% near the nitrided layer-substrate interface, with an enhanced surface microhardness. Hydrogen is found to enhance the nitriding efficiency. Without hydrogen, a high voltage polarization provides a supplementary amount of implanted nitrogen available for further diffusion and the sputtering of the surface passive oxide. So, with limited high voltages, thicker layers with higher amounts of nitrogen can be achieved by PBII compared to plasma nitriding. However, with higher voltages, the sputtering becomes too important and the nitride layer is thinner.

Tribocorrosion of Pulsed Plasma-Nitrided CoCrMo Implant Alloy

“…These peaks are characteristic of the nitrogen occupying octahedral interstitial sites in supersaturated phase (c N ), which is an expanded structure of the c phase. This phase has been also found in nitrided austenitic stainless steels and CoCrMo [4][5][6][7]. For PPN-II, besides the c N phase, two broad peaks at 38°a nd 45°are observed, the first one corresponding to CrN [5] and the second to Cr 2 N [4].…”

“…In the case of PPN-I there is no evidence of chromium nitride precipitates. The temperature at which the c N phase forms in CoCrMo alloys is not commonly agreed in the literature [3][4][5][6][7]15]. The lowest temperature of c N formation has been found by Lutz et al [15] at 230°C during plasma immersion ion implantation of CoCr alloys, having its maximum at 370°C.…”

“…Wei et al [3,4] used a highintensity plasma ion-nitriding process (HIPIN) to treat CoCrMo at different temperatures. Ç elik et al [5] investigated the effect of the plasma-nitriding temperature and time duration on a CoCrMo alloy, and Pichon et al [6] investigated the influence of the hydrogen content in the gas mixture and the ion energy using nitrogen implantation on CoCrMo. These studies highlighted the importance of process temperature on the formation of different surface structures on CoCrMo: a hard chromium nitride layer is formed at temperatures above 500°C, while at temperatures below 500°C, nitrogen diffuses in the alloy to form a new structure, the so-called c N phase, which is a nitrogensupersaturated fcc structure [7].…”

Use of combination of accelerator-based ion-beam analysis techniques to the investigation of the corrosion behavior of CoCrMo alloy

“…The XRD and GIXRD investigation showed in the case of CoCrMo + N sample the formation of a c N expanded austenite phase [11,12]. The formation of a c N expanded austenite phase was also observed in the case of the CoCrMo + N + O sample.…”

“…100 nm whereas on the initial material its content was low (ca. 1%) fact connected to the treatment procedure (nitridation and oxidation) [12].…”