Low pressure plasma nitrided CoCrMo alloy utilising HIPIMS discharge for biomedical applications

Abstract

CoCrMo is a biomedical grade alloy which is widely used in the manufacturing of orthopaedic implants such as hip and knee replacement joints because of it has high hardness, high corrosion resistance, and excellent biocompatibility. However, the release of metal ions due to corrosion and wear of the alloy over time may cause allergic or other adverse reactions in some patients. To date, various surface modification techniques including nitriding, have been used to improve the performance of CoCrMo (F75) alloy.15 m 3 N -1 m -1 (-1100 V), which were one order of magnitude lower than the untreated substrate, Kc = 6 ×10 -14 m 3 N -1 m -1 . The Knoop microhardness (HK) of nitrided samples significantly increased by a factor of 5 (HK= 2750 at -1100 V) as compared to the untreated substrate, HK=525, demonstrating the high efficiency of the process. The samples nitrided at -700 V and -900 V exhibited enhanced corrosion resistance as compared to untreated alloy by avoiding the formation of CrN based compounds which adversely affect the corrosion performance.

Effect of nitriding voltage on the impact load fatigue and fracture toughness behaviour of CoCrMo alloy nitrided utilising a HIPIMS discharge

“…Apart from the Co2-3N compound layer (Co4N+Co2-3N), presence of CrN and Cr2N was also detected. A detailed investigation of elemental composition and texture of these nitrided layers has been presented in a previous publication [13]. The XRD data (not presented here) for these specimens confirmed the presence of both Co4N and Co2-3N, also termed as expanded austenite phase γN(111) and γN(200).…”

“…The process temprature and pressure were set constant at 400 °C and 8.3×10 -3 mbar respectively for all the experiments. Detailed information on the plasma nitriding processes using HIPIMS discharge can be found in the previous publication [13].…”

“…In recent years, surface hardening techniques such as gas nitriding, plasma nitriding/ carburising [3,8], Ion Implantation [9][10][11], boronisation [12], oxidising and very recently High Power Impulse Magnetron Sputtering (HIPIMS) nitriding [13] have been used to improve the wear resistance and surface hardness of these CoCrMo alloys. In general, a few micrometres thick nitrided layer on the surface improves wear resistance by one order of magnitude and significantly reduces the coefficient of friction [14,15].…”

“…This facilitates higher diffusion rates and in general a rapid nitriding rate, however at lower temperatures than conventional nitriding. In the previous studies, the authors had demonstrated the use of HIPIMS discharge for plasma nitriding of CoCrMo alloys [13]. It was found that the microstructure was sensitive to the nitriding voltage (-700 V to -1100 V) applied to the specimens and not to the temperature as it was fixed at 400 °C.…”

Improved Adhesion of the DLC Coating Using HiPIMS with Positive Pulses and Plasma Immersion Pretreatment

“…They increase the resistance to abrasive and adhesive wear, while reducing the coefficient of friction. The wear residues are biocompatible, which makes this coating a perfect choice for medical implants [6].…”