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

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. In the current work, a new low-pressure plasma nitriding process is described. Unlike conventional plasma nitriding, the process utilises High Power Impulse Magnetron Sputtering (HIPIMS) discharge, sustained on one Cr target at low power, to further enhance the ionisation of the gas in the vacuum chamber and to avoid coating deposition. The nitriding of CoCrMo alloy has been carried out in a wide range of nitriding voltages (from -500 V to -1100 V) at 400 °C for duration of 4 h. The chemical and phase composition of the nitrided layer has been studied by various advanced surface analyses techniques. The X-ray diffraction data of all the nitrided samples revealed the formation of expanded austenite (γN) phase. Texture analyses revealed that at lower nitriding voltages (-700 V) the predominant crystallographic orientation of the compound layer is (200) whereas at higher voltages (-900 V to -1100 V) the layer develops mixed (111) and (200) texture. For samples nitrided at a lower bias voltage of - 500 V, diffraction peaks for CrN/NbN and Cr₂N were also observed due to the deposition of target materials (Cr and Nb). However, no coating deposition on the substrate surface was observed at higher bias voltages (-700 V and higher) due to sufficient re-sputtering effect. The results obtained from glow discharge optical emission spectroscopy (GDOES) depth profiling showed that the depth of nitriding increased from approximately 0.7 μm at -500 V to 6 μm at -1100 V. In the pin-on-disc tribological test nitrided samples showed low coefficient of friction μ in the range of 0.6-0.7, compared to μ = 0.8 recorded for the untreated substrate. The wear coefficients (Kc) were found to be between 1.79 × 10^-15 m³N^-1m^-1 (-700 V) and 4.62 × 10^-15 m³N^-1m^-1 (-1100 V), which were one order of magnitude lower than the untreated substrate, Kc = 6×10^-14 m³N^-1m^-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.