Nitriding of medical grade CoCrMo alloys using HIPIMS discharge

The aim of this work was to develop a plasma nitriding process for a bio-grade CoCrMo alloy using a High Power Impulse Magnetron Sputtering (HIPIMS) discharge. An industrial-sized PVD machine (Hauzer techno Coating 1000-4, The Netherlands) machine equipped with two HIPIMS and two DC power supplies was utilised for this objective. Low-pressure plasma nitriding was carried out in a H2:N2 (15:85) environment with different nitriding voltages (-500 V to -1100 V). Nitrided specimens thoroughly were characterised using various techniques. To understand the effect of nitriding voltage on the phase composition and texture (T*) x-ray diffraction (XRD) was used. Microstructural changes due to the variation in nitriding voltage w investigated using a focused ion beam Scanning electron microscope (FIB-SEM) equipped with various detectors such as secondary electron (SE) and back scattered (BSE). Extent of nitrogen diffusion was analysed using GDOES and SIMS. Elemental analysis of the virgin nitrided layer and analysed surfaces were performed using point, line, and colour mapping electron diffraction spectroscopy (EDS). Nano/Micro hardness testers were used to understand the effect of nitriding voltage on hardness (Hp and HV) and fracture toughness (KIc). A high frequency macro impact load tester was used to understand the effect of nitriding voltage on impact fatigue behaviour of the compound layers. Fractured surfaces were analysed using SEM and FIBSEM after static and cyclic loading to understand the deformation mechanism such as fatigue cracking underneath nitrided layer. Potentiodyanmic polarisation study was performed in 3.5 wt.% NaCl and Hank’s solution to analyse the corrosion performance of these nitrided layers. At the end, HIPIMS plasma nitrided specimens were compared with benchmarked DC plasma nitrided specimens. Sample nitrided using HIPIMS discharge at -900 V exhibited novel phase composition, superior hardness and fracture toughness, better wear resistance, improved corrosion resistance and reduced metal ion release compared to the benchmarked DCPN.