A Study of TiN Coatings on Medical Implants Deposited by HiPIMS

Titanium nitride coatings (TiN) deposited by the promising technique high power impulse magnetron sputtering (HiPIMS) has been investigated as a mean of improving the surface properties of the CoCr-based medical implant grade (ASTM F75). HiPIMS is a physical vapour deposition (PVD) technique that applies very short pulses of very high power to the target followed by periods of no applied power. The HiPIMS plasma discharge contains a large fraction of target ions (in this work Ti-ions). By applying a negative bias to the substrate the Ti-ions can be accelerated towards the substrate and used for etching away oxides on the substrate surface. This technique of pre-treating the substrate prior to deposition of TiN coatings was extensively examined by a series of pre-treatments with variations in the applied negative bias and duration time. Each pre-treatment was followed by the deposition of a TiN coating. The pre-treatments positive effect on the TiN-coating adhesion was undoubtedly shown by comparative scratch testing of pre-treated and not pre-treated samples. Utilizing the pre-treatment that gave the best coating adhesion a series of TiN coatings was deposited with variations in applied substrate bias and temperature. The elemental composition in the coatings was analyzed with x-ray photoelectron spectroscopy (XPS). The morphology and the topography were studied by scanning electron microscopy (SEM) and interferometry. Residual stress and coating texture analysis was investigated with x-ray diffraction (XRD). The coating to substrate adhesion was studied by scratch testing. Some samples were also evaluated on corrosion resistance by cyclic potentiodynamic polarization (CP). Finally, the coatings were also evaluated on their tribological properties using pin-on-disk technique. To deduce strengths and weaknesses with the HiPIMS technique one reference coating was deposited with direct current magnetron sputtering (dcMS) technique and then characterized on the same material properties as the HiPIMS coatings. Scratch test analysis of the TiN coatings deposited with HiPIMS showed variations in coating adhesion from average to very good. The HiPIMS coatings showed in general high degrees of compressive residual stress with one coating exhibiting tensile stress. The texture and morphology in HiPIMS coatings was tuneable by changing the applied substrate bias and temperature. In comparison with the coating deposited with dcMS the coatings deposited with HiPIMS had less defects with smoother surfaces. This was also apparent in the pin-on-disk and interferometry analysis where the HiPIMS coatings show lower coefficient of friction and Ra values than the dcMS coating. One selected HiPIMS coating was together with the dcMS coating investigated with CP, which showed similar corrosion resistance between the coatings.Validerat; 20110221 (anonymous