INVITED: Development of nanoscale multilayer CrN/NbN coatings for joint replacements deposited by High Power Impulse Magnetron Sputtering

The demand for reliable coating on medical implants is ever growing. In this research, enhanced performance of medical implants was achieved by a CrN/NbN coating utilising nanoscale multilayer structure. The advantages of the novel High Power Impulse Magnetron Sputtering technology, namely its unique highly ionised plasma were exploited to deposit dense and strongly adherent coatings on Co-Cr implants.  TEM analyses revealed coating's nanolayer structure with bi-layer thickness of 3.5 nm. CrN/NbN deposited on Co-Cr samples showed exceptionally high adhesion, critical load values of L_C2= 50 N in scratch adhesion tests. Nanoindentation tests showed high hardness of 34 GPa and Young's modulus of 447 GPa. Low coefficient of friction (µ) 0.49 and coating wear coefficient (K_C) 4.94 x 10^-16 m³N^-1m^-1 were recorded in dry sliding tests. Metal ion release studies showed a reduction in Co, Cr and Mo release at physiological and elevated temperatures to undetectable levels (<1 ppb). Rotating beam fatigue testing showed a significant increase in fatigue strength from 349±59 MPa (uncoated) to 539±59 MPa (coated). In vitro biological testing has been performed in order to assess the safety of the coating in biological environment, cytotoxicity, genotoxicity and sensitisation testing have been performed, all showing no adverse effects.