The Effect of Coating Density on Functional Properties of SiN_x Coated Implants

Ceramic coatings may be applied onto metallic components of joint replacements for improved wear and corrosion resistance as well as enhanced biocompatibility, especially for metal-sensitive patients. Silicon nitride (SiN_x) coatings have recently been developed for this purpose. To achieve a high coating density, necessary to secure a long-term performance, is however challenging, especially for sputter deposited SiN_x coatings, since these coatings are insulating. This study investigates the time-dependent performance of sputter-deposited SiN_x based coatings for joint applications. SiN_x coatings with a thickness in the range of 4.3-6.0 µm were deposited by reactive high power impulse magnetron sputtering onto flat discs as well as hip heads made of CoCrMo. SiN_x compositional analysis by X-ray photoelectron spectroscopy showed N/Si ratios between 0.8 and 1.0. Immersion of the flat disks in fetal bovine serum solution over time as well as short-term wear tests against ultra-high molecular weight polyethylene (UHMWPE) discs showed that a high coating density is required to inhibit tribocorrosion. Coatings that performed best in terms of chemical stability were deposited using a higher target power and process heating.