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

Abstract

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 (SiNx) 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 SiNx coatings, since these coatings are insulating. This study investigates the time-dependent performance of sputter-deposited SiNx based coatings for joint applications. SiNx 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. SiNx 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.

Current status and future potential of wear-resistant coatings and articulating surfaces for hip and knee implants

“…plasma nitriding. Because of its potential for strong adhesion CrN is also sometimes used as an interlayer between the substrate and a top coating [ 46 , 52 ].…”

“…However, a number of novel experimental coatings have been reported (e.g. silicon nitride (SiN) [ [46] , [47] , [48] , [49] , [50] , [51] , [52] , [53] ], multilayer structured coatings [ 42 , 44 , 54 ]) as well as commercially available coatings (oxidized zirconium (OxZr) [ 55 ] and titanium nitride (TiN) [ 56 , 57 ]), which are utilised in interfaces of joint prostheses, and currently, there is no comprehensive review available. This review aims to provide readers with a comprehensive overview of coatings that are (i) commercially available and (ii) currently being researched, and to summarize the current status and future potential of such coatings for hip and knee joint implants.…”

The Effect of N, C, Cr, and Nb Content on Silicon Nitride Coatings for Joint Applications

“…Here, the reduction in coating density and the pronounced growth of columns led to incorporation of O as the coatings were exposed to air prior to analysis. Further, a reduced morphological density was observed as Cr and Nb were added to the process [40]. This was mirrored in higher O and C contents in the corresponding coatings.…”

“…Figures 3 and 5 show that coatings with no apparent reaction or coating failure, and that gave low pin wear rates, were those with an initially high hardness (22.5-28.4 GPa), and, hence, presumably higher density and lower reactivity and/or a high N content (N-low, N-high, Bias-medium, Bias-high, and Si Power-high), with the exception being the Standard (H = 23.4 GPa) coating, which, however, contained more oxygen than the well-performing coatings (Table 2), suggesting a higher reactivity. Coating wear through contact with UHMWPE during the tests was not expected, and coatings failing would rather be associated to a higher reactivity and subsequent dissolution [40].…”

“…According to our previous work, SiN x based coatings have shown potential as an alternative for joint bearings due to their biocompatibility, high wear resistance and hardness, and reduced metal ion release [36][37][38][39]. However, it is challenging to achieve an optimal combination of adhesion, coating density, and reactivity in SiN x coatings; a high coating density resulting in a lower reactivity may give an insufficient adhesion to the substrate due to high residual stresses [40]. Alloying with a third element may be an option to improve the chemical stability while maintaining a balance in coating density and adhesion.…”