Fe and C additions decrease the dissolution rate of silicon nitride coatings and are compatible with microglial viability in 3D collagen hydrogels.

Silicon nitride (SiN) coatings may reduce unwanted release of metal ions from metallic implants. However, as SiN slowly dissolves in aqueous solutions, additives that reduce this dissolution rate would likely increase the lifetime and functionality of implants. Adding iron (Fe) and carbon (C) permits tuning of the SiN coatings' mechanical properties, but their effect on SiN dissolution rates, and their capacity to reduce metal ion release from metallic implant substrates, have yet to be investigated. Such coatings have recently been proposed for use in spinal implants; therefore, it is relevant to assess their impact on the viability of cells expected at the implant site, such as microglia, the resident macrophages of the central nervous system (CNS). To study the effects of Fe and C on the dissolution rate of SiN coatings, compositional gradients of Si, Fe and C in combination with N were generated by physical vapor deposition onto CoCrMo discs. Differences in composition did not affect the surface roughness or the release of Si, Fe or Co ions (the latter from the CoCrMo substrate). Adding Fe and C reduced ion release compared to a SiN reference coating, which was attributed to altered reactivity due to an increase in the fraction of stabilizing Si-C or Fe-C bonds. Extracts from the SiN coatings containing Fe and C were compatible with microglial viability in 2D cultures and 3D collagen hydrogels, to a similar degree as CoCrMo and SiN coated CoCrMo reference extracts. As Fe and C reduced the dissolution rate of SiN-coatings and did not compromise microglial viability, the capacity of these additives to extend the lifetime and functionality of SiN-coated metallic implants warrants further investigation.

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

Hip and knee joint replacements are common and largely successful procedures that utilise implants to restore mobility and relieve pain for patients suffering from e.g. osteoarthritis. However, metallic ions and particles released from both the bearing surfaces and non-articulating interfaces, as in modular components, can cause hypersensitivity and local tissue necrosis, while particles originating from a polymer component have been associated with aseptic loosening and osteolysis. Implant coatings have the potential to improve properties compared to both bulk metal and ceramic alternatives. Ceramic coatings have the potential to increase scratch resistance, enhance wettability and reduce wear of the articulating surfaces compared to the metallic substrate, whilst maintaining overall toughness of the implant ensuring a lower risk of catastrophic failure of the device compared to use of a bulk ceramic. Coatings can also act as barriers to inhibit ion release from the underlying material caused by corrosion. This review aims to provide a comprehensive overview of wear-resistant coatings for joint replacements - both those that are in current clinical use as well as those under investigation for future use. While the majority of coatings belong predominantly in the latter group, a few coated implants have been successfully marketed and are available for clinical use in specific applications. Commercially available coatings for implants include titanium nitride (TiN), titanium niobium nitride (TiNbN), oxidized zirconium (OxZr) and zirconium nitride (ZrN) based coatings, whereas current research is focused not only on these, but also on diamond-like-carbon (DLC), silicon nitride (SiN), chromium nitride (CrN) and tantalum-based coatings (TaN and TaO). The coating materials referred to above that are still at the research stage have been shown to be non-cytotoxic and to reduce wear in a laboratory setting. However, the adhesion of implant coatings remains a main area of concern, as poor adhesion can cause delamination and excessive wear. In clinical applications zirconium implant surfaces treated to achieve a zirconium oxide film and TiNbN coated implants have however been proven comparable to traditional cobalt chromium implants with regards to revision numbers. In addition, the chromium ion levels measured in the plasma of patients were lower and allergy symptoms were relieved. Therefore, coated implants could be considered an alternative to uncoated metal implants, in particular for patients with metal hypersensitivity. There have also been unsuccessful introductions to the market, such as DLC coated implants, and therefore this review also attempts to summarize the lessons learnt.

Tailoring the dissolution rate and in vitro cell response of silicon nitride coatings through combinatorial sputtering with chromium and niobium.

Ceramic coatings have been widely investigated as a means to reduce wear and metallic ion release from joint implants. Silicon nitride-based coatings have been a topic of interest specifically due to their solubility in aqueous solutions. This could imply a reduced adverse immune response since the generated debris would dissolve. However, there are concerns regarding the dissolution rate and adhesion of these silicon nitride-based coatings. This study attempts to address the concern of dissolution rate as well as coating adhesion of silicon nitride coatings. We hypothesized that alloying with chromium and niobium would affect the adhesion, dissolution rate, and the resulting ion release and cell response to the coatings. A combinatorial approach was used to deposit sputtered coatings with compositional gradients both with and without a CrN interlayer. Compositional gradients were achieved for all the investigated elements: Si (38.6-46.9 at%), Nb (2.2-4.6 at%) and Cr (1.9-6.0 at%). However, while the presence of an interlayer reduced the delamination during adhesion testing, the differences in composition in the top coating did not affect the adhesion. Nor did the top coating's composition affect the surface roughness or the coatings' inherent mechanical properties (elastic modulus and hardness). All coating compositions were associated with a low Co release from the underlying metal and points with a higher Cr content (4.3-6.0 at%) gave an overall lower release of Si, Cr and Nb ions, possibly due to the formation of a stable oxide, which reduced the dissolution rate of the coating. Optimum chromium contents were furthermore found to give an enhanced in vitro fibroblast cell viability. In conclusion, the results indicate a possibility to tailor the ion release rate, which lends promise to further investigations such as tribocorrosive tests towards a future biomedical application.

Si-Fe-C-N Coatings for Biomedical Applications: A Combinatorial Approach.

Ceramic coatings may prolong the lifetime of joint implants. Certain ions and wear debris may however lead to negative biological effects. SiN-based materials may substantially reduce these effects, but still need optimization for the application. In this study, a combinatorial deposition method enabled an efficient evaluation of a range of Si-Fe-C-N coating compositions on the same sample. The results revealed compositional gradients of Si (26.0-33.9 at.%), Fe (9.6-20.9 at.%), C (8.2-13.9 at.%) and N (39.7-47.2 at.%), and low oxygen contaminations (0.3-0.6 at.%). The mechanical properties varied with a hardness (H) ranging between 13.7-17.3 GPa and an indentation modulus (M) between 190-212 GPa. Both H and M correlated with the Si (H and M increased as Si increased) and Fe (H and M decreased as Fe increased) content. A slightly columnar morphology was observed in cross-sections, as well as a surface roughness in the nm range. A cell study revealed adhering pre-osteogenic MC3T3 cells, with a morphology similar to that of cells seeded on a tissue culture plastic control. The investigated coatings could be considered for further investigation due to the ability to tune their mechanical properties while maintaining a smooth surface, together with their promising in vitro cell response.

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

Ceramic coatings deposited on orthopedic implants are an alternative to achieve and maintain high wear resistance of the metallic device, and simultaneously allow for a reduction in metal ion release. Silicon nitride based (SiNx) coatings deposited by high power impulse magnetron sputtering (HiPIMS) have shown potential for use in joint replacements, as a result of an improved chemical stability in combination with a good adhesion. This study investigated the effect of N, C, Cr, and Nb content on the tribocorrosive performance of 3.7 to 8.8 µm thick SiNx coatings deposited by HiPIMS onto CoCrMo discs. The coating composition was assessed from X-ray photoelectron spectroscopy and the surface roughness by vertical scanning interferometry. Hardness and Young's modulus were measured by nanoindentation and coating adhesion was investigated by scratch tests. Multidirectional wear tests against ultrahigh molecular weight polyethylene pins were performed for 2 million cycles in bovine serum solution (25%) at 37 °C, at an estimated contact pressure of 2.1 MPa. Coatings with a relatively low hardness tended to fail earlier in the wear test, due to chemical reactions and eventually dissolution, accelerated by the tribological contact. In fact, while no definite correlation could be observed between coating composition (N: 42.6-55.5 at %, C: 0-25.7 at %, Cr: 0 or 12.8 at %, and Nb: 0-24.5 at %) and wear performance, it was apparent that high-purity and/or -density coatings (i.e., low oxygen content and high nitrogen content) were desirable to prevent coating and/or counter surface wear or failure. Coatings deposited with a higher energy fulfilled the target profile in terms of low surface roughness (Ra < 20 nm), adequate adhesion (Lc2 > 30 N), chemical stability over time in the tribocorrosive environment, as well as low polymer wear, presenting potential for a future application in joint bearings.

Influence of Substrate Heating and Nitrogen Flow on the Composition, Morphological and Mechanical Properties of SiNx Coatings Aimed for Joint Replacements.

Silicon nitride (SiNx) coatings are promising for joint replacement applications due to their high wear resistance and biocompatibility. For such coatings, a higher nitrogen content, obtained through an increased nitrogen gas supply, has been found to be beneficial in terms of a decreased dissolution rate of the coatings. The substrate temperature has also been found to affect the composition as well as the microstructure of similar coatings. The aim of this study was to investigate the effect of the substrate temperature and nitrogen flow on the coating composition, microstructure and mechanical properties. SiNx coatings were deposited onto CoCrMo discs using reactive high power impulse magnetron sputtering. During deposition, the substrate temperatures were set to 200 °C, 350 °C or 430 °C, with nitrogen-to-argon flow ratios of 0.06, 0.17 or 0.30. Scanning and transmission electron spectroscopy revealed that the coatings were homogenous and amorphous. The coatings displayed a nitrogen content of 23-48 at.% (X-ray photoelectron spectroscopy). The surface roughness was similar to uncoated CoCrMo (p = 0.25) (vertical scanning interferometry). The hardness and Young's modulus, as determined from nanoindentation, scaled with the nitrogen content of the coatings, with the hardness ranging from 12 ± 1 GPa to 26 ± 2 GPa and the Young's moduli ranging from 173 ± 8 GPa to 293 ± 18 GPa, when the nitrogen content increased from 23% to 48%. The low surface roughness and high nano-hardness are promising for applications exposed to wear, such as joint implants.

Morphology and Dissolution Rate of Wear Debris from Silicon Nitride Coatings.

Silicon nitride (SiNx) coatings have recently been introduced as a potential material for joint implant bearing surfaces, but there is no data on wear debris morphology nor their dissolution rate, something that could play a central role to implant longevity. In this study, wear debris was generated in a ball-on-disc setup in simulated body fluid. After serum digestion the debris was analyzed with scanning electron microscopy and energy-dispersive X-ray spectroscopy. The particle dissolution rate was evaluated using inductively coupled plasma techniques, on model SiNx particles. The wear debris from SiNx coatings was found to be round, in the nm range and formed agglomerates in the submicrometer to micrometer range. Model particles dissolved in simulated body fluid at a rate of: c(t) = 39.45[1 - exp(-1.11 × 10-6t)], where [c(t)] = mg/L and [t] = s. This study can be used as a preliminary prediction of size, shape, and dissolution rate of wear debris from SiNx coatings.