Improving fretting corrosion resistance of CoCrMo alloy with TiSiN and ZrN coatings for orthopedic applications

Abstract

Total hip replacement is the most effective treatment for late stage osteoarthritis. However, adverse local tissue reactions (ALTRs) have been observed in patients with modular total hip implants. Although the detailed mechanisms of ALTRs are still unknown, fretting corrosion and the associated metal ion release from the CoCrMo femoral head at the modular junction has been reported to be a major factor. The purpose of this study is to increase the fretting corrosion resistance of the CoCrMo alloy and the associated metal ion release by applying hard coatings to the surface. Cathodic arc evaporation technique (arc-PVD) was used to deposit TiSiN and ZrN hard coatings on CoCrMo substrates. The morphology, chemical composition, crystal structures and residual stress of the coatings were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffractometry. Hardness, elastic modulus, and adhesion of the coatings were measured by nano-indentation, nano-scratch test, and the Rockwell C test. Fretting corrosion resistance tests of coated and uncoated CoCrMo discs against Ti6Al4V spheres were conducted on a four-station fretting testing machine in simulated body fluid at 1Hz for 1 million cycles. Post-fretting samples were analyzed for morphological changes, volume loss and metal ion release. Our analyses showed better surface finish and lower residual stress for ZrN coating, but higher hardness and better scratch resistance for TiSiN coating. Fretting results demonstrated substantial improvement in fretting corrosion resistance of CoCrMo with both coatings. ZrN and TiSiN decreased fretting volume loss by more than 10 times and 1000 times, respectively. Both coatings showed close to 90% decrease of Co ion release during fretting corrosion tests. Our results suggest that hard coating deposition on CoCrMo alloy can significantly improve its fretting corrosion resistance and could thus potentially alleviate ALTRs in metal hip implants.

Introduction

Osteoarthritis is one of the most prevailing chronic diseases in the current world. It is a joint disease affecting more than 250 million people worldwide and the number is expected to increase over the next few decades (Hunter et al., 2014). The most effective treatment for late stage osteoarthritis would be total hip replacement. It is reported that more than 1 million total hip replacements are performed annually worldwide (Pivec et al., 2012). Current artificial hip implants typically consist of a titanium femoral stem press-fitted into a CoCrMo hemispherical femoral head that articulates with a titanium acetabular cup with a highly crosslinked polyethylene liner at the bearing surface, which is referred to as metal-on-polyethylene (MoP) implants (Eltit et al., 2019). Although hip arthroplasty is generally a successful procedure, a significant number of patients have been observed with adverse local tissue reactions (ALTRs) to the implants, which are inflammatory lesions that destroy the soft tissues of the hip joint, affecting the prognosis of further clinical solutions (Whitehouse et al., 2015; Picardo et al., 2011; Matharu et al., 2016; Eltit et al., 2017). Since MoP is the most commonly used hip implant system today (National Joint Registry for England, 2018; >Canadian Institute for Health Information, 2018), there is an urgent need to address this issue.

Although the exact mechanism of ALTRs is still unclear, it has been widely attributed to the metal species released from the CoCrMo implant as a result of wear and corrosion at the modular junction where the trunnion of the Ti stem is fitted into the bore of the CoCrMo head (Cooper et al., 2013; Kop and Swarts, 2009; Collier et al., 1992; Mathiesen et al., 1991). This type of wear and corrosion at the head-neck interface has been given the specific term “trunnionosis” (Pastides et al., 2013; Berstock et al., 2018; Shulman et al., 2015). Many studies have identified fretting corrosion to be the main degradation mechanism and concern at the modular junction (Gilbert et al., 1993, 1994; Hallab and Jacobs, 2003; Geringer et al., 2005). Fretting corrosion is a type of corrosion induced by cyclic micro-abrasion between two contacting surfaces. This would disrupt the formation of passive oxide films on implant surfaces thereby leading to accelerated release of metal ions and metal oxide particles (Hallab and Jacobs, 2003; Swaminathan and Gilbert, 2012). The reformation of oxide layers would result in local oxygen depletion and form a more acidic environment which could further induce crevice and pitting corrosion (Brown et al., 1995; Panigrahi et al., 2014). Therefore, protection on the modular junction surfaces to reduce fretting corrosion should potentially prevent ALTRs.

Researchers have been studying the application of various protective layers to alleviate wear and corrosion on orthopedic implants in order to increase their lifespan since the 1990s (Sella et al., 1991; Günzel et al., 1999). Nitride coatings including silicon nitrides (Pettersson et al., 2013; Filho et al., 2020; Skjoldebrand et al., 2017) and superlattice coatings, which are coatings with multiple alternating nitride layers in nanometer scale (Garza-Maldonado et al., 2017; Hovsepian et al., 2016; Gallegos-Cantu et al., 2015), are studied most extensively for biomedical applications. An effective coating must be uniform, corrosion resistant and bonded strongly to its substrate. Most importantly, for biomedical application, the coatings need to be biocompatible. Transition metal nitride coatings have been widely studied because of their excellent biocompatibility, corrosion resistance, high hardness and low friction coefficient (Braic et al., 2012; Pohrelyuk et al., 2013; Subramanian et al., 2011; Lohberger et al., 2020; Ragone et al., 2019; Doring et al., 2019; Dinu et al., 2020). Hendry and Pilliar (2001) previously utilized nitride coatings to specifically study fretting corrosion prevention on Ti6Al4V alloy. Better fretting corrosion resistance with TiN and ZrN coatings by PVD was demonstrated in their work. However, CoCrMo alloy rather than Ti6Al4V has been linked to the development of ALTRs in recent clinical studies (Eltit et al., 2017; Cooper et al., 2012, 2013). Although several studies have been undertaken on fretting corrosion behavior of CoCrMo alloys (Swaminathan and Gilbert, 2012; Ocran et al., 2015; Sun et al., 2009), limited studies have yet to explore the strategy of depositing hard coatings on CoCrMo. Therefore, a comprehensive fretting corrosion analysis of coatings on CoCrMo alloy is needed.

In this study, ZrN and TiSiN coatings were chosen to improve fretting corrosion resistance of CoCrMo alloy. ZrN was selected due to its distinguished chemical and physical properties among different transition metal nitrides (TiN, CrN, HfN, TaN, etc) such as high chemical, thermal stability (Wu et al., 1997) and low electrical resistivity (Wang et al., 1995). It has also been proven to be effective in improving wear and corrosion resistance of magnesium alloys (Xin et al., 2009) and stainless steels (Chou et al., 2003). Hubler et al. even observed greater wear and corrosion resistant enhancement of ZrN compared with TiN on 316L femoral implant alloy (Hübler et al., 2001). For TiN coatings, the latest development has been the incorporation of other elements such as B, C or Si to further improve mechanical properties. These nanocomposite coatings consist of a nano-crystalline phase TiN embedded in an amorphous B, C or Si nitride matrix. These coatings possess exceptionally high hardness, usually in the range of 40–50 GPa (Karvankova et al., 2006; Ma et al., 2005). TiSiN coating is of particular interest in biomedical applications due to its high hardness, elastic modulus, low friction coefficient and high wear and corrosion resistance (Diserens et al., 1998; Cheng et al., 2010; Balasubramanian et al., 2012). Nanocomposite coatings can be synthesized through various physical vapor deposition (PVD) techniques such as magnetron sputtering, cathodic arc evaporation, electron ion plating. Among these techniques, cathodic arc evaporation (arc-PVD) has received extensive attention due to its high ionization rate and high current density which lead to high deposition rate and good adhesion of coatings (Yang et al., 2007).

This study intends to fill in the gap in the literature by addressing the fretting corrosion issue that was commonly found in hip implants. As previously noted, limited studies have been reported on fretting corrosion testing and mechanistic analyses of CoCrMo with hard coatings. One objective of this study is to simulate fretting corrosion that might occur in-vivo at the trunnion-femoral head junction of MoP type implants and investigate the possible improvement in fretting corrosion resistance of CoCrMo alloy with either ZrN or TiSiN coatings. The fretting corrosion tests were conducted using Ti6Al4V against unmodified and arc-PVD modified CoCrMo alloy in simulated body fluid (SBF), followed by comprehensive material and mechanistic analyses. To the authors’ knowledge, this is one of the first studies in a laboratory setting to recreate similar fretting corrosion results as found in retrieval studies (Cooper et al., 2013). More importantly, this study proposes and demonstrates the TiSiN nanocomposite coating on the CoCrMo alloy as an effective way of reducing Co ion release caused by fretting corrosion in modular total hip implants.