Duplex surface engineered CoCrMo orthopaedic alloy using commercial treatments : a corrosion-wear investigation

Abstract

The increase in global population and life expectancy are leading to a rise in the need for total hip arthroplasty (THA). While metal-on-metal (MoM) implants offer a cost-effective solution, the corrosion-wear prone environment in which these operate causes the release of metallic ions and debris, and ensuing complications. In an effort to improve the corrosion-wear performance of MoM implants, in this work a low-carbon wrought ASTM F1537 CoCrMo alloy and a low-carbon cast StelliteTM 21 alloy (similar to ASTM F75 CoCrMo alloy) were surface engineered using commercially available treatments, namely low temperature carburising, and a high-power impulse magnetron sputtered Cr2N coating. The aim of applying such treatments was to limit oxidation-related losses on CoCrMo alloys due to the cyclic formation and damage of the passive film (or Type I corrosion-wear), and suppress coatingsubstrate interface corrosion leading to coating blistering (or Type II corrosion-wear). The dense Cr2N coating was deposited to inhibit any electrolytic pathway to the underlying substrate, while the low temperature carburised diffusion layer was added to reduce the propensity to coating delamination and failure by enhancing the load support to the above Cr2N layer and avoid micro-cracking while further ensuring that any electrolyte passing through the coating does not cause any localised corrosion. A corrosion-wear evaluation of the untreated, carburised, Cr2N-coated, and duplex Cr2N-coated (Cr2N/Carburised) samples was conducted using a custom-built reciprocating sliding tribometer. A novel testing configuration was employed whereby both the disc and counterface materials (jointly referred to as a “tribopair”) were metallic and surface engineered to better replicate the MoM implant conditions. Self-mated tribopairs were made to slide against each other under open circuit potential (OCP) and anodic potential (AP) conditions in both Ringer’s and diluted bovine serum (DBS) solutions at a temperature of 37 ± 1 °C under elastic contact conditions (maximum contact pressure of 437 MPa). Whilst the disc material was always composed from a wrought alloy, the use of both cast and wrought counterface CoCrMo alloys were considered. The untreated Wrought-Cast (W-C) tribopairs exhibited a superior corrosion-wear performance with lower dynamic anodic currents, coefficient of friction (CoF), and total volume losses, compared to the untreated Wrought-Wrought (W-W) tribopairs. The improved corrosion-wear behaviour is being attributed to the in situ formation of an oxidised layer that suppresses oxidation losses on the W-C tribopairs. Testing in DBS under AP conditions resulted in a reduction of around 35% and 90% in Type I corrosion-wear (𝐶𝑊) for the untreated W-C and W-W tribopairs, respectively, when compared to tests conducted in Ringer’s solution. The reduction in 𝐶𝑊 is ascribed to the spontaneous formation of a protein-rich layer which diminishes anodic dissolution and lubricates the tribological interface. When compared to the untreated tribopairs, low temperature carburising did not offer a marked improvement in either W-C and W-W configurations and test solutions. Compared to the untreated W-C tribopairs, the carburised W-C tribopairs displayed higher dynamic anodic currents under AP conditions (~20 μA compared to ~3 μA in untreated W-C), and higher CoF and material losses under both electrochemical conditions, due to the inability of the latter to form an oxidised layer. Similar dynamic anodic currents, CoF and material losses were obtained between the untreated and carburised W-W tribopairs. In Ringer’s solution, the carburised tribopairs exhibited a higher metal ion release than the untreated tribopairs, while in DBS an opposite trend was noted. The Cr2N and Cr2N/Carburised-coated materials successfully mitigated Type I when compared to the untreated and carburised tribopairs, and were also resistant to Type II corrosion-wear. The inherent chemical inertness of the Cr2N coating resulted in extremely low dynamic anodic currents (~2 μA), low metal ion release and low CoF in both test electrolytes. In the absence of third-body particles originating from coating failure either on the disc or counterface, the corrosion-wear scars were characterised by smooth polishing wear free from wear debris. Nano-scratch results and corrosion-wear testing confirm that the underlying carburised layer offers an enhanced load-bearing support to the Cr2N coating with less Cr2N/Carburised samples exhibiting coating failure than the Cr2N samples. The observed coating failures on the counterfaces were attributed to delamination wear occurring upon severe coating thinning. The hard coating debris generated upon coating failure was found to trigger roughening of the otherwise smooth scars via three-body wear. Corrosion-wear testing of ultra-high molecular weight polyethylene sliding against wrought CoCrMo alloy in a metal-on-polymer (MoP) configuration resulted in low dynamic anodic currents, low CoF and negligible material losses. Such results suggest that surface engineering the metallic alloy in a MoP configuration may not impart any substantial benefits. However, this work has clearly shown that the addition of a carburising diffusion layer beneath a dense Cr2N coating layer is promising in MoM configurations.