Nano-scale tribocorrosion of CoCrMo biomedical alloys

Abstract

Tribocorrosion plays a significant role in the performance and failure of implantable metallic devices. The damage caused by tribocorrosion has been reported previously in several implantable devices such as disk replacements, bone plates, the interface between the fixation cement and the metallic stem in cemented hip arthroplasties and also in the taper-trunnion contact area of hip replacements. The origin of the tribocorrosion processes is produced at the nanoscale as a result of the micromotion between hard single asperities and the metallic components at their contacting interfaces. Hard particles cause deformation and wear of the surfaces and depassivation, opening new metallic areas to corrosive attack. The combination of both processes produces the liberation of metallic ions and metal debris responsible for adverse reactions within the body, causing pain and the need for implant revision.

Significant research has been undertaken to understand the wear-corrosion mechanisms at the macro and micro scale. The focus has been on different experimental conditions and the effects of the environment (protein-phosphate contents or pH), the role played by the microstructure of CoCrMo alloys and the contact conditions or the wear mode (sliding, fretting, scratching). However, no work has been undertaken into the interaction of plastic deformation and corrosion mechanisms at the nano-scale. For a better understanding of the combined effects of the corrosion and the deformation processes, a new electrochemistry cell was designed in combination with the nanoindenter systemto simulate a single asperity in contact with a metallic surface. Four CoCrMo alloys with different manufacturing and thermal histories were analysed toobtain a deformation-corrosion map which summarizes their scratch-corrosion performance at the nano-scale.

The microstructure, chemical composition and mechanical properties of the Forged, As Cast (AC), As Cast thermal treated (AC-TT) and As Cast with low carbon content (AC-LC) Co based alloys were studied. Grain size, carbide morphology, carbon content and crystallographic phases present were analysed by metallographic preparation, Scanning Electron Microscopy (SEM) observations and Electron Backscattered Diffraction (EBSD) techniques respectively. Hardness (H) and Young’s Modulus (E) were calculated by indentation and nano-indentation techniques. Static corrosion behaviour of the four alloys immersed in 0.9 wt.% NaCl solution was studied using open circuit potential (OCP) and potentiodynamic (PD) polarization experiments to understand the corrosion mechanisms affecting the alloys without the interference of the plastic deformation and to estimate the minimum stabilization time required to reach a steady potential to be used in the nano-scratch corrosion experiments.

The deformation processes under fretting, reciprocating sliding and scratch experiments in dry conditions were also characterized by measuring tangential friction forces, coefficient of friction and plastic deformation values. Post experimental surface analysis was performed to analyse the oxide layer formation and the wear scar morphology.

The four CoCrMo alloys were tested under several loading conditions using the new electrochemistry cell performing single scratch-corrosion experiments. The results demonstrated that the crystallographic orientation of the grains produced characteristic deformation features. These features were directly linked to therepassivation times and current densities and were governing the deformation corrosion processes at the nano-scale and the liberation of metallic ions.

This work establishes a novel experimental technique that gives a better understanding of the deformation-corrosion processes occurring at the nano-scale in CoCrMo alloys. In addition the results obtained will be useful to help interpret the failure mechanisms observed in retrieved implants and improve the design and development of new materials and material pairing selection for future implants. Moreover the technique developed as part of this work can be extended not just to biomedical applications but also to any other applications in the materials science field where passivated metals are used in a corrosive environment and single asperity contacts act to deform metallic surfaces.

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Identifiers

ORCID for Richard Cook: orcid.org/0000-0002-2468-5820

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