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Coatings from diamond-like carbon (DLC) have been proven to be an excellent choice for wear reduction in many technical applications. However, for successful adaption to the total joint replacement field, layer performance, stability and adhesion in realistic physiological setups are quintessential and these aspects have not been consistently researched. In our team's efforts to develop long-term stable DLC implant coatings, test results gained from a simplified linear spinal simulator setup are presented. It is shown that metal-on-metal (MoM) pairs perform well up to 7 million loading cycles, after which they start to generate wear volumes in excess of 20 times those of DLC-coated implants. This is attributed to the roughening observed on unprotected metal surfaces. Furthermore, we illustrate that in contrast to DLC-on-DLC, MoM tribopairs require protein-containing media to establish low-friction conditions. Finally, results of defect monitoring during testing are presented, showing catastrophic failure of layers whose interfaces are too weak with respect to the stress-corrosion-cracking mechanism encountered in vivo., (Copyright 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Abstract: Diamond-like carbon (DLC) coatings are promising candidates for improving the mechanical wear and chemical resistance of load-bearing implants. However, in view of medical data showing severe problems due to film delamination, the application of DLC layers to the medicotechnical field is not yet recognized. In an extension to an earlier publication, in which we established that coating delamination in phosphate-buffered saline (PBS) solution is driven by stress–corrosion cracking, we present for the first time a detailed structural and chemical analysis of the interface region of DLC-coated CoCrMo. High-resolution and scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy shows that a 5nm thick carbide layer with an average Me:C stoichiometry of 2:1 is formed at this interface. It is shown that the failure of DLC-coated CoCrMo in PBS is due to the instability of Co carbides at the film–implant interface. [Copyright &y& Elsevier]

Abstract: Low friction and wear resistant coatings have a long history of successful applications in industry. It has long been hoped that these coatings, especially diamond-like carbon (DLC), could also be used successfully in load-bearing joint implants, extending implant life time considerably. However, despite several medical studies carried out so far, no regular DLC-coated implants are available on the market. In most cases, failure was due to insufficient long-term stability of the adhesion of such coatings on implants in vivo. That is because introducing a coated implant not only brings the coating into contact with the body environment but also the interface that controls the adhesion. This usually reactively formed interface must be considered to be at least one additional material which must be not only biocompatible, but also unsusceptible to corrosive attack. The aim of this paper is to analyze in detail the interface, i.e., the transition region between the substrate and the coating. This knowledge is necessary in order to find the right measures to ensure the long-term stability of the adhesion. Results for DLC coatings on a cobalt–chromium–molybdenum alloy are presented. It is shown that a very thin interface layer is formed, with the alloy on one side and the carbon film on the other side. This layer consists of a mixture of carbides from all the metals of the base material. This result is obtained by means of measuring depth profiles using X-ray photoelectron spectroscopy because these spectra yield not only the chemical composition of the interface but a detailed analysis provides information on the chemical states across the interface. [Copyright &y& Elsevier]