Nanomechanical properties of energetically treated polyethylene surfaces
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L. Pruitt Department of Mechanical Engineering and Department of Bioengineering, University of California, Berkeley, California 94720
K. Komvopoulos Department of Mechanical Engineering, University of California, Berkeley, California 94720 (Received 31 May 2001; accepted 20 November 2001)
The effects of energetic treatments, crosslinking, and plasma modification on the surface mechanical properties and deformation behavior of ultrahigh molecular weight polyethylene (UHMWPE) were examined in light of nanoindentation experiments performed with a surface force microscope. Samples of UHMWPE were subjected to relatively high-dose gamma irradiation, oxygen ion implantation, and argon ion beam treatment. A range of crosslinking was achieved by varying the radiation dose. In addition, low-temperature plasma treatment with hexamethyldisiloxane/O2 and C3F6 was investigated for comparison. The surface mechanical properties of the treated UHMWPE samples are compared with those of untreated UHMWPE samples used as controls. Surface adhesion measurements obtained from the nanoindentation material responses are also discussed in terms of important treatment parameters. Results demonstrate that high-dose oxygen ion implantation, argon ion beam treatment, and low-temperature C3F6 plasma modification are effective treatments for enhancing the surface mechanical properties of UHMWPE.
I. INTRODUCTION
Recent advances in polymer materials processing and surface property characterization have been largely due to the increasing use of polymeric materials in various biological and medical applications. Despite significant improvements of the mechanical properties and surface chemical behavior of various polymers, several obstacles must be overcome before the use of these materials can be further extended or introduced in different technologies. For example, a significant limitation in total joint replacements is the small particles generated by articulation and the subsequent biological response to polymer wear debris. Recently, ionizing radiation of ultrahigh molecular weight polyethylene (UHMWPE) has been reported to improve the wear behavior observed in hip simulator studies.1,2 Ion implantation techniques have also been used to increase the wear resistance of polymers.3–5 An important consequence of wear reduction is the likelihood for increased life of the orthopedic component in vivo. The majority of the research in this area has been focused on the wear behavior of crosslinked UHMWPE in simulated gait analysis studies. However, J. Mater. Res., Vol. 17, No. 2, Feb 2002
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much less work has been done to identify the exact mechanism responsible for the enhanced tribological performance and associated surface properties. Recent investigations have demonstrated that both the morphology and the texture evolution in UHMWPE are strongly related to the wear performance.6,7 These studies suggest that it is the reduced modes of plasticity caused by crosslinking that increase the wear re
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