Investigation of characteristic rippling topology produced during UHMWPE sliding
Ultra-high molecular weight polyethylene (UHMWPE) remains the sole polymer bearing material for use in orthopedic implants and its behavior has been studied extensively. Various techniques, such as crosslinking, have been employed to reduce the total wear volume. However, little is known about the actual wear mechanisms that contribute to the removal of debris from the bulk. A nearly universal observation of the wear surfaces after multidirectional sliding of UHMWPE is the presence of regularly-spaced parallel ripples whose wavelength closely matches debris size. In spite of the fact that this phenomenon has been observed for decades in both retrieved implants as well as wear simulation studies, the specific causes of the rippled topology have not yet been identified. Proposed mechanisms have included abrasion, crystallinity effects, fatigue, stick-slip behavior, thermal softening, and Schallamach waves. This investigation involved an experimental and computational factorial test design to determine the role of various parameters involved in the rippling phenomenon. The authors employed a film buckling theoretical model and applied it to UHMWPE using a finite element modeling approach. Wear testing was performed on a two-axis tribometer to expose the polymer to multi-directional sliding. Characteristic ripples developed on all wear surfaces, but with varying periodicity. Two regimes of the ripple phenomenon were observed: localized long wavelength (2.2–3.2 µm) ripples which appeared to dominate before steady state wear was established, and long-range shorter-wavelength (approximately 1 µm) ripples which indicated a steady-state condition. Surprisingly, feature wavelength showed little dependence on roughness or temperature but were slightly affected by contact pressure. The wear surfaces were cleaved and examined by SEM to gauge the thickness of the plastically deformed quasi-film layer. These layers were on the order of 10–20 nm thick. Further examination of the surfaces confirms the development of a deformed layer which acts similarly to a buckled thin film.
This is a manuscript of an article published as Plumlee, K. G., and C. J. Schwartz. "Investigation of characteristic rippling topology produced during UHMWPE sliding." Wear 426-427, Part A (2019): 171-179. DOI: 10.1016/j.wear.2019.01.049. Posted with permission.