Title

Micron-scale wear mechanisms in ultra high molecular weight polyethylene

Date of Award

2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Jeremy L. Gilbert

Keywords

UHMWPE, Wear, Strain analysis, Surface mechanics, Polyethylene

Subject Categories

Biomedical Engineering and Bioengineering | Engineering

Abstract

Wear of ultra high molecular weight polyethylene (UHMWPE) is a limiting factor in the longevity of joint replacements. Therefore there is a desire to both create new materials and enhance processing conditions associated with current materials to reduce wear. This requires an understanding of the effects of processing on the performance of implants as well as an in-depth understanding of the micron-scale wear mechanisms ongoing. The ultimate goal is to generate predictive methodologies of material success, in vivo . Two methodologies are of interest: experimental modeling of wear behavior, and creation of rapid mechanical assays to determine wear resistance. This work develops quantitative understanding of the interplay between microstructure, wear-induced deformation and local mechanical property evolution, information necessary for the theoretical modeling of UHMWPE wear behavior by exploring the processing-structure-properties relationship (PSP) of four UHMWPE types with variations in crosslinking, molecular weight, and crystal size. This thesis also presents idealized single asperity-based wear tests that provide fundamental methods to study the nano to micron scale wear behavior.

First, a surface strain analysis technique is developed to measure permanent strain due to asperity scratch deformations. The methodology is then expanded to three dimensions and permanent strain is correlated with local surface mechanical properties obtained by microindentation. Second, assessments of structural aspects, fracture mechanics, characteristic wear feature sizes, and lamellar alignment due to strain level are quantified across four UHMWPE types. These results help to understand and develop the PSP relationships. Finally, a novel adhesive/abrasive wear simulation is developed to study plasticity-induced adhesive wear mechanisms, and to serve as a predictive measure of material success.

Residual surface strains were seen to increase with asperity load and were dependent on UHMWPE processing, with a highly crystalline form exhibiting the most plastic strain and a crosslinked form exhibiting the least. Local surface mechanical properties across the asperity region were seen to correlate inversely with the strain, and level of change was material dependent. Strains were also shown to induce lamellar alignment in indentations and asperity scratches, influenced by characteristic lamellar size and deformation mechanism. Finally, adhesive wear mechanisms, including fibril formation and extent of adhesive wear, were seen to vary across material type.

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