Experimental and Numerical Modeling of Variable Friction Between Nanoregions in Conventional and Crosslinked UHMWPE

[+] Author and Article Information
Sunita P. Ho

Department of Preventive and Restorative Dental Sciences, University of California San Francisco, San Francisco, CA 94143

Paul F. Joseph

Department of Mechanical Engineering, Clemson University, Clemson, SC 29634

Michael J. Drews

School of Materials Science and Engineering, Clemson University, Clemson, SC 29634

Thomas Boland, Martine LaBerge

Department of Bioengineering, Clemson University, Clemson, SC 29634

J Biomech Eng 126(1), 111-119 (Mar 09, 2004) (9 pages) doi:10.1115/1.1645530 History: Received February 25, 2003; Revised October 14, 2003; Online March 09, 2004
Copyright © 2004 by ASME
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Schematic of Hertzian contact including friction variability at the contact
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Representative frictional load vs. normal load curves defining the onset of plastic deformation for uncrosslinked and crosslinked UHMWPE using the stiffer cantilever with 2.1 N/m as spring constant. Notice the normal load at which the sudden rise in friction (onset of plastic deformation) B was observed. A was the beginning of the test and C was the end of the test.
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3-D plot of normalized effective stress distribution illustrating stress concentrations when the counterpart traverses from: 3(a). Crystalline μ=0.20 to amorphous μ=0.35 nanoregions for uncrosslinked UHMWPE (maximum σeff=1.1), 3(b). Crystalline μc=0.20 to crosslinked μc=0.61 nanoregions for crosslinked UHMWPE (maximum σeff=2.8) within a tribological system. The variability in friction being sudden (x1=−1e−05,x2=1e−05,θ=[tan−12−μ1)/(x2−x1)]=89.9 deg) for crosslinked UHMWPE.
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2-D plot of normalized effective stresses on the surface of the substrate with a sudden friction variability (θ=89.9 deg) as the indenter moves from (a) Crystalline (μ=0.2) to an amorphous (μ=0.35) nanoregion within an uncrosslinked UHMWPE, maximum effective stress=1.1, (b). Crystalline (μ=0.2) to a crosslinked (μ=0.61) nanoregions within a crosslinked UHMWPE, maximum effective stress=2.8.
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2-D plot of normalized effective stress on surface of the substrate with friction variability from crystalline (μ=0.2) to crosslinked (μ=0.61) nanoregions. This plot illustrates a decrease in maximum effective stress as friction gradient changes from sudden (θ=89.9 deg) to gradual (θ=22 deg). One of the ways we could explain the higher wear resistance of crosslinked compared to uncrosslinked UHMWPE is that instead of a sudden friction variability, there could be gradual transition from μ=0.2 to μ=0.61. This implies that the interfacial or transitional zone is broadened between the two nanoregions as shown here. This not only minimizes the effective stresses but also eliminates stress concentrations there by increasing the wear resistance of the material.
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2-D plot of normalized effective stress on surface of the substrate with uniform friction of 0.2 and 0.6
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AFM scans of UHMWPE before and after testing showing ploughing wear track produced at the interface of the tribocontact due to loads placed on the A type, V-shaped cantilever with a nominal spring constant of 2.1 N/m




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