Design and construction of a microtribometer and its use in the study of the micro/nanotribological behavior of ultra-high molecular weight polyethylene (UHMWPE)

The effect of surface roughness of ultra-high molecular weight polyethylene (UHMWPE) on the friction behavior and onset of wear in a UHMWPE/silicon nitride interface in both dry ambient and bovine serum environments is explored. A ball-on-flat microtribometer using a silicon nitride ball was constructed in order to examine tribological properties of multi-asperity contacts on the microscale. Performance specifications and construction details are given. The results from the tribometer are compared with topography and nanoscale single asperity friction data obtained using an atomic force microscope (AFM). Friction was found to increase with a decrease in surface roughness of the UHMWPE sample in air, which is due to an increase in real area of contact. This trend was seen to disappear or even reverse in serum. This effect is due to an increase in the interfacial shear stress of the UHMWPE surface when exposed to the serum. This increase is believed to be caused by an adhered layer of protein on the UHMWPE surface. Wear was observed to decrease with a decrease in surface roughness, and due to the serum environment. A non-linear relationship between wear factor and surface roughness of the UHMWPE itself was proposed which would reduce the importance of roughness on wear as the surface becomes smoother. This study shows that using a microtribometer and AFM in combination to study the friction and wear behavior of an interface can yield definite information on contributions to the tribological behavior from real area of contact and interfacial shear stress