A tribometer, especially designed to simulate the high contact stresses, reciprocating sliding, and plowing conditions observed for the non-conformal mating wear couple of a total knee prosthesis is described. This tribometer was used to evaluate the tribological behavior of Co-Cr, Ti-6Al-4V, yttria-stabilized tetragonal zirconia, and monoclinic zirconia-coated zirconium alloy pins in cyclic sliding contact with UHMWPE.
The tribometer described employs a right cylinder on flat geometry. The metallic and ceramic pins were in the form of cylinders which were 9 mm in diameter and 19 mm long. The cylinders were wear tested in reciprocating sliding contact (15 mm stroke length, 2.1 Hz) with 12.7 mm wide UHMWPE strips. The wear tests were conducted in deionized water for two million cycles. The wear of the UHMWPE strips and cylinders was characterized using an appropriate combination of weight and thickness change measurements, optical stereomicroscopy, scanning electron microscopy (SEM), and measurement of UHMWPE debris and metal ion concentrations in the deionized water solutions.
The lowest friction coefficients (0.02 – 0.04) occurred during the run-in period for all materials tested. The observed increases in COF values coincided with the formation of UHMWPE transfer layers and abrasion of the cylinder surface. Transfer layer formation and an increase in the COF, as well as cylinder abrasion and the resultant increase in surface roughness, correlated with an increase in wear of the UHMWPE strips and an increase in the concentration of UHMWPE wear debris and metal ion levels in the deionized water solutions. The results of this series of wear tests show that this tribometer is capable of obtaining reliable and accurate dynamic friction data and that measured changes in the COF correlate well with observed changes in surface roughness and chemistry of the contact surfaces.