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In this study the friction and wear behavior of medical grade ultra-high molecular weight polyethylene (UHMWPE) (GUR 1050 resin) were evaluated as a function of polymer crystallinity. Crystallinity was controlled by heating UHMWPE samples to a temperature above its melting point and varying the hold time and cooling rates. Degree of crystallinity of the samples was evaluated using differential scanning calorimetry (DSC). Quantitative friction experiments were conducted at two different scales. A custom-made microtribometer with commercially available spherical Si3N4 probes in dry conditions was used to test friction at the microscale. An atomic force microscope with commercially available Si3N4 probes under dry conditions was used for nanoscale experiments. A higher degree of crystallinity in the UHMWPE resulted in lower friction force and an increase in scratch resistance at both scales. Reciprocating wear tests preformed using the tribometer show that higher crystallinity also results in lower friction, as well as lower wear depth and width.

In this study, the friction and wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) were evaluated as a function of polymer crystallinity in the presence of the phospholipid dipalmitoyl phosphatidylcholine (DPPC) dissolved in ethanol. Samples of UHMWPE were separately heat treated to get high and low crystallinity samples. Degree of crystallinity was evaluated using differential scanning calorimetry. Quantitative friction and wear experiments were conducted using a custom-made microtribometer with commercially available spherical Si(3)N(4) probes in controlled and phospholipid-dissolved lubricants. The higher crystallinity sample exhibited slightly lower friction than the lower crystallinity in the control and decreased significantly when phospholipids were present. The higher crystallinity sample showed a higher wear resistance than the lower crystallinity sample during all reciprocating wear tests. DPPC acting as a lubricant had a marginal effect on the wear resistance of high crystallinity UHMWPE, whereas the low crystallinity sample became more prone to wear. Atomic force microscopy topography images and contact angle measurements of both samples before and after phospholipid exposure indicate that the higher crystallinity sample absorbed a greater density of DPPC. Increasing crystallinity is a way of escalating adsorption of surface active phospholipids onto UHMWPE to make it a more wear-resistant load-bearing material for total joint replacements.

The friction behavior of two different materials, mica and ultra-high molecular weight polyethylene (UHMWPE), was evaluated at the nanoscale with an atomic force microscope and with a custom-built ball-on-flat microtribometer at the microscale. The same counterface (Si 3 N 4 probe), environmental conditions (25°C, RH \ 10%), and similar load ranges were maintained for all experiments. The friction-force data obtained were analyzed for contact-area dependence. Friction force between silicon nitride and mica at the nanoscale showed initial non-linearity with normal load up to a certain load, beyond which surface damage was observed resulting in a linear dependence of friction force on normal load. At the microscale, the friction force of the mica-silicon nitride interface exhibited linear dependence on normal load. Friction force between silicon nitride and UHMWPE exhibited non-linearity with normal load at both the length scales, for the applied load ranges of our experiment. An appropriate contact mechanics theory was applied to calculate an interfacial shear strength value for the material pair at both the scales. The values at both the scales were similar, when the conditions were carefully maintained to be the same across scales.

In this work, we have compared the friction behavior of two different materials (a) mica and (b) ultra-high molecular weight polyethylene (UHMWPE) at two length scales. The friction experiments were carried out at the nanoscale with an atomic force microscope (AFM) and at the microscale, with a custom-built microtribometer. The material interface (Si3N4 probe) and the environmental conditions (RH < 10%) were kept the same at both the scales. The friction data obtained were analyzed for dependence on normal load or contact area, based on which, a coefficient of friction has been reported or an appropriate contact mechanics theory was applied and an interfacial shear strength value was calculated for the material pair. Friction between a silicon nitride and UHMWPE interface resulted in contact area dependence at both the length scales, for the applied load ranges of our experiment. Friction between silicon nitride and mica at the nanoscale showed an initial nonlinearity and then exhibited damage and linearity with normal load beyond certain loads. At the microscale, the mica-silicon nitride interface resulted in a linear friction behavior.

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In this study we evaluate the interfacial shear strength and scratch resistance of medical grade ultra-high molecular weight polyethylene (UHMWPE) (GUR 1050 resin) as a function of polymer crystallinity. Crystallinity was controlled by heating UHMWPE samples to a temperature above its melting point and varying the hold time and cooling rates. Degree of crystallinity of the samples was evaluated using differential scanning calorimetry (DSC). Quantitative nanoscale friction experiments were conducted using an atomic force microscope with commercially available Si 3 N 4 probes under dry conditions. A higher crystallinity resulted in lower friction force and lower interfacial shear strength as well as increased scratch resistance. The trend in friction response was observed in microscale friction measurements. EXPERIMENTAL DETAILS MaterialsCommercially available, ram extruded GUR 1050, rod-stock; medical grade UHMWPE (Poly Hi Solidur, Fort Wayne, Indiana) was cut into small square (30 mm) pieces. The melting Mater. Res. Soc. Symp. Proc. Vol. 977