Experimental and Computational Thermal Modeling of In Vitro Pin-on-Disk Tests of Ultra High Molecular Weight Polyethylene

Frictional heating occurring during pin-on-flat tribotesting of ultrahigh molecular weight polyethylene (UHMWPE) pins was measured and modeled. A full factorial experiment was conducted to determine if testing parameters can produce sufficient frictional heat to alter tribological properties of the bovine serum used as lubricant in the system. Temperature of the surrounding bovine serum was monitored during tribotests using varying pin sizes and sliding speeds to determine typical temperature rises due to frictional heating. This work examined two sliding speeds (40 mm/s and 80 mm/s) and two pin diameters (6.35 mm and 9.5 mm) at a single static load. Gravimetric analysis for wear determination and coefficient of friction measurement were performed for each test. Results showed that frictional heating increased the bulk temperature of the surrounding serum and correlated to sliding speed and average coefficient of friction. No correlation was seen at this temperature range between serum temperature rise and wear rate, providing evidence that the tested parameters are acceptable for tribotesting of UHMWPE. A computational model was developed to predict bulk serum temperature increase. This model closely predicted the temperature increase to within 2 °C, which is sufficient accuracy for identifying if bovine serum protein precipitation is likely during tribotesting. This work serves as an initial estimate and prediction for appropriate testing parameters based on lubricant responses to frictional heating.