Advanced in-situ Measurements within Sliding Contacts

• In recent years the field of polymer tribology experienced a tremendous development leading to an increased demand for highly sophisticated in-situ measurement methods. Therefore, advanced measurement techniques were developed and established in this study. Innovative approaches based on dynamic thermocouple, resistive electrical conductivity, and confocal distance measurement methods were developed in order to in-situ characterize both the temperature at sliding interfaces and real contact area, and furthermore the thickness of transfer films. Although dynamic thermocouple and real contact area measurement techniques were already used in similar applications for metallic sliding pairs, comprehensive modifications were necessary to meet the specific demands and characteristics of polymers and composites since they have significantly different thermal conductivities and contact kinematics. By using tribologically optimized PEEK compounds as reference a new measurement and calculation model for the dynamic thermocouple method was set up. This method allows the determination of hot spot temperatures for PEEK compounds, and it was found that they can reach up to 1000 °C in case of short carbon fibers present in the polymer. With regard to the non-isotropic characteristics of the polymer compound, the contact situation between short carbon fibers and steel counterbody could be successfully monitored by applying a resistive measurement method for the real contact area determination. Temperature compensation approaches were investigated for the transfer film layer thickness determination, resulting in in-situ measurements with a resolution of ~0.1 μm. In addition to a successful implementation of the measurement systems, failure mechanism processes were clarified for the PEEK compound used. For the first time in polymer tribology the behavior of the most interesting system parameters could be monitored simultaneously under increasing load conditions. It showed an increasing friction coefficient, wear rate, transfer film layer thickness, and specimen overall temperature when frictional energy exceeded the thermal transport capabilities of the specimen. In contrast, the real contact area between short carbon fibers and steel decreased due to the separation effect caused by the transfer film layer. Since the sliding contact was more and more matrix dominated, the hot spot temperatures on the fibers dropped, too. The results of this failure mechanism investigation already demonstrate the opportunities which the new measurement techniques provide for a deeper understanding of tribological processes, enabling improvements in material composition and application design.

$Rev: 13581$