The reflection of ultrasound can be used to determine oil film thickness in elastohydrodynamic lubricated (EHL) contacts if the opposing surfaces are fully separated by the liquid layer. The proportion of the wave amplitude reflected depends on the stiffness of the liquid layer, which is a function of its bulk modulus and thickness. However, in many practical applications, boundary or mixed film lubrication is a common occurrence as the nominal thickness of the separating film is of a similar order to the height of the surface asperities. The reflection is then dependent on both the liquid contact and solid contact parts and the total interfacial stiffness is the controlling parameter. In this paper an investigation was carried to study the reflection of ultrasonic waves from the lubricated contact between a sliding steel ball and a flat steel disc when substantial solid contact occurs. To interpret the ultrasonic reflection results, a mixed regime model for a circular point contact was established. The liquid film stiffness was calculated by using a predicted film thickness and a bulk modulus estimated from published rheological models of lubricants under high pressure. Solid contact stiffness was predicted using a statistical rough surface contact model. Under all operating conditions, the prediction of fluid stiffness was found to be much greater than the solid contact stiffness. The total stiffness predicted by the model showed good agreement with experimental measurements for kinematic cases. The model was used to separate the stiffness contributions from the asperity contact part and lubricant layer part from the experimental data. For contact pressures ranging from 0.42 to 0.84 GPa and sliding speed from zero to 2 m/s, the film thickness was found to vary from 0.01 to 0.8 μm, and the proportion of the load supported by asperity contact varied from 50% to 0%.

In this study, bearing bushes made of polyetheretherketone (PEEK), 30 wt % carbon fibre reinforced PEEK, 30 wt % glass fibre reinforced PEEK, each 10 wt % of PTFE, graphite and carbon fibre modified PEEK were investigated on a purpose built pin joint test rig. The unlubricated friction and wear behaviour was assessed in sliding contact with a 300M shaft, subjected to a nominal pressure of 93 MPa, articulating sliding speed of 45 °/s. The worn surface and the subsurface layer were studied using optical profilometry and scanning electron microscopy (SEM). Due to thermal sensitivity of PEEK composites, friction energy and temperature rise were analysed for determining the friction and wear mechanism. The bush made of PTFE, graphite and carbon fibre (each 10 wt %) modified PEEK presented the best performance for friction coefficient, wear loss, friction energy and temperature rise. Current work demonstrated that reinforcement modified PEEK composite possesses desirable properties to perform as a load bearing bush in certain tribological applications.