Material Characterization and Influence of Sliding Speed and Pressure on Friction and Wear Behavior of Self-Lubricating Bearing Materials for Hydropower Applications

Abstract

Nowadays, hydropower plants are forced to have more frequent power control and the self-lubricated bearings used in the applications are one of the most critical components affected by the continuously changing operating conditions. In this study, microstructure and composition of two commercially available bearing materials (Orkot TXM Marine and Thordon ThorPlas) used in hydropower turbines were studied. In addition, the influence of sliding speed and applied pressure on the friction and wear behavior of the materials was investigated systematically for dry sliding conditions. The bearing materials were characterized using X-ray microtomography, Nuclear Magnetic Resonance (NMR) spectroscopy and Inductively Coupled Plasma-Sector Field Mass Spectrometry (ICP-SFMS) techniques. Friction and wear tests were carried out with a polymer pin sliding against a stainless steel (SS2333) plate with a linear reciprocating motion. Test conditions were: room temperature, 9-28 MPa pressure and 10-40 mm/s sliding speed ranges. Surface analysis of the polymer pins and the wear tracks were performed by optical profilometry, Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) techniques. Test results show that, for both materials, the coefficient of friction (COF) is decreasing at higher pressures. Surface analysis reveals higher concentrations of solid lubricants in the transfer layers formed at higher pressures, explaining the decrease in COF. Furthermore, the specific wear rate coefficients are increasing at higher sliding speeds, especially at lower pressures. Results of this study demonstrate that, under dry sliding conditions, changes in sliding speed and pressure have a significant influence on the tribological behavior of these bearing materials.

A Novel Reciprocating Tribometer for Friction and Wear Measurements with High Contact Pressure and Large Area Contact Configurations

“…At low contact pressure (26 MPa), the deviation in average height reduction between repeat tests is only 1.6%. The deviation increases at higher contact pressures with a maximum deviation of 9.6%, which is within the range of what has previously been reported for the thermoset material [15,18,19]. Based on the height reduction curves at different contact pressures (Figure 11), it can be stated that the repeatability of the wear measurements for the test rig is good.…”

“…It is clear from the enveloping curves that the long-term repeatability of friction measurements for the test rig is good at all contact pressures. Both the mean and maximum coefficients of friction have a maximum deviation of 5% between repeat tests, which is within the range shown previously for the thermoset material [15][16][17][18][19].…”

“…For the study regarding the influence of contact pressure, For the tribological tests, pin specimens with dimensions of 18 × 18 × 10 mm 3 (l × w × h) were used, machined from the two bearing materials. This gives a 20 times larger nominal contact area in comparison to previous studies by the authors [8,15] where pins with dimensions of 4 × 4 × 4 mm 3 (l × w × h) were used, illustrated in Figure 7a. In order to minimize the edge effect and to remove fibers that protrude from the surface and machining marks, the edges of the load-carrying side of the polymer pins were manually ground using SiC abrasive paper of grit size #600.…”

“…The thermoplastic material (Thordon ThorPlas Blue) (Thordon Bearings Inc., Burlington, ON, Canada) is a homogeneous polymer blend with incorporated additives and solid lubricants such as PTFE (Figure 7b). The characteristics and a more detailed analysis of the composition and microstructure of the materials are presented in [15].…”

“…Based on the hardness measurements reported for the thermoset material [19], a standard deviation of more than 10% can be seen between the measurements. This is attributed to the structural heterogeneity of the thermoset with nonuniform spacing between the woven fibers and larger pores situated on different locations in the material revealed by tomographic reconstruction of the material at different magnifications [15]. This is one of the explanations for the deviation in wear measurements of the thermoset between the repeat tests.…”