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Research Papers: Friction and Wear

J. Tribol. 2017;139(5):051601-051601-13. doi:10.1115/1.4035344.

The nonlubricated sliding wear of SiC–B4C–Si cermets against a diamond indenter was studied. The cermets containing 2, 5, 10, and 20 wt.% of Si were fabricated by both conventional sintering and spark plasma sintering (SPS) techniques. It has been observed that wear depth, volume of the wear debris, and wear rate increases with increasing applied load for both cases. Minimum wear depth and lowest wear rate was obtained for the cermet containing 10 wt.% Si. Three-body abrasion is the main wear mechanism which results in surface delamination, and formation of grooves and cavities on the damaged surface.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051602-051602-10. doi:10.1115/1.4035340.

The effect of bifilm oxides on the dry sliding wear behavior of Fe-rich (1.5 wt.%) F332 Al–Si alloy under as-cast and T6 heat-treated conditions was investigated. Toward this end, the surface oxides were intentionally incorporated into the molten alloy by surface agitation. The results showed that, after sliding under the applied load of 75 N, due to the presence of bifilms, the wear rate of base (0.2 wt.% Fe) and 1.5 wt.% Fe-containing alloys increased by almost 22% and 14%, respectively. The results also indicated that, despite the positive effect on the hardness, T6 heat treatment adversely affected the wear resistance of alloys made under surface turbulence condition. This negative effect can be attributed to the expansion of bifilms which, during heat treatment, are converted to the potential sites for initiation and propagation of subsurface microcracks. However, the strengthening effect exerted by the thermally modified β-Al5FeSi platelets showed that it can compensate the negative effects of bifilm oxides because it improves the wear rate of 1.5 wt.% Fe-containing F332-T6 alloy by about 5% under the applied load of 75 N.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;139(5):051603-051603-5. doi:10.1115/1.4035341.

The static and dynamic friction properties of a steel pin on polyoxymethelyne homopolymer disk were studied at temperatures ranging from 22 to 160 °C. Samples were tested at externally applied normal loads ranging from 20 to 80 N. Under this range of temperatures, the friction coefficients displayed a linearly increasing dependence on the load. The load dependence is attributed to an enhanced contribution of the plowing friction mechanism at higher loads. As load increases, the pin asperities penetrate into the hard, injection mold-induced skin layer, causing an increase in the frictional plowing. The coefficient of friction was observed to decrease from 0.08 at 22 °C to 0.05 at 50 °C, and subsequently rise to 0.07 at 160 °C. The initial drop was caused by a decrease in the modulus of elasticity attributed to the rise in molecular mobility with increased available thermal energy. As the temperature increased to 160 °C, however, the further decrease in modulus allowed the penetration of the pin asperities to increase significantly, requiring increased material displacement to initiate frictional motion.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2017;139(5):054501-054501-5. doi:10.1115/1.4035156.

This study investigated the use of pressure sensitive paint (PSP) as new measuring technique for measuring the pressure distribution of a gas bearing. An externally pressurized circular thrust gas bearing with single gas supply hole was used as the test bearing to investigate the suitability of this technique. The test bearing was 30 mm in diameter, with a gas supply hole of diameter 0.7 mm. A coat of PtTFPP, the substance used as the PSP, was applied to the bearing surface using an air-assisted spray. The PSP luminescence characteristics were calibrated before the tests because of their dependency on temperature and pressure. The pressure distribution was obtained by averaging 50 images captured by a 12-bit complementary metal-oxide semiconductor (CMOS) camera. These experimental results were compared with the results of a numerical analysis based on the divergence formulation method. There was good agreement between the experimental and analytical results, thus demonstrating the effectiveness of using PSP for pressure distribution measurements.

Commentary by Dr. Valentin Fuster

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