Continued from page 17
Combining the results of the wear-scar diameter presented in Fig. 6 with the friction curves from Fig. 5, it is safe to conclude that Oil C, in spite of its low frictional resistance, performs poorly in terms of wear protection. The better performance of Oils A and B is in both cases due to the formation of tribofilms at the mating interface; the tribofilms act as a sacrificial layer, thus protecting the metallic surfaces from wear. While these films offer wear protection, they also cause an increase the frictional resistance. The MCR Tribometer’s ability to distinguish such minor differences in the friction and wear behaviour of the tribosystem makes it an indispensible tool for screening materials for their desired properties.
Figure 3. Coefficient of friction plotted as a function of rotational speed from the 1st Run of the Stribeck tests.
Wear Tests
Each of the wear tests was carried out with a fresh set of specimens. Fig. 5 shows the evolution of frictional resistance in the systems with increasing sliding distance. While Oil A does not show any significant change in the friction coefficient over the entire test duration, Oil B shows a slight decrease over the first half of the test, and a rather constant value in the second half. However, in the test with Oil C, the coefficient of friction drops from an initial value of ~ 0.13 to ~ 0.075 by the end of the third quarter of the test.
Conclusion Tribological tests were carried out on an MCR Tribometer to investigate the friction and wear performance of three engine oils with slightly varying composition. The results show a marked difference in the initial running-in processes with the three different oils. In addition, results from the 2nd
run reveal the
effect of the tribofilm formation on the mating interface. The results presented here clearly reflect the capability of the current test methodology to discern minute differences in the tribological behaviour of lubricated systems, especially in the static friction and ultra-low-speed regime. Typical applications for this methodology can be found in the automotive industry for testing engine oils, gear oils, special additives, etc.
Authors: 1 Dr. Kartik Pondicherry, 2 Dr. Frederik Wolf 1Anton Paar GmbH, Department Rheology, Tribometry, Graz, Austria
2Anton Paar Germany GmbH, Department Rheology, Tribometry, Ostfildern, Germany
LINK
www.anton-paar.com
Figure 5. Coefficient of friction plotted as a function of sliding distance for the wear tests.
Figure 6. Average wear-scar diameter recorded on the surface of the pins.
18
LUBE MAGAZINE NO.130 DECEMBER 2015
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