Mechanism of Friction and Wear in MoS2 and ZDDP/F-PTFE Greases under Spectrum Loading Conditions

Abstract

Two different greases formulated using MoS2 and a combination of ZDDP and functionalized PTFE (F-PTFE) were examined under spectrum loading conditions where loads, frequency, and duration of the steps were treated as variables. Combination of ZDDP and F-PTFE were synergistic resulting in a significant reduction in the wear and friction under spectrum loading condition. Decreasing the time step during the ramp up and ramp down cycles resulted in larger wear for the grease containing MoS2 particles in comparison to ZDDP/F-PTFE in grease. The tribofilm formed on the surface was analyzed using various characterization techniques like SEM, EDS, and Stereo Optical Microscopy. Tribofilms from MoS2 additives had extensive amounts of abrasive and adhesive wear and showed the formation of MoS2 on the surface on the other hand the tribofilms from ZDDP/F-PTFE had smaller amounts of severe wear and exhibited patchy tribofilms of Zn-phosphates as well as sulfides of Zn and Fe.

Wear and Friction of Greases Containing Organic and Inorganic Sulfur Carriers

“…Variation in load conditions in 'cyclic loading' tests is a good way to understand wear and friction response of additives. Effects of variation in load and sliding speed have been studied in previous Table 1 Formulations of blend for sulfurized and non-sulfurized additives all in lithium complex NLGI#2 base grease [16,17], on microscopic MoS2 where it was seen that the load cycles and the absolute load applied on the interacting surfaces affect the wear & friction performance of the chemistries, while sliding speed had minimal effect on the tribological performance. A Four Ball Tribometer from Plint Tribology (TE 92) was used to conduct the wear tests.…”

“…Figures (3-6) show the SEM images for all samples that were subjected to both test protocols. Previous studies [1,9,10,16,17,30] that characterize wear surface and wear mechanisms form the basis for the current work to distinguish between different wear mechanisms such as abrasive wear, polishing wear and adhesive wear. Higher magnification regions were focused on region of interest (ROI) that revealed presence of tribofilm on the wear surface so as to give an insight into the wear mechanism prevalent during tribometer tests.…”

“…The appropriate treat rate employed in the formulation was about 3 mass% for the EP properties to be effective. Treat rates higher or lower than 3 mass% in greases couldn't achieve optimal performance [1,10,16,17].…”

“…Earlier studies have shown that spectrum loading under varying conditions of load, speed or temperature could be beneficial to evaluate tribological performance of the greases [1,16,17]. A study by Suresh et al [10] demonstrated synergism of sulfur based additives when used in conjunction with ZDDP and functionalized-PTFE and its contribution in improving the antiwear and extreme pressure properties of grease.…”

“…A study by Suresh et al [10] demonstrated synergism of sulfur based additives when used in conjunction with ZDDP and functionalized-PTFE and its contribution in improving the antiwear and extreme pressure properties of grease. Bagi et al [1,16,17] showed that variation in loading conditions plays a major role in determining wear and friction performance of EP and antiwear additives. This study is a continuation of previous efforts to understand effects of test parameters on wear and friction behavior of grease blends formulated with different sulfur components, comprising of MoS2, MoDTC, WS2 and sulfur free organo-tungstate additives.…”