Effect Of Additive Morphology & Chemistry On Wear & Friction Of Greases Under Spectrum Loading
Bagi, Sujay Dilip
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The primary focus of grease development is to minimize friction and improve on wear and load bearing capacity. Traditional method to evaluate wear behavior of grease is based on ASTM D2266 which evaluates the performance of grease at 75°C, 40Kg and 1200 rpm for 1 hour. However, actual applications may require bearings to be subjected to spectrum loading conditions where-in rotations per minute (rpm), load and duration of test are variables. A series of six tests were conducted for all the grease blends; load was treated as the variable in four of the tests while maintaining the frequency constant and in the other two tests frequency was treated as the variable while keeping the load constant. Four different blends of greases were formulated using techfine or unmilled MoS2, milled MoS2, the third one consisting of a combination of PTFE & ZDDP and the fourth one consisting of a combination of PTFE, ZDDP and MoDTC, which were then tested under spectrum loading conditions where loads, frequency and duration of the tests were treated as variables. MoS2 has long been considered as an Extreme Pressure (EP) additive; however untreated MoS2 at lower loads behaves as pro-abrasive agent leading to an increase in the wear numbers. Milled MoS2 was primarily used to study the deteriorating role played by the sharp edges or corners of the MoS2. The third blendviwhich used a combination of PTFE (Polytetrafluoroethylene) & ZDDP (Zinc Dialkyldithiophosphate) was used as PTFE acts as a friction modifier and ZDDP as an anti-wear additive. The fourth blend which consisted of PTFE, ZDDP & MoDTC (Molybdenum Dithiocarbamate) was tested to study the effect of friction modifier on the combination of PTFE & ZDDP. With intent to replace the MoS2 based greases the combination of PTFE, ZDDP and the combination of PTFE, ZDDP & MoDTC were proven to be synergistic and reported very low wear and friction numbers. Four ball tribometer was used to carry out the wear tests. E52100 steel balls as specified by the ASTM D2266 standard were used. The role played by the test variables on wear and friction was examined; the tribofilm formed on the surface was analyzed using various characterization techniques like SEM, EDS and Stereo Optical Microscopy. This research is focused on understanding the interactions of various additive chemistries and milling conditions under the influence of load, temperature and rpm.Ball milling of MoS2 leads to significant changes in the morphology of the particles owing to which a significant improvement in the wear behavior and friction coefficient is seen. Functionalization of PTFE particles with electron beam irradiation makes the PTFE particles highly polar and increases the surface affinity of the particles leading to a better tribofilm resulting in significantly reduced wear. The addition of a friction modifier like MoDTC to a mixture of PTFE and ZDDP further reduces friction and wear due to its synergistic interaction with the additives.