Tribological characteristics of greases with and without metallo-organic friction-modifiers

An important focus of grease development is to minimize friction and wear while improving load bearing capacity. ASTM D2266 test method is commonly used to evaluate performance of grease at 75 degrees C, 40 kg and 1200 rpm for 1 hour. However, actual applications may require bearings to be subjected to cyclic loading and variable frequency conditions wherein rotations per minute (rpm), load and duration of test are variables. Five different blends of greases were formulated using ZDDP (3 wt.%), PTFE (2 wt.%), MoDTC (2 wt.%), combination of ZDDP/PTFE in a weight ratio of 3:2 and a combination of ZDDP/PTFE/MoDTC in 3:2:2 weight ratios. They were tested under ASTM D2266 test method as well as under cyclic loading and variable frequency conditions where loads, frequency and duration of the tests were treated as variables. It was found that the combination of ZDDP/PTFE/MoDTC results in significant improvement in the wear and friction under cyclic loading as well as ASTM D2266 test conditions. It was also demonstrated that MoDTC accelerated the tribochemical degradation of ZDDP that resulted in the formation of a protective tribofilm layer on the interacting surfaces. The analysis of the tribofilm formed indicated that when MoDTC was used together with ZDDP and PTFE, a combination of MoS2, phosphates and sulfates of Zn and Fe are formed whereas when only ZDDP and PTFE was used the tribofilms were largely composed of phosphates and sulfates of Zn and Fe.