Effect of Test Parameters on the 4-Ball Wear and Weld Performance of Greases with MoS2 and without MoS2 as EP Additives

摘要

Molybdenum Disulfide (MoS2) based greases have been in use for a very long time and have found application extensively in extreme pressure situations. The lamellar structure of MoS2 provides very good weld protection by forming a layer that can be easily sheared under the application of extreme pressures. However, it has been found that at lower loads the MoS2 sometime behaves as an abrasive and results in enhanced wear. The 4 ball wear test has been used extensively to examine the wear behavior of greases and an ASTM standard ASTM 2266 has been developed to predict the wear behavior of greases. The standard uses one specific experimental condition where the test is conducted using a 4 ball method wherein the load is kept constant at 40 Kg and the grease is heated to 75°C and the test is conducted at a speed of 1200 rpm for a period of 1 hour and the wear scar is measured at the end of this period. While this method is very useful to compare several greases tested under fixed test conditions it is not always representative of conditions in the field where different speeds, duration and loads are applied. In this study we have attempted to use a design of experiments approach to systematically vary the test conditions such as applied load, duration of the test and speed of the test to examine the outcome in the wear tests. We have used the DOE software to predict the extent of wear within these broad set of test conditions and determined the interaction between the test conditions and the extent of wear in both low and high MoS2 based grease. The results have indicated that wear performance is strongly dependent of the applied load and duration of the test with the higher loads and longer duration resulting in significantly higher levels of wear.^