Spectrum loading in milled MoS2 greases using four ball wear test

Abstract

The relationship between the milled and unmilled MoS2 (molybdenum disulfide) greases to its tribological properties such as coefficient of friction, wear and chemical-mechanical properties of tribofilms is examined for constant extreme pressure loading and spectrum loading, where load is treated as variable, that simulates actual conditions under 1200 and 600 rpm rotational speeds using four ball test with chromium steel ball bearing quality and aircraft grade E52100. In this study, A series of tests were conducted for all loading conditions after heating the grease to 75°C and applying the desired load and speed via Plint four-ball wear tester (Model number TE92) software where a program is written for each condition to adjust for the speed and loading. Design of Experiments (DOE) approach was used to analyze the different loadings and speeds at a specific duration of 36000 revolutions to examine the lithium base grease wear behavior with milled and unmilled molybdenum disulfide (MoS2) powder. Results indicated that ball milled MoS2 grease tests showed significant improvement in the wear behavior and friction coefficient under all conditions especially spectrum or actual loading. Unmilled MoS2 powder exhibited worse wear outcomes than the milled one. SEM (scanning electron microscopy) with EDS (energy dispersive spectroscopy) and AES analysis of the actual loading tribofilms indicate that a protective tribofilm is formed on the wear surface of the milled powder grease initially at lower loads in the ramp-up tests that protects the wear surface from wear and abrasion at higher loads.