Minimum quantity lubrication of M50/M50 and M50/Si3N4 tribological interfaces

Hybrid rolling element bearings are being considered for use in flight critical aerospace applications. The following work was undertaken to lay the foundation for future work that would permit the recommendation of appropriate quantities of lubrication for these advanced bearing systems. This was accomplished by focusing on the tribological performance of M50/M50 (or AISI M-50) and M50/Si3N4 interfaces with minimum quantities of lubrication at two levels of speed and slide-to-roll ratio (SRR). Ball-on-ring tribological testing was conducted with stepped reductions of lubricant pump rate into the tribological contacts at initial lambda values of approximately 1.7 and 2.5, as calculated from the Hamrock and Dowson film thickness equation and surface finish measurements. Experiments were conducted with 5% and 45% slide-to-roll ratio (or SRR). Starting from these high initial lambda levels, the lubricant flow rate into the contact was reduced until severe damage occurred. Tests were then suspended due to excessive traction (traction coefficient greater than 0.2). The results of this work are in agreement with the starved lubrication literature and the bearing test literature. They provide some solid insight into the minimum quantity of lubrication recommended for M50/M50 and M50/Si3N4 interfaces. In all of the tests conducted there was a clear trend that higher speed and increased sliding both require an increased supply of lubricant to the contact. The M50/M50 interface needed an order of magnitude higher lubricant pump rate into the contact for adequate lubrication as compared to the M50/Si3N4 interface in the initial lambda similar to 1.7 experiments. At lambda similar to 2.5 with 5% SRR, the M50/M50 interface needed three times the lubricant pump rate that the M50/Si3N4 interface needed. However, both M50/M50 and M50/Si3N4 needed a similar amount of lubricant at lambda similar to 2.5 with 45% SRR, in which the interfaces were severely damaged due to the extreme test conditions. These tribological test results are interpreted with an analysis of the wear morphology and surface chemistry of the test articles. (C) 2011 Elsevier B.V. All rights reserved.