Viscosity Loss in PFPE Lubricant for Space Applications under EHL Conditions

Abstract

An unbranched perfluoropolyether (PFPE) 815Z is the current ball bearing lubricant for space applications. Measurements of elastohydrodynamic lubrication (EHL) oil film thickness have been carried out to assess the lubricating performance of PFPE with average molecular weight of 9200 using an optical interferometric technique under mean Hertzian pressure 0.45 GPa. The film thickness of 815Z became less than predicted film thickness from Hamrock and Dowson formula for EHL central film thickness. There are two main explanations why PFPE is inferior to mineral oil in their ability to form EHL films, temporary viscosity loss and permanent viscosity loss. In order to elucidate the results, measurements of permanent viscosity loss under mean Hertzian pressure from 0.41 GPa to 2.67 GPa have been carried out using the thrust ball bearing. There results show that the degree of the permanent viscosity loss depends on Hertzian pressure, occurrence of permanent viscosity loss of 36 % with 2.67 GPa and 2 % with 0.41 GPa.

The state of art on lubrication methods in space environment

“…PFPE B and SHF exhibited the high temperature dependence of viscosity index, while PFPE L showed the stability of viscosity index for larger temperature range. The viscosity loss of PFPE based lubricant 815 Z was investigated by Mia et al [36] under mean Hertzian contact pressure from 0.36 to 2.67 GPa, the permanent and temporary viscosity losses were reported. The permanent viscosity loss occurred rapidly with time and with increase in pressure beyond 1.95 GPa, which could be ascribed to decomposition of 815Z into (-OCF 2 O-) acetal group present in lubricant, at high pressure, adding to it, viscosity loss increases with change of phase from liquid to elastic-plastic solid phase.…”

Effects of SiO2 Contaminant on Thermo-Mechanical/Chemical Properties and Lubricity of PFPE Lubricants

“…Its design adaptability enables it to be applied for other applications, including automotive applications, metal cutting, space applications, oil and gas industries, electronic industries, sheet metal forming industries, etc. [18][19][20][21][22][23][24][25]. A single PFPE lubricant chain is composed of backbone and functional end groups.…”