Evaluation Of Vapor Pressure And Ultra-High Vacuum Tribological Properties Of Ionic Liquids (2) Mixtures and Additives

Ionic liquids are salts, many of which are typically viscous fluids at room temperature. The fluids are characterized by negligible vapor pressures under ambient conditions. These properties have led us to study the effectiveness of ionic liquids containing both organic cations and anions for use as space lubricants. In the previous paper we have measured the vapor pressure and some tribological properties of two distinct ionic liquids under simulated space conditions. In this paper we will present vapor pressure measurements for two new ionic liquids and friction coefficient data for boundary lubrication conditions in a spiral orbit tribometer using stainless steel tribocouples. In addition we present the first tribological data on mixed ionic liquids and an ionic liquid additive. Post mortem infrared and Raman analysis of the balls and races indicates the major degradation pathway for these two organic ionic liquids is similar to those of other carbon based lubricants, i.e. deterioration of the organic structure into amorphous graphitic carbon. The coefficients of friction and lifetimes of these lubricants are comparable to or exceed these properties for several commonly used space oils. INTRODUCTION The first Apollo missions including a vehicle (lunar roving vehicle, LRV) to transport the astronauts (Apollo 15 in 1971 and Apollo 17 in 1972) had specifications for operation in an environment of +120C for as much as 2 years. In order to expedite the delivery schedule, these requirements were eventually reduced to operation in 0-100C for only a few hours. In order to provide adequate lubrication to the wheel drive unit (traction drive assembly) with lubricants available at the time, the units were sealed with a nitrogen atmosphere at approximately 1⁄2 atm. (5x10 Pa) pressure. Two separate reports list the lubricant for the traction drive assembly as KrytoxTM 143AZ oil [1] or KrytoxTM 143AC oil [2], both of which are oils used for space lubrication. Other lubricating materials included KrytoxTM 240AC grease in the traction drive and steering motors as well as silicone base oil in the suspension damper. Dry lubrication was also employed. In 2004, President George W. Bush set forth a Vision for Space Exploration Program in which we would return to the moon. The specifications for lubrication of rovers and other moving mechanical assemblies include a five year life time during which the lubricant may have to endure the temperature extremes of the lunar diurnal cycle (-170 to 110C at equator). Current liquid lubricants do not have attractive vapor pressures at the peak temperatures and become too viscous at the lower temperature extremes making them undesirable. Furthermore, future missions will involve longer lunar traverses than the few miles driven during Apollo 15 and Apollo 17 [3] in what is now known to be an abrasive environment created by the fine, electrically charged or magnetic particulate lunar regolith. Planned exploration may include traverses into totally shaded craters where ice may reside, imposing a lubricant requirement of operation at a temperature of -233C. For these reasons we have undertaken a search for more suitable base oils to be used for liquid lubricants on the moon 1 https://ntrs.nasa.gov/search.jsp?R=20090013712 2017-09-14T01:25:46+00:00Z