William R. Jones

A New Apparatus to Evaluate Lubricants for Space Applications: The Spiral Orbit Tribometer (SOT)

Lubricants used in space mechanisms must be thoroughly tested prior to their selection for critical applications. Traditionally, two types of tests have been used: accelerated and full-scale. Accelerated tests are rapid, economical, and provide useful information for gross screening of candidate lubricants. Although full-scale tests are more believable, because they mimic actual spacecraft conditions, they are expensive and time consuming. The spiral orbit tribometer compromises between the two extremes. It rapidly determines the rate of tribochemically induced lubricant consumption, which leads to finite test times, under realistic rolling/pivoting conditions that occur in angular contact bearings.

In-situ, On-demand Lubrication System for Space Mechanisms

Many of todays spaceraft have long mission lifetimes. Whatever the lubrication method selected, the initial lubricant charge is required to last the entire mission. Fluid lubricant losses are mainly due to evaporation, tribo-degradation, and oil creep out of the tribological regions. In the past, several techniques were developed to maintain the appropriate amount of oil in the system. They were based on oil reservoirs (cartridges, impregnated porous parts), barrier films, and labyrinth seals. Nevertheless, all these systems have had limited success or have not established a proven record for space missions. The system reported here provides to the ball-race contact fresh lubricant in-situ and on demand when the ball bearing is close to failure. The lubricant is stored in a porous cartridge attached to the inner or the outer ring of a ball bearing. The oil is released by heating the cartridge to eject oil, taking advantage of the greater thermal expansion of the oil compared to the porous network. The heating may be activated by torque increases that signal the depletion of oil in the contact. The low surface tension of the oil compared to the ball bearing material is utilized and the close proximity of the cartridge to the moving balls allows the lubricant to reach the ball-race contacts. This oil re-supply system avoided a mechanism failure, reduced torque to an acceptable level, and extended the life of the component.

The Effect of Stress and TiC Coated Balls on Lifetime of a Perfluoropolyalkylether Using a Vacuum Rolling Contact Tribometer

A vacuum spiral orbit tribometer (SOT) was used to determine the relative lifetimes of a branched perfluoropolyalkylether (PFPAE) on 440C stainless steel. The effect of varying the mean Hertzian stress (0.75, 1.0, 1.5 and 2.0 GPa) and the use of TiC coated balls on lubricant lifetime was studied. Other conditions included: ∼100 rpm, ∼50 μg of lubricant, an initial vacuum level of < 1.3 × 10−6 Pa (< 1.0 × 10−8 Torr), and room temperature (∼23 °C). Increasing the mean Hertzian stress from 0.75 to 2.0 GPa results in an exponential decrease in lubricant lifetime for both material combinations. However, substituting a TiC ball for the 440C ball quadrupled lifetime at low stress levels (0.75 and 1.0 GPa) and doubled life at higher stresses (1.5 and 2.0 GPa). The reduced reactivity of the TiC surface with the PFPAE lubricant is considered to be the reason for this enhancement. Decreasing lifetime with increasing stress levels correlated well with energy dissipation calculations. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 1–4, 2000

X‐ray photoelectron spectroscopy study of the stability of Fomblin Z25 on the native oxide of aluminum

Thin films of Fomblin Z25, a perfluoropolyalkylether lubricant, were vapor deposited onto clean, oxidized aluminum, and sapphire surfaces, and their behavior at different temperatures was studied using x‐ray photoelectron spectroscopy and temperature desorption spectroscopy (TDS). It was found that the interfacial fluid molecules decompose on the native oxide at room temperature, and continue to decompose at elevated temperatures, as previous studies had shown to occur on the clean metal. TDS indicated that different degradation mechanisms were operative for clean and oxidized aluminum. On sapphire substrates, no reaction was observed at room temperature. Our conclusion is that the native oxide of aluminum is neither passive nor protective towards Fomblin Z25. At higher temperatures (150 °C) degradation of the polymer on sapphire produced a debris layer at the interface with a chemical composition similar to the one formed on aluminum oxide. Rubbing a Fomblin film on a single crystal sapphire also induced...

Volatility and Wear Characteristics of a Variety of Liquid Lubricants for Space Applications

The vapor pressures and wear characteristics are critical properties for liquid lubricants to assure long-term reliability and performance in space applications. Vapor pressures, obtained using a Knudsen cell technique, and wear properties, obtained using a vacuum four-ball apparatus, were measured for a series of unformulated liquid lubricants. These included: two multiply alkylated cyclopentanes (MACs) (X-1000 and X-2000), two linear perfluoropolyalkylethers (PFPAEs) (Z-25 and 815Z), and four silahydrocarbons (a tri-, a tetra- and two pentas). Vapor pressures were measured at three elevated temperatures (423, 448 and 498K) and extrapolated to room temperature 298K. The lowest 298K vapor pressure of 5.7 × 10−10 Pa, was obtained with the PFPAE fluid (815Z) and the highest value with the low molecular weight MAC (X-1000) at 3.6 × 10−7 Pa. In addition, vacuum wear rates were determined for some of the lubricants. The lowest wear rates (approximately 3 × 10−11 mm3 lmm) were observed for three of the silahydrocarbons while the highest wear rates (approximately 2 × 10−9 mm3 lmm) were observed with the two PFPAE fluids (Z-25 and 815Z). The MAC (X-2000) yielded a wear rate of about 10−10 mm lmm. The results indicated that the silahydrocarbon class of liquid lubricants offers the better potential for space applications. Presented at the 55th Annual Meeting Nashville, Tennessee May 7–11, 2000

A Preliminary Study of Ester Oxidation on an Aluminum Surface Using Chemiluminescence

The oxidation characteristics of a pure ester (trimethyolpropane triheptanoate) were studied by using a chemiluminescence technique. Tests were run in a thin film microoxidation apparatus with an aluminum alloy catalyst. Conditions included a pure oxygen atmosphere and a temperature range of 176 to 206 C. Results indicated that oxidation of the ester (containing .001 M diphenylanthracene as an intensifier) was accompanied by emission of light. The maximum intensity of light emission was a function of the amount of ester, the concentration of intensifier, and the test temperature. The induction period, or the time to reach one-half of maximum intensity was inversely proportional to test temperature. Decreases in light emission at the later stages of a test were caused by depletion of the intensifier.

Determination of the Thermal Stability of Fluids by Tensimetry: Instrumentation and Procedure

A computerized tensimeter and experimental procedure for determination of the thermal decomposition temperature (Td) of perfluoroalkylethers have been developed and tested. Both the apparatus and the procedure are described in detail. Results of testing with bis (2-ethylhexyl) phthalate and trimethylolpropane triheptanoate demonstrate that the reciprocal of the decomposition temperature is a linear function of the logarithm of the gas volume/heated liquid volume ratio. The Td obtained for each compound at a gas volume/heated liquid volume ratio of one was similar to the value previously reported using an isoteniscope technique. Results of testing with a polymer of hexafluoropropylene oxide demonstrate that this instrument and procedure can be used to determine the Td of perfluoroalkylethers. Presented at the 35th STLE/ASME Tribology Conference in Fort Lauderdale, Florida October 16–19, 1989

Enhanced Performance of Pennzane® Greases for Space Applications by Both Additive Formulations and Smooth Hard Coatings

Tribological requirements of the moving mechanical assemblies (MMAs) of spacecraft are usually satisfied by a variety of lubricants and materials. When the lubricant elastohydrodynamic film is broken, metal-to-metal contact occurs in the MMAs. This may lead to lubricant overheating, and breakdown, and then to increased wear and failure. Wear related failure can also occur due to evaporation and/or creep of the lubricant over the lifetime of space assembly. As requirements for spacecraft performance and lifetime increase, improved lubrication systems for MMAs are needed. A considerable amount of progress has been made in developing improved lubricants with advanced additives; however, their performance has not been evaluated and ranked. In the present work, four-ball and reciprocating tribometer tests were conducted to evaluate and rank the performance of various Pennzane® based greases. Employing the reciprocating tribometer technique, Pennzane® based greases were also evaluated with hard coatings such as titanium nitride (TiN) and titanium carbonitride (TiCN) in a metal-on-coating configuration. Viability of a filtered cathodic arc technique for obtaining very smooth, hard coatings is demonstrated. The importance of coating deposition temperature for certain bearing steel materials is also discussed. It is demonstrated that wear is substantially reduced with an optimized Pennzane® grease formulation on a smooth, hard TiCN surface coating.

Thermal-Oxidative Pretreatment and Evaluation of Poly(hexafluoropropene Oxide) Fluids

Two commercial poly(hexafluoropropene oxide) fluids were thermally pretreated at 343 C in pure oxygen. IR and nuclear magnetic resonance(NMR) spectra indicate that this pretreatment was effective in removing hydrogen end-capped impurities. A decrease in the quantity of volatile material produced during thermal oxidative decomposition and increase in the thermal decomposition temperature indicated improvement in the stability of the fluids. However, this pretreatment failed to render the fluids completely stable in oxidizing atmospheres at 316 C in the presence of metal alloys.