Stephen V. Pepper

Interfacial chemistry of a perfluoropolyether lubricant studied by X-ray photoelectron spectroscopy and temperature desorption spectroscopy

The interfacial chemistry of Fomblin Z25, a commercial perfluoropolyether used as lubricant for space applications, with different metallic surfaces—440 C steel, gold, and aluminum—was studied. Thin layers of Fomblin Z25 were evaporated onto the oxide‐free substrates and the interfacial chemistry studied using x‐ray photoelectron spectroscopy and temperature desorption spectroscopy. The reactions were induced by heating the substrate and by rubbing the substrate with a steel ball. Gold was found to be completely unreactive towards Fomblin at any temperature. Reaction at room temperature was observed only in the case of the aluminum substrate, the most reactive towards Fomblin Z25 of the substrates studied. It was necessary to heat the 440C steel substrate to 190 °C to induce decomposition of the fluid. The degradation of the fluid was indicated by the formation of a debris layer at the interface. This debris layer, composed of inorganic and organic reaction products, when completely formed, passivated the...

Study of copper on graphite with titanium or chromium bond layer

Improvement of copper to graphite adhesion by thin interfacial films of titanium and chromium was investigated. Graphite fibers and highly oriented pyrolytic graphite flats were sputter-coated first with 10 nm of titanium or chromium and then with 50 nm of copper. After annealing to 970 °C in argon/5%-hydrogen at atmospheric pressure for 5 min, copper without an interfacial bond layer agglomerated into nearly spherical particles, copper with the chromium bond layer agglomerated into particles with a contact angle less than 90°, indicating improvement in adhesion, and copper with the titanium bond layer exhibited a continuous metal film. In the latter case, most of the interfacial titanium was observed to have migrated into the copper and to the free surface, where the titanium reacted with contaminants in the annealing ambient.

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.

Spiral Orbit Tribometry — Part II: Evaluation of Three Liquid Lubricants in Vacuum

The coefficients of friction and relative degradation rates of three lubricants run in the boundary regime in vacuum are evaluated in a Spiral Orbit Tribometer. This tribometer subjected the lubricants to rolling contact conditions similar to those found in angular contact ball bearings. A multiply alkylated cyclopentane (MAC) hydrocarbon lubricant suffered degradation at a rate almost two orders of magnitude less than the degradation rate of two perfluoropolyalkylether (PFPE) lubricants.

A new technique for Auger analysis of surface species subject to electron-induced desorption.

A method is presented to observe surface species subject to electron‐induced desorption by Auger electron spectroscopy. The surface to be examined is moved under the electron beam at constant velocity, establishing a time‐independent condition and eliminating the time response of the electron spectrometer as a limiting factor. The dependence of the Auger signal on the sample velocity, incident electron current, beam diameter, and desorption cross section is analyzed. It is shown that it is advantageous to analyze the moving sample with a high beam current, in contrast to the usual practice of using a low beam current to minimize desorption from a stationary sample. The method is illustrated by the analysis of a friction transfer film of PTFE, in which the fluorine is removed by electron‐induced desorption. The method is relevant to surface studies in the field of lubrication and catalysis.

Spiral orbit tribometry - Part I: Description of the tribometer

A new rolling contact tribometer based on a planar thrust bearing geometry is described. The bearing “races” are flat plates that drive a ball into a near-circular, spiral path. The spiraling hall is returned to its initial radius each revolution around the race by a “guide plate” backed by a force transducer. The motions of the ball are analyzed and the force exerted by the ball on the guide plate is related to the friction coefficient of the system. The experimental characteristics of the system are presented and the system is shown to exhibit the behavior expected for a tribometer.

X‐ray photoelectron and mass spectroscopic study of electron irradiation and thermal stability of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) was subjected to 3 keV electron bombardment and then heated in vacuum to 300 °C. The behavior of the material as a function of radiation dose and temperature was studied by x‐ray photoelectron spectroscopy (XPS) of the surface and mass spectroscopy of the species evolved. A quantitative comparison of the radiation dose rate with that in other reported studies showed that, for a given total dose, the damage observed by XPS is greater for higher dose rates. Lightly damaged material heated to 300 °C evolved saturated fluorocarbon species, whereas unsaturated fluorocarbon species evolved from heavily damaged material. After heating the heavily damaged material, those features in the XPS that were associated with damage diminished, giving the appearance that the radiation damage annealed. The observations were interpreted by incorporating mass transport of severed chain fragments and thermal decomposition of severely damaged material into the branched and cross‐linked network mod...

Elemental Analysis of a Friction and Wear Surface During Sliding Using Auger Spectroscopy

An Auger cylindrical mirror spectrometer has been incorporated into a friction and wear apparatus. The spectrometer can analyze for all elements heavier than helium present on a friction and wear surface. The instrument response is sufficiently fast to allow a chemical analysis of the surface in 0.10 second. The surface can be continuously analyzed while the experiment is in progress, and better insight into those chemical factors influencing friction and wear can thereby be provided. In sliding friction experiments, transfer of silver, aluminum, and copper to a steel surface was observed. Transfer took place without any attempt to clean the steel surface. This occurred in the first pass of the rider across the steel. Variations in the mode of metal transfer could also be detected. Gases containing oxygen were adsorbed to a copper transfer film, and its subsequent friction-initiated desorption was monitored. Presented at the 27th ASLE Annual Meeting in Houston, Texas, May 1–4, 1972

Tribological properties of Ag/Ti films on Al2O3 ceramic substrates☆

Abstract Silver solid lubricant films with a thin titanium interlayer for enhanced adhesion, were sputter deposited on Al 2 O 3 substrate disks to reduce friction and wear. The dual Ag/Ti films were tested at room temperature in a pin-on-disk tribometer sliding against bare, uncoated Al 2 O 3 pins under a load of 4.9 N at a sliding velocity of 1 m s -1 . The Ag/Ti films reduced the friction coefficient by 50% to about 0.42 compared to unlubricated baseline specimens. Pin wear was reduced by a factor of 140 and disk wear was reduced by a factor of 2.5 compared to the baseline. These films retain their good tribological properties, including adhesion, after heat treatment in argon at 850°C and thus may be able to lubricate over a wide temperature range. This lubrication technique is applicable to space lubrication, advanced heat engines and advanced transportation systems.

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.