Kenneth W. Street

Sliding wear and fretting wear of diamondlike carbon-based, functionally graded nanocomposite coatings

Abstract Improving the tribological functionality of diamondlike carbon (DLC) films-developing good wear resistance, low friction, and high load-carrying capacity-was the aim of this investigation. Nanocomposite coatings consisting of an amorphous DLC (a-DLC) top layer and a functionally graded titanium–titanium carbide–diamondlike carbon (Ti–Ti x C y –DLC) underlayer were produced on AISI 440C stainless steel substrates by the hybrid technique of magnetron sputtering and pulsed-laser deposition. The resultant DLC films were characterized by Raman spectroscopy, scanning electron microscopy, and surface profilometry. Two types of wear experiment were conducted in this investigation: sliding friction experiments and fretting wear experiments. Unidirectional ball-on-disk sliding friction experiments were conducted to examine the wear behavior of an a-DLC/Ti–Ti x C y –DLC-coated AISI 440C stainless steel disk in sliding contact with a 6-mm diameter AISI 440C stainless steel ball in ultrahigh vacuum, in dry nitrogen, and in humid air. Although the wear rates for both the coating and ball were low in all three environments, the humid air and dry nitrogen caused mild wear with burnishing in the a-DLC top layer, and the ultrahigh vacuum caused relatively severe wear with brittle fracture in both the a-DLC top layer and the Ti–Ti x C y –DLC underlayer. For reference, amorphous hydrogenated carbon (H-DLC) films produced on a-DLC/Ti–Ti x C y –DLC nanocomposite coatings by using an ion beam were also examined in the same manner. The H-DLC films markedly reduced friction even in ultrahigh vacuum without sacrificing wear resistibility. The H-DLC films behaved much like the a-DLC/Ti–Ti x C y –DLC nanocomposite coating in dry nitrogen and humid air, presenting low friction and low wear. Fretting wear experiments were conducted in humid air (approximately 50% relative humidity) at a frequency of 80 Hz and an amplitude of 75 μm on an a-DLC/Ti–Ti x C y –DLC-coated AISI 440C disk and on a titanium–6 wt.% aluminum–4 wt.% vanadium (Ti–6Al–4V) flat, both in contact with a 9.4-mm diameter, hemispherical Ti–6Al–4V pin. The resistance to fretting wear and damage of the a-DLC/Ti–6Al–4V materials pair was superior to that of the Ti–6Al–4V/Ti–6Al–4V materials pair.

Carbon Nanostructure Examined by Lattice Fringe Analysis of High Resolution Transmission Electron Microscopy Images

The dimensions of graphitic layer planes directly affect the reactivity of soot towards oxidation and growth. Quantification of graphitic structure could be used to develop and test correlations between the soot nanostructure and its reactivity. Based upon transmission electron microscopy images, this paper provides a demonstration of the robustness of a fringe image analysis code for determining the level of graphitic structure within nanoscale carbon, i.e., soot. Results, in the form of histograms of graphitic layer plane lengths, are compared to their determination through Raman analysis.

Tribological Performance of Some Pennzane®-Based Greases for Vacuum Applications

Commercial greases for space applications usually fulfill the requirements imposed by the severe conditions of use (load, ultrahigh vacuum, speed, etc.). The main requirement is their ability to create an elastohydrodynamic lubrication film, boundary film, or both, for the speed, load, and temperature conditions under which the mechanisms will operate. Three greases, all based on a multiply alkylated cyclopentane (Pennzane®) base oil, were studied. The thickeners were an n-octadecylterephthalamate soap, a lithium soap, and a urea derivative. A four-ball tribometer and a spiral-orbit tribometer were employed to evaluate the greases under ultrahigh vacuum. The results indicated that all three greases yielded very low wear rates and extended lifetimes. In addition, routine physical property data are reported for each grease.

Three-Body Abrasion Testing Using Lunar Dust Simulants to Evaluate Surface System Materials

Numerous unexpected operational issues relating to the abrasive nature of lunar dust, such as scratched visors and spacesuit pressure seal leaks, were encountered during the Apollo missions. To avoid reoccurrence of these unexpected detrimental equipment problems on future missions to the Moon, a series of two- and three-body abrasion tests were developed and conducted in order to begin rigorously characterizing the effect of lunar dust abrasiveness on candidate surface system materials. Two-body scratch tests were initially performed to examine fundamental interactions of a single particle on a flat surface. These simple and robust tests were used to establish standardized measurement techniques for quantifying controlled volumetric wear. Subsequent efforts described in the paper involved three-body abrasion testing designed to be more representative of actual lunar interactions. For these tests, a new tribotester was developed to expose samples to a variety of industrial abrasives and lunar simulants. The work discussed in this paper describes the three-body hardware setup consisting of a rotating rubber wheel that applies a load on a specimen as a loose abrasive is fed into the system. The test methodology is based on ASTM International (ASTM) B611, except it does not mix water with the abrasive. All tests were run under identical conditions. Abraded material specimens included poly(methyl methacrylate) (PMMA), hardened 1045 steel, 6061-T6 aluminum (Al) and 1018 steel. Abrasives included lunar mare simulant JSC- 1A-F (nominal size distribution), sieved JSC-1A-F (<25 m particle diameter), lunar highland simulant NU-LHT-2M, alumina (average diameter of 50 m used per ASTM G76), and silica (50/70 mesh used per ASTM G65). The measured mass loss from each specimen was converted using standard densities to determine total wear volume in cm3. Abrasion was dominated by the alumina and the simulants were only similar to the silica (i.e., sand) on the softer materials of aluminum and PMMA. The nominal JSC- 1A-F consistently showed more abrasion wear than the sieved version of the simulant. The lunar dust displayed abrasivity to all of the test materials, which are likely to be used in lunar landing equipment. Based on this test experience and pilot results obtained, recommendations are made for systematic abrasion testing of candidate materials intended for use in lunar exploration systems and in other environments with similar dust challenges.

Developing Abrasion Test Standards for Evaluating Lunar Construction Materials

Operational issues encountered by Apollo astronauts relating to lunar dust were catalogued, including material abrasion that resulted in scratches and wear on spacesuit components, ultimately impacting visibility, joint mobility and pressure retention. Standard methods are being developed to measure abrasive wear on candidate construction materials to be used for spacesuits, spacecraft, and robotics. Calibration tests were conducted using a standard diamond stylus scratch tip on the common spacecraft structure aluminum, Al 6061-T6. Custom tips were fabricated from terrestrial counterparts of lunar minerals for scratching Al 6061-T6 and comparing to standard diamond scratches. Considerations are offered for how to apply standards when selecting materials and developing dust mitigation strategies for lunar architecture elements.

Application of Carbon Based Nano-Materials to Aeronautics and Space Lubrication

Publisher Summary Carbon nanoparticles and carbon films, such as diamond and graphite-like carbons, are making new inroads into lubrication applications. The carbon-based nanoparticles are heat resistant, radiation hard, and durable and provide low coefficient of friction (CoF) with a number of tribocouples. A careful examination of the internal structure of the carbon materials reveals that the nanostructure is highly variable and depends upon the starting material and processing conditions—which is also true of carbon black. The significance with respect to oxidation of the internal structure of carbon is its effect upon reactivity. Graphitic carbon is characterized by layer planes with large in-plane dimensions. The connection between layer plane dimensions and oxidation is due to the anisotropic reactivity of the graphitic segments comprising carbon. Carbon atoms within basal plane sites, surrounded by other carbon atoms, exhibit a far lower oxidative reactivity than those located at the periphery of such segments.

Liquid Space Lubricants Examined by Vibrational Microspectroscopy

Abstract Considerable effort has been expended to develop liquid lubricants for satellites and space exploration vehicles. These lubricants must often perform under a range of harsh conditions such as vacuum, radiation, and temperature extremes while in orbit or in transit and in extremely dusty environments at destinations such as the moon and Mars. Historically, oil development was guided by terrestrial application, which did not provide sufficient space lubricants. Novel fluids such as perfluorinated polyethers provided some relief but are far from ideal. With each new fluid proposed to solve one problem, other problems have arisen. Much of the work performed at National Aeronautics and Space Adminstrations (NASA) Glenn Research Center, in elucidating mechanisms by which chemical degradation of space oils occur, has been done by infrared and Raman microspectroscopy, which this review details. Fundamental lubrication studies are presented as well as actual case studies, in which vibrational spectroscopy led to millions of dollars in savings and potentially prevented loss of mission.

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.

Transfer Layers: A Comparison across SWNTs, DWNTs, Graphite, and an Ionic Fluid

Lubrication is the science of friction at moving interfaces. Nanomaterials acting as interfacial modifiers can minimize friction and thereby improve energy efficiency. To test this hypothesis, single- (SWNT) and double-walled (DWNT) carbon nanotubes and an ionic fluid are tested individually and compared to SWNTs and graphite as additives within the ionic fluid. The minimum coefficient of friction is correlated with the longest lifetime using a ball-on-disc tribometer, in air, at atmospheric pressure. Results are interpreted in terms of the nanotubes mechanical properties and the formation of transfer layers upon the tribosurfaces.

Grease Degradation in Critical Helicopter Drivetrain Bearings

An investigation of critical aviation bearings lubricated with MIL-PRF-81322 grease was conducted to derive an understanding of the mechanisms of grease degradation and the loss of lubrication over time. Chemical analysis was performed on grease samples from fielded bearings and compared to fresh grease and samples taken from bearings run for extended times in a laboratory environment. Size exclusion chromatography and Fourier transform infrared spectroscopy were used to investigate the condition of the grease, and evidence of additive depletion, oil evaporation, and thickener degradation were seen, consistent with results reported by other authors. Given the relatively light loading conditions experienced by the test bearings, they were able to continue operating at high temperature despite having most of the original oil depleted from the grease.Copyright