Jeffrey H. Sanders

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.

Lubrication using a microstructurally engineered oxide: performance and mechanisms

Oxide coatings have the potential to lubricate over a wide range of environmental conditions. However, oxides are typically brittle, form abrasive wear debris, and have high friction. ZnO is no exception; hot-pressed 1–2 µm ZnO has a friction coefficient of about 0.6 and causes extensive wear on steel counterfaces. Microstructural engineering may be used to permit plastic deformation and the formation of lubricious transfer films. The work presented here focuses on controlling the microstructure and chemistry within ZnO to provide low-friction and long-life coatings (e.g., µ=0.1−0.2, 1M+ sliding cycles). Coatings having a (0001) columnar texture with good crystallinity along the c-axis wear quickly and generate substantial wear debris. Depositions that create a (0001) texture with a mosaic substructure within the columns deform plastically. Here, nanocrystalline structures may enhance grain boundary sliding and contribute to plastic deformation and low friction. Dislocation motion within ZnO is enhanced by oxygen adsorption, which may further reduce friction by lowering shear strength. In addition, it is likely that defects arising from oxygen deficiency and the high surface-to-volume ratio of nanostructures, promote adsorption of water and/or oxygen. The adsorbed species can reduce friction through passivation of dangling or strained bonds. The complex interaction of mechanical and surface chemical effects result in millions of dry sliding cycles on nanostructured coatings in 50% RH air. In addition, the coatings have low friction in vacuum. Coating characterization and performance are discussed and a mechanism to explain the tribological properties is proposed.

Ionic-Liquid Lubrication of Sliding MEMS Contacts: Comparison of AFM Liquid Cell and Device-Level Tests

Lubrication of microelectromechanical systems (MEMS) became very critical as the devices became complex and its reliability began to deteriorate. In this paper, ionic liquids (ILs) with low volatility and high environmental stability were investigated as lubricants for sliding MEMS devices. A method that is based on atomic force microscopy (AFM) with a liquid cell was developed to study friction and wear properties of surfaces lubricated with ILs, having a systematic variation in molecular geometry and chemistry. Six-member pyridinium and five-member imidazolium rings are compared as cations in ethyl methyl pyridinium and ethyl methyl imidazolium ethyl sulfate; influence of short and long alkyl chain lengths on lubrication is studied with butyl methyl pyrrolidinium and hexyl methyl pyrrolidinium bis(trifluro methyl sulfonyl) imide. Formation of a surface-screening cation layer was discovered and linked to low friction and wear of IL-lubricated hydrogenated-silicon (H-Si) substrates. Several promising IL lubricants were identified from the AFM study and were tested in real MEMS motor devices. The friction and wear data obtained for these tests showed good correlation with the failure life span of lubricated MEMS motors. This supports a conclusion that the AFM-liquid-cell technique can be used in screening IL lubricants for MEMS devices.

Lubrication of microelectromechanical systems radio frequency switch contacts using self-assembled monolayers

Contact failures in microelectromechanical systems (MEMS) switches prevent widespread use of MEMS technology for current handling in miniature devices. A self-assembled monolayer (SAM) lubricant was applied to MEMS switch surfaces in this paper as a possible approach for preventing contact failure. Chemical and physical processes on SAM lubricated contact surfaces were investigated at low (10 μA) and high (1 mA) current using a micro/nanoadhesion apparatus as a switch simulator with in situ monitoring of contact resistance and adhesion force. This was coupled with ex situ analytical analyses of the contacts using x-ray photoelectron spectroscopy (XPS) and micro-Raman techniques. Diphenyl disulfide was chosen as a lubricant due to its thermal stability, enhanced conductivity, and its ability to form a 3.4 A thick SAM on the gold electrode surface. Hot switching experiments were conducted in humid air (45% RH) and dry nitrogen using a MEMS-scale contact force of 200 μN and 5 Hz frequency. At low current, lu...

Sliding behavior of multifunctional composite coatings based on diamond-like carbon

AbstractThe sliding behavior of several coatings based on non-hydrogenated diamond-like carbon (DLC) is described. Coatings were producedbyusingamagnetronsputter-assistedpulsedlaserdepositionprocessdevelopedattheAirForceResearchLaboratory.Resultsarecomparedfor two types of coatings: DLC with WC nanoparticles and DLC with both WC particles and WS 2 (“WCS”). Sliding tests were done in air,nitrogen and vacuum and for alternating periods in different environments, e.g., cycling between air and vacuum conditions. The frictionforce and the signal from an in situ Kelvin probe were monitored during sliding. Friction coefficients ranging from near 0.01 to 0.6 havebeen observed.The Kelvin probe detected transients lasting from ten minutes to more than one hour. Post-test characterization included SEM/EDS,Raman and TEM. The role of transfer and mixing is discussed.© 2003 Elsevier Science B.V. All rights reserved. Keywords: Composite coatings; DLC coatings; Sliding friction; WS 2 1. IntroductionTypical solid lubricant materials are relatively soft andtherefore not abrasion resistant. Hard coatings are often em-ployed to protect surfaces from abrasive and erosive wear.However, hard coatings may not have low friction and arefrequently brittle. There is a need to design hard coatingsthat also provide low friction, exhibit increased toughnessand remain compatible with their substrates. For aerospaceapplications, such coatings should also operate reliably overa wide range of loads, temperatures and environmental con-ditions and should survive cycling of these variables. Forexample, the coating might be required to operate well inhumid air at a launch site as well as for extended periodsin the vacuum of space. Coatings that maintain low frictionwould assure that energy use is minimized during extendedoperation or when resuming relative motion after periods ofrest during on–off cycles. They would also be expected toprovide increased lifetimes of tribological systems.To produce effective and reliable coatings that might sat-isfy the requirements outlined above, the multidisciplinarycoatings research group at AFRL has chosen a strategy thatincludes all of the following:

Tribological behavior of a multialkylated cyclopentane oil under ultrahigh vacuum conditions

An ultrahigh vacuum, ball-on-flat test apparatus has been built to study the performance of candidate oils intended for spacecraft applications. Tests have been conducted on a multialkylated cyclopentane base oil using steel balls and disks. Different results are obtained when this oil is tested under vacuum conditions than when it is tested under a nitrogen environment. These differences are dramatic when the tests are conducted under starved conditions. Analyses of gases evolved during rubbing reveal that large quantities of methane are evolving from the process. A mechanism is proposed whereby oxide-free steel surfaces combine with tribological activity to crack the hydrocarbon oil to produce CHx radicals. These CHx radicals abstract hydrogen from the surrounding oil to produce methane. The increased volatility of the oil fragments remaining after methane formation leads to material loss by evaporation, thereby explaining the differences in vacuum and nitrogen performance of the oil.

Chemical characterization of antiwear films generated by Tris-[p-(perfluoroalkylether)phenyl] phosphine using X-ray absorption spectroscopy

Abstract Perfluoropolyalkylethers (PFPAEs) are primary candidates as high temperature oils for the next generation of turbine engines due to their chemical and thermal stability. However, the usefulness of the PFPAE base fluids are hindered by corrosive wear in dry environments. This problem can be minimized and overall wear properties improved by the addition of soluble additives, such as Tris-[ p -(perfluoroalkylether)phenyl] phosphine (PH3). Currently, little work has been reported on the mechanism by which this additive actually improves overall wear performance. This paper provides critical insight regarding the interactions of the PFPAE additive PH3 with Fe-based alloys in a pin-on-flat tribological environment. It is found that the PH3 decomposes on the surface, within the wear track, forming a tribofilm composed of a polyphosphate glassy material. At low relative humidity (∼0%), the polyphosphate antiwear film substantial improves the wear performance of the fluid which is reflected by a decrease of ∼325% in width of the measured wear scar. Contrasting this result, at high relative humidity (∼50%), little improvement is found in the wear properties of the fluid. This is due to the formation of carboxylate multilayers produced by PFPAEs in a moist environment, which serve as their own antiwear film. The formation and protective properties of these films are controlled by three important environmental factors. First, oxygen must be present in order to form the polyphosphate. Second, tribomechanical scission and hydrolysis of the additive is required to drive the reaction to completion. At low humidity, a large amount of unreacted and intermediate material was found within the wear track. Third, the test temperature combined with the relative humidity was shown to control the overall useful lifetime of the additive. In order to gain some understanding on how this additive works, a series of tribological experiments were performed at different temperatures, relative humidities and rubbing times. The worn specimens were examined by X-ray absorption near edge structure spectroscopy (XANES) and imaging photoelectron spectromicroscopy (MEPHISTO).

Degradation of Cu-Al Coating on Ti-6AI-4V Substrate under Fretting Fatigue Conditions

The degradation process of a Cu-Al coating on Ti-6Al-4V substrate was investigated under fretting and fretting fatigue loading conditions. Wear and coefficient of friction (COF) of the coating were investigated as a function of fretting and fretting fatigue cycles. Damage of the coating was also characterized using scanning electron microscopy, energy dispersive spectrometry, and profilometery. COF decreased due to self-lubrication effects from the debris formation under fretting and fretting fatigue loading conditions. At a given normal load applied to fretting pad, coating wear increased with the increasing number of cycles and applied load to the specimen, with the lowest wear occurring under fretting loading conditions. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

Surface chemistry of new lubrication systems for high-speed spacecraft bearings

Multialkylated cyclopentane (MAC) and silahydrocarbon (SiHC) are primary candidates for future spacecraft applications due to their high viscosity and good viscosity–temperature profile, low vapor pressure and good lubricating properties. In this work, the friction, wear and associated tribochemistry of these fluids, both unformulated and formulated with 2% aryl phosphate ester (TPP), were investigated. A Plint reciprocating wear rig equipped with an environmental chamber that was filled with dry air or nitrogen was used to produce boundary lubrication conditions. The resulting specimens were examined by X-ray absorption near-edge structure (XANES) spectroscopy in order to gain some understanding of how the base fluid and additive function. Several relationships were discovered among friction, wear and tribochemistry within the wear scar. First, the wear rate in both unformulated and formulated fluids was higher in a dry nitrogen environment than in dry air. Second, when tested in air, unformulated silahydrocarbon acts as its own antiwear additive by decomposing to a silicon oxide glass within the wear track thereby eliminating additive issues such as solubility, evaporation and concentration effects. Third, the antiwear properties of silahydrocarbon oil are hindered by the presence of a phosphate additive. Both the oil and additive form an oxide glass within the wear track and compete for active growth sites. Fourth, the chain length of the polyphosphate glass formed in the wear track controls the antiwear performance of the film. The phosphate additive in multialkylated cyclopentane decomposed to a polyphosphate glass in both dry air to generate a good antiwear film (short to medium chain length polyphosphate) and nitrogen to form a poor antiwear film (long chain length polyphosphate).

Fretting Fatigue Behavior of Cu-Al-Coated Ti-6Al-4V

Fretting fatigue of a Cu-Al coating deposited on an alumina-gritted Ti-6Al-4V substrate was investigated. Two types of tests were conducted: one series involved fretting fatigue tests at different bulk stress amplitudes under a constant contact load and the second series was run at different contact loads under a constant bulk stress amplitude. The coefficient of friction (COF) was lower on the coated substrate than on the bare substrate before fretting. However, COFs of the coated and the bare (i.e., uncoated) substrates were identical after the exposure to fretting fatigue cycles since fretting fatigue caused wear of the coating. The coating thickness as a function of fretting fatigue cycles was monitored. The coating damage increased as the applied bulk stress amplitude increased. At lower bulk stress amplitudes, the coating damage was gradual and it survived over one million cycles. However, the coating delaminated from the gritted surface and/or caused premature specimen failure at higher stress amplitudes. At the lowest contact load used in the present study, the contact condition was gross slip and the life of the coating was the shortest due to fretting wear. On the other hand, the higher contact loads induced a partial slip contact condition that caused less coating damage, and the coating life increased with increasing contact load. *The views expressed in this article are those of the authors and do not reflect the official policy or position of the United State Air Force, Department of Defense, or the U.S. government