Vapor Phase Lubrication Research Articles

Tribochemical Polymerization of Adsorbed n-Pentanol on SiO2 during Rubbing: When Does It Occur and Is It Responsible for Effective Vapor Phase Lubrication?

The origin and role of tribochemical reaction products formed while sliding silicon oxide surfaces in the presence of adsorbed alcohol molecules in equilibrium with the vapor phase were studied. Wear and friction coefficient studies with varying contact loads and n-pentanol vapor environments were used to determine under what operating conditions the tribochemical reaction species was produced. Imaging time-of-flight secondary ion mass spectrometry and microinfrared spectroscopy found that hydrocarbon species with a molecular weight higher than the starting vapor molecules are produced when there is wear of the SiO(2) surface. When the n-pentanol vapor lubrication is effective and the silicon oxide surface does not wear, then the tribochemical polymerization products are negligible. These results imply that the tribochemical polymerization is associated with the substrate wear process occurring due to insufficient adsorbate supply or high mechanical load. The tribochemical reactions do not seem to be the primary lubrication mechanism for vapor phase lubrication of SiO(2) surfaces with alcohol, although they may lubricate the substrate momentarily upon failure of the alcohol vapor delivery to the sliding contact.

Large Scale Molecular Dynamics Simulations of Vapor Phase Lubrication for MEMS

While alkylsilane monolayers reduce both adhesion and friction in MEMS, experiments and simulations have shown that they are easily damaged by momentary contact even at low loads. Vapor phase alcohols appear to provide a potential solution to this problem, reducing friction in MEMS with no noticeable wear, and allowing devices to run for billions of cycles without failure. The underlying mechanisms behind both the reduction in friction as well as the healing of damage are however unclear. We report on the results of large scale molecular dynamics simulations aimed at understanding the tribology of vapor phase alcohols in contact with amorphous silica substrates. The healing mechanism is investigated by simulating asperity contact with a model AFM tip in contact with a monolayer of propanol on an amorphous silica substrate. We find that because of the low vapor pressure, alcohol molecules removed by shear contact remain close to the substrate, moving around the contact region to replenish molecules removed from the damage site. For comparison, the tribology of propanol and water confined between two opposing flat silica surfaces is also studied.

Macro- to Nanoscale Wear Prevention via Molecular Adsorption

As the size of mechanical systems shrinks from macro- to nanoscales, surface phenomena such as adhesion, friction, and wear become increasingly significant. This paper demonstrates the use of alcohol adsorption as a means of continuously replenishing the lubricating layer on the working device surfaces and elucidates the tribochemical reaction products formed in the sliding contact region. Friction and wear of native silicon oxide were studied over a wide range of length scales from macro- to nanoscales using a ball-on-flat tribometer (millimeter scale), sidewall microelectromechanical system (MEMS) tribometer (micrometer scale), and atomic force microscopy (nanometer scale). In all cases, the alcohol vapor adsorption successfully lubricated and prevented wear. Imaging time-of-flight secondary ion mass spectrometry analysis of the sliding contact region revealed that high molecular weight oligomeric species were formed via tribochemical reactions of the adsorbed linear alcohol molecules. These tribochemical products seemed to enhance the lubrication and wear prevention. In the case of sidewall MEMS tests, the lifetime of the MEMS device was radically increased via vapor-phase lubrication with alcohol.

Activation of SiC Surfaces for Vapor Phase Lubrication by Chemical Vapor Deposition of Fe

Vapor phase lubrication (VPL) has been proposed as a method for lubrication of high temperature engines. During VPL, lubricants such as tricresylphosphate (TCP), (CH3–C6H4O)3P=O, are delivered through the vapor phase to high temperature engine parts and react on their surfaces to deposit a thin, solid, lubricating film. Although ceramics such as SiC are desirable materials for high temperature applications, their surfaces are unreactive for the decomposition of TCP and thus not amenable to VPL. As a means of activating the SiC surface for TCP decomposition we have used chemical vapor deposition of Fe from Fe(CO)5. Modification of the SiC surface with adsorbed Fe accelerates subsequent decomposition of TCP and deposition of P and C onto the surface. In the temperature range 500–800 K, m-TCP decomposes more readily on Fe-coated SiC surfaces than on SiC surfaces. The C and P deposition rates depend on the thickness of the Fe film and are further enhanced by oxidation of the Fe. This work provides a proof-of-concept demonstration of the feasibility of using VPL for ceramics

Vapor Lubricant Transport in MEMS Devices

Vapor phase lubrication is considered one of the most likely means of lubricating MEMS devices with replenishable solid films. Since gas phase diffusion is the primary means of vapor lubricant transport into the long, narrow channels of MEMS devices, it is necessary to determine that this is sufficient to replenish lubricant films worn at high rates. The diffusion of vapor phase lubricant into a long, narrow channel has been modeled in order to solve for the lubricant partial pressure distribution along such a channel in a MEMS device. This has been used to estimate the maximum channel length for which vapor phase transport can replenish adsorbed lubricant films. Using reasonable parameters for description of the channel, the vapor lubricant properties, and the diffusion constant, it can be shown that gas phase diffusion is sufficient to support the replenishment of vapor lubricant films being worn at rates as high as 1 monolayer(s) in channels with aspect ratios of 100 or higher.

The surface chemistry of vapor phase lubricants: tricresylphosphate on Ni(1 0 0)

Tricresylphosphate (TCP) is known to serve as an excellent high temperature vapor phase lubricant with some metals such as Fe but not with others such as Ni. The surface chemistry of m-TCP has been studied on clean and phosphorous covered Ni(1 0 0) surfaces in order to understand the differences between its reactivity on Fe and Ni. Our results show that upon heating to 800 K m-TCP decomposes on the clean Ni(1 0 0) surface to deposit carbon and phosphorous with the evolution of H 2, CO, benzene, and toluene into the gas phase. During further heating to 1000 K, all the carbon on the surface dissolves into the Ni bulk leaving only phosphorous. The adsorption and heating of m-TCP on the phosphorous-modified Ni surface does not result in significant decomposition. The clean Ni substrate is able to activate TCP decomposition in much the same way that it decomposes on the clean Fe surface. The lack of significant differences in the chemistry of TCP on the clean metal surfaces cannot explain the fact that TCP vapor will form thick lubricating films on Fe but not on Ni.

Vapor-Phase Lubrication: Reaction of Phosphate Ester Vapors with Iron and Steel

Aromatic phosphate esters such as triphenyl phosphate, tricresyl phosphate (TCP), and tri(tert-butylphenyl) phosphate, have been degraded in the presence of pure iron or metal alloys such as M-50 or 52100 steel. Among these volatile degradation products are those generated from the addition of an aromatic ring to the phosphate ester. Other products, which have been identified, include substituted biphenyls and diphenyl ethers derived from the decomposition of the above-mentioned addition product. Still other products are fused ring aromatic compounds such as anthracene, which arise from secondary reactions of the initial decomposition reactions. The decomposition reactions leave a nonvolatile phosphate film on the surface of the metal. Characterization of the film with Auger spectroscopy suggests iron phosphate as the product. X-ray photoelectron spectroscopy shows the presence of a bound organic layer at the surface. A mechanism that explains many of the decomposition products and the formation of a bound glassy iron phosphate film is proposed.

The surface chemistry of alkyl and arylphosphate vapor phase lubricants on Fe foil

The surface chemistry of tributylphosphate (TBP) and tricresylphosphate (TCP) on a polycrystalline Fe surface was studied using temperature programmed reaction spectroscopy and Auger electron spectroscopy to illustrate some of the initial steps in the reaction mechanisms of alkyl and arylphosphate vapor phase lubricants. During heating, TBP [(C 4H 9O) 3P=O] adsorbed on the Fe surface decomposes via C–O bond scission to give butyl surface intermediates [C 4H 9–] that react via β-hydride elimination to desorb as 1-butene [CH 3CH 2CH=CH 2] and H 2 without appreciable carbon deposition onto the surface. The thermal decomposition of 1-iodobutane [I-C 4H 9] on Fe was observed to proceed via the same β-hydride elimination mechanism. In contrast to tributylphosphate, meta-tricresylphosphate ( m-TCP) [(CH 3–C 6H 4O) 3P=O] decomposes on Fe via P–O bond scission to produce methylphenoxy intermediates [CH 3–C 6H 4O–]. During heating to 800 K, methylphenoxy intermediates either desorb as m-cresol [CH 3–C 6H 4–OH] via hydrogenation or decompose further to generate tolyl intermediates [CH 3–C 6H 4–]. Some of the tolyl intermediates desorb as toluene [CH 3–C 6H 5] via hydrogenation but the majority decompose resulting in H 2 and CO desorption and carbon deposition onto the Fe surface. The P–O bond scission mechanism of m-TCP was verified by showing that the temperature programmed reaction spectra of m-cresol yield products that are almost identical to those of m-TCP. These results provide insight into the origin of the differences in the performance of alkyl and arylphosphates as vapor phase lubricants. The alkylphosphates decompose via alkyl intermediates that readily undergo β-hydride elimination and desorb into the gas phase as olefins, thus removing carbon from the surface. In contrast, the arylphosphates generate aryloxy intermediates by P–O bond scission and aryl intermediates by further C–O bond scission. Neither of these intermediates can undergo β-hydride elimination and thus they decompose to deposit carbon onto the Fe surface. The higher efficiency for carbon deposition may be the primary reason for the superior performance of the arylphosphates over alkylphosphates as vapor phase lubricants.

Geometric effects in high‐temperature vapour‐phase lubrication using hydrocarbon feed gases

AbstractThe effectiveness of ‘far‐field’ vapour‐phase lubrication, in which areas of a bearing surface that are cycled through the contact are exposed to vapour while outside the contact, has been demonstrated in both sliding and combined roll slide tests using acetylene vapours to deposit pyrolytic graphite. Friction coefficients as low as μ = 0.008 have been measured for steel at 540°C with far‐field acetylene concentrations as low as 5%. Effective vapour‐phase lubrication depends on solid lubricant deposition that exceeds the contact's capacity to remove solid lubricant through wear. While the rate of removal is increased by increasing the sliding velocity, in far‐field vapour‐phase lubrication the rate of lubricant deposition, and therefore the lubrication effectiveness, is augmented by increased areas available for far‐field deposition, such as those provided by performing wear tests with increased wear‐track diameters. These geometric concepts may be considered in rolling‐element bearing and gear set applications where vapour‐phase lubrication is to be employed.

Numerical Investigation of the Effect of Carbon-Carbon Composite Cages on High-Speed Bearing Operating Temperatures

In addition to geometry and environmental factors, operating temperatures of bearings are directly affected by applied load, operating speed, friction torque, and lubricant type and viscosity. Without adequate heat dissipation, bearing temperatures will increase to the point that the lubricant is distressed, ultimately leading to bearing failure. Numerically investigating bearing operating temperatures for specific environment, geometry, and lubricant conditions provides the opportunity to design bearings for improved thermal performance. This is critical for high-speed bearings when using mist or vapor phase lubrication with reduced heat sink capability. For these conditions, heat generation can lead to temperatures too extreme for reliable operation. Interactions between the bearing cage and the bearing races are the primary component of the total heat generation in these bearings. As the loading during these interactions is a function of centrifugal forces, it has been predicted that a lower density cage material will provide thermal performance benefits due to reduced loading. If this lower density cage material also demonstrates acceptable strength and traction characteristics, it may be an excellent replacement for conventional cage materials. Carbon-carbon composites meet the desired cage properties and the effects of using C-C cages on bearing operating temperature have been investigated using the finite element method. Predicted heat generation and steady-state temperatures for a 30 mm ball bearing operating with mist lubricant delivered at 325°C for several shaft operating speeds are presented to demonstrate differences in thermal performance due to cage material. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

Thermal decomposition of tricresylphosphate isomers on Fe

decomposed on the Fe surface to deposit carbon and phosphorous and produce gas phase H2, CO, toluene and cresol. The amounts of carbon and phosphorous deposited onto the Fe surface by TCP (arylphosphate) decomposition were compared to those deposited by decomposition of tributylphosphate (TBP), an alkylphosphate. Thermal decomposition of all three TCP isomers deposits substantial amounts of carbon ono the Fe surface while TBP decomposition does not. We suggest that it is the lack of -CH bonds in the methylphenoxy intermediates and the high reactivity of the tolyl intermediates generated during TCP decomposition are the primary reasons for the differences in behavior of TCP and TBP and the root cause of the differences in behavior of arylphosphates and alkyphosphates as vapor phase lubricants.

Nanotribology of a Vapor-Phase Lubricant: A Quartz Crystal Microbalance Study of Tricresylphosphate (TCP) Uptake on Iron and Chromium

Adsorption measurements of tricresylphosphate (TCP) on high-purity iron and chromium surfaces have been performed in ultra-high vacuum with a quartz crystal microbalance in conjunction with Auger electron spectroscopy, yielding values for gas uptake rates, molecular slippage, and tribofilm stress levels in the temperature range 25-400°C. At room temperature, TCP uptake is observed to be limited to two layers of intact molecules that are likely to be physisorbed. Above 200°C, the data recorded on both iron and chromium substrates are consistent with far greater uptake levels and extensive interdiffusion of TCP fragments with the substrate. The most noteworthy difference between the two substrates is TCP's fragmentation upon impact on iron, but not chromium, at elevated temperatures.

Differential application of wear models to fractional thin films

Global application of bulk wear models, originally developed for monolithic bodies, to pre-deposited thin films and coatings can lead to the paradoxical prediction of wear removal rates exceeding the rate of film introduction into the tribological contact. A thin-film wear model is developed which resolves this paradox through differential wear model application, coupled with the fractional nature of such thin films which may exist as they are worn through and the corresponding fractional normal load that film supports. The model predicts the state of the wearing film as a function of position within the contact. A corresponding description of friction coefficient of contacts of such wearing fractional thin films is also developed, for purely sliding as well as combined rolling/sliding contacts. Furthermore, it is demonstrated that prediction of film state through the contact, from inlet to exit, enables subsequent prediction of the evolving global wear and friction behavior with time. The model is compared to examples of experimental friction data for thin films taken from the literature. The manuscript closes with a discussion of extension of the model to cases where such thin films are continuously replenished, such as in vapor phase lubrication.

Reaction mechanisms in organophosphate vapor phase lubrication of metal surfaces

This paper discusses the surface chemistry of a number of reactants and species that are probable intermediates in vapor phase lubrication chemistry. In order to understand the mechanism by which arylphosphates [(RO) 3PO] such as tricresylphosphate [R=CH 3C 6H 4–] react to form lubricating films on metal surfaces, several of the likely elementary steps are illustrated using experiments with model compounds on the Cu(111) surface. Trimethylphosphite [(CH 3O) 3P] has been used as a simple model for the phosphates and experiments suggest that the initial step in the decomposition mechanism is P–O bond breaking to form alkoxy groups. Other evidence suggests that under some conditions the decomposition of arylphosphates may be initiated by either P–O or the C–O cleavage to produce adsorbed aryloxy or aryl intermediates. The decomposition of aryloxy groups or aryl groups then leads to the deposition of carbon into the lubricating films. The chemistry of aryl, alkyl, aryloxy, and alkoxy species has been investigated on the Cu(111) surface. In either the alkoxy groups produced by P–O cleavage or the alkyl groups produced by C–O cleavage the presence of β-CH bonds has a strong influence on the rate of carbon deposition onto the surface. In the arylphosphates which would produce either aryl or aryloxy groups, the lack of β-CH bonds leads to greater rates of carbon deposition onto the surface than for either alkyl or alkoxy groups.

The decomposition mechanism of trimethylphosphite on Ni(1 1 1)

The chemistry of trimethylphosphite ((CH 3O) 3P) adsorbed on the Ni(1 1 1) surface has been studied in order to gain insight into the initial steps in the decomposition of alkyl and aryl phosphates commonly used as vapor phase lubricants. The primary products of trimethylphosphite decomposition on the Ni(1 1 1) surface are H 2 and CO which desorb at 355 and 400 K with kinetics that are similar to those observed for methoxy (CH 3O (ad)) decomposition. In addition a CO desorption feature appears at 620 K consistent with recombination of C and O atoms that must be formed by dissociation of the C–O bonds in some fraction of the adsorbed trimethylphosphite. Other minority species that are observed desorbing from the surface include trimethylphosphite itself, methanol, and formaldehyde. This suggests that the major pathway for decomposition is one in which the P–O bonds in trimethylphosphite are cleaved initially to yield the methoxy species. Of the methoxy groups in trimethylphosphite one third undergo C–O dissociation, either as methoxy groups on the Ni(1 1 1) surface or while still bonded to phosphorus. Following heating to temperatures in excess of 700 K the Ni(1 1 1) surface is left clean of all adsorbed species other than phosphorus atoms. The use of trimethylphosphite serves as a simple, clean method for controlled deposition of phosphorus onto the Ni(1 1 1) surface.

Activation of the SiC surface for vapor phase lubrication by Fe chemical vapor deposition from Fe(CO) 5

The feasibility is demonstrated of a new approach to the vapor phase lubrication of ceramics using organophosphorus compounds. The surface of SiC is shown to be unreactive for the decomposition of trimethylphosphite, (CH3O)3P, a simple model for organophosphorus vapor phase lubricants such as tricresylphosphate. In order to activate the surface of SiC it has been exposed to Fe(CO) 5 at a temperature of 600 K. Chemical vapor deposition serves as a means of depositing Fe on the SiC surface. The Fe-modified SiC surface is then shown to induce the decomposition of adsorbed (CH3O)3P. The mechanism of (CH3O)3P decomposition is similar to that observed on Fe(110) surfaces modified by the presence of oxygen. It is initiated by P‐O bond cleavage to produce adsorbed methoxy groups, CH 3O(ad), which then decompose by β-hydride elimination resulting in H2 ,C O, H 2CO, and CH3OH desorption. It is suggested that chemical vapor deposition of metals using high vapor pressure metal-containing compounds such as Fe(CO)5 can serve as a mechanism for continuous, in situ activation of ceramic surfaces for vapor phase lubrication in high temperature engines.

Vapor-Phase Lubrication in Combined Rolling and Sliding Contacts: Modeling and Experimentation

The in situ vapor-phase lubrication of M50 steel, in combined rolling and sliding contacts at 540°C using nitrogen atmospheres containing acetylene, is achieved. Acetylene partial pressures of 0.05 atmospheres are capable of providing continuous lubrication to combined rolling and sliding contacts through pyrolytic carbon deposition. In these tests, friction coefficients as low as μ=0.01 are found for contacts at 2.0 m/s rolling speed, 10 cm/s sliding speed, 100 N load (1.3 GPa Hertzian contact pressure), and ambient temperature of 540°C, with even lower values observed at more modest sliding speeds. One example of a model for vapor phase lubrication of combined rolling and sliding contacts is developed which predicts the lubricant steady-state fractional coverage of the contact surfaces, and from this makes friction coefficient predictions using a linear rule-of-mixture. Friction coefficient responses to step changes in acetylene partial pressure, sliding speed, and disk wear-track diameter are measured. Increased partial pressure of acetylene and increased area available for deposition are observed to be beneficial, while increased sliding speed is detrimental to lubrication performance. Shapes and trends of steady-state friction coefficient versus acetylene partial pressure, sliding speed, and disk wear-track diameter are described and curve-fit by the model. In combined rolling and sliding this example model predicts large regions of operating conditions over which friction coefficient is independent of rolling speed, as well as regions of independence of vapor partial pressure. In the special case of pure sliding, a region of friction coefficient independence of a ratio of partial pressure to sliding speed and another region of independence of a ratio of partial pressure to the product of sliding speed and normal load are predicted.

Vapor Phase Lubrication for Expendable Gas Turbine Engines

Vapor phase lubrication (VPL) is an emerging technology that is currently targeted for application in limited life, expendable engines. It has the potential to cut 90 percent of the cost and weight of the lubrication system, when compared to a conventional liquid lubricated system. VPL, is effective at much higher temperatures than conventional liquid lubrication (600°C versus 200°C), so considerably less cooling for the bearing is required, to the extent that the bearing materials often dictate the maximum upper temperature for its use. The hot no. 8 bearing and the cold no. 1 bearing of the T63 engine were used to evaluate the applicability of this technology to the expendable engine environment. The no. 8 bearing was a custom made hybrid with T15 steel races, silicon nitride balls, and a carbon–carbon composite cage; it was run for 10.7 h at a race temperature of 450°C at full power, without incident. Prior to engine tests, a bearing rig test of the no. 8 bearing demonstrated an 18.6 h life at a race temperature of 500°C at engine full power speed of 50,000 rpm. Cold bearing performance was tested with the standard no. 1 bearing, which consisted of 52100 steel races and balls, and a bronze cage; it was run for 7.5 h at a race temperature of 34°C at flight idle power, without incident. A self-contained lubricant misting system, running off compressor bleed air, provided lubricant at flow rates of 7–25 ml/h, depending on engine operating conditions. These tests have demonstrated for the first time that a single self-contained VPL system can provide adequate lubrication to both the hot and cold bearings for the required life of an expendable cruise missile engine. [S0742-4795(00)01302-2]

Electron Affinity Calculations For Thioethers

Previous work indicated that polyphenyl thioethers possessed chemical properties, related to their electron affinities, which could allow them to function as vapor phase lubricants (VPL). Indeed, preliminary tribological tests revealed that the thioethers could function as vapor phase lubricants but not over a wide temperature and Hertzian pressure range. Increasing the electron affinity of the thioethers may improve their VPL properties over this range. Adding a substituent group to the thioether will alter its electron affinity in many cases. Molecular orbital calculations were undertaken to determine the effect of five different substituent groups on the electron affinity of polyphenyl thioethers. It was found that the NO2, F, and I groups increased the thioethers electron affinity by the greatest amount. Future work will involve the addition of these groups to the thioethers followed by tribological testing to assess their VPL properties. Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998

The Carbon Deposition Mechanism in Vapor Phase Lubrication

The study of various alkoxy species on metal surfaces has provided insight into the mechanism by which carbon is deposited into the lubricating films of polyphosphate glass generated by arylphosphate vapor phase lubricants. Previous work using trimethylphosphite has suggested that one of the initial steps in the decomposition of organophosphorus compounds on metal surfaces is cleavage of P-O bonds to produce adsorbed alkoxy species. In the case of alky I alkoxy groups (poor ligands for vapor phase lubrication) there are reaction mechanisms such as β-hydride elimination which efficiently remove hydrocarbons from the surface. In the case of the aryl alkoxy groups (good ligands for vapor phase lubricants) such mechanisms cannot be active due to the lack of β-hydrogen atoms. Experiments using various alkoxy species demonstrate these differences and reveal that phenoxy groups deposit carbon onto the surface quite efficiently while cyclohexanoxy and t-butanoxy groups do not. Presented at the 55th Annual Meeting Nashville, Tennessee May 7–11, 2000