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     

Initial steps in the surface chemistry of vapor phase lubrication by organophosphorus compounds

The surface chemistry of trimethylphosphite (CH₃O)₃P has been studied on Cu(111) and Ni(111) surfaces in order to model the initial steps in the reactions of vapor phase lubrication by organophosphorus compounds. The initial reactions involve scission of the P–O bonds to deposit methoxy groups (CH₃O_(ad)) on the surfaces. On the Cu(111) surface the formation of CH₃O_(ad) species occurs only after oxidation of the surface. The CH₃O_(ad) groups on Cu(111) decompose by β‐hydride elimination to produce formaldehyde (O=CH₂) and adsorbed hydrogen. CH₃O_(ad) groups are formed from (CH₃O)₃P on the clean Ni(111) surface and decompose by complete dehydrogenation to CO and adsorbed hydrogen atoms. This chemistry is very similar to that observed for CH₃OH on these surfaces. These results suggest that alkoxides are important intermediates in the decomposition of vapor phase lubricants on metal surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Decomposition of Tricresylphosphate Isomers on Fe

Triresylphosphate (TCP) [(CH₃-C₆H₄O)₃P=O] is the most commonly tested vapor phase lubricant and is readily available as a mixture containing all three isomers (ortho-, meta-, and para-) of the cresyl groups. The surface chemistry of ortho-, meta-, and para-TCP on Fe foil was studied in order to examine the possible differences in decomposition mechanisms among the isomers. All three TCP isomers decomposed by the same reaction mechanisms and with roughly the same kinetics on Fe. Upon heating, they 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 alkylphosphates as vapor phase lubricants.     

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 [(C₄H₉O)₃P=O] adsorbed on the Fe surface decomposes via C–O bond scission to give butyl surface intermediates [C₄H₉–] that react via β-hydride elimination to desorb as 1-butene [CH₃CH₂CH=CH₂] and H₂ without appreciable carbon deposition onto the surface. The thermal decomposition of 1-iodobutane [I-C₄H₉] on Fe was observed to proceed via the same β-hydride elimination mechanism. In contrast to tributylphosphate, meta-tricresylphosphate (m-TCP) [(CH₃–C₆H₄O)₃P=O] decomposes on Fe via P–O bond scission to produce methylphenoxy intermediates [CH₃–C₆H₄O–]. During heating to 800 K, methylphenoxy intermediates either desorb as m-cresol [CH₃–C₆H₄–OH] via hydrogenation or decompose further to generate tolyl intermediates [CH₃–C₆H₄–]. Some of the tolyl intermediates desorb as toluene [CH₃–C₆H₅] via hydrogenation but the majority decompose resulting in H₂ 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.     

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=CH₃C₆H₄–] 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 [(CH₃O)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 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 H₂, 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.     

The Difference in Running-In Period and Friction Coefficient Between Self-Mated Si3N4 and SiC Under Water Lubrication

Running-in periods and friction coefficients of SiC and Si₃N₄ sliding against themselves under water lubrication were investigated with a pin-on-disk apparatus at sliding speed of 120 mm/s and a normal load of 5 N under ambient conditions. It was found that the running-in period of self-mated Si₃N₄ is much shorter than that of self-mated SiC, and also that the steady-state friction coefficient of self-mated Si₃N₄ was lower (0.0035) than that of self-mated SiC (0.01). The difference in mechanism was analyzed from the point of view of electronic structure and surface chemistry.     

Investigation of wear mechanism of SiC particulate-reinforced Al-20Si-3Cu-1Mg aluminium matrix composites under dry sliding and water lubrication

Unreinforced Al-20Si-3Cu-1Mg (ASCM) aluminium alloy and SiC particle reinforced Al-20Si-3Cu-1Mg (ASCM-SiC) aluminium matrix composites were fabricated by powder metallurgy (). The samples were slid against 4Cr13 stainless steel in a reciprocal friction tester under a load of 25 N to 175 N and sliding velocity of 0.3 to 1.2 m s⁻¹ at ambient conditions. The results show that SiC particulate-reinforced aluminium matrix composites possess good wear resistance at dry sliding and less wear resistance under water lubrication. Ploughing wear is the dominant wear mechanism at dry sliding and tribochemical wear is dominant under water lubrication. SEM, AES and XPS were used to examine the wear morphology and surface chemistry.     

The effect of fluorination on the surface chemistry of alkyl ethers on Cu(111)

By using small fluorinated ethers as models for perfluoropolyalkylether (PFPAE) lubricants, we have been able to determine the effect of fluorination on the bonding of the alkyl ethers adsorbed on the Cu(111) surface. The desorption energies have been determined by using temperature programmed desorption (TPD). The model compounds studied were dioxolane , diethyl ether ((CH₃CH₂)₂O), dimethoxymethane ((CH₃O)₂CH₂), dimethyl ether ((CH₃)₂O) and their perfluorinated analogues. All of the molecules studied adsorb molecularly and reversibly on the Cu(111) surface exhibiting first-order desorption kinetics. Upon fluorination of the alkyl ethers, the adsorbate-metal bond was weakened by 14 to 8 kcal/mol. The intermolecular interaction parameters for the hydrogenated ethers indicated repulsive adsorbate-adsorbate interactions, while the fluorinated ethers have attractive adsorbate-adsorbate interactions.     

Reduction of adhesion and friction of silicon oxide surface in the presence of <Emphasis Type="Italic">n</Emphasis>-propanol vapor in the gas phase

The equilibrium adsorption of gas phase alcohol molecules has been proposed as a new means of in-use anti-stiction and lubrication for MEMS devices. Adhesion and friction of silicon oxide surfaces as a function of n-propanol vapor pressure in the ambient gas were invesitigated using atomic force microscopy. As the vapor pressure increases, the adsorbed n-propanol layer thickness increases. The adhesion and friction significantly decrease with very little addition of n-propanol vapor.     

The comparisons of sliding speed and normal load effect on friction coefficients of self-mated Si3N4 and SiC under water lubrication

The effects of sliding speed and normal load on friction coefficients of self-mated Si₃N₄ and SiC sliding in water after running-in in water were investigated with pin-on-disk apparatus at sliding speeds of 30 to 120 mm/s, normal loads of 1 to 14 N in ambient condition. The results showed that, after running-in in water, for two kinds of self-mated ceramics, friction coefficient increases with both decreasing sliding speed and increasing normal load when normal load is larger than a critical normal load. Friction coefficient was independent of normal load when normal load is smaller than the critical load. The lubrication film of Si₃N₄ under water lubrication exhibited larger load carrying capacity than that of SiC did. Stribeck curves indicated that, for self-mated Si₃N₄ ceramics, hydrodynamic lubrication will change into boundary lubrication abruptly when the sommerfeld number is less than a critical value; while for self-mated SiC ceramics, hydrodynamic lubrication will change into mixed lubrication and then into boundary lubrication gradually when the sommerfeld number is below critical value.     

Experimental simulation of phosphites additives tribochemical reactions by gas phase lubrication

Abstract

Tribochemical reactions of phosphites additives on steel surface have been simulated by gas phase lubrication. Trimethylphosphite (TMPi), P(OCH₃)₃, has been used as model molecule for phosphites additives. It has been introduced under gas phase up to 5 hPa in a new tribometer dedicated to gas phase lubrication. Friction tests have been carried out at ambient temperature and 100°C. Chemical analyses by X-ray photoelectron spectroscopy and by Auger electron spectroscopy have been conducted inside and outside of the track. Two kinds of analysis have been carried out: ex situ and in situ surface analyses after tribological test. Indeed, a new environmentally controlled tribometer allows friction test then accurate analyses without air exposure of the formed tribofilm. Tribotests conducted under TMPi gas phase show a reduction of friction coefficient until 0˙2 instead of 1˙4 under high vacuum. Jointly, formation of tribofilm has been confirmed by optical microscopy and ex situ chemical analysis. Comparison between analyses performed inside and outside of the wear scar indicates that the friction induces the formation of phosphide compound that could reduce friction. Moreover analyses show the formation of methoxy group (CH₃O) and carbonate originally from the decomposition of TMPi under friction into H₂ and CO. In situ analyses clearly show the importance to investigate an uncontaminated tribofilm in order to obtain a better characterisation of it and then a better comprehension of the tribochemical mechanisms.     

The tribological properties and tribochemical analysis of blends of poly alpha-olefins with neopentyl polyol esters

The tribological properties of blends of poly alpha-olefins with neopentyl polyol esters were evaluated with a four-ball tester. Results show blends display better tribological properties than pure neopentyl polyol esters. So blends can replace pure neopentyl polyol esters to reduce cost. The film-forming properties were also investigated. The worn surfaces were analyzed by SEM, LSCM and XPS. The surface analysis reveals chemical adsorption films and chemical reaction films composed of Fe₃O₄, FeO, Fe₂O₃ and [Fe(CH₃C(O)CHC(O)CH₃)₃] were formed on worn surfaces. Also, PAOs and neopentyl polyol esters get a synergistic boundary lubrication effect under both antiwear and extreme pressure conditions.     

Effect of Humidity on Lubricated Carbon Overcoats

An apparatus has been designed and built to measure the adsorption of contaminants on the surfaces of lubricated carbon overcoats such as those used on magnetic data storage media. The device is based on a quartz crystal microbalance housed in a high vacuum chamber and can be used to make rapid measurements of both the amounts and the rates of contaminant adsorption from the gas phase during exposure. Initial measurements of the adsorption of water during exposure to water vapor indicate that at room temperature and moderate humidity levels (50% RH) the amount of water on a surface is of the order of one adsorbed monolayer. Adsorption and desorption is remarkably rapid indicating that equilibrium with ambient humidity is reached on timescales of minutes. Finally, the comparison of H₂O and D₂O adsorption indicates that the adsorbed water either forms a hydrogen bonded network on the surface or is hydrogen bonded to some species exposed at the surface of the carbon overcoat.     

Surface chemistry of fluorine containing ionic liquids on steel substrates at elevated temperature using Mössbauer spectroscopy

Ionic liquids have properties that make them attractive as solvents for many chemical synthesis and catalysis reactions. Consequently, research has focused on their application as advanced solvents. Recently, ionic liquids were shown to have promise as a lubricant due to many of the same properties that make them useful as solvents. The focus of this paper is to study the surface chemistry of ionic liquid lubricated steel in sliding contact to temperatures from room to 300 °C. Tribological properties were evaluated using a pin on disk tribometer with high temperature capability (up to 800 °C). Chemistry was studied using Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Samples used for tribological evaluation were 1 inch diameter polished M50 disks. Samples used for studying the surface chemistry were enriched ⁵⁷Fe grown via thermal evaporation. Some ⁵⁷Fe samples were oxidized to Fe₂O₃ and Fe₃O₄ prior to treatment with ionic liquids. The metallic and oxidized ⁵⁷Fe samples were then reacted with ionic liquids at elevated temperatures. Three ionic liquids were used in this study; 1-n-ethyl-3-methylimidazolium tetrafluoroborate (BF₄), 1,2-di-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (TFMS), and 1,2-di-methyl-3-butylimidazolium hexafluorophosphate (PF₆). This study was focused on understanding the high temperature stability of the liquids in contact with metal and under tribological stress. Therefore, the friction data was collected in the boundary (or mixed boundary/EHL) lubrication region to enhance surface contact. BF₄ provided a friction coefficient of 0.04 for both the room and 100 °C tests and varied between 0.07 and 0.2 for the 300 °C test. The results from TFMS lubrication showed a friction coefficient of 0.025 at room temperature and 0.1 at 100 °C. The 300 °C test friction coefficient ranged between 0.1 and 0.3. Chemical analysis of the surface revealed corrosion of the surface due to reaction between the ionic liquids and steel/iron substrates.     

Mechanism of high-temperature lubrication of ceramics by deposits from organic vapors

While lubrication of hot-pressed silicon nitride surfaces at temperatures between 350 and 650°C by carbons deposited from carbonaceous gases and vapors has been demonstrated to be very effective (friction coefficients of less than 0.02 and negligible wear), a chemical mechanism has not yet been established. Tribometer experiments followed by surface spectroscopies of Raman, fluorescence, Auger and XPS are consistent with a heterogeneous mechanism of vapor decomposition. Laser-induced photoluminescence determined with a Raman spectrometer using 514 nm argon ion excitation showed a nearly 1 : 1 correspondence between the intensity of the broad maximum near 4000 cm^-1 with carbon adsorption and lubrication efficiency: surfaces of low fluorescence could not be lubricated by ethylene or similar gases. Carbon was found to adsorb preferentially on wear tracks, which also exhibited very high fluorescence. Wear track surfaces generally exhibited a lower N/Si surface composition ratio than the surrounding surfaces. The reduced nitrogen content could be responsible for dangling bonds and structural defects, which would result in more chemisorption and strong fluorescence.Professor Emeritus, Rensselaer Polytechnic Institute, where the work described was started. Currently Researcher at the Center for Magnetic Recording Research, University of California, San Diego.     

Electron stimulated decomposition of fluorocarbons on amorphous hydrogenated carbon (<Emphasis Type="Italic">a</Emphasis>-CH<Subscript>x</Subscript>) overcoats used in data storage media

The electron-induced surface chemistry of perfluoropolyalkylether (PFPE) lubricants on a-CH_x films has been probed by studying the impact of free electrons on perfluorodiethylether, (CF₃CF₂)₂O, and 2,2,2-trifluoroethanol, CF₃CH₂OH, as models of the chemical functionality of PFPE lubricants such as Fomblin Zdol. Electron-stimulated decomposition of (CF₃CF₂)₂O and CF₃CH₂OH on fresh and oxidized a-CH_x is observed when the sample is unbiased and in the presence of 70 eV free electrons. Electron-induced decomposition is indicated by the deposition of fluorine onto the surface of the a-CH_x film following desorption of molecular (CF₃CF₂)₂O and CF₃CH₂OH by heating in front of a mass spectrometer. Biasing the sample to −80 V successfully eliminates the decomposition by preventing the impingement of electrons onto the surface. The electron-stimulated decomposition of PFPE lubricants may contribute to lubricant decomposition during normal drive operation.     

The interaction of short-chain model lubricants with the surfaces of hydrogenated amorphous carbon films

The adsorption of water and small model perfluorinated lubricants on hydrogenated amorphous carbon (aC-H) films of varying hydrogen content was investigated using thermal desorption spectroscopy (TDS). Hydrogen content of the carbon films was measured by Rutherford back scattering (RBS) and elastic recoil spectroscopy (ERS) and correlated to changes in surface free energies measured by contact angle analysis. Hydrogenated carbon films exhibiting the highest surface free energy provided a greater attractive interaction for the model lubricants. All model lubricant species studied - water (D₂O), perfluorodiethyl ether (CF₃CF₂OCF₂CF₃), perfluoropentane (CF₃(CF₂)₃CF₃), perfluorooctane (CF₃(CF₂)₆CF₃), 2,2,2-trifluoroethanol (CF₃CH₂OH), and 1,1,7-H-perfluoroheptanol (CF₂H(CF₂)₅CH₂OH)—reversibly adsorbed to the carbon surface with little chemical reaction. Increases in desorption energies with increasing chain length were observed among the adsorbates and are ascribed to increasing van der Waals interactions. Incorporation of alcoholic end groups provided an avenue of hydrogen bonding to the surface and produced an ~20 kJ/mol increase in desorption energy relative to a perfluorinated alkane of the same chain length. Ether linkages within the model lubricant provide little increase in desorption energy as fluorine substituents effectively screen the oxygen. Together these findings implicate a predominantly physisorbed state for perfluorinated lubricants on hydrogenated carbon surfaces.     

Additive interaction and lubrication performance in a polar additive binary system

Additive interactions were studied between polar compounds, one of which decomposes at the frictional surface, on the lubrication performance of a binary system for polar straight chain hydrocarbons and aromatic compounds.As a comparison, an additive single component system was also studied. The lubricant performance of the blended oils was evaluated using a Falex lubricant testing machine and the adsorption characteristics of the additives on an iron surface were analysed.In the single component system, good correlation was obtained between the adsorption isotherm and the anti-seizure characteristics of the additives for both 1-octadecanol and 2-naphthol. In the binary system of 2-naphthol and 1-octadecanol, the lubrication characteristics of 2-naphthol appeared, producing heavy decomposition products of 2-naphthol because 2-naphthol was irreversibly (or chemically) adsorbed while 1-octadecanol was reversibly (or physically) adsorbed.Conversely, in the binary system of β-naphthol and stearic acid, the lubrication performance was poorer than that of the individual additive, since neither the decomposition products of the aromatics nor the adsorption film of stearic acid was sufficient against seizure. This was owing to the comparable adsorbability of the two compounds.