High Temperature Solid Lubricant Research Articles

Characterization of annealed pulsed laser deposited (PLD) thin films of cesium oxythiomolybdate (Cs 2MoOS 3)

Cesium oxythiomolybdate (Cs 2MoOS 3) is under consideration as a high temperature solid lubricant for silicon nitride bearings. However, Cs 2MoOS 3 oxidizes below the proposed maximum use temperature of 800 °C, and the oxidation process is complex. In the presence of silicon nitride, the oxidation reactions change. The purpose of the present research was to determine the chemistry of Cs 2MoOS 3 coatings grown by pulsed laser deposition (PLD) on several substrates (Si 3N 4, SiC, Inconel, Al 2O 3 and ZrO 2), and to determine the changes in chemistry and crystal structure after annealing in air to temperatures up to 800 °C. Many of the oxidation products identified are expected to be lubricious at elevated temperatures.

Thermal Processing Effects on the Adhesive Strength of PS304 High Temperature Solid Lubricant Coatings

The effects of post deposition heat treatments on the strength properties ofPS304, a plasma sprayed nickel-chrome based, high temperature solid lubricant coating are studied. Coating samples were exposed in air at temperatures from 432 to 650°C for up to 500 hr to promote residual stress relief, enhance particle to particle bonding and increase coating to substrate bond strength. Coating pull off strength was measured using a commercial adhesion tester. The as-deposited (untreated) samples either delaminated at the coating-substrate interface or failed internally (cohesive failure) at about 17 MP a. Samples heat treated at temperatures above 540°Cfor 100 hr or at 600°C or above for more than 24 hr exhibited strengths above 31 MPa. Coating failure occurred inside the body of the coating (cohesive failure) for nearly all of the heat-treated samples and only occasionally at the coating substrate interface (adhesive failure). A microstructural change is believed to be partially responsible for the coating strength increase. Increasing the heat treatment temperature, exposure time or both accelerate the heat treatment process. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

The Effect of Journal Roughness and Foil Coatings on the Performance of Heavily Loaded Foil Air Bearings

Foil air bearing load capacity tests were conducted to investigate if a solid lubricant coating applied to the surface of the bearing's top foil can function as a break-in coating. Two foil coating materials, a conventional soft polymer film (polyimide) and a hard ceramic (alumina), were independently evaluated against as-ground and worn (run-in) journals coated with NASA PS304, a high-temperature solid lubricant composite coating. The foil coatings were evaluated at journal rotational speeds of 30,000 rpm and at 25 °C. Tests were also performed on a foil bearing with a bare (uncoated) nickel-based superalloy top foil to establish a baseline for comparison. The test results indicate that the presence of a top foil solid lubricant coating is effective at increasing the load capacity performance of the foil bearing. Compared to the uncoated baseline, the addition of the soft polymer coating on the top foil increased the bearing load coefficient by 120 percent when operating against an as-ground journal surface and 85% against a run-in journal surface. The alumina coating increased the load coefficient by 40 percent against the as-ground journal but did not have any effect when the bearing was operated with the run-in journal. The results suggest that the addition of solid lubricant films provide added lubrication when the air film is marginal, indicating that as the load capacity is approached foil air bearings transition from hydrodynamic to mixed and boundary lubrication. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

Thermal chemistry of a high temperature solid lubricant, cesium oxythiomolybdate. Part I. Thermo-oxidative stability of Cs2MoOS3

Cesium oxythiomolybdate (Cs2MoOS3) is a potential high temperature solid lubricant. It undergoes complex oxidation reactions at elevated temperatures, but continues to provide lubrication above the oxidation temperature. Therefore, in order to determine the nature of the lubricant at elevated temperature, it is necessary to understand the thermal chemistry of Cs2MoOS3in an air environment. The thermo-oxidative stability of Cs2MoOS3was evaluated between room temperature and 800°C in air. Melting and phase transition temperatures were determined. X-ray photoelectron spectroscopy, micro-Raman scattering and x-ray diffraction were used to identify the chemical species evolved at increasing temperatures. As-received Cs2MoOS3was not pure. It also contained cesium molybdates, molybdenum oxides, and Cs2SO4. Between 300–400°C, the material began to decompose forming Cs2SO4and MoS2. Between 400–600°C, Cs2MoOS3also formed cesium molybdates and molybdenum oxides. In addition, the Cs2SO4began to oxidize to cesium oxides (which melted) and SOxgas. Also, MoS2oxidized to MoO3. At approximately 700°C, MoO3began to sublime. Upon cooling from 800°C, the material was primarily cesium oxides and Cs2MoO4, with small amounts of complex cesium molybdates and molybdenum oxides.

Solid Lubrication of Silicon Nitride with Cesium-Based Compounds: Part I — Rolling Contact Endurance, Friction and Wear

The high temperature rolling contact endurance, friction, and wear of 16 cesium-based compounds with solid lubricating properties were investigated on silicon nitride (Si3N4). Some were also investigated on bearing tool steels and several state-of-the-art high temperature solid lubricants were investigated for comparison. Experiments were conducted in air at temperatures up to 650°C, contact stresses up to 4.34 GPa, and a pure rolling surface speed of 1.8 m/s. Although all of the cesium-based compounds exhibited self-lubricating properties, the best overall performance was achieved with a cesium silicate reaction film formed in-situ (Cs2O·xSiO2) and a hydrated cesium silicate bonded coating (Cs2O·3SiO2·nH2O). Bonded coatings of cesium oxythiotungstate + tungsten disulfide mixture (Cs2WOS3 + WS2) and cesium hydroxide (CsOH) also performed well. It is hypothesized that high temperature chemical reactions between the cesium-containing compounds and the silicon nitride surface form a lubricious cesium silicate film. Presented at the 55th Annual Meeting Nashville, Tennessee May 7–11, 2000

The Evaluation of a Modified Chrome Oxide Based High Temperature Solid Lubricant Coating for Foil Gas Bearings

This paper describes the friction and wear performance of PS304, a modified chrome oxide based coating, for foil gas bearings. PS304 contains 60 wt. % NiCr binder, 20 wt. % Cr203 hardener, and 10 wt. % each Ag and BaF2ICaF2 lubricants. For evaluation, the coating is plasma spray deposited onto test journals which are slid against a superalloy partial arc foil bearing. The test load was 10 KPa (1.5 psi) and the bearings were run under start/stop cyclic conditions. The data show good wear performance of the bearing especially at temperatures above 25 °C. Bearing friction was moderate (μ ≤≈0.4) over the entire temperature range. Based upon the results obtained, the PS304 coating has promise for high temperature, oil-free turbomachinery applications. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

Comparison on the microstructure and mechanical strength of a Ni-based alloy with and without 3wt.% CeF3 addition

Abstract The need for high temperature self-lubricating composites with excellent tribological properties as well as mechanical strengths leads to the survey for new types of high temperature solid lubricant material. The addition of CeF3 to a Ni-based alloy showed promise for providing good mechanical strength as well as tribological properties. In the paper, based on the authors' previous work, comparison on the microstructure of a Ni-based alloy with and without 3 wt.% (4.3 vol.%) CeF3 addition was made. Finally, the correlation between the microstructure and mechanical strength is discussed.

Calcium sulfate as a high temperature solid lubricant

Both composite coatings and layered structures of CaF 2 and WS 2 were grown via pulsed laser deposition (PLD) on steel substrates and TiN coated steel substrates. Films were found to be lubricious at temperatures up to 500°C. XPS analysis of specimens indicated that the CaF 2 and WS 2 materials interacted to form CaSO 2, among other compounds. CaSO 1 was then evaluated as a solid lubricant coating and found to have an excellent coefficient of friction of ∼ 0.15 at 500°C. Comparison of CaSO 1 films with CaF 2 films showed CaSO 4 films performed better than CaF 2 coatings both alone and in layered structures with WS 2 or Ag.

Characterization of air-annealed, pulsed laser deposited ZnO-WS 2 solid film lubricants by transmission electron microscopy

ZnO-WS 2 is a candidate high temperature solid film lubricant for aerospace applications that exhibits adaptive lubricant behavior. In the as-deposited state, room temperature (RT) pulsed laser deposited (PLD) ZnO-WS 2 films are amorphous, but when wear-tested, the crystalline phases WS 2, WO 3 and ZnWO 4 are produced. Of these, WS 2 is a lubricant phase at low temperatures (≤ ∼ 450°C) while ZnWO 4, which is formed by reaction of the film with air, becomes lubricious above 600°C. If this material is to be used at elevated temperatures, the characterization of the microstructural and chemical changes that occur when these films are heated in air is extremely important to the understanding of the dynamics of this system. As-deposited films and films heated in air at increasing temperatures to 800°C were examined by transmission electron microscopy (TEM), Raman spectroscopy, and scanning electron microscopy (SEM). Cross-sectional TEM (XTEM) of the as-deposited RT-PLD ZnO-WS 2 films showed that they were fully dense and amorphous. A periodic structure was seen that was due to density variations and was attributed to a smaller angular distribution of W in the plume compared to the other elements. At 500°C, an approximately 37 nm-thick film of WO 3 and ZnWO 4 formed at the surface. At 600°C, a 150–200 nm-thick mixed oxide layer of ZnWO 4 and WO 3 formed at the surface with the WS 2 phase forming below it. The volume fraction of WS 2 decreased with increasing depth from the surface. Above 600°C, surface roughening of the film was seen as well as significant grain growth of the WO 3 and ZnWO 4 phases. The ZnO phase was not detected in any of the films heated in air. The dynamics of the nucleation of these lubricant phases are advantageous with respect to applications: the high temperature lubricant phase, ZnWO 4, is available at the surface while the low temperature phase, WS 2, remains intact to provide lubrication when the temperature is lowered.

Ion-beam deposited Cu-Mo coatings as high temperature solid lubricants

Thin coatings of Cu-Mo were deposited on alumina substrates via ion-beam deposition (IBD). Structure and composition of the coatings were examined using X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. Sliding tests of coated and uncoated substrates were performed in air under reciprocating sliding conditions against alumina ball counterfaces; mean Hertzian contact pressures were 1.0 and 1.6 GPa, and test temperatures were between 25 and 650 °C. The as-deposited coatings were metallic and amorphous, but after exposure to temperatures ±530 °C, they converted to oxides containing CuMoO 4 and MoO 3. With increasing temperature, the friction coefficients of the IBD Cu-Mo coatings decreased from ∼0.5 to ∼0.2. At high temperature, the coatings were capable of sustaining low friction sliding (μ0.2) for 3000 cycles. At high load and temperature, the ball wear coefficient against the IBD Cu-Mo-coated alumina was 20–100 times less than that of uncoated alumina. The low friction observed at high temperatures was attributed to transformation of the coatings to crystalline oxides, which are known to be lubricious at elevated temperatures.

The effect of counterface on the tribological performance of a high temperature solid lubricant composite from 25 to 650°C

The effect of counterface selection on the tribological performance of A Ag/BaF 2CaF 2 containing composite coating are studied. Ceramic (Al 2O 3) and metal (Inconel X-750) pins are slid against PS300 (a metal bonded chrome oxide coating with Ag and BaF 2/CaF 2 lubricant additives) in a pin-on-disk tribometer at 25, 500 and 650°C. Compared to the ceramic counterface, the metal counterface generally exhibited lower friction and wear at 25°C but higher friction and wear at 650°C. Friction coefficients, for example, for the Al 2O 3/PS300 combination at 25°C were 0.64 compared to 0.23 for the Inconel/PS300 sliding couple. At 650°C the ranking was reversed: the Al 2O 3/PS300 combination gave a friction coefficient of 0.19 while the friction for the metal counterface increased slightly to about 0.3. Based upon these tribological results and other information found in the literature, it appears that the performance of each counterface/PS300 combination is affected by the ability of the solid lubricant additives to form an adequate transfer film. The effects of surface wettability and tribological compatability are discussed in relation to the observed tribological results.

The correlation among the tribological characteristic parameters of rare earth trifluorides and their electron structures, bond parameters, and thermodynamic parameters

Compared with other metal fluorides, the rare earth trifluorides (mainly refers to the trifluorides of light rare earths such as La, Ce, Gd, etc.) are more thermally and chemically stable. Therefore, some rare earth trifluorides have been used as high temperature solid lubricants. For example, LaF 3 and CeF 3 have been applied in lubricating greases and bonded films recently. But the related research work was confined to LaF 3, CeF 3, and NdF 3. Moreover, the friction and wear behaviours of rare earth trifluorides as lubricating additives need to be disclosed. Especially the correlation among the tribological characteristic parameters of rare earth trifluorides and their electron structures, bond parameters, and thermodynamic parameters need to be revealed. In addition, more work should be done so as to realize the action mechanisms of the rare earth trifluorides as lubricating additives. Because the measurement of some thermodynamic parameters of heavy rare earth trifluorides is confined owing to the experimental difficulties related to the chemistry of fluorine, so in this work, the light rare earth trifluorides such as LaF 3, CeF 3, PrF 3, SmF 3, EuF 3, GdF 3, and the mixtures of these rare earth trifluorides were selected as the additive samples. The antiwear and extreme pressure properties of these compounds as additives in lithium grease were evaluated on a four-ball friction and wear tester and an Optimol SRV friction and wear tester. Meanwhile, the correlation among the tribological characteristic parameters of these rare earth trifluorides and their electron structures, bond parameters, and thermodynamic parameters were investigated as well.

Characterization of pulsed laser deposited Zno-WS2 thin solid film lubricants

ZnO-WS2 is a potential high temperature solid film lubricant for aerospace applications that exhibits adaptive lubricant behavior. An adaptive lubricant undergoes phase and/or chemical changes in response to thermal, environmental, and tribological conditions; with the resulting phase or wear debris also being lubricious. Pulsed laser deposited (PLD) ZnO-WS2 thin films deposited at room temperature (RT) and wear-tested at room temperature have been shown to have coefficients of friction of 0.04 or less which are comparable to WS2 films, but have much longer wear lives. In the as-deposited state, PLD ZnO-WS2 films are amorphous, but when wear-tested, the phases WS2, WO3, and ZnWO4 are produced. Of these, WS2 is a lubricant phase at low temperatures (⪯ ~450°C) while ZnWO4 is a lubricant phase above about 600°C. The purpose of this work was to characterize the microstructural and chemical changes that occur when the RT-PLD ZnO-WS2 films are heated in air.The RT-PLD ZnO-WS2 films were deposited in a system having a base pressure of 9×l0-7 Pa with a typical pressure during deposition of 6×10-5 Pa.

The Use of Silver in Self-Lubricating Coatings for Extreme Temperatures

The advantages and disadvantages of elemental silver as a tribological material are discussed. It is demonstrated that the relatively high melting point of 961 deg C, softness, marked plasticity, and thermochemical stability of silver combine to make this metal useful in thin film solid lubricant coatings over a wide temperature range. Disadvantages of silver during sliding, except when used as a thin film, are shown to be gross ploughing due to plastic deformation under load with associated high friction and excessive transfer to counterface surfaces. This transfer generates an irregular surface topography with consequent undesirable changes in bearing clearance distribution. Research to overcome these disadvantages of element silver is described. A comparison is made of the tribological behavior of pure silver with that of silver formulated with other metals and high-temperature solid lubricants. The composite materials are prepared by co-depositing the powdered components with an airbrush followed by furnace heat treatment or by plasma-spraying. Composite coatings were formulated which are shown to be self-lubricating over repeated, temperature cycles from low temperature to about 900 deg C.