Tribochemical lubrication of ceramics by carbon from organic vapors

Carbons deposited on frictional contacts between ceramic surfaces as a result of the injection of an organic gas or vapor have been shown to have lubricating properties. In a pinon-disc apparatus--sapphire pin on discs of Sialon, silicon carbide, zirconia or zirconia/alumina--friction coefficients as low as 0.02 could be maintained in at least some temperature ranges between 20 and 700°C as long as the contact region was surrounded by a carbonaceous gas supplied a[ ambient temperature at flow speeds high enough to keep the rate of carbon deposition above the wear rate. Mean contact pressures were 300 MPa. Evidence was also obtained to show that low friction could be maintained similarly in a simulated nickel-coated silicon nitride-ball and silicon nitride-race roller bearing operated at 1.5% slip at 450°C at a mean contact pressure of 2 GPa. Raman and Auger spectroscopies of the carbonaceous deposits formed in the wear tracks showed significant differences depending on the temperature range and the feed gases or vapors supplied. In general, friction and wear reduction occurred in two stages at different rates. The tribochemistry involved is likely to comprise hydrocarbon acid-catalytic cracking at low temperatures and pyrolysis at high temperatures. Recent efforts toward greater efficiency in engines have been focusing on reduced weight and higher operating temperatures and therefore on ceramic materials as replacements of metals. Ideally the cooling system is eliminated, resulting in even higher operating temperatures. A major stumbling block in these developments has been the lack of appropriate high-temperature lubricants. Ideally the lubricants should be solids that can be continuously replenished during engine operation. Replenishment is no great problem with liquid lubricants: pumping systems can be used. Solid lubricants can be resupplied as vapors or mists, which can adsorb on and react with the tribo-surfaces, or--and that has been our approach--as thermally stable gases capable of reacting a, m t with the tribosurfaces to form solid lubricants. The former approach is that of Klaus and coworkers [I]; it can work over a wide temperature range, since a lubricant is always present, and with a great variety of materials but it requires an elaborate system of preheating and conduction of the heated vapors through heated tubes and it wears the solid tribosurfaces chemically. Our approach is much simpler and essentially not destructive of the solid wear surfaces but restricted to certain kinds of metallic and ceramic surfaces. Our early work seemed to require metallic catalytic surfaces, notably nickel and palladium to decompose our feed of ethylene gas to lubricating carbon and hydrogen [2]. Recently, however, we were able to remove most of the restrictions--ethylene can be replaced by other gases and vapors, notably propane, benzene, and even propyl alcohol, though not with equal efficiency, and metallic surfaces can be replaced by ceramics such as silicon carbide, Sialon (silicon nitride with 10% alumina), zirconia or zirconia-alumina without surface overcoats [3]. Acid catalysis could play an essential role in the formation of lubricating carbon, especially at low temperatures. Very low friction and wear coefficients have been measured with these ceramics, in fact, friction coefficients so low (< 0.02) that the lubrication regime could be hydrodynamic. Our concept of , lubrication and replenishment has been demonstrated . to work between ambient and 700"C.provided air or oxygen can be excluded from the contact region into which a (flammable) carbonaceous gas is injected. This condition can be achieved by maintaining a strong flow of the active gas into the contact region or, as in the case in aircraft turbine bearings, submersion in fuel-derived gases. Air or oxygen would not need to be excluded if the lubricant-former are not flammable. Klaus and coworkers [I] have avoided the flammability problem by the use of non-flammable phosphate esters. The work reported in this paper has been concerned Copyright @ 1990 American Institute of Aeronautics and Astronautics, Inc. All rights reserved primarily with the mechanism of this lubrication by lubricants formed at the surfaces and the nature of these lubricants for different gas and vapor feeds and solid surfaces. Auger electron spectroscopy (AES) and Raman spectroscopy (RS) have been our principal analytical tools besides the friction and wear records themselves.