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Exhaust gases from internal combustion engines contain appreciable quantities of various hydrocarbons and carbon monoxide. Solid carbon formed by decomposition of these gases on tribosurfaces could replace circulating lubricating oil, especially in low heat rejection engines. Experiments with silicon nitride in a pin-on-disc tribometer operating at 520°C, 2.2GPa contact pressure and 4.4cm/s sliding speed have demonstrated that friction and wear can be reduced by over 90% of their unlubricated values when ethylene, acetylene, and carbon monoxide and hydrogen are directed into the contacts. The lubricating deposit composition and morphology have been evaluated and compared in terms of effectiveness.

For lubrication and reduced wear of friction couples at high temperatures, such as those required by the “adiabatic” or low heat rejection engine, solid lubricants are the materials of choice. Their replenishment under operating conditions is, however, more difficult than that of liquid lubricants. Two principal approaches have been suggested: (i) reaction of the boundary surfaces with vaporized liquid lubricants [1]∗ and (ii) dissociation of a gas, stable at high temperature, at the boundary surfaces to produce a lubricating carbon [2]. Continuing work by the latter approach has demonstrated its feasibility at temperatures between 400 and 650°C with both a metallic (NiAℓ) and a ceramic surface (Si3N4 · Aℓ2O3) in a pin-on-disc tribometer for ethylene gas. Friction coefficients dropped to < 0.02.

All organic materials become very unstable at high temperatures. They will crack, coke, polymerize, especially on hot solid surfaces and on some more so than on others. They can also react with the surfaces. Some of these pyrolysis or reaction products can be good solid lubricants. They don't last long, but then again under friction and wear new active surfaces and more lubricants can be formed. This is a concept of solid lubricant regeneration. Our work has proved experimentally that this concept has merit, perhaps as a result of partial graphitization, under selected conditions. In particular, in an environmental chamber, on heated nickel or nickel alloy and palladium surfaces in inert atmospheres, friction and wear coefficients were found to drop by an order of magnitude or more when as little as 1% of ethylene gas was introduced. Diffusion of elemental carbon through the metal lattice appears to be the rate-controlling step in the process.

The wear process in bearings generates a clean active surface. Carbon is known to form readily on catalytic surfaces through the reduction of carbon monoxide or hydrocarbons. Carbon, through the adsorption of hydrocarbons, water vapor, or oxygen, becomes an effective lubricant. If these three phenomena can be made to work together, a new concept of high temperature lubrication would be available for combustion engines. This paper covers initial laboratory investigations towards the development of this concept. Carbon has been successfully produced through catalytic reduction of ethylene on a variety of metallic and ceramic surfaces containing nickel. This carbon has been shown to reduce friction at a sliding interface.