High Temperature Lubrication by Carbon Continuously Replenished by Surface Reaction with Carbonaceous Gases-Comparison of Metallic (Ni) and Ceramic (Si3N4) Surfaces

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. A lubricating carbon was formed on both types of surfaces, but of different properties: that on nickel aluminide was 0.6 - 1.0μm thick and dendritic outside but flaky inside the wear track, while that on Sialon was about 30 Å thick and was found only in the wear track. Both of these carbons stuck tenaciously to their substrates. Raman and Auger spectra were substantially different.