LUBRICATION OF CERAMIC CONTACTS BY SURFACE-DEPOSITED PYROLYTIC CARBON

Pyrolytic carbon is shown to be an efficient lubricant of ceramic bear ing surfaces over a wide temperature range and at contact pressures ev en in the gigapascal range. It can be continuously replenished by a st ream of carbonaceous gases made to pyrolyze at hot (greater than 700-d egrees-C) surfaces. At ambient temperatures around 500-degrees-C, whic h are projected for advanced engine operations, the temperature peaks in bearing contacts can exceed 700-degrees-C and therefore cause pyrol ysis to occur. At high surface velocities the time available at the pe ak temperatures can be inadequate for both nucleation and growth of py rolytic lubricating carbon. This problem was solved by providing a sep arate hot surface to cause nucleation of carbon ahead of the contact. Pin-on-disc and four-ball test data were consistent with practical use of this concept of solid lubrication with lubricant replenishment by gaseous feed. The carbonaceous deposits were identified by Raman spect roscopy as combinations of diamond- and graphite-like (sp3 and sp2) ca rbons.