Lubrication of ceramic contacts by surface-deposited pyrolytic carbon

Pyrolytic carbon is shown to be an efficient lubricant of ceramic bearing surfaces over a wide temperature range and at contact pressures even in the gigapascal range. It can be continuously replenished by a stream of carbonaceous gases made to pyrolyze at hot (greater than 700°C) surfaces. At ambient temperatures around 500°C, which are projected for advanced engine operations, the temperature peaks in bearing contacts can exceed 700°C and therefore cause pyrolysis to occur. At high surface velocities the time available at the peak temperatures can be inadequate for both nucleation and growth of pyrolytic lubricating carbon. This problem was solved by providing a separate 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 spectroscopy as combinations of diamond- and graphite-like (sp³ and sp²) carbons.