This report describes experiments on friction of Si3N4and SiC sliding against themselves and against each other in water at room temperature. The friction coefficient of Si3N4against itself is f = 0.7 at low speeds (0.2 cm/s). At speeds above 6.5 cm/s and 4.91-N load, hydrodynamic lubrication sets in, with f = <0.002 and an estimated water film thickness of 70 nm. Between 0.2 and 5 cm/s, mixed lubrication occurs. This remarkably thin hydrodynamic film is made possible by the tribochemical wear of Si3N4which occurs by dissolution in water and an absence of wear particles. The friction coefficient of SiC, against SiC or Si3N4in water at low speeds is f = 0.26. Wear occurs by dissolution and fracture pitting. Small wear particles are observed. The friction coefficient of SiC against SiC is independent of sliding velocity between 0.2 and 20 cm/s. Hydrodynamic lubrication is obtained with a Si3N4pin on SiC plate near 10 cm/s, mixed lubrication is obtained with a SiC in on Si3N4plate at v < 20 cm/s.Presented at the 41st Annual Meeting in Toronto, Ontario, Canada May 12–15, 1986