Abstract: The friction and wear of ceramics Si 3N 4 against gray cast iron and 0.8% carbon steel under four lubricated conditions of dry friction,distilled water, emulsion and 10 # machinery oil have been studied in a ring block type wear tester. It is concluded that friction factor and wear are deceased in the order of dry friction,distilled water,emulsion and 10 # machine oil.Specially, friction factor is about 0.13 in emulsion and down to 0.02 in distilled water because the lubricating film containing graphite is formed on worn surface of gray cast iron in distilled water.The graphite can be taken as a solid lubricant in air and distilled water,but it can not in emulsion and oil.