Abstract: Friction and wear,phase transformation and brittle-ductile behavior of single crystal silicon against Si_3N_4 under lubrication at low contact stress were studied.Friction and wear tests were performed on a UMT-2MT test rig under linear velocity of 3 mm/s and load of 1 N under boundary lubrication.Pentadecane,anhydrous ethanol and distilled water were used as lubricants during the tests.Single crystal silicon showed the lowest friction coefficient(0.136) and wear volume under lubrication of pentadecane.The morphologies and phase compositions of the worn surface were analyzed by means of scanning electron microscopy and Raman micro-spectroscopy.It was found that the worn surface of single crystal silicon lubricated by pentadecane was characterized by ductile regime.Raman micro-spectrum analysis indicated that phase transformation of Si-I to a-Si was involved on the worn surface.Under lubrication of anhydrous ethanol,the phase transformation of Si-I to a-Si on the worn surface could be identified although a-Si is much less than that under lubrication of pentadecane.Coexistence of brittle and ductile characteristic could be found on the worn surface.A sight transformation of Si-I to Si-III on the worn surface of single crystal silicon lubricated by distilled water was,detected by Raman micro-spectrum.In addition,the worn surface became rough and a large number of micro-fracture was found.Polarity of the lubricants might be one of the main factors for phase transformation as well as brittle and ductile behavior of single crystal silicon.