Abstract:A systematic investigationwas carried out on a universal ballondisk tribometer with stainless steel/Si3N4 as frictional pairs to study the tribological behaviour of tannin acid as waterbased lubricating additive under undirectional sliding.The adsorption of tannin acid on the surface of stainless steel substrate was studied by means of electrochemical workstation and contact angle meter,and the friction reducing and antiwear mechanism of tannin acid was discussed.It is found that adding a small amount of tannin acid to deionized water can significantly improve the tribological performances of water.However,when the concentration of tannin acid in water is more than 5 g/L,larger wear is found for stainless steel.The friction reducing and antiwear mechanism of tannin acid is due to the formation of irontannin acid complex on stainless steel surface during the sliding.