Using an optical interferometry system,the elastohydrodynamic lubrication (EHL) films in a ballondisc contact were observed under pure sliding and zero entrainment velocity (ZEV) condition.The results are inconsistent with the prediction of traditional EHL theory,which is attributed to wall slippage effect.Two possibilities of wall slippage,i.e.the limiting shear stress of lubricants and the critical shear stress of interface were discussed.It is shown that the critical shear stress between solidliquid interfaces may be responsible for the present observations.In order to validate the mechanism of critical shear stress,the movements of an entrapped lubricant in EHL contact were studied.The results show good agreement with the above assumption.By measuring the dimple core velocities under the conditions of ZEV and simple ball sliding,it is shown that the limiting shear stress of lubricant cannot be exceeded.Therefore,it is indicated that the glass interface coating Cr layer employs the lowest critical shear stress.