Contact-fluid interfacial slippage in hydrodynamic lubricated contacts

Recent experiments [E. Bonaccurso, H.J. Butt, V.S.J. Craig, Surface Roughness and Hydrodynamic Boundary Slip of a Newtonian Fluid in a Completely Wetting System, Phys. Rev. Lett., 2003, 90, No. 144501.] [R. Pit, H. Hervet, L. Leger, Direct Experimental Evidence of Slip in Hexadecane: Solid Interfaces, Phys. Rev. Lett., 2000, 85, pp. 980-983.] [V.S.J. Craig, C. Neto, D.R.M. Williams, Shear-Dependent Boundary Slip in an Aqueous Newtonian Liquid, Phys. Rev. Lett., 2001, 87, No. 054504.] [Y.X. Zhu, S. Granick, Rate-Dependent Slip of Newtonian Liquid at Smooth Surfaces, Phys. Rev. Lett., 2001, 87, No. 096105.] and molecular dynamics simulations [P.A. Thompson, S.M. Troian, A General Boundary Condition For Liquid Flow at Solid Surfaces, Nature, 1997, 389, pp. 360-362.] [M. Sun, C. Ebner, Molecular Dynamics Study of Flow at a Fluid-Wall Interface, Phys. Rev. Lett., 1992, 69, pp. 3491-3494.] showed that the contact-fluid interfacial slippage can actually occur in a practical lubrication in concentrated contacts not only of non-wetting or partial-wetting systems but also of completely wetting systems, due to the weak interaction strength and then the low shear strength between the contact-fluid interface. This slippage was universally shown to greatly reduce the load-carrying capacity of a lubrication in concentrated contacts in the condition of low fluid film thickness. These findings qualitatively agree with the theories developed in [Y.B. Zhang, et al., About the Load-Carrying Capacity of Elastohydrodynamic Lubrication Film, Tribol. Trans., 2001, 44, pp. 1-10.] [Y.B. Zhang, et al., A Lubrication Deviation From the Classical EHL Theory by the Lubricant Visoplasticity: Part I-Film Thickness Dependence, Tribol. Trans., 2001, 44, pp. 224-232.] [Y.B. Zhang, et al., A Lubrication Deviation From the Classical EHL Theory by the Lubricant Viscoplasticity: Part II-Boundary of Lubrication Regimes, Tribol. Trans., 2001, 44 , pp. 305-309.] [Y.B. Zhang, et al., An Analysis of Elastohydrodynamic Lubrication with Limiting Shear Stress: Part I-Theory and Solutions, Tribol. Trans., 2002, 45, pp. 135-144.] [Y.B. Zhang, et al., An Analysis of Elastohydrodynamic Lubrication with Limiting Shear Stress: Part II-Load Influence, Tribol. Trans., 2002, 45, pp. 211-216.] [Y.B. Zhang, et al., EHL Film Thickness Limitation Theory Under a Limiting Shear Stress, Tribol. Trans., 2002, 45, pp. 531-539.] for elastohydrodynamic lubrication (EHL) based on the contact-fluid interfacial slippage. It was actually proven that in a hydrodynamic lubrication the contact-fluid interfacial shear strength is usually low for high interfacial temperature and the contact-fluid interfacial slippage often happens and has a critical importance in a hydrodynamic lubrication due to this low interfacial shear strength [Y.B. Zhang, Contact-Fluid Interfacial Shear Strength and Its Critical Importance in Elastohydrodynamic Lubrication, Ind. Lubr. Tribol., 2006, 58, 1.], [Y.B. Zhang, Shear Strength of a Fluid in the Whole Ranges of Pressure and Temperature, Ind. Lubr. Tribol., 2006, 58, 2.]. It was also shown that in elastohydrodynamic lubrication of smooth concentrated contacts, when the contact-fluid interfacial shear strength in the inlet zone is low enough, the contact-fluid interfacial slippage widely occurs in the contact and significantly reduces the EHL load-carrying capacity especially in severe operating conditions i.e. for high rolling speeds, heavy loads, large slide-roll ratios and high bulk fluid temperatures [Y.B. Zhang, Mixed Rheologies in Elastohydrodynamic Lubrication, Ind. Lubr. Tribol., 2004, 56, pp. 88-106.]. As a result, viscoelastic fluid film, viscoplastic fluid film and non-continuum boundary film can respectively occur from the inlet zone to the Hertzian zone due to this slippage and the severe EHL film thinning in the Hertzian zone [Y.B. Zhang, Mixed Rheologies in Elastohydrodynamic Lubrication, Ind. Lubr. Tribol., 2004, 56, pp. 88-106.]. Therefore, due to the contact-fluid interfacial slippage, mixed rheologies occur in a smooth EHL contact in severe operating conditions and a mode of mixed lubrication is formed in this contact. New EHL and mixed EHL theories thus need to be established by considering the contact-fluid interfacial slippage. Furthermore, contact surface roughness was experimentally found to have an increasing effect on the contact-fluid interfacial slippage in a practical hydrodynamic lubricated concentrated contact [E. Bonaccurso, H.J. Butt, V.S.J. Craig, Surface Roughness and Hydrodynamic Boundary Slip of a Newtonian Fluid in a Completely Wetting System, Phys. Rev. Lett., 2003, 90, No. 144501.]. It was analytically shown that contact surface roughness can also increase the contact-fluid interfacial slippage and then further thin the film in an EHL contact [Y.B. Zhang, Contact Surface Irregularity and Contact-Fluid Interfacial Slip Effects in Elastohydrodynamic Lubrication, Jour. Bal. Trib. Assoc., 2004, 10, pp. 368-382.]. This especially occurs in severe operating conditions [Y.B. Zhang, Contact Surface Irregularity and Contact-Fluid Interfacial Slip Effects in Elastohydrodynamic Lubrication, Jour. Bal. Trib. Assoc., 2004, 10, pp. 368-382.]. A new mixed EHL theory is thus obviously required by considering contact-fluid interfacial slippage, contact surface roughness and mixed rheologies as shown in Ref. [Y.B. Zhang, Mixed Rheologies in Elastohydrodynamic Lubrication, Ind. Lubr. Tribol., 2004, 56, pp. 88-106.]. This is why it is so urgent to write lubrication theory allowing interfacial slippage incorporated into elastohydrodynamic lubrication. The present paper reviews the developments in the study of contact-fluid interfacial slippage and its effect in hydrodynamic lubricated concentrated contacts by both experiments and theories in recent years, and discusses about the importance and necessity of developing new EHL and mixed EHL theories based on the contact-fluid interfacial slippage, based on the previous research results on this subject.