Transient High Friction Dominated by High Shear Strength Residual Water Film

Over the past few years, the importance of nanoscale technology in industries, such as data storage, micro-electro-mechanical systems (MEMs), and conventional sliding and rolling element bearings, has increased significantly. This is due to increased performance criteria and emerging technologies at smaller scales. One way to increase tribological performance of such applications is through nanoscale surface texturing. These textures will allow for precise control of the performance of lubricated surfaces with very thin films. This work examines how the behavior of the lubricant changes as the geometry of the texture is decreased toward the nanoscale. This work uses existing scale dependent lubrication theories to model the hydrodynamic lubrication of textured surfaces in attempt to predict how nanoscale textures will perform. The theoretical results show that the scale effects of a lubricant between textured surfaces can decrease the load carrying capacity while also decreasing the friction force. Overall, the friction force decreases more than the load carrying capacity and so the effective friction coefficient is decreased. It should be noted that relative to larger scale textured surfaces, the load support can also decrease with the decreasing scale of the texture. DOI: 10.1115/1.4001105

A Scale Dependent Simulation of Liquid Lubricated Textured Surfaces

High shear rate response of thin lubricant films in a slider-disk interface

Over the past few years, the importance of nanoscale technology in industries, such asdata storage, micro-electro-mechanical systems (MEMs), and conventional sliding androlling element bearings, has increased signiﬁcantly. This is due to increased perfor-mance criteria and emerging technologies at smaller scales. One way to increase tribo-logical performance of such applications is through nanoscale surface texturing. Thesetextures will allow for precise control of the performance of lubricated surfaces with verythin ﬁlms. This work examines how the behavior of the lubricant changes as the geometryof the texture is decreased toward the nanoscale. This work uses existing scale dependentlubrication theories to model the hydrodynamic lubrication of textured surfaces in at-tempt to predict how nanoscale textures will perform. The theoretical results show thatthe scale effects of a lubricant between textured surfaces can decrease the load carryingcapacity while also decreasing the friction force. Overall, the friction force decreasesmore than the load carrying capacity and so the effective friction coefﬁcient is decreased.It should be noted that relative to larger scale textured surfaces, the load support canalso decrease with the decreasing scale of the texture.

A Scale Dependent Simulation of Liquid Lubricated Textured

Liquid Bridge Modulated Friction: From Millimeter to Nanometer

Historically, the lubrication of solid surfaces has been characterized as comprising four major regimes: boundary lubrication, mixed lubrication, EHD, and hydrodynamic lubrication. One generally expects that the presence of the lubricant serves to reduce the friction of the interface. However, for very smooth surfaces, the introduction of lubricant can cause a substantial increase in friction over that of unlubricated sliding. While this increase in friction has been attributed to adhesive and/or viscous effects, it has been difficult to predict which of these contributions is most important to a given sliding regime. In the present work, an attempt is made to delineate regimes of lubrication where one or the other of the mechanisms is predominant. Measurements of friction are carried out for several perfluoropolyether lubricants in a thickness range of 0 to 80 nm and at sliding speeds of 0.25 to 250 mm/s. Three distinct regimes of lubrication are observed. The first, an “adhesion” regime, is characterized by large friction at low speeds and a decreasing friction with increasing sliding speed. The second, a hydrodynamic regime, is well described by classical lubrication theory and exists at higher sliding velocities than the first regime. The third regime, identified by an abrupt change in the dependence of friction on velocity, exists at relatively large sliding speeds and indicates substantial shear-thinning of the lubricant. The onset of the third regime is predicted from consideration of the tensile stresses in the lubricant film. It is also found modelling the lubricant as a continuum is valid at least down to 23 nm for the materials studied.

J.P. Gerhardstein

Papers

Lubrication Regimes for Nanometer-scale Lubricant Films with Capillary Effects