In-Contact Molecular Spectroscopy of Liquid Lubricant Films
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The Application of In-Contact Methods to Tribology Research Our world today relies on lubrication (or, more generally, tribology, which is the science of rubbing surfaces) to keep it moving. This is not just true of engines and bearings but also applies to many other aspects of everyday life, from ice skating to synovial lubrication of hip and ankle joints. The tribological triumvirate of friction (force required to overcome resistance to motion), lubrication (interfacial film formation), and wear (material loss at surfaces) characterize the rubbing process. Whenever a mechanism relies on the relative motion of two surfaces under load, the interfacial lubricant film plays a critical role in determining the performance of the system. The fric-
tion, wear, and, ultimately, service life of the component are determined by the properties of this film and the nature of the rubbing surfaces. In most systems, the role of the lubricant is to separate the surfaces, thus reducing damage and controlling friction. However, in some cases, controlled material loss at the surface is required; an important example of such an application is railtrack wear. In this case, controlled wear of the rail surface is necessary to truncate crack growth that otherwise could result in track failure due to fatigue. Similarly, in most applications, the aim is to reduce friction; this is particularly important, for example, in improving energy efficiency and reducing fuel
MRS BULLETIN • VOLUME 33 • DECEMBER 2008 • www.mrs.org/bulletin
consumption in internal combustion engines. However, for some applications, such as brakes and traction drives, the opposite is required. Thus, to optimize the performance of a component, it might be necessary to increase or reduce both the friction and the surface wear, and the lubricant plays an important role in achieving this goal. The mechanisms of formation and the properties of the interfacial film, particularly relative thickness and chemical composition, are the determining factors. Thus, an understanding of the film properties is important for developing new lubricant formulations and optimizing performance and component life. As a liquid lubricant passes into the contact zone formed by the loaded surfaces, it experiences a rapidly changing environment that depends on the nature of the rubbing surfaces and the operating conditions (primarily load, temperature, and mean speed). The features of a lubricated contact are shown in a simple schematic diagram in Figure 1a. The contact is formed by two surfaces that are loaded together and are in relative motion. The lubricant is drawn into the converging gap at the entrance to the loaded zone by the movement of the surfaces. The lubricant experiences rapidly increasing shear and pressure fields as it passes into the loaded region, where it is required to form an interfacial film carrying the load and separating the surfaces. Figure 1a shows the contact conditions experienced by the lubricant as it passes through a loaded contact for idealized smooth surfaces. I
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