Accessing Inaccessible Interfaces: In Situ Approaches to Materials Tribology
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Inaccessible Interfaces: In Situ Approaches to Materials Tribology
W. Gregory Sawyer and Kathryn J. Wahl, Guest Editors Abstract The field of materials tribology has entered a phase of instrumentation and measurement that involves accessing and following the detailed chemical, structural, and physical interactions that govern friction and wear. Fundamental tribological research involves the development of new experimental methods capable of monitoring phenomena that occur within the life of a sliding contact. Measuring friction phenomena while the process is ongoing is a major improvement over earlier techniques that required the surfaces to be separated and analyzed, thereby interrupting the frictioncausing event and modifying surface conditions. In the past, MRS Bulletin has highlighted how in situ approaches can greatly enhance our understanding of materials structure, processing, and performance. This issue highlights in situ approaches as applied to materials tribology, namely, the study of contacting surfaces and interfaces in relative motion.
Introduction Tribology is a field of study that is focused on the fundamental investigations of friction and wear. As recently summarized in a report on the “Frontiers of Fundamental Tribology,”1 new tools are needed to monitor tribological phenomena that are occurring within buried interfaces. These tools are essential for fundamental studies of friction and wear because they are not intrinsic properties of a material; rather, they are functions of the tribological system (which includes the contacting surfaces that are in relative motion, the local environment, the background temperature, the surface roughness and preparation, the sliding speeds and loads, and a host of other contributors). Over the past half century, tribological systems have been discussed and described in terms of three basic groups of thematically linked elements:2 (1) the types
of materials in contact and the contact geometry; (2) the operating conditions, including the gross motion, loads, stresses, and duration of operation; and (3) the environment and surface conditions, including the surface chemistry, surface topography, and ambient temperature. The incredibly large number of factors affecting tribological performance makes fundamental studies of materials tribology exceedingly difficult. Energy and material losses in moving mechanical devices as a result of friction and wear impose an enormous cost on the national economy. Engineering tribology involves the designs of bearings, bushings, and a wide variety of interfaces that support our everyday mobility and often aims to simultaneously reduce both friction and wear. Practical solutions to mitigate friction and wear have traditionally
MRS BULLETIN • VOLUME 33 • DECEMBER 2008 • www.mrs.org/bulletin
been through the use of fluid lubricants such as oils and greases. However, there are a number of applications where traditional fluid lubrication strategies are either precluded or undesirable.3,4 Materials tribology, and in particular solid
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