Friction in the Presence of Molecular Lubricants and Solid/Hard Coatings
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MRS BULLETIN/JUNE 1998
In general, advances in experimental studies have been realized through developing a greater sensitivity to the relatively few number of species found within a tribological contact. Recent developments in the areas of simulation and theoretical calculations of tribological interfaces have been realized largely through advances in computer technology, increasing the number of atoms within the contact and allowing a greater specification of parameters within the system. Despite the difficulties in simulating or measuring molecular events found within tribological interfaces, significant progress is now being made in understanding the relevant parameters associated with interfaces and relating the results of molecular-level simulations and measurements. Examples presented in this article, atomic-force-microscope (AFM) measurements and moleculardynamics (MD) simulations, are the result of such progress and represent the molecular level of detail currently being provided by modern research tools. Drawing upon these examples, the frictional properties of lubricated and hardcoating surfaces will be discussed in terms of energy dissipation, the chemical state of the interface, and the occurrence of tribochemical reactions.
Experimental and Theoretical Methods As previously mentioned, the development of a number of experimental and theoretical methods has fostered the re-
cent progress in tribology research. Experimentally, techniques that are providing molecularly specific information include the surface forces apparatus, 1 the AFM, 2 " the quartz-crystal microbalance,5 and optical-spectroscopy experiments being performed within the tribological contact.6 Theoretically, simple analytic models have had a long history in the study of friction.7 Whereas these models have yielded insight into friction, their main drawback is the simplifying assumptions that must be made. Thus unanticipated results may be overlooked. More recently, first-principles totalenergy techniques 8 and moleculardynamics simulations have been used to examine tribological properties.7'"15 In this article, we present examples from our own research—thus limited to studies performed with the AFM and with MD simulations—in an effort to highlight the areas in which experiment and theory agree in the field of molecular tribology and in which there is room for more work. An AFM can be used to measure tribological properties on the microscopic scale by measuring forces between the tip and sample that give rise to twisting or torsion of the tip. To obtain meaningful results, one must have knowledge of the tip shape and radius, the cantilever force constants, and the specifics of nanometer-scale contact mechanics.16 Tribological properties also can be studied on the atomic scale using MD simulations. These simulations have their basis in thermodynamics and classical mechanics, and involve tracking the motion of atoms as a function of time. It is natural to choose the constant-NVT (number of particles, volume, and temperature) ensemble for
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