Effects of Vapor Adsorption Isotherm On Adhesion, Friction, and Wear of Solid Interfaces

In tribology, there are many examples where we expect certain adhesion, friction and wear behaviors based on mechanical properties of solids or structures of molecules involved; but find drastically different results. There are clear demonstrations that fundamental understandings of tribological systems at nano- or molecular scales are still far from realistic. For example, it is often expected that friction and wear behaviors of glass and silica are similar since both are based on amorphous Si-O networks and their moduli are similar. Thus, many scientists study silica surfaces as a model system for more complicated glasses. However, their wear behaviors are drastically different in humid environments. We expect the wear is always lower when the friction coefficient is lower; but diamond-like carbon (DLC) exhibits no wear in alcohol vapor environments even though its friction coefficient is much higher than the values measured in dry conditions in which hydrogenated DLC surface shows wear. These are a few examples where conventional solid contact mechanics or tribochemistry models cannot explain the interfacial processes. These observations can be explained only when we fully understand the mechanochemical interplay at nanoscales between the solid surfaces and the molecules adsorbed from the environment. This talk will address adsorption isotherms of vapor molecules and their effects on mechanical properties of solid interfaces.