Direct observation of tribological recrystallization

The fundamental processes of tribology related to friction, wear, and adhesion [1–3] are still not well understood [4,5]. When a single asperity touches and slides on a surface, complex phenomena such as local rearrangements of atoms, permanent change of the surface structure [6], and more macroscopic phenomena such as dislocation motion or microstructural changes can occur. As the interface is buried, very few studies that directly observe microstructural changes during sliding have been performed, and much of our existing knowledge is based upon the forensic analysis of probable processes taking place. Related studies have been done using in situ transmission electron microscopy (TEM) by controlling the temperature and/or stress on polycrystalline materials, enabling the study of dynamic phenomena in greater detail [7,8]. These have used a strain holder (with or without heating) mounted inside the TEM system [9–11]. While they provide invaluable information about global processes, it is not completely clear how one should translate this to a single asperity problem. More relevant, in situ nanoindentation studies have been performed, for instance induced grain growth in polycrystalline aluminum [7]; again, nanoindentation is not the same as tribological sliding. For a recent review of relevant in situ studies, see [12]. In this study, an external tungsten tip was gently slid on a polycrystalline gold film and mechanically induced site-specific recrystallization and grain growth were observed. This allows site-specific systematic studies of local changes in the film microstructure related to wear, scratching, and sliding.