Tribology in Full View
- PDF / 580,706 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 97 Downloads / 268 Views
Introduction A fundamental aspect of tribology is the modification of the nanoscale structure at the sliding interface, in terms of both mechanical changes during sliding and the consequences of what is, in effect, nanoindentation. As illustrated in Figure 1, a sliding interface includes both regions where there is sliding (S) and regions where there is nanoindentation (N). Experiments in tribology have long suffered from the inability to directly observe what takes place at this sliding contact—the classic buriedinterface problem. Sliding interfaces have been studied in a detailed manner by scanning probe microscopy,1,2 quartz crystal microbalance,3,4 and surface-force apparatus techniques.5 In addition, a number of techniques have been developed that can obtain some in situ information such as optical spectroscopies6–9 and x-ray photoelectron spectroscopy.10 In a few cases, dynamic processes at monolayer interfaces have been observed, for instance, motion
N S
Figure 1. Schematic of a sliding interface showing regions where there are both sliding contact (S) and nanoindentation (N).
1168
of misfit dislocations11 (i.e., dislocations present to account for the different lattices across an interface). Although these methods have identified many friction phenomena on the nanoscale, interpretation issues can arise because of the indirect or ex situ characterization of the contact surfaces or because the techniques are performing volume-averaged measurements, rather than giving direct insight into what is taking place at a single asperity–asperity contact. To put this difficulty into context, consider that many of the main processes associated with mechanical deformation of materials have been revealed by transmission electron microscopy, either by in situ examination or by examination of representative regions postmortem. Unless the two materials have, in effect, welded together (which, for friction, is typically the least desirable of all processes), preserving the interfacial structure so that it can be examined later is exceedingly difficult. Much more common is to separate the two sliding pieces and then perform a retroactive analysis of the transfer layer (material transferred from one surface to another) or examine the wear debris to try to reconstruct the wear events. Although such an analysis has certainly led to enormous improvements in our scientific understanding, at the same time, one always has to worry that the interfacial region might have changed or that the models of the underlying processes that
one generates from retroactive analysis might not be fully correct. An ideal experiment would be to slide a single asperity against a surface, image the event in real time at the atomic scale at both the structural and chemical levels, and correlate this information with all of the applied forces and surface tractions of the system. Although such an experiment cannot yet be performed in detail, it can be approached by the use of transmission electron microscope (TEM) holders (typically rod-like insertion de
Data Loading...
