Direct TEM observation of the welding of asperities between two single-crystal gold films

To understand the microscopic interaction between two metal surfaces, a direct transmission electron microscopic (TEM) examination of metal-to-metal contacts was performed. Gold was chosen for this study to eliminate complications due to surface oxides and to gain insight into the anomalously high coefficient of friction between two nonlubricated gold surfaces. The formation of grain boundaries between two gold films in a contact was directly observed on a hot stage in a TEM at a temperature that was less than 0.4 of the gold melting point. It is noted that this observation is the first direct evidence of metal-to-metal welding by the formation of grain boundaries at touching asperities. These welds can contribute to the frictional force of a sliding contact because grain boundaries of pure metals have the same mechanical strength as the bulk of the material. In addition, shear deformation of these joined asperities can contribute to the adhesive wear in sliding. This would be relevant to the gold alloys used in electrical sliding contacts. The experimental results are reported in detail