Wear of mica under aqueous environments:direct observation of defect nucleation by AFM

The generation of defects at surfaces in sliding contacts is the catalyst for the eventual wear of the materials. Here, the wear of muscovite mica has been investigated under aqueous environments using atomic force microscopy (AFM). Through concomitant acquisition of topography, friction, and adhesion data under controlled pH conditions, defect nucleation on the atomic scale prior to gross wear may be directly observed. Nucleation is found to present itself initially as charging of the surface due to stress-induced tribochemical bond scission as OH- breaks open the surface terminating Si-O-Si or Si-O-Al bonds. As the surface bonds are continually cleaved, an ensemble of defects (e.g., Si-OH/Al-OH and Si-O-) contribute to a crystal lattice reconstruction (from approximately 5.2 to approximately 3 A), as observed in AFM topographic and frictional force micrographs. Following lattice restructuring, displacement/abstraction of mica surface materials ensues, yielding readily discernible wear scars ranging from approximately 2 to 10 A in depth. The environmental OH- concentration profoundly affects the efficacy of this sequence of events leading to wear and is illustrated by the acceleration or inhibition of wear with adjustment of pH under identical load and scan conditions.