Mica Surfaces: Charge Nucleation and Wear
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The area of nanotribology has advanced greatly in the past 17 years with the introduction of scanned probe microscopies for the characterization of surface properties. The ability to probe the details of structure, friction, and adhesion on a local atomic/molecular scale affords a definitive approach for understanding the mechanisms of wear at the most fundamental level. Muscovite mica, a layered aluminosilicate in the form of KAl2(Si3AlO10)(OH)2, has been a suitable standard for atomic force microscopy (AFM) investigations of the atomic scale relationship between friction, adhesion, and wear due to the ability to readily generate large areas of atomically smooth surface. The rich solution chemistry of mica, attributed to the Brønsted acid sites [Si–O(H)–Al] within the basal (001) plane and its alternating sheet structure, combine with the vast atomically flat surface areas created when cleaved, to give a model substrate for examining a variety of phenomena. These include studies of double layer forces, van der Waals forces, ion binding and mobility, monitoring the recoil tracks of alpha particles, the characteristics of polymers at interfaces, and ordering of discrete water layers.
This review will be limited to a discussion on the wear of mica surfaces, with instructive comparisons drawn to other systems. The overall scope and outline of the chapter is detailed as follows.
As the area of nanotribology is too vast for us to cover in this review, we shall endeavor to provide a perspective on studies as related to the mica surface, with a focus on direct observations of charge nucleation and defect formation, further limited to scanned probe microscopy studies. Relevant oxide and inorganic systems that have exhibited behavior paralleling that of mica will also be discussed. We will first begin with a description of the structure and chemistry of the mica surface followed by an overview of the scanned probe methodologies employed in the investigation of nanotribological properties of mica surfaces. This includes the operating principles of the atomic force microscope (AFM), lateral force (or friction) measurements by AFM, as well as adhesion measurements. Moreover, as one wishes often to quantify such details we will also describe methodologies used to calibrate the signals retrieved from such experiments and then provide a brief description of a standardized approach for the measurement of wear at surfaces, whereby the details of the tip contact area will be explicitly taken into consideration when evaluating the number of scans needed to induce wear at an interface. After describing these approaches, we will then focus on the wear of the mica surface, including the impact of water, and functionalization of the mica surface with alkylsilanes.