Measurement of surface topography and area-specific nanohardness in the scanning force microscope
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(Received 26 September 1994; accepted 8 June 1995) A Scanning Force Microscope (SFM) is employed to indent and image surfaces with sub-micron resolution. The SFM image shows the area and depth of each indentation as well as its location with respect to nearby topographic surface features. The image also reveals the surface roughness, which can set a lower limit on useful nanoindentation size. A cross section of a nitrided steel surface is measured to illustrate the method. The use of the SFM with separate tip-cantilever structures for indenting and imaging has significant advantages over other nanohardness methods for the study of samples with lateral inhomogeneities.
1. INTRODUCTION
Recent years have seen increasing interest in the study of mechanical material properties on a micron or smaller length scale. Among the reasons for this are the growing technological importance of composite materials, thin films, and surface treatments. Not surprisingly, this increasing interest has been accompanied by the development of new instruments and techniques for measuring these properties. These instruments can be conveniently divided into several categories. One important type of instrument measures the load versus displacement curve as a sharp indenter is pressed into a surface and then In this way, much can be learned about the plastic and elastic response of the surface. These instruments do not produce an image of the indented area, so Scanning Electron Microscopy (SEM)s or Scanning Force Microscopy (SFM)6,7 often accompanies this technique. Related instruments have been developed for measurements of microscratch,8 microimpact,' and microwear," and SFM or SEM is a useful adjunct to these as well. Another important group of instruments is SFM's (also known as Atomic Force Microscopes) operated with tip-sample forces high enough to plastically deform the sample surface."-I5 In some cases information can be obtained from the curve of cantilever deflection versus sample displacement, but usually creep and hysteresis in the piezoelectric actuator causes unacceptable distortion.'6 A more common method is to make an indentation with a high force and then image it by scanning with a much lower force. This technique has the additional advantage that the SFM image provides information about the area around the indentations as well. Conventional microhardness measurements are limited by the resolution of optical microscopes, but the SFM can go well below this barrier. Performing a scaleddown version of the commonly used Knoop and Vickers J. Mater. Res., Vol. 10, No. 10, Oct 1995 http://journals.cambridge.org
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microhardness tests, the SFM can conveniently produce sub-micron indentations and provide a detailed image showing their size and location relative to the local surface topography.
II. EXPERIMENTAL
A Nanoscope I1 SFM (Digital Instruments Inc., Santa Barbara, CA) was used to make indentations and SFM images. The tip-cantilever structure used for imaging was the Park Ultralever (Park Scientific Inst
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