Surface characterization by computer analysis of microtopographer data

Abstract

The Gould microtopographer is an instrument for investigating surface features in the microinch range by scanning the surface with a diamond stylus. It has the capability of scanning a raster of parallel lines and providing a topographic representation of the surface on an XY plotter. Its uses and limitations are reviewed, and the question of stylus damage is addressed. The use of the instrument was greatly enhanced by automatic data collection and analysis using first an 1800 computer and, more recently, an S/7, with analysis performed on the 360/195. Up to 10k data points were collected during several successive sweeps of a surface, with a sampling interval as low as 1 μm (40 μin.). Programs were written to calculate, for a specified roughness width cut-off, the center-line average and root mean square roughness of each sweep, and to plot an autocorrelation function and a histogram of surface heights. These programs have been in regular use for surface characterization by the General Products Division Materials Research Group in San Jose. A special program was written to calculate the radius of curvature of the tips of asperities on a surface. The numerical technique involves a leastsquares fit of a parabola to the five data points which describe a peak. On smooth machined surfaces the radii of "asperities" were found to be much larger than expected (> 50 μm). Only on a fractured surface (such as ground glass) were small radii found (∼ 10 μm). The lower limit is the radius of the stylus itself (2.5 μm). © 1977.