The measurement and characterization of surface topography

Abstract

The concept of surface topography is introduced and its relevance to the production and functional behaviour of engineering components is highlighted. Some aspects of the measurement and characterization of surface topography are reviewed. The strengths and weaknesses of particular techniques are identified.
A system developed by the author which measures surface topography is described. It employs a stylus transducer and is designed to gather areal data from nominally flat surfaces using a multiple parallel traversing technique. The system is computer controlled and makes use of an original sampling technique known as 'sampling in space. ' This permits the use of signal averaging to control the level of ambient noise in the data and implementation of algorithms to reject transient noise spikes. An accurate specimen relocation device is included. The performance of this equipment is analysed and found to be better than that of other systems which use the conventional 'sampling in time' technique.
The uses of two-dimensional spectral analysis in the characterization of areal measurements of surface topography are investigated.
Computer software for calculating areal spectra is developed and used to evaluate the power spectra of measurements from surfaces produced by a wide variety of manufacturing processes. A technique which involves sampling power spectra to form 'surface signatures' is described. This is found to be an effective method of simplifying spectra in order to observe characteristic features.
The distribution of variance within power spectra is examined. It is found that for many classes of surface a majority of variance is described by a relatively small proportion of the coefficients of the spectrum. Consequently a good approximation of a surface can be obtained by applying the inverse Fourier transformation to a small group of selected coefficients. A catalogue of Fourier coefficients for constructing numerical models in this way is presented.