The accuracy of fast 3D topography measurements

One important factor in surface engineering is the evaluation of the surface topography, i.e. measurement and analysis. These measurements of the surface roughness are normally done by stylus instruments and 2D profiles due to available standards. 3D surface topography evaluation is, however, in some cases necessary and frequently used, a standard is under development, and newly developed measurement instruments are used. Specially interferometric instruments are of interest here since measuring time is critical and their accuracy and comparability to stylus instruments must be evaluated. Therefore, in this study a number of engineering surfaces have been measured and evaluated: plateau-hones cylinder liners for engines, polished steel roller for bearings, grounded and honed gear surfaces from gear boxes, and steel sheet surfaces used in car bodies. By the use of a precise relocation technique, an exact comparison was possible between an interferometric instrument (Wyko RST Plus) and a 3D stylus instrument (Somicronic 3D). To be able to judge and analyze differences between the instruments an Atomic Force Microscope (AFM) was used. The AFM is extremely accurate for this type of surfaces, but limited in its measuring range. The result shows a very good agreement between the instruments with deviations of approximately 5-20% depending on the parameter evaluated. The stylus in general gives lower values. The results from the AFM are generally found between the parameters of these instruments, and for soft surfaces, such as sheets or smooth surfaces, the interference instrument has a slightly better agreement to the AFM. This is due to the stylus tip geometry and the relatively high contact pressure having difficulties measuring small features and also damaging softer surfaces. The result from an interferometric measurement occasionally shows optically introduced artifacts caused by local surface slopes which exaggerate the topography. By knowing the degree of influence from these peculiarities it is still possible to use fast interferometric instruments and to estimate its deviation from what would have been the result with a stylus instrument. (C) 2001 Elsevier Science Ltd. All rights reserved.