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Optical profilometer

The characterization of surface micro- topography is important in the study of interfacial phenomena, such as fr ict ion and wear, or thermal and electrical contact resistance. As readers of Precision Engineering are aware, the major thrust has been on defining and measuring surface topography para- meters that affect surface interaction (see King and Spedding, this issue).

Understanding the surface topo- graphy of magnetic head-medium interfaces is of great interest in conven- tional magnetic recording, where read/write functions are accomplished by the relative motion of a magnetic head and the medium. Continuous contact occurs between a head and a medium during start-stop and, in the case of tapes or rigid discs, a hydro- dynamic f i lm is generated when the moving member reaches a l i f t-off speed. Close proximity of the read/write head and the medium is essential and this generally results in occasional contact at the high asperities.

Stylus instruments can adequately measure the surface topography of hard materials. In the case of polymers (magnetic media), however, the stylus digs into the surface and the results do

Adaptors are available to a l l ow remote viewing through the F- IM1 micro- in te r fe rometer wi th a video camera.

not truly represent the surface. Two other commonly-used techniques - the pneumatic and hydraulic methods - measure only 'average' roughness. For soft surfaces, non-contacting techniques such as optical and interference methods are preferable, and many techniques have been put forward.

Among the many papers presented at the 38th Annual Meeting of ASLE - 24-28 April 1983, Houston, TX - was one describing a new non-contact measurement tech- nique which uses the principle of Mirau interferometry, electronic- phase-measurement techniques, and computer analysis*. The system con- sists of a standard Leitz reflection microscope, a Mirau interferometer controlled by a piezoelectric trans- ducer, a linear array of photodiode detectors, and a microcomputer.

The authors state that the Mirau interferometer offers distinct advan- tages over other microscope-based two- beam interferometers, for example the Nomarski and DIC interferometers. The advantages cited include:

• non-contact • compact • optically simple • common path through micro-

scope objective • three different reflectivity

reference surfaces available wi thout removing components

• yields surface errors rather than the slope of the surface errors.

The last two points are interesting: the Nomarski and DIC interferometers are easy to operate and are almost vibration insensitive, but they have the disadvantage that they both measure what is essentially the slope of the surface errors, rather than the surface errors themselves. Furthermore, the authors point out, because they measure surface slope errors in only one direction, sample orientation is very important.

The electronic-phase-measurement technique is described in detail, as are the methods used for data analysis. Examples of results are given in the form of surface profiles, histograms of surface heights, probability plots of surface height, autocovariance func- tions, surface slopes and curvatures, and peak data.

Lateral resolution of the instru- ment is said to be of the order of 2 #m. Repeatability is given as 1 -10 angstroms rms, depending on the surface roughness and reflectivity. The accuracy of the profilometer was determined by com- paring results of the measurement of a superior polished surface with measure- ments made by Jean Bennett at the Naval Weapons Center using a modified Talystep instrument. The two measure- ments agreed to wi thin 0.7 angstroms rms. It is acknowledged that the optical profilometer and the Talystep measure different characteristics: the Talystep, in fact, has a lateral resolution of 0.1 /~m. Overall, however, the authors are satisfied that the accuracy of their measurements is of the order of 10 angstroms.

*Wyant J.C., Koliopoulos C.L., Bhushan B. and George O.E. An optical profilo- meter for surface characterization of magnetic media, ASLE Preprint 83-AM-6A- I

James Wyant and Chris Koliopoulos are at the University of Arizona, Tucson, AZ 85721, USA. Bharat Bhushan and Orrin George are with the IBM Corporation, Tucson, AZ, USA

Evaluating surface texture

'Functional' versus 'geometric' rough- ness measurement is the subject of a leaflet with the above tit le from Dimensions. The leaflet has actually been produced to give information on the Surf-Chek equipment - which uses a functional approach, evaluating the frictional quality and average slope of irregularities - but contains some comparative information which may be of interest to 'lay' readers.

Dimensions, 1495 Old Deerfield Road, Highland Park, IL 60035, USA

Measuring equipment The latest catalogue from Carl Mahr contains two hundred pages of infor- mation on the range of dimensional metrology equipment produced by Mahr, Feinpruf and Perthen. Each item is illustrated; for many, examples of use are also given.

Carl Mahr, POB 147, D-7300 Esslingen a.N, FRG

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