Trends in surface roughness

Pergamon Plh

Int. J. Mach. Tools Manufact. Vol. 38, Nos 5-6. pp. 405-41 I, 1998 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0890-6955(97)00084-9 0890-6955/98 $19.00 + 0.00

TRENDS IN SURFACE ROUGHNESS T. R. Thomas Avalon Technology, P. O. Box 21, Coulby Newham, Cleveland TS7 0YR, United Kingdom

ABSTRACT The recent past of roughness measurement and characterisation is briefly reviewed and some interesting unresolved problems are discussed. Gaps in the present instrumental coverage of range-resolution space are identified. Progress of in-process and inspection techniques is reviewed, and it is noted that existing measurement techniques are restricted to single-valued surfaces. The lack of compatibility of proprietary software packages is deplored and timely development of soft gauges is advocated. Support is expressed for current proposals for a consolidated 3D parameter set. Finally two remaining problem areas of 3D characterisation are identified as the description of lay and the connectedness of surface features. © 1998 ElsevierScienceLtd

1.

INTRODUCTION

1oo00

Recent developments and advances in the measurement and characterisation of surface roughness are both an excuse and an opportunity to take stock of current trends in the field. The number of published papers on roughness is increasing exponentially with time (Figure 1), encouraged by the explosion of commercially available techniques such as AFM and scanning interferometry. The present paper is a very personal attempt to comment on some current trends and to identify a few problems which may be of a wider interest, some of them academic, other more practical and pressing.

1000

100

1970

~

1980

1990

2000

Figure 1. Cumulative numbers of publications on surface roughness (Thomas 1997).

405

2.

MEASUREMENT

2.1

Current and future coverage

The expression of the performance of an instrument as an area in rangeresolution space (Stedman 1987) is now a generally accepted presentation of its vertical and horizontal range and It is interesting to resolution. construct a Stedman diagram to examine summarily the global coverage of topographical measurement A single techniques (Figure 2). envelope covers the current range of instruments, roughness measuring stylus, optical and AFM. Electron microscopy extends this coverage to shorter wavelengths and larger amplitudes, though it is difficult (though not impossible; for a discussion see Whitehouse 1994) to extract quantitative height information. At the upper end of the roughness range, coordinate measuring machines take over. A final envelope of surveying techniques covers everything from autocollimators at the low-range end to satellite ranging techniques at the upper end.

Figure 2.

Glohul Stedman diugram

Figure 2 is not intended comprehensive or definitive, and sake of simplicity many measurement techniques have omitted. It does make the

to be for the useful been point,

however. that there are large areas in range-resolution space which are not accessible to any current technique. Does this matter? The lower right area of Figure 2 represents small amplitudes at long wavelengths. At the moment there does not seem to be any technological requirement in this area. but it is worth noting that the error in the Hubble telescope mirror (Parks 1991) would only just come within a shaded area; a future generation of Hubbies might well fall outside. The upper left area perhaps represents a more pressing practical that of the problem, unavailability of for techniques measuring large amplitudes at short wavelengths. It is certainly possible to think of existing artefacts whose topography falls in this area, for instance a hairbrush! More importantly, there are very many biological structures with large vertical small but horizontal dimensions. starting on a cellular scale (Boyan et al 1996). continuing up through growing crops (Gilley & Kottwitz 1994) and ending with forest canopies (Gallagher et al. 1992). Many such structures are of great economic importance, and at the moment we have no way of describing their topography comprehensively. This task would demand a much greater ratio of vertical range to resolution than is available from current instruments. But when we reflect on the improvement in this ratio in recent years, the implications for the future are promising. At the start of the 1980s few roughness instruments offered a ratio of better than lOE3: 1 (Farago 1982). The current state of the art provides examples both of stylus instruments (Garratt 1982) and optical instruments (Caber et al. 1993) with

Trends in surface roughness ratios of better than 10E5:l. There does not seem to be any fundamental law of instrument design preventing further improvements, if not in these techniques then perhaps in newer ones; chemical balances, for instance, have for many years been constructed with a range 10E8 times their resolution (Cook & Rabinowicz 1963).

2.2 In-process measurement The desirability of the in-process measurement of surface roughness, as part of a unified system of adaptive control of machining, has long been evident (Spurgeon & Slater 1974), and various techniques have been proposed (Table 1). Considerations of speed suggest optical techniques (Young et al. 1980), but methods relying on diffraction or scattering can only yield limited information about the detailed topography of the surface. The techniques of Table 1 all claim to measure actual surface prof'des, some at quite high speeds. The problem remains that, while instruments for offline measurement are readily available from commercial suppliers, in-process measurement is still confined to the laboratory.

Techniq ue

Speed ( m/s )

Reference

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0.025

Garbini

Stylus

1.1

Ultraso und

5

Pneum

51.8

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et

al.

1988

ance

Zhao & Webster 1989 Blessing & Eitzen 1988 Woolley 1991

Table 1. In-process measuring techniques in increasing order of speed.

A similar problem applies to inspection. Of course the majority of roughness instruments sold are used for inspection purposes, usually off-line and on a statistical sampling basis. But modern trends of quality control increasingly require 100% inspection (Kennedy et al. 1987), and this is now the norm for many other product parameters. Most existing roughness instruments are too slow for 100% inspection of massproduced components. For a component produced at a rate of 3000 an hour, not excessive for many production lines, the total time available for setting up, data acquisition and processing is not much more than a second. Although stylus instruments have made impressive advances in speed (Morrison 1995), the only current techniques which can approach the required speeds are electromagnetic (optical or capacitance). For many comparative purposes these work well enough; difficulties arise when it is necessary to validate their performance in terms of legal standards, which are still exclusively written in terms of stylus instruments and their limitations. Prediction by non-stylus techniques even of Ra may be in error by a factor of 5, while successful prediction of Rz is out of the question (Figure 3).

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features on the surface which overhang, or fold back on themselves, or in some other way produce a local microgeometry where there may be two or more discrete values of height corresponding to a single position on the surface. This is why, for instance, when we apply fractal mathematics to the description of surfaces, we are compelled to describe them as selfaffine (single-valued) rather than selfsimilar (multi-valued) (Russ 1994). The physical reason is that the instrument probe, whether a stylus, conventional or AFM, or a beam of incident radiation, is reacting with an envelope of local summits rather than with the true surface.

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Figure 3. Difficulty of reconciling non-stylus techniques with stylusmeasured parameters. Upper: Ra by capacitance (Lieberman et al. 1988); lower: Rz by glossmeter (Brodmann 1986).

2.3 Limitations of measurement techniques

existing

Standards (e.g. BS 1134:1988 Part 1) distinguish between the "real" surface which is the actual boundary between a body and its environment, and the "effective" surface which is the nearest we can approximate to the real surface with our measuring instruments. This is not just a conceit of standards committees; all the instruments which we have available at the moment produce a record of the surface which is single-valued. In other words, they cannot "see" re-entrants or other

But many real surfaces are not in fact single-valued, as we can see (but regrettably not easily measure) from electron micrographs. Many machining processes tear the surface into irregular fragments, and wear processes such as delamination produce overlapping plates of material (Suh 1986). On a small scale it may well be that most technical surfaces are multivalued: this is certainly the inference to be drawn from chemical measurements, which suggest that on a molecular scale the real area of surfaces is many times their nominal area (Grigor'ev et al. 1988). Our lack of appropriate instrumentation may well be concealing from us important information about, for instance, features which tend to promote lubricant retention.

3. C H A R A C T E R I S A T I O N

3.1 Software problems The proliferation of commercial software packages for roughness analysis has created as many problems for workers in the field as it has solved.

Trends in surface roughness For obvious commercial reasons, source code is rarely available to users, and by the time queries have been filtered through sales engineers and office staff it is often very difficult to establish exactly how a particular parameter is calculated or even defined (Figure 4). The difficulty is compounded if, as sometimes happens, the instrument manufacturer has subcontracted the data analysis to a software house, whose programmers may not be familiar with roughness literature or standards. If users lucky enough to have access to several packages try to compare their analyses of the same data set, they will generally find that few of these packages have mutually compatible data formats. Stout et al. (1993) have attempted to remedy this situation by proposing a common data format for roughness measurement, but this has not yet received widespread implementation. There is an urgent need to promote such a common format and to develop and circulate "soft gauges", robust data sets whose parameters are established from accessible source code so that they can be used to benchmark commercial software. The onus would then be on manufacturers to explain and justify anomalous results.

Ridge

J SUMMIT

NO SUMMIT

Saddle point

7Z SUMMIT

NO SUMMIT

Figure 4. Discrepancies in summit definition. A and B are higher than their 4 nearest neighbours, but not higher than their 8 nearest neighbours; 22% of 5-point summits are not 9point summits (Greenwood 1984).

In 3D analysis there is of course a related problem, that there is as yet no formal agreement on a set of 3D parameters. Again there is a proposal on the table, the so-called "Birmingham set" of 14 parameters (Stout et al. 1993), which provides a basis for a de facto standard. The Birmingham 14 may not be the ideal parameter set, and no doubt time and experience will prune them to 10 or even 6, but they deserve general support if the community wants to avoid a rerun of the 2D problems of the seventies famously anathematised as the "parameter rash" (Whitehouse 1982). Already there are ominous signs that individual workers are starting to invent their own 3D parameters for specialised applications. Referees have a particular responsibility here.

3.2 Missing parameters However, there are two important areas in 3D measurement where some new mathematical description would be a real advantage. The first is the description of the lay. Existing rigorous mathematical descriptions perform best with isotropic textures. But the majority of machining processes produce anisotropic textures. Some anisotropic textures such as shaping or turning can be treated for many practical purposes as if they were 2-dimensional. Others such as grinding can be adequately described by their "long-crestedness" (Longuet-

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Higgins 1962), in effect a ratio of the moments of the power spectrum in two principal directions. More complicated textures, such as endmilled or plateau-honed surfaces, are not currently readily describable (though see Boudreau & Raja 1992). The second is the description of the "connectedness" of summits or valleys on a random surface. It is possible to predict the number of summits or valleys and their mean size, as a function of height from the mean line, from existing theories. But there is no easy way of predicting how summits join together to form ridges, or how valleys link up. In geographical terms, it is as if we knew the average area of a valley, but had no way of finding the easiest pass to the next valley, or the most direct route through a range of hills by means of interconnecting valleys. Knowledge of the connectedness of summits would be interesting from the point of view of contact mechanics. Knowledge of the connectedness of valleys would be useful for lubrication problems, and more than useful for sealing problems. Nayak (1973) touched on the subject of connectedness in a discussion of plastic contact; more recently Scott (1996) has begun a different but promising approach.

REFERENCES Blessing, G.V. and Eitzen, D.G., "Surface roughness sensed by ultrasound", Surface Topography, 1, pp.143-158 (1988) Boudreau, B. D.; Raja, J., "Analysis of lay characteristics of three-dimensional surface maps", Int. J. Mach. Tools Manufact. 32, 171-177 (1992).

Boyan, B. D., Hummert, T. W., Dean, D. D., Schwarz, Z., "Role of material surfaces in regulating bone and cartilage cell response", Biomaterials 17, 137-146 (1996). Brodmann, R., "Roughness waviness measurement by light-scattering." Engineering,.8, pp. 221-226,

form and means of

Precision (1986).

Caber, P. J., Martinek, S. J., Niemann, R. J., "A new interferometric profiler for smooth and rough surfaces", Proc. SPIE 2088 (1993). Cook, N. H., and Rabinowicz, E.,

Physical measurement and analysis, Addison-Wesley, Palo Alto (1963). F.T., Handbook of dimensional measurement 2e, Industrial Press, New York (1982) Farago,

Gallagher, M. W.; Beswick, K. M.; Choularton, T. W.,"Measurement and modelling of cloudwater deposition to a snow-covered forest canopy", Atmospheric Environment 26A p 28932903 (1992). Garbini, J,L., Jorgensen, J.E., Downs, R.A. and Kow, S.P. "Fringe-field capacitive profllometry", Surface Topography, l, pp.131-142 (1988) Garratt, J.D., "Applications for a wide range stylus instrument in surface metrology", 2nd Int.Conf on Met. & Prop. of Eng.Surf. pp.ll Apr. 14-It (t982). Gilley, J.E.; Kottwitz, E.R.," DarcyWeisbach roughness coefficients for selected crops", Transactions of the ASAE 37 p 467-471 (1994). Greenwood, J.A., "Unified theory of surface roughness." Proceedings of Royal Society of London Series A.,.393, pp. 133-157, (1984)

Trends in surface roughness Grigor'ev, A.Ya., Myshkin, N.K., Semenyuk, N.F. and Kholodilov, O.V., "Evaluating specific surface area by the secondary electron emission method", Trenie i lznos, 9, 5, pp.793-798 (1988) Kennedy, C. W., Hoffman, E. G., Bond, S. D., Inspection and gaging 6e, Industrial Press, New York (1987). Lieberman, A.G., Vorburger, T.V., Giaque, C.H.W., Risko, D.G. and Rathbun, K.R.,"Comparison of capacitance and stylus measurements of surface roughness", Surface Topography, 1, pp.115-130 (1988). Longuet-Higgins, M.S., "The statistical geometry of random surfaces", Proc. of the 13th Symp. on Appl. Maths, Amer.Maths.Soc., pp. 105-43. (1962). Morrison, E., "A prototype scanning stylus profilometer for rapid measurement of small surface area", Int. J. Mach. Tools Manufact. 35, 325331 (1995). Nayak, P.R., "Random process model of rough surfaces in plastic contact", Wear, 26, pp.305-33. (1973). Parks, R. E., "The Hubble space telescope investigation", Optics & Photonics News 2, 28 (1991). Russ, J. C., Fractal surfaces, Plenum Press, New York (1994). Scott, P. J., "Recent advances in areal characterisation", International Surface Colloquium, TU Chemnitz-Zwickau, p.151-159 (1994). Spurgeon, D. and Slater, R.A.C.,"Inprocess indication of surface roughness using a fibre-optics transducer", Proc. of the 15th Int. Machine Tool Des. & Res. Conf., Birmingham, U.K., , pp.339-47. (1974).

Stedman, M., "Basis for comparing the performance of surface-measuring machines", Prec. Eng., 9, 3, pp.149152 (1987) Stout, K.J., Sullivan, P.J., Dong, W.P., Mainsah, E., Luo, N., Mathia, T. and Zahouani, H., The development of methods for the characterisation of roughness in 3 dimensions. Sch. Man. & Mech. Eng., Univ. of Birmingham, UK , EC Contract No.3374/1/0/170/90/2, Phase II Report, (March 1993) Suh, N. P., Tribophysics, Prentice-Hall, New Jersey (1986). Thomas, T. R., "Bibliography of Surface Roughness", Avalon Technology, Middlesbrough (1997). Whitehouse, D. J., Handbook of Surface Metrology, IOP, London, 1994. Whitehouse, D.J. "The parameter rash is there a cure?", 2nd Int.Confon Met.& Prop.ofEng.Surf pp. Apr. 1-16 (1982). Woolley, R. W., "Pneumatic method for making fast, high resolution noncontacting measurement of surface topography", Proc. SPIE 1573, 205215 (1991). Young, Russell D.; Vorburger, Theodore, V.; Teague, E. Clayton, "Inprocess and on-line measurement of surface finish", Ann CIRP 29 p 435440 (1980). Zhao, Y.W. and Webster, J., "An inprocess roughness measuring system for adaptive control of plunge grinding", Surface Topography, 2, pp.247-261 (1989)

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