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Advantages and Industrial Applications of Three-Dimensional Surface Roughness Analysis
Advantages and Industrial Applications of Three-Dimensional Surface Roughness Analysis L. De Chiffre (2), S. Christiansen, S. Skade, Institute of Manufacturing Engineering, Technical University of Denmark Received on January 14,1994
Based on more than a decade's work with digital surface roughness analysis techniques, this paper describes the advantages as well as the range of application of three-dimensional surface roughness analysis. Conventional as well as optical equipment is used for surface mapping, and a number of specially developed software packages perform the data processing, including three-dimensional surface filtering and computation of three-dimensional parameters to describe selected functional aspects of surface topography. Several cases of industrial applications are referred, ranging from seal leakage to die wear and human skin diseases. Keywords: Surface roughness, digital analysis, industrial applications.
Introduction Since the mid-seventies,three-dimensionalmeasuringsystems for surface topography analysis have been developedand used at many laboratories /1-7/. While the main objectives of these efforts originally were the to create network reproductions of the surfaces in three dimensions, several features as coloured plots, extensive filtering facilities, computation of threedimensional parameters as well as volume and area computation facilities have been added and are now in use in different systems. The technology is nowadays ready for use in an increasing number of industrialapplicationsand some systems are now commercially available / 6 / . Furthermore, the basic work for a standardizationof the methods has been carried out, due to the pressure from industrial companies that are interested in surface topography /9/. Based on more than a decade's work with digital surface roughness analysis techniques, this paper describes the advantages as well as the range of application of three-dimensional surface analysis, with special reference to industrial applications.
Generation
Surface engineering
-Honing -Lapping -Coating
-Sealing
-Instrument -Measurement
Fig. 1 The three elements in surface metrology: generation, characterizationand function.Also showing the threeinterrelatedfactors: process control, quality control and surface engineering.
Advantages of threedimensional surface characterization Figure 1 illustrates the interaction between the three elements involved with a surface: generation, characterization and function. The three elements are interdependent and for each specific application it is necessary to consider each aspect carefully. In normative standards, roughnessand waviness are generally defined in terms of the production process used in manufacturing the component. In the last few years, however, a functional approach has become more and more common, because it has been recognized that the parameters relating to the production process are not necessarily related to specific functions /10-12/. Efforts are made to relate specific features of the surface to the surface's function and to develop special functional parameters, although this requires a large number of new parameters. A fundamental step in this direction is threedimensional surface analysis. Generally, characterization of surfaces in three dimensions is needed if the surface comprises an inhomogeneous t o p graphy or one characterized by isolated features such as holes, ridges or furrows. A typical case where two-dimensional surface roughness analysis performed on the basis of conventional pro
Annals of the ClRP Vol. 43/1/1994
Fig-2 Grey tone contour plots of the surface on a hydraulic piston rod. (a) ground and chromium plated; (b) ceramic coated. Length of horizontal axis: apprm, 0.2 mm (100 samples). Length of vertical axis: apprm. 0.13 mm (75 samples).
473
filometry cannot be applied, is shown in Fig.2, where the topography of a ground ceramic coated piston rod used for hydraulic cylinders is compared to that of a ground and chromium plated piston rod. It can be easily seen on this threedimensional plot, that the chromium plated surface (a) is characterized by the parallel grinding marks while the ceramic coated surface (b) is characterized by single asperities and randomly distributed pores. Two-dimensional profiles cannot distinguish between the two surfaces though these clearly have very different functional properties when used as components in a sealing system.
established technique that is known to be stable. In this connection it is very important to select the right polynomial order, since a wrong choice may remove some possible waviness that could be an important functional feature, or even introduce some waviness.
Although conventionaltwo-dimensional profilometry combined with visual inspection in a microscopecan cover some applications, three-dimensional surface topography gives, in a large number of applications, a tool to quantify different aspects of surface features. Referring to Fig.2, parameters describing the extension of peaks and valleys and other surface features can be computed from the data. Nowadays, commercial instruments are born as three-dimensional systems and act as digital microscopes provided with powerful characterization software
Visual characterization is available by the use of coloured axonometric and contour plots.
/9/.
Three dimensional surface characterization At the authors' university, development, acquisition and use of different systems for digital analysis of surfaces has been undertakensince 1981, including a specially developed system for threedimensional surface mappingand analysis,which now exists in its fourth generation as well as a commercial optical instrument /6,13-18/. Depending on the specific task and on the object material, the stylus instrument or the optical roughness equipment will be chosen to cover the normal roughness range. In general the stylus instrument will be selected unless the stylus is expected to deform the surface. The optical instrument, which is of the focus detect type, is most useful for investigations on soft materials but unsuitable for measurements on e.g. ceramic materials. These latter include sharp edges which cause the focus detector to overshoot, as shown by Hillmann /19/. Data collection is carried out as a raster scan technique where the object and the probe are moved relatively to each other in parallel traces making a raster scan with sample area, sample' interval and vertical range as main, partly interdependent, parameters. We have found it most appropriate to let area and interval be the independent parameters because functional features and machining marks in the surface, e.g. physical distances between grooves and ridges, are related to the area and to the resolution. In general the guidelines proposed in /9/ are followed but in practical cases choice of the measuring conditions is based on compromis. Data processing normally includes the following three steps /16/: 1) form error removal; 2) datum choice and 3)characterization. Form removal is a two stage process. The first step is to evaluate the form of the measured surface and in the second stage the fitted form is subtracted from the measured surface by subtraction of the "N'th order least squares polynomial fit" from the raw set of data. Least squares form fit is a well
474
To provide a quantitative evaluation of the surface roughness, a reference or datum level is needed. Different methods for definition of a reference plane have been examined and the most suitable plane has been found to be the least squares mean plane.
Two groups of surface parameters have been found to be most relevant: (i) S ,, S, and S,, that are related to the general roughness of the surface, and (ii) S,k, sk, S,, which are related to specific functional properties primarily tribological properties. These parameters, which correspond to the twodimensional parameters R,, %(DIN), R,k, Rpk, Rk and &, are defined in /9/. Their implementation in the system, as well as implementation of other surface parameters, was carried out with software developed by the authors /16/, integrated with commercial packages.
-
In particular, in contrast to recommendations in /9/,it was decided to use the three-dimensional generalisation of the bearing curve parameters defined in DIN 4776 /20/.These parameters subdivide the surface into 3 levels: top, core and valleys. Each parameter is related to one level and the three parameterstogether'aresuitable for characterization of surfaces with stratified structure, e.g. plateau honed surfaces and surfaces of ceramic materials. Another set of parameters that are under development comprises surface high peaks count, furrow and pit identification, furrow separation and other texture identification entities.
Traceabiliiy Traceability is a fundamental aspect of all measurements, including three-dimensional surface analysis. The authors' department is accredited for calibration and testing in the field of roughness. Traceability covering twodimensional measurements is provided by PTB through the use of IS0 5436 Type A groove standards /21/, and a best measuring conditions uncertainty of 2.5% on conventional two-dimensional parameters is used. Work regarding the uncertainty of three-dimensional measurements is undeway, in which the stylus instrument is used as a reference.
Ranges of industrial application In a series of applications, three-dimensional surface roughness analysis has been found by the authors to be a powerful and versatile concept, compared to two-dimensional profilometry used in connection with visual inspection through a microscope. Some examples of industrial applications of threedimensional surface analysis related to the following functional features are presented here:
a) Leakage and wear on seals b) Wear on deep drawing dies c) Paintability of sand blasted surfaces d) Efficiency of cutting fluids e) Integrity of human skin
Wear and leakage in PTFE sealing systems A Danish company that develops and produces sealing systems wanted to develop their knowledge of sealing surfaces, in order to be able to recommend suitable surface parameters to their customers. In particular, they were most concerned about PolyTetraFluoroEthylene (PTFE) seals used in connection with hydraulic cylinder rods. Up to now, only the parameters Ra and R, have been used for characterization of rod surfaces. These parameters are generally sufficient when the rods are ground, but when new manufacturing processes are introduced in connection with conventional steel surfaces, the surface structure may change and unexpected seal failures occur. Another aspect of this problem is that ceramic coated rods induce excessive seal wear and subsequent leakage, even when R, and R, are kept within the normal limits. In cooperation with the authors, an investigation was carried out 1171 on the influence of rod surface structure on sealing properties. For these investigations a test rig earlier developed by the company was used. The rig enables measurement of wear, leakage and friction over a specified number of cycles, for selected pressure conditions.
A number of induction hardened rods having different surfaces were investigated:
-
a reference surface, ground and chromium coated a ground surface, grinding marks parallel to rod movement a honed surface, structure of arks crossing the seal at an angle of approximately 45' to rod movement a surface with etched holes, rough random structure a sand blasted and polished rod, medium random structure a glass blasted and polished rod, fine random structure.
By evaluating the surface roughness of the rods before and after the test, an indication of rod wear was achieved. This wear was determined to be less than one micrometer. Seal wear, measured in a contour projector, lay in the range from 0.1 to 0.5 mm, which means that none of the original seal surface was left. Resultsfromsurface roughness measurements were presented as three-dimensional plots, height distribution curves and surface bearing area curves. Furthermore, surface roughness parameters were calculated. The results, Fig.3, show that sealing wear increases with rod spk. It was assumed on this basis that the peaks scratch and wear the seal, presumably because they penetrate through the lubricating oil film thickness. It was concluded that if the surface is engineered to an Spk value less than 0.1 pm then the rod will not be worn, and spk will not change during the wear process. It was also concluded that the lubricating oil film thickness in the sealing system must be approximately equal to 0.1 pm. If spk> 0.1 pm, Spk will decrease during a running-in. Although the use of the twodimensional parameter Rpk essentially has given the same
results, it was concluded from the investigation that the threedimensional analysis is more universal and robust.
Glass blasted
0 0.0
0.2
0.4 Rod %k
0.6
0.8
1.0
[pml
Fig. 3 Wear on PTFE seal for differentvalues of Sw Steel rod machined using different processes /17/.
A second series of experiments focused on ceramic coated rods 1161. Two coating materials were examined, Al2O3 and TiOg. The surface structure of the ceramics differs significantly from the traditionally used ground and polished steel surfaces, as already shown in Fig.2. The ground surfaces are dominated by parallel grinding marks while the ceramic coated surfaces have randomly distributed cavities in a substantially plane surface. A number of rod surfaces were prepared by centerless grinding, honing, super finishing and lapping, and manufacturing was carefully undertaken to achieve the finest possible surface finishes. Irrespective of the surface preparation process, the random structure of cavities was predominant. Differences between surfaces were found in their stratified composition, as the lapped and the superfinished rods had a lesser peak height than the honed and the ground rods.
The ceramic rods were run on the test rig mentioned above, until a severe increase in leakage was observed, and the sealing system was then defined to be worn out. Test results were arranged in two groups, rods run together with seals that had lasted less than approximately 1.GOOstrokes and rods that could run for more than 20.000 strokes. The variation in performance was explained by the differently stratified structures. When the results from the previous investigation are kept in mind, the peak heights could be expected to influence the wear rate of a particular rod. In Fig.4 the upper section of the surface bearing curve for a honed ceramic coated rod and a honed steel rod before and after the test are plotted. The following observations were made from the experiments 1161. Ceramic rods that performed well had a considerably lower peak height than rods that failed to perform satisfactorily. With the ceramic rods, no initial wear in terms of reduction in peak height was observed, as could be seen with the steel rods and in view of the hardness, it was concluded that no running-in of these rods would take place. Peaks that penetrated the lubricant film continued to wear the seal, and therefore the rods with the lowest peak height resulted in the lowest seal wear rate.
475
information about initial wear and the type of wear is lost when only profile measurements are used, as discussed in more details in 1161.
a1 I
0
I
1
0
2
0
~
4
0
Material ratio [%I
5
b
6
0
Fig. 4 Upper portions of three-dimensional bearing curves for rods, showing differences between curves before and after wear tests. Single hatching shows differencerelated to ceramic coated rods. Cross hatching shows wear related to chromiumplated steel rods
/ I 6/.
The two investigations have led to a better understanding of surface characteristics that influence the functional properties of PTFE-seals: wear, friction and leakage.
Wear on deep drawing dies A Danish company that manufactures large quantities of stainless steel sheets requested an investigation into the influence of tool surfaces on tool wear progression. Traditionally, wear on dies has been investigated by measuring profiles continuously during the wear process, and using light microscopes and SEM. However these latter instruments give no information on the actual heights in surface topographies.
F&. 5 Axonometric plot of die surface after drawing of 5 strips /16/
An investigation was carried out on a strip test evaluation equipment, on which a die section was worn by pulling through 250 steel strips, and the progress of wear monitored.
:
At selected intervals, the die surface was measured using twodimensionalas well as three-dimensional roughness measuring equipment. From two-dimensional profiles it can only be concluded that roughness increases with use. But the mechanisms involved can not be determined, e.g. whether the roughness increase is caused by galling or ploughing. This information can be obtained from surface measurements. A SEM or a light microscope may give the same information as to the type of wear, but only three-dimensional profilometrygives information about the magnitude of wear. From surface plots it was concluded that the wear process was already initiated after 5 strips, by the single ploughing shown in Fig.5. As more strips were drawn, more ploughing traces appeared on the die surface. As the die was worn, either too many scratches appeared on the strip or an increase in friction occurred preventing the process from being carried out any further. After 250 strips the surface was worn considerably, though the die was not yet worn out. Parameter calculationsat different stages of wear are plotted in Fig.6. The results indicate that S, is an effective parameter for wear prediction, due to the fact that wear is dominated by ploughing 1161. In this example wear in the drawing process was examined by profile and surface measurements, and it was concluded that
476
: 0
1
Y
6
0.8 0.4 0.2 50
1 m 1 3 0 m m : Number of drawn parts
Fig. 6 Surface bearing curve parameters for the die as a function of the number of drawn strips /16/.
Sand blasted surface paintability Steel sheets used in ship building are sand blasted before painting, to generate a clean surface with good adherence properties. Usually these surfaces are characterized by visual examination or through simple roughness parameters (Ra, F$). These methods are inadequate since small, but still significant, variations in process conditions cannot be detected by visual
inspection and conventional roughness parameters do not correlate to paint adherence. Therefore a Danish paint manufacturer experienced difficulties in characterizing sand blasted surfaces by the conventional roughness parameters Ra and q.
In an investigationinvolvingthree-dimensional surface analysis, paint adherence was evaluated by an impact test resulting in three graduations: 0 = good, 1 = average and 2 = bad. At 0 the paint is intact while the graduation 2 means that the paint is totally removed from the surface. Table 1 shows a number of selected roughness parameters compared to paint adherence for four different surfaces.
Unfolded area Traced area
Table 7
13.5
1.14
10.0
1.15
12.3
1.12
Standard deviation of surface slopes distribution
Adherence
Adherence of paint on sand blasted surfaces.
From the results it was deduced that Sa relates poorly to paint adherence while the unfolded area ratio relates to adherence. A larger area ratio results in higher adherence, though only small variations in the unfolded surface area were experienced. The dispersion of surface slope distribution (u) relates to paint adherence, as shown in Table 1. If the surface height distribution is narrow, i.e. the dispersion is small, only a limited number of slopes are present and the surface structure appears shiny.
Cutting fluid efficiency A Danish company producing additives for metalworking fluids wanted a simple method to evaluate lubricant action in cutting. A reaming test was developed at the authors’ university 1221 and three-dimensionalmapping was used to documentate the definition of efficiency in terms of degree of replication of the surface determined by the tool geometry and movement. Fig.7 shows the contour plots of ream.ed surfaces, including a plot of the ideal surface generated synthetically by a special signal generation software developed by the authors 116,171. The plots show clearly that, while reaming under good lubrication conditions generates a surface which is similar to the theoretical pattern, not many of the typical furrows can be recognized on the surface obtained using a poor lubricant. Conventional two-dimensional analysis can be used in this case, due to the banded structure produced by reaming, but three-dimensional analysis proved itself as a convincing tool for understanding the conditions in the general case of any arbitrary surface.
Fig. 7 Grey tone maps of reamed stainless steel surfaces. Length of horizontalaxis: 0.8 mm (200 samples). length of vertical axis: 0.4 mm (100 samples). (a) Theoretical surface; [b) Reamed surface, using a good lubricant; (c) Reamed surface, poor lubricant/22/,
Human skin integrity The last industrial example concerns characterizations of human skin surfaces. A Danish company producing medicaments for skin diseases was interested in a quantitative assessment of dermatologists’quantitative evaluation. The company had invested in an optical instrument but the investigation was undertaken at the authors’ university. Human skin replicas were taken and the optical surface measurement instrument was used for measuring these. Roughness measurements enabled the quantifying of the progress of skin deseases and the effect of liniments. An investigation of 16 skin replicas were carried out 1231. Area samples of 5.6 x 5.6 mm2 were measured with the optical profilometer and analyzed by means of the described three-dimensional parameters. In this investigationthe parameters Sa, S,, S, and S, were calculated using the software facilities developed by the authors. The surface plots obtained from the optical profilometer were divided into three categories by means of visual inspection: Category I : Category 1 I : Category 111 :
Characteristic waviness, with or without craters. No characteristic waviness, with craters. No characteristic waviness, without craters.
477
Figure 8(a) shows an example of a profile plot from category I, and figure 8(b) shows a replica from category 111. By using the three-dimensional parameters mentioned above it was possible to place a replica in one of the categories in accordance with the visual inspection. Table 2 shows the combination of parameters and parameter values found in this investigation.A calculation of the parameters Sa, St, s k and S, for a certain profile plot permits determination of which category the replica belongs to. All four parameter values are needed to place the replica in the correct category.
Conclusions At the authors’ university, based on more than a decade’s work with digital surface roughness analysis techniques, a comprehensive system based on stylus and optical instruments and versatile software has been developed and used to undertake many surface analyses.
Three-dimensionalsurface characterization has proven itself to be invaluable in a number of industrial applications, ranging from leakage and wear on seals to wear on deep drawing dies, paintability of sand blasted surfaces, efficiency of cutting fluids, and integrity of human skin. In some cases, three-dimensional analysis is the only possibility. In other cases it is a valid alternative to two-dimensionalprofilometry and subsidiary techniques. In all cases it provides a modern analysis tool which is expected to increase in power, as computers develop in capacity and speed.
Waviness, with or without craters
References ~
Sa II
No waviness, with
st
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‘k
sa=20 2 3 0 4 ,~ 2 8 0 6o
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without craters
Three-dimensionalparametersfor characterizationof skin/23/.
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Sayles, R.A., Thomas, T.R., 1976, Mapping of Small area of a surface, J. Phys. E., vol. 9, p. 855-861. Clayton Teague, E., 1976, Evaluation, revision and application of the NBS stylus/computer system for the measurement of surface roughness. National Bureau of Standards, Washington. N.N., 1979, Le rugosimbtre tridimensionnel p w r une meilleure evaluation des Btats de surface. Centre technique des industries mhniques. Saint-Etienne. Tsukada, T., Sasajima, K., 1981, A three-dimensional measuring technique for surface asperities, Wear, vol. 71, p. 1-14. Bengtson, A., Ronnberg, A., 1984, Wide range three-dimensional roughness measuring system, Prec. Eng., vol. 6, p. 141-147. De Chiffre, L., Nielsen, H.S., 1987, A digital system for surface roughness analysis of plane and cylindricalparts, Prec. Eng., vol. 9, p. 59-64. Stout, K.J., Sullivan, P.J., 1992, The use of 3-D topographic analysis to determine the microgeometrytransfer characteristics of textured sheet surfaces through rolling, Annals of the CIRP, vol. 4111, p. 621-624. Stout, K.J., et al., 1991, The development of an integrated approach to 3D surface finish assessment, Interim Report, EC Contract No. 3374/1/0/170/90/2. Stout, K.J., et el., 1993, The development methods for the characterization of roughness in 3 dimensions, Phase I1 report, vol. 1 and 2. EC Contract No. 3374/1/0/170/90/2. Schneider, U., et al., 1988, An approach to the evaluation of surface profiles by separating them into functionally different parts, Surface Topography, vol. 1. p. 71-83. Scott,P.J., 1988, Developments in surface texture measurement, Surface Topography, vol. 1, p. 153-163. de Bruin, W., 1986, STC mSurfaces‘:Surface and Function, Annals of the CIRP, vol. 3512, p. 551-559. Nielsen, H.S., 1988, New approaches to surface roughness evaluation of special surfaces, Prec. Eng., vol. 10, p. 209313. Larsen, I.B., De Chffre, L.. Asmussen, E., 1990, A new method for measuring wear of restorations in vivo, J. Dent. Res., vol. 69, Spec. Iss., No. 138. Andreasen, J.L., 1992. Monitoring and control of machined surfaces, Ph.D. thesis, Technical University of Denmark. Christiansen, S., 1994, Function-related characterization of surface topography, Ph.D. thesis, Technical University of Denmark. Skade, S., 1994, Engineering of surface topographies, Ph.D. thesis, Technical University of Denmark. Efsen. J., et al., 1994, Chapter 8: Laser profilometry, In vivo examination of the skin: a handbook of non-invasive methods, CRC Press, Florida. Hillmann, W., 1990, Surface profiles obtained by means of optical methods Are they true representations of me real surface?, Annak of the CIRP. vol. 3911, p. 581-583. DIN 4776, 1990, KenngrBssen Rk, R Q, M,,, M, zur Beschreibung des Materialanteils im Rau\&sprofil. Nielsen. H.S., 1989, Calibration of surface roughness instruments, VDI Berichte nr. 761, p.365370. De Chffre, L., et al., 1992, A reaming test for cutting ffuid evaluation, Proc. 8th Int. Coil. Trib., TAE, Esslingen, p. 18.9. Efsen, J., Hansen, H.N., 1993, Optisk ruhedsmaing, M.Sc. thesis, Technical University of Denmark (in Danish).
-
,
Based on more than a decade's work with digital surface roughness analysis techniques, this paper describes the advantages as well as the range of application of three-dimensional surface roughness analysis. Conventional as well as optical equipment is used for surface mapping, and a number of specially developed software packages perform the data processing, including three-dimensional surface filtering and computation of three-dimensional parameters to describe selected functional aspects of surface topography. Several cases of industrial applications are referred, ranging from seal leakage to die wear and human skin diseases. Keywords: Surface roughness, digital analysis, industrial applications.
Introduction Since the mid-seventies,three-dimensionalmeasuringsystems for surface topography analysis have been developedand used at many laboratories /1-7/. While the main objectives of these efforts originally were the to create network reproductions of the surfaces in three dimensions, several features as coloured plots, extensive filtering facilities, computation of threedimensional parameters as well as volume and area computation facilities have been added and are now in use in different systems. The technology is nowadays ready for use in an increasing number of industrialapplicationsand some systems are now commercially available / 6 / . Furthermore, the basic work for a standardizationof the methods has been carried out, due to the pressure from industrial companies that are interested in surface topography /9/. Based on more than a decade's work with digital surface roughness analysis techniques, this paper describes the advantages as well as the range of application of three-dimensional surface analysis, with special reference to industrial applications.
Generation
Surface engineering
-Honing -Lapping -Coating
-Sealing
-Instrument -Measurement
Fig. 1 The three elements in surface metrology: generation, characterizationand function.Also showing the threeinterrelatedfactors: process control, quality control and surface engineering.
Advantages of threedimensional surface characterization Figure 1 illustrates the interaction between the three elements involved with a surface: generation, characterization and function. The three elements are interdependent and for each specific application it is necessary to consider each aspect carefully. In normative standards, roughnessand waviness are generally defined in terms of the production process used in manufacturing the component. In the last few years, however, a functional approach has become more and more common, because it has been recognized that the parameters relating to the production process are not necessarily related to specific functions /10-12/. Efforts are made to relate specific features of the surface to the surface's function and to develop special functional parameters, although this requires a large number of new parameters. A fundamental step in this direction is threedimensional surface analysis. Generally, characterization of surfaces in three dimensions is needed if the surface comprises an inhomogeneous t o p graphy or one characterized by isolated features such as holes, ridges or furrows. A typical case where two-dimensional surface roughness analysis performed on the basis of conventional pro
Annals of the ClRP Vol. 43/1/1994
Fig-2 Grey tone contour plots of the surface on a hydraulic piston rod. (a) ground and chromium plated; (b) ceramic coated. Length of horizontal axis: apprm, 0.2 mm (100 samples). Length of vertical axis: apprm. 0.13 mm (75 samples).
473
filometry cannot be applied, is shown in Fig.2, where the topography of a ground ceramic coated piston rod used for hydraulic cylinders is compared to that of a ground and chromium plated piston rod. It can be easily seen on this threedimensional plot, that the chromium plated surface (a) is characterized by the parallel grinding marks while the ceramic coated surface (b) is characterized by single asperities and randomly distributed pores. Two-dimensional profiles cannot distinguish between the two surfaces though these clearly have very different functional properties when used as components in a sealing system.
established technique that is known to be stable. In this connection it is very important to select the right polynomial order, since a wrong choice may remove some possible waviness that could be an important functional feature, or even introduce some waviness.
Although conventionaltwo-dimensional profilometry combined with visual inspection in a microscopecan cover some applications, three-dimensional surface topography gives, in a large number of applications, a tool to quantify different aspects of surface features. Referring to Fig.2, parameters describing the extension of peaks and valleys and other surface features can be computed from the data. Nowadays, commercial instruments are born as three-dimensional systems and act as digital microscopes provided with powerful characterization software
Visual characterization is available by the use of coloured axonometric and contour plots.
/9/.
Three dimensional surface characterization At the authors' university, development, acquisition and use of different systems for digital analysis of surfaces has been undertakensince 1981, including a specially developed system for threedimensional surface mappingand analysis,which now exists in its fourth generation as well as a commercial optical instrument /6,13-18/. Depending on the specific task and on the object material, the stylus instrument or the optical roughness equipment will be chosen to cover the normal roughness range. In general the stylus instrument will be selected unless the stylus is expected to deform the surface. The optical instrument, which is of the focus detect type, is most useful for investigations on soft materials but unsuitable for measurements on e.g. ceramic materials. These latter include sharp edges which cause the focus detector to overshoot, as shown by Hillmann /19/. Data collection is carried out as a raster scan technique where the object and the probe are moved relatively to each other in parallel traces making a raster scan with sample area, sample' interval and vertical range as main, partly interdependent, parameters. We have found it most appropriate to let area and interval be the independent parameters because functional features and machining marks in the surface, e.g. physical distances between grooves and ridges, are related to the area and to the resolution. In general the guidelines proposed in /9/ are followed but in practical cases choice of the measuring conditions is based on compromis. Data processing normally includes the following three steps /16/: 1) form error removal; 2) datum choice and 3)characterization. Form removal is a two stage process. The first step is to evaluate the form of the measured surface and in the second stage the fitted form is subtracted from the measured surface by subtraction of the "N'th order least squares polynomial fit" from the raw set of data. Least squares form fit is a well
474
To provide a quantitative evaluation of the surface roughness, a reference or datum level is needed. Different methods for definition of a reference plane have been examined and the most suitable plane has been found to be the least squares mean plane.
Two groups of surface parameters have been found to be most relevant: (i) S ,, S, and S,, that are related to the general roughness of the surface, and (ii) S,k, sk, S,, which are related to specific functional properties primarily tribological properties. These parameters, which correspond to the twodimensional parameters R,, %(DIN), R,k, Rpk, Rk and &, are defined in /9/. Their implementation in the system, as well as implementation of other surface parameters, was carried out with software developed by the authors /16/, integrated with commercial packages.
-
In particular, in contrast to recommendations in /9/,it was decided to use the three-dimensional generalisation of the bearing curve parameters defined in DIN 4776 /20/.These parameters subdivide the surface into 3 levels: top, core and valleys. Each parameter is related to one level and the three parameterstogether'aresuitable for characterization of surfaces with stratified structure, e.g. plateau honed surfaces and surfaces of ceramic materials. Another set of parameters that are under development comprises surface high peaks count, furrow and pit identification, furrow separation and other texture identification entities.
Traceabiliiy Traceability is a fundamental aspect of all measurements, including three-dimensional surface analysis. The authors' department is accredited for calibration and testing in the field of roughness. Traceability covering twodimensional measurements is provided by PTB through the use of IS0 5436 Type A groove standards /21/, and a best measuring conditions uncertainty of 2.5% on conventional two-dimensional parameters is used. Work regarding the uncertainty of three-dimensional measurements is undeway, in which the stylus instrument is used as a reference.
Ranges of industrial application In a series of applications, three-dimensional surface roughness analysis has been found by the authors to be a powerful and versatile concept, compared to two-dimensional profilometry used in connection with visual inspection through a microscope. Some examples of industrial applications of threedimensional surface analysis related to the following functional features are presented here:
a) Leakage and wear on seals b) Wear on deep drawing dies c) Paintability of sand blasted surfaces d) Efficiency of cutting fluids e) Integrity of human skin
Wear and leakage in PTFE sealing systems A Danish company that develops and produces sealing systems wanted to develop their knowledge of sealing surfaces, in order to be able to recommend suitable surface parameters to their customers. In particular, they were most concerned about PolyTetraFluoroEthylene (PTFE) seals used in connection with hydraulic cylinder rods. Up to now, only the parameters Ra and R, have been used for characterization of rod surfaces. These parameters are generally sufficient when the rods are ground, but when new manufacturing processes are introduced in connection with conventional steel surfaces, the surface structure may change and unexpected seal failures occur. Another aspect of this problem is that ceramic coated rods induce excessive seal wear and subsequent leakage, even when R, and R, are kept within the normal limits. In cooperation with the authors, an investigation was carried out 1171 on the influence of rod surface structure on sealing properties. For these investigations a test rig earlier developed by the company was used. The rig enables measurement of wear, leakage and friction over a specified number of cycles, for selected pressure conditions.
A number of induction hardened rods having different surfaces were investigated:
-
a reference surface, ground and chromium coated a ground surface, grinding marks parallel to rod movement a honed surface, structure of arks crossing the seal at an angle of approximately 45' to rod movement a surface with etched holes, rough random structure a sand blasted and polished rod, medium random structure a glass blasted and polished rod, fine random structure.
By evaluating the surface roughness of the rods before and after the test, an indication of rod wear was achieved. This wear was determined to be less than one micrometer. Seal wear, measured in a contour projector, lay in the range from 0.1 to 0.5 mm, which means that none of the original seal surface was left. Resultsfromsurface roughness measurements were presented as three-dimensional plots, height distribution curves and surface bearing area curves. Furthermore, surface roughness parameters were calculated. The results, Fig.3, show that sealing wear increases with rod spk. It was assumed on this basis that the peaks scratch and wear the seal, presumably because they penetrate through the lubricating oil film thickness. It was concluded that if the surface is engineered to an Spk value less than 0.1 pm then the rod will not be worn, and spk will not change during the wear process. It was also concluded that the lubricating oil film thickness in the sealing system must be approximately equal to 0.1 pm. If spk> 0.1 pm, Spk will decrease during a running-in. Although the use of the twodimensional parameter Rpk essentially has given the same
results, it was concluded from the investigation that the threedimensional analysis is more universal and robust.
Glass blasted
0 0.0
0.2
0.4 Rod %k
0.6
0.8
1.0
[pml
Fig. 3 Wear on PTFE seal for differentvalues of Sw Steel rod machined using different processes /17/.
A second series of experiments focused on ceramic coated rods 1161. Two coating materials were examined, Al2O3 and TiOg. The surface structure of the ceramics differs significantly from the traditionally used ground and polished steel surfaces, as already shown in Fig.2. The ground surfaces are dominated by parallel grinding marks while the ceramic coated surfaces have randomly distributed cavities in a substantially plane surface. A number of rod surfaces were prepared by centerless grinding, honing, super finishing and lapping, and manufacturing was carefully undertaken to achieve the finest possible surface finishes. Irrespective of the surface preparation process, the random structure of cavities was predominant. Differences between surfaces were found in their stratified composition, as the lapped and the superfinished rods had a lesser peak height than the honed and the ground rods.
The ceramic rods were run on the test rig mentioned above, until a severe increase in leakage was observed, and the sealing system was then defined to be worn out. Test results were arranged in two groups, rods run together with seals that had lasted less than approximately 1.GOOstrokes and rods that could run for more than 20.000 strokes. The variation in performance was explained by the differently stratified structures. When the results from the previous investigation are kept in mind, the peak heights could be expected to influence the wear rate of a particular rod. In Fig.4 the upper section of the surface bearing curve for a honed ceramic coated rod and a honed steel rod before and after the test are plotted. The following observations were made from the experiments 1161. Ceramic rods that performed well had a considerably lower peak height than rods that failed to perform satisfactorily. With the ceramic rods, no initial wear in terms of reduction in peak height was observed, as could be seen with the steel rods and in view of the hardness, it was concluded that no running-in of these rods would take place. Peaks that penetrated the lubricant film continued to wear the seal, and therefore the rods with the lowest peak height resulted in the lowest seal wear rate.
475
information about initial wear and the type of wear is lost when only profile measurements are used, as discussed in more details in 1161.
a1 I
0
I
1
0
2
0
~
4
0
Material ratio [%I
5
b
6
0
Fig. 4 Upper portions of three-dimensional bearing curves for rods, showing differences between curves before and after wear tests. Single hatching shows differencerelated to ceramic coated rods. Cross hatching shows wear related to chromiumplated steel rods
/ I 6/.
The two investigations have led to a better understanding of surface characteristics that influence the functional properties of PTFE-seals: wear, friction and leakage.
Wear on deep drawing dies A Danish company that manufactures large quantities of stainless steel sheets requested an investigation into the influence of tool surfaces on tool wear progression. Traditionally, wear on dies has been investigated by measuring profiles continuously during the wear process, and using light microscopes and SEM. However these latter instruments give no information on the actual heights in surface topographies.
F&. 5 Axonometric plot of die surface after drawing of 5 strips /16/
An investigation was carried out on a strip test evaluation equipment, on which a die section was worn by pulling through 250 steel strips, and the progress of wear monitored.
:
At selected intervals, the die surface was measured using twodimensionalas well as three-dimensional roughness measuring equipment. From two-dimensional profiles it can only be concluded that roughness increases with use. But the mechanisms involved can not be determined, e.g. whether the roughness increase is caused by galling or ploughing. This information can be obtained from surface measurements. A SEM or a light microscope may give the same information as to the type of wear, but only three-dimensional profilometrygives information about the magnitude of wear. From surface plots it was concluded that the wear process was already initiated after 5 strips, by the single ploughing shown in Fig.5. As more strips were drawn, more ploughing traces appeared on the die surface. As the die was worn, either too many scratches appeared on the strip or an increase in friction occurred preventing the process from being carried out any further. After 250 strips the surface was worn considerably, though the die was not yet worn out. Parameter calculationsat different stages of wear are plotted in Fig.6. The results indicate that S, is an effective parameter for wear prediction, due to the fact that wear is dominated by ploughing 1161. In this example wear in the drawing process was examined by profile and surface measurements, and it was concluded that
476
: 0
1
Y
6
0.8 0.4 0.2 50
1 m 1 3 0 m m : Number of drawn parts
Fig. 6 Surface bearing curve parameters for the die as a function of the number of drawn strips /16/.
Sand blasted surface paintability Steel sheets used in ship building are sand blasted before painting, to generate a clean surface with good adherence properties. Usually these surfaces are characterized by visual examination or through simple roughness parameters (Ra, F$). These methods are inadequate since small, but still significant, variations in process conditions cannot be detected by visual
inspection and conventional roughness parameters do not correlate to paint adherence. Therefore a Danish paint manufacturer experienced difficulties in characterizing sand blasted surfaces by the conventional roughness parameters Ra and q.
In an investigationinvolvingthree-dimensional surface analysis, paint adherence was evaluated by an impact test resulting in three graduations: 0 = good, 1 = average and 2 = bad. At 0 the paint is intact while the graduation 2 means that the paint is totally removed from the surface. Table 1 shows a number of selected roughness parameters compared to paint adherence for four different surfaces.
Unfolded area Traced area
Table 7
13.5
1.14
10.0
1.15
12.3
1.12
Standard deviation of surface slopes distribution
Adherence
Adherence of paint on sand blasted surfaces.
From the results it was deduced that Sa relates poorly to paint adherence while the unfolded area ratio relates to adherence. A larger area ratio results in higher adherence, though only small variations in the unfolded surface area were experienced. The dispersion of surface slope distribution (u) relates to paint adherence, as shown in Table 1. If the surface height distribution is narrow, i.e. the dispersion is small, only a limited number of slopes are present and the surface structure appears shiny.
Cutting fluid efficiency A Danish company producing additives for metalworking fluids wanted a simple method to evaluate lubricant action in cutting. A reaming test was developed at the authors’ university 1221 and three-dimensionalmapping was used to documentate the definition of efficiency in terms of degree of replication of the surface determined by the tool geometry and movement. Fig.7 shows the contour plots of ream.ed surfaces, including a plot of the ideal surface generated synthetically by a special signal generation software developed by the authors 116,171. The plots show clearly that, while reaming under good lubrication conditions generates a surface which is similar to the theoretical pattern, not many of the typical furrows can be recognized on the surface obtained using a poor lubricant. Conventional two-dimensional analysis can be used in this case, due to the banded structure produced by reaming, but three-dimensional analysis proved itself as a convincing tool for understanding the conditions in the general case of any arbitrary surface.
Fig. 7 Grey tone maps of reamed stainless steel surfaces. Length of horizontalaxis: 0.8 mm (200 samples). length of vertical axis: 0.4 mm (100 samples). (a) Theoretical surface; [b) Reamed surface, using a good lubricant; (c) Reamed surface, poor lubricant/22/,
Human skin integrity The last industrial example concerns characterizations of human skin surfaces. A Danish company producing medicaments for skin diseases was interested in a quantitative assessment of dermatologists’quantitative evaluation. The company had invested in an optical instrument but the investigation was undertaken at the authors’ university. Human skin replicas were taken and the optical surface measurement instrument was used for measuring these. Roughness measurements enabled the quantifying of the progress of skin deseases and the effect of liniments. An investigation of 16 skin replicas were carried out 1231. Area samples of 5.6 x 5.6 mm2 were measured with the optical profilometer and analyzed by means of the described three-dimensional parameters. In this investigationthe parameters Sa, S,, S, and S, were calculated using the software facilities developed by the authors. The surface plots obtained from the optical profilometer were divided into three categories by means of visual inspection: Category I : Category 1 I : Category 111 :
Characteristic waviness, with or without craters. No characteristic waviness, with craters. No characteristic waviness, without craters.
477
Figure 8(a) shows an example of a profile plot from category I, and figure 8(b) shows a replica from category 111. By using the three-dimensional parameters mentioned above it was possible to place a replica in one of the categories in accordance with the visual inspection. Table 2 shows the combination of parameters and parameter values found in this investigation.A calculation of the parameters Sa, St, s k and S, for a certain profile plot permits determination of which category the replica belongs to. All four parameter values are needed to place the replica in the correct category.
Conclusions At the authors’ university, based on more than a decade’s work with digital surface roughness analysis techniques, a comprehensive system based on stylus and optical instruments and versatile software has been developed and used to undertake many surface analyses.
Three-dimensionalsurface characterization has proven itself to be invaluable in a number of industrial applications, ranging from leakage and wear on seals to wear on deep drawing dies, paintability of sand blasted surfaces, efficiency of cutting fluids, and integrity of human skin. In some cases, three-dimensional analysis is the only possibility. In other cases it is a valid alternative to two-dimensionalprofilometry and subsidiary techniques. In all cases it provides a modern analysis tool which is expected to increase in power, as computers develop in capacity and speed.
Waviness, with or without craters
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Sayles, R.A., Thomas, T.R., 1976, Mapping of Small area of a surface, J. Phys. E., vol. 9, p. 855-861. Clayton Teague, E., 1976, Evaluation, revision and application of the NBS stylus/computer system for the measurement of surface roughness. National Bureau of Standards, Washington. N.N., 1979, Le rugosimbtre tridimensionnel p w r une meilleure evaluation des Btats de surface. Centre technique des industries mhniques. Saint-Etienne. Tsukada, T., Sasajima, K., 1981, A three-dimensional measuring technique for surface asperities, Wear, vol. 71, p. 1-14. Bengtson, A., Ronnberg, A., 1984, Wide range three-dimensional roughness measuring system, Prec. Eng., vol. 6, p. 141-147. De Chiffre, L., Nielsen, H.S., 1987, A digital system for surface roughness analysis of plane and cylindricalparts, Prec. Eng., vol. 9, p. 59-64. Stout, K.J., Sullivan, P.J., 1992, The use of 3-D topographic analysis to determine the microgeometrytransfer characteristics of textured sheet surfaces through rolling, Annals of the CIRP, vol. 4111, p. 621-624. Stout, K.J., et al., 1991, The development of an integrated approach to 3D surface finish assessment, Interim Report, EC Contract No. 3374/1/0/170/90/2. Stout, K.J., et el., 1993, The development methods for the characterization of roughness in 3 dimensions, Phase I1 report, vol. 1 and 2. EC Contract No. 3374/1/0/170/90/2. Schneider, U., et al., 1988, An approach to the evaluation of surface profiles by separating them into functionally different parts, Surface Topography, vol. 1. p. 71-83. Scott,P.J., 1988, Developments in surface texture measurement, Surface Topography, vol. 1, p. 153-163. de Bruin, W., 1986, STC mSurfaces‘:Surface and Function, Annals of the CIRP, vol. 3512, p. 551-559. Nielsen, H.S., 1988, New approaches to surface roughness evaluation of special surfaces, Prec. Eng., vol. 10, p. 209313. Larsen, I.B., De Chffre, L.. Asmussen, E., 1990, A new method for measuring wear of restorations in vivo, J. Dent. Res., vol. 69, Spec. Iss., No. 138. Andreasen, J.L., 1992. Monitoring and control of machined surfaces, Ph.D. thesis, Technical University of Denmark. Christiansen, S., 1994, Function-related characterization of surface topography, Ph.D. thesis, Technical University of Denmark. Skade, S., 1994, Engineering of surface topographies, Ph.D. thesis, Technical University of Denmark. Efsen. J., et al., 1994, Chapter 8: Laser profilometry, In vivo examination of the skin: a handbook of non-invasive methods, CRC Press, Florida. Hillmann, W., 1990, Surface profiles obtained by means of optical methods Are they true representations of me real surface?, Annak of the CIRP. vol. 3911, p. 581-583. DIN 4776, 1990, KenngrBssen Rk, R Q, M,,, M, zur Beschreibung des Materialanteils im Rau\&sprofil. Nielsen. H.S., 1989, Calibration of surface roughness instruments, VDI Berichte nr. 761, p.365370. De Chffre, L., et al., 1992, A reaming test for cutting ffuid evaluation, Proc. 8th Int. Coil. Trib., TAE, Esslingen, p. 18.9. Efsen, J., Hansen, H.N., 1993, Optisk ruhedsmaing, M.Sc. thesis, Technical University of Denmark (in Danish).
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