- Email: [email protected]
The application of laser scanning confocal microscopy to tribological research
Wear 251 (2001) 1159–1168
The application of laser scanning confocal microscopy to tribological research D.N. Hanlon a,∗ , I. Todd a , E. Peekstok b , W.M. Rainforth c , S. van der Zwaag a,b a
The Netherlands Institute for Metals Research, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Laboratory for Materials Science, Delft Technical University, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Department of Engineering Materials, The University of Sheffield, The Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK b
c
Abstract Since the introduction of the first commercial systems in the early 1980s laser scanning confocal microscopy (LSCM) has become an established technique in biological and medical fields of research. To date the application of LSCM to metallurgical and tribological fields of research has been extremely limited. However, largely as a result of recent rapid advances in computer processing power, the modern LSCM system has become a flexible research tool with a broad range of capabilities, which are well suited to metallurgical research. In this article, the application of LSCM to the study of worn surfaces is discussed. Illustrations are presented which show how post-processing of confocal image stacks can be used to achieve a greatly extended depth of field thus enabling clear images of rough tribological surfaces to be constructed. Furthermore, illustrations of the use of LSCM for the quantification of surface topography are also presented which demonstrate that surface height profiles which faithfully reproduce the geometry of the real surface can be measured with a high degree of accuracy. The accuracy of profile surface roughness measurements via the LSCM has also been systematically investigated and compared with the results obtained from profiles measured using contact mode atomic force microscopy (AFM). The results presented show that, after suitable data post-processing to correct for tilt and extraneous signal noise, the results obtained using both techniques are in good agreement. Finally, the results of a study of rolling sliding contact wear surfaces of two high chromium content white cast irons are presented. The combination of extended focal depth imaging and topographical quantification afforded by the LSCM has proven capable of firmly establishing the wear mechanisms operating in these materials. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser scanning confocal microscopy; High alloy white cast irons; Abrasive wear; Spray casting; Surface topography
1. Introduction The concept of confocal microscopy is attributable to Mervin Minski who developed the first confocal optical systems in the 1950s [1]. However, it was not until the 1980s that the first truly commercial confocal systems were developed. The principal reasons for the delay in the wider adoption of the technique were the unavailability of affordable compact lasers, modern image storage devices and sufficiently fast processors. In the intervening two decades, the modern laser scanning confocal microscope (LSCM) has become an established technique in biological and medical fields of research. The ability to work with stable laser light sources which provide high intensity monochromatic light affords significant advantages, particularly in connection with fluorescent microscopy. By comparison the application of LSCM to metallurgical and tribological research has been extremely limited except for a few notable exam∗ Corresponding author. Tel.: +31-15-278-2189. E-mail address: [email protected] (D.N. Hanlon).
ples, e.g. characterization of corroded surfaces [2], surface roughness measurement [3] and the in situ observation of intermetallic formation in liquid steels [4] or moving interphase boundaries during the ␦ to ␥ transformation in steels [5,6]. However, a number of the inherent features of LSCM make the technique ideally suited to the characterization of worn metallurgical samples. For instance high intensity, short wavelength light enables the attainment of resolution close to the theoretical limit for optical systems. Furthermore, the effective depth of field is independent of the numerical aperture of the objective lens and is many times greater than that of a conventional system enabling observations to be made on even the roughest of surfaces. In addition, there is no vacuum requirement, as in a SEM and samples may be viewed directly, with a minimum of preparation even in situ in wet media. Finally, the recent rapid advances in computational power have allowed incorporation of powerful image processing software which enables the construction of three-dimensional (3D) images, topographical maps, stereo anaglyphs and the quantification of surface topography down to a few tens of nanometers. The purpose
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of this paper is to illustrate, by means of a number of worked examples, the suitability of LSCM to tribological research. The principle of the technique, extended focal depth imaging, three-dimensional image processing and quantification of surface roughness are discussed in turn.
2. The principle of LSCM The fundamental principal of confocal microscopy is to form images using light from a discrete focal plane. In practice, a collimated point light source is directed at the object plane from where it is reflected back through the objective lens as shown in the light path diagram in Fig. 1. In the LSCM, the point light source is scanned across the specimen surface to form the complete image. The reflected light from each position on the surface is directed to a photomultiplier via a detector pinhole. The detector pinhole is conjugated to the objective aperture such that only light from a discrete focal plane is allowed to reach the detector and all off focal plane light is discarded. The depth of the optical section produced in this way is a function of the pinhole diameter and the wavelength of the incident light. By means of a high precision motorized focusing stage, the objective can be displaced along the vertical axis (z-axis) and a number of images (optical sections) produced from a series of
Fig. 1. A schematic diagram illustrating the principle of operation of the LSCM.
regularly spaced focal planes (a so-called z-series). The images in this series represent a volume of material for which a three-dimensional distribution of intensities is known (as illustrated in Fig. 2). The column of pixels at each x, y coordinate can be processed to create projected images from the maximum or mean intensities, create 3D visualizations of the object, create topographical (height-coded maps, or render 3D stereo anaglyphs.
3. Extended focus imaging The depth of field (Z) of an aberration free diffraction limited optical system is determined by the numerical aperture NA as expressed through the relationship Z =
nλ NA2
(1)
For λ = 488 nm and using a dry lens (refractive index, n = 1) of numerical aperture 0.95, Z equals 540 nm. In the case of the LSCM, despite the fact that the system is designed to produce very shallow optical sections, the fact that projected images may be produced by post-processing of multiple optical sections in a z-series leads, paradoxically, to a greatly extended effective depth of field. The depth of focus is determined only by the limits of the mechanical focusing table and the working distance of the objective lens. The former represents the total z-range over which the z-series can be acquired under ideal circumstances whilst the latter reflects the practical limitation that the attainable z-range is often determined by mechanical contact between the lens and the sample surface. The danger of contact is, of course, greater for higher magnification objective lenses with a shorter working distance. In practice, an extended depth of focus of the order of 200 m can be achieved by such a post-processing treatment. The principal advantage of an instrument with an extended depth of focus is that it enables irregular, rough surfaces to be observed. The LSCM is thus particularly suited for the imaging of worn surfaces. Fig. 3 shows a z-series of confocal images (Fig. 3a) and corresponding maximum
Fig. 2. A schematic illustration of optical sectioning and the formation of a z-series of confocal images in the LSCM.
D.N. Hanlon et al. / Wear 251 (2001) 1159–1168
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Fig. 3. LSCM and conventional optical images of a 17Cr white cast iron wear surface illustrating the extended depth of field available by image post-processing a z-series: (a) a z-series of optical sections; (b) a maximum intensity projection (50× objective magnification); (c) a maximum intensity projection of the area depicted in (b) taken at higher objective magnification (100× objective magnification); (d) a conventional optical micrograph of the area shown in (b) and (c) (50× objective magnification); (e) a conventional optical micrograph of the area shown in (b) and (c) (100× objective magnification).
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intensity projections (Fig. 3b and c) of a 17Cr white cast iron worn surface. As reported in detail elsewhere [7,8] the wear surface of this material is characterized by deep abrasive grooving and oxide prows, which protrude considerably above the abraded surface. The maximum intensity projection in Fig. 3b shows a typical oxide prow and it is apparent that both the surface of the prow and the surrounding abraded surface are in sharp focus. This is equally true of the image shown in Fig. 3c, which was taken at a higher objective magnification. For the purpose of comparison, the same region imaged in the conventional optical microscope as shown in Fig. 3d and e. Comparison of these images with the confocal projections clearly illustrates the dramatic increase in effective depth of field which may be achieved in the LSCM. In the case of the conventional optical micrographs, even the prow surface is not clearly focused across its entire area.
4. Quantification of surface topography Another significant advantage of the LSCM arises as a direct consequence of the manner in which images are recorded in the LSCM. Since the optical sections in the z-series are recorded at known relative z displacements, pixels in the observation volume are height-coded. The 3D nature of the intensity data thus enables the construction of topological images and consequently the quantification of surface topography. Height-coded images (gray or color scale topological maps) may be constructed according to the simple relation Gx,y = 255
Z − Z0 Zm − Z 0
(2)
where Gx,y is the height-coded gray level at a coordinate position x and y, Z the height of the maximum intensity pixel (effectively the surface), Zm the height of the highest section taken and Z0 is the average height. The full range of gray levels can be exploited by interpolation. From such a height-coded image the measurement of a height profile is simple. Topography of surfaces can be measured with 50 nm relative accuracy. Fig. 4 shows an example of such a topological image. The subject in question is a standard Vickers micro hardness indentation in the matrix of a commercial martensitic white cast iron. In this image, black pixels represent the minimum height whilst white pixels represent the maximum. A height profile across the indent diagonal obtained from such an image is shown in Fig. 5. That the profile is a faithful representation of the true indentation geometry can be ascertained since the geometry of a standard Vickers indentor is well known. The root angle between two faces of a Vickers diamond is 136◦ whilst that determined in the present work is on average (134 ± 3)◦ . Furthermore, the measured depth of the indentation can be compared to that calculated from the length of the indentation diagonals. Fig. 6 shows such a
Fig. 4. A topological height-coded gray scale image of a Vickers hardness indentation in the martensitic matrix of a white cast iron.
comparison of measured and calculated indentation depths for a range of standard indentation sizes and objective magnifications for the same material shown in Fig. 4. The data reveals a generally good correlation at all objective magnifications across the entire range of indentation sizes considered. The exception is for the 50× objective magnification where the measured values consistently underestimate the true depth of the indents. The error does however appear to be a constant relative error and such a systematic variation may be accurately determined and a corrective calibration applied in the quantification of cross-sectional profiles. The accuracy of profile surface roughness measurements via the LSCM has also been systematically investigated
Fig. 5. A height profile measured across the diagonal of a Vickers hardness indentation in the martensitic matrix of a white cast iron.
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Rq =
Fig. 6. Comparison of measured and calculated indentation depths for a range of indentation sizes and objective magnifications (50, 100 and 150×). In all cases, the z-step size was held constant at 0.41 m.
and compared with the results obtained from profiles measured using an atomic force microscope (AFM, Digital Instruments, NanoScope IIIA) operating in contact mode. To compare the results obtained using the two different microscopical techniques, surface roughness parameters were determined for LSCM topographic data obtained using both a 50 and 100× objective lens from carbon–manganese steel sheet that had been unidirectionally ground to 60, 240 or 600 grit (denoted treatment 1, 2 and 3, respectively). Cross-sections perpendicular to the direction of grinding were obtained and the LSCM data was corrected for tilting using a least squares method before quantification. The roughness analysis was performed both on data corrected for specimen tilt only and for this same data after smoothing via a moving average algorithm to remove extraneous signal noise. The AFM data was corrected for tilt and smoothed using the DI instrument software. Profiles taken from an AFM image, and topological data generated by the LSCM using the 50× objective lens are shown for comparison in Fig. 7(a and b, respectively) as are the same LSCM data after smoothing (Fig. 7c). It is clear the data produced by the two methods is qualitatively in good agreement. To quantify the surface roughness and the level of agreement, two amplitude parameters obtained from the roughness profiles have been used in this study, the average roughness, Ra and the RMS roughness Rq , which are given by nx 1 Ra = |Z(i) − Zave | (3) nx i=1
nx 2 i=1 |Z(i) − Zave |
nx
(4)
where Z(i) denotes the topography data for the surface after tilt correction, Zave the average surface height, i corresponds to the measurement in the x-direction and the total number of such measurements is given by nx . The results of these analyses along with the results generated by the AFM software are given in Table 1. It is clear that the overall correlation of the results obtained by the two techniques and also between the results obtained via LSCM using the two different objective lenses is good, with the only significant deviation observed being for the Ra and Rq determined for condition 2 using the 50× objective. In general, there was no significant difference between the Ra and Rq obtained using either the smoothed or the unsmoothed data. The only notable exception was the aforementioned topographic data obtained via the 50× objective for the condition 2 material that exhibited a low signal-to-noise ratio unsmoothed. After a noise reducing smoothing operation, the data was found to fall into line with the overall trend in roughness results (Table 1). These findings corroborate those of Gjønnes [3] who has shown, by comparison with stylus profilometry, that the LSCM is an ideal tool for performing quantification of surface quality on rolled aluminum sheet products. In contrast, however, Lindseth and Bardal [9] have claimed that topological data from LSCM and AFM gave rise to very different surface roughness results. The differences observed may, however, be attributable to differences in data post-processing methodologies for the techniques under consideration in their study.
5. Characterization of high chromium cast iron worn surfaces in the LSCM In order to illustrate better the application of LSCM to the characterization of tribological samples, a short investigation of real worn surfaces has been conducted. The materials under investigation, a series of high chromium cast iron samples, have been the subject of earlier extensive investigations and the details of their manufacture and the testing methods used have been published in detail elsewhere [7,8]. The basic microstructure of these materials in the service condition comprises a distribution of hard chromium carbides embedded within a tempered martensitic matrix. In these earlier publications it has been reported that spray casting leads to a microstructure which is dramatically refined compared with that of a conventionally cast material. Comparison of conventionally cast with spray cast microstructures reveals that the spray cast material comprises a fine, uniform distribution of carbides (in the size range 1–8 m) whereas in the conventionally cast material the carbides where distributed in a continuous network in the interdendritic regions (interdendritic separations where of the order of 200–500 m).
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Fig. 7. Cross-sectional profiles of a C–Mn steel sheet unidirectionally ground to 60 grit. The profiles shown were generated from: (a) AFM; (b) LSCM using a 50× objective and (c) this same LSCM data after smoothing to remove extraneous signal noise.
Table 1 Comparison of the surface roughness statistics parameters: (a) Ra and (b) Rq obtained by analysis of the cross-sectional profiles measured by AFM and by LSCM with a 50 and 100× objective Surface condition
1 2 3
Ra
Rq
LSCM 50× unsmoothed
LSCM 50× smoothed
LSCM 100× unsmoothed
AFM
LSCM 50× unsmoothed
LSCM 50× smoothed
LSCM 100× unsmoothed
AFM
0.30 0.26 0.08
0.30 0.16 0.07
0.26 0.15 0.07
0.26 0.18 0.06
0.37 0.36 0.10
0.38 0.20 0.10
0.30 0.18 0.09
0.34 0.23 0.08
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This refinement of microstructure was shown to give rise to a five-fold increase in rolling sliding wear resistance across a broad range of testing temperatures from 20 to 400◦ C, whilst at higher temperatures (500–700◦ C) the wear resistance was dramatically reduced and both materials exhibited an almost identical wear response. In these earlier studies the wear surfaces were observed to comprise oxide prows and deep abrasive grooves suggesting that the principal wear processes included mildly oxidative and three body abrasive mechanisms. The improvement of wear response at low to intermediate temperatures exhibited by the spray cast material was attributed in part to the greater resistance to fracture exhibited by these smaller discrete carbides and also to the shorter mean free path between the carbides resulting from the more uniform carbide distribution. The reduction in mean free path (effectively from the
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diameter of the carbide denuded interdendritic region in the conventionally cast material which was 200–300 m to the intercarbide spacing which was 10 m in the spray cast material) was envisaged to lead to more effective protection of the matrix from third body abrasives. The increase in wear rate at higher temperatures was attributed to a dramatic softening of the matrix which occurred at temperatures above 500◦ C. This led to a wear process dominated by the plasticity of the matrix rather than by the mechanical stability of the carbides and thus to a similar wear response. In both materials, the wear mechanism was found to be a complex combination of severe abrasion and oxidation. In this investigation worn surfaces of conventionally cast and spray formed material produced at both room temperature and 700◦ C have been compared. As in the previous study, the surface was found to comprise oxide prows and
Fig. 8. LSCM maximum projections of worn 17Cr cast iron surfaces after rolling sliding contact wear at 700◦ C: (a) conventionally cast 17Cr cast iron at low magnification (A denotes oxide prow); (b) spray cast 17Cr cast iron at low magnification (A denotes oxide prow); (c) the trailing edge of the oxide prow in Fig. 3a at higher magnification and (d) the trailing edge of the oxide prow in Fig. 3b at higher magnification.
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abrasive grooves. Fig. 8a and b shows low magnification LSCM maximum intensity projections of spray cast and conventionally cast materials tested at 700◦ C. In each image, an oxide prow is visible at the top of the image whilst immediately behind each prow there is an “comet tail” of deep abrasive grooving. Fig. 8c and d shows the tail end of each of the prows at higher magnification and in each case the oxide can been seen to be extensively cracked. The spray cast and conventionally cast materials appear identical supporting the previous observation of their identical wear response. This pattern of cracking and abrasive wear was associated with the tail end of all prows observed. Fig. 9 shows a typical surface height profile taken along the sliding axis across the prow shown in Fig. 8a. The surface of the prow can be seen to be globally smooth (A and B) whilst in the cracked region at the tail, a large section of the oxide plate is inclined (B and C). Immediately behind the prow there is a shear drop (C and D) and a plateau (D and E) which is followed by a deep gouge (E and F). This pattern of behavior was remarkably consistent and such profiles were
Fig. 9. A height profile measured along the sliding direction over the oxide prow (A) shown in Fig. 8a.
Fig. 10. LSCM maximum projections of worn 17Cr cast iron surfaces after rolling sliding contact wear at 20◦ C: (a) spray cast 17Cr cast iron at low magnification (A denotes oxide prow); (b) conventionally cast 17Cr cast iron showing extensive surface cracking and (c) conventionally cast 17Cr cast iron showing pitting.
D.N. Hanlon et al. / Wear 251 (2001) 1159–1168
observed for the majority of prows investigated. Based on these topological observations, a sequence of micromechanisms leading to the destruction of the oxide prows and consequent liberation of wear debris can be proposed. Bending stresses due to asperity contacts and surface tensile stresses due to friction forces lead to cracking and pull-off of oxide at the tail of the prows where constraint is absent. The oxide debris thus produced is ploughed through the surface region immediately behind the tail of the prow. This leads to the formation of deep abrasive grooving and the formation of the gouge. The plateau behind the prow represents a region which is less severely abraded either by virtue of its being in the shadow of the prow (and thus protected from asperity
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contact with the counterface) or due to its recent exposure (thus having been exposed to abrasion for a limited period). The worn surfaces tested at 20◦ C were also characterized by oxide patches but as might be expected abrasive grooving was less apparent. Fig. 10a shows an LSCM image of such a worn surface where a number of oxide patches are clearly visible (the example shown is taken from spray cast material). In the case of the conventionally cast material areas of extensive surface cracking (see Fig. 10b) and deep pits of approximately 50–100 m in diameter (Fig. 10c) were observed. Such features were not found on the surfaces of spray cast materials. Surface profiles taken across the cracked region in Fig. 10b and across the pit in Fig. 10c are shown in Fig. 11a and b, respectively. The step changes in height visible at points A, B, C and D in Fig. 11a may indicate that a delamination mechanism is in operation whilst the pit in Fig. 11b may represent direct evidence of enhanced wear in the carbide denuded interdendritic regions (the diameter of the pits is of the order of size of the interdendritic spacing). Whilst the reasons for enhanced wear resistance have already been inferred from earlier SEM and optical observations on these samples [8], the quantitative information obtained in the LSCM puts the explanation on a much sounder scientific basis.
6. Conclusions 1. The LSCM is well suited for imaging rough surfaces such as those encountered in tribological research. 2. The present findings indicate that there is a good agreement between surface roughness statistics obtained by the LSCM and AFM techniques over a wide range of surface finishes (R a = 0.25–0.07 m). This, combined with the excellent level of accuracy afforded by LSCM in the measurement of deeper surface topological features, as witnessed by the results of hardness indentation depth measurements, would indicate that LSCM is readily applicable to the analysis and quantification of surface topography. 3. The combination of extended focal depth imaging and topographical measurement afforded by the LSCM has proven capable of firmly establishing wear mechanisms operating in two high chromium white cast irons. This indicates that the LSCM is an ideal tool for use in the characterization and interpretation of surface damage arising from wear.
References Fig. 11. Height profiles taken across: (a) the cracked area shown in Fig. 10b and (b) the pitted area shown in Fig. 10c.
[1] J.B. Pawley, Handbook of Biological Confocal Microscopy, Plenum Press, New York, 1999.
1168 [2] [3] [4] [5] [6]
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J.M. Brown, C.J. Newton, Br. Corros. J. 29 (1994) 261–269. L. Gjønnes, Met. Mater. Trans. A 27 (1996) 2338–2346. N. Yuki, H. Shibata, T. Emi, ISIJ Int. 38 (1998) 317–323. H. Yin, T. Emi, H. Shibata, ISIJ Int. 38 (1998) 794–801. H. Yin, T. Emi, H. Shibata, Acta Mater. 47 (1999) 1523–1534.
[7] D.N. Hanlon, W.M. Rainforth, C.M. Sellars, R. Price, H.T. Gisborne, J. Forrest, J. Mater. Sci. 34 (1999) 2291–2301. [8] D.N. Hanlon, W.M. Rainforth, C.M. Sellars, Wear 225–229 (1999) 587–599. [9] I. Lindseth, A. Bardal, Surf. Coatings Tech. 111 (1999) 276–286.
The application of laser scanning confocal microscopy to tribological research D.N. Hanlon a,∗ , I. Todd a , E. Peekstok b , W.M. Rainforth c , S. van der Zwaag a,b a
The Netherlands Institute for Metals Research, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Laboratory for Materials Science, Delft Technical University, Rotterdamseweg 137, 2628 AL Delft, The Netherlands Department of Engineering Materials, The University of Sheffield, The Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK b
c
Abstract Since the introduction of the first commercial systems in the early 1980s laser scanning confocal microscopy (LSCM) has become an established technique in biological and medical fields of research. To date the application of LSCM to metallurgical and tribological fields of research has been extremely limited. However, largely as a result of recent rapid advances in computer processing power, the modern LSCM system has become a flexible research tool with a broad range of capabilities, which are well suited to metallurgical research. In this article, the application of LSCM to the study of worn surfaces is discussed. Illustrations are presented which show how post-processing of confocal image stacks can be used to achieve a greatly extended depth of field thus enabling clear images of rough tribological surfaces to be constructed. Furthermore, illustrations of the use of LSCM for the quantification of surface topography are also presented which demonstrate that surface height profiles which faithfully reproduce the geometry of the real surface can be measured with a high degree of accuracy. The accuracy of profile surface roughness measurements via the LSCM has also been systematically investigated and compared with the results obtained from profiles measured using contact mode atomic force microscopy (AFM). The results presented show that, after suitable data post-processing to correct for tilt and extraneous signal noise, the results obtained using both techniques are in good agreement. Finally, the results of a study of rolling sliding contact wear surfaces of two high chromium content white cast irons are presented. The combination of extended focal depth imaging and topographical quantification afforded by the LSCM has proven capable of firmly establishing the wear mechanisms operating in these materials. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser scanning confocal microscopy; High alloy white cast irons; Abrasive wear; Spray casting; Surface topography
1. Introduction The concept of confocal microscopy is attributable to Mervin Minski who developed the first confocal optical systems in the 1950s [1]. However, it was not until the 1980s that the first truly commercial confocal systems were developed. The principal reasons for the delay in the wider adoption of the technique were the unavailability of affordable compact lasers, modern image storage devices and sufficiently fast processors. In the intervening two decades, the modern laser scanning confocal microscope (LSCM) has become an established technique in biological and medical fields of research. The ability to work with stable laser light sources which provide high intensity monochromatic light affords significant advantages, particularly in connection with fluorescent microscopy. By comparison the application of LSCM to metallurgical and tribological research has been extremely limited except for a few notable exam∗ Corresponding author. Tel.: +31-15-278-2189. E-mail address: [email protected] (D.N. Hanlon).
ples, e.g. characterization of corroded surfaces [2], surface roughness measurement [3] and the in situ observation of intermetallic formation in liquid steels [4] or moving interphase boundaries during the ␦ to ␥ transformation in steels [5,6]. However, a number of the inherent features of LSCM make the technique ideally suited to the characterization of worn metallurgical samples. For instance high intensity, short wavelength light enables the attainment of resolution close to the theoretical limit for optical systems. Furthermore, the effective depth of field is independent of the numerical aperture of the objective lens and is many times greater than that of a conventional system enabling observations to be made on even the roughest of surfaces. In addition, there is no vacuum requirement, as in a SEM and samples may be viewed directly, with a minimum of preparation even in situ in wet media. Finally, the recent rapid advances in computational power have allowed incorporation of powerful image processing software which enables the construction of three-dimensional (3D) images, topographical maps, stereo anaglyphs and the quantification of surface topography down to a few tens of nanometers. The purpose
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 7 2 7 - X
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of this paper is to illustrate, by means of a number of worked examples, the suitability of LSCM to tribological research. The principle of the technique, extended focal depth imaging, three-dimensional image processing and quantification of surface roughness are discussed in turn.
2. The principle of LSCM The fundamental principal of confocal microscopy is to form images using light from a discrete focal plane. In practice, a collimated point light source is directed at the object plane from where it is reflected back through the objective lens as shown in the light path diagram in Fig. 1. In the LSCM, the point light source is scanned across the specimen surface to form the complete image. The reflected light from each position on the surface is directed to a photomultiplier via a detector pinhole. The detector pinhole is conjugated to the objective aperture such that only light from a discrete focal plane is allowed to reach the detector and all off focal plane light is discarded. The depth of the optical section produced in this way is a function of the pinhole diameter and the wavelength of the incident light. By means of a high precision motorized focusing stage, the objective can be displaced along the vertical axis (z-axis) and a number of images (optical sections) produced from a series of
Fig. 1. A schematic diagram illustrating the principle of operation of the LSCM.
regularly spaced focal planes (a so-called z-series). The images in this series represent a volume of material for which a three-dimensional distribution of intensities is known (as illustrated in Fig. 2). The column of pixels at each x, y coordinate can be processed to create projected images from the maximum or mean intensities, create 3D visualizations of the object, create topographical (height-coded maps, or render 3D stereo anaglyphs.
3. Extended focus imaging The depth of field (Z) of an aberration free diffraction limited optical system is determined by the numerical aperture NA as expressed through the relationship Z =
nλ NA2
(1)
For λ = 488 nm and using a dry lens (refractive index, n = 1) of numerical aperture 0.95, Z equals 540 nm. In the case of the LSCM, despite the fact that the system is designed to produce very shallow optical sections, the fact that projected images may be produced by post-processing of multiple optical sections in a z-series leads, paradoxically, to a greatly extended effective depth of field. The depth of focus is determined only by the limits of the mechanical focusing table and the working distance of the objective lens. The former represents the total z-range over which the z-series can be acquired under ideal circumstances whilst the latter reflects the practical limitation that the attainable z-range is often determined by mechanical contact between the lens and the sample surface. The danger of contact is, of course, greater for higher magnification objective lenses with a shorter working distance. In practice, an extended depth of focus of the order of 200 m can be achieved by such a post-processing treatment. The principal advantage of an instrument with an extended depth of focus is that it enables irregular, rough surfaces to be observed. The LSCM is thus particularly suited for the imaging of worn surfaces. Fig. 3 shows a z-series of confocal images (Fig. 3a) and corresponding maximum
Fig. 2. A schematic illustration of optical sectioning and the formation of a z-series of confocal images in the LSCM.
D.N. Hanlon et al. / Wear 251 (2001) 1159–1168
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Fig. 3. LSCM and conventional optical images of a 17Cr white cast iron wear surface illustrating the extended depth of field available by image post-processing a z-series: (a) a z-series of optical sections; (b) a maximum intensity projection (50× objective magnification); (c) a maximum intensity projection of the area depicted in (b) taken at higher objective magnification (100× objective magnification); (d) a conventional optical micrograph of the area shown in (b) and (c) (50× objective magnification); (e) a conventional optical micrograph of the area shown in (b) and (c) (100× objective magnification).
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intensity projections (Fig. 3b and c) of a 17Cr white cast iron worn surface. As reported in detail elsewhere [7,8] the wear surface of this material is characterized by deep abrasive grooving and oxide prows, which protrude considerably above the abraded surface. The maximum intensity projection in Fig. 3b shows a typical oxide prow and it is apparent that both the surface of the prow and the surrounding abraded surface are in sharp focus. This is equally true of the image shown in Fig. 3c, which was taken at a higher objective magnification. For the purpose of comparison, the same region imaged in the conventional optical microscope as shown in Fig. 3d and e. Comparison of these images with the confocal projections clearly illustrates the dramatic increase in effective depth of field which may be achieved in the LSCM. In the case of the conventional optical micrographs, even the prow surface is not clearly focused across its entire area.
4. Quantification of surface topography Another significant advantage of the LSCM arises as a direct consequence of the manner in which images are recorded in the LSCM. Since the optical sections in the z-series are recorded at known relative z displacements, pixels in the observation volume are height-coded. The 3D nature of the intensity data thus enables the construction of topological images and consequently the quantification of surface topography. Height-coded images (gray or color scale topological maps) may be constructed according to the simple relation Gx,y = 255
Z − Z0 Zm − Z 0
(2)
where Gx,y is the height-coded gray level at a coordinate position x and y, Z the height of the maximum intensity pixel (effectively the surface), Zm the height of the highest section taken and Z0 is the average height. The full range of gray levels can be exploited by interpolation. From such a height-coded image the measurement of a height profile is simple. Topography of surfaces can be measured with 50 nm relative accuracy. Fig. 4 shows an example of such a topological image. The subject in question is a standard Vickers micro hardness indentation in the matrix of a commercial martensitic white cast iron. In this image, black pixels represent the minimum height whilst white pixels represent the maximum. A height profile across the indent diagonal obtained from such an image is shown in Fig. 5. That the profile is a faithful representation of the true indentation geometry can be ascertained since the geometry of a standard Vickers indentor is well known. The root angle between two faces of a Vickers diamond is 136◦ whilst that determined in the present work is on average (134 ± 3)◦ . Furthermore, the measured depth of the indentation can be compared to that calculated from the length of the indentation diagonals. Fig. 6 shows such a
Fig. 4. A topological height-coded gray scale image of a Vickers hardness indentation in the martensitic matrix of a white cast iron.
comparison of measured and calculated indentation depths for a range of standard indentation sizes and objective magnifications for the same material shown in Fig. 4. The data reveals a generally good correlation at all objective magnifications across the entire range of indentation sizes considered. The exception is for the 50× objective magnification where the measured values consistently underestimate the true depth of the indents. The error does however appear to be a constant relative error and such a systematic variation may be accurately determined and a corrective calibration applied in the quantification of cross-sectional profiles. The accuracy of profile surface roughness measurements via the LSCM has also been systematically investigated
Fig. 5. A height profile measured across the diagonal of a Vickers hardness indentation in the martensitic matrix of a white cast iron.
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Rq =
Fig. 6. Comparison of measured and calculated indentation depths for a range of indentation sizes and objective magnifications (50, 100 and 150×). In all cases, the z-step size was held constant at 0.41 m.
and compared with the results obtained from profiles measured using an atomic force microscope (AFM, Digital Instruments, NanoScope IIIA) operating in contact mode. To compare the results obtained using the two different microscopical techniques, surface roughness parameters were determined for LSCM topographic data obtained using both a 50 and 100× objective lens from carbon–manganese steel sheet that had been unidirectionally ground to 60, 240 or 600 grit (denoted treatment 1, 2 and 3, respectively). Cross-sections perpendicular to the direction of grinding were obtained and the LSCM data was corrected for tilting using a least squares method before quantification. The roughness analysis was performed both on data corrected for specimen tilt only and for this same data after smoothing via a moving average algorithm to remove extraneous signal noise. The AFM data was corrected for tilt and smoothed using the DI instrument software. Profiles taken from an AFM image, and topological data generated by the LSCM using the 50× objective lens are shown for comparison in Fig. 7(a and b, respectively) as are the same LSCM data after smoothing (Fig. 7c). It is clear the data produced by the two methods is qualitatively in good agreement. To quantify the surface roughness and the level of agreement, two amplitude parameters obtained from the roughness profiles have been used in this study, the average roughness, Ra and the RMS roughness Rq , which are given by nx 1 Ra = |Z(i) − Zave | (3) nx i=1
nx 2 i=1 |Z(i) − Zave |
nx
(4)
where Z(i) denotes the topography data for the surface after tilt correction, Zave the average surface height, i corresponds to the measurement in the x-direction and the total number of such measurements is given by nx . The results of these analyses along with the results generated by the AFM software are given in Table 1. It is clear that the overall correlation of the results obtained by the two techniques and also between the results obtained via LSCM using the two different objective lenses is good, with the only significant deviation observed being for the Ra and Rq determined for condition 2 using the 50× objective. In general, there was no significant difference between the Ra and Rq obtained using either the smoothed or the unsmoothed data. The only notable exception was the aforementioned topographic data obtained via the 50× objective for the condition 2 material that exhibited a low signal-to-noise ratio unsmoothed. After a noise reducing smoothing operation, the data was found to fall into line with the overall trend in roughness results (Table 1). These findings corroborate those of Gjønnes [3] who has shown, by comparison with stylus profilometry, that the LSCM is an ideal tool for performing quantification of surface quality on rolled aluminum sheet products. In contrast, however, Lindseth and Bardal [9] have claimed that topological data from LSCM and AFM gave rise to very different surface roughness results. The differences observed may, however, be attributable to differences in data post-processing methodologies for the techniques under consideration in their study.
5. Characterization of high chromium cast iron worn surfaces in the LSCM In order to illustrate better the application of LSCM to the characterization of tribological samples, a short investigation of real worn surfaces has been conducted. The materials under investigation, a series of high chromium cast iron samples, have been the subject of earlier extensive investigations and the details of their manufacture and the testing methods used have been published in detail elsewhere [7,8]. The basic microstructure of these materials in the service condition comprises a distribution of hard chromium carbides embedded within a tempered martensitic matrix. In these earlier publications it has been reported that spray casting leads to a microstructure which is dramatically refined compared with that of a conventionally cast material. Comparison of conventionally cast with spray cast microstructures reveals that the spray cast material comprises a fine, uniform distribution of carbides (in the size range 1–8 m) whereas in the conventionally cast material the carbides where distributed in a continuous network in the interdendritic regions (interdendritic separations where of the order of 200–500 m).
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Fig. 7. Cross-sectional profiles of a C–Mn steel sheet unidirectionally ground to 60 grit. The profiles shown were generated from: (a) AFM; (b) LSCM using a 50× objective and (c) this same LSCM data after smoothing to remove extraneous signal noise.
Table 1 Comparison of the surface roughness statistics parameters: (a) Ra and (b) Rq obtained by analysis of the cross-sectional profiles measured by AFM and by LSCM with a 50 and 100× objective Surface condition
1 2 3
Ra
Rq
LSCM 50× unsmoothed
LSCM 50× smoothed
LSCM 100× unsmoothed
AFM
LSCM 50× unsmoothed
LSCM 50× smoothed
LSCM 100× unsmoothed
AFM
0.30 0.26 0.08
0.30 0.16 0.07
0.26 0.15 0.07
0.26 0.18 0.06
0.37 0.36 0.10
0.38 0.20 0.10
0.30 0.18 0.09
0.34 0.23 0.08
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This refinement of microstructure was shown to give rise to a five-fold increase in rolling sliding wear resistance across a broad range of testing temperatures from 20 to 400◦ C, whilst at higher temperatures (500–700◦ C) the wear resistance was dramatically reduced and both materials exhibited an almost identical wear response. In these earlier studies the wear surfaces were observed to comprise oxide prows and deep abrasive grooves suggesting that the principal wear processes included mildly oxidative and three body abrasive mechanisms. The improvement of wear response at low to intermediate temperatures exhibited by the spray cast material was attributed in part to the greater resistance to fracture exhibited by these smaller discrete carbides and also to the shorter mean free path between the carbides resulting from the more uniform carbide distribution. The reduction in mean free path (effectively from the
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diameter of the carbide denuded interdendritic region in the conventionally cast material which was 200–300 m to the intercarbide spacing which was 10 m in the spray cast material) was envisaged to lead to more effective protection of the matrix from third body abrasives. The increase in wear rate at higher temperatures was attributed to a dramatic softening of the matrix which occurred at temperatures above 500◦ C. This led to a wear process dominated by the plasticity of the matrix rather than by the mechanical stability of the carbides and thus to a similar wear response. In both materials, the wear mechanism was found to be a complex combination of severe abrasion and oxidation. In this investigation worn surfaces of conventionally cast and spray formed material produced at both room temperature and 700◦ C have been compared. As in the previous study, the surface was found to comprise oxide prows and
Fig. 8. LSCM maximum projections of worn 17Cr cast iron surfaces after rolling sliding contact wear at 700◦ C: (a) conventionally cast 17Cr cast iron at low magnification (A denotes oxide prow); (b) spray cast 17Cr cast iron at low magnification (A denotes oxide prow); (c) the trailing edge of the oxide prow in Fig. 3a at higher magnification and (d) the trailing edge of the oxide prow in Fig. 3b at higher magnification.
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abrasive grooves. Fig. 8a and b shows low magnification LSCM maximum intensity projections of spray cast and conventionally cast materials tested at 700◦ C. In each image, an oxide prow is visible at the top of the image whilst immediately behind each prow there is an “comet tail” of deep abrasive grooving. Fig. 8c and d shows the tail end of each of the prows at higher magnification and in each case the oxide can been seen to be extensively cracked. The spray cast and conventionally cast materials appear identical supporting the previous observation of their identical wear response. This pattern of cracking and abrasive wear was associated with the tail end of all prows observed. Fig. 9 shows a typical surface height profile taken along the sliding axis across the prow shown in Fig. 8a. The surface of the prow can be seen to be globally smooth (A and B) whilst in the cracked region at the tail, a large section of the oxide plate is inclined (B and C). Immediately behind the prow there is a shear drop (C and D) and a plateau (D and E) which is followed by a deep gouge (E and F). This pattern of behavior was remarkably consistent and such profiles were
Fig. 9. A height profile measured along the sliding direction over the oxide prow (A) shown in Fig. 8a.
Fig. 10. LSCM maximum projections of worn 17Cr cast iron surfaces after rolling sliding contact wear at 20◦ C: (a) spray cast 17Cr cast iron at low magnification (A denotes oxide prow); (b) conventionally cast 17Cr cast iron showing extensive surface cracking and (c) conventionally cast 17Cr cast iron showing pitting.
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observed for the majority of prows investigated. Based on these topological observations, a sequence of micromechanisms leading to the destruction of the oxide prows and consequent liberation of wear debris can be proposed. Bending stresses due to asperity contacts and surface tensile stresses due to friction forces lead to cracking and pull-off of oxide at the tail of the prows where constraint is absent. The oxide debris thus produced is ploughed through the surface region immediately behind the tail of the prow. This leads to the formation of deep abrasive grooving and the formation of the gouge. The plateau behind the prow represents a region which is less severely abraded either by virtue of its being in the shadow of the prow (and thus protected from asperity
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contact with the counterface) or due to its recent exposure (thus having been exposed to abrasion for a limited period). The worn surfaces tested at 20◦ C were also characterized by oxide patches but as might be expected abrasive grooving was less apparent. Fig. 10a shows an LSCM image of such a worn surface where a number of oxide patches are clearly visible (the example shown is taken from spray cast material). In the case of the conventionally cast material areas of extensive surface cracking (see Fig. 10b) and deep pits of approximately 50–100 m in diameter (Fig. 10c) were observed. Such features were not found on the surfaces of spray cast materials. Surface profiles taken across the cracked region in Fig. 10b and across the pit in Fig. 10c are shown in Fig. 11a and b, respectively. The step changes in height visible at points A, B, C and D in Fig. 11a may indicate that a delamination mechanism is in operation whilst the pit in Fig. 11b may represent direct evidence of enhanced wear in the carbide denuded interdendritic regions (the diameter of the pits is of the order of size of the interdendritic spacing). Whilst the reasons for enhanced wear resistance have already been inferred from earlier SEM and optical observations on these samples [8], the quantitative information obtained in the LSCM puts the explanation on a much sounder scientific basis.
6. Conclusions 1. The LSCM is well suited for imaging rough surfaces such as those encountered in tribological research. 2. The present findings indicate that there is a good agreement between surface roughness statistics obtained by the LSCM and AFM techniques over a wide range of surface finishes (R a = 0.25–0.07 m). This, combined with the excellent level of accuracy afforded by LSCM in the measurement of deeper surface topological features, as witnessed by the results of hardness indentation depth measurements, would indicate that LSCM is readily applicable to the analysis and quantification of surface topography. 3. The combination of extended focal depth imaging and topographical measurement afforded by the LSCM has proven capable of firmly establishing wear mechanisms operating in two high chromium white cast irons. This indicates that the LSCM is an ideal tool for use in the characterization and interpretation of surface damage arising from wear.
References Fig. 11. Height profiles taken across: (a) the cracked area shown in Fig. 10b and (b) the pitted area shown in Fig. 10c.
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