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The nature of mechanically polished surfaces of copper: Polishing with fine diamond abrasives
149
M E T A L L O G R A P H Y 18:149-160 (1985)
The Nature of Mechanically Polished Surfaces of Copper: Polishing with Fine Diamond Abrasives
D. M. T U R L E Y AND L. E. S A M U E L S
Materials Research Laboratories, Defence Science and Technology Organization, P.O. Box 50, Ascot Vale, Victoria 3032. Australia
Surfaces of copper polished with 0.1 p.m size d i a m o n d abrasive were e x a m i n e d by transmission electron microscopy. There was no evidence of an a m o r p h o u s layer of the type k n o w n as the Beilby layer; the surfaces were crystalline and showed evidence of plastic deformation. Slab-shaped cells were present at the surface, and appear to correspond to the low strain microbands that have been observed in copper cold-rolled to small reductions. F r o m c o m p a r i s o n with previous results it was apparent that there was a progressive reduction in the m a x i m u m surface strain and depth of the strained layer as the size of the d i a m o n d abrasive decreased from 6 ¢tm to 1 ¢tm to 0.1 ~tm.
Introduction The structure of surfaces of copper which have been polished with 6 ~m and 1 Ixm grades of monocrystalline diamond abrasive has been investigated previously, using the techniques of transmission electron microscopy [1]. Briefly, it was found after polishing with the 6 o,m abrasive that cell structures were present in the surface layers, structures which appeared to be similar to either the microbands or the shear bands observed by Malin and Hatherly [2] in cold-rolled copper. Some small recrystallized grains were also present, indicating that some relaxation and modification of the cell structure had occurred, which could be attributed to the high plastic strains and mild surface heating. These structures were similar in principle, but different in degree, from those found in abraded surfaces, which is in agreement with the view [3] that both abrasion and polishing occurs by a chip-cutting mechanism in which the abrasive particles act as machining tools. However, Samuels and Wallace [4] have since shown that a different mechanism of material removal begins to predominate when finer abra© Elsevier Science Publishing C o . , I n c . , 1985 52 Vanderbilt Ave., N e w York, NY 10017
0026-0800/85/$03.30
150
D. M. Turley and L. E. Samuels
sives are used for polishing. The new mechanism is one in which small plate-like particles are removed from the surface, presumably by a delamination mechanism [5]. It becomes increasingly important when the mean particle size of a diamond abrasive falls below 3 ~m and is virtually the sole mechanism with particles 0.5 ixm or smaller. In this regard, Turley and Samuels [1] found that surfaces polished with a 1 Ixm abrasive were on average less severely strained than those polished with 6 ~m abrasive, and it now seems possible that this might be due to the decreasing importance of the chip-cutting mechanism of material removal. If so, it would seem possible that surfaces polished with even finer abrasives where delamination predominates over chip cutting would be even less severely strained. The purpose of the present investigation was to explore this possibility.
EXPERIMENTAL PROCEDURE The copper used was an oxygen-flee high-conductivity grade which was annealed at 650°C for 2 h. The annealed material had a grain size of -0.15 mm mean diameter and a hardness of -35 HV. The annealing treatment largely destroyed the cellular dislocation substructure which was present in the as-received material. Reference surfaces were prepared by mechanical metallographic methods known to be capable to producing surfaces that by normal standards are free of any deformation introduced by the preparation procedures [3]. After the polishing procedure the surface was given a light etch, and was then termed a reference surface, being regarded as strain-flee. These surfaces were then polished unidirectionally on a napped cloth charged with 0.1 ~m monocrystalline diamond abrasive to produce what are termed experimental surfaces. The diamond abrasive paste used was purchased from a reputable commercial supplier. Examination by transmission electron microscopy indicated that most of the abrasive particles were thin plates with an angular outline, sometimes, a hexagonal outline (Fig. 1). The maximum lateral dimension of these particles was about 0.1 Ixm. However, a small but significant number of particles were present which were much larger than this, the maximum lateral dimension of these particles ranging up to 1 ~m. Several batches of monocrystalline diamond of this grade have been examined at various times and all were found to contain some of these larger particles. The polishing procedure was similar to that used in earlier experiments [1]. Specimens were held by hand on a rotating disc (pressure: - 5 x 10 -z M N m - 2; linear speed: - 1 m sec- 1) which was covered with a proprietary
Polishing Ca with Fine Diamond Abrasives
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FIG. 1. Transmission electron micrograph of particles (arrowed) of the abrasive that was used for polishing. Inset is a selected area diffraction pattern of these particles, confirming that they are particles of diamond.
cloth composed of rayon fibers cemented to a w o v e n cotton-polyester backing. Polishing was done unidirectionally and kerosene was used as the polishing fluid. Debris was collected from the polishing track for examination by transmission electron microscopy [1]. Some small sections of the polishing track were also examined directly for debris by scanning electron microscopy. The technique used to prepare thin films from the experimental surfaces for examination by transmission electron microscopy has also been previously described [1]. Briefly, thin films were prepared from longitudinal and normal sections after first copper plating the surface. Both longitudinal and normal sections were taken perpendicular to the surface, the longitudinal sections being taken parallel to the polishing scratches and the normal sections being taken transverse to the scratch direction. Thin films were prepared parallel to the polished surface by first coating the unplated experimental surface with " L a c o m i t " lacquer and then electropolishing from the unlacquered side until perforation occurred.
152
D. M. Turley and L. E. Samuels
~E :~ili~!ill ~¸ ~
~
i¸¸
,, ~ ~i~
'~ ~
i¸
FIG. 2. Transmission electron micrograph of a normal section of a reference surface showing that there was no change in dislocation density at the surface. Inset is the selected area diffraction pattern from the surface layer.
The dotted line in all of the accompanying transmission electron micrographs of longitudinal and normal sections marks the junction between the surface and the electrodeposit, the electrodeposit being in all cases above the dotted line. An example is given in Fig. 2, which is a normal section of a reference surface. This micrograph confirms that the dislocation density of the annealed material was low and that preparation of the reference surface had not caused any change in dislocation density at the surface. Furthermore, selected area diffraction of the surface layers of this section gave a single orientation diffraction pattern (inset Fig. 2), which did not change across the junction or in the adjoining electrodeposit. Results STRUCTURE OF THE POLISHED SURFACES A typical transmission electron micrograph of a longitudinal section of a polished surface is shown in Fig. 3. Here the new substructure in the
Polishing Ca with Fine Diamond Abrasives
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surface layer consisted essentially of a narrow band of tangled dislocations, the band running parallel to and -0.1 ~m below the surface. This narrow band of tangled dislocations formed, in conjunction with the surface, an elongated cell structure, which contained many dislocations. Sometimes there was another band of dislocations -0.1 p~m further below this region, though this band was usually less well developed and defined, and, in conjunction with the band of dislocations above it, formed another row of less well-defined elongated cells. As shown in Fig. 4, an elongated cell structure was also discernible at the surface in normal sections. The cell boundaries that run parallel to and approximately 0.05 ~m below the surface are quite sharp compared to the cell boundary visible in Fig. 3. Individual dislocations can, however, just be resolved in the region arrowed in Fig. 4. A more diffuse cell boundary in which individual dislocations are discernible is present -0.1 ~m below the arrowed boundary in Fig. 4 and, in some instances, another diffuse cell boundary was present 0.1 ~m below this again. It is also apparent from Figs. 3 and 4 that the dislocation substructures in normal and longitudinal sections are similar (consisting of elongated cells), indicating that the cells at the surface are slab-shaped and contain many dislocations. In summary, there was always at least one layer of cells at the surface. Sometimes, a second layer of cells was present and, even less frequently, a third layer. The second and third layers of cells, if present, had diffuse and increasingly less delineated boundaries. The range of substructures observed is illustrated schematically in Fig. 5.
FiG. 3. T r a n s m i s s i o n electron micrograph of a longitudinal section of a polished surface. A narrow band of tangled dislocations runs parallel to and approximately 0.1 ~ m below the surface. The inset is the selected area diffraction pattern from the surface layer.
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D. M. Turley and L. E. Samuels
Tilting the thin film in the electron microscope indicated that many dislocations that emerged at the surface extended epitaxially into the electrodeposit. This is shown more clearly in Fig. 4, and as also shown in Fig. 4, it was often observed that the cells at the surface and the contiguous grains in the electrodeposit exhibited similar contrast, indicating that the orientation of both was very similar. This is evidence that the electrodeposit was epitaxed with respect to the polished surface. Selected area diffraction of the surface layers illustrated in Figs. 3 and 4 gave a single-orientation pattern (inset Fig. 3). There was little arcing of the diffraction spots, indicating that the misorientation across the cell walls was small. Selected area diffraction also showed that the orientations of the cells in the surface layers and the contiguous grains in the electrodeposit were virtually the same, though in some cases there was a small rotation of the diffraction pattern. As previously found [1], the interpretation of parallel sections was difficult, and a definite dislocation substructure was not distinguishable. Extinction contours associated with the deformation structures were present, and many of these were aligned so as to delineate the polishing direction. Selected area diffraction invariably gave single orientation diffraction patterns with very little arcing of the spots. There was no evidence of small recrystallized grains similar to those previously observed in surfaces polished with 6 ~m diamond abrasives [1].
FIG.4. Transmission electron micrograph of a normal section of a polished surface. The cell boundary that is adjacent to the surface is quite sharp, though individual dislocations can be just resolved in the region arrowed.
Polishing Ca with Fine Diamond Abrasives /~_
155
POLISHING 131RFC,TINN Stab-shaped Cells
ldaries
(a)
POLISHING DIRECTION
Slab-shaped Ce"s Sharper Boundaries
Diffuse Boundaries
(b) FIG. 5. Schematic sketches illustrating the range of substructures observed beneath polished surfaces. (a) and (b) are primarily based on Figs. 3 and 4, respectively.
NATURE OF THE POLISHING DEBRIS A small amount of debris could be recovered from the polishing tracks of the cloths which could be examined by transmission electron microscopy (Fig. 6). Most of the debris appeared to have a plate-like morphology consistent with a delamination mechanism of material removal. Particles with a morphology resembling that of a machining chip were occasionally observed, which might have been produced by the larger particles present in this abrasive. Selected area diffraction of the debris (inset Fig. 6) showed that it consisted of a mixture of metallic copper and cuprous oxide. A considerably
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D. M. Turley and L. E. Samuels
FIG. 6. Transmission electron micrograph of the debris recovered from a polishing cloth. Most of the particles (arrowed) have a plate-like morphology. The inset is the selected area diffraction pattern from the debris, which indicates that it is a mixture of metallic copper and cuprous oxide.
larger proportion of cuprous oxide was present than for debris recovered after coarser polishing operatings [1]. The oxide could be either particles that had been removed from the (oxidized) specimen surface itself or metallic debris that had oxidized after having been removed from the specimen surface, or a mixture of the two. Significant oxidation of such small debris particles certainly would seem to be a possibility. However, scanning electron microscopy failed to detect any debris on the small sections of the polishing track that were examined. This is perhaps not surprising because, as determined by weight loss, a thickness of only - 1 Ixm had been removed from the surface of the specimen. However, debris in the form of very small plate-shaped particles have been detected on the polishing tracks, formed under similar polishing conditions, on which materials having a greater polishing rate had been polished for a longer time [4].
Polishing Ca with Fine Diamond Abrasives
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TOPOGRAPHY OF THE POLISHED SURFACE The polished surface was composed of narrow shallow grooves (Fig. 7). Occasional wider and deeper grooves were present (one is arrowed in Fig. 7), which presumably were produced by the abnormally large particles present in this abrasive. However, no indications were found of shallow depressions of the type that might have been expected to result from a delamination material-removal process. Discussion The surface layers of even these very finely polished surfaces undoubtedly were crystalline. In fact, they were more perfectly crystalline than more coarsely polished or abraded surfaces. There was no evidence of the presence of an amorphous-like layer of the type known as the Beilby Layer.
Fl6. 7. Scanning electron micrograph of the polished surface showing that the surface topography consists of many grooves. A deeper groove presumably caused by a larger size diamond abrasive particle is arrowed.
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D. M. Turley and L. E. Samuels
The dislocation structures that were observed in the surface layers were analogous to those observed by Malin and Hatherly [2] in copper deformed by cold rolling. The slab-shaped cells at the surface appeared to be the microbands described by Malin and Hatherly [2], bands which start to form at strains as small as 2% reduction and are a major feature of the deformation at 20% reduction [2, 6]. Thus, the cells observed in the present investigation are considered to be microbands formed at small strains in this range, because individual dislocations could be resolved at the boundaries. Some rearrangement and recovery must have occurred, however, to form the sharper of the cell boundaries. No evidence was found of recrystallized grains in the surfaces, as was found for more coarsely polished and abraded surfaces [1]. It is also apparent that there was indeed a progressive reduction in both the magnitude of the maximum surface strain and the depth of the strained layer as the size of the polishing abrasive decreases from 6 p~m to 1 ~m to 0.1 p~m. This change correlates well with a decline in the importance of chipcutting as a material-removal mechanism. The results have important practical implications in the finish polishing of surfaces for metallographic purposes (see Appendix).
Appendix PRACTICAL IMPLICATIONS FOR METALLOGRAPHIC PREPARATION The surfaces produced with fine abrasives are much less severely strained than surfaces prepared using coarser abrasives. Moreover, the strained layer is so shallow that it could be expected to be removed completely by most metallographic etching treatments. The features seen after etching would then be those characteristic of a virtually strain-free surface. Full advantage cannot be taken of this possibility with an abrasive of the type used in the present investigation because the abrasive contains occasional abnormally large particles. These particles produce largerthan-average scratches in the surface, scratches that must be expected to have associated with them locally a more severely strained layer, characteristic of the particular size of abrasive particle. However, a supply of polycrystalline diamond abrasive became available that is classified as having a particle size range of 0-k ~m and has been found not to contain any abnormally large particles. It can reasonably be assumed that the strained layer produced by this abrasive would not be too dissimilar from that described in the paper.
Polishing Ca with Fine Diamond Abrasives
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FIG. 8. Optical micrographs of annealed 70-30 brass etched in an alcoholic ferric chloride reagent. Left side: after finish polishing on 0.1 ~tm gamma alumina abrasive on a " s e l v y t cloth. Right side: after finish polishing on 0 - ] Izm polycrystalline diamond abrasive on a synthetic suede cloth.
Trials were carried out in which the polycrystalline diamond was used to finish polish specimens of annealed 70-30 brass, the polished surface being etched in an alcohol-base ferric chloride reagent. This is a sensitive trial because the presence of polishing deformation in surfaces of 70-30 brass causes marked degradation in the etching contrast between grains and in the enhancement of any polishing scratches [3]. An example of these consequences after a conventional final polishing treatment is given in Fig. 8 (left side). A result obtained after polishing with the 0-~ ~xm polycrystalline diamond abrasive by comparison is shown in Fig. 8 (right side). This result is a very considerable improvement. The grain contrast is, in fact, almost as good as that obtained with an electropolished surface. References 1. D. M. Turley and L. E. Samuels, The nature of mechanically polished surfaces of copper, Metallography 14:275-294 (1981). 2. A. S. Malin and M. Hatherly, Microstructure of cold-rolled copper, Met. Sci. 13:463472 (1979).
160
D. M . Turley a n d L . E. S a m u e l s
3. L. E. Samuels, Metallographic Polishing by Mechanical Methods, 3rd ed. (ASM 1982). 4. L. E. Samuels and B. Wallace, Effects of type and size of diamond abrasives on material removal rates in metallographic polishing, Metallography 17:19-42 (1984). 5. N. P. Suh, An overview of the delamination theory of wear, Wear. 44:1-16 (1977). 6. M. Hatherly, Deformation at high strains, in Strength of Metals and Alloys, Proc. 6th Int. Conf. on Strength of Metals and Alloys, (R. C. Gifkins, Ed.) Pergamon Press (1983), Vol. 3, pp. 1181-1195.
Received July 1984; accepted November 1984.
M E T A L L O G R A P H Y 18:149-160 (1985)
The Nature of Mechanically Polished Surfaces of Copper: Polishing with Fine Diamond Abrasives
D. M. T U R L E Y AND L. E. S A M U E L S
Materials Research Laboratories, Defence Science and Technology Organization, P.O. Box 50, Ascot Vale, Victoria 3032. Australia
Surfaces of copper polished with 0.1 p.m size d i a m o n d abrasive were e x a m i n e d by transmission electron microscopy. There was no evidence of an a m o r p h o u s layer of the type k n o w n as the Beilby layer; the surfaces were crystalline and showed evidence of plastic deformation. Slab-shaped cells were present at the surface, and appear to correspond to the low strain microbands that have been observed in copper cold-rolled to small reductions. F r o m c o m p a r i s o n with previous results it was apparent that there was a progressive reduction in the m a x i m u m surface strain and depth of the strained layer as the size of the d i a m o n d abrasive decreased from 6 ¢tm to 1 ¢tm to 0.1 ~tm.
Introduction The structure of surfaces of copper which have been polished with 6 ~m and 1 Ixm grades of monocrystalline diamond abrasive has been investigated previously, using the techniques of transmission electron microscopy [1]. Briefly, it was found after polishing with the 6 o,m abrasive that cell structures were present in the surface layers, structures which appeared to be similar to either the microbands or the shear bands observed by Malin and Hatherly [2] in cold-rolled copper. Some small recrystallized grains were also present, indicating that some relaxation and modification of the cell structure had occurred, which could be attributed to the high plastic strains and mild surface heating. These structures were similar in principle, but different in degree, from those found in abraded surfaces, which is in agreement with the view [3] that both abrasion and polishing occurs by a chip-cutting mechanism in which the abrasive particles act as machining tools. However, Samuels and Wallace [4] have since shown that a different mechanism of material removal begins to predominate when finer abra© Elsevier Science Publishing C o . , I n c . , 1985 52 Vanderbilt Ave., N e w York, NY 10017
0026-0800/85/$03.30
150
D. M. Turley and L. E. Samuels
sives are used for polishing. The new mechanism is one in which small plate-like particles are removed from the surface, presumably by a delamination mechanism [5]. It becomes increasingly important when the mean particle size of a diamond abrasive falls below 3 ~m and is virtually the sole mechanism with particles 0.5 ixm or smaller. In this regard, Turley and Samuels [1] found that surfaces polished with a 1 Ixm abrasive were on average less severely strained than those polished with 6 ~m abrasive, and it now seems possible that this might be due to the decreasing importance of the chip-cutting mechanism of material removal. If so, it would seem possible that surfaces polished with even finer abrasives where delamination predominates over chip cutting would be even less severely strained. The purpose of the present investigation was to explore this possibility.
EXPERIMENTAL PROCEDURE The copper used was an oxygen-flee high-conductivity grade which was annealed at 650°C for 2 h. The annealed material had a grain size of -0.15 mm mean diameter and a hardness of -35 HV. The annealing treatment largely destroyed the cellular dislocation substructure which was present in the as-received material. Reference surfaces were prepared by mechanical metallographic methods known to be capable to producing surfaces that by normal standards are free of any deformation introduced by the preparation procedures [3]. After the polishing procedure the surface was given a light etch, and was then termed a reference surface, being regarded as strain-flee. These surfaces were then polished unidirectionally on a napped cloth charged with 0.1 ~m monocrystalline diamond abrasive to produce what are termed experimental surfaces. The diamond abrasive paste used was purchased from a reputable commercial supplier. Examination by transmission electron microscopy indicated that most of the abrasive particles were thin plates with an angular outline, sometimes, a hexagonal outline (Fig. 1). The maximum lateral dimension of these particles was about 0.1 Ixm. However, a small but significant number of particles were present which were much larger than this, the maximum lateral dimension of these particles ranging up to 1 ~m. Several batches of monocrystalline diamond of this grade have been examined at various times and all were found to contain some of these larger particles. The polishing procedure was similar to that used in earlier experiments [1]. Specimens were held by hand on a rotating disc (pressure: - 5 x 10 -z M N m - 2; linear speed: - 1 m sec- 1) which was covered with a proprietary
Polishing Ca with Fine Diamond Abrasives
151
FIG. 1. Transmission electron micrograph of particles (arrowed) of the abrasive that was used for polishing. Inset is a selected area diffraction pattern of these particles, confirming that they are particles of diamond.
cloth composed of rayon fibers cemented to a w o v e n cotton-polyester backing. Polishing was done unidirectionally and kerosene was used as the polishing fluid. Debris was collected from the polishing track for examination by transmission electron microscopy [1]. Some small sections of the polishing track were also examined directly for debris by scanning electron microscopy. The technique used to prepare thin films from the experimental surfaces for examination by transmission electron microscopy has also been previously described [1]. Briefly, thin films were prepared from longitudinal and normal sections after first copper plating the surface. Both longitudinal and normal sections were taken perpendicular to the surface, the longitudinal sections being taken parallel to the polishing scratches and the normal sections being taken transverse to the scratch direction. Thin films were prepared parallel to the polished surface by first coating the unplated experimental surface with " L a c o m i t " lacquer and then electropolishing from the unlacquered side until perforation occurred.
152
D. M. Turley and L. E. Samuels
~E :~ili~!ill ~¸ ~
~
i¸¸
,, ~ ~i~
'~ ~
i¸
FIG. 2. Transmission electron micrograph of a normal section of a reference surface showing that there was no change in dislocation density at the surface. Inset is the selected area diffraction pattern from the surface layer.
The dotted line in all of the accompanying transmission electron micrographs of longitudinal and normal sections marks the junction between the surface and the electrodeposit, the electrodeposit being in all cases above the dotted line. An example is given in Fig. 2, which is a normal section of a reference surface. This micrograph confirms that the dislocation density of the annealed material was low and that preparation of the reference surface had not caused any change in dislocation density at the surface. Furthermore, selected area diffraction of the surface layers of this section gave a single orientation diffraction pattern (inset Fig. 2), which did not change across the junction or in the adjoining electrodeposit. Results STRUCTURE OF THE POLISHED SURFACES A typical transmission electron micrograph of a longitudinal section of a polished surface is shown in Fig. 3. Here the new substructure in the
Polishing Ca with Fine Diamond Abrasives
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surface layer consisted essentially of a narrow band of tangled dislocations, the band running parallel to and -0.1 ~m below the surface. This narrow band of tangled dislocations formed, in conjunction with the surface, an elongated cell structure, which contained many dislocations. Sometimes there was another band of dislocations -0.1 p~m further below this region, though this band was usually less well developed and defined, and, in conjunction with the band of dislocations above it, formed another row of less well-defined elongated cells. As shown in Fig. 4, an elongated cell structure was also discernible at the surface in normal sections. The cell boundaries that run parallel to and approximately 0.05 ~m below the surface are quite sharp compared to the cell boundary visible in Fig. 3. Individual dislocations can, however, just be resolved in the region arrowed in Fig. 4. A more diffuse cell boundary in which individual dislocations are discernible is present -0.1 ~m below the arrowed boundary in Fig. 4 and, in some instances, another diffuse cell boundary was present 0.1 ~m below this again. It is also apparent from Figs. 3 and 4 that the dislocation substructures in normal and longitudinal sections are similar (consisting of elongated cells), indicating that the cells at the surface are slab-shaped and contain many dislocations. In summary, there was always at least one layer of cells at the surface. Sometimes, a second layer of cells was present and, even less frequently, a third layer. The second and third layers of cells, if present, had diffuse and increasingly less delineated boundaries. The range of substructures observed is illustrated schematically in Fig. 5.
FiG. 3. T r a n s m i s s i o n electron micrograph of a longitudinal section of a polished surface. A narrow band of tangled dislocations runs parallel to and approximately 0.1 ~ m below the surface. The inset is the selected area diffraction pattern from the surface layer.
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D. M. Turley and L. E. Samuels
Tilting the thin film in the electron microscope indicated that many dislocations that emerged at the surface extended epitaxially into the electrodeposit. This is shown more clearly in Fig. 4, and as also shown in Fig. 4, it was often observed that the cells at the surface and the contiguous grains in the electrodeposit exhibited similar contrast, indicating that the orientation of both was very similar. This is evidence that the electrodeposit was epitaxed with respect to the polished surface. Selected area diffraction of the surface layers illustrated in Figs. 3 and 4 gave a single-orientation pattern (inset Fig. 3). There was little arcing of the diffraction spots, indicating that the misorientation across the cell walls was small. Selected area diffraction also showed that the orientations of the cells in the surface layers and the contiguous grains in the electrodeposit were virtually the same, though in some cases there was a small rotation of the diffraction pattern. As previously found [1], the interpretation of parallel sections was difficult, and a definite dislocation substructure was not distinguishable. Extinction contours associated with the deformation structures were present, and many of these were aligned so as to delineate the polishing direction. Selected area diffraction invariably gave single orientation diffraction patterns with very little arcing of the spots. There was no evidence of small recrystallized grains similar to those previously observed in surfaces polished with 6 ~m diamond abrasives [1].
FIG.4. Transmission electron micrograph of a normal section of a polished surface. The cell boundary that is adjacent to the surface is quite sharp, though individual dislocations can be just resolved in the region arrowed.
Polishing Ca with Fine Diamond Abrasives /~_
155
POLISHING 131RFC,TINN Stab-shaped Cells
ldaries
(a)
POLISHING DIRECTION
Slab-shaped Ce"s Sharper Boundaries
Diffuse Boundaries
(b) FIG. 5. Schematic sketches illustrating the range of substructures observed beneath polished surfaces. (a) and (b) are primarily based on Figs. 3 and 4, respectively.
NATURE OF THE POLISHING DEBRIS A small amount of debris could be recovered from the polishing tracks of the cloths which could be examined by transmission electron microscopy (Fig. 6). Most of the debris appeared to have a plate-like morphology consistent with a delamination mechanism of material removal. Particles with a morphology resembling that of a machining chip were occasionally observed, which might have been produced by the larger particles present in this abrasive. Selected area diffraction of the debris (inset Fig. 6) showed that it consisted of a mixture of metallic copper and cuprous oxide. A considerably
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D. M. Turley and L. E. Samuels
FIG. 6. Transmission electron micrograph of the debris recovered from a polishing cloth. Most of the particles (arrowed) have a plate-like morphology. The inset is the selected area diffraction pattern from the debris, which indicates that it is a mixture of metallic copper and cuprous oxide.
larger proportion of cuprous oxide was present than for debris recovered after coarser polishing operatings [1]. The oxide could be either particles that had been removed from the (oxidized) specimen surface itself or metallic debris that had oxidized after having been removed from the specimen surface, or a mixture of the two. Significant oxidation of such small debris particles certainly would seem to be a possibility. However, scanning electron microscopy failed to detect any debris on the small sections of the polishing track that were examined. This is perhaps not surprising because, as determined by weight loss, a thickness of only - 1 Ixm had been removed from the surface of the specimen. However, debris in the form of very small plate-shaped particles have been detected on the polishing tracks, formed under similar polishing conditions, on which materials having a greater polishing rate had been polished for a longer time [4].
Polishing Ca with Fine Diamond Abrasives
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TOPOGRAPHY OF THE POLISHED SURFACE The polished surface was composed of narrow shallow grooves (Fig. 7). Occasional wider and deeper grooves were present (one is arrowed in Fig. 7), which presumably were produced by the abnormally large particles present in this abrasive. However, no indications were found of shallow depressions of the type that might have been expected to result from a delamination material-removal process. Discussion The surface layers of even these very finely polished surfaces undoubtedly were crystalline. In fact, they were more perfectly crystalline than more coarsely polished or abraded surfaces. There was no evidence of the presence of an amorphous-like layer of the type known as the Beilby Layer.
Fl6. 7. Scanning electron micrograph of the polished surface showing that the surface topography consists of many grooves. A deeper groove presumably caused by a larger size diamond abrasive particle is arrowed.
158
D. M. Turley and L. E. Samuels
The dislocation structures that were observed in the surface layers were analogous to those observed by Malin and Hatherly [2] in copper deformed by cold rolling. The slab-shaped cells at the surface appeared to be the microbands described by Malin and Hatherly [2], bands which start to form at strains as small as 2% reduction and are a major feature of the deformation at 20% reduction [2, 6]. Thus, the cells observed in the present investigation are considered to be microbands formed at small strains in this range, because individual dislocations could be resolved at the boundaries. Some rearrangement and recovery must have occurred, however, to form the sharper of the cell boundaries. No evidence was found of recrystallized grains in the surfaces, as was found for more coarsely polished and abraded surfaces [1]. It is also apparent that there was indeed a progressive reduction in both the magnitude of the maximum surface strain and the depth of the strained layer as the size of the polishing abrasive decreases from 6 p~m to 1 ~m to 0.1 p~m. This change correlates well with a decline in the importance of chipcutting as a material-removal mechanism. The results have important practical implications in the finish polishing of surfaces for metallographic purposes (see Appendix).
Appendix PRACTICAL IMPLICATIONS FOR METALLOGRAPHIC PREPARATION The surfaces produced with fine abrasives are much less severely strained than surfaces prepared using coarser abrasives. Moreover, the strained layer is so shallow that it could be expected to be removed completely by most metallographic etching treatments. The features seen after etching would then be those characteristic of a virtually strain-free surface. Full advantage cannot be taken of this possibility with an abrasive of the type used in the present investigation because the abrasive contains occasional abnormally large particles. These particles produce largerthan-average scratches in the surface, scratches that must be expected to have associated with them locally a more severely strained layer, characteristic of the particular size of abrasive particle. However, a supply of polycrystalline diamond abrasive became available that is classified as having a particle size range of 0-k ~m and has been found not to contain any abnormally large particles. It can reasonably be assumed that the strained layer produced by this abrasive would not be too dissimilar from that described in the paper.
Polishing Ca with Fine Diamond Abrasives
159
FIG. 8. Optical micrographs of annealed 70-30 brass etched in an alcoholic ferric chloride reagent. Left side: after finish polishing on 0.1 ~tm gamma alumina abrasive on a " s e l v y t cloth. Right side: after finish polishing on 0 - ] Izm polycrystalline diamond abrasive on a synthetic suede cloth.
Trials were carried out in which the polycrystalline diamond was used to finish polish specimens of annealed 70-30 brass, the polished surface being etched in an alcohol-base ferric chloride reagent. This is a sensitive trial because the presence of polishing deformation in surfaces of 70-30 brass causes marked degradation in the etching contrast between grains and in the enhancement of any polishing scratches [3]. An example of these consequences after a conventional final polishing treatment is given in Fig. 8 (left side). A result obtained after polishing with the 0-~ ~xm polycrystalline diamond abrasive by comparison is shown in Fig. 8 (right side). This result is a very considerable improvement. The grain contrast is, in fact, almost as good as that obtained with an electropolished surface. References 1. D. M. Turley and L. E. Samuels, The nature of mechanically polished surfaces of copper, Metallography 14:275-294 (1981). 2. A. S. Malin and M. Hatherly, Microstructure of cold-rolled copper, Met. Sci. 13:463472 (1979).
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D. M . Turley a n d L . E. S a m u e l s
3. L. E. Samuels, Metallographic Polishing by Mechanical Methods, 3rd ed. (ASM 1982). 4. L. E. Samuels and B. Wallace, Effects of type and size of diamond abrasives on material removal rates in metallographic polishing, Metallography 17:19-42 (1984). 5. N. P. Suh, An overview of the delamination theory of wear, Wear. 44:1-16 (1977). 6. M. Hatherly, Deformation at high strains, in Strength of Metals and Alloys, Proc. 6th Int. Conf. on Strength of Metals and Alloys, (R. C. Gifkins, Ed.) Pergamon Press (1983), Vol. 3, pp. 1181-1195.
Received July 1984; accepted November 1984.