The direct observation in the transmission electron
microscope of the heavily deformed surface layer of a copper
pin after dry sliding against a steel ring
Citation for published version (APA):
Dijck, van, J. A. B. (1977). The direct observation in the transmission electron microscope of the heavily deformed surface layer of a copper pin after dry sliding against a steel ring. Wear, 42, 109-117.
https://doi.org/10.1016/0043-1648(77)90172-7
DOI:
10.1016/0043-1648(77)90172-7 Document status and date: Published: 01/01/1977
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Wear, 42 (1977) 109 - 117
0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
109
THE DIRECT OBSERVATION IN THE TRANSMISSION ELECTRON
MICROSCOPE OF THE HEAVILY DEFORMED SURFACE LAYER OF
A COPPER PIN AFTER DRY SLIDING AGAINST A STEEL RING
J. A. B. VAN DIJCK
Mechanical Engineering Materials Laboratory, University of Technology, Eindhouen (Netherlands)
(Received May 6, 1976; in final form July 6, 1976)
Summary
A thin-foil technique has been developed to obtain more information about the material in the neighbourhood of the contact surface of a worn copper pin. With this it is clearly shown that a crystal structure is present in the contact surface. Depending on the place of sectioning, grain sizes of 250 - 600 A have been observed, which, using the linear intercept method and taking into account the original grain size, result in an effective deforma- tion of 230.
1. Introduction
Under certain test conditions, wear only results from plastic flow in and near the contact surface of a copper pin sliding against a steel 60 ring [ 1 - 33. The plastic deformation which leads to the displacement of the pin material can be evaluated by grain size me~urements. With copper pins worn in this way it appeared that the crystal structure of the sub-surface material up to 6 E.trn from the contact surface could not be resolved by electron microscopy, despite all the metallographic etching processes used. This may have been due to the crystals present being so small that the etching reagents would not separate them, to the resolving power of the replica employed being too small or to the absence of a crystal structure. As many investigators have assumed that this layer possesses different physical properties from the bulk material, it appeared that further investigation of such surface layers was necessary. For this purpose a thin-foil technique was developed for transmis- sion electron microscope examination.
2. Experimental procedure
In order to get a better insight into the grain size and shape of a surface layer, two cross sections require to be prepared, one perpendicular to the
110
a b
d
e
Fig. 1. (a) Worn pin after copper plating, (b) copper-plated pin after sectioning, (c) spark- eroding specimens from a slice of the pin material, (d) polishing holder with slit, covered with adhesive tape, and (e) cut parallel to contact surface.
contact surface and parallel to the sliding direction and one parallel to the contact surface. The copper pins used for the investigation were held at a low temperature immediately after the wearing process in order to prevent
recovery or recrystallization. Also, the time before preparation and the preparation of the thin foil was kept as short as possible.
2.1. Cross section perpendicular to the con tact surface and parallel to the sliding direction
The worn copper pin was copper-plated in two steps with a layer of at least 1.5 mm thickness by the common galvanizing technique. The first step in an alkaline copper bath provided a thin layer. The second in an acid copper bath produced a thick layer. The pin (Fig. l(a)) is cut by spark- erosion (Fig. l(b)) into slices of thickness 0.2 - 0.3 mm. These were sawn off, marked, stuck on a block with double adhesive tape and surface-sanded on both sides in running water with progressively finer-grained grinding paper to a final thickness of 0.15 - 0.2 mm. The disks were carefully loosened from the adhesive tape by a solvent. Circular specimens approximately 3.2 mm in diameter were spark-eroded from the disks. Before this was done, the spec- imens were marked to indicate their location (Fig. l(c)). The eroded disks were pre-polished electrolytically in a polishing machine (Struers Tenupol) in the space where the polishing holder is normally placed. The specimens were held where marked with stainless-steel tweezers and then polished for 2 min under the conditions prescribed for 3 mm test disks; the separating layer between the electroplated copper and the copper pin was now visible.
The specimen was placed in the 3 mm polishing holder which had been covered on both sides with opaque plastic tape so that only a zone containing the interface between pin material and the electrodeposited layer came into
111
Fig. 2. Photomicrograph showing a general view of the areas A and D.
Fig. 3. Electron micrograph showing a general view of the areas A, B and C.
contact with the polishing liquid on both sides (Fig. l(d)). The specimen was then polished according to the instructions. When a hole appeared in the specimen, it was rinsed as rapidly as possible in methanol, removed from the holder and examined in methanol with an optical microscope. If the hole was not in the desired position, the specimen was again polished until a hole was obtained in the desired position ready for electron microscopy.
2.2. Cross section p~r~~l~~ to the con tact surface
A slice 0.2 - 0.3 mm thick was spark-eroded from the worn end of the pin (Fig. l(e)), stuck by the contact surface on to a block with double
. . .
113
Fig. 5. Electron micrograph of the areas E, F and G (general view).
adhesive tape and ground in running water to a thickness in the centre of approximately 0.1 mm. Specimens 3 mm in diameter were spark-eroded from it.
Before polishing the specimen, one of the two holes on the inside of the polishing holder was covered with clear plastic tape. The contact surface side of the specimen was placed on the plastic tape inside the polishing holder and electrolytically polished for electron microscopy.
3. Results and discussion
Figure 2 shows a thin foil; across the middle (at A and D) the contact surface is clearly visible. Increased deformation is visible in the area adjacent to D. The sliding direction is from A to D. Examination in the electron micro- scope revealed that the areas about A and D are suitable for transmission electron microscopy. Figure 3 shows area A of Fig. 2; electroplated copper layers on the pin material can be seen, together with deformed pin material on the contact surface. Between A and C the increase in the shear angle is shown but the magnification is too low for the true metal grains to be discernible.
Figure 4(a) shows area A at higher magnification. Oblong crystals with their longitudinal axes in the sliding direction are shown. The thickness of the crystals varies from 200 to 600 A and represents the smallest copper crystals measured in a worn pin. Electron diffraction (Fig. 4(b)) from area A of approximately 1 pm2 confirms the very fine crystalline structure. Figures 4(c) and 4(d) show irradiated regions B and C where the electron beam passed through at 8 pm and approximately 11 pm from the contact surface
(dj Fig. 6. (a) Electron micrograph of area E (2 - 3 E_“m from the contact surface). (b) Selected area (approximately 1 pm2) of E for electron diffraction (Fig. 3). (c) Electron micrograph of area F (6 km from the contact surface). (d) Electron micrograph of area G (8 lrn from the contact surface}.
115
Fig. 7. Electron micrograph of the contact surface.
respectively (for an accurate location of B and C see Fig. 3). Comparing Figs. 4(a), 4(c) and 4(d), the grain size increases with distance from the contact surface.
Figure 5 shows region D of Fig. 2; the shear angle varies with distance from the contact surface. Figure 6(a) shows area E of Fig. 5 at 2 - 3 pm from the contact surface. A diffraction photograph from this area of approximate- ly 1 pm2 (Fig. 6(b)) also shows that the copper is finely crystalline. Figures 6(c) and 6(d) show areas at distances of 6 pm and 8 pm, respectively, from the contact surface. Figure 7 shows the actual contact surface. The foil was obtained by the method described in Section 2.2, but the period of time between the wear test and the preparation of the pin was too long and re- crystallization occurred in the contact surface.
When examining thin foils it must be taken into account that the copper may recrystallize owing to heating by the electron beam. Figure 8 shows a thin foil which has not recrystallized where it was irradiated (at the centre of the photograph) but which has recrystallized at the edges. Thus specimens which have been irradiated in the electron microscope are not suitable for further investigation. Before being studied in the electron microscope, all specimens are examined in methanol with an optical microscope (see Section 2.1) to establish whether the spots suitable for irradiation are in suitable areas for examination. If not, the foil may be repolished since optical micro- scope observation does not cause recrystallization. Table 1 summarizes the various observed linear intercepts* of the crystal sizes in several areas as
*Linear intercept is the average length of the lines between two consecutive cuts of the grain boundaries by a line perpendicular to the contact surface.
116
Fig. 8. Photomicrograph of a thin foil after irradiation in the electron microscope.
TABLE 1
Area Distance to Linear intercept
contact surface of the crystals
Mm) (8) A O-1 250 230 B 8 650 89 c 11 770 75 E 2-3 570 101 F 6 800 72 G 8 665 87
functions of the distance from the contact surface. The table also contains the effective deformation c given by
where fi is the linear intercept of the incipient grain size (10 - 15 pm) and C is the linear intercept of the grain size in the corresponding area. The table shows that deformation in the neighbourhood of the contact surface can be very large. Using the data from Table 1, Fig. 9 provides information which could not be previously determined [l] . It should be noted that these data are not averages of large numbers.
Conclusions
A technique has been developed for preparing thin foils of material in the neighbourhood of the contact surface. The technique supplements
117
I I
Q LIGHTMICROSCOPICAL OBSERVATIONS I . El_ECTRONMICROSW’lCAL OBSERVATIONS OF REPLICAS x E~CT~~~CROSCGPI~L 06SERVATlGNS CF THIN FOILS
- DISTANCE FROM SURFACE IN vrn
Fig. 9. Effective deformation of copper as a function of the distance from the contact surface.
information obtained by the repiica technique. The results of the investiga- tion show that material near the contact surface remains crystalline up to the contact surface. The grains are the smallest so far measured in a deformation process and the effective deformations are the largest ever measured.
Acknowledgment
The author thanks J. H. Dautzenberg for his encouragement and helpful comments.
References
J. H. Dautzenberg and J. H. Zaat, Quantitative determination of deformation by sliding wear, Wear, 23 (1973) 9 - 19.
J. H. Dautzenberg and J. H. Zaat, Model1 fiir Gleitverschleiss bei Trockenreibung, 1st European Tribology Congress, London, 1973, Paper no. C 276173, Inst. Mech. Eng., London, 1975, pp. 147 - 154.
J. H. Dautzenberg and J. I-I. Zaat, Model of strain distribution by sliding wear, Wear, 26 (1973) 105 - 119.
