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Generation of wear elements and origin of tribomagnetization phenomenon
Wear 269 (2010) 491–497
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Generation of wear elements and origin of tribomagnetization phenomenon Hiroshi Mishina a,∗ , Hayato Iwase a , Alan Hase b a b
Artificial Systems Science (Tribology Laboratory), Department of Graduate School of Engineering, Chiba University, 1-33, Yayoi, Inage-ku, Chiba 263-8522, Japan Department of Mechanical Engineering, Faculty of Engineering, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, Japan
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Article history: Received 4 January 2010 Received in revised form 6 May 2010 Accepted 14 May 2010 Available online 9 June 2010 Keywords: Wear Wear elements Tribomagnetization Magnetic force microscopy Paleogeomagnetism
a b s t r a c t Experiments showed that a generation of wear elements, that means elemental debris of wear particles, induced a spontaneous magnetization of the sliding surface (i.e. tribomagnetization phenomenon) when the ferromagnetic materials iron, nickel and cobalt were subjected to sliding friction in the absence of an external magnetic field. Origin of the tribomagnetization phenomenon that arose through the generation of wear elements with a size of about 15 nm to a few tens of nanometer, a size that was of the same order as that of a single magnetic domain particle of ferromagnetic materials, was revealed by using a magnetic force microscope (MFM). The magnetic flux density that varied in both strength and direction over the entire sliding surface was identified by using a Tesla meter with a three-axis or single-axis probe. From experimental results the relation between a generation of wear elements in tribological process and the original process in mechanism of the tribomagnetization phenomenon was discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A recent research on tribology of materials has revealed a nanoscale tribological phenomenon at the contacting surfaces. The nanoscale phenomena that result from friction have been widely investigated by using a frictional force microscopy (FFM) and an atomic force microscopy (AFM) [1,2], and nanoscale adhesion at a real contacting area was reported using transmission electron microscopy (TEM) [3]. As a result of sliding friction and wear, when the particle with a nanometer size is generated [4], it alters physical and chemical properties of rubbing surfaces. We have reported a tribomagnetization phenomenon by which ferromagnetic materials undergo spontaneous magnetization as a result of tribological actions when they are subjected to sliding friction, even when this is performed in the absence of an external magnetic field [5,6]. In our previous studies, we speculated that the generation of magnetic flux on a sliding surface as a result of tribomagnetization phenomenon was closely related to the generation of elemental debris of wear particles with a size of 16–25 nm which was similar size to a single magnetic domain particle, which were formed in the initial stages of friction and wear. As for magnetic properties in tribological phenomenon, some investigations have been focused on the effect of an external magnetic field on the friction and wear of ferromagnetic materials [7,8]. The tribomagnetization phenomenon reported in this paper is much different from these phenomena,
∗ Corresponding author. Tel.: +81 43 290 3034; fax: +81 43 290 3034. E-mail address: [email protected] (H. Mishina). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.05.004
because tribomagnetization is the phenomenon which is caused by a spontaneous magnetization as a result of friction and is occurred in the absence of an external magnetic field. However, to understand the origin of tribomagnetization phenomenon, we need to identify whether each nanoparticle is really a significant magnetic property. In this report, we demonstrate by using a magnetic force microscopy (MFM) that the particle with a size of tens of nanometers has a significant magnetism and is of a similar size to a particle with a single magnetic domain predicted theoretically by Kittel [9,10]. Furthermore, a random distribution of magnetic-flux density on the sliding surface is observed in both properties of strength and direction using Tesla meter with a threeaxis-probe for the ferromagnetic materials. By using an in situ observation system with sliding devices, the changes in magnetic domain patterns occurred beneath the sliding surface as well as the plastic deformation are directly observed when cobalt is subjected to sliding friction. The aims of this study are to confirm the origin of the tribomagnetization phenomenon by observation of the initial stage of wear by using MFM, and to determine the relation between a generation of wear elements and a tribomagnetization phenomenon as a result of friction.
2. Experimental The friction experiments were performed by using a pinon-block sliding system which was set in a magnetic shield. A schematic of the experimental setup is shown in Fig. 1, and the experimental procedure was the same as described in previous papers [6,11]. Sliding friction occurred when the pin was made to
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Fig. 2. In situ observation system and a powder patterns method.
Fig. 1. Schematic diagram of a pin-on-block system inside the magnetic shield [6].
slide along the surface of the linearly moving block mounted on an XY-stage. The sliding friction continued on the same track through one to thirty strokes of about 12 mm each in a constant direction on the block surface. Sliding was conducted at room temperature in air of 30–50% relative humidity, without a lubricant, under a load of 10.0 N and at a sliding velocity of 4.0 mm/s. In each case, the pin and block both consisted of the same ferromagnetic material. The tribomagnetization phenomenon was examined by measuring the density of the magnetic flux B at the same points on the surface before and after the frictional experiments. The magnetic flux density was determined by using two kinds of Hall generatortype Tesla meter with a three-axis and a single-axis probe with a generator probe diameter of approximately 0.76 mm. The error in measurement with the probe was less than ±0.25% per given value of the density of magnetic flux. In the experiments, the effects of the external magnetic field around the experimental apparatus, which are produced by the terrestrial magnetism on the Earth or magnetized steel materials used in the experimental apparatus, could have caused erroneous results. To minimize any errors caused by external magnetic forces, frictional experiments and measurements of magnetic flux density were performed inside a magnetic shield made of ferrite, as shown in Fig. 1. The magnetic flux density measured inside the shield was less than 0.1 T in all directions. Furthermore, a magnetic force microscopy (MFM) was used to map the magnetic forces on the sliding surface. Measurements of MFM were conducted in the phase-detection (PD) mode with a probe lift (that was the distance between probe-tip and surface) of 20 nm by using cobalt alloy-coated silicon probe with a tip radius of 25–50 nm. The probe (tip) polarity was north, and a positive value of the magnetic flux density indicates a magnetic flux that passed from the inside to the outside through the surface of the substance. The coercive force of the probe used in the measurements of magnetic force was about 3.2 × 104 A/m. The phase angle detected in the PD-mode of the MFM was converted into a magnetic flux density by calibration using a standard sample. An atomic force microscopy (AFM) was also used to determine the wear elements generated in adhesive wear process. To visualize what was occurring on and beneath the surfaces of test samples during sliding, changes in magnetic domain patterns or plastic deformation occurring beneath the sliding surface through in situ observation was made by using an optical microscope system with sliding devices. A schematic of the in situ observation system is shown in Fig. 2, and details of the experimental procedure have been reported in previous publications [12–15]. On the stage of the optical microscope, a pin slid along
the X-axis of the XZ-surface close to the bottom XY-face of the block, and microscopic observations were performed on the XYsurface. By this means, we were able to perform direct observations of the plastic deformation as a result of sliding friction and movements of domain walls just beneath the sliding surface from a side view. The movements of domain walls were visualized by using a colloidal suspension of 10 nm magnetite particles applied to the surface being observed. For the in situ studies, the sliding velocity was set to a very low value of 0.02 mm/s to permit detailed observations to be made of movements during the frictional process under a load of 4.0 N. Three ferromagnetic materials, nickel (purity 99.7%, Vickers hardness 192 HV , Curie temperature 631 K), iron (purity 99.9%, Vickers hardness 98 HV , Curie temperature 1043 K), and cobalt (purity 99.99%, Vickers hardness 295 HV , Curie temperature 1390 K) were used in the experiments. The shape of tip of the pin was a hemisphere 4 mm in diameter. The block was a rectangular parallelepiped measuring 30 mm × 15 mm × 10 mm. The surfaces of the metals were mechanically finished by using #2000-SiC emery paper to a surface roughness of less than 0.5 m and then degreased by ultrasonic washing in acetone. The surfaces were polished with alumina powder of a grid size of 0.05 m before microscopic observation of the movement of the magnetic walls or the observations with MFM and AFM. Because the ferromagnetic materials were easily magnetized during the surface-finishing process, residual magnetization was removed by means of a thermal demagnetization method in which the material was heated higher than its Curie temperature in flowing argon with a purity of 99.999% at a pressure of about 105 Pa. Thermal demagnetization removed more than 99% of residual magnetization for nickel or iron, and more than 95% for cobalt. 3. Results and discussion 3.1. Tribomagnetization measurements on nickel and iron surfaces The friction experiments and measurements of magnetic flux density B on the sliding surface were first performed with a nickel and iron. Fig. 3 shows values of changes in the magnetic flux density B on frictional sliding surfaces of the two materials using Tesla meter with a single-axis-probe after frictional sliding of twenty cycles of sliding of the pin on the same track. In Fig. 3 magnetic flux density is shown only a perpendicular ingredient of magnetic flux to the sliding surface because it was measured by using Tesla meter with a single-axis-probe. B was the changes in the value of the magnetic flux density B at the same points on the block surfaces before and after the frictional experiments. A positive value of the magnetic flux density indicated a magnetic flux that passed from the inside to the outside through the surface of the substance; a neg-
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Fig. 3. Changes in the magnetic-flux density B of surfaces as a result of tribomagnetization for nickel and iron blocks rubbed with pins of the same material, sliding velocity was 4.0 mm/s and load was 10.0 N. The magnetic flux is shown only a perpendicular ingredient of magnetic flux to the sliding surface by using Tesla meter with a single-axis-probe.
ative value indicated a flux in the opposite direction. We can see that the surface of ferromagnetic materials was clearly magnetized as a result of sliding friction. The magnetized area was confined within the sliding area of about 12 mm for the two materials, and the magnetic flux density was randomly distributed on the surface although magnitude was depended on the substance. For the iron block slid contact with an iron pin, the surface was magnetized to B values of between −2.0 T and +7.5 T. Nickel was higher magnetized rather than iron with the maximum value of about +28 T. The strength of magnetization depends on the properties of the materials that are rubbed together, because the phenomenon is caused by spontaneous magnetization and it depends on such properties as the saturation magnetization and the coercive force of each material. Note that magnetization as a result of friction did not occur evenly over the entire sliding area, but that was randomly distributed at various points, although some periodicity was shown for the nickel. To clarify the precise distribution of strength and direction of magnetic flux density on the sliding surface, measurements of magnetic flux density by using Tesla meter with a three-axis-probe was performed. Fig. 4 shows the changes in magnitude and direction of magnetic flux density on the sliding surface of nickel block. The measurements were performed after the nickel pin was slid on the same trace for thirty cycles in a constant x-axis direction. A measurement area was 20 mm long; including the sliding area of about
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12 mm. In the area without sliding friction, B was small less than 1.0 T and the direction of magnetic flux drawn with arrows was randomly distributed. In the sliding area, on the other hand, the magnitude of magnetic flux density became high value and the surface was randomly magnetized. Some periodicity in magnitude of magnetic flux density was observed over the sliding area in Fig. 4(a), but it had not an exact interval as similar to that shown in Fig. 3. The direction of magnetic flux caused by friction was seemed to have a random distribution along the x-axis which was the sliding direction of the pin [Fig. 4(a)], but it seemed to be similar in the yz-face view in Fig. 4(b). Fig. 5 shows the micrographs of the surfaces corresponding to each area symbolized by “A”, “B”, and “C” in Fig. 4(a). The relatively high magnetization was detected at the point A or B in Fig. 4(a) where the transfer particles adhered to the sliding surface (Fig. 5A and B). As shown in Fig. 5C and Fig. 4, on the other hand, the area without frictional sliding was far less magnetized. Both properties of the magnitude and distribution of magnetic flux resulted from the changes in surface state which was caused by the formation of transfer particles during friction; the detail is discussed in the followings. The measured value in Fig. 4 was relatively lower than the result using Tesla meter with a single-axis-probe in Fig. 3, because the distance between the surface and the detector of a single-axis-probe was less than that of a three-axis-probe. 3.2. Observations of MFM and AFM To investigate the original phenomenon of tribomagnetization, magnetized area was observed by using MFM. Fig. 6 shows the magnetization map images on a nickel surface subjected to sliding friction recorded by using of MFM after nickel pin slid on a single cycle. Fig. 6(a) shows an image recorded with AFM and Fig. 6(b) shows with the MFM in the same area. The probe polarity of MFM was north, and that attractive magnetic force was negative direction of magnetic flux when the magnetized surface was as south pole. In the MFM image, the colors from red to violet correspond to magnetic flux values from −18 T (attractive force) to +18 T (repulsive force), respectively; a yellowish green color corresponds to a zero flux. In the AFM image we can see the presence of a large number of particles with a size of the order of a few tens of nanometers to 100 nm adhering to the sliding surface; these particles were formed in wear processes by sliding friction. Furthermore (and this is an important point), we can see in the MFM image in Fig. 6(b) that the magnetic-force distribution is quite localized in particles of a similar size to the particles on the sliding surface in the AFM. The magnetized particles with magnetic fluxes of various directions are
Fig. 4. Overall magnitude and direction of the magnetic flux density on nickel surface by using Tesla meter with a three-axis-probe.
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Fig. 5. Observations of the sliding surfaces on the magnetized nickel block. The symbols A, B, C are corresponding to the positions on the block in Fig. 4 as the same symbols.
Fig. 6. Nanoscale distribution of magnetic force on the sliding surface on a nickel surface as mapped by using MFM after single sliding with a nickel pin. (a) Image recorded by AFM. (b) Image from MFM at the same position of (a). (c) Enlargement of the MFM image of the area marked with the white square “A” in (b). The magnetic force measured by a phase-detection (PD) mode was converted into a magnetic-flux density by calibration with a standard sample (a phase angle of 1◦ in PD mode corresponds to a magnetic-flux density of 18 T). The probe polarity of MFM was north, and that attractive magnetic force was negative direction of magnetic flux.
distributed on the sliding surface and have magnetic flux densities of ±18 T or more. That is, the magnetization on the sliding surface originates from randomly distributed fine particles in its initial stage, as shown in the MFM image. Fig. 6(c) is an enlargement of the MFM image of the area marked with the white square “A” in Fig. 6(b). The minimum size of particle shown in Fig. 6(c) is about 15 nm to a few tens of nanometers; the particles are marked by arrows (a white arrow corresponds to a positive direction of the
magnetic flux and a black one corresponds to a negative direction). Particle with about 100 nm as symbolized by “B”, for example, was consisted of fine debris with a size from a few tens of nanometers or less. Therefore, we can consider that magnetization as a result of friction for ferromagnetic materials was originated by the fine debris. The fine debris was generated in the friction of metals between sliding surfaces, which we referred to such particles as “wear elements” or “elemental debris of wear particles” in the pre-
Fig. 7. AFM observation of the sliding surface of iron block after single sliding with iron pin. (a) Image on the sliding surface. (b) Wear elements and transfer particles of iron.
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Fig. 8. In situ observation of the deformation beneath the sliding surface and the movements of magnetic domain patterns beneath the sliding surface for the cobalt block with the progress of frictional sliding of the cobalt pin from micrograph (a) to (c) using a powder pattern method; sliding velocity was 0.02 mm/s and load was 4.0 N.
vious papers [4,5,16]. An origin of tribomagnetization phenomenon is closely related to the formation of these wear elements. The argument that tribomagnetization is caused by the generation of wear elements was verified in the previous papers. To examine the effects of the transfer particles that remained on the friction surface, the magnetic flux densities before and after removing wear particles were examined, and we found that the magnetic flux density decreased on removal of the particles, although a density of magnetic flux on the surface did not become zero because the surface was still magnetized after removing the particles [11]. Furthermore, we reported in another paper that a surface with very little deposited particles such as mild wear surface had very little magnetization [6]. These results as well as the magnetization map in Fig. 6 indicate that the tribomagnetization phenomenon is induced by the formation of the elemental or transfer particles observed in this paper. In the sliding friction between iron pin and iron block the particle with a similar size to nickel was observed. Fig. 7 shows the observation from AFM on the iron block surface after a single sliding of iron pin. We can see a transfer particle with a size of 300–500 nm (symbolized by “A”, for example), and the particle was consisted of some fine particles with a size less than 100 nm as same manner as nickel. The transfer particle was generated after accumulation of fine particles such as wear elements in the adhesive wear process. In Fig. 7(b) we demonstrate real image of wear elements of iron with a size of a few tens of nanometer for the iron. Consequently, we can explain the phenomenon of tribomagnetization was originated from the magnetized fine particles with a size of about 15 nm to a few tens of nanometer that were the wear elements generated in adhesive wear. 3.3. In situ observation of plastic deformation and changes in magnetic domains as a result of sliding friction Plastic deformation and changes in magnetic domains beneath the sliding surface as a result of sliding friction were observed. Among the three ferromagnetic materials (nickel, iron, and cobalt) we demonstrate the result of cobalt block, because the movement of magnetic domains was clearly observed. Fig. 8 comprises a series of micrographs recorded in situ; these show the movements of continuous changes in deformation and magnetic domains just beneath the sliding surface of a cobalt block during a single sliding movement of a cobalt pin from a position at the right in Fig. 8(a) toward a position at the left in Fig. 8(c). We can see a wedge-shaped domain which is parallel to c-axis of cobalt crystals in the center of the photos. As the pin moved toward the left, new area with a different direction of magnetic domains appeared from the surface with the progress of sliding friction of the pin. The direction of new mag-
netic domains was different from that of the original substance. The particle symbolized by “P” was interposed between pin and block surface. The particle was transfer particle generated in the sliding process, and we can consider that it was magnetized like as the particles observed in Fig. 6. So the magnetic domains beneath the sliding surface change by the magnetized transfer particles as well as the plastic deformation as a result of sliding friction. The formation of new domains beneath the surface in Fig. 8 was depended on both effects of the plastic deformation subjected to the tribological actions and the tribomagnetization process on the sliding surface. The magnetic flux density on the cobalt surface was very high value reached to 100 T as reported in the previous paper [6]. As for the induction of surface magnetization by friction, we can explain the mechanism of surface magnetization as follows. Fig. 9 illustrates the magnetization mechanism on and beneath the surface in frictional sliding. When the transfer particle is formed between the sliding surfaces the particle has already magnetized through the above process and the particle has a magnetic field around it, then the surface is locally magnetized by this magnetic field. And that the magnetic domains near the surface are affected by the produced magnetic field of particles as shown in Fig. 8. 3.4. Mechanism of tribomagnetization through a generation of wear elements In the investigation an important relation between a generation of wear elements with a size of about 15 nm to a few tens of nanometer and an origin of tribomagnetization phenomenon was confirmed from the measurements of density of magnetic flux and observation from FMF or AFM. Because a spot with high magnetization on the sliding surface was the result of the formation of a particle of wear elements or transfer particles, as shown in
Fig. 9. Schematic illustrate of magnetization of the sliding surface caused by the generation of elemental and transfer particles in wear process.
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Figs. 4 and 5, and the MFM image in Fig. 6, we can understand the elemental mechanism of tribomagnetization as follows. As shown in Figs. 6 and 7, the tribomagnetization phenomenon is originated from the generation of a fine particle in the wear process, and the magnetization has random distribution which depends on the location of the particles adhered to the sliding surface. The most significant particle which induces magnetization of the sliding surface is wear elements with a size of about 15 nm to a few tens of nanometer as unitary debris of wear particles; because we consider that a tribomagnetization phenomenon is induced by the physical property of the particle. In particular, the size of the wear elements is significant to understand a magnetization phenomenon. In the previous paper, we discussed a mechanism of the tribomagnetization, and suggested that it resulted from the formation of a single magnetic domain particle as a result of friction [5]. When nickel was subjected to sliding friction, the size of particle observed on the sliding surface was 16–25 nm [5], which was quite similar to the theoretical critical size for a single magnetic domain particle calculated by Kittel from the surface energy of the boundary between domains (Bloch wall) and surface energy per unit area [9,10]; after the initial prediction of the existence of this particle by Frenkel and Dorfman [17]. Furthermore, the size of the single magnetic domain particles has been experimentally measured by Elmore or others to be ∼20 nm [18–20]. The present result from MFM image in Fig. 6 shows that the particle with a size of about 15 nm to a few tens of nanometer has certain magnetism individually, and area on the surface without any particle was not magnetized (the area had yellowish green color where magnetic flux density was about zero), consequently we can assume that the wear element has a single magnetic domain. Here, we can clearly explain that the tribomagnetization of surfaces measured in this study was induced from the originally magnetized wear elements which are of same size as a single magnetic domain particle, and by the secondary process of formation of magnetized transfer particles which are formed by aggregation of wear elements. In the tribomagnetization phenomenon, the surface was randomly magnetized in the initial stages of formation of wear elements as shown in Fig. 6, and after several cycles of sliding were performed, the elemental debris aggregated so that the direction of the magnetic flux tended to become aligned in individual transfer particle observed as symbolized “B” in Fig. 6(c) and wear particles [11], although this depended on the distribution of the particles adhering to the surface. As a result, tribological actions essentially changed the magnetic properties at the surface or subsurface area, including changes in magnetic domains of ferromagnetic materials as well as plastic deformation caused by the sliding friction (Fig. 8). The detail of formation process of wear elements has been discussed in previous papers, and involves the generation of elemental slip (which corresponds to a slip interval of about 15 nm in the initial stages of plastic deformation) and the adhesive force between surface asperities [4,12,21–23]. The size of wear elements of ferromagnetic materials is determined by both effects of formation of nanosize slip at the contact point described above and physical properties of wear elements such as magnetic energy and surface energy of the particle. Our current investigations lead to an interesting note in the field of general science in relation to studies of paleogeomagnetism of ferromagnetic materials (or ferrimagnetic magnetite) that will be an important factor in terms of determining the past record of a rock magnetization of planets, because iron and magnetite made up a large proportion of bodies such as the Earth’s crust or other planets such as the Moon [24,25]. When friction occurs in deposits of these materials in the Earth’s crust as a result of tectonic movements or as the earthquakes, for example, the readings of the magnetization record of the strength and direction of the magnetic flux could be directly affected by friction, and it is possible that the magnetic
field could be changed as a result. Our results show that if the surface contains debris with a size of about a few tens nanometer, or agglomerations of such particles, the surface is likely to have been subjected to sliding friction in the past, and there is a possibility that its magnetism has been changed. Furthermore, our current investigation shows that it is quite difficult to eliminate the magnetization phenomenon in polishing or surface-finishing process of the ferromagnetic materials, because magnetized wear elements and transfer particles are intrinsically and randomly generated and adhere to the surface as a result of friction, and that it causes surface damages. It can be only minimized the degree of tribomagnetization of a frictional surface by suppressing the generation of these particles in mild wear mode [6,26,27]. 4. Conclusion We investigated the tribomagnetization phenomenon of ferromagnetic materials iron, nickel and cobalt in reference to a generation of wear elements and transfer particles in adhesive wear process. The magnetization of wear elements with a size of about 15 nm to a few tens of nanometer was identified from MFM. The subsequent process of magnetization of the surface as a result of tribological actions was revealed by using Tesla meter and surface observations. The results can be summarized as follows. The strength of magnetic flux density was randomly distributed on the surface, and a high magnetization flux density was detected on the wear elements and transfer particles adhering to the sliding surface. The mechanism of magnetization in tribological process was discussed in relation to the wear process of materials; that is, the magnetization begins with the generation of magnetized wear elements with a size of about 15 nm to a few tens of nanometers, a size that is similar to that of a single magnetic domain particle. The formation of the wear elements and of transfer particles induced the magnetization of materials without the presence of any external magnetic field. The direction and magnitude of the magnetic flux density were randomly distributed over the sliding area, which was depended on the location of these particles. The degree of magnetization was dependent on the materials. The magnetic flux density of nickel reached −15 to 28 T and −2 to 7.5 T on iron surface. The in situ observation was capable of visualizing the movements of domain walls beneath the sliding surface, and we clarified the marked changes in the direction of magnetic domains and the plastic deformation that occurred as a result of tribological actions. The magnetic induction mechanism on the surface was caused by magnetized particles formed between the sliding surfaces. Acknowledgements The authors gratefully acknowledge the help of K. Watanabe of The Institute of Physical and Chemical Research (RIKEN) and T. Murakami of the National Institute of Advanced Industrial Science and Technology (AIST) in measurements made using the MFM and in the preparation of materials. References [1] B. Bhushan, Nanotribology and Nanomechanics, Springer, Berlin, 2005. [2] L. Jansen, A. Schirmeisen, J.L. Hedrick, M.A. Lantz, A. Knoll, R. Cannara, B. Gotsmann, Nanoscale friction dissipation into shear-stress polymer relaxations, Phys. Rev. Lett. 102 (2009) 236101–236104. [3] T. Kizuka, Atomic process of point contact in gold studies by time-resolved high-resolution transmission electron microscopy, Phys. Rev. Lett. 81 (1998) 4448–4451. [4] A. Hase, H. Mishina, Wear elements generated in the elementary process of wear, Tribol. Int. 42 (2009) 1684–1690. [5] H. Mishina, Magnetization of ferromagnetic material surfaces by tribological process, J. Appl. Phys. 92 (2002) 6721–6727.
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Generation of wear elements and origin of tribomagnetization phenomenon Hiroshi Mishina a,∗ , Hayato Iwase a , Alan Hase b a b
Artificial Systems Science (Tribology Laboratory), Department of Graduate School of Engineering, Chiba University, 1-33, Yayoi, Inage-ku, Chiba 263-8522, Japan Department of Mechanical Engineering, Faculty of Engineering, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, Japan
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Article history: Received 4 January 2010 Received in revised form 6 May 2010 Accepted 14 May 2010 Available online 9 June 2010 Keywords: Wear Wear elements Tribomagnetization Magnetic force microscopy Paleogeomagnetism
a b s t r a c t Experiments showed that a generation of wear elements, that means elemental debris of wear particles, induced a spontaneous magnetization of the sliding surface (i.e. tribomagnetization phenomenon) when the ferromagnetic materials iron, nickel and cobalt were subjected to sliding friction in the absence of an external magnetic field. Origin of the tribomagnetization phenomenon that arose through the generation of wear elements with a size of about 15 nm to a few tens of nanometer, a size that was of the same order as that of a single magnetic domain particle of ferromagnetic materials, was revealed by using a magnetic force microscope (MFM). The magnetic flux density that varied in both strength and direction over the entire sliding surface was identified by using a Tesla meter with a three-axis or single-axis probe. From experimental results the relation between a generation of wear elements in tribological process and the original process in mechanism of the tribomagnetization phenomenon was discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction A recent research on tribology of materials has revealed a nanoscale tribological phenomenon at the contacting surfaces. The nanoscale phenomena that result from friction have been widely investigated by using a frictional force microscopy (FFM) and an atomic force microscopy (AFM) [1,2], and nanoscale adhesion at a real contacting area was reported using transmission electron microscopy (TEM) [3]. As a result of sliding friction and wear, when the particle with a nanometer size is generated [4], it alters physical and chemical properties of rubbing surfaces. We have reported a tribomagnetization phenomenon by which ferromagnetic materials undergo spontaneous magnetization as a result of tribological actions when they are subjected to sliding friction, even when this is performed in the absence of an external magnetic field [5,6]. In our previous studies, we speculated that the generation of magnetic flux on a sliding surface as a result of tribomagnetization phenomenon was closely related to the generation of elemental debris of wear particles with a size of 16–25 nm which was similar size to a single magnetic domain particle, which were formed in the initial stages of friction and wear. As for magnetic properties in tribological phenomenon, some investigations have been focused on the effect of an external magnetic field on the friction and wear of ferromagnetic materials [7,8]. The tribomagnetization phenomenon reported in this paper is much different from these phenomena,
∗ Corresponding author. Tel.: +81 43 290 3034; fax: +81 43 290 3034. E-mail address: [email protected] (H. Mishina). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.05.004
because tribomagnetization is the phenomenon which is caused by a spontaneous magnetization as a result of friction and is occurred in the absence of an external magnetic field. However, to understand the origin of tribomagnetization phenomenon, we need to identify whether each nanoparticle is really a significant magnetic property. In this report, we demonstrate by using a magnetic force microscopy (MFM) that the particle with a size of tens of nanometers has a significant magnetism and is of a similar size to a particle with a single magnetic domain predicted theoretically by Kittel [9,10]. Furthermore, a random distribution of magnetic-flux density on the sliding surface is observed in both properties of strength and direction using Tesla meter with a threeaxis-probe for the ferromagnetic materials. By using an in situ observation system with sliding devices, the changes in magnetic domain patterns occurred beneath the sliding surface as well as the plastic deformation are directly observed when cobalt is subjected to sliding friction. The aims of this study are to confirm the origin of the tribomagnetization phenomenon by observation of the initial stage of wear by using MFM, and to determine the relation between a generation of wear elements and a tribomagnetization phenomenon as a result of friction.
2. Experimental The friction experiments were performed by using a pinon-block sliding system which was set in a magnetic shield. A schematic of the experimental setup is shown in Fig. 1, and the experimental procedure was the same as described in previous papers [6,11]. Sliding friction occurred when the pin was made to
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Fig. 2. In situ observation system and a powder patterns method.
Fig. 1. Schematic diagram of a pin-on-block system inside the magnetic shield [6].
slide along the surface of the linearly moving block mounted on an XY-stage. The sliding friction continued on the same track through one to thirty strokes of about 12 mm each in a constant direction on the block surface. Sliding was conducted at room temperature in air of 30–50% relative humidity, without a lubricant, under a load of 10.0 N and at a sliding velocity of 4.0 mm/s. In each case, the pin and block both consisted of the same ferromagnetic material. The tribomagnetization phenomenon was examined by measuring the density of the magnetic flux B at the same points on the surface before and after the frictional experiments. The magnetic flux density was determined by using two kinds of Hall generatortype Tesla meter with a three-axis and a single-axis probe with a generator probe diameter of approximately 0.76 mm. The error in measurement with the probe was less than ±0.25% per given value of the density of magnetic flux. In the experiments, the effects of the external magnetic field around the experimental apparatus, which are produced by the terrestrial magnetism on the Earth or magnetized steel materials used in the experimental apparatus, could have caused erroneous results. To minimize any errors caused by external magnetic forces, frictional experiments and measurements of magnetic flux density were performed inside a magnetic shield made of ferrite, as shown in Fig. 1. The magnetic flux density measured inside the shield was less than 0.1 T in all directions. Furthermore, a magnetic force microscopy (MFM) was used to map the magnetic forces on the sliding surface. Measurements of MFM were conducted in the phase-detection (PD) mode with a probe lift (that was the distance between probe-tip and surface) of 20 nm by using cobalt alloy-coated silicon probe with a tip radius of 25–50 nm. The probe (tip) polarity was north, and a positive value of the magnetic flux density indicates a magnetic flux that passed from the inside to the outside through the surface of the substance. The coercive force of the probe used in the measurements of magnetic force was about 3.2 × 104 A/m. The phase angle detected in the PD-mode of the MFM was converted into a magnetic flux density by calibration using a standard sample. An atomic force microscopy (AFM) was also used to determine the wear elements generated in adhesive wear process. To visualize what was occurring on and beneath the surfaces of test samples during sliding, changes in magnetic domain patterns or plastic deformation occurring beneath the sliding surface through in situ observation was made by using an optical microscope system with sliding devices. A schematic of the in situ observation system is shown in Fig. 2, and details of the experimental procedure have been reported in previous publications [12–15]. On the stage of the optical microscope, a pin slid along
the X-axis of the XZ-surface close to the bottom XY-face of the block, and microscopic observations were performed on the XYsurface. By this means, we were able to perform direct observations of the plastic deformation as a result of sliding friction and movements of domain walls just beneath the sliding surface from a side view. The movements of domain walls were visualized by using a colloidal suspension of 10 nm magnetite particles applied to the surface being observed. For the in situ studies, the sliding velocity was set to a very low value of 0.02 mm/s to permit detailed observations to be made of movements during the frictional process under a load of 4.0 N. Three ferromagnetic materials, nickel (purity 99.7%, Vickers hardness 192 HV , Curie temperature 631 K), iron (purity 99.9%, Vickers hardness 98 HV , Curie temperature 1043 K), and cobalt (purity 99.99%, Vickers hardness 295 HV , Curie temperature 1390 K) were used in the experiments. The shape of tip of the pin was a hemisphere 4 mm in diameter. The block was a rectangular parallelepiped measuring 30 mm × 15 mm × 10 mm. The surfaces of the metals were mechanically finished by using #2000-SiC emery paper to a surface roughness of less than 0.5 m and then degreased by ultrasonic washing in acetone. The surfaces were polished with alumina powder of a grid size of 0.05 m before microscopic observation of the movement of the magnetic walls or the observations with MFM and AFM. Because the ferromagnetic materials were easily magnetized during the surface-finishing process, residual magnetization was removed by means of a thermal demagnetization method in which the material was heated higher than its Curie temperature in flowing argon with a purity of 99.999% at a pressure of about 105 Pa. Thermal demagnetization removed more than 99% of residual magnetization for nickel or iron, and more than 95% for cobalt. 3. Results and discussion 3.1. Tribomagnetization measurements on nickel and iron surfaces The friction experiments and measurements of magnetic flux density B on the sliding surface were first performed with a nickel and iron. Fig. 3 shows values of changes in the magnetic flux density B on frictional sliding surfaces of the two materials using Tesla meter with a single-axis-probe after frictional sliding of twenty cycles of sliding of the pin on the same track. In Fig. 3 magnetic flux density is shown only a perpendicular ingredient of magnetic flux to the sliding surface because it was measured by using Tesla meter with a single-axis-probe. B was the changes in the value of the magnetic flux density B at the same points on the block surfaces before and after the frictional experiments. A positive value of the magnetic flux density indicated a magnetic flux that passed from the inside to the outside through the surface of the substance; a neg-
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Fig. 3. Changes in the magnetic-flux density B of surfaces as a result of tribomagnetization for nickel and iron blocks rubbed with pins of the same material, sliding velocity was 4.0 mm/s and load was 10.0 N. The magnetic flux is shown only a perpendicular ingredient of magnetic flux to the sliding surface by using Tesla meter with a single-axis-probe.
ative value indicated a flux in the opposite direction. We can see that the surface of ferromagnetic materials was clearly magnetized as a result of sliding friction. The magnetized area was confined within the sliding area of about 12 mm for the two materials, and the magnetic flux density was randomly distributed on the surface although magnitude was depended on the substance. For the iron block slid contact with an iron pin, the surface was magnetized to B values of between −2.0 T and +7.5 T. Nickel was higher magnetized rather than iron with the maximum value of about +28 T. The strength of magnetization depends on the properties of the materials that are rubbed together, because the phenomenon is caused by spontaneous magnetization and it depends on such properties as the saturation magnetization and the coercive force of each material. Note that magnetization as a result of friction did not occur evenly over the entire sliding area, but that was randomly distributed at various points, although some periodicity was shown for the nickel. To clarify the precise distribution of strength and direction of magnetic flux density on the sliding surface, measurements of magnetic flux density by using Tesla meter with a three-axis-probe was performed. Fig. 4 shows the changes in magnitude and direction of magnetic flux density on the sliding surface of nickel block. The measurements were performed after the nickel pin was slid on the same trace for thirty cycles in a constant x-axis direction. A measurement area was 20 mm long; including the sliding area of about
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12 mm. In the area without sliding friction, B was small less than 1.0 T and the direction of magnetic flux drawn with arrows was randomly distributed. In the sliding area, on the other hand, the magnitude of magnetic flux density became high value and the surface was randomly magnetized. Some periodicity in magnitude of magnetic flux density was observed over the sliding area in Fig. 4(a), but it had not an exact interval as similar to that shown in Fig. 3. The direction of magnetic flux caused by friction was seemed to have a random distribution along the x-axis which was the sliding direction of the pin [Fig. 4(a)], but it seemed to be similar in the yz-face view in Fig. 4(b). Fig. 5 shows the micrographs of the surfaces corresponding to each area symbolized by “A”, “B”, and “C” in Fig. 4(a). The relatively high magnetization was detected at the point A or B in Fig. 4(a) where the transfer particles adhered to the sliding surface (Fig. 5A and B). As shown in Fig. 5C and Fig. 4, on the other hand, the area without frictional sliding was far less magnetized. Both properties of the magnitude and distribution of magnetic flux resulted from the changes in surface state which was caused by the formation of transfer particles during friction; the detail is discussed in the followings. The measured value in Fig. 4 was relatively lower than the result using Tesla meter with a single-axis-probe in Fig. 3, because the distance between the surface and the detector of a single-axis-probe was less than that of a three-axis-probe. 3.2. Observations of MFM and AFM To investigate the original phenomenon of tribomagnetization, magnetized area was observed by using MFM. Fig. 6 shows the magnetization map images on a nickel surface subjected to sliding friction recorded by using of MFM after nickel pin slid on a single cycle. Fig. 6(a) shows an image recorded with AFM and Fig. 6(b) shows with the MFM in the same area. The probe polarity of MFM was north, and that attractive magnetic force was negative direction of magnetic flux when the magnetized surface was as south pole. In the MFM image, the colors from red to violet correspond to magnetic flux values from −18 T (attractive force) to +18 T (repulsive force), respectively; a yellowish green color corresponds to a zero flux. In the AFM image we can see the presence of a large number of particles with a size of the order of a few tens of nanometers to 100 nm adhering to the sliding surface; these particles were formed in wear processes by sliding friction. Furthermore (and this is an important point), we can see in the MFM image in Fig. 6(b) that the magnetic-force distribution is quite localized in particles of a similar size to the particles on the sliding surface in the AFM. The magnetized particles with magnetic fluxes of various directions are
Fig. 4. Overall magnitude and direction of the magnetic flux density on nickel surface by using Tesla meter with a three-axis-probe.
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Fig. 5. Observations of the sliding surfaces on the magnetized nickel block. The symbols A, B, C are corresponding to the positions on the block in Fig. 4 as the same symbols.
Fig. 6. Nanoscale distribution of magnetic force on the sliding surface on a nickel surface as mapped by using MFM after single sliding with a nickel pin. (a) Image recorded by AFM. (b) Image from MFM at the same position of (a). (c) Enlargement of the MFM image of the area marked with the white square “A” in (b). The magnetic force measured by a phase-detection (PD) mode was converted into a magnetic-flux density by calibration with a standard sample (a phase angle of 1◦ in PD mode corresponds to a magnetic-flux density of 18 T). The probe polarity of MFM was north, and that attractive magnetic force was negative direction of magnetic flux.
distributed on the sliding surface and have magnetic flux densities of ±18 T or more. That is, the magnetization on the sliding surface originates from randomly distributed fine particles in its initial stage, as shown in the MFM image. Fig. 6(c) is an enlargement of the MFM image of the area marked with the white square “A” in Fig. 6(b). The minimum size of particle shown in Fig. 6(c) is about 15 nm to a few tens of nanometers; the particles are marked by arrows (a white arrow corresponds to a positive direction of the
magnetic flux and a black one corresponds to a negative direction). Particle with about 100 nm as symbolized by “B”, for example, was consisted of fine debris with a size from a few tens of nanometers or less. Therefore, we can consider that magnetization as a result of friction for ferromagnetic materials was originated by the fine debris. The fine debris was generated in the friction of metals between sliding surfaces, which we referred to such particles as “wear elements” or “elemental debris of wear particles” in the pre-
Fig. 7. AFM observation of the sliding surface of iron block after single sliding with iron pin. (a) Image on the sliding surface. (b) Wear elements and transfer particles of iron.
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Fig. 8. In situ observation of the deformation beneath the sliding surface and the movements of magnetic domain patterns beneath the sliding surface for the cobalt block with the progress of frictional sliding of the cobalt pin from micrograph (a) to (c) using a powder pattern method; sliding velocity was 0.02 mm/s and load was 4.0 N.
vious papers [4,5,16]. An origin of tribomagnetization phenomenon is closely related to the formation of these wear elements. The argument that tribomagnetization is caused by the generation of wear elements was verified in the previous papers. To examine the effects of the transfer particles that remained on the friction surface, the magnetic flux densities before and after removing wear particles were examined, and we found that the magnetic flux density decreased on removal of the particles, although a density of magnetic flux on the surface did not become zero because the surface was still magnetized after removing the particles [11]. Furthermore, we reported in another paper that a surface with very little deposited particles such as mild wear surface had very little magnetization [6]. These results as well as the magnetization map in Fig. 6 indicate that the tribomagnetization phenomenon is induced by the formation of the elemental or transfer particles observed in this paper. In the sliding friction between iron pin and iron block the particle with a similar size to nickel was observed. Fig. 7 shows the observation from AFM on the iron block surface after a single sliding of iron pin. We can see a transfer particle with a size of 300–500 nm (symbolized by “A”, for example), and the particle was consisted of some fine particles with a size less than 100 nm as same manner as nickel. The transfer particle was generated after accumulation of fine particles such as wear elements in the adhesive wear process. In Fig. 7(b) we demonstrate real image of wear elements of iron with a size of a few tens of nanometer for the iron. Consequently, we can explain the phenomenon of tribomagnetization was originated from the magnetized fine particles with a size of about 15 nm to a few tens of nanometer that were the wear elements generated in adhesive wear. 3.3. In situ observation of plastic deformation and changes in magnetic domains as a result of sliding friction Plastic deformation and changes in magnetic domains beneath the sliding surface as a result of sliding friction were observed. Among the three ferromagnetic materials (nickel, iron, and cobalt) we demonstrate the result of cobalt block, because the movement of magnetic domains was clearly observed. Fig. 8 comprises a series of micrographs recorded in situ; these show the movements of continuous changes in deformation and magnetic domains just beneath the sliding surface of a cobalt block during a single sliding movement of a cobalt pin from a position at the right in Fig. 8(a) toward a position at the left in Fig. 8(c). We can see a wedge-shaped domain which is parallel to c-axis of cobalt crystals in the center of the photos. As the pin moved toward the left, new area with a different direction of magnetic domains appeared from the surface with the progress of sliding friction of the pin. The direction of new mag-
netic domains was different from that of the original substance. The particle symbolized by “P” was interposed between pin and block surface. The particle was transfer particle generated in the sliding process, and we can consider that it was magnetized like as the particles observed in Fig. 6. So the magnetic domains beneath the sliding surface change by the magnetized transfer particles as well as the plastic deformation as a result of sliding friction. The formation of new domains beneath the surface in Fig. 8 was depended on both effects of the plastic deformation subjected to the tribological actions and the tribomagnetization process on the sliding surface. The magnetic flux density on the cobalt surface was very high value reached to 100 T as reported in the previous paper [6]. As for the induction of surface magnetization by friction, we can explain the mechanism of surface magnetization as follows. Fig. 9 illustrates the magnetization mechanism on and beneath the surface in frictional sliding. When the transfer particle is formed between the sliding surfaces the particle has already magnetized through the above process and the particle has a magnetic field around it, then the surface is locally magnetized by this magnetic field. And that the magnetic domains near the surface are affected by the produced magnetic field of particles as shown in Fig. 8. 3.4. Mechanism of tribomagnetization through a generation of wear elements In the investigation an important relation between a generation of wear elements with a size of about 15 nm to a few tens of nanometer and an origin of tribomagnetization phenomenon was confirmed from the measurements of density of magnetic flux and observation from FMF or AFM. Because a spot with high magnetization on the sliding surface was the result of the formation of a particle of wear elements or transfer particles, as shown in
Fig. 9. Schematic illustrate of magnetization of the sliding surface caused by the generation of elemental and transfer particles in wear process.
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Figs. 4 and 5, and the MFM image in Fig. 6, we can understand the elemental mechanism of tribomagnetization as follows. As shown in Figs. 6 and 7, the tribomagnetization phenomenon is originated from the generation of a fine particle in the wear process, and the magnetization has random distribution which depends on the location of the particles adhered to the sliding surface. The most significant particle which induces magnetization of the sliding surface is wear elements with a size of about 15 nm to a few tens of nanometer as unitary debris of wear particles; because we consider that a tribomagnetization phenomenon is induced by the physical property of the particle. In particular, the size of the wear elements is significant to understand a magnetization phenomenon. In the previous paper, we discussed a mechanism of the tribomagnetization, and suggested that it resulted from the formation of a single magnetic domain particle as a result of friction [5]. When nickel was subjected to sliding friction, the size of particle observed on the sliding surface was 16–25 nm [5], which was quite similar to the theoretical critical size for a single magnetic domain particle calculated by Kittel from the surface energy of the boundary between domains (Bloch wall) and surface energy per unit area [9,10]; after the initial prediction of the existence of this particle by Frenkel and Dorfman [17]. Furthermore, the size of the single magnetic domain particles has been experimentally measured by Elmore or others to be ∼20 nm [18–20]. The present result from MFM image in Fig. 6 shows that the particle with a size of about 15 nm to a few tens of nanometer has certain magnetism individually, and area on the surface without any particle was not magnetized (the area had yellowish green color where magnetic flux density was about zero), consequently we can assume that the wear element has a single magnetic domain. Here, we can clearly explain that the tribomagnetization of surfaces measured in this study was induced from the originally magnetized wear elements which are of same size as a single magnetic domain particle, and by the secondary process of formation of magnetized transfer particles which are formed by aggregation of wear elements. In the tribomagnetization phenomenon, the surface was randomly magnetized in the initial stages of formation of wear elements as shown in Fig. 6, and after several cycles of sliding were performed, the elemental debris aggregated so that the direction of the magnetic flux tended to become aligned in individual transfer particle observed as symbolized “B” in Fig. 6(c) and wear particles [11], although this depended on the distribution of the particles adhering to the surface. As a result, tribological actions essentially changed the magnetic properties at the surface or subsurface area, including changes in magnetic domains of ferromagnetic materials as well as plastic deformation caused by the sliding friction (Fig. 8). The detail of formation process of wear elements has been discussed in previous papers, and involves the generation of elemental slip (which corresponds to a slip interval of about 15 nm in the initial stages of plastic deformation) and the adhesive force between surface asperities [4,12,21–23]. The size of wear elements of ferromagnetic materials is determined by both effects of formation of nanosize slip at the contact point described above and physical properties of wear elements such as magnetic energy and surface energy of the particle. Our current investigations lead to an interesting note in the field of general science in relation to studies of paleogeomagnetism of ferromagnetic materials (or ferrimagnetic magnetite) that will be an important factor in terms of determining the past record of a rock magnetization of planets, because iron and magnetite made up a large proportion of bodies such as the Earth’s crust or other planets such as the Moon [24,25]. When friction occurs in deposits of these materials in the Earth’s crust as a result of tectonic movements or as the earthquakes, for example, the readings of the magnetization record of the strength and direction of the magnetic flux could be directly affected by friction, and it is possible that the magnetic
field could be changed as a result. Our results show that if the surface contains debris with a size of about a few tens nanometer, or agglomerations of such particles, the surface is likely to have been subjected to sliding friction in the past, and there is a possibility that its magnetism has been changed. Furthermore, our current investigation shows that it is quite difficult to eliminate the magnetization phenomenon in polishing or surface-finishing process of the ferromagnetic materials, because magnetized wear elements and transfer particles are intrinsically and randomly generated and adhere to the surface as a result of friction, and that it causes surface damages. It can be only minimized the degree of tribomagnetization of a frictional surface by suppressing the generation of these particles in mild wear mode [6,26,27]. 4. Conclusion We investigated the tribomagnetization phenomenon of ferromagnetic materials iron, nickel and cobalt in reference to a generation of wear elements and transfer particles in adhesive wear process. The magnetization of wear elements with a size of about 15 nm to a few tens of nanometer was identified from MFM. The subsequent process of magnetization of the surface as a result of tribological actions was revealed by using Tesla meter and surface observations. The results can be summarized as follows. The strength of magnetic flux density was randomly distributed on the surface, and a high magnetization flux density was detected on the wear elements and transfer particles adhering to the sliding surface. The mechanism of magnetization in tribological process was discussed in relation to the wear process of materials; that is, the magnetization begins with the generation of magnetized wear elements with a size of about 15 nm to a few tens of nanometers, a size that is similar to that of a single magnetic domain particle. The formation of the wear elements and of transfer particles induced the magnetization of materials without the presence of any external magnetic field. The direction and magnitude of the magnetic flux density were randomly distributed over the sliding area, which was depended on the location of these particles. The degree of magnetization was dependent on the materials. The magnetic flux density of nickel reached −15 to 28 T and −2 to 7.5 T on iron surface. The in situ observation was capable of visualizing the movements of domain walls beneath the sliding surface, and we clarified the marked changes in the direction of magnetic domains and the plastic deformation that occurred as a result of tribological actions. The magnetic induction mechanism on the surface was caused by magnetized particles formed between the sliding surfaces. Acknowledgements The authors gratefully acknowledge the help of K. Watanabe of The Institute of Physical and Chemical Research (RIKEN) and T. Murakami of the National Institute of Advanced Industrial Science and Technology (AIST) in measurements made using the MFM and in the preparation of materials. References [1] B. Bhushan, Nanotribology and Nanomechanics, Springer, Berlin, 2005. [2] L. Jansen, A. Schirmeisen, J.L. Hedrick, M.A. Lantz, A. Knoll, R. Cannara, B. Gotsmann, Nanoscale friction dissipation into shear-stress polymer relaxations, Phys. Rev. Lett. 102 (2009) 236101–236104. [3] T. Kizuka, Atomic process of point contact in gold studies by time-resolved high-resolution transmission electron microscopy, Phys. Rev. Lett. 81 (1998) 4448–4451. [4] A. Hase, H. Mishina, Wear elements generated in the elementary process of wear, Tribol. Int. 42 (2009) 1684–1690. [5] H. Mishina, Magnetization of ferromagnetic material surfaces by tribological process, J. Appl. Phys. 92 (2002) 6721–6727.
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