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Effect of the adhesion force on the equation of adhesive wear and the generation process of wear elements in adhesive wear of metals
Wear 432-433 (2019) 202936
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Effect of the adhesion force on the equation of adhesive wear and the generation process of wear elements in adhesive wear of metals
T
Hiroshi Mishinaa,∗, Alan Haseb a b
Graduate School of Engineering, Chiba University, 1-33 Yayoi, Inage-ku, Chiba, 263-8522, Japan Department of Mechanical Engineering, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama, 369-0293, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Adhesive wear Metal wear Wear equation Wear elements Adhesion force FFM–FDC method
The effects of the adhesion force on the equation of adhesive wear were investigated by examining the relationship between the adhesion force of actual sliding surfaces and the number of wear elements generated at a junction. The number of wear elements and their generation mechanism in the elementary wear processes were determined by nanoscale observations and by analysis of the sliding surfaces by scanning electron microscopy and atomic-force microscopy. In addition, the adhesion force between actual sliding metal surfaces was measured by a method combining friction-force microscopy and force–distance curve measurement. The adhesion force between metal-probe cantilevers of nine different metals and the surfaces of blocks of the same metal were measured. We propose a modified equation for adhesive wear that takes into account the effect of the adhesion force, and we discuss the precise process for the generation of wear elements.
1. Introduction For many years, investigation into the mechanism and the wear equation of adhesive wear have relied on Holm's well-known wear equation, V = Z·(P·ℓ/pm), where V is the amount of wear, P is the applied load, pm is yield stress of the softer material, ℓ is sliding distance, and Z is the wear coefficient [1,2]. Here, in the idea of Holm, Z is the probability of removing atomic-size particles or layers to form wear particles. However, Holm's equation does not take into account the exact effects of such factors as the physical and chemical properties of the sliding surfaces on the adhesive wear. The influence of these factors and the wear mechanism are usually considered to be included in the wear coefficient Z, but an accurate interpretation for these factors has not been made. With regard to adhesive wear, we have previously proposed a wear equation that we established from the elementary wear process (the mechanism of generation of a wear element, which is the unitary debris of an adhesive wear particle) [3–6]. This wear equation is written as follows [3]:
V=
1 n ⎛ b ⎞3 ⎛ P·ℓ ⎞ ·⎛ ⎞· ·⎜ ⎟ 3 ⎝ λ ⎠ ⎝ a ⎠ ⎝ pm ⎠
(1)
where a is the mean radius of the junctions, b is the mean radius of the wear elements, and V, P, ℓ, and pm have the same meaning as in Holm's
∗
equation; Furthermore, n is the number of wear elements generated at a junction and λ is a factor that takes into account chemical effects, such as chemisorption of surrounding molecules, that determine the wear mode. The detailed meaning of each factor has been explained previously [3]. In the wear model, n is predicted to be related to a physical factor, such as the adhesion force at the sliding surface. However, the exact relationship between the number of wear elements generated in a junction and the adhesion force has not yet been clarified. The purpose of the current study was to measure the number of wear elements generated in adhesive wear, together with the adhesion force of the actual sliding metal surface, and to determine the exact relationship between the number of wear elements n in Eq. (1) and the adhesion force. Of course, the adhesion force between sliding surfaces is an important property in relation to understanding the phenomena of friction and adhesive wear. At an actual sliding surface, the real contact point (junction) is plastically deformed due to the close contact and sliding motion during the tribological process. In investigating adhesive wear, it is necessary to determine the adhesion force between the actual sliding surfaces rather than that between ideal clean and/or elastic contact surfaces. To investigate the adhesion force between actual sliding metal surfaces, we measured the adhesion force by a combination of friction-force microscopy (FFM) and force–distance curve (FDC) measurements, known as the FFM–FDC method [7]. In this method, the
Corresponding author. E-mail address: [email protected] (H. Mishina).
https://doi.org/10.1016/j.wear.2019.202936 Received 2 December 2018; Received in revised form 5 June 2019; Accepted 11 June 2019 Available online 14 June 2019 0043-1648/ © 2019 Published by Elsevier B.V.
Wear 432-433 (2019) 202936
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experimental procedure for the observation of deformation beneath the sliding surface was similar to that for our in situ microscopic observations with an applied load of 2.0 N [3,9,10].
adhesion force is determined by the pull-off force determined from the FDC measured on the sliding surface immediately after performing the sliding friction in FFM mode. Details of the FFM–FDC method have been reported in our previous papers [7,8]. In the present experiments, the adhesion force was measured for same-metal combinations of nine pure metals: molybdenum, titanium, cobalt, nickel, iron, copper, silver, gold, and zinc. Furthermore, to clarify the influence of the adhesion force on the macroscopic amount of adhesive wear, the adhesion force was measured for three different-metal combinations that differed in their mutual metallurgical solubilities: Cu–Ni, Ag–Fe, and Cu–Al. In addition to the measurements of the adhesion force of metals, the number and critical size of wear elements generated in the elementary process of adhesive wear were also investigated. We have previously reported and discussed the process for the formation of wear elements, as determined by microscopy studies of the region beneath the sliding surface of metals [3,4,9,10]. However, because the deformation beneath the sliding surface was observed in situ, the observation was limited to optical microscopy levels. Previous reports have explained that the size of wear elements is determined by the distance between slip lines (or bands) produce by plastic deformation immediately beneath the sliding surface. However, the critical size of the wear element could not be determined by using an optical microscope. In this study, nanoscale observations of the wear element and plastic deformation beneath the sliding surface were performed by scanning electron microscopy (SEM) and atomic-force microscopy (AFM) to reveal the exact size and the mechanism of generation of wear elements.
2.2. Measurements of the adhesion forces of metal surfaces by using the FFM–FDC method In experiments to measure the adhesion force between actual sliding metal surfaces, we used the FFM–FDC method, which combined a sliding friction motion in the FFM mode with measurements of the pulloff force by FDC using a metal cantilever probe. The experimental procedure was similar to that described in previous reports [7,8]. To perform this FFM–FDC method, an SPM cantilever with a colloidal probe consisting of a particle of one of nine pure metals (Mo, Ti, Co, Ni, Fe, Cu, Ag, Au, or Zn) with an average diameter of 29 ± 8 μm (15 μm for Ni) was used. Each metal particle was stuck onto the tipless cantilever by using an acrylic adhesive. An all-in-one tipless cantilever (BudgetSensors; Sofia) with a force constant of 2.7 N/m, a width of 30 μm, and a length of 210 μm was selected, as this permitted the measurement of the adhesion force between the metal surfaces. A force constant of 2.7 N/m was selected from preliminary experiments performed with various force constants from 0.2 to 40 N/m. The FFM and FDC in the AFM modes of the SPM were performed by using a JSPM–5200 microscope (JEOL Ltd., Tokyo). All experiments were performed in air at 23 °C and 25–30% RH. In the FFM–FDC method, the cantilever of the metal probe was in contact with the surface of the metal block, and frictional movement was achieved in the FFM mode by using a back-and-forth motion with a sliding distance of 12–15 μm. After sliding for one reciprocation by means of the FFM, the same probe was used to measure the pull-off force of the FDC on the sliding track of the block surface. In this study, after repeating the FFM–FDC cycle a sufficient number of times for the pull-off force to become nearly stable, the adhesion force of each metal was determined. The required number of repetitions of the measurement was determined by the strength of adhesion of the particles to the cantilever in a preliminary experiment. Adherent particles of about 30 μm in size were not removed during 100 cycles of measurement, and the pull-off force reached a nearly stable value before the measurement were repeated 45 times. From these preliminary experiments, we determined that the optimal number of repetitions of the cycle of FFM–FDC measurements was 50 for eight of the metals. In the case of Ni, whose probe particles were smaller in size (15 μm) than those of the other eight metals, the strength of adhesion of the glued particles was weak and they detached from the cantilever after 35–37 repetitions in five preliminary experiments; the pull-off force was almost stable after about 25 repetitions of the FFM–FDC measurement. Therefore, in the case of Ni, we used data obtained after 37 repeated measurements [Fig. 5c].
2. Experimental 2.1. Piezoelectric-type pin-on-block wear test apparatus for observation and determination of the number of wear elements Wear experiments were performed by using a piezoelectric-type pinon-block wear test apparatus (Fig. 1). In this experiment, the block specimen of the pin-on-block system was held directly in the sample holder of the scanning probe microscope (SPM) to permit immediate observation by AFM. Driven by a piezoelectric actuator, the pin specimen was moved along the sliding track on the block specimen (30 × 15 × 10 mm in size) at a sliding velocity of 113 μm/s and with an applied load of 0.1 N in air at 23 °C and 25–30% RH. The pin had a hemispherical shape with a diameter of 4 mm at the contact surface. The pin specimen was slid in one direction over a distance of 113 μm without lubricant. After sliding the pin specimen once, the sliding surface of the block was observed immediately by AMF using a standard silicon probe, and the sliding surface was examined to count the number of generated wear elements. In addition to counting the number of wear elements, SEM and AFM observations of the plastic deformation (slip lines or bands) beneath the sliding surface of the block material were performed to determine the exact distance between slip lines (or bands) and formation process of wear elements. The
3. Materials The probe materials were colloidal particles of nine metals (Mo, Ti, Co, Ni, Fe, Cu, Ag, Au, and Zn) of commercial purity; these metals were selected because it was possible to obtain spherical particle for measurements of the adhesion force and because these metals have a range of different physical and chemical properties. Mo, Ti, Co, Ni, and Fe are all transition metals with relatively high interatomic bonding energies [11] that are highly active in the chemisorption of atmospheric materials [12–14]. On the other hand, Cu, Ag, Au, and Zn are metals with relatively low bonding energies and low chemisorption activities. The probes of the eight metals had an average diameter of 29 ± 8 μm, except for Ni, where this was value was 15 μm, because spherical particles ∼30 μm in diameter could not be obtained. The diameter of the particles was chosen on the basis of its similarity in size to the width of the tipless cantilever. The particles were produced commercially by gas atomization. The roughness of the particles attached to the AFM
Fig. 1. Piezoelectric-type pin-on-block wear test apparatus. 2
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Table 1 Properties of the metallic probe particles, the metals of the pin and block specimens, and metal combinations. Metallic probe particles Metals Mo Purity, % 99.8 Probe diameter, μm 29 ± 8 Pin and block specimen Metals Mo Purity, % 99.7 Vickers hardness of block specimen 232 Roughness of pin and block(Ra), nm 1.4–5.1 Metal combinations Same-metal combinations: Mo, Ti, Co, Ni, Fe, Cu, Ag, Au,
Ti 99.98
Co 99.9
Fe 99.9
Cu 99.9
Ag 99.9
Au 99.99
Zn 99.9
Ni 99.9 15
Ti 99.7 189
Co 99.9 188
Fe 99.9 110
Cu 99.99 86
Ag 99.9 87
Au 99.9 75
Zn 99.9 36
Ni 99.9 123
Al 99.99 26
Zn
Three different-metal combinations with different metallurgical mutual solubility. Cu–Ni; Complete mutual solubility. Ag–Fe; No solubility in the solid and liquid phases. Cu–Al; An intermetallic compound system (intermediary solubility).
4. Experimental results
cantilever could not be measured in the experiment. Table 1 shows the purity of the metallic probe particles and the Vickers hardness and purity of the block metals, including an Al block. The reported Vickers hardness is the average value from five measurements at a load of 4.9 N. The pin specimen used for the pin-on-block test was made from the same metal as the block specimen. The surfaces of the blocks and pins were polished with diamond paste powder and finished to reduce their roughness to 1.4–5.1 nm·Ra; they were then degreased with acetone in an ultrasonic cleaner. In the experiments, each of the nine metals was used as both pin and block. In addition to measurements of the same metal, we also measured the adhesion force for three combinations different metals (Cu–Ni, Ag–Fe, and Cu–Al) to elucidate the effects of the adhesion force on the amount of adhesive wear. In these experiments, the three differentmetal combinations shown in Table 1 were selected because they differ in their metallurgical mutual solubilities and in their tribological properties of adhesive wear [15–17]. The combination of Cu and Ni has complete mutual solubility and shows a large amount of wear. On the other hand, the combination of Ag and Fe does not exhibit solubility in the solid or liquid phases, and wear is very low. The combination of Cu and Al exhibits intermediate solubility, forms an intermetallic compound system, and shows intermediate wear. From the experiments with these three different-metal combinations, the influence of adhesion force on the macroscopic amount of adhesive wear reported in the literature [15] is discussed.
4.1. Measurement and observation of wear elements in the elementary wear process First, we examined the number of wear elements generated in the elementary wear process of the nine metals. To determine the number of wear elements (n in Eq. (1)) generated at a junction of mean area πa2, AFM observations of the sliding surface were performed, and the number of wear elements per unit area of the sliding surface nunit was counted from the AFM image. Here, assuming that n corresponds to the number of wear elements present in the area of πa2, n is expressed by n ≈ πa2nunit. As previously described, the wear element is the unitary debris of an adhesive wear particle, and the size of wear element was ranging from a few nanometers to a few tens of nanometers by AFM observation [3–6,8]. With respect to the size of the wear elements, it has previously been discussed this is comparable to the size of singlemagnetic-domain particles when the metal is a ferromagnetic metal (Ni, Fe, or Co) [5,6]. Furthermore, in the case of other metals, the size of the wear element is similar to that of an ultrafine particle [3,4,8]. In the experiments, the number of wear elements was determined by counting the number of particles with sizes ranging from a few nanometers to a few tens of nanometers from the AFM image of each metal. The transfer particles formed by aggregation of the wear elements were not included in the counting of the number nunit because the transfer particle is not an original element but the secondary by-product particle of the wear elements. Fig. 2A shows an AFM image of a sliding surface of the Ni block after sliding the Ni pin once using the piezoelectric-type wear test apparatus. In the figure, wear elements are indicated by the white triangles with
Fig. 2. AFM image of the sliding surface of the nickel block after one sliding pass of a nickel pin in the piezoelectric-type pin-on-block wear test apparatus; typical wear elements are indicated by white triangle and the particle size (A), and the profile of a wear element (B). 3
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Table 2 Number of wear elements per unit area on the wear surfaces. Metals
Mo
Ti
Co
Ni
Fe
Cu
Ag
Au
Zn
Number of wear elements per unit area of sliding surface nunit, μm−2
11
9
4
10
7
5
4
5
6
the respective particle size noted, and the sizes of these wear elements of Ni ranged from 15 nm to a few tens of nanometers. Fig. 2B shows a typical surface profile around a wear element as measured by AFM. The wear element had a height of 1–2 nm and a length of 15–20 nm. The number of wear elements per unit area nunit for each of the nine metals is listed in Table 2. In Fig. 2, in addition to the wear elements, many particles with a size larger than a few tens of nanometers were observed on the sliding surface. As previously explained for the wear mechanism of adhesive wear, these are transfer particles formed by aggregation of wear elements and of other transfer particles between the sliding surfaces through mutual transfer-and-growth processes [3,4,15–17]. The AFM images of the surfaces for the same metal combinations of Fe [3,4], Cu [3], Ti [18], Zn [19], Ni [5,6], and different metal combinations including Ag–Fe [4] have already been reported in previous papers. From the images of these metals, the difference in the presence of wear elements of each metal can be observed. Next, through nanoscale observations by SEM and AFM, we investigated the precise size and generation process of wear element in the elementary process of adhesive wear. The generation process of wear elements in the elementary processes has previously been discussed on the basis of in situ observations made with a microscope [3,4,9,10]. In the wear process of adhesive wear, the real contact area is plastically deformed by both the compressive stress of the applied load and by the shear stress of the sliding motion, as the applied stress exceeds the elastic limit of the material at the contact point. Furthermore, in the case of the pin-on-block sliding system, the position of the stress point moves on the crystal grain on the surface and, simultaneously, the direction of the stress in the crystal grain continuously changes as the pin moves on the block surface. As a result, some of the crystal grains immediately beneath the sliding surface are distorted by the synthetic stress generated from both the applied load and the shearing force due to the sliding motion [3,9,10]. In this study, plastic deformation was accurately observed by SEM and AFM to permit the determination of the actual distance between the slip lines (slip bands). Fig. 3 shows SEM images of the sliding surface and the region beneath the surface of the iron block after sliding the iron pin once on the block surface. The upper part of the SEM image of Fig. 3A shows the sliding surface, and the lower part shows the region beneath the sliding surface. Fig. 3B is an enlarged view of the region inside the rectangle in Fig. 3A. It can be seen that the slip lines (or slip bands) resulting from plastic deformation cross each other beneath the sliding surface as the sliding motion progresses, as was previously found in optical microscopic images [3,4]. In Fig. 3, the first slip occurred from the surface in the direction (a) from the surface, and a second slip then occurred in another direction (b). As a result, the slip lines (or bands) crossed each other beneath the sliding surface. As in the AFM observations in Fig. 2A, many transfer particles were seen on the sliding surface; these are shown at the points marked by small arrows in the figure. More-accurate observations to determine the distance between the slip lines (or bands) were made by AFM. From the AFM image of iron in Fig. 4, the distance between the slip lines (or bands) was found to be of the order of a few tens of nanometers. If the wear element is generated in the crossed area immediately beneath the contact point, the size of the wear elements of iron should be a few tens of nanometers. This value was similar to the size of the wear elements observed in previous studies. In terms of the size distribution of the wear elements of metals, the size, of between a few nanometers and a few tens of nanometers, was not dependent on the metal [4]. This means that the distance between
Fig. 3. SEM image of the sliding surface and beneath the surface of the iron block after one sliding pass of the iron pin on the block surface. Image B is an enlarged view of the rectangle in image A.
Fig. 4. Nanosize slip lines (or bands) beneath a sliding surface of the iron block by AFM imaging.
slip lines (or bands) of plastic deformation beneath the sliding surface is similar in the case where adhesive wear occurs between metals. The details of the relationship between the plastic deformation beneath the sliding surfaces and the applied stress in adhesive wear are discussed below. 4.2. Determination of the adhesion forces of metal sliding surfaces by using the FFM–FDC method The adhesion forces for same-metal combinations of nine metals were measured by the FFM–FDC method [Fig. 5(a)–(c)]. From Fig. 5, the adhesion force for each metal was determined as follows. As previously pointed out [7], the pull-off force in the FFM–FDC method generally increases as the number of the sliding-contact cycles increases, eventually reaching a nearly stable value that depends on the metal, although there are some fluctuations due to changes in the sliding surface state due to each friction cycle of the FFM [7]. As shown in Fig. 5, the pull-off forces for Ti, Fe, Zn, Mo, Co, Ag, Ni, Au, and Cu reached nearly stable values after 13, 43, 43, 38, 42, 40, 24, 38, and 42 or more repetitions, respectively. As mentioned in a previous report, it is thought that the pull-off force becomes almost stable after the removal of a surface film or contamination from the surface after repeated sliding friction cycles of the FFM. In the experiment, however, it was not possible to detect what type of surface film was removed from the 4
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Fig. 6. SEM image of the surface of the copper probe after 50 cycles of FFMFDC measurements.
method are listed in Table 3. The adhesion forces of Ni, Ti, Mo, and Fe had relatively large values of 600 nN or more; those of Au, Zn and Cu were in the range 500–600 nN; and, those of Co and Ag were less than 400 nN. Fig. 6 shows an image of the surface of the copper probe after 50 cycles of FFM–FDC measurements. In the contact portion, shown by the white circle, plastic deformation occurred due to sliding friction by the FFM and, at the same time, small particles adhered to the surface. The relationship between the number of wear elements per unit area nunit in Table 2 and the adhesion force Fad measured by the FFM–FDC method in Table 3 is shown in Fig. 7. The number of wear elements per unit area tended to increase linearly with increasing adhesion force of the metal. Metals with large adhesion forces generate a relatively large number of wear elements in the elementary process of adhesive wear. The line in Fig. 7 is approximately straight and can be written as follows:
n unit = 0.018(Fad – 190)
(2)
Details of the modification of the wear equation are discussed below. In addition to measuring the adhesion forces for the nine samemetal combinations, the FFM–FDC method was used to examine the adhesion force for combinations of different metals. The different-metal combinations were selected to have different metallurgical properties, as described in Table 1. The results for the adhesion forces of the three different-metal combinations Cu–Ni, Ag–Fe, and Cu–Al measured by the FFM–FDC method are shown in Table 3. The combination of Cu and Ni, which has complete mutual solubility, showed a relatively high adhesion force of 515 nN, which is similar to that of the same-metal combination Cu–Cu. On the other hand, the combination of Ag and Fe, which have immiscible solid and liquid phases, showed a low adhesion force of 220 nN; this was the lowest among all the combinations of metals examined in the experiment. The combination of Cu and Al, which form an intermetallic compound phase, exhibited an intermediate adhesion force of 334 nN. Compared with the number of wear elements generated by the combination of Ag and Fe, measured in a previous paper, the number of wear elements nunit of Ag–Fe was 2 μm−2 [4]. The combination of metals with a low adhesion force of about
Fig. 5. (a) Pull-off forces for the same-metal combinations of titanium, iron, and zinc measured by the FFM-FDC method: (b) Same-metal combinations of molybdenum, cobalt, and silver, (c) Same-metal combinations of nickel, gold, and copper.
sliding surface. In this study, the adhesion force of the sliding surface of each metal was taken as the average of the last five values from Fig. 5 (the average of the values from 46 through 50 cycles for Ti, Mo, Fe, Au, Zn, Cu, Co and Ag, or that from 33 through 37 cycles for Ni). The adhesion forces of the nine metals Fad measured by using FFM-FDC
Table 3 Adhesion forces measured by the FFM-FDC method for nine same-metal combinations and three different-metal combinations. Metals
Mo
Ti
Co
Ni
Fe
Cu
Ag
Au
Zn
Adhesion force of same-metal combinations Fad, nN Different-metal combinations, Adhesion force Fad, nN
665 Cu–Ni 515
670
380
754 Ag–Fe 220
656
504
376
538 Cu–Al 334
500
5
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motion progresses [3,4,9,10]. Furthermore, in the plastic-deformation process of adhesive wear, the shear stress of the sliding motion distorts some grains. In our experiments, the nine metals that we used to observe the wear elements and plastic deformation beneath the sliding surface had three different types of crystal structure. Whereas slip in a single crystal due to plastic deformation occurs in a particular direction on the crystal plane, the onset of plastic deformation in a polycrystalline metal is influenced by the number of available slip systems, and several slip systems can exist, corresponding to incompatibilities at the grain boundaries under the conditions of applied stresses. As mentioned above, the most important point regarding adhesive wear is that the stress due to the load and the shear force due to the sliding motion change continuously in direction at the contact point through the plastic deformation process. Because the angle of the direction of the slip line (or band) to the sliding motion depends on the crystal structure of the grain and on the grain boundaries of adjacent grains, the angle varies from grain to grain, as previously observed [9,10]. When wear elements are generated in the crossed area of slip lines (or bands) immediately beneath the contact point, the size of the wear elements is determined by the distance d (d ≈ 2b) of the slip lines, as shown in Fig. 8. From the AFM image of iron shown in Fig. 4, the distance between the slip lines (or bands) was deduced to be of the order of a few tens of nanometers, which was about the same size as that of the wear elements [3,4]. On the basis of the above process of adhesive wear, Equation (1) was corrected according to the results of this study. In our previous model [3], we predicted that n (the number of wear elements generated at a junction) is related to various physical factors, such as the adhesion force between sliding surfaces. According to our present measurements of the adhesion force by the FFM–FDC method and our observation of the presence of wear elements, the number of wear elements per unit area nunit is given by Equation (2) as a function of the adhesion force Fad. Here, if we assume that n in Equation (1) is determined by multiplying the number of wear elements per unit area nunit by the area of each junction (≈πa2), then n increases in the same manner as nunit as the adhesion force Fad increases. Consequently, we can express the wear equation by using n = k (Fad – 190), where k is a constant (k ≈ 0.018πa2). That is,
Fig. 7. The relationship between the number of wear elements per unit area nunit and the adhesion force Fad measured by using the FFM–FDC method for nine metals.
190 nN resulted in the formation of very few wear elements during adhesive wear. The close relationship between the adhesion force, the amount of wear in adhesive wear, and the metallurgical mutual solubility of the metals is explained in Section 5. 5. Discussion The mechanism for the generation of wear elements in the elementary process and the subsequent process of accumulation of wear elements to form wear particles of adhesive wear has been previously discussed [3–6,9,10]. An overview of the elementary process, extracted from these previous reports, is shown in Fig. 8 [3]. In the process of adhesive wear, when stresses due to load and shearing forces of sliding motion are both applied to the sliding surface, the actual contact area deforms plastically because the applied stress exceeds the elastic limit of the metal. The plastic deformation that occurs in the crystal grains immediately beneath the sliding surface through the action of the compressive stress in both the vertical and the sliding directions is explained below. First, in a pin-on-block sliding system, as the pin approaches a crystal grain on the block, a compressive stress is applied to the grain in the sliding direction, and slip due to plastic deformation occurs in the grain located in front of the pin. The first slip from the surface then occurs in direction (a) in Figs. 3, 4 and 8. As the pin specimen slides further, the plastically deformed area moves through the grain and, at the same time, the density of the slip lines increases. Secondly, as the contact point moves over the grain, a compressive stress due to the load acting perpendicularly to the sliding surface causes slip in a different direction [direction (b) in Figs. 3, 4 and 8]. Finally, as the pin passes through, the slip lines (or bands) of (a) and (b) cross one another. In this process, as shown in Figs. 3 and 4 and in other reports, slip lines (or bands) are formed parallel to one another immediately beneath the sliding surface. However, from previous observations, in many cases the slip lines run in a curve or run along grain boundaries as the sliding
V=
1 k (Fad − 190) ⎛ b ⎞3 ⎛ P·ℓ ⎞ · · ·⎜ ⎟ λ 3 ⎝ a ⎠ ⎝ pm ⎠
(3)
From this wear equation, the amount of adhesive wear can be expressed by using the physical adhesion factor k (Fad – 190), the chemical factor (λ), the size factor (b/a), the applied load (P), the sliding distance (ℓ), and the yield stress of the softer material (pm). In this study, only the physical properties were investigated, but previous reports have discussed the chemical factor λ and have reported that chemical activity determines whether the wear mode is mild or severe [3,13,14]. As mentioned in the previous papers, the adhesion force affects the aggregation of wear elements, while the chemical activity such as chemisorption of surrounding molecules is more important in the aggregation process of the wear elements to form transfer particles
Fig. 8. An overview of the elementary process and a schematic model for the generation process of wear elements at a junction; (a) shows the first slip and (b) shows the second slip beneath the sliding surface; d is the distance between slip lines (or bands). 6
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and wear particles in the process of adhesive wear. In Holm's equation V = Z·(P·ℓ/pm), the wear coefficient Z is interpreted as the probability of removal of small debris from the junction, and small debris particles are removed directly from the junction to form a wear particle [2]. On the other hand, we can interpret the wear coefficient Z chemically and physically as a factor that is determined by the adhesion force Fad, the chemical properties, and the ratio of the size of the wear elements to the size of the junction (b/a). In other words, we propose that Holm's wear coefficient Z is not a probability but a physicochemical property of the material. As shown in Fig. 7 and by Equation (3), it is interesting to note that wear elements will not be generated if the adhesion force is less than 190 nN. In combination with very low adhesion force of less than 190 nN, shear of the junction due to sliding motion occurs preferentially at the interface of two metals rather than in the bulk, the generation of wear elements is limited, and the amount of wear is very low. However, very few metals have low adhesion forces of less than 190 nN under normal conditions in the absence of a lubricant. The FFM–FDC method is important for the measurement of adhesion forces of actual sliding surfaces. When considering adhesive wear, it is necessary to know not only the adhesion force between ideal and elastic contact surfaces, such those as calculated by the Johnson–Kendall–Roberts, Derjaguin–Muller–Toporov, or other approximation models [20–22], but also the adhesion force between actual sliding surfaces. When discussing adhesive wear based on the adhesion force, adhesion essentially occurs at the plastically deformed junction of the sliding surfaces, and the adhesion force depends on two factors: the size of the real contact area resulting from plastic deformation, and the interatomic cohesion force in the real contact area. As shown in Tables 1 and 3, for example, the softest metal Zn has the lowest Vickers hardness of 36 and the lowest cohesion energy of 130 kJ/mol among the nine metals that we studied [11], whereas it has a relatively large adhesion force of 500 nN. On the other hand, the hard transition metal Mo, which has a hardness of 232 and a high cohesion energy of 648 kJ/mol, exhibits an adhesion force of 665 nN. Zn exhibits an unexpectedly high adhesion force, comparable to that of hard metals with higher cohesion energies. Because Zn is a soft metal, it has a relatively large real contact area (about six times that of Mo) under the applied load in adhesive wear. Therefore, although Zn has a low cohesion energy, it has a relatively large adhesion force due to the large real contact area, because the adhesion force of a sliding surface is related to both the number of interatomic bonds and to the strength of cohesion of the metals in the real contact area. Furthermore, as shown in Fig. 7, it is considered that a difference in the crystal structures of metals does not affect the relationship between the number of wear elements and the adhesion force. In general, the amount of wear is closely dependent on the combination of metals that are subject to the adhesive wear. To explain the effect of changing the combination of metals on their wear properties, one of us previously investigated the relationship between the amount of wear and the metallurgical mutual solubility of two contacting metals. Fig. 9 shows a typical wear–distance curve for three combinations of metals with different mutual solubilities, the data are extracted from the previous report [15]. The wear test was performed by using a pinon-disk apparatus (using a pin with flat ended shape) with a sliding velocity of 80.8 cm/s and a load of 11.1 N. The amount of wear correlated to the mutual solubility. That is, Cu–Ni, which have complete mutual solubility, exhibited a large amount of wear, and a considerable amount of transfer occurred between the sliding surfaces. On the other hand, Ag–Fe, which are immiscible in the solid and liquid phases, showed very little wear and a produced a very small number of transfer particles, whereas Cu–Al, which form an intermetallic compound system, showed intermediate wear [15–17]. In the present experiments on the adhesion force measured by the FFM–FDC method, Cu–Ni, Cu–Al, and Ag–Fe showed adhesion forces of 515, 334, and 220 nN, respectively. The order of these values of the adhesion force
Fig. 9. Typical wear-distance curve for three combinations of metals with different mutual-solubility properties: Cu–Ni, Al–Cu, and Ag–Fe. The wear tests were performed by using a pin-on-disk apparatus at the sliding velocity of 80.8 cm/s and the load of 11.1 N without lubricant (data are extracted from Ref. [15]).
Fig. 10. The relationship between the wear of the pin and disk and the adhesion force measured by the FFM–FDC method for three different-metal combinations with different metallurgical mutual solubilities.
[(Cu–Ni) > (Al–Cu) > (Ag–Fe)] corresponds to that of the amount of adhesive wear of these three combinations. Fig. 10 shows the relationship between the adhesion force of the combinations and the wear of the pin and disk. Here, the amount of wear was determined after a sliding distance of 0.97 km. The amount of wear in adhesive wear depends directly on the adhesion force of the sliding surface. When a metal combination has a large adhesion force and a high mutual solubility, its amount of wear increases because the wear elements are actively generated and growth of transfer particles occurs on the sliding surface. We assume, therefore, that the previous qualitative description of the effect of the mutual solubility of the sliding components on the adhesive wear translates into the effect of adhesion force between the sliding surfaces. In Fig. 10, the wear was very small when the adhesion force decreased to about 200 nN, such as that shown by the combination Ag–Fe (Fad = 220 nN). This is in good agreement with the previous reports that wear of the combination of Ag and Fe is very low and few wear elements are generated shown in the AFM image of Ag–Fe (the value of nunit for Ag–Fe was 2 μm−2) [4,15–17]. 6. Conclusions The number of wear elements generated through the elementary process of adhesive wear was examined by means of nanoscale observations by AFM. Nanoscale observations by SEM and AFM clarified 7
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the precise mechanism of the generation of wear elements and their exact size through observations of the slip lines (or bands) caused by plastic deformation. Furthermore, the adhesion force of the actual sliding surface of metals was measured by using the FFM–FDC method. From these results, the relationship between the number of wear elements generated at a junction n and the adhesion force Fad was shown to be n = k (Fad – 190); moreover, a correction of the wear equation to account for adhesive wear was proposed. Measurements of the adhesion forces of combinations of different metals with different metallurgical mutual solubilities revealed that the amount of adhesive wear was directly dependent on the adhesion force of the sliding metals. The values of the adhesion force showed the relationship (Cu–Ni) > (Al–Cu) > (Ag–Fe); this order also corresponds to that of the amount of adhesive wear and to that of the mutual metallurgical solubility of the three combinations.
process, J. Appl. Phys. 92 (2002) 6721–6727 https://doi.org/10.1063/1.1519340. [7] H. Mishina, Y. Ichimoto, H. Kobayashi, T. Uehara, T. Ohmori, A. Hase, measurements of adhesion force between sliding surfaces by means of FFM-FC system, Proc. Int. Tribology Conf. 2015, Tokyo, 2015, pp. 449–450. [8] A. Hase, H. Mishina, Study on elementary process of adhesive wear using scanning probe microscopy, Tribol. Online 11 (2) (2016) 114–120 https://doi.org/10.2474/ trol.11.114. [9] H. Mishina, Surface deformation and formation of original element of wear particles in sliding friction, Wear 215 (1998) 10–17 https://doi.org/10.1016/S00431648(97)00295-0. [10] H. Mishina, S. Kondoh, In-situ observation of original wear process: deformation of surface crystal grains and origin of elemental wear debris in pure iron, J. Japan Soc. Tribol. 44 (1999) 528–535 (in Japanese). [11] C. Kittel, Introduction to Solid State Physics, eighth ed., Wiley, New York, 2012. [12] B.M.W. Trapnell, The activities of evaporated metal films in gas chemisorption, Proc. Roy. Soc. Lond. A 23 (1953) 566–577 https://doi.org/10.1098/rspa.1953. 0125. [13] H. Mishina, Atmospheric characteristics in friction and wear of metals, Wear 152 (1992) 99–110 https://doi.org/10.1016/0043-1648(92)90207-O. [14] H. Mishina, Chemisorption of diatomic gas molecules and atmospheric characteristics in adhesive wear and friction of metals, Wear 180 (1995) 1–7 https://doi.org/ 10.1016/0043-1648(95)80003-4. [15] T. Sasada, S. Norose, H. Mishina, Effect of mutual metallic solubility on adhesive wear, J. Japan Soc. Lubr. Eng. 22 (1977) 169–176 (in Japanese). [16] T. Sasada, S. Norose, H. Mishina, The behavior of adhered fragments interposed between sliding surfaces and the formation process of wear particles, Trans. ASME. J. Lubr. Technol. 103 (1981) 195–202. [17] H. Mishina, H. Makita, T. Sasada, S. Norose, Various modes of the origin and growth process of wear particles in pure metal–metal rubbing, J. Japan Soc. Lubr. Eng. 24 (1979) 466–472 (in Japanese). [18] H. Mishina, K. Chiba, A. Hase, Generation of ammonia during wear processes in adhesive wear of titanium, Tribol. Online 10 (2) (2015) 201–206 https://doi.org/ 10.2474/trol.10.201. [19] H. Mishina, A. Kohno, Y. Akamatsu, Discovery of elemental debris of adhesive wear particles and primary process of wear, J. Japan Soc. Tribol. 48 (2003) 307–314 (in Japanese). [20] K.L. Johnson, K. Kendall, A.D. Roberts, Surface energy and the contact of elastic solids, Proc. Roy. Soc. Lond. A 324 (1971) 301–313 https://doi.org/10.1098/rspa. 1971.0141. [21] B.V. Derjaguin, V.M. Müller, Y.P. Toporov, Effect of contact deformations on the adhesion of particles, J. Colloid Interface Sci. 53 (1975) 314–326 https://doi.org/ 10.1016/0021-9797(75)90018-1. [22] Y.I. Rabinovich, J.J. Adler, A. Ata, R.K. Singh, B.M. Moudgil, Adhesion between nanoscale rough surfaces: I. Role of asperity geometry, J. Colloid Interface Sci. 232 (2000) 10–16 https://doi.org/10.1006/jcis.2000.7167.
Acknowledgements The authors gratefully acknowledge the help of Dr. T. Ohmori, Y. Tsuchiya, Y. Ichimoto, H. Kobayashi, T. Uehara, H. Ueda, and T. Kitakoga (all of Chiba University); we also thank Hitachi Power Solutions Co., Ltd. for the SEM observations. References [1] R. Holm, Über metallische kontaktwiderstände, Wiss. Veröff. Siemens-Werk 7 (2) (1929) 217–258. [2] R. Holm, Electric Contacts: Electric Contacts: Theory and Application, Springer, Berlin, 1946. [3] H. Mishina, A. Hase, Wear equation for adhesive wear established through elementary process of wear, Wear 308 (2013) 186–192 https://doi.org/10.1016/j. wear.2013.06.016. [4] A. Hase, H. Mishina, Wear elements generated in the elementary process of wear, Tribol. Int. 42 (2009) 1684–1690 https://doi.org/10.1016/j.triboint.2009.02.006. [5] H. Mishina, H. Iwase, A. Hase, Generation of wear elements and origin of tribomagnetization phenomenon, Wear 269 (2010) 491–497 https://doi.org/10.1016/j. wear.2010.05.004. [6] H. Mishina, Magnetization of ferromagnetic material surfaces by tribological
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Effect of the adhesion force on the equation of adhesive wear and the generation process of wear elements in adhesive wear of metals
T
Hiroshi Mishinaa,∗, Alan Haseb a b
Graduate School of Engineering, Chiba University, 1-33 Yayoi, Inage-ku, Chiba, 263-8522, Japan Department of Mechanical Engineering, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama, 369-0293, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Adhesive wear Metal wear Wear equation Wear elements Adhesion force FFM–FDC method
The effects of the adhesion force on the equation of adhesive wear were investigated by examining the relationship between the adhesion force of actual sliding surfaces and the number of wear elements generated at a junction. The number of wear elements and their generation mechanism in the elementary wear processes were determined by nanoscale observations and by analysis of the sliding surfaces by scanning electron microscopy and atomic-force microscopy. In addition, the adhesion force between actual sliding metal surfaces was measured by a method combining friction-force microscopy and force–distance curve measurement. The adhesion force between metal-probe cantilevers of nine different metals and the surfaces of blocks of the same metal were measured. We propose a modified equation for adhesive wear that takes into account the effect of the adhesion force, and we discuss the precise process for the generation of wear elements.
1. Introduction For many years, investigation into the mechanism and the wear equation of adhesive wear have relied on Holm's well-known wear equation, V = Z·(P·ℓ/pm), where V is the amount of wear, P is the applied load, pm is yield stress of the softer material, ℓ is sliding distance, and Z is the wear coefficient [1,2]. Here, in the idea of Holm, Z is the probability of removing atomic-size particles or layers to form wear particles. However, Holm's equation does not take into account the exact effects of such factors as the physical and chemical properties of the sliding surfaces on the adhesive wear. The influence of these factors and the wear mechanism are usually considered to be included in the wear coefficient Z, but an accurate interpretation for these factors has not been made. With regard to adhesive wear, we have previously proposed a wear equation that we established from the elementary wear process (the mechanism of generation of a wear element, which is the unitary debris of an adhesive wear particle) [3–6]. This wear equation is written as follows [3]:
V=
1 n ⎛ b ⎞3 ⎛ P·ℓ ⎞ ·⎛ ⎞· ·⎜ ⎟ 3 ⎝ λ ⎠ ⎝ a ⎠ ⎝ pm ⎠
(1)
where a is the mean radius of the junctions, b is the mean radius of the wear elements, and V, P, ℓ, and pm have the same meaning as in Holm's
∗
equation; Furthermore, n is the number of wear elements generated at a junction and λ is a factor that takes into account chemical effects, such as chemisorption of surrounding molecules, that determine the wear mode. The detailed meaning of each factor has been explained previously [3]. In the wear model, n is predicted to be related to a physical factor, such as the adhesion force at the sliding surface. However, the exact relationship between the number of wear elements generated in a junction and the adhesion force has not yet been clarified. The purpose of the current study was to measure the number of wear elements generated in adhesive wear, together with the adhesion force of the actual sliding metal surface, and to determine the exact relationship between the number of wear elements n in Eq. (1) and the adhesion force. Of course, the adhesion force between sliding surfaces is an important property in relation to understanding the phenomena of friction and adhesive wear. At an actual sliding surface, the real contact point (junction) is plastically deformed due to the close contact and sliding motion during the tribological process. In investigating adhesive wear, it is necessary to determine the adhesion force between the actual sliding surfaces rather than that between ideal clean and/or elastic contact surfaces. To investigate the adhesion force between actual sliding metal surfaces, we measured the adhesion force by a combination of friction-force microscopy (FFM) and force–distance curve (FDC) measurements, known as the FFM–FDC method [7]. In this method, the
Corresponding author. E-mail address: [email protected] (H. Mishina).
https://doi.org/10.1016/j.wear.2019.202936 Received 2 December 2018; Received in revised form 5 June 2019; Accepted 11 June 2019 Available online 14 June 2019 0043-1648/ © 2019 Published by Elsevier B.V.
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experimental procedure for the observation of deformation beneath the sliding surface was similar to that for our in situ microscopic observations with an applied load of 2.0 N [3,9,10].
adhesion force is determined by the pull-off force determined from the FDC measured on the sliding surface immediately after performing the sliding friction in FFM mode. Details of the FFM–FDC method have been reported in our previous papers [7,8]. In the present experiments, the adhesion force was measured for same-metal combinations of nine pure metals: molybdenum, titanium, cobalt, nickel, iron, copper, silver, gold, and zinc. Furthermore, to clarify the influence of the adhesion force on the macroscopic amount of adhesive wear, the adhesion force was measured for three different-metal combinations that differed in their mutual metallurgical solubilities: Cu–Ni, Ag–Fe, and Cu–Al. In addition to the measurements of the adhesion force of metals, the number and critical size of wear elements generated in the elementary process of adhesive wear were also investigated. We have previously reported and discussed the process for the formation of wear elements, as determined by microscopy studies of the region beneath the sliding surface of metals [3,4,9,10]. However, because the deformation beneath the sliding surface was observed in situ, the observation was limited to optical microscopy levels. Previous reports have explained that the size of wear elements is determined by the distance between slip lines (or bands) produce by plastic deformation immediately beneath the sliding surface. However, the critical size of the wear element could not be determined by using an optical microscope. In this study, nanoscale observations of the wear element and plastic deformation beneath the sliding surface were performed by scanning electron microscopy (SEM) and atomic-force microscopy (AFM) to reveal the exact size and the mechanism of generation of wear elements.
2.2. Measurements of the adhesion forces of metal surfaces by using the FFM–FDC method In experiments to measure the adhesion force between actual sliding metal surfaces, we used the FFM–FDC method, which combined a sliding friction motion in the FFM mode with measurements of the pulloff force by FDC using a metal cantilever probe. The experimental procedure was similar to that described in previous reports [7,8]. To perform this FFM–FDC method, an SPM cantilever with a colloidal probe consisting of a particle of one of nine pure metals (Mo, Ti, Co, Ni, Fe, Cu, Ag, Au, or Zn) with an average diameter of 29 ± 8 μm (15 μm for Ni) was used. Each metal particle was stuck onto the tipless cantilever by using an acrylic adhesive. An all-in-one tipless cantilever (BudgetSensors; Sofia) with a force constant of 2.7 N/m, a width of 30 μm, and a length of 210 μm was selected, as this permitted the measurement of the adhesion force between the metal surfaces. A force constant of 2.7 N/m was selected from preliminary experiments performed with various force constants from 0.2 to 40 N/m. The FFM and FDC in the AFM modes of the SPM were performed by using a JSPM–5200 microscope (JEOL Ltd., Tokyo). All experiments were performed in air at 23 °C and 25–30% RH. In the FFM–FDC method, the cantilever of the metal probe was in contact with the surface of the metal block, and frictional movement was achieved in the FFM mode by using a back-and-forth motion with a sliding distance of 12–15 μm. After sliding for one reciprocation by means of the FFM, the same probe was used to measure the pull-off force of the FDC on the sliding track of the block surface. In this study, after repeating the FFM–FDC cycle a sufficient number of times for the pull-off force to become nearly stable, the adhesion force of each metal was determined. The required number of repetitions of the measurement was determined by the strength of adhesion of the particles to the cantilever in a preliminary experiment. Adherent particles of about 30 μm in size were not removed during 100 cycles of measurement, and the pull-off force reached a nearly stable value before the measurement were repeated 45 times. From these preliminary experiments, we determined that the optimal number of repetitions of the cycle of FFM–FDC measurements was 50 for eight of the metals. In the case of Ni, whose probe particles were smaller in size (15 μm) than those of the other eight metals, the strength of adhesion of the glued particles was weak and they detached from the cantilever after 35–37 repetitions in five preliminary experiments; the pull-off force was almost stable after about 25 repetitions of the FFM–FDC measurement. Therefore, in the case of Ni, we used data obtained after 37 repeated measurements [Fig. 5c].
2. Experimental 2.1. Piezoelectric-type pin-on-block wear test apparatus for observation and determination of the number of wear elements Wear experiments were performed by using a piezoelectric-type pinon-block wear test apparatus (Fig. 1). In this experiment, the block specimen of the pin-on-block system was held directly in the sample holder of the scanning probe microscope (SPM) to permit immediate observation by AFM. Driven by a piezoelectric actuator, the pin specimen was moved along the sliding track on the block specimen (30 × 15 × 10 mm in size) at a sliding velocity of 113 μm/s and with an applied load of 0.1 N in air at 23 °C and 25–30% RH. The pin had a hemispherical shape with a diameter of 4 mm at the contact surface. The pin specimen was slid in one direction over a distance of 113 μm without lubricant. After sliding the pin specimen once, the sliding surface of the block was observed immediately by AMF using a standard silicon probe, and the sliding surface was examined to count the number of generated wear elements. In addition to counting the number of wear elements, SEM and AFM observations of the plastic deformation (slip lines or bands) beneath the sliding surface of the block material were performed to determine the exact distance between slip lines (or bands) and formation process of wear elements. The
3. Materials The probe materials were colloidal particles of nine metals (Mo, Ti, Co, Ni, Fe, Cu, Ag, Au, and Zn) of commercial purity; these metals were selected because it was possible to obtain spherical particle for measurements of the adhesion force and because these metals have a range of different physical and chemical properties. Mo, Ti, Co, Ni, and Fe are all transition metals with relatively high interatomic bonding energies [11] that are highly active in the chemisorption of atmospheric materials [12–14]. On the other hand, Cu, Ag, Au, and Zn are metals with relatively low bonding energies and low chemisorption activities. The probes of the eight metals had an average diameter of 29 ± 8 μm, except for Ni, where this was value was 15 μm, because spherical particles ∼30 μm in diameter could not be obtained. The diameter of the particles was chosen on the basis of its similarity in size to the width of the tipless cantilever. The particles were produced commercially by gas atomization. The roughness of the particles attached to the AFM
Fig. 1. Piezoelectric-type pin-on-block wear test apparatus. 2
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Table 1 Properties of the metallic probe particles, the metals of the pin and block specimens, and metal combinations. Metallic probe particles Metals Mo Purity, % 99.8 Probe diameter, μm 29 ± 8 Pin and block specimen Metals Mo Purity, % 99.7 Vickers hardness of block specimen 232 Roughness of pin and block(Ra), nm 1.4–5.1 Metal combinations Same-metal combinations: Mo, Ti, Co, Ni, Fe, Cu, Ag, Au,
Ti 99.98
Co 99.9
Fe 99.9
Cu 99.9
Ag 99.9
Au 99.99
Zn 99.9
Ni 99.9 15
Ti 99.7 189
Co 99.9 188
Fe 99.9 110
Cu 99.99 86
Ag 99.9 87
Au 99.9 75
Zn 99.9 36
Ni 99.9 123
Al 99.99 26
Zn
Three different-metal combinations with different metallurgical mutual solubility. Cu–Ni; Complete mutual solubility. Ag–Fe; No solubility in the solid and liquid phases. Cu–Al; An intermetallic compound system (intermediary solubility).
4. Experimental results
cantilever could not be measured in the experiment. Table 1 shows the purity of the metallic probe particles and the Vickers hardness and purity of the block metals, including an Al block. The reported Vickers hardness is the average value from five measurements at a load of 4.9 N. The pin specimen used for the pin-on-block test was made from the same metal as the block specimen. The surfaces of the blocks and pins were polished with diamond paste powder and finished to reduce their roughness to 1.4–5.1 nm·Ra; they were then degreased with acetone in an ultrasonic cleaner. In the experiments, each of the nine metals was used as both pin and block. In addition to measurements of the same metal, we also measured the adhesion force for three combinations different metals (Cu–Ni, Ag–Fe, and Cu–Al) to elucidate the effects of the adhesion force on the amount of adhesive wear. In these experiments, the three differentmetal combinations shown in Table 1 were selected because they differ in their metallurgical mutual solubilities and in their tribological properties of adhesive wear [15–17]. The combination of Cu and Ni has complete mutual solubility and shows a large amount of wear. On the other hand, the combination of Ag and Fe does not exhibit solubility in the solid or liquid phases, and wear is very low. The combination of Cu and Al exhibits intermediate solubility, forms an intermetallic compound system, and shows intermediate wear. From the experiments with these three different-metal combinations, the influence of adhesion force on the macroscopic amount of adhesive wear reported in the literature [15] is discussed.
4.1. Measurement and observation of wear elements in the elementary wear process First, we examined the number of wear elements generated in the elementary wear process of the nine metals. To determine the number of wear elements (n in Eq. (1)) generated at a junction of mean area πa2, AFM observations of the sliding surface were performed, and the number of wear elements per unit area of the sliding surface nunit was counted from the AFM image. Here, assuming that n corresponds to the number of wear elements present in the area of πa2, n is expressed by n ≈ πa2nunit. As previously described, the wear element is the unitary debris of an adhesive wear particle, and the size of wear element was ranging from a few nanometers to a few tens of nanometers by AFM observation [3–6,8]. With respect to the size of the wear elements, it has previously been discussed this is comparable to the size of singlemagnetic-domain particles when the metal is a ferromagnetic metal (Ni, Fe, or Co) [5,6]. Furthermore, in the case of other metals, the size of the wear element is similar to that of an ultrafine particle [3,4,8]. In the experiments, the number of wear elements was determined by counting the number of particles with sizes ranging from a few nanometers to a few tens of nanometers from the AFM image of each metal. The transfer particles formed by aggregation of the wear elements were not included in the counting of the number nunit because the transfer particle is not an original element but the secondary by-product particle of the wear elements. Fig. 2A shows an AFM image of a sliding surface of the Ni block after sliding the Ni pin once using the piezoelectric-type wear test apparatus. In the figure, wear elements are indicated by the white triangles with
Fig. 2. AFM image of the sliding surface of the nickel block after one sliding pass of a nickel pin in the piezoelectric-type pin-on-block wear test apparatus; typical wear elements are indicated by white triangle and the particle size (A), and the profile of a wear element (B). 3
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Table 2 Number of wear elements per unit area on the wear surfaces. Metals
Mo
Ti
Co
Ni
Fe
Cu
Ag
Au
Zn
Number of wear elements per unit area of sliding surface nunit, μm−2
11
9
4
10
7
5
4
5
6
the respective particle size noted, and the sizes of these wear elements of Ni ranged from 15 nm to a few tens of nanometers. Fig. 2B shows a typical surface profile around a wear element as measured by AFM. The wear element had a height of 1–2 nm and a length of 15–20 nm. The number of wear elements per unit area nunit for each of the nine metals is listed in Table 2. In Fig. 2, in addition to the wear elements, many particles with a size larger than a few tens of nanometers were observed on the sliding surface. As previously explained for the wear mechanism of adhesive wear, these are transfer particles formed by aggregation of wear elements and of other transfer particles between the sliding surfaces through mutual transfer-and-growth processes [3,4,15–17]. The AFM images of the surfaces for the same metal combinations of Fe [3,4], Cu [3], Ti [18], Zn [19], Ni [5,6], and different metal combinations including Ag–Fe [4] have already been reported in previous papers. From the images of these metals, the difference in the presence of wear elements of each metal can be observed. Next, through nanoscale observations by SEM and AFM, we investigated the precise size and generation process of wear element in the elementary process of adhesive wear. The generation process of wear elements in the elementary processes has previously been discussed on the basis of in situ observations made with a microscope [3,4,9,10]. In the wear process of adhesive wear, the real contact area is plastically deformed by both the compressive stress of the applied load and by the shear stress of the sliding motion, as the applied stress exceeds the elastic limit of the material at the contact point. Furthermore, in the case of the pin-on-block sliding system, the position of the stress point moves on the crystal grain on the surface and, simultaneously, the direction of the stress in the crystal grain continuously changes as the pin moves on the block surface. As a result, some of the crystal grains immediately beneath the sliding surface are distorted by the synthetic stress generated from both the applied load and the shearing force due to the sliding motion [3,9,10]. In this study, plastic deformation was accurately observed by SEM and AFM to permit the determination of the actual distance between the slip lines (slip bands). Fig. 3 shows SEM images of the sliding surface and the region beneath the surface of the iron block after sliding the iron pin once on the block surface. The upper part of the SEM image of Fig. 3A shows the sliding surface, and the lower part shows the region beneath the sliding surface. Fig. 3B is an enlarged view of the region inside the rectangle in Fig. 3A. It can be seen that the slip lines (or slip bands) resulting from plastic deformation cross each other beneath the sliding surface as the sliding motion progresses, as was previously found in optical microscopic images [3,4]. In Fig. 3, the first slip occurred from the surface in the direction (a) from the surface, and a second slip then occurred in another direction (b). As a result, the slip lines (or bands) crossed each other beneath the sliding surface. As in the AFM observations in Fig. 2A, many transfer particles were seen on the sliding surface; these are shown at the points marked by small arrows in the figure. More-accurate observations to determine the distance between the slip lines (or bands) were made by AFM. From the AFM image of iron in Fig. 4, the distance between the slip lines (or bands) was found to be of the order of a few tens of nanometers. If the wear element is generated in the crossed area immediately beneath the contact point, the size of the wear elements of iron should be a few tens of nanometers. This value was similar to the size of the wear elements observed in previous studies. In terms of the size distribution of the wear elements of metals, the size, of between a few nanometers and a few tens of nanometers, was not dependent on the metal [4]. This means that the distance between
Fig. 3. SEM image of the sliding surface and beneath the surface of the iron block after one sliding pass of the iron pin on the block surface. Image B is an enlarged view of the rectangle in image A.
Fig. 4. Nanosize slip lines (or bands) beneath a sliding surface of the iron block by AFM imaging.
slip lines (or bands) of plastic deformation beneath the sliding surface is similar in the case where adhesive wear occurs between metals. The details of the relationship between the plastic deformation beneath the sliding surfaces and the applied stress in adhesive wear are discussed below. 4.2. Determination of the adhesion forces of metal sliding surfaces by using the FFM–FDC method The adhesion forces for same-metal combinations of nine metals were measured by the FFM–FDC method [Fig. 5(a)–(c)]. From Fig. 5, the adhesion force for each metal was determined as follows. As previously pointed out [7], the pull-off force in the FFM–FDC method generally increases as the number of the sliding-contact cycles increases, eventually reaching a nearly stable value that depends on the metal, although there are some fluctuations due to changes in the sliding surface state due to each friction cycle of the FFM [7]. As shown in Fig. 5, the pull-off forces for Ti, Fe, Zn, Mo, Co, Ag, Ni, Au, and Cu reached nearly stable values after 13, 43, 43, 38, 42, 40, 24, 38, and 42 or more repetitions, respectively. As mentioned in a previous report, it is thought that the pull-off force becomes almost stable after the removal of a surface film or contamination from the surface after repeated sliding friction cycles of the FFM. In the experiment, however, it was not possible to detect what type of surface film was removed from the 4
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Fig. 6. SEM image of the surface of the copper probe after 50 cycles of FFMFDC measurements.
method are listed in Table 3. The adhesion forces of Ni, Ti, Mo, and Fe had relatively large values of 600 nN or more; those of Au, Zn and Cu were in the range 500–600 nN; and, those of Co and Ag were less than 400 nN. Fig. 6 shows an image of the surface of the copper probe after 50 cycles of FFM–FDC measurements. In the contact portion, shown by the white circle, plastic deformation occurred due to sliding friction by the FFM and, at the same time, small particles adhered to the surface. The relationship between the number of wear elements per unit area nunit in Table 2 and the adhesion force Fad measured by the FFM–FDC method in Table 3 is shown in Fig. 7. The number of wear elements per unit area tended to increase linearly with increasing adhesion force of the metal. Metals with large adhesion forces generate a relatively large number of wear elements in the elementary process of adhesive wear. The line in Fig. 7 is approximately straight and can be written as follows:
n unit = 0.018(Fad – 190)
(2)
Details of the modification of the wear equation are discussed below. In addition to measuring the adhesion forces for the nine samemetal combinations, the FFM–FDC method was used to examine the adhesion force for combinations of different metals. The different-metal combinations were selected to have different metallurgical properties, as described in Table 1. The results for the adhesion forces of the three different-metal combinations Cu–Ni, Ag–Fe, and Cu–Al measured by the FFM–FDC method are shown in Table 3. The combination of Cu and Ni, which has complete mutual solubility, showed a relatively high adhesion force of 515 nN, which is similar to that of the same-metal combination Cu–Cu. On the other hand, the combination of Ag and Fe, which have immiscible solid and liquid phases, showed a low adhesion force of 220 nN; this was the lowest among all the combinations of metals examined in the experiment. The combination of Cu and Al, which form an intermetallic compound phase, exhibited an intermediate adhesion force of 334 nN. Compared with the number of wear elements generated by the combination of Ag and Fe, measured in a previous paper, the number of wear elements nunit of Ag–Fe was 2 μm−2 [4]. The combination of metals with a low adhesion force of about
Fig. 5. (a) Pull-off forces for the same-metal combinations of titanium, iron, and zinc measured by the FFM-FDC method: (b) Same-metal combinations of molybdenum, cobalt, and silver, (c) Same-metal combinations of nickel, gold, and copper.
sliding surface. In this study, the adhesion force of the sliding surface of each metal was taken as the average of the last five values from Fig. 5 (the average of the values from 46 through 50 cycles for Ti, Mo, Fe, Au, Zn, Cu, Co and Ag, or that from 33 through 37 cycles for Ni). The adhesion forces of the nine metals Fad measured by using FFM-FDC
Table 3 Adhesion forces measured by the FFM-FDC method for nine same-metal combinations and three different-metal combinations. Metals
Mo
Ti
Co
Ni
Fe
Cu
Ag
Au
Zn
Adhesion force of same-metal combinations Fad, nN Different-metal combinations, Adhesion force Fad, nN
665 Cu–Ni 515
670
380
754 Ag–Fe 220
656
504
376
538 Cu–Al 334
500
5
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motion progresses [3,4,9,10]. Furthermore, in the plastic-deformation process of adhesive wear, the shear stress of the sliding motion distorts some grains. In our experiments, the nine metals that we used to observe the wear elements and plastic deformation beneath the sliding surface had three different types of crystal structure. Whereas slip in a single crystal due to plastic deformation occurs in a particular direction on the crystal plane, the onset of plastic deformation in a polycrystalline metal is influenced by the number of available slip systems, and several slip systems can exist, corresponding to incompatibilities at the grain boundaries under the conditions of applied stresses. As mentioned above, the most important point regarding adhesive wear is that the stress due to the load and the shear force due to the sliding motion change continuously in direction at the contact point through the plastic deformation process. Because the angle of the direction of the slip line (or band) to the sliding motion depends on the crystal structure of the grain and on the grain boundaries of adjacent grains, the angle varies from grain to grain, as previously observed [9,10]. When wear elements are generated in the crossed area of slip lines (or bands) immediately beneath the contact point, the size of the wear elements is determined by the distance d (d ≈ 2b) of the slip lines, as shown in Fig. 8. From the AFM image of iron shown in Fig. 4, the distance between the slip lines (or bands) was deduced to be of the order of a few tens of nanometers, which was about the same size as that of the wear elements [3,4]. On the basis of the above process of adhesive wear, Equation (1) was corrected according to the results of this study. In our previous model [3], we predicted that n (the number of wear elements generated at a junction) is related to various physical factors, such as the adhesion force between sliding surfaces. According to our present measurements of the adhesion force by the FFM–FDC method and our observation of the presence of wear elements, the number of wear elements per unit area nunit is given by Equation (2) as a function of the adhesion force Fad. Here, if we assume that n in Equation (1) is determined by multiplying the number of wear elements per unit area nunit by the area of each junction (≈πa2), then n increases in the same manner as nunit as the adhesion force Fad increases. Consequently, we can express the wear equation by using n = k (Fad – 190), where k is a constant (k ≈ 0.018πa2). That is,
Fig. 7. The relationship between the number of wear elements per unit area nunit and the adhesion force Fad measured by using the FFM–FDC method for nine metals.
190 nN resulted in the formation of very few wear elements during adhesive wear. The close relationship between the adhesion force, the amount of wear in adhesive wear, and the metallurgical mutual solubility of the metals is explained in Section 5. 5. Discussion The mechanism for the generation of wear elements in the elementary process and the subsequent process of accumulation of wear elements to form wear particles of adhesive wear has been previously discussed [3–6,9,10]. An overview of the elementary process, extracted from these previous reports, is shown in Fig. 8 [3]. In the process of adhesive wear, when stresses due to load and shearing forces of sliding motion are both applied to the sliding surface, the actual contact area deforms plastically because the applied stress exceeds the elastic limit of the metal. The plastic deformation that occurs in the crystal grains immediately beneath the sliding surface through the action of the compressive stress in both the vertical and the sliding directions is explained below. First, in a pin-on-block sliding system, as the pin approaches a crystal grain on the block, a compressive stress is applied to the grain in the sliding direction, and slip due to plastic deformation occurs in the grain located in front of the pin. The first slip from the surface then occurs in direction (a) in Figs. 3, 4 and 8. As the pin specimen slides further, the plastically deformed area moves through the grain and, at the same time, the density of the slip lines increases. Secondly, as the contact point moves over the grain, a compressive stress due to the load acting perpendicularly to the sliding surface causes slip in a different direction [direction (b) in Figs. 3, 4 and 8]. Finally, as the pin passes through, the slip lines (or bands) of (a) and (b) cross one another. In this process, as shown in Figs. 3 and 4 and in other reports, slip lines (or bands) are formed parallel to one another immediately beneath the sliding surface. However, from previous observations, in many cases the slip lines run in a curve or run along grain boundaries as the sliding
V=
1 k (Fad − 190) ⎛ b ⎞3 ⎛ P·ℓ ⎞ · · ·⎜ ⎟ λ 3 ⎝ a ⎠ ⎝ pm ⎠
(3)
From this wear equation, the amount of adhesive wear can be expressed by using the physical adhesion factor k (Fad – 190), the chemical factor (λ), the size factor (b/a), the applied load (P), the sliding distance (ℓ), and the yield stress of the softer material (pm). In this study, only the physical properties were investigated, but previous reports have discussed the chemical factor λ and have reported that chemical activity determines whether the wear mode is mild or severe [3,13,14]. As mentioned in the previous papers, the adhesion force affects the aggregation of wear elements, while the chemical activity such as chemisorption of surrounding molecules is more important in the aggregation process of the wear elements to form transfer particles
Fig. 8. An overview of the elementary process and a schematic model for the generation process of wear elements at a junction; (a) shows the first slip and (b) shows the second slip beneath the sliding surface; d is the distance between slip lines (or bands). 6
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and wear particles in the process of adhesive wear. In Holm's equation V = Z·(P·ℓ/pm), the wear coefficient Z is interpreted as the probability of removal of small debris from the junction, and small debris particles are removed directly from the junction to form a wear particle [2]. On the other hand, we can interpret the wear coefficient Z chemically and physically as a factor that is determined by the adhesion force Fad, the chemical properties, and the ratio of the size of the wear elements to the size of the junction (b/a). In other words, we propose that Holm's wear coefficient Z is not a probability but a physicochemical property of the material. As shown in Fig. 7 and by Equation (3), it is interesting to note that wear elements will not be generated if the adhesion force is less than 190 nN. In combination with very low adhesion force of less than 190 nN, shear of the junction due to sliding motion occurs preferentially at the interface of two metals rather than in the bulk, the generation of wear elements is limited, and the amount of wear is very low. However, very few metals have low adhesion forces of less than 190 nN under normal conditions in the absence of a lubricant. The FFM–FDC method is important for the measurement of adhesion forces of actual sliding surfaces. When considering adhesive wear, it is necessary to know not only the adhesion force between ideal and elastic contact surfaces, such those as calculated by the Johnson–Kendall–Roberts, Derjaguin–Muller–Toporov, or other approximation models [20–22], but also the adhesion force between actual sliding surfaces. When discussing adhesive wear based on the adhesion force, adhesion essentially occurs at the plastically deformed junction of the sliding surfaces, and the adhesion force depends on two factors: the size of the real contact area resulting from plastic deformation, and the interatomic cohesion force in the real contact area. As shown in Tables 1 and 3, for example, the softest metal Zn has the lowest Vickers hardness of 36 and the lowest cohesion energy of 130 kJ/mol among the nine metals that we studied [11], whereas it has a relatively large adhesion force of 500 nN. On the other hand, the hard transition metal Mo, which has a hardness of 232 and a high cohesion energy of 648 kJ/mol, exhibits an adhesion force of 665 nN. Zn exhibits an unexpectedly high adhesion force, comparable to that of hard metals with higher cohesion energies. Because Zn is a soft metal, it has a relatively large real contact area (about six times that of Mo) under the applied load in adhesive wear. Therefore, although Zn has a low cohesion energy, it has a relatively large adhesion force due to the large real contact area, because the adhesion force of a sliding surface is related to both the number of interatomic bonds and to the strength of cohesion of the metals in the real contact area. Furthermore, as shown in Fig. 7, it is considered that a difference in the crystal structures of metals does not affect the relationship between the number of wear elements and the adhesion force. In general, the amount of wear is closely dependent on the combination of metals that are subject to the adhesive wear. To explain the effect of changing the combination of metals on their wear properties, one of us previously investigated the relationship between the amount of wear and the metallurgical mutual solubility of two contacting metals. Fig. 9 shows a typical wear–distance curve for three combinations of metals with different mutual solubilities, the data are extracted from the previous report [15]. The wear test was performed by using a pinon-disk apparatus (using a pin with flat ended shape) with a sliding velocity of 80.8 cm/s and a load of 11.1 N. The amount of wear correlated to the mutual solubility. That is, Cu–Ni, which have complete mutual solubility, exhibited a large amount of wear, and a considerable amount of transfer occurred between the sliding surfaces. On the other hand, Ag–Fe, which are immiscible in the solid and liquid phases, showed very little wear and a produced a very small number of transfer particles, whereas Cu–Al, which form an intermetallic compound system, showed intermediate wear [15–17]. In the present experiments on the adhesion force measured by the FFM–FDC method, Cu–Ni, Cu–Al, and Ag–Fe showed adhesion forces of 515, 334, and 220 nN, respectively. The order of these values of the adhesion force
Fig. 9. Typical wear-distance curve for three combinations of metals with different mutual-solubility properties: Cu–Ni, Al–Cu, and Ag–Fe. The wear tests were performed by using a pin-on-disk apparatus at the sliding velocity of 80.8 cm/s and the load of 11.1 N without lubricant (data are extracted from Ref. [15]).
Fig. 10. The relationship between the wear of the pin and disk and the adhesion force measured by the FFM–FDC method for three different-metal combinations with different metallurgical mutual solubilities.
[(Cu–Ni) > (Al–Cu) > (Ag–Fe)] corresponds to that of the amount of adhesive wear of these three combinations. Fig. 10 shows the relationship between the adhesion force of the combinations and the wear of the pin and disk. Here, the amount of wear was determined after a sliding distance of 0.97 km. The amount of wear in adhesive wear depends directly on the adhesion force of the sliding surface. When a metal combination has a large adhesion force and a high mutual solubility, its amount of wear increases because the wear elements are actively generated and growth of transfer particles occurs on the sliding surface. We assume, therefore, that the previous qualitative description of the effect of the mutual solubility of the sliding components on the adhesive wear translates into the effect of adhesion force between the sliding surfaces. In Fig. 10, the wear was very small when the adhesion force decreased to about 200 nN, such as that shown by the combination Ag–Fe (Fad = 220 nN). This is in good agreement with the previous reports that wear of the combination of Ag and Fe is very low and few wear elements are generated shown in the AFM image of Ag–Fe (the value of nunit for Ag–Fe was 2 μm−2) [4,15–17]. 6. Conclusions The number of wear elements generated through the elementary process of adhesive wear was examined by means of nanoscale observations by AFM. Nanoscale observations by SEM and AFM clarified 7
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the precise mechanism of the generation of wear elements and their exact size through observations of the slip lines (or bands) caused by plastic deformation. Furthermore, the adhesion force of the actual sliding surface of metals was measured by using the FFM–FDC method. From these results, the relationship between the number of wear elements generated at a junction n and the adhesion force Fad was shown to be n = k (Fad – 190); moreover, a correction of the wear equation to account for adhesive wear was proposed. Measurements of the adhesion forces of combinations of different metals with different metallurgical mutual solubilities revealed that the amount of adhesive wear was directly dependent on the adhesion force of the sliding metals. The values of the adhesion force showed the relationship (Cu–Ni) > (Al–Cu) > (Ag–Fe); this order also corresponds to that of the amount of adhesive wear and to that of the mutual metallurgical solubility of the three combinations.
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Acknowledgements The authors gratefully acknowledge the help of Dr. T. Ohmori, Y. Tsuchiya, Y. Ichimoto, H. Kobayashi, T. Uehara, H. Ueda, and T. Kitakoga (all of Chiba University); we also thank Hitachi Power Solutions Co., Ltd. for the SEM observations. References [1] R. Holm, Über metallische kontaktwiderstände, Wiss. Veröff. Siemens-Werk 7 (2) (1929) 217–258. [2] R. Holm, Electric Contacts: Electric Contacts: Theory and Application, Springer, Berlin, 1946. [3] H. Mishina, A. Hase, Wear equation for adhesive wear established through elementary process of wear, Wear 308 (2013) 186–192 https://doi.org/10.1016/j. wear.2013.06.016. [4] A. Hase, H. Mishina, Wear elements generated in the elementary process of wear, Tribol. Int. 42 (2009) 1684–1690 https://doi.org/10.1016/j.triboint.2009.02.006. [5] H. Mishina, H. Iwase, A. Hase, Generation of wear elements and origin of tribomagnetization phenomenon, Wear 269 (2010) 491–497 https://doi.org/10.1016/j. wear.2010.05.004. [6] H. Mishina, Magnetization of ferromagnetic material surfaces by tribological
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