Wear 386–387 (2017) 49–57
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Severe plastic deformation of four FCC metals during friction under lubricated conditions A. Moshkovicha, I. Lapskera, Y. Feldmanb, L. Rapoporta, a b
Holon Institute of Technology, Department of Science, Holon, Israel Weizmann Institute of Science, Department of Surface and Interface, Rehovot, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords: Friction Wear Plastic deformation Grain size
The eﬀects of stacking fault energy (SFE) on the sliding friction and wear of FCC metals (Ag, Cu, Ni, and Al) against Type 1040 steel were studied using a pin-on-disk rig. Friction results were presented as Stribeck curves. The transition from elasto-hydrodynamic lubrication (EHL) to boundary lubrication (BL) was studied. Friction and wear coeﬃcients and the temperature near the contact were measured during the tests. It was found that friction in the BL region is characterized by formation of a nanocrystalline structure with a steady-state hardness (stress) for all studied metals. It was shown that the wear coeﬃcient correlates well with the steady-state hardness/stress (σs) and SFE. It was found that SFE and melting temperature play import roles in the wear process. However, the grain size or contact temperature do not directly indicate a decreasing or increasing wear rate. Friction and wear of the investigated FCC metals is mainly determined by deformation hardening and dynamic recovery. The annihilation of dislocations occurs much more easily under Ni friction (high SFE) than the other metals. Annealing of Ni and Cu at relatively low temperature (~0.14 Tm) suggests that a recovery process occurred during frictional contact. The diﬀerence in behavior of Al compared with the other FCC metals is explained by its low melting temperature.
1. Introduction Severe plastic deformation (SPD) during friction and wear was studied over a long period of time. The dislocation structure and hardening of surface layers during mainly unlubricated friction were analyzed [1–10]. Application of preliminary hardening by grain reﬁnement before the friction tests signiﬁcantly improves the tribological properties of rubbed surfaces [11–15]. It is known that SPD methods provide formation of ultraﬁne grained (UFG) and nanocrystalline (NC) structure (d = 100–200 nm) with relatively high strength [16–18]. Diﬀerent methods of SPD such as equal channel angular pressing or extrusion (ECAP or ECAE), high-pressure torsion (HPT), cold rolling, dynamic plastic deformation (DPD), accumulative roll-bonding (ARB) and some others have been widely utilized both in industry and research laboratories for increasing the mechanical properties of metals and alloys. One of the dominant parameters studied in the SPD processes is grain size. The grain size usually decreases with increasing strain and obtains steady state value, ds that is preserved with further straining. The parameters responsible for the formation of steady-state grain sizes, ds and steady-state hardness, Hs, have been analyzed in several works
Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.wear.2017.05.018 Received 14 December 2016; Received in revised form 28 May 2017; Accepted 29 May 2017 Available online 31 May 2017 0043-1648/ © 2017 Elsevier B.V. All rights reserved.
[19–25]. Steady-states grain size, ds and Hs in accordance with some physical parameters such as melting temperature, diﬀusivity, valence electrons, speciﬁc heat capacity and activation energy for self-diﬀusion, shear modulus and stacking fault energy (SFE) were investigated [26–28]. It was concluded that the steady-state grain size is insensitive to the melting temperature and no systematic correlations exist with the SFE and valence electrons for pure metals. From other side, the important role of SFE in the formation of dislocation structure and hardening was noted [29–32]. Reducing the SFE inhibits dislocation activities and promotes deformation twinning, leading to more signiﬁcant grain reﬁnement and more homogeneous microstructures. It is seen that the correlations between the steady-state grain size and hardness from one side and the physical properties of pure metals after SPD processing are not well understood. Recently, a similarity between the eﬀect of SPD obtained by diﬀerent methods of grain reﬁnement from one side and friction and wear of Cu in lubricant conditions from other side was shown . SPD of surface layers under friction of Cu in the conditions of boundary lubrication is also accompanied by preservation of steadystate grain size and hardness [9,10,33,34]. Steady state during large plastic deformation as a balance between dislocation accumulation and
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μm. The eﬀects of load on the friction coeﬃcient, wear coeﬃcient, and average temperature near the interface were studied. Since the metals with diﬀerent virgin hardness were compared, non-dimensional Archard wear coeﬃcient as: k= W·H or (V·H)/(N·S), where, W is dimensional wear coeﬃcient (mm3/N m), V is wear volume, H – hardness after friction in the BL region, N – load and S – sliding way. The running-in process occurred at load of 8 N and was continued during about 150 min while close to 90% of the surface of pin was in contact. Sliding velocity was constant, 0.37 m/s (300 rpm). Since Ni and Cu samples are about two times harder than Ag and Al after annealing, a loading after the running-in process was continued by steps of 25 N and 12.5 N per 20 min for two pairs of materials, respectively. The load was increased by steps while the BL region is obtained. Then, the eﬀect of load in the BL region (3–6 points) was analyzed. Six drops of PAO-4 oil with viscosity of 18 mPa s at 40 °C (PAZ) were supplied in the interface each minute. Pure oil was chosen in order to avoid the eﬀect of additives in oil on friction and wear of surface layers. Wear particles obtained in the BL region were collected, rinsed, dried and then studied by scanning electron microscopy (SEM). The wear coeﬃcient was calculated based on the measurement of wear by the displacement sensor. The application of this sensor was associated with two main causes: the wear of soft pin-hard disk pair is determined mainly by the wear of pin. Moreover, preliminary analysis of rubbed surfaces after friction in the BL region showed the limited number of thin transferred ﬁlms in comparison to that observed during dry friction. Therefore, it was found the displacement sensor can be used for the measurement of wear loss during friction of studied contact pairs rubbed in lubricant conditions. A standard deviation was not more than 10%. The friction power, h, was calculated as: h=μ·N·v, where μ is the friction coeﬃcient, N is the load, and v is the sliding velocity. Five friction tests were performed for each of the materials in each load. In order to evaluate the transition from the EHL to BL region the Stribeck curves were analyzed similar to that used in our previous works (e.g. [14,34,35]).
recovery appeared in the SPD processing. It is expected that a similar balance should be performed during friction and wear. However, the balance between deformation hardening and dynamic recovery from one side and friction and wear properties from other side is not well understood up to now. It is expected that structural balance during friction is determined by some physical parameters similar to that observed in SPD processing. In this case it is important to analyze the inﬂuence of the structural parameters as homological temperature, stress state, strain and stacking fault energy, (SFE) on the steady-state grain size and hardness/stress during friction, as well as a connection between these parameters and friction and wear properties. The results of this analysis will be compared with the steady-state grain size and hardening obtained during diﬀerent SPD processes. A lot of publication on the deformation hardening and damage development of studied metals under diﬀerent SPD processes facilitates a comparison with observed friction results. The purpose of the present work is threefold: First, the study of friction and wear properties of pure FCC metals such as: Ag, Cu, Ni and Al in the transition from elasto-hydrodynamic lubrication (EHL) to boundary lubrication (BL); second, the evaluation of grain size and hardness after friction in the BL region; third, a correlation between some physical parameters and steady-state grain size and hardness during friction in the BL region; fourth, the connection between structural parameters and friction and wear properties. The main assumption of this work is that the friction and wear properties of FCC metals in the BL region are associated with the plastic deformation and damage development of surface rubbed layers but not with a formation of triboﬁlms as usually observed in the application of diﬀerent additives to pure oil. 2. Experimental procedure 2.1. Friction and wear tests
2.2. Structure and morphology of surface layers after friction in the BL region
All friction tests were conducted under laboratory conditions (temperature, T = 25 °C, humidity ~ 50%) using home -made pin-ondisk rig expended in our previous works (e.g. [14, 34, 35]). Flat shape pins were chosen in order to provide constant pressure with time at each load. It is especially important in the analysis of the eﬀect of load on structure evaluation with preservation of constant pressure with time. In order to avoid the eﬀect of solute atoms on plastic deformation, the grain size and hardness of pure FCC metals such as Ag, Cu, Ni and Al were chosen as the materials for pins. The hardness of samples was measured using microhardness tester Matsuzawa MXT 50 (Japan) at loads of 10–25 g. The hardness was evaluated at virgin state, Hi, (after annealing) and after friction in the BL region, Hf. Some parameters as melting point, Tm, shear modulus, G, burger vector, b, stacking fault energy, γ, purity, %, thermal conductivity, kt, and virgin hardness Hi, of studied materials are presented in Table 1. The pins rubbed against the steel disks (AISI 1040) hardened up to HRc = 45. The surface of pin and disk were originally ground to roughness, Ra ~ 0.08 μm. After grinding the pins were annealed at the constant homological temperature, T/Tm = 0.6 during 2 h in vacuum, T is the temperature of annealing. The average grain size was close to 30
The surface morphology of pins and disks after the friction tests was analyzed by SEM. Diﬀraction measurements were carried out in reﬂection geometry using a TTRAX III (Rigaku, Japan) diﬀractometer equipped with a rotating Cu anode, operating at 50 kV/200 mA. A bent graphite monochromator and scintillation detector were aligned in the diﬀracted beam. θ/2θ scans were performed under specular conditions in the Bragg-Brentano mode with variable slits. The 2θ scanning range was 35–150 degrees with step size 0.02 degrees and scan speed 0.5 degree per minute. Phase analysis was performed using the PDF-4+ 2015 database (ICDD) and Jade 9.5 software (Materials Data, Inc.), where peak positions and widths were determined by a self-consistent proﬁle–ﬁtting procedure. Average crystallite size (coherent scattering length) and micro-strain were estimated in a Williamson – Hall plot and then reﬁned using the Whole Pattern Fitting /Rietveld reﬁnement module of Jade 9.5 in conjunction with the ICSD 2015 (FIZ, Karlsruhe) database. SEM was used in order to study the morphology of wear surfaces.
Table 1 Melting point, Tm, shear modulus, G, burger vector, b, stacking fault energy (SFE), γ, purity, wt.%, thermal conductivity, kt, and virgin hardness, Hi. Metal
Tm, K [36,37]
G, GPa [36,37]
b, nm 
γSFE, mJm−2 [39,40]
Purity, wt. %
Ag Cu Ni Al
1234 1357 1728 933
27 48.3 75 26.2
0.2889 0.2556 0.2492 0.2864
16 45 125 166
99.95 99.999 99.999 99.9965
406 401 91 205
0.27 ± 0.01 0.44 ± 0.02 0.78 ± 0.03 0.19 ± 0.02
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Table 3 The average values of steady-state grain sizes, ds and steady-state hardness/ stress, σs = Hf/3 after friction in the BL region. Metal
ds, nm (XRD)
Al Ag Cu Ni
440 ± 75 33 ± 3 63 ± 1 140 ± 3
0.28 ± 0.03 0.78 ± 0.12 1.5 ± 0.2 2.63 ± 0.25
0.09 0.26 0.5 0.88
. Steady-state grain sizes, ds measured using XRD analysis, hardness after friction, Hf and stress, σs = H/3 are presented in Table 3. 3.2. Morphology of rubbed FCC metals and wear debris after friction in the BL region The typical surfaces of pins and wear particles were analyzed after friction in the BL region. The surfaces are presented in accordance with increasing the stacking fault energy: Ag, Cu, Ni, Al. A SEM image of an Ag pin is outlined in Fig. 2. Friction of Ag is accompanied by shearing and delamination of thin surface layers. Magniﬁed image demonstrates one of the possible mechanisms of wear particle formation due to joint of pores and cracks. Morphology of the Cu surface after friction in the BL region is shown in Fig. 3. Although the load conditions are principally diﬀerent under friction of Ag and Cu, the wear mechanisms are quite similar. The morphology of friction surface of Ni sample is principally diﬀerent, (Fig. 4). As seen, the friction of Ni is accompanied by ploughing, severe plastic deformation with crack formation and delamination in small contact spots. Magniﬁed image demonstrates the damaged place with small wear particles inserted. The morphology of Al surface after friction in the BL region is outlined in Fig. 5. Although the SFE of Al is maximal (γ = 166 mJm−2) the morphology of the surface is very similar to the metals with relatively low SFE (Ag, Cu). The wear particles after friction in the BL region for all studied materials are presented in Fig. 6. Many wear particles are from the relatively soft pins. The EDS and XPS analyses of rubbed surface and wear particles will be presented in our future work. It should be noted that many wear particles are found in an agglomerated state and a deﬁnition of a real size of single wear particles is very complex problem. However, the magniﬁed images show the size of small wear particles is changed from about 200 nm to 5–6 μm. Statistical analysis of wear particle distribution will allow more detail characterization.
Fig. 1. The typical Stribeck curves in the transition from EHL to BL region for studied metals. The insert demonstrates the details of the friction behavior in the BL region (explanation below). In the X-axis, η - viscosity, v - sliding velocity and P - contact pressure.
3. Results 3.1. Friction experiments The typical Stribeck curves of the studied metals are shown in Fig. 1. Friction coeﬃcient in the EHL region for all metals except Al is close to 0.02. With increasing load the friction coeﬃcient is increased to μ ~ 0.1 in the BL region. Maximal value of the friction coeﬃcient is observed for Al, μ ~ 0.16. It is clear that the transition to BL region occurs at diﬀerent loads for studied materials. The results of friction tests as the maximal values of load, N, in the BL region, coeﬃcient of friction (CoF), bulk temperature of pin Tb,average temperature of contact, Tavr, Tavr = Tb+Tf; where Tf is the calculated value of ﬂash temperature, (see calculation below) and homological temperature, Tavr/Tm are presented in Table 2. It is seen that maximal and minimal loads are observed for Ni and Al, respectively. Maximal values of the friction coeﬃcient and wear coeﬃcient appear for Al while the best tribological properties are revealed for Ni. The temperature of rubbed surface is an important parameter both in friction and in plastic deformation of surface layers. Although the maximal loads in the BL region are diﬀerent for studied materials, the homological temperature is close the same (~ 0.3) except for Al (> 0.3). Since temperature plays an important role in the friction and wear behavior, the ﬂash temperature, Tf, was calculated. For simplicity and with the suggestion that contact heights are found in the direct contact in the BL region, the average ﬂash temperature developed by Archard  was used:
Tf = μ
3.3. Interaction between friction, wear and structural parameters
Friction and wear properties mainly depend on the grain size, steady-state stress and SFE. The dependences of the friction and wear coeﬃcients on steady-state stress, σs, are shown in Fig. 7. It is seen that both the friction and wear coeﬃcients are decreased with an increase in the steady-state stress. For instance, maximal and minimal values of the wear coeﬃcients are revealed for Al and Ni,
where μ is the friction coeﬃcient, Hf hardness after friction, N is - load, kt is the thermal conductivity. The steady-state stress, σs was calculated based on the value of hardness after friction in the BL region, σs= H/3
Table 2 Maximal load in the BL region, N, coeﬃcient of friction, CoF, wear coeﬃcient, k, real area of contact, A= N/Hf, bulk temperature of pin, Tb, average temperature of contact, Tavr, and homological temperature of rubbed metals in the BL region, Tavr/Tm.
Al Ag Cu Ni
Wear coeﬃcient, k
194 ± 34 223 ± 23 1273 ± 57 1820 ± 200
0.16 ± 0.02 0.1 ± 0.02 0.09 ± 0.01 0.08 ± 0.02
1.05·10−6 7.05·10−7 2.99·10−7 1.88·10−8
0.69 0.29 0.85 0.69
317 312 356 376
332 320 382 530
0.36 0.26 0.28 0.31
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Fig. 2. SEM image of Ag surface after friction in the BL region (a); magniﬁed image of wear particles formation (b), N = 212 N, μ = 0.09.
Fig. 3. SEM image of Cu sample after friction in the BL region (a); magniﬁed image of shear bands on the rubbed surfaces (b), N =1285 N, μ = 0.08.
Fig. 4. SEM image of Ni surface after friction in the BL region (a); magniﬁed image of the wear track (b), N=1640 N, μ = 0.09.
Fig. 5. SEM images of the Al surface after friction in the BL region (a); magniﬁed image of the wear track (b), N= 202 N, μ = 0.16.
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Fig. 6. SEM images of wear particles after friction in the BL region: (a, b) – Ag; (c, d) – Cu, (e, f) – Ni; (g, h) – Al.
In this case, Ni with a larger SFE value in comparison to Ag appears to have the best wear properties. Non-expected results were obtained for the connection between the friction power and the wear coeﬃcient. Usually, an increase of the friction power is accompanied by increasing
respectively. Inverse proportional connection is observed between the wear coeﬃcient, friction power and non-dimensional SFE parameter: the larger the value of the SFE and friction power, the smaller the value of the wear coeﬃcient, except Al, (Fig. 8). 53
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Fig. 11. The dependence of the steady-state stress on melting temperature.
Fig. 7. The dependences of the friction and wear coeﬃcients on the steady-state (saturated) stress.
wear loss with a loading for deﬁnite material. However, in the comparison of metals with diﬀerent SFEs and contact temperature a similar connection is not observed. Ni with the maximal friction power reveals a minimal value of the wear coeﬃcient. The eﬀect of steady-state grain size, ds, and homological temperature on the wear coeﬃcient is outlined in Fig. 9. Inverse proportional connection appeared between ds and the wear coeﬃcient, except for Al: the smaller the grain size (Ag), the larger the wear rate. It should be noted that all FCC metals, except Al rubbed at close to the same homological temperature (≤ 0.3) in the BL region and thus it can be concluded that at the close the same value of homological temperature the wear coeﬃcients are diﬀerent. Then, the eﬀect of melting temperature on the wear coeﬃcient was evaluated, (Fig. 10). It can be seen that with Tm increasing the wear coeﬃcient is decreased. It is interesting that increasing both Tm and Tavr leads to decreasing the wear loss. Usually, increasing the temperature of contact during friction indicates an increasing the wear rate. However, a similar connection is absent in the comparison of the wear behavior of FCC metals in the BL region. Since the stress and melting temperature are important parameters in plastic deformation during friction, the dependence of steady-state stress on the melting temperature was considered, (Fig. 11). It is seen that increasing the Tm leads to an increase the value of steady-state stress.Since the SFE strongly aﬀects plastic deformation and thus the grain size, the interaction between these parameters was considered, (Fig. 12). It may be seen that Ag with a minimal SFE value reﬁned up to minimal value of grain size during severe plastic deformation in the BL region, while the maximal value of grain size appears for Al with maximal value of the SFE. The interaction between stress (ΔH = Hf – Hi) and grain size after friction in the BL region (Hall-Petch equation) was evaluated, Fig. 13. Inverse proportional correlation is observed: Ni with grain size, ds = 140 nm demonstrates maximal hardness in
Fig. 8. The connection between the SFE (γ), friction power (h), and the wear coeﬃcient (k).
Fig. 9. The eﬀect of the steady-state grain size, ds and the homological temperature on the wear coeﬃcient.
Wear coeﬃcient, k
1.E-09 0 W=f(Tm)
Tavr, Tm, K
Fig. 10. The eﬀect of contact and melting temperatures on the wear coeﬃcient. Fig. 12. The interaction between the grain size and the stacking fault energy, γ.
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2000 1800 1600 1400 1200 1000 800 600 400 200 0
dislocations and subsequent rearrangement by dynamic recovery . Dynamic recovery by the climbing and cross-slipping is a thermally activated processes of the rearrangement and annihilation of dislocations [49–51]. Because high value of SFE in Ni, it is natural to suggest that the recovery process for this metal will be achieved much more easily than for Ag.
4.2. Deformation hardening-softening model of friction for FCC metals
Ag Al 0.00
Formation of the nanocrystalline structure during friction in the BL region as well as in other grain-reﬁned processes obtained by SPD should decrease the temperature of the recovery. Therefore, the jumps of the friction force under rubbing of Ni and Cu in the BL region can be associated with the process of recovery in surface layers at relatively high contact temperature. In order to better understand the process of recovery occurring in the maximal contact temperature during friction in the BL region, heating of samples by the steps of 20 ° C with a measurement of hardness at each step was performed. It was found that the hardness of grain-reﬁned Ni after friction in the BL and following annealing at T=240 °C (~ 0.14Tm) decreased from 2700 MPa to 1700 MPa. At similar homological temperature the hardness of Cu was decreased from 1450 MPa to 850 MPa. For comparison, the dynamic recrystallization of reﬁned Cu grains after ECAP process observed at T= 153 °C . It is suggested that dynamic recrystallization at relatively high contact temperature in the BL region for metal with high SFE (Ni) is accompanied by softening, leading to an increase of the real contact area and much easier shearing of surface layers. At these conditions the friction force and the temperature of the rubbed pair are decreased. As a result of these processes the viscosity of oil and thus the thickness of lubricant ﬁlm are increased. With following loadings during one-two steps the temperature increases again up to the maximal value and the recovery process is repeated leading to jumps in the values of friction force, (Fig. 1, inset). This phenomenon in Ni but with much smaller jumps is observed for Cu, while these jumps are absent under loading of Ag. Deformation hardening-softening process has been described in one of the most essential approaches – Kocks–Mecking model (K–M) :
ds-1/2, nm-1/2 Fig. 13. The dependence of the steady-state hardness variation on the inverse of the square root of the grain size.
comparison to Ag, ds = 35 nm. As it can be seen, Hall-Petch equation does not performed under friction of FCC metals in the BL region. It is suggested that the stress in the surface friction layers is much complex parameter. 4. Discussion 4.1. The eﬀect of the SFE on the structure, friction and wear It is known that the steady-state grain size, ds, and hardness, Hf (σs) are obtained at large strain in diﬀerent SPD processes [21–29] as well as during friction in the BL region . The experiments with FCC metals indicate a preservation of steady-state grain size and hardness (saturated stress) under friction in the BL region. Steady-state grain size and stress are determined mainly by the temperature and stacking fault energy. The direct correlation observed between grain size and SFE corresponds to the reported experimental results on the connection between ds and γSFE [43,44]. This connection is presented as:
γ q d min = A⎛ ⎞ b ⎝ Gb ⎠
dρ = MT (k1 ρ1/2 − k2 ρ) dε
Where A is the parameter depending on the structure and q = 1 , or q = 0.5 . In our experiments q = 1. Grain reﬁnement usually increases hardness and strength and sometimes improves ductility during tension/compression or fatigue tests at room temperature. In friction, especially when the contact temperature is relatively high, the minimal grain size does not provide high level of hardness/stress and minimal value of wear loss. It is expected that wear of FCC metals is determined both by deformation hardening and dynamic recovery in surface layers depending strongly on the SFE and temperature. It is known that γSFE is mainly responsible for cross slip as one of the dominant rate controlling mechanisms in the plastic deformation of FCC metals [32,44–46]. The reduction in the SFE promotes the splitting of the full dislocations into two partials containing a wide stacking fault ribbon and thus limits cross-slip. As a result of this process high dislocation density, minimal grain size, and maximal deformation hardening appeared during SPD of FCC metals restricting thus the dynamic recovery. For instance, the deformation hardening of Ag (ΔH = Hf – Hi) = 510 MPa in comparison to Al (ΔH=90 MPa) when the loads in the BL region for these materials are close the same. It is expected that strong hardening of Ag accompanied by the localization of deformation leads to formation of shear bands and ﬁnally to instability associated with crack development and wear particle formation. It was shown that low SFE metals develop more shear bands and at an earlier stage during deformation . In fact, instability in plastic deformation during friction of Ag is accompanied by the maximal wear rate. With increasing SFE (Cu, Ni) the wear rate is decreased. For these metals dislocation slip is the primary mode of deformation and grain reﬁnement is achieved through the accumulation of
where k1 and k2 are the coeﬃcients associated with the athermal storage of moving dislocations and dynamic recovery, respectively, ρ is density of dislocations. The k2 is temperature and strain rate dependent, and therefore the recovery mechanism is thermally activated. Thus, the evolution of dislocation density with strain depends generally on two components: multiplication and annihilation of dislocation. The annihilation of dislocation (hardness decreasing) at relatively high contact temperature occurs much more easily for metals with high SFE (Ni). A decrease in SFE reduces the dislocation annihilation rate during severe plastic deformation of metals . In this case, the localization of plastic deformation occurs much slower and therefore the damage develops much earlier. It is clear that the evaluation of dislocation is connected to variation of stress. This connection can be presented as :
σtotal ∞ρtotal = ρstorage − ρannihilation
Thus, with ρannihilation increasing the critical stress for wear particle formation realized much later. In the development of the K–M model, two-dimensional and three-dimensional dislocation models have been proposed [56,57]. Recently, based on the published works some new models describing the dynamic recovery separately in the walls and cells of grains during SPD processes were developed [55,58,59]. Some new constitutive relationships for the deformation of metals were proposed [60–62] Although the recent models agree well with the experimental data, the quantitative description of the dynamic recovery remains to be a complex problem. 55
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The wear coeﬃcient correlates well with the steady-state hardness/ stress (σs), the SFE and melting temperature: increasing the σs, Tm and SFE leads to decreasing the wear coeﬃcient. 2. Direct correlation between the SFE and steady state grain size, ds was observed: the smaller the value of SFE, the smaller is the grain size. It was shown that friction and wear behavior of FCC metals rubbed in the BL region does not connect directly to the minimal grains size or maximal contact temperature. 3. The behavior of FCC metals under friction in the BL region is mainly determined by deformation hardening and dynamic recovery in surface layers similar to that occurring during tension/compression and fatigue of grain reﬁned by SPD processes. The recovery process was revealed as the jumps in the friction force occurred during friction of Ni and Cu rubbed in the BL region. The recovery process is conﬁrmed by annealing the samples after SPD in the BL region. The hardness was signiﬁcantly decreased at relatively low temperature (~0.14Tm). This phenomenon was not observed for Ag. The behavior of Al is diﬀerent than the other FCC studied metals and can be explained by low melting temperature.
Table 4 Comparison of the grain sizes, hardness and yield/steady-state stress obtained during SPD by diﬀerent techniques and after friction of FCC metals. Method
Our test ECAE ECAP HPT Our test HPT Our test HPT Our test HPT
Cu Cu Cu Cu Ni Ni Ag Ag Al Al
Number of passes
4 8 5
Grain size, XRD, nm 63 58
Grain size, TEM, nm
220 ~200 120
140 170 33 220 440 1500
Hardness, Hf, MPa
500 415 ~420 ~420 880
  
~1400 2630 3020 780 940 280 310
 260  90 
One of the important parameters responsible for dynamic recovery is melting temperature. It was shown that an increase in the dislocation density of cell walls with increasing Tm (or decreasing SFE) improves the generation of dislocations . Hence, the model showed that the intrinsic parameters such as SFE and Tm, not only inﬂuence the annihilation of dislocations in the cell interiors and walls, but also aﬀect the work hardening implicitly. Our experiments conﬁrm the eﬀect of melting temperature on the wear coeﬃcient: Ni with maximal melting point demonstrates maximal hardness. From other side, a decrease of Tm for Al is similar to the eﬀect of SFE increase. Then, the morphology of friction surface for Al similar to the materials with relatively low SFE value (Ag, Cu) becomes more clearly, Fig. 5. Moreover, it is the reason that practically all studied dependences do not include the Al point. The study of wear particles showed that many particles are found in the agglomerated state. However, the magniﬁed images of single wear particles revealed that the small size of nanoparticles is changed from 200 nm to 5–6 μm. The dominant nano-sized wear particles were recently also revealed in pin-on-disk tests of wheel–rail contacts . It is suggested that the wear behavior of these studied metals is mainly determined by the SFE, melting temperature and steady-state stress. It is the main reason of the absent of correlation between the wear rate and simple parameters as grain size, contact temperature and friction power under friction of studied FCC metals. Therefore, a comparison of friction and wear behavior of FCC metals and alloys in the BL region should be based on the analysis of deformation hardening and dynamic recovery. A comparison between the structural parameters obtained by different methods of grain reﬁnement and SPD after friction of FCC metals in the BL region is presented in Table 4. Much larger information was found for steady-state hardness of Cu. It is seen that the steady-state grain size and hardness of FCC metals after SPD obtained by diﬀerent technique and friction in the BL region are close the same. Therefore the values of grain size and hardness/ stress indicate a similarity the deformation hardening and dynamic recovery occurring during reﬁnement by SPD obtained by diﬀerent techniques or friction in the BL region. At this case, the values of steadystate hardness after SPD in the BL region can be used, for example, in Archard equation for the evaluation of wear loss.
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