Severe plastic deformation of four FCC metals during friction under lubricated conditions

Severe plastic deformation of four FCC metals during friction under lubricated conditions

Wear 386–387 (2017) 49–57 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Severe plastic deformation ...

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Wear 386–387 (2017) 49–57

Contents lists available at ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

Severe plastic deformation of four FCC metals during friction under lubricated conditions A. Moshkovicha, I. Lapskera, Y. Feldmanb, L. Rapoporta, a b

MARK



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] (L. Rapoport).

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 [33]. 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 [38]

γSFE, mJm−2 [39,40]

Purity, wt. %

kt, W/m·K

Hi, GPa

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

50

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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)

Hf, GPa

σs, GPa

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

[42]. 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 [41] was used:

Tf = μ

3.3. Interaction between friction, wear and structural parameters

πHf N

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,

(1)

8kt

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

Load, N

CoF

Wear coefficient, k

A, mm2

Tb, K

Tavr, K

Tavr/Tm

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-06

Al

Al

Ag

Ag

Cu

Cu 1.E-07

1.E-08

Ni

Ni

1.E-09 0 W=f(Tm)

500 W=f(Tavr)

1000

1500

2000

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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MPa

A. Moshkovich et al.

2000 1800 1600 1400 1200 1000 800 600 400 200 0

dislocations and subsequent rearrangement by dynamic recovery [48]. 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.

Ni

Cu

4.2. Deformation hardening-softening model of friction for FCC metals

Ag Al 0.00

0.05

0.10

0.15

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 [52]. 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) [53]:

0.20

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 [33]. 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 ⎠

(2)

dρ = MT (k1 ρ1/2 − k2 ρ) dε

Where A is the parameter depending on the structure and q = 1 [43], or q = 0.5 [44]. 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 [47]. 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

(3)

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 [54]. 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 [55]:

σtotal ∞ρtotal = ρstorage − ρannihilation

(4)

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

Material

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

σy (σs)

Reference

1500

500 415 ~420 ~420 880

[55] [65] [66]

~1400 2630 3020 780 940 280 310

[18] 260 [54] 90 [67]

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 [63]. 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 [64]. 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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5. Conclusions Based on the results of this work the following new insights concerning the nature of lubricated wear of FCC metals were: 1. Friction and wear tests of four FCC metals (Ag, Cu, Ni and Al) showed a dependence of the tribological properties on the SFE. A transition from EHL to BL regions occurred at minimal load and with maximal wear for Ag (minimal SFE) in comparison to Cu and Ni. 56

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