Triboxidation and softening induced by dry sliding of copper coupling with steel at 3.36 m s−1 speeds

In this paper, the copper coupled with steel under dry sliding are comparatively sdudied at 0.56 m s−1 and 3.36 m s−1 condition from the morphology, structure and composition of tribolayer of the copper. It is found that the wear rate of Cu at 3.36 m s−1 is twice that at 0.56 m s−1, and the temperature of Cu block at 3.36 m s−1 (240 °C) is much higher than that at 0.56 m s−1 (75 °C). The results show that when the grinding speed is 0.56 m s−1, the grain distortion occurs in the top area of the subsurface layer of Cu block. When the grinding speed is 3.36 m s−1, the grain refinement zone appears below the subsurface layer of Cu block wear. The study further proves that the softening caused by friction heat occurs in the grain refinement zone, and a large number of softened metal copper tends to transfer to the opposite side. At the same time, the friction heat changes the mechanical and thermal deformation, material transfer and friction oxidation of the contact surface, which has a great influence on the dry sliding behavior of Cu.


Introduction
When the two coupling materials rub each other, the friction force will be distributed on the contact surface.Part of the friction force, acting on the surface of the two coupling materials, produces plastic deformation or initiates secondary processes (such as acoustic emission and luminescence emission); the other part will appear in the form of thermal energy [1,2].In particular, under the condition of high-speed dry friction, the friction heat will lead to the formation of temperature gradient near the contact surface and the occurrence of thermal softening, accompanied by recrystallization, phase transformation and element diffusion [3][4][5].
The researchers [6][7][8][9][10][11][12][13] studied the non-lubrication wear behavior of copper.Emge A et al studied the tribological behavior [8,9] of oxygen-free highly conductive (OFHC) copper under vacuum conditions, different sliding speeds (0.05 ∼ 1 m s −1 ) and low loads (1.5 N).They believe that the difference in friction coefficient is related to the difference in the transfer mode of the coupling to the other side.K Lu et al mainly studied the fretting wear behavior of electrodeposited nanocrystalline Cu under 5 Hz fretting frequency and different loads (5 ∼ 40 N) [12,13].It is concluded that the improvement of the wear resistance of nanocrystalline Cu is related to the high hardness and low work hardening rate of nanocrystalline structure.In these experiments, the sliding speed and normal load of the copper block are very low, so the friction heat is less, that is, the surface/subsurface deformation is less affected.
The existing research shows that the tribological behavior and failure of sliding parts have an important relationship with the contact temperature generated by friction heat [14,15].As the temperature increases, the structure and properties of the contact material will change, the surface is easy to oxidize and the contact solid is easy to soften, and the friction and wear behavior of the material will change.
Under dry friction conditions, both sliding speed and normal load will cause friction heat, and the sliding speed has a greater influence.Copper has good thermal conductivity and ductility, therefore, copper is selected as the research object in this paper.The purpose is to study the effect of friction heat on its dry sliding behavior.
At the same time, the morphology, structure and composition of the worn surface/subsurface were characterized, and the wear mechanism of Cu under the action of friction heat was further studied.

Sliding wear tests
Sliding wear tests were carried out on a pin-on-disk friction and wear tester under non-lubrication conditions.The metal Cu (T2) was used as the fixed pin (Φ5 × 12 mm), and the carbon steel was used as the rotating disc (Φ50 × 10 mm).Before the wear test, the rotating disc was machined and polished to remove the surface oxide layer.The pin is mechanically polished to Ra = 0.6 μm at the contact tip.The steel paltes are polished to Ra = 0.6 μm, too.Both the pins and plates were ultrasonically cleaned with anhydrous ethanol for 10 min and allowed to dry before the experiment.The sliding wear tests of 0.56, 1.12, 1.68, 2.24, 2.80 and 3.36 m s −1 were carried out respectively.In order to simplify the test, the load was kept constant at 10 N, and the lowest speed and the highest speed were selected for comparative study.The wear tests of 0.56 m s −1 and 3.36 m s −1 were repeated three times.During the dry sliding process, thermocouples were used to measure the overall temperature during the sliding distance (about 1000 m).The thermocouple is fixed at a distance of 5 mm from the contact tip of the Cu needle.Worn surface/subsurface characterization The worn surface/subsurface morphologies of Cu samples were observed by scanning electron microscopy (SEM JSM-5600LV) and optical microscopy (OM Carl Zeiss).The chemical composition of the worn surface was analyzed by laser Raman spectroscopy (LRS Invia Renishaw).The nanoindentation experiment was performed by the Hysitron Tribo Indenter with a Belkovich tip.The speed is 5 mN s −1 and the load is 5 mN.A precision saw blade (Biller ISO MET-1000) is used to cut the cross-section specimens of the Cu specimen perpendicular to the worn surface and parallel to the sliding direction, then they are mounted on the resin and polished metallographically with a (Biller Phoenix Beta grinder-polishing machine) to obtain a mirror surface.

Results and discussion
Friction behavior and Cu bulk temperature Three parallel tests were carried out on copper blocks at sliding speeds of 0.56 m s −1 and 3.36 m s −1 .It can be seen from figure 1 that the average friction coefficient is 0.860 ± 0.085 at 0.56 m s −1 and 0.899 ± 0.084 at 3.36 m s −1 .The wear rate of Cu at 3.36 m s −1 is twice that at 0.56 m s −1 .According to the change of copper body temperature with sliding distance, after sliding 400 m at the grinding speed of 0.56 m s −1 , the copper body temperature reaches the highest temperature of 75 °C, and after sliding 400 m at the grinding speed of 3.36 m s −1 , the copper body temperature tends to be stable and the highest temperature reaches 240 °C.The dynamic friction coefficient between copper and steel can be divided three stages [16].At the initial stage, the friction coefficient rise rapidly soon because of contact asperity tips and hard convex plough effect.When the sliding distance reaches to 200 m, the contact surface is relatively flat and oxidation occurs, and the friction coefficient is stables relatively.After the sliding distance is 300-1000 m, the friction coefficient fluctuates and it is because the adhesive wear and thermal softenin.By comparing the friction coefficient in figures 1(a) and (b), it is found that during the sliding process, the coupling of copper and steel at a speed of 0.56 m s −1 has a wider fluctuation range (0.7-1.1) than that at a speed of 3.36 m s −1 .

Morphology of worn surfaces
By comparing the OM and SEM morphologies in figures 2 and 3, it can be found that the wear surface of Cu presents a 'gray' morphology at 0.56 m s −1 (figure 2(a)); at 3.36 m s −1 , the worn surface of Cu exhibits a 'colorful' morphology (figure 2(b)).It can be seen from the SEM magnification diagram that at 0.56 m s −1 , the wear surface presents randomly extended cracks.The surface layer breaks and forms small debris after sliding, as shown in figure 3(a).As shown in figure 3(b), at 3.36 m s −1 , the wear surface appears obvious smear morphology and plastic flow.
By comparing the OM and SEM morphologies in figures 2 and 3, it can be found that the wear surface of Cu exhibits a 'gray-black' morphology at 0.56 m s −1 (figure 2(a)); at 3.36 m s −1 , the worn surface of Cu exhibits a 'colorful' morphology (figure 2(b)).From the SEM magnification diagram, it can be seen that at 0.56 m s −1 , the wear surface presents randomly extended cracks.After sliding, the surface is broken to form small debris, as shown in figure 3(a).As shown in figure 3(b), at 3.36 m s −1 , the wear surface appears obvious 'flowing morphology.Under the contact load and sliding condition, strain of the material underneath the firctional surface is engough to begin deformationl.It can be judged that the material has local flowing in the contact area, and the material can modify the wear trace in a more stable wear mode.
From the morphology we can see, at 0.56 m s −1 , the wear surface of Cu is covered with a 'gray-black' color layer (figure 2 The Raman peak corresponding to the friction speed at 3.36 m s −1 is sharper than that at 0.56 m s −1 , which indicates that when the friction speed is 3.36 m s −1 , the iron oxide has higher crystallinity.

Morphology and composition of the worn subsurface
The OM micrographs in figure 5 show the longitudinal cross-section morphology of Cu in the differential interference contrast (DIC) mode.It can be seen from figure 5(a) that at 0.56 m s −1 , the grains are bent along the sliding direction; it can be seen from figure 5(b) that at 3.36 m s −1 , slight plastic deformation occurs below the contact surface, and the plastic strain can develop from the block to the surface.From figure 5(b), the marked area can be seen and present a fine-grained layer with a thickness of 40-55 μm underneath the contact surface.The elemental analysis results in this area are in figure 6, where Cu is the main element and Fe is less.This layer is the mechanical mixing zone in the composition.

Morphology of wear debris
From the observation results of wear debris (figure 7), it can be seen that the wear debris generated at 0.56 m s −1 grinding speed is small in size and irregular in shape.The color debris generated at 3.36 m s −1 is large in size,  striped in shape, and some cracks can be seen on the exfoliated debris.The wear scar morphology of the body steel was observed, and it was found that the width of the wear scar on the steel surface at 0.56 m s −1 was smaller than that at 3.36 m s −1 .As shown in figure 8, more Cu is transferred to the steel and covered on the wear surface, and the Cu transfer layer on the steel surface is scratched with high strength and is not easy to peel off from the steel matrix.

Discussion
As mentioned above, the wear surface/subsurface and wear debris produced at 0.56 m s −1 and 3.36 m s −1 show different morphologies and structures.Under the same load and sliding distance, we believe that friction heat and substrate temperature are important factors.The thermal physical properties of Cu and steel were compared: the thermal conductivity of Cu was 397 W/(m•K), and the thermal conductivity of steel was 50 W/ (m•K).Therefore, when the friction work at the interface is converted into heat energy, more friction heat is transmitted and dissipated in the Cu bulk.At 0.56 m s −1 , the temperature (tb) of Cu bulk is 25 ∼ 75 °C, indicating that there is little friction heat generated during the sliding process.A small amount of copper is transferred to the coupling steel.At 3.36 m s −1 , the Cu bulk temperature (tb) is 25 ∼ 230 °C, indicating that a large amount of frictional heat is generated during the sliding process.Although the bulk temperature recorded near the Cu contact surface is lower than the initial softening point of copper (about 400 °C), the real contact surface temperature of the Cu bulk is higher than the measured temperature.Accordingly, thermal softening can occur at the top of the subsurface layer of Cu, and once the copper softens, it is easy to transfer to the coupling steel.
The work hardening is partly caused by the plastic deformation of the worn surface, but the thermal softening is caused when the surface material is at high temperature.Work hardening and thermal softening work together on the contact surface [18][19][20].At 0.56 m s −1 , the grain distortion is serious, it is inferred that the work hardening effect is better than the thermal softening effect.At 0.56 m s −1 , the hardened layer is prone to cracks under the shear stress of the worn surface.At 3.36 m s −1 , the thermal softening effect is dominant.Therefore, thermal softening has a greater effect on the tribological behavior than friction oxidation, and thermal softening changes the morphology and structure of the contact material, and the softened Cu is easy to transfer to the dual steel.
When the friction speed is 0.56 m s −1 and 3.36 m s −1 , Fe 2 O 3 and Fe 3 O 4 are the main oxides of Cu wear surface.It is known that magnetite (Fe 3 O 4 ) will transform into maghemite (γ-Fe 2 O 3 ) or hematite (α-Fe 2 O 3 ) [17] at high temperature.The formation of iron oxides on the contact surface should have played a lubricating role in the sliding process, but the thickness of the oxide layer was not enough to retain on the contact surface.In addition, thermal softening causes copper to diffuse and expand the contact surface, thereby gathering the adhesion between the couple surfaces, and a large amount of softened copper is transferred to the counterpart.
As shown in figure 9, the temperature change of the copper block grinding interface is indirectly understood by observing the temperature of the grinding rotating disc.The maximum temperature of the grinding rotating disc interface at 3.36 m s −1 during the grinding process is significantly higher than the maximum interface temperature at 0.56 m s −1 .It can be seen from figure 5 that when the friction speed is 3.36 m s −1 , the fine grain zone can be clearly seen in the subsurface of the Cu block, and the strength and hardness of the fine grain zone are evaluated by the indentation test.At 3.36 m s −1 , the hardness of the fine grain zone is lower than that of the Cu matrix, softening partially happens at the region underneath the contact surface.Softening can be achieved by Coble creep [21] or grain boundary sliding [22][23][24] or dislocation-free accumulation at ultrafine grain size [25].However, there is no consensus on the softening phenomenon in fine-grained or nanocrystalline materials.It has been confirmed that the formation of softening is mainly caused by stress corrosion coupled thermal activation deformation [22] at grain boundaries.When the contact surface temperature is high, the grain boundary is viscous the ability to resist deformation is reduced, so the grain boundary shear becomes the main deformation mode.In addition, high temperature and low strain can allow grain boundary annihilation to invade dislocations and induce softening [26][27][28][29][30][31].The softening effect changes the morphology of the contact material and the migration of Cu, which induces the dry sliding behavior.
As shown in figure 10, when the rotation speed is 0.56 m s −1 and the sliding distance is 1007 m, the maximum equivalent stress of the copper pin sample is 469.6 MPa, and the maximum equivalent strain is 0.088.When the rotation speed is 3.36 m s −1 and the sliding distance is 1007 m, the maximum equivalent stress of the copper pin sample is 478 MPa, and the maximum equivalent strain becomes 0.103.When the rotational speed increases from 0.56 m s −1 to 3.36 m s −1 , the maximum equivalent stress, maximum equivalent strain and maximum temperature increase by 101.79%, 117.05% and 113.49%, respectively.Therefore, it is inferred that the sliding speed has a great influence on the stress state and temperature of the dynamic surface of the friction contact, and feeds back the change of the friction coefficient and wear amount.

Conclusions
1.The wear rate of Cu at 3.36 m s −1 is twice as much as that at 0.56 m s −1 in present study.The temperature of Cu bulk at 3.36 m s −1 (240 °C) is much higher than that at 0.56 m s −1 (75 °C).At 0.56 m s −1 and 3.36 m s −1 , metal copper, Fe 3 O 4 and Fe 2 O 3 are the main components of Cu wear surface.
2. A grain-refined zone with softening characteristic is presented underneath Cu worn surface at 3.36 m s −1 .The stress-assisted thermally activated deformation in the grain boundary is dominant.
3. The thermal softening and triboxidation can be promoted by frictional heating and can affect the dry sliding behavior greatly.

Figure 1 .
Figure 1.Variation of dynamic friction coefficient and bulk temperature of Cu during sliding against steel (a) 0.56 m s −1 and (b) 3.36 m s −1 .

Figure 2 .
Figure 2. OM morphologies of worn surface of Cu sliding against steel at (a) 0.56 m s −1 and (b) 3.36 m s −1 .

Figure 3 .
Figure 3. SEM morphologies of worn surface of Cu sliding against steel at (a) 0.56 m s −1 and (b) 3.36 m s −1 .
(a)); at 3.36 m s −1 , a 'color' layer appears on the worn surface of Cu (figure 2(b)).According to the color, the black substance (figure 2(a)) is Fe 3 O 4 compound or CuO compound, and the brown substance (figure 2(b)) is Fe 2 O 3 compound or Cu 2 O compound.It can be seen from the Raman spectrum detection in figure 4 that the black and brown substances are Fe 3 O 4 and Fe 2 O 3 .

Figure 4 .
Figure 4. Raman spectra on the worn surface of Cu sliding against steel at (a) 0.56 m s −1 and (b) 3.36 m s −1 .

Figure 5 .
Figure 5. OM micrographs of longitudinal sections of Cu sliding against steel at (a) 0.56 m s −1 and (b) 3.36 m s −1 .

Figure 7 .
Figure 7. OM morphologies of worn debris of Cu sliding against steel at (a) 0.56 m s −1 and (b) 3.36 m s −1 .

Figure 9 .
Figure 9. Temperature field diagram of sliding wear zone of steel plate (a) 0.56 m s −1 and (b) 3.36 m s −1 .