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The effect of active and passive third body film in friction of carbons Part 2: Characterization of friction film
Tribology Research: From Model Experiment to Industrial Problem G. Dalmaz et al. (Editors) 9 2001 Elsevier Science B.V. All rights reserved.
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T h e effect o f a c t i v e a n d p a s s i v e third b o d y f i l m in f r i c t i o n o f c a r b o n s P a r t 2: C h a r a c t e r i z a t i o n o f f r i c t i o n f i l m K. Lafdi and K.-M. Teo, NSF-Industry-University Center for Advanced Friction Studies Southern Illinois University, Carbondale, Illinois 62901-4343, USA In the part I of this study, friction tests were conducted using both pin-on-disk and disk-on-disk setups. Despite the nature of the friction scales, similar results were found. In the friction of carbon materials, it has been found that the coefficient of friction is dependent on the total energy absorbed by a given mass and the rate of the energy dissipation. We observed that at high energy conditions, the occurrence of friction transition does not appear related to moisture effects. In this part of the study, the friction surface was examined using various macro and nanoscopic techniques to understand the evolution of friction film before, during and after friction transition. Examination of the friction surfaces display two distinct surface characteristics: shiny and dull bands. Structural changes of friction film have been observed from graphitic allotropic form to amorphous type structure. This sudden change leads to an increase of the coefficient of friction. It seems that the inter-laminar shearing and disruption of the film might cause the friction transition. In this paper, friction behavior of a graphitic carbon at high energy had been studied. At high power input, hot spots had been generated. The current study shows that both the friction transition and hot spots had occurred because of friction film break-off and powdered slits had formed. The presence of these particles surface contributes in the increase of the coefficient of friction (~t > 1) and its instabilities. 1. L I T E R A T U R E R E V I E W
The mechanism of how friction is acting at the interface between two sliding bodies is not thoroughly understood today even though intensive and exhaustive researches have already been performed. There are three basic elements involved in the friction process [1]. They are real contact surface, contact bond and it strength at the contact area, and deformation of surface asperity during sliding. It was reported that the friction coefficient due to asperity properties, shear stresses and microstructures is not sufficient to cover the actual sliding condition [2]. Transition phenomena at the surface have been described as accumulation of subsurface fatigue damage, wear-through of surface film, build-up of transfer layers, agglomeration of debris and change of material properties [3]. He also mentioned that the magnitude of the friction coefficient during transition is proportional to the introduction of debris. Humidity, oxygen and some contaminants are believed to affect the friction and wear process [4]. Water vapor appears to have a lubrication effect on non-graphitic carbon [5]. It was reported that the
friction coefficient of graphite remained low only in the presence of water or other condensable vapors [6-8]. Indeed, the friction coefficient of carbon was reported ten times higher at vacuum condition. It was shown that the inter-laminar bonding (crosslinking) of graphite is high under vacuum condition [9]. This inter-laminar bonding decreased in the presence of air or water. It was also suggested that water might reduce the binding energy of graphite by bonding with the available electrons. It was suggested that the lubrication properties of graphite increase when moisture is absorbed between the carbon basal planes. Friction surfaces of carbon-carbon composites at high-energy friction test exhibit two types of surface morphology: a dull-looking gray surface and mirrorlike black surface, which was ascribed to contact pressure alteration [10]. The contact pressure is higher at the gray area, which leads to a rougher surface without continuous wear debris film coverage. It was reported that these contact pressure differences are caused by non-uniform thermal expansion due to frictional heating [10]. These pressure differences play an important role in the stress field of the brakes. It was found that friction and wear decrease with increasing temperature to
352
950~ due to chemisorption of impurities to form lubricating film, and to oxidation [ 11 ]. Dry sliding friction was determined to depend on two basic factors: interfacial bond and material deformation [1, 12]. Adhesive bond at the interface was said to be the main factor of frictional energy. It was reported that the coefficient of friction of carbon brakes increased with increasing heat treatment temperature and Young's modulus [13]. It was found that the wear of composites exhibiting a coefficient of friction above 0.4 was dominated by an abrasion mechanism. Other research suggested that more breakage and loss of fiber meant that higher frictional energy is spent as mechanical energy [ 14]. In this study, we will be emphasizing on friction transition that occurs regardless the gas environment and magnitude of the surface temperature. It concerns a friction transition that is directly related to materials properties of carbon materials. The overall goal of this paper is only to examine the morphological and structural changes that may occur at friction surface. We will characterize at various scales using multiple techniques both frictions film and wear debris before, during and after friction transition. 2. S T R U C T U R A L ANALYSES From the previous paper (part I), we observed a similar friction behavior regardless the friction test (pin-on-disk and sub-scale dynamometer). Depending on the testing conditions, the coefficient of friction profile as a function of time exhibits either single or multiple friction transitions. Figure 1 shows a typical trend in friction coefficient during friction transition. This trend is divided into four stages during the evolution of surface during a rubbing process. These stages were described in the previous paper. In this paper, structural changes during the evolution of the friction surface will be presented.
3
1
Time Figure 1: A model illustrating multiple stages of the coefficient of friction variation as a function of time (Evolution of friction transition with respect to time). Friction surfaces for all experiments exhibit some specific characteristics in which they can be summarized as follows: 1) Friction surface before friction testing 2) Friction surface before friction transition 3) Friction surface at the friction transition 4) Friction surface when hot spot is generated. 2.1 Friction surface before friction testing. Before each friction test, both the sample and the rotor disk surfaces are sanded using sandpaper up to the grid size of 800. For all experiments, same surface finish is managed. Also, the surface profile is taken before and after each test. The surface is relatively smooth with some visible voids. 2.2 Friction surface before friction transition As the friction test progressed, friction film was built up (Figure 2a and b). The details of the coke grains were masked. Visual examination shows that the entire friction surface appears very shiny. In addition the surface topography appears very smooth with very tiny waviness (Figure 2c). The friction film is extremely tiny and no motion of this film has been observed. At this level, the film bonds to the surface and the film appears to be passive. Furthermore, transmission electron microscopy characterization shows that the friction film is amorphous.
353
Figure 3" Surface topography of the friction surface at the transition stage (a) Smooth part of the surface at transition stage; (b) Rough part of the surface at transition stage
354
2.3 Friction surface at the friction transition
At the first indication of the transition, the coefficient of friction starts to increase and some surface alterations are observed. The friction surface of the sample appears to be made of two distinct bands or wear tracks: one is shiny (i.e. smooth) with some relative waviness and the second is dull with some random surface irregularity (i.e. rough). For comparison, surface topographies at the smooth and rough areas are shown in Figure 3. During the friction transition, dull slits (powdered wear track) appear with localized roughness, cracks and voids as confirmed by the SEM images shown in Figure 4. On the other hand, the smooth part was observed to have polished bands and/or bands with some granular structure. This smooth part was made of a smeared friction film in which agglomeration of tiny
particulates has occurred as shown in Figure 5a. These particulates with narrow range of sizes and shapes are highly compacted. As shown in both images (Figure 5b and c) the structure of the smeared film is made of low order carbon. The selected diffraction pattern showed a very diffuse halo (inset of Figure 5b) and we observe that few graphene layers remain unchanged (arrows in Figure 5c). Indeed, there is a structural change from very straight and anisotropic structure in form of graphitic structure into quasi-less crystalline and amorphous type structure. We believe the degree of compaction is strongly dependent on testing conditions (i.e. normal load, time and initial velocity). However, this smeared friction film might often be disrupted and contain cracks.
Figure 4: SEM images of the powdered wear track (dull slit) at the beginning of friction transition
355
Figure 5: (a) High magnification SEM images of the smeared friction film, (b) Bright field TEM image of the smeared friction film with the diffraction pattern of the smeared film at the lower left corner; (c) High resolution TEM image (Lattice fringe of TEM modes) of a single particle of the corresponding smeared film. The arrows show distorted graphene layers Characterization of the friction film using the Atomic Force Microscopy (AFM) in both topography and lateral force microscopy mode (LFM mode) shows that the friction film is very shiny along the wear track and it is relatively smooth (Figure 6a). The average roughness is about 42 A.
Since the surface roughness is smaller than 200 nm, the lateral force microscopy mode can be used to see if there is any friction coefficient gradient within this smooth friction film. We used Lateral Force Microscopy (LFM) to measure the frictional forces resulting from the AFM probe scanning over a
356
sample. When the probe crosses an area of the sample, which has a higher drag or friction force, the cantilever will display a greater amount of torque. The opposite is true for areas where lower frictional forces are generated. Here 'high' and 'low' are relative terms, for instance an adhesive or charged particle on a carbon surface may exhibit a 'higher' frictional force than the surrounding substrate. Thus, the particulates appear brighter in the phase image. Below that, where the particulate could be said to be 'slippery' (having a low frictional coefficient), the particulate appears darker in the friction image. As shown in the LFM image (Figure 6b), we observed that the surface is made of narrow range of gray scale of brightness. The bright area means that the
area has a higher friction property than the dark area. Scanning of an area of 2 x 2 ~tm shows that the size of the particulates are in the range of 100 nm. Their shape appears to be elliptical, elongated or spherical particulates (Figure 6c and d). However the LFM images show that most of these particulates were slightly covered by a bright thin layer which indicates that this layer located on the skin of these particulates is stiffer and presents higher coefficient of friction than the dark part located on the core (Figure 6d). These AFM data confirmed the TEM observations from our previous works. It seems that a rolling mechanism has a great effect on the friction of carbons.
Figure 6: AFM characterization of the friction film (a) Topography image of the friction film using AFM; (b) LFM image of the film; (c) High resolution LFM images of the film; (d) Variation of the friction using LFM
357
For a qualitative purpose and after calibration of the tip probe, LFM mode should lead us to determine the distribution of surface attractive force within the same type of friction film. But in any case, we always recorded Gaussian distributions for the surface roughness and the frictional force along the wear tracks on the smooth area (Figure 7).
stresses and others). However there is always a fair matching at macro-metric scale between friction surfaces of the rotor and stator (Figure 8). The particulates that has been generated during the cracks of the surface (stage 3) are subjected to high shearing action. Then, these particulates tend to be rolled and give either spherical or cylindrical types (Figure 9). These rolled particulates are always dispersed into lamellar and amorphous medium. The change of friction film structure from amorphous into turbostratic carbons would enhance an increase in the coefficient of friction.
Figure 7: Friction Analysis of the film using LFM mode of the AFM. In summary, the film is made of compacted particulates with narrow range of sizes and shapes in which the bonding force appears to be very large. At some locations, the interfaces between particulates can be observed without any apparent gap between them even at nanometric scale. However, the dull part of the friction surface is clearly made of disrupted film particulates. This area started to get rougher in comparison to the shiny part. The number and the width of these dull tracks are strongly dependent on the testing conditions (normal load, time and velocity) and gas environment (argon, humidity and air). At some locations in the dull area, cracks and holes are very obvious. The cracks are either empty or filled with wear debris. The extension and the magnitude of these bands and their shapes depend on many parameters (oxidation, thermal expansion, thermal
Figure 9: Friction Film Model 2.4 Friction surface when hot spot was generated
We observed that during hot spotting, powdered debris is generated on the wear tracks. We assumed that the area of contact where hot spots had occurred might be in the range of powdered wear track dimension, located on the surface. It appeared that the powdered wear tracks were about 1-5 % of the total surface area. In fact, during hot spotting the contact is extremely localized. However, the real contact area during hot spotting is unknown.
358
During hot spotting, two effects are dominant: oxidation reaction and thermal expansion. We observe a rapid friction transition followed by an increase of bulk temperature and finally the hot spot on the surface. Figure 10 shows the hot spot ring captured during the tests. When a hot spot starts to appear, the hot spot ring changes from red into intensive white color. The magnitude and extent of this hot spot lasts for only few seconds. As shown at low magnification in Figure 10a, the friction surface of the sample appears to be made of distinct bands or wear tracks: one is shiny (i.e. smooth) and the
second is dull (i.e. rough, see single arrow in Figure 10b). Examination of the friction surface at high magnifications using SEM shows that the shiny part consists of polished bands (Figure 1 l a) and bands with some granular structure (Figure 1 l b). These smooth bands are made of smeared friction film in which agglomeration and compaction of tiny particulates has occurred. However, some pits and holes are clearly present. This smeared area can be disrupted and contains some tiny cracks (Figure 11 c)
Figure 11: SEM images of the shiny band on the friction surface (a) Highly polished and compacted friction film; (b) Friction film with discontinuity and some granular structures; (c) Disrupted friction film with cracks and voids
359
Figure 12: SEM images of the dull band on the friction surface (a) Area covered by particulates of different sizes; (b) Area with cracks and voids; (c) Empty voids due to oxidation However, the dull part is clearly made of two major features: the area is covered with fluffy particulates of wide range of size and shapes (Figure 12a) and with cracks and voids (Figure 12b). The size of particulates is so tiny that can be flushed from the surface by local increase of the intemal pressure or removed by fast oxidation and combustion. We assume that two distinct effects are taking place at these specific locations: oxidation and thermal expansion. The surface temperature is expected to be higher than 1200 ~ due the white color of the hot spot ring. The extension and the magnitude of these dull bands depend on the testing conditions. The voids on the surface can be caused by an increase of the oxidation reaction due to a sudden increase of surface temperature. These voids are either filled with wear particulates or empty (Figure 12c). The surface temperature is probably high enough to oxidize any type of carbons (graphitic or non-graphitic carbons start to oxidize at moderate rate when the temperature reaches 500 ~ Oxidation may quickly gasify the wear particulates of large reaction surfaces inside the voids and leave the voids empty, especially along the hot spot ring. 3. C O N C L U S I O N In this study, we observed that regardless the test scale (pin on disc "FAST" or sub-scale dynamometer), we obtained the same friction trend and behaviors. All friction surfaces exhibit the same type of structure. The sample surfaces are examined
using various techniques. For given time and energy absorbed, we observed a structural change from graphitic to amorphous carbon has occurred on the surface of the specimen (stage 1). However, for an extended friction conditions (stages 2 and 3), wear tracks created at the transition stage are very porous due to surface damage. The classical explanation for the low friction of graphite relates friction to the weak binding energy of the graphite lattice, therefore little energy is needed to induce the cleavage or shear, and so the coefficient of friction is low if the graphite is oriented parallel to the direction of rubbing. However this easy cleavage is true only when wet and condensable vapors are present in the environment. If there is any substantial rise in surface temperatures, the water saturation decreases and the carbon surface is physically desorbed, leading to an increase of coefficient of friction. High coefficient of friction and wear may also be induced at room temperatures if the combination of load and velocity produce an increase of surface temperature in the order of 600 ~ or greater. Apart from the water effect, there are other factors that influence the friction transition and the magnitude of the coefficient of friction. Finely divided debris from the wear process forms the friction film made of nanometric amorphous sheared particles. This friction film may enhance the preferred crystallographic orientation and direction of easy shear of carbon crystallites or rolled particulates. It has reported that this effect is occurring in a cyclic manner during the sliding of carbon. It was reported
360
that when the friction rises, the surface stresses become sufficient to disrupt the layer-consolidated contact area decreases. This leads to a sudden increase in the friction coefficient [ 15]. However, we observe that an increase of friction coefficient is due to the inter-laminar shear of particulates formed from the wear debris. During the run-in process, an amorphous friction film is built up. The shear stress within the film increases leads to either mechanical or thermal failure of the surface (friction transition) and causes an increase in the friction coefficient. The stresses increase until it disrupts the film and causes a sudden change in friction coefficient. However, the disruption of the film releases the wear particulates and caused a decrease in true contact area. Decrease of contact area keeps the friction coefficient high but decreases gradually as film is being built again. The shear stress may propagate and crack the bulk material. The crack in the bulk material will affect the behavior of the friction film and being affected by it.
debris. Then, the surface is roughened and the real 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14.
REFERENCES 1. Tabor, D., J. Lubr. Tech. 103, (1981) 169-179. 2. Kuhlmann-Wilsdorf, D., Fundamentals of Friction and Wear of Materials, Rigney, D. A., ASM, Metal Part, (1981) 119-186.
15.
Blau, P. J., Joumal of Tribology, 109, (1987) 537-544. Brunt, C. V. and Savage, R.H., Gen. Elec. 47, (1944) 16. Ramadanoffand, D. and Glass, S.W., Trans. AIEE, 6, (1944) 825. Savage, R. H., J. Appl. Phys. 19, (1948) 136. Savage, R. H., J. App. Phys. 19, No. 1, (1948) 1. Savage, R.H. and Shaffer, D.L., J. Appl. Phys., 27, (1956) 136. Bryant, P.J., Gutshall, P. C. and Taylor, L. H., Mechanisms of Solid Friction, Elsevier Amsterdam, (1964) 118. Yen, B. K. and Ishihara, T., Wear 174, (1994) 111-117. Li, C. C. and Sheehan, J. E., Int. Conf., ASME, (1981) 525-533. Kragelskii, I. V., Butterworths, London (1965). Kimura, S., Yasuda, E., Narita, N., JSLE, Int. Ed. 5, (1984) 11-16. Kim, D. G., Kweon, D. W., Lee, J. Y. and Mater, J., Sci. Lett. 12, (1993) 8-10. Midley, J.W. and Teer, D.G., Trans. ASME, 85, (1963) 488.
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T h e effect o f a c t i v e a n d p a s s i v e third b o d y f i l m in f r i c t i o n o f c a r b o n s P a r t 2: C h a r a c t e r i z a t i o n o f f r i c t i o n f i l m K. Lafdi and K.-M. Teo, NSF-Industry-University Center for Advanced Friction Studies Southern Illinois University, Carbondale, Illinois 62901-4343, USA In the part I of this study, friction tests were conducted using both pin-on-disk and disk-on-disk setups. Despite the nature of the friction scales, similar results were found. In the friction of carbon materials, it has been found that the coefficient of friction is dependent on the total energy absorbed by a given mass and the rate of the energy dissipation. We observed that at high energy conditions, the occurrence of friction transition does not appear related to moisture effects. In this part of the study, the friction surface was examined using various macro and nanoscopic techniques to understand the evolution of friction film before, during and after friction transition. Examination of the friction surfaces display two distinct surface characteristics: shiny and dull bands. Structural changes of friction film have been observed from graphitic allotropic form to amorphous type structure. This sudden change leads to an increase of the coefficient of friction. It seems that the inter-laminar shearing and disruption of the film might cause the friction transition. In this paper, friction behavior of a graphitic carbon at high energy had been studied. At high power input, hot spots had been generated. The current study shows that both the friction transition and hot spots had occurred because of friction film break-off and powdered slits had formed. The presence of these particles surface contributes in the increase of the coefficient of friction (~t > 1) and its instabilities. 1. L I T E R A T U R E R E V I E W
The mechanism of how friction is acting at the interface between two sliding bodies is not thoroughly understood today even though intensive and exhaustive researches have already been performed. There are three basic elements involved in the friction process [1]. They are real contact surface, contact bond and it strength at the contact area, and deformation of surface asperity during sliding. It was reported that the friction coefficient due to asperity properties, shear stresses and microstructures is not sufficient to cover the actual sliding condition [2]. Transition phenomena at the surface have been described as accumulation of subsurface fatigue damage, wear-through of surface film, build-up of transfer layers, agglomeration of debris and change of material properties [3]. He also mentioned that the magnitude of the friction coefficient during transition is proportional to the introduction of debris. Humidity, oxygen and some contaminants are believed to affect the friction and wear process [4]. Water vapor appears to have a lubrication effect on non-graphitic carbon [5]. It was reported that the
friction coefficient of graphite remained low only in the presence of water or other condensable vapors [6-8]. Indeed, the friction coefficient of carbon was reported ten times higher at vacuum condition. It was shown that the inter-laminar bonding (crosslinking) of graphite is high under vacuum condition [9]. This inter-laminar bonding decreased in the presence of air or water. It was also suggested that water might reduce the binding energy of graphite by bonding with the available electrons. It was suggested that the lubrication properties of graphite increase when moisture is absorbed between the carbon basal planes. Friction surfaces of carbon-carbon composites at high-energy friction test exhibit two types of surface morphology: a dull-looking gray surface and mirrorlike black surface, which was ascribed to contact pressure alteration [10]. The contact pressure is higher at the gray area, which leads to a rougher surface without continuous wear debris film coverage. It was reported that these contact pressure differences are caused by non-uniform thermal expansion due to frictional heating [10]. These pressure differences play an important role in the stress field of the brakes. It was found that friction and wear decrease with increasing temperature to
352
950~ due to chemisorption of impurities to form lubricating film, and to oxidation [ 11 ]. Dry sliding friction was determined to depend on two basic factors: interfacial bond and material deformation [1, 12]. Adhesive bond at the interface was said to be the main factor of frictional energy. It was reported that the coefficient of friction of carbon brakes increased with increasing heat treatment temperature and Young's modulus [13]. It was found that the wear of composites exhibiting a coefficient of friction above 0.4 was dominated by an abrasion mechanism. Other research suggested that more breakage and loss of fiber meant that higher frictional energy is spent as mechanical energy [ 14]. In this study, we will be emphasizing on friction transition that occurs regardless the gas environment and magnitude of the surface temperature. It concerns a friction transition that is directly related to materials properties of carbon materials. The overall goal of this paper is only to examine the morphological and structural changes that may occur at friction surface. We will characterize at various scales using multiple techniques both frictions film and wear debris before, during and after friction transition. 2. S T R U C T U R A L ANALYSES From the previous paper (part I), we observed a similar friction behavior regardless the friction test (pin-on-disk and sub-scale dynamometer). Depending on the testing conditions, the coefficient of friction profile as a function of time exhibits either single or multiple friction transitions. Figure 1 shows a typical trend in friction coefficient during friction transition. This trend is divided into four stages during the evolution of surface during a rubbing process. These stages were described in the previous paper. In this paper, structural changes during the evolution of the friction surface will be presented.
3
1
Time Figure 1: A model illustrating multiple stages of the coefficient of friction variation as a function of time (Evolution of friction transition with respect to time). Friction surfaces for all experiments exhibit some specific characteristics in which they can be summarized as follows: 1) Friction surface before friction testing 2) Friction surface before friction transition 3) Friction surface at the friction transition 4) Friction surface when hot spot is generated. 2.1 Friction surface before friction testing. Before each friction test, both the sample and the rotor disk surfaces are sanded using sandpaper up to the grid size of 800. For all experiments, same surface finish is managed. Also, the surface profile is taken before and after each test. The surface is relatively smooth with some visible voids. 2.2 Friction surface before friction transition As the friction test progressed, friction film was built up (Figure 2a and b). The details of the coke grains were masked. Visual examination shows that the entire friction surface appears very shiny. In addition the surface topography appears very smooth with very tiny waviness (Figure 2c). The friction film is extremely tiny and no motion of this film has been observed. At this level, the film bonds to the surface and the film appears to be passive. Furthermore, transmission electron microscopy characterization shows that the friction film is amorphous.
353
Figure 3" Surface topography of the friction surface at the transition stage (a) Smooth part of the surface at transition stage; (b) Rough part of the surface at transition stage
354
2.3 Friction surface at the friction transition
At the first indication of the transition, the coefficient of friction starts to increase and some surface alterations are observed. The friction surface of the sample appears to be made of two distinct bands or wear tracks: one is shiny (i.e. smooth) with some relative waviness and the second is dull with some random surface irregularity (i.e. rough). For comparison, surface topographies at the smooth and rough areas are shown in Figure 3. During the friction transition, dull slits (powdered wear track) appear with localized roughness, cracks and voids as confirmed by the SEM images shown in Figure 4. On the other hand, the smooth part was observed to have polished bands and/or bands with some granular structure. This smooth part was made of a smeared friction film in which agglomeration of tiny
particulates has occurred as shown in Figure 5a. These particulates with narrow range of sizes and shapes are highly compacted. As shown in both images (Figure 5b and c) the structure of the smeared film is made of low order carbon. The selected diffraction pattern showed a very diffuse halo (inset of Figure 5b) and we observe that few graphene layers remain unchanged (arrows in Figure 5c). Indeed, there is a structural change from very straight and anisotropic structure in form of graphitic structure into quasi-less crystalline and amorphous type structure. We believe the degree of compaction is strongly dependent on testing conditions (i.e. normal load, time and initial velocity). However, this smeared friction film might often be disrupted and contain cracks.
Figure 4: SEM images of the powdered wear track (dull slit) at the beginning of friction transition
355
Figure 5: (a) High magnification SEM images of the smeared friction film, (b) Bright field TEM image of the smeared friction film with the diffraction pattern of the smeared film at the lower left corner; (c) High resolution TEM image (Lattice fringe of TEM modes) of a single particle of the corresponding smeared film. The arrows show distorted graphene layers Characterization of the friction film using the Atomic Force Microscopy (AFM) in both topography and lateral force microscopy mode (LFM mode) shows that the friction film is very shiny along the wear track and it is relatively smooth (Figure 6a). The average roughness is about 42 A.
Since the surface roughness is smaller than 200 nm, the lateral force microscopy mode can be used to see if there is any friction coefficient gradient within this smooth friction film. We used Lateral Force Microscopy (LFM) to measure the frictional forces resulting from the AFM probe scanning over a
356
sample. When the probe crosses an area of the sample, which has a higher drag or friction force, the cantilever will display a greater amount of torque. The opposite is true for areas where lower frictional forces are generated. Here 'high' and 'low' are relative terms, for instance an adhesive or charged particle on a carbon surface may exhibit a 'higher' frictional force than the surrounding substrate. Thus, the particulates appear brighter in the phase image. Below that, where the particulate could be said to be 'slippery' (having a low frictional coefficient), the particulate appears darker in the friction image. As shown in the LFM image (Figure 6b), we observed that the surface is made of narrow range of gray scale of brightness. The bright area means that the
area has a higher friction property than the dark area. Scanning of an area of 2 x 2 ~tm shows that the size of the particulates are in the range of 100 nm. Their shape appears to be elliptical, elongated or spherical particulates (Figure 6c and d). However the LFM images show that most of these particulates were slightly covered by a bright thin layer which indicates that this layer located on the skin of these particulates is stiffer and presents higher coefficient of friction than the dark part located on the core (Figure 6d). These AFM data confirmed the TEM observations from our previous works. It seems that a rolling mechanism has a great effect on the friction of carbons.
Figure 6: AFM characterization of the friction film (a) Topography image of the friction film using AFM; (b) LFM image of the film; (c) High resolution LFM images of the film; (d) Variation of the friction using LFM
357
For a qualitative purpose and after calibration of the tip probe, LFM mode should lead us to determine the distribution of surface attractive force within the same type of friction film. But in any case, we always recorded Gaussian distributions for the surface roughness and the frictional force along the wear tracks on the smooth area (Figure 7).
stresses and others). However there is always a fair matching at macro-metric scale between friction surfaces of the rotor and stator (Figure 8). The particulates that has been generated during the cracks of the surface (stage 3) are subjected to high shearing action. Then, these particulates tend to be rolled and give either spherical or cylindrical types (Figure 9). These rolled particulates are always dispersed into lamellar and amorphous medium. The change of friction film structure from amorphous into turbostratic carbons would enhance an increase in the coefficient of friction.
Figure 7: Friction Analysis of the film using LFM mode of the AFM. In summary, the film is made of compacted particulates with narrow range of sizes and shapes in which the bonding force appears to be very large. At some locations, the interfaces between particulates can be observed without any apparent gap between them even at nanometric scale. However, the dull part of the friction surface is clearly made of disrupted film particulates. This area started to get rougher in comparison to the shiny part. The number and the width of these dull tracks are strongly dependent on the testing conditions (normal load, time and velocity) and gas environment (argon, humidity and air). At some locations in the dull area, cracks and holes are very obvious. The cracks are either empty or filled with wear debris. The extension and the magnitude of these bands and their shapes depend on many parameters (oxidation, thermal expansion, thermal
Figure 9: Friction Film Model 2.4 Friction surface when hot spot was generated
We observed that during hot spotting, powdered debris is generated on the wear tracks. We assumed that the area of contact where hot spots had occurred might be in the range of powdered wear track dimension, located on the surface. It appeared that the powdered wear tracks were about 1-5 % of the total surface area. In fact, during hot spotting the contact is extremely localized. However, the real contact area during hot spotting is unknown.
358
During hot spotting, two effects are dominant: oxidation reaction and thermal expansion. We observe a rapid friction transition followed by an increase of bulk temperature and finally the hot spot on the surface. Figure 10 shows the hot spot ring captured during the tests. When a hot spot starts to appear, the hot spot ring changes from red into intensive white color. The magnitude and extent of this hot spot lasts for only few seconds. As shown at low magnification in Figure 10a, the friction surface of the sample appears to be made of distinct bands or wear tracks: one is shiny (i.e. smooth) and the
second is dull (i.e. rough, see single arrow in Figure 10b). Examination of the friction surface at high magnifications using SEM shows that the shiny part consists of polished bands (Figure 1 l a) and bands with some granular structure (Figure 1 l b). These smooth bands are made of smeared friction film in which agglomeration and compaction of tiny particulates has occurred. However, some pits and holes are clearly present. This smeared area can be disrupted and contains some tiny cracks (Figure 11 c)
Figure 11: SEM images of the shiny band on the friction surface (a) Highly polished and compacted friction film; (b) Friction film with discontinuity and some granular structures; (c) Disrupted friction film with cracks and voids
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Figure 12: SEM images of the dull band on the friction surface (a) Area covered by particulates of different sizes; (b) Area with cracks and voids; (c) Empty voids due to oxidation However, the dull part is clearly made of two major features: the area is covered with fluffy particulates of wide range of size and shapes (Figure 12a) and with cracks and voids (Figure 12b). The size of particulates is so tiny that can be flushed from the surface by local increase of the intemal pressure or removed by fast oxidation and combustion. We assume that two distinct effects are taking place at these specific locations: oxidation and thermal expansion. The surface temperature is expected to be higher than 1200 ~ due the white color of the hot spot ring. The extension and the magnitude of these dull bands depend on the testing conditions. The voids on the surface can be caused by an increase of the oxidation reaction due to a sudden increase of surface temperature. These voids are either filled with wear particulates or empty (Figure 12c). The surface temperature is probably high enough to oxidize any type of carbons (graphitic or non-graphitic carbons start to oxidize at moderate rate when the temperature reaches 500 ~ Oxidation may quickly gasify the wear particulates of large reaction surfaces inside the voids and leave the voids empty, especially along the hot spot ring. 3. C O N C L U S I O N In this study, we observed that regardless the test scale (pin on disc "FAST" or sub-scale dynamometer), we obtained the same friction trend and behaviors. All friction surfaces exhibit the same type of structure. The sample surfaces are examined
using various techniques. For given time and energy absorbed, we observed a structural change from graphitic to amorphous carbon has occurred on the surface of the specimen (stage 1). However, for an extended friction conditions (stages 2 and 3), wear tracks created at the transition stage are very porous due to surface damage. The classical explanation for the low friction of graphite relates friction to the weak binding energy of the graphite lattice, therefore little energy is needed to induce the cleavage or shear, and so the coefficient of friction is low if the graphite is oriented parallel to the direction of rubbing. However this easy cleavage is true only when wet and condensable vapors are present in the environment. If there is any substantial rise in surface temperatures, the water saturation decreases and the carbon surface is physically desorbed, leading to an increase of coefficient of friction. High coefficient of friction and wear may also be induced at room temperatures if the combination of load and velocity produce an increase of surface temperature in the order of 600 ~ or greater. Apart from the water effect, there are other factors that influence the friction transition and the magnitude of the coefficient of friction. Finely divided debris from the wear process forms the friction film made of nanometric amorphous sheared particles. This friction film may enhance the preferred crystallographic orientation and direction of easy shear of carbon crystallites or rolled particulates. It has reported that this effect is occurring in a cyclic manner during the sliding of carbon. It was reported
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that when the friction rises, the surface stresses become sufficient to disrupt the layer-consolidated contact area decreases. This leads to a sudden increase in the friction coefficient [ 15]. However, we observe that an increase of friction coefficient is due to the inter-laminar shear of particulates formed from the wear debris. During the run-in process, an amorphous friction film is built up. The shear stress within the film increases leads to either mechanical or thermal failure of the surface (friction transition) and causes an increase in the friction coefficient. The stresses increase until it disrupts the film and causes a sudden change in friction coefficient. However, the disruption of the film releases the wear particulates and caused a decrease in true contact area. Decrease of contact area keeps the friction coefficient high but decreases gradually as film is being built again. The shear stress may propagate and crack the bulk material. The crack in the bulk material will affect the behavior of the friction film and being affected by it.
debris. Then, the surface is roughened and the real 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14.
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15.
Blau, P. J., Joumal of Tribology, 109, (1987) 537-544. Brunt, C. V. and Savage, R.H., Gen. Elec. 47, (1944) 16. Ramadanoffand, D. and Glass, S.W., Trans. AIEE, 6, (1944) 825. Savage, R. H., J. Appl. Phys. 19, (1948) 136. Savage, R. H., J. App. Phys. 19, No. 1, (1948) 1. Savage, R.H. and Shaffer, D.L., J. Appl. Phys., 27, (1956) 136. Bryant, P.J., Gutshall, P. C. and Taylor, L. H., Mechanisms of Solid Friction, Elsevier Amsterdam, (1964) 118. Yen, B. K. and Ishihara, T., Wear 174, (1994) 111-117. Li, C. C. and Sheehan, J. E., Int. Conf., ASME, (1981) 525-533. Kragelskii, I. V., Butterworths, London (1965). Kimura, S., Yasuda, E., Narita, N., JSLE, Int. Ed. 5, (1984) 11-16. Kim, D. G., Kweon, D. W., Lee, J. Y. and Mater, J., Sci. Lett. 12, (1993) 8-10. Midley, J.W. and Teer, D.G., Trans. ASME, 85, (1963) 488.