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C composites under tribological loading
Wear 269 (2010) 104–111
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Fiber–matrix unbonding and plastic deformation in C/C composites under tribological loading Haytam Kasem a,b,∗ , Sylvie Bonnamy a , Yves Berthier b , Pascale Jacquemard c a b c
CRMD, UMR 6619 CNRS-Université d’Orléans, 1B rue de la Férollerie, 45071 Orléans Cedex 2, France LaMCoS, UMR 5514, CNRS-INSA de Lyon, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex, France Messier-Bugatti, 7 Avenue du Bel Air, 69627 Villeurbanne Cedex, France
a r t i c l e
i n f o
Article history: Received 30 September 2009 Received in revised form 10 March 2010 Accepted 15 March 2010 Available online 20 March 2010 Keywords: Carbon/carbon composite Tribology Surface damages Fiber–matrix unbonding Plastic deformation
a b s t r a c t Carbon/carbon composites exhibit specific tribological behaviour during friction, i.e. the friction coefficient and wear rate decrease as temperature in the contact increases. In order to better understand the wear mechanisms involved at low and high temperatures, friction experiments were performed under air on a tribometer in disc-on-disc configuration and the worn surfaces were characterized at different length scales by OM, SEM and AFM techniques. It was seen that the high wear rate occurring at low temperature was due to severe mechanical abrasion. Worn and scratched surfaces were observed, with the presence of fiber–matrix unbonding and broken and torn-out fibers and the formation of high amounts of third body, demonstrating the fragile behaviour of the C/C composite at low temperature. On the contrary, the low wear rate obtained at high temperature was due to a soft mechanical process, since the main phenomena observed were pyrocarbon matrix exfoliation and expansion and plastic deformation of Z-fiber surfaces, modifying the stress field close to the rubbed surfaces. This involved the detachment of small particles, leading to smooth surfaces. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon/carbon (C/C) composite materials are the optimal choice for aircraft brake discs because of their remarkable properties, i.e. low density (ranging from 1.8 to 2 g/cm3 ), thermo-mechanical properties at over 2000 ◦ C, high thermal capacities and thermal conductivity, low thermal expansion, and excellent ablative and friction properties [1,2]. Since the 1960s, they have undergone significant development and improvement; however, in spite of a large number of papers in the literature dealing with their tribological behaviour, the mechanisms governing their friction and wear are not yet fully understood and are still subject to debate. It is now well known that parameters such as the atmosphere (humidity and oxygen partial pressure) have a considerable effect on the tribological behaviour of C/C composites [2–5], especially concerning the occurrence of abrupt friction transition [6] which is highly dependent on the desorption of H2 O from the rubbed surfaces. It has also been reported that friction is greatly influenced by the existence of adhesive forces between the two opposing surfaces in contact [7]. Giltrow and Lancaster [8,9] suggested that wear
∗ Corresponding author at: Université des Sciences et Technologies de Lille, Polytech’Lille, bâtiment D - bureau 238, Avenue Paul Langevin, 59655 Villeneuve d’Ascq Cedex, France. Tel.: +33 03 20 43 65 14; fax: +33 03 20 33 71 53. E-mail address: [email protected] (H. Kasem). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.03.016
strongly depends on the roughness of the opposing surfaces. They also pointed out that roughness can be modified during friction by carbon transfer and abrasion. Other authors have tried to quantify the effect of carbon oxidation on tribological behaviour during friction [10–12]. They showed that oxidation occurs only at high friction regimes associated with high wear rates and that carbon gas production depends on the C/C material used and the size of the detached particles. As for the influence of operational mechanical conditions on wear and friction behaviour, this has been dealt with by many papers [5,13,14]. Among them, Yen and Ishihara observed that a load increase during friction under air leads to the onset of transition and increased wear rate [5]. Similarly, others papers have pointed out that an increase of sliding speed also leads to a friction transition and thus increases wear rates [13,14]. However, following friction transition, increasing temperature up to 500 ◦ C leads to a decrease in wear rate [1,10]. Worn surfaces [15,16] and polished cross sections cut perpendicularly to the rubbed surface [10,17] have often been characterized to identify the mechanisms involved in the friction and wear of C/C composite. It has been seen that the damaged depth of the material due to friction is deeper when the wear rate is higher. The aim of this paper is to first describe and analyze the damages occurring at the surface and close to the rubbed surface of the first bodies and then to correlate the types of damage to the tribological data (friction coefficient and wear rate). To achieve this,
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Fig. 1. Polished surface of the C/C composite observed under optical microscopy between crossed polarizers and with the addition of a retarder plate.
friction experiments were performed on a disc-on-disc tribometer simulating complete braking stops. After friction, the worn surfaces were characterized at different scales, i.e. by laser profilometer, optical microscopy, scanning electron microscopy and atomic force microscopy (OM, SEM and AFM). 2. Experimental 2.1. Material The material used was a carbon/carbon PAN-CVI composite. It consists of a three-directional needled preform made of polyacrylonitrile-based carbon fibers (PAN) is densified by chemical vapour infiltration (CVI), resulting in a pyrocarbon matrix with rough lamellar texture. In the C/C composite, the longitudinal fibers are oriented in the frictional plane while the Z-fibers are those oriented perpendicularly to the rubbing surface (Fig. 1). No further heat treatment at high temperature was performed for this composite after CVI densification over 1000 ◦ C. 2.2. Experimental set-up Friction experiments were performed on a disc-on-disc tribometer configuration. The discs (rotor and stator) had outer and inner diameters of 133 mm and 98 mm respectively, with a thickness of 14 mm. In the present study, the experimental conditions simulate complete braking stops at reduced scale. The friction experiments were performed under an imposed relative humidity of about 20%. In addition, two external heat sources (one for the rotor and the second for the stator) were used to control the initial temperatures. The experimental parameters were: • Initial linear sliding speed of 4.7 ms−1 , calculated at the center of the friction track, • Applied nominal pressure of 0.32 MPa, • Initial temperatures: 60, 100, 140, 180 and 220 ◦ C, • Preliminary running-in of the braking system precedes the experiments to ensure good contact conformity and reproducibility of the tribological behaviour. After this running-in stage, identical braking cycles were repeated 200 times for each experiment. Tribological behaviour (friction profile and wear rate) was reproducible for each cycle. The following parameters were measured and calculated for each experiment: • For a single braking cycle, the average friction coefficient was calculated for the whole braking duration (in one cycle). It was
Fig. 2. Tribological results: decrease of average friction coefficient and average wear rate (expressed in m per face and per stop) as initial temperature increases in the contact.
obtained by dividing the tangential force by the normal one. The tangential force was deducted from the torque generated by the friction. Finally, the average friction coefficient for the 200 brakings was calculated. • The average wear rate (expressed in m/face/stop) was calculated from the average thickness lost (rotor and stator) over the number of brakings (200). The average thickness lost was measured from the distance, i.e. separation, between the upper rotor surface and a fixed reference. This wear rate only took into account the decrease in thickness of the couple of discs and neglects the weight loss due to oxidation. • The rotor and the stator temperatures were measured by thermocouples located about 1 mm from the friction surface. 2.3. Characterization techniques After the friction experiments, the worn surfaces were characterized at different length scales by using: • 2-Axis laser profilometer (UBM: lateral resolution 1 m, height resolution 10 nm). Worn surface profiles were studied radially (perpendicular to the sliding direction). • Optical microscopy (OM, Leica DM IRM). OM images were taken between crossed polarizers with the addition of a retarder plate ( plate) that introduces a phase shift of 551 nm. This configuration permits investigating the average aromatic layer directions and orientations. • Scanning electron microscopy (SEM, Hitachi S4200 equipped with a field emission gun). SEM images were taken at a reduced acceleration voltage (1 kV) in order to limit the secondary electron analysis to the outermost surface. Atomic force microscopy (Molecular Imaging Pico) was used in tapping mode. 3. Results and discussion 3.1. Tribological data Five friction experiments, each corresponding to 200 braking operations, were performed at an initial temperature ranging from 60 to 220 ◦ C. The tribological data, reported in Fig. 2, show that both the friction coefficient and wear rate decrease as the initial temperature in the contact increases. The error bars express the maximum dispersion of the friction coefficient and wear rate on the 200 brakings. These results agree well with the data in the literature [1,10].
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Fig. 3. 2D laser profilometer observations, radial profiles of the rubbed surfaces after friction experiments performed at (a) low initial temperature 60 ◦ C and (b) high initial temperature 220 ◦ C.
However, the friction coefficient and wear rate did not decrease in the same ratio. Indeed, by comparing data obtained at the lowest and highest temperatures, a 36% decrease was observed for the friction coefficient, whereas an 80% decrease was obtained for the wear rate. This clearly underlines that friction and wear are not systematically correlated and already suggests that different mechanisms are involved at low and high temperatures.
3.2. Characterization of the worn surfaces In order to identify the wear mechanisms involved, the worn surfaces resulting from friction tests performed at 60 and 220 ◦ C were systematically characterized at different length scales to highlight the several types of damages undergone by the C/C composites during friction.
Fig. 4. OM characterization of worn surfaces after friction at (a) low temperature: occurrence of damaged surfaces and (b) high temperature: occurrence of smooth surface.
Fig. 5. (a) SEM and (b) OM images of the same area of a worn surface after friction at low temperature (high friction coefficient associated with high wear rate): {1} the presence of third body in large quantities, {2} damaged pyrocarbon and {3} degraded and broken fibers.
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Fig. 6. (a) SEM and (b) OM images of the same area of a worn surface after friction at high temperature (low friction coefficient associated with low wear rate): {1} small amount of third body {2}, “smooth” pyrocarbon surface and {3} as if polished fibers.
Fig. 7. SEM characterization of worn surfaces rubbed at low temperature: (a) Z-fiber/matrix unbonding, (b) the presence of broken third body agglomerates, a part is on the pulled Z-fibers, the other part is on the pyrocarbon matrix surrounding the fibers.
3.2.1. Worn surface morphologies In the case of high wear resulting from friction at low initial temperature, 2-axis laser profilometry performed radially showed that the surfaces were mostly grooved and rough (Fig. 3a). In the contrary, the surfaces resulting from friction at high temperature were relatively smooth (Fig. 3b). Optical microscopy images taken at low magnification show that the surfaces rubbed at low tem-
perature were degraded due to quite severe abrasion, exhibited by numerous scratches strongly ground into the worn surface (Fig. 4a). Conversely, during friction at high initial temperature associated with a low wear rate, the surfaces were smooth and the scratches were not evident (Fig. 4b).
Fig. 8. Schematic explanation of the presence of Z-fibers above the rubbed surface: (a) during friction, elastic buckling of long Z-fibers and (b) relaxation of Z-fiber after opening the contact.
Fig. 9. Schematic explanation of the presence of Z-fibers below the rubbed surface: (a) during friction, compression of the composite and (b) relaxation of the composite after opening the contact.
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Fig. 10. SEM characterization of worn surfaces rubbed at high temperature. Exfoliation of the pyrocarbon matrix is oriented perpendicular to the friction direction.
Since OM and SEM are complementary imaging techniques useful for characterizing C/C composites [18], a comparison of the worn surface images was systematically carried out for both techniques at the same magnification, thereby limiting errors in the interpretation of surface damage. In the case of high wear resulting from friction at low temperature, Fig. 5 confirms that the surfaces are mostly degraded, with the fibers and matrix textures becoming hardly recognizable. Indeed, the pyrocarbon matrix shows severe wear with bits of the matrix removed, leaving voids (see {2} in Fig. 5), and the fibers are mainly broken or torn out (see {3} in Fig. 5). As for the porosities and holes initially present or formed during friction, they are totally filled with third body particles (see {1}in Fig. 5). In the case of low wear resulting from friction at high temperature (220 ◦ C), the surface is smooth, with both the pyrocarbon matrix and fibers presenting a rather polished appearance (Fig. 6). Only fibers torn out locally and a small amount of third body filled the porosities 3.2.2. Fiber–matrix unbonding SEM images at high magnification underline other important phenomena regarding the unbonding between the Z-fibers and the pyrocarbon matrix observed after friction at low temperature (Fig. 7a). It should be noted that the Z-fibers either protrude or are retracted below the surface. When they protrude from the sur-
face, the same agglomerates of third body particles cover both the Z-fibers surface and the surrounding matrix (Fig. 7b). This means that during friction the fibers were compressed in the contact and it was only upon opening the latter that they relaxed, leading to the fragmentation of the third body agglomerates. It is well known that carbon fibers have a very low thermal expansion coefficient that can even be negative up to 400 ◦ C, and over this temperature it is: ˛// ∼ 0.3 × 10−6 and ˛⊥ ∼ 36 × 10−6 . Taking into account these data, they cannot explain the presence of the Z-fibers above or below the worn surface after opening the contact. This is probably due to fiber–matrix unbonding in the first strata (close to the worn surface), whereas the Z-fibers remain united (in adhesion) in the strata below the rubbing surface: • In the case of the Z-fibers observed above the worn surface, they should be long and undergo elastic buckling during friction (Fig. 8a). Due to unbonding, the normal load on these fibers remains very low which explains their low wear rate. Once the contact is opened, the Z-fibers initially buckling relax and protrude from the rubbed surface (Fig. 8b). • As for Z-fibers observed below the rubbed surface, they should be relatively short and pulled towards the rubbing surface as a consequence of the elastic compression of the bulk composite due to the presence of macro-porosities (Fig. 9a). When the contact is opened, the strata below the surface relax towards the bulk of the
Fig. 11. AFM characterization of a cleaned worn surface after friction at high temperature. Expansion of the pyrocarbon matrix surrounding longitudinal fibers.
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composite, stretching with them the short Z-fibers that therefore appear below the rubbed surface (Fig. 9b). This fiber–matrix unbonding is considerably greater in the case of friction at low temperature. In a previous work, it is shown that unbonding is more accentuated in the case of C/C composite heat-treated after densification [17]. This confirms that unbonding strongly depends on the temperatures that the fiber and matrix have undergone during the manufacturing process. 3.2.3. Pyrocarbon exfoliation and expansion SEM observations at high magnification showed a pyrocarbon matrix exfoliation only on the worn surfaces resulting from friction at high temperature. It is always localized around the Z-fibers and accentuated in the areas where the pyrocarbon aromatic layers are oriented perpendicular to the friction direction (Fig. 10). Stacks of thick aromatic layers are completely folded and close to the point of being removed in the friction direction (Fig. 10). This exfoliation shows evidence of a local plastic deformation in the matrix. Surface damage was also observed on polished section cut perpendicular to the rubbed surface. Data were reported in a previous paper [10] describing that matrix exfoliation affects the C/C composite to an average depth of 3 m. This type of first body degradation (plastic deformation) is quite similar to the surface tribological transformation (STT) which has been identified and well studied in the case of metals [19]. To investigate damage at micrometric length scale, the components (longitudinal and Z-fibers as well as pyrocarbon around the
Fig. 12. High magnification OM image of a cleaned worn surface after friction at high temperature showing an oxidized pyrocarbon around longitudinal fibers.
longitudinal and Z-fibers) of the worn C/C composite were observed by AFM at high magnification. Previous to their characterization, the third body particles trapped on the worn surface were eliminated by ultrasound in order to obtain surfaces devoid of all types of wear debris capable of blurring AFM observations. These observations clearly showed that, after friction at high temperature, the pyrocarbon is always in relief with regard to the average friction surface; in particular that surrounding the longitu-
Fig. 13. (a) AFM characterization of a cleaned worn surface after friction at high temperature: (b) 3D representation of a high magnification of a wrinkled Z-fiber surface. It shows the occurrence of plastic deformation and/or micro-impact wear on the Z-fiber surface; (c) and (d) are the profiles of the wrinkled fiber surface, as drawn in (a). They are oriented respectively in parallel and perpendicular to the sliding direction.
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dinal fibers was observed to be 30–50 nm above the rubbed surface (Fig. 11). Since it is well known that pyrocarbon is more compressible than fibers [20], it compresses more readily in the average frictional plane during friction. Once the load is removed (opening of the contact area after friction), the pyrocarbon can expand and thus rise above the rubbed surface. The fact that the pyrocarbon expansion is not observed after friction at low temperature can be explained by the high wear rate at low temperature and by the contribution of high oxidation at high temperature in increasing pyrocarbon compressibility (Fig. 12).
3.2.4. Damage of Z-fiber extremities In addition to pyrocarbon expansion, AFM observations highlighted the existence of regular and smooth wrinkle marks observed on the Z-fiber surfaces (Fig. 13a and b). They are oriented perpendicular to the sliding direction and are no more than 10 nm in depth (see profiles in Fig. 13c and d). As this morphology is observed only after friction at high temperature, it can be explained by the occurrence of plastic deformation at the extremities of the Z-fibers during friction. The self-induced plasticity of PAN-based carbon fibers has already been evidenced by in situ mechanical tests performed at high temperature, showing mechanical changes from fragile to ductile at high temperature [21,22]. These wrinkle marks could also result from locally high temperature combined with severe tribological stresses (compression and shearing). Indeed, the elastic modulus of carbon fibers is higher according to their axis (Z direction), about 27 GPa, than in the longitudinal one or in the pyrocarbon matrix, where the elastic modulus is in the range 9–14 GPa [20]. Therefore, the Z-fibers undergo most of the compression stresses during friction [23]. Consequently, they warm up more than the other components of the composite. Thus, during friction at high initial temperature (220 ◦ C), the temperature of the surface occupied by the Z-fibers would be the highest, explaining why these wrinkle marks are only observed on the Zfibers. On the one hand, the elevated temperature may favour the local occurrence of plastic deformation close to the rubbing surface, analogous with surface tribological transformation STT [19]. On the other hand, it may cause thermoelastic friction instabilities [24] that usually lead to a dynamic response characterized by the propagation on the contacting surface of sliding, sticking and separation areas (stick-slip-separation waves) [25], thus leading to micro-impact wear.
4. Conclusion In order to better understand the wear mechanisms of C/C composites during friction at low and high temperatures, characterization of the damage of the first bodies occurring at the surface and close to the worn surface was performed. Then, the damage was correlated to the friction coefficient and the wear rate, which decreased as temperature increased in the contact. The results showed that: – During friction at low temperature (high wear rate), most of the surface was degraded, resulting from particularly severe abrasion. The latter was apparent in the form of numerous scratches, broken and torn-out fibers, a strongly worn pyrocarbon matrix, many fiber–matrix unbonding, and a high amount of third body filling the holes (resulting from abrasion) and the porosities (initially present or formed during friction). All this mechanical damage is evident of fragile behaviour of the C/C composite during friction. Consequently, a large amount of large particles were removed, leading to a high source flow and to damaged surfaces, with an increase in the friction coefficient and wear rate [10].
– During friction at high temperature (low wear rate), the surfaces were very smooth due to the detachment of smaller size particles occurring at the outermost surface of the materials. SEM and AFM observations at high magnification highlighted three important phenomena: exfoliation of the pyrocarbon matrix around the Zfibers, perpendicular to the friction direction, which expresses a plastic deformation; expansion of the pyrocarbon surrounding the longitudinal fibers; and plastic deformation and/or microimpact wear which affect only the extremities of the Z-fibers during friction at high temperature. Although these damages affect the material to a shallow depth, they limit the stress field close to the rubbing surface, limiting the depth of the composite affected by friction. This change in mechanical behaviour occurring during friction at high temperature is coupled with a modification of the characteristics of the third body [17], with both contributing to reducing the wear rate at high temperature. Acknowledgements The authors greatly thank Annie Richard (Centre de Microscopie Electronique, Université d’Orléans, France) and Marylène Vayer (CRMD, France) for their help, respectively, in SEM imaging and AFM characterization. References [1] S. Awasthi, J.L. Wood, C/C composite materials for aircraft brakes, Adv. Ceram. Mater. 3 (1988) 449–451. [2] C. Blanco, J. Bermejo, Chemical and physical properties of carbon as related to brake performance, Wear 213 (1–2) (1997) 1–12. [3] M. Krkoska, P. Filip, Humidity and frictional performance of C/C composites, Ceram. Eng. Sci. Proc. 26 (8) (2005) 139–155. [4] B.K. Yen, Influence of water vapor and oxygen on the tribology of carbon materials with sp2 valence configuration, Wear 192 (1996) 208–215. [5] B.K. Yen, T. Ishihara, An investigation of friction and wear mechanisms of carbon–carbon composites in nitrogen and air at elevated temperatures, Carbon 34 (1995) 489–498. [6] H. Kasem, S. Bonnamy, Y. Berthier, P. Dufrénoy, P. Jacquemard, Tribological, physicochemical and thermal study of the abrupt friction transition during carbon/carbon composite friction, Wear 267 (2009) 846–852. [7] I.C. Roselman, D. Tabor, The friction of carbon fibres, J. Phys. D: Appl. Phys. 9 (1976) 2517–2532. [8] J.P. Giltrow, J.K. Lancaster, The role of the counterface in the friction and wear of carbon fibre reinforced thermosetting resins, Wear 16 (5) (1970) 359–374. [9] J.K. Lancaster, W.T. Clark, Breakdown and surface fatigue of carbons during repeated sliding, Wear 6 (6) (1963) 467–482. [10] H. Kasem, S. Bonnamy, B. Rousseau, H. Estrade-Szwarckopf, Y. Berthier, P. Jacquemard, Interdependence between wear process, size of detached particles and CO2 production during carbon/carbon composite friction, Wear 263 (2007) 1220–1229. [11] M. Gouider, Y. Berthier, P. Jacquemard, B. Rousseau, S. Bonnamy, H. Estrade-Szwarckopf, Mass spectrometry during C/C composite friction: carbon oxidation associated with high friction coefficient and high wear rate, Wear 256 (2004) 1082–1087. [12] H.W. Chang, Correlation of wear with oxidation of carbon–carbon composite, Wear 80 (1982) 7–14. [13] J.D. Chen, C.P. Ju, Effect of sliding speed on the tribological behaviour of a PANpitch carbon–carbon composite, Mater. Chem. Phys. 39 (1995) 174–179. [14] J.R. Gomes, O.M. Silva, The effect of sliding speed and temperature on the tribological behaviour of carbon/carbon composites, Wear 249 (2001) 240–245. [15] N. Murdie, C.P. Ju, J. Don, F.A. Fortunato, Microstructure of worn pitch/resin/CVI C–C composites, Carbon 29 (3) (1991) 335–342. [16] B.K. Yen, T. Ishihara, The surface morphology and structure of carbon–carbon composites in high-energy sliding contact, Wear 174 (1–2) (1994) 111–117. [17] H. Kasem, Etude du comportement tribologique de composites C/C sous sollicitations de freinage aéronautique. Approches mécanique et physico-chimique, Ph.D. Thesis, University of Orléans-France, 2008. [18] B. Rousseau, H. Estrade-Szwarckopf, S. Bonnamy, M. Gouider, Y. Berthier, P. Jacquemard, Optical and scanning electron microscopies cross-fertilization: application to worn carbon/carbon composite surface studies, Carbon 43 (2005) 1334–1337. [19] A. Eleod, F. Oucherif, J. Devecz, Y. Berthier, Conception of numerical and experimental tools for study of the tribological transformation of surface TTS, Tribology 36 (1998) 673–682. [20] P. Diss, J. Lamon, L. Carpentier, J.L. Loubet, Ph. Kapsa, Sharp indentation behaviour of carbon/carbon composites and varieties of carbon, Carbon 40 (2002) 2567–2579.
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composites Carbone/Carbone utilisés en freinage aéronautique, Ph.D. Thesis, INSA de Lyon-France, 2007. [24] J.R. Barber, Thermoelastic instabilities in the sliding of conforming solids, Proc. Roy. Soc. A312 (1969) 381–394. [25] V. Linck, L. Baillet, Y. Berthier, Dry friction: influence of local dynamic aspect on contact pressure, kinematics and friction, Tribol. Interface Eng. Ser. 43 (2003) 545–552.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Fiber–matrix unbonding and plastic deformation in C/C composites under tribological loading Haytam Kasem a,b,∗ , Sylvie Bonnamy a , Yves Berthier b , Pascale Jacquemard c a b c
CRMD, UMR 6619 CNRS-Université d’Orléans, 1B rue de la Férollerie, 45071 Orléans Cedex 2, France LaMCoS, UMR 5514, CNRS-INSA de Lyon, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex, France Messier-Bugatti, 7 Avenue du Bel Air, 69627 Villeurbanne Cedex, France
a r t i c l e
i n f o
Article history: Received 30 September 2009 Received in revised form 10 March 2010 Accepted 15 March 2010 Available online 20 March 2010 Keywords: Carbon/carbon composite Tribology Surface damages Fiber–matrix unbonding Plastic deformation
a b s t r a c t Carbon/carbon composites exhibit specific tribological behaviour during friction, i.e. the friction coefficient and wear rate decrease as temperature in the contact increases. In order to better understand the wear mechanisms involved at low and high temperatures, friction experiments were performed under air on a tribometer in disc-on-disc configuration and the worn surfaces were characterized at different length scales by OM, SEM and AFM techniques. It was seen that the high wear rate occurring at low temperature was due to severe mechanical abrasion. Worn and scratched surfaces were observed, with the presence of fiber–matrix unbonding and broken and torn-out fibers and the formation of high amounts of third body, demonstrating the fragile behaviour of the C/C composite at low temperature. On the contrary, the low wear rate obtained at high temperature was due to a soft mechanical process, since the main phenomena observed were pyrocarbon matrix exfoliation and expansion and plastic deformation of Z-fiber surfaces, modifying the stress field close to the rubbed surfaces. This involved the detachment of small particles, leading to smooth surfaces. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon/carbon (C/C) composite materials are the optimal choice for aircraft brake discs because of their remarkable properties, i.e. low density (ranging from 1.8 to 2 g/cm3 ), thermo-mechanical properties at over 2000 ◦ C, high thermal capacities and thermal conductivity, low thermal expansion, and excellent ablative and friction properties [1,2]. Since the 1960s, they have undergone significant development and improvement; however, in spite of a large number of papers in the literature dealing with their tribological behaviour, the mechanisms governing their friction and wear are not yet fully understood and are still subject to debate. It is now well known that parameters such as the atmosphere (humidity and oxygen partial pressure) have a considerable effect on the tribological behaviour of C/C composites [2–5], especially concerning the occurrence of abrupt friction transition [6] which is highly dependent on the desorption of H2 O from the rubbed surfaces. It has also been reported that friction is greatly influenced by the existence of adhesive forces between the two opposing surfaces in contact [7]. Giltrow and Lancaster [8,9] suggested that wear
∗ Corresponding author at: Université des Sciences et Technologies de Lille, Polytech’Lille, bâtiment D - bureau 238, Avenue Paul Langevin, 59655 Villeneuve d’Ascq Cedex, France. Tel.: +33 03 20 43 65 14; fax: +33 03 20 33 71 53. E-mail address: [email protected] (H. Kasem). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.03.016
strongly depends on the roughness of the opposing surfaces. They also pointed out that roughness can be modified during friction by carbon transfer and abrasion. Other authors have tried to quantify the effect of carbon oxidation on tribological behaviour during friction [10–12]. They showed that oxidation occurs only at high friction regimes associated with high wear rates and that carbon gas production depends on the C/C material used and the size of the detached particles. As for the influence of operational mechanical conditions on wear and friction behaviour, this has been dealt with by many papers [5,13,14]. Among them, Yen and Ishihara observed that a load increase during friction under air leads to the onset of transition and increased wear rate [5]. Similarly, others papers have pointed out that an increase of sliding speed also leads to a friction transition and thus increases wear rates [13,14]. However, following friction transition, increasing temperature up to 500 ◦ C leads to a decrease in wear rate [1,10]. Worn surfaces [15,16] and polished cross sections cut perpendicularly to the rubbed surface [10,17] have often been characterized to identify the mechanisms involved in the friction and wear of C/C composite. It has been seen that the damaged depth of the material due to friction is deeper when the wear rate is higher. The aim of this paper is to first describe and analyze the damages occurring at the surface and close to the rubbed surface of the first bodies and then to correlate the types of damage to the tribological data (friction coefficient and wear rate). To achieve this,
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Fig. 1. Polished surface of the C/C composite observed under optical microscopy between crossed polarizers and with the addition of a retarder plate.
friction experiments were performed on a disc-on-disc tribometer simulating complete braking stops. After friction, the worn surfaces were characterized at different scales, i.e. by laser profilometer, optical microscopy, scanning electron microscopy and atomic force microscopy (OM, SEM and AFM). 2. Experimental 2.1. Material The material used was a carbon/carbon PAN-CVI composite. It consists of a three-directional needled preform made of polyacrylonitrile-based carbon fibers (PAN) is densified by chemical vapour infiltration (CVI), resulting in a pyrocarbon matrix with rough lamellar texture. In the C/C composite, the longitudinal fibers are oriented in the frictional plane while the Z-fibers are those oriented perpendicularly to the rubbing surface (Fig. 1). No further heat treatment at high temperature was performed for this composite after CVI densification over 1000 ◦ C. 2.2. Experimental set-up Friction experiments were performed on a disc-on-disc tribometer configuration. The discs (rotor and stator) had outer and inner diameters of 133 mm and 98 mm respectively, with a thickness of 14 mm. In the present study, the experimental conditions simulate complete braking stops at reduced scale. The friction experiments were performed under an imposed relative humidity of about 20%. In addition, two external heat sources (one for the rotor and the second for the stator) were used to control the initial temperatures. The experimental parameters were: • Initial linear sliding speed of 4.7 ms−1 , calculated at the center of the friction track, • Applied nominal pressure of 0.32 MPa, • Initial temperatures: 60, 100, 140, 180 and 220 ◦ C, • Preliminary running-in of the braking system precedes the experiments to ensure good contact conformity and reproducibility of the tribological behaviour. After this running-in stage, identical braking cycles were repeated 200 times for each experiment. Tribological behaviour (friction profile and wear rate) was reproducible for each cycle. The following parameters were measured and calculated for each experiment: • For a single braking cycle, the average friction coefficient was calculated for the whole braking duration (in one cycle). It was
Fig. 2. Tribological results: decrease of average friction coefficient and average wear rate (expressed in m per face and per stop) as initial temperature increases in the contact.
obtained by dividing the tangential force by the normal one. The tangential force was deducted from the torque generated by the friction. Finally, the average friction coefficient for the 200 brakings was calculated. • The average wear rate (expressed in m/face/stop) was calculated from the average thickness lost (rotor and stator) over the number of brakings (200). The average thickness lost was measured from the distance, i.e. separation, between the upper rotor surface and a fixed reference. This wear rate only took into account the decrease in thickness of the couple of discs and neglects the weight loss due to oxidation. • The rotor and the stator temperatures were measured by thermocouples located about 1 mm from the friction surface. 2.3. Characterization techniques After the friction experiments, the worn surfaces were characterized at different length scales by using: • 2-Axis laser profilometer (UBM: lateral resolution 1 m, height resolution 10 nm). Worn surface profiles were studied radially (perpendicular to the sliding direction). • Optical microscopy (OM, Leica DM IRM). OM images were taken between crossed polarizers with the addition of a retarder plate ( plate) that introduces a phase shift of 551 nm. This configuration permits investigating the average aromatic layer directions and orientations. • Scanning electron microscopy (SEM, Hitachi S4200 equipped with a field emission gun). SEM images were taken at a reduced acceleration voltage (1 kV) in order to limit the secondary electron analysis to the outermost surface. Atomic force microscopy (Molecular Imaging Pico) was used in tapping mode. 3. Results and discussion 3.1. Tribological data Five friction experiments, each corresponding to 200 braking operations, were performed at an initial temperature ranging from 60 to 220 ◦ C. The tribological data, reported in Fig. 2, show that both the friction coefficient and wear rate decrease as the initial temperature in the contact increases. The error bars express the maximum dispersion of the friction coefficient and wear rate on the 200 brakings. These results agree well with the data in the literature [1,10].
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Fig. 3. 2D laser profilometer observations, radial profiles of the rubbed surfaces after friction experiments performed at (a) low initial temperature 60 ◦ C and (b) high initial temperature 220 ◦ C.
However, the friction coefficient and wear rate did not decrease in the same ratio. Indeed, by comparing data obtained at the lowest and highest temperatures, a 36% decrease was observed for the friction coefficient, whereas an 80% decrease was obtained for the wear rate. This clearly underlines that friction and wear are not systematically correlated and already suggests that different mechanisms are involved at low and high temperatures.
3.2. Characterization of the worn surfaces In order to identify the wear mechanisms involved, the worn surfaces resulting from friction tests performed at 60 and 220 ◦ C were systematically characterized at different length scales to highlight the several types of damages undergone by the C/C composites during friction.
Fig. 4. OM characterization of worn surfaces after friction at (a) low temperature: occurrence of damaged surfaces and (b) high temperature: occurrence of smooth surface.
Fig. 5. (a) SEM and (b) OM images of the same area of a worn surface after friction at low temperature (high friction coefficient associated with high wear rate): {1} the presence of third body in large quantities, {2} damaged pyrocarbon and {3} degraded and broken fibers.
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Fig. 6. (a) SEM and (b) OM images of the same area of a worn surface after friction at high temperature (low friction coefficient associated with low wear rate): {1} small amount of third body {2}, “smooth” pyrocarbon surface and {3} as if polished fibers.
Fig. 7. SEM characterization of worn surfaces rubbed at low temperature: (a) Z-fiber/matrix unbonding, (b) the presence of broken third body agglomerates, a part is on the pulled Z-fibers, the other part is on the pyrocarbon matrix surrounding the fibers.
3.2.1. Worn surface morphologies In the case of high wear resulting from friction at low initial temperature, 2-axis laser profilometry performed radially showed that the surfaces were mostly grooved and rough (Fig. 3a). In the contrary, the surfaces resulting from friction at high temperature were relatively smooth (Fig. 3b). Optical microscopy images taken at low magnification show that the surfaces rubbed at low tem-
perature were degraded due to quite severe abrasion, exhibited by numerous scratches strongly ground into the worn surface (Fig. 4a). Conversely, during friction at high initial temperature associated with a low wear rate, the surfaces were smooth and the scratches were not evident (Fig. 4b).
Fig. 8. Schematic explanation of the presence of Z-fibers above the rubbed surface: (a) during friction, elastic buckling of long Z-fibers and (b) relaxation of Z-fiber after opening the contact.
Fig. 9. Schematic explanation of the presence of Z-fibers below the rubbed surface: (a) during friction, compression of the composite and (b) relaxation of the composite after opening the contact.
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Fig. 10. SEM characterization of worn surfaces rubbed at high temperature. Exfoliation of the pyrocarbon matrix is oriented perpendicular to the friction direction.
Since OM and SEM are complementary imaging techniques useful for characterizing C/C composites [18], a comparison of the worn surface images was systematically carried out for both techniques at the same magnification, thereby limiting errors in the interpretation of surface damage. In the case of high wear resulting from friction at low temperature, Fig. 5 confirms that the surfaces are mostly degraded, with the fibers and matrix textures becoming hardly recognizable. Indeed, the pyrocarbon matrix shows severe wear with bits of the matrix removed, leaving voids (see {2} in Fig. 5), and the fibers are mainly broken or torn out (see {3} in Fig. 5). As for the porosities and holes initially present or formed during friction, they are totally filled with third body particles (see {1}in Fig. 5). In the case of low wear resulting from friction at high temperature (220 ◦ C), the surface is smooth, with both the pyrocarbon matrix and fibers presenting a rather polished appearance (Fig. 6). Only fibers torn out locally and a small amount of third body filled the porosities 3.2.2. Fiber–matrix unbonding SEM images at high magnification underline other important phenomena regarding the unbonding between the Z-fibers and the pyrocarbon matrix observed after friction at low temperature (Fig. 7a). It should be noted that the Z-fibers either protrude or are retracted below the surface. When they protrude from the sur-
face, the same agglomerates of third body particles cover both the Z-fibers surface and the surrounding matrix (Fig. 7b). This means that during friction the fibers were compressed in the contact and it was only upon opening the latter that they relaxed, leading to the fragmentation of the third body agglomerates. It is well known that carbon fibers have a very low thermal expansion coefficient that can even be negative up to 400 ◦ C, and over this temperature it is: ˛// ∼ 0.3 × 10−6 and ˛⊥ ∼ 36 × 10−6 . Taking into account these data, they cannot explain the presence of the Z-fibers above or below the worn surface after opening the contact. This is probably due to fiber–matrix unbonding in the first strata (close to the worn surface), whereas the Z-fibers remain united (in adhesion) in the strata below the rubbing surface: • In the case of the Z-fibers observed above the worn surface, they should be long and undergo elastic buckling during friction (Fig. 8a). Due to unbonding, the normal load on these fibers remains very low which explains their low wear rate. Once the contact is opened, the Z-fibers initially buckling relax and protrude from the rubbed surface (Fig. 8b). • As for Z-fibers observed below the rubbed surface, they should be relatively short and pulled towards the rubbing surface as a consequence of the elastic compression of the bulk composite due to the presence of macro-porosities (Fig. 9a). When the contact is opened, the strata below the surface relax towards the bulk of the
Fig. 11. AFM characterization of a cleaned worn surface after friction at high temperature. Expansion of the pyrocarbon matrix surrounding longitudinal fibers.
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composite, stretching with them the short Z-fibers that therefore appear below the rubbed surface (Fig. 9b). This fiber–matrix unbonding is considerably greater in the case of friction at low temperature. In a previous work, it is shown that unbonding is more accentuated in the case of C/C composite heat-treated after densification [17]. This confirms that unbonding strongly depends on the temperatures that the fiber and matrix have undergone during the manufacturing process. 3.2.3. Pyrocarbon exfoliation and expansion SEM observations at high magnification showed a pyrocarbon matrix exfoliation only on the worn surfaces resulting from friction at high temperature. It is always localized around the Z-fibers and accentuated in the areas where the pyrocarbon aromatic layers are oriented perpendicular to the friction direction (Fig. 10). Stacks of thick aromatic layers are completely folded and close to the point of being removed in the friction direction (Fig. 10). This exfoliation shows evidence of a local plastic deformation in the matrix. Surface damage was also observed on polished section cut perpendicular to the rubbed surface. Data were reported in a previous paper [10] describing that matrix exfoliation affects the C/C composite to an average depth of 3 m. This type of first body degradation (plastic deformation) is quite similar to the surface tribological transformation (STT) which has been identified and well studied in the case of metals [19]. To investigate damage at micrometric length scale, the components (longitudinal and Z-fibers as well as pyrocarbon around the
Fig. 12. High magnification OM image of a cleaned worn surface after friction at high temperature showing an oxidized pyrocarbon around longitudinal fibers.
longitudinal and Z-fibers) of the worn C/C composite were observed by AFM at high magnification. Previous to their characterization, the third body particles trapped on the worn surface were eliminated by ultrasound in order to obtain surfaces devoid of all types of wear debris capable of blurring AFM observations. These observations clearly showed that, after friction at high temperature, the pyrocarbon is always in relief with regard to the average friction surface; in particular that surrounding the longitu-
Fig. 13. (a) AFM characterization of a cleaned worn surface after friction at high temperature: (b) 3D representation of a high magnification of a wrinkled Z-fiber surface. It shows the occurrence of plastic deformation and/or micro-impact wear on the Z-fiber surface; (c) and (d) are the profiles of the wrinkled fiber surface, as drawn in (a). They are oriented respectively in parallel and perpendicular to the sliding direction.
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dinal fibers was observed to be 30–50 nm above the rubbed surface (Fig. 11). Since it is well known that pyrocarbon is more compressible than fibers [20], it compresses more readily in the average frictional plane during friction. Once the load is removed (opening of the contact area after friction), the pyrocarbon can expand and thus rise above the rubbed surface. The fact that the pyrocarbon expansion is not observed after friction at low temperature can be explained by the high wear rate at low temperature and by the contribution of high oxidation at high temperature in increasing pyrocarbon compressibility (Fig. 12).
3.2.4. Damage of Z-fiber extremities In addition to pyrocarbon expansion, AFM observations highlighted the existence of regular and smooth wrinkle marks observed on the Z-fiber surfaces (Fig. 13a and b). They are oriented perpendicular to the sliding direction and are no more than 10 nm in depth (see profiles in Fig. 13c and d). As this morphology is observed only after friction at high temperature, it can be explained by the occurrence of plastic deformation at the extremities of the Z-fibers during friction. The self-induced plasticity of PAN-based carbon fibers has already been evidenced by in situ mechanical tests performed at high temperature, showing mechanical changes from fragile to ductile at high temperature [21,22]. These wrinkle marks could also result from locally high temperature combined with severe tribological stresses (compression and shearing). Indeed, the elastic modulus of carbon fibers is higher according to their axis (Z direction), about 27 GPa, than in the longitudinal one or in the pyrocarbon matrix, where the elastic modulus is in the range 9–14 GPa [20]. Therefore, the Z-fibers undergo most of the compression stresses during friction [23]. Consequently, they warm up more than the other components of the composite. Thus, during friction at high initial temperature (220 ◦ C), the temperature of the surface occupied by the Z-fibers would be the highest, explaining why these wrinkle marks are only observed on the Zfibers. On the one hand, the elevated temperature may favour the local occurrence of plastic deformation close to the rubbing surface, analogous with surface tribological transformation STT [19]. On the other hand, it may cause thermoelastic friction instabilities [24] that usually lead to a dynamic response characterized by the propagation on the contacting surface of sliding, sticking and separation areas (stick-slip-separation waves) [25], thus leading to micro-impact wear.
4. Conclusion In order to better understand the wear mechanisms of C/C composites during friction at low and high temperatures, characterization of the damage of the first bodies occurring at the surface and close to the worn surface was performed. Then, the damage was correlated to the friction coefficient and the wear rate, which decreased as temperature increased in the contact. The results showed that: – During friction at low temperature (high wear rate), most of the surface was degraded, resulting from particularly severe abrasion. The latter was apparent in the form of numerous scratches, broken and torn-out fibers, a strongly worn pyrocarbon matrix, many fiber–matrix unbonding, and a high amount of third body filling the holes (resulting from abrasion) and the porosities (initially present or formed during friction). All this mechanical damage is evident of fragile behaviour of the C/C composite during friction. Consequently, a large amount of large particles were removed, leading to a high source flow and to damaged surfaces, with an increase in the friction coefficient and wear rate [10].
– During friction at high temperature (low wear rate), the surfaces were very smooth due to the detachment of smaller size particles occurring at the outermost surface of the materials. SEM and AFM observations at high magnification highlighted three important phenomena: exfoliation of the pyrocarbon matrix around the Zfibers, perpendicular to the friction direction, which expresses a plastic deformation; expansion of the pyrocarbon surrounding the longitudinal fibers; and plastic deformation and/or microimpact wear which affect only the extremities of the Z-fibers during friction at high temperature. Although these damages affect the material to a shallow depth, they limit the stress field close to the rubbing surface, limiting the depth of the composite affected by friction. This change in mechanical behaviour occurring during friction at high temperature is coupled with a modification of the characteristics of the third body [17], with both contributing to reducing the wear rate at high temperature. Acknowledgements The authors greatly thank Annie Richard (Centre de Microscopie Electronique, Université d’Orléans, France) and Marylène Vayer (CRMD, France) for their help, respectively, in SEM imaging and AFM characterization. References [1] S. Awasthi, J.L. Wood, C/C composite materials for aircraft brakes, Adv. Ceram. Mater. 3 (1988) 449–451. [2] C. Blanco, J. Bermejo, Chemical and physical properties of carbon as related to brake performance, Wear 213 (1–2) (1997) 1–12. [3] M. Krkoska, P. Filip, Humidity and frictional performance of C/C composites, Ceram. Eng. Sci. Proc. 26 (8) (2005) 139–155. [4] B.K. Yen, Influence of water vapor and oxygen on the tribology of carbon materials with sp2 valence configuration, Wear 192 (1996) 208–215. [5] B.K. Yen, T. Ishihara, An investigation of friction and wear mechanisms of carbon–carbon composites in nitrogen and air at elevated temperatures, Carbon 34 (1995) 489–498. [6] H. Kasem, S. Bonnamy, Y. Berthier, P. Dufrénoy, P. Jacquemard, Tribological, physicochemical and thermal study of the abrupt friction transition during carbon/carbon composite friction, Wear 267 (2009) 846–852. [7] I.C. Roselman, D. Tabor, The friction of carbon fibres, J. Phys. D: Appl. Phys. 9 (1976) 2517–2532. [8] J.P. Giltrow, J.K. Lancaster, The role of the counterface in the friction and wear of carbon fibre reinforced thermosetting resins, Wear 16 (5) (1970) 359–374. [9] J.K. Lancaster, W.T. Clark, Breakdown and surface fatigue of carbons during repeated sliding, Wear 6 (6) (1963) 467–482. [10] H. Kasem, S. Bonnamy, B. Rousseau, H. Estrade-Szwarckopf, Y. Berthier, P. Jacquemard, Interdependence between wear process, size of detached particles and CO2 production during carbon/carbon composite friction, Wear 263 (2007) 1220–1229. [11] M. Gouider, Y. Berthier, P. Jacquemard, B. Rousseau, S. Bonnamy, H. Estrade-Szwarckopf, Mass spectrometry during C/C composite friction: carbon oxidation associated with high friction coefficient and high wear rate, Wear 256 (2004) 1082–1087. [12] H.W. Chang, Correlation of wear with oxidation of carbon–carbon composite, Wear 80 (1982) 7–14. [13] J.D. Chen, C.P. Ju, Effect of sliding speed on the tribological behaviour of a PANpitch carbon–carbon composite, Mater. Chem. Phys. 39 (1995) 174–179. [14] J.R. Gomes, O.M. Silva, The effect of sliding speed and temperature on the tribological behaviour of carbon/carbon composites, Wear 249 (2001) 240–245. [15] N. Murdie, C.P. Ju, J. Don, F.A. Fortunato, Microstructure of worn pitch/resin/CVI C–C composites, Carbon 29 (3) (1991) 335–342. [16] B.K. Yen, T. Ishihara, The surface morphology and structure of carbon–carbon composites in high-energy sliding contact, Wear 174 (1–2) (1994) 111–117. [17] H. Kasem, Etude du comportement tribologique de composites C/C sous sollicitations de freinage aéronautique. Approches mécanique et physico-chimique, Ph.D. Thesis, University of Orléans-France, 2008. [18] B. Rousseau, H. Estrade-Szwarckopf, S. Bonnamy, M. Gouider, Y. Berthier, P. Jacquemard, Optical and scanning electron microscopies cross-fertilization: application to worn carbon/carbon composite surface studies, Carbon 43 (2005) 1334–1337. [19] A. Eleod, F. Oucherif, J. Devecz, Y. Berthier, Conception of numerical and experimental tools for study of the tribological transformation of surface TTS, Tribology 36 (1998) 673–682. [20] P. Diss, J. Lamon, L. Carpentier, J.L. Loubet, Ph. Kapsa, Sharp indentation behaviour of carbon/carbon composites and varieties of carbon, Carbon 40 (2002) 2567–2579.
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