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silicon-carbide composite
Composites Science and Technology 61 (2001) 417±423
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High-speed tribological behaviour of a carbon/silicon-carbide composite J.-Y. Paris *, L. Vincent, J. Denape Laboratoire GeÂnie de Production, EÂcole Nationale d'IngeÂnieurs de Tarbes, Avenue d'Azereix, BP 1629, 65016 Tarbes Cedex, France Received 20 July 1999; received in revised form 2 November 1999; accepted 16 May 2000
Abstract Increasing attention is being paid to carbon-®bre/SiC-matrix composites (C/SiC) for their friction and wear performance because the carbon-®bre reinforcement enables a signi®cant reduction in the coecient of friction through self-lubricating properties. This study was performed by using a C/SiC composite at high-speed (15 m sÿ1). Two distinct types of tribological behaviour, as a function of the rubbing pin features (100Cr6 or alumina), were found. Analysis of both the surface degradation and velocity adaptation mechanisms has made it possible to establish the major role of debris trapped in the contact zone. The wear debris from the 100Cr6-C/SiC pair is a heterogeneous layer adhering strongly to the friction track. Consequently, the velocity adaptation is a direct eect of the shearing of the transfer layers and fracturing of bulk materials, resulting in severe wear. In contrast, with an alumina-C/SiC pair, wear particles are ®nely ground owing to the fact that they remain for a long time in the contact zone. The velocity adaptation here mainly occurs through debris rolling and shearing within the powder bed, thus resulting in reduced wear. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbon-®bre/SiC-matrix composite (C/SiC); 100Cr6 steel; Alumina; B. Friction/wear; The role of debris
1. Introduction In the last decade, there has been increasing interest in ceramic composites, in part because the use of monolithic ceramic pairs is not a viable option under dry friction conditions (ambient air) since they must be used along with solid lubricants such as graphite, sulphides, etc. [1±3]. On the other hand, the reinforcements of ceramic components (particles, whiskers and ®bres) reduce the risk of fractures and/or, through their selflubricating properties, reduce the coecient of friction. Some timely examples include composites with carbon ®bres [4±8]. However, the self-lubricating qualities of these materials varies, depending on their homogeneity (porosity, interface ®bre/matrix), on the ®brous structure of the carbon reinforcement (degree of graphitization, orientation) and the experimental friction conditions (pressure, speed, temperature and environment), as shown for composites of type C/C and C/SiC [9,10]. Somewhat abandoned to the bene®t of C/C composites, * Corresponding author. Fax: +33-5-6244-2708. E-mail addresses: [email protected] (J.-Y. Paris), [email protected] (J. Denape).
the C/SiC composites are at present showing a clear revival of interest owing to their lower sensibility to surroundings and to oxidisation. This study deals with the behaviour of a C/SiC composite, examining friction and wear at a speed of 15 m sÿ1. Studies conducted prior to this one have used speeds of either below 10 or above 30 m sÿ1. We speci®cally wanted to establish the substantial eect of the velocity adaptation mechanisms on the debris [11]. An analysis of the mechanisms of degradation of the composite C/SiC was therefore conducted while distinguishing between the different outputs of material in the contact zone: source output (debris detached from the surfaces), internal output (debris circulating between the surfaces) and wear output (debris fully eliminated from the contact zone). 2. Materials and experimental procedures 2.1. Materials The composite chosen for our study consists of a matrix of silicon carbide and of carbon ®bres. This composite C/SiC was achieved by a continuous rolling
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00124-X
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J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423
(onto a mandrel) of the carbon ®bres pre-impregnated, through soaking, with SiC. After sintering, the tube thus produced is sliced into discs (``donuts'') of the required ®nal sample dimensions. The ®bres are thereby all oriented more or less parallel to the rubbing surface. Observation of the surface quality by SEM (raw after slicing: Ra=3.1 mm) revealed the existence of a high interstratum porosity (about 23 vol%) associated with an irregular presence of silicon carbide (Fig. 1). The volume fraction of ®bre and matrix are about 0.43 and 0.34, respectively. We also recorded generalized fracturing of surface ®bres, probably as a result of the slicing, and micro cracking of the matrix caused by a dierence in the expansion of the ®bres and the matrix during their handling. Two types of pins were used against the C/SiC: . a pin of high purity alumina (aAl2O3 of 99.7 wt.%) characterized by high mechanical and chemical stability and showing a microstructure of bimodal distribution of the grain sizes (2±5 and 10±20 mm);
. a pin of 100Cr6 (AISI 52100) martensitic steel showing high hardness (807 HV20), but also with a marked sensitivity to increases in temperature. The surface condition of these two materials corresponds to a polished state for the ceramic (Ra=0.07 mm) and a ground state for the steel (Ra=0.27 mm). 2.2. Experimental apparatus and test conditions The test apparatus was a pin-on-disc rotating tribometer. A cylindrical pin of diameter 10 mm chamfered at 45 over 1 mm (contact diameter 8 mm) was applied to a rotating disc of 80 mm diameter and 5 mm thickness. The tribometer instrumentation allowed real time collecting and visualizing of the information relative to coecient of friction (strain gauge for traction/ compression force), cumulated height wear of the samples (displacement gauge LVDT calibrated by keys), and temperature (K-type thermometer, diameter 0.6 mm). For the alumina pins, two thermocouples were glued to the surface of the pin-holder, against the pin and approximately 2 mm from the rubbing surface (one in front of the contact zone, one behind). The tests were performed at a speed of 15 m sÿ1 for 1 h (distance covered: 54 km) in the laboratory atmosphere with the applied load (dead load) ranging from 10 to 200 N (0.2±0.4 MPa). The samples were subject to a rigorous cleaning procedure for over 30 min before the tests so as to eliminate as much surface contamination as possible (grease, carbon dust retained in the coarse surface of the C/SiC). 3. Tribological behaviour 3.1. Friction behaviour
Fig. 1. (a) SEM micrograph of the surface of a C/SiC composite disc; (b) detail of a strand of carbon ®bres.
After a transitory running in period, when the coecient of friction increased rapidly (the period varying from 50 to 650 s for steel and 500 to 1000 s for alumina), the recordings showed a more regular period resulting in a slow but continuous decrease in the coef®cient of friction. However this period remained unstable during the whole test from 40 N with the pair 100Cr6-C/SiC, and from 60 N with the pair alumina-C/ SiC, with cyclic variations (a gradual increase followed by a drop in the coecient of friction) which lasted several minutes. The coecients of friction varied from 0.4 to 0.6 with steel whereas they varied from 0.04 to 0.2 with alumina, according to the loads applied (Fig. 2). The values recorded for alumina under light loads are comparable to those generally observed in a lubricated environment.
J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423
3.2. Wear The values for the wear rate of the C/SiC discs showed the same tendencies as for the coecients of friction: they were high when using 100Cr6 and increased with the load, whereas they were lower and constant when using alumina (Table 1). The short-term tests (240 s) done with the pair 100Cr6-C/SiC led to closer wear rates than those shown
419
previously. These latter results show that the surface degradation appeared very rapidly, from the start of the transitory period (Table 2), and was not limited to the removal of the natural screens (contaminant oxide layers, etc.). The continuous recording of cumulative height wear on the samples gives additional information concerning the development of degradation during the test. The general shape may be described by distinguishing two phases: . The ®rst phase corresponds to the rise in temperature of the system: a certain number of elements of the tribometer such as the pin (steel or alumina) and the pin-holder (stainless steel), dilated and led to a lifting of the loading system. As the temperature is higher for higher loads, it then takes longer to reach thermal equilibrium thereby lengthening this phase. This expansion phase masks the wear aecting the samples (ascertained visually and con®rmed experimentally). . The second phase is characterized by a linear increase in displacement. As a consequence, the cumulative wear on the two opposed materials is regular until the end of the test. This was the case for the two pairs studied. However, the slope depends on which pair, and varies with the normally applied load. 3.3. Temperature
Fig. 2. Change in coecient of friction with applied load: (a) Steel-C/ SiC pairs and (b) Alumina-C/SiC pairs.
Table 1 Change in wear rate of C/SiC disc with both applied load and pin features (1-h tests) Applied load (N)
10
30
40
50
60
90
120
200
Steel [10ÿ14 m3/(N m)] Alumina [10ÿ14 m3/(N m)]
5.7
7.8
15.1
32.4
±
±
±
±
±
0.7
±
±
1.6
2.5
2.5
1.8
Table 2 Change in wear rate of C/SiC disc with applied load (100 Cr6-C/SiC, 240 s) Applied load (N)
3.5
10
30
100
Wear rate [10ÿ14 m3/(N m)]
9
9
6
100
During the tests, the temperature evolved as the coef®cient of friction following two distinct regimes: for the ®rst a more or less rapid increase in temperature according to the load; a relatively stable development for the second. The coecient of friction and the temperature are closely connected (Fig. 3): a change in the coecient of friction is always associated with a change in the temperature, this also applies to the alumina where the thermocouples are glued to the pin-holder. The average values calculated integrating the two regimes (Table 3) still remain far below the temperatures actually existing in the contact zone. However, their increase, as a function of the load, is in agreement with the observations made during the tests: incandescent points (red/orange) appear in the contact zone, and for the highest loads (from 90 N for the set alumina-C/SiC) almost whole of the contact zone remains incandescent during the test. 4. Damage mechanisms The variations in the coecient of friction, f, are relevant to the change in the sliding resistance and aect the instantaneous power, P, (P=f.p.v where p is the applied load, and v is the slip speed) dissipated in the
420
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Fig. 3. Connected behaviour between coecient of friction and temperature (100Cr6-C/SiC, 1 h and 40 N).
Table 3 Change in average temperature with applied load (about 1 mm from the contact area in the case of steel and 2 mm for alumina) Applied load (N)
10
30
40
50
60
90
120
200
Steel ( C) Alumina ( C)
160 ±
± 65
475 ±
560 ±
± 120
± 220
± 230
± 260
contact zone. The variations in the coecient of friction also explain the change of the velocity adaptation mode in the contact zone (leading, or not, to wear). . The transitory period of the tests (establishing the coecient of friction, rise in temperature and thermal phenomena) is always very unstable. It determines the adaptation of the contact surfaces: the natural screens on the ®rst bodies are initially eliminated, then the very rough disc surface is partially levelled, the debris packing into its irregularities. . The steady period, with its slow cyclic variations of both coecient of friction and temperature, therefore corresponds to a phenomenon of stress accumulation over several hundred passes followed by a sudden relaxation, in close relationship to a displacement of the surface adaptation areas and the debris features. 4.1. Wear of the C/SiC The high frequency of passage of the pin on the C/SiC disc (77 Hz during this study) gives a cyclic succession of mechanical and thermal phenomena throughout the test. Under the pin, the C/SiC surface undergoes a sudden stress increase followed by a steep increase in the surface
temperature as a result of the large amount of energy dissipated in the contact zone. Outside the contact zone, the C/SiC surface is subject to rapid cooling in the friction track. These cycles provoke fatigue phenomena, which makes the material produce debris (source output). These cyclical normal loadings induce repeated ¯exing in the contact zone. Because of the high porosity of the composite, a redistribution of internal stresses occurs ®rst by elastic deformation of the ®bres and then by a plattering of the whole friction track. The subsequent ®bre failure leads to ®bre/matrix transfers of load, which causes local micro-mechanisms of ®bre/matrix de-cohesion, ®bre pull-out (Fig. 4a), and grain detachment from the brittle ceramic matrix (Fig. 4b), initially weaken by cracks resulting from the preparation process. Tangential stresses produced by friction cause similar degradations but more localized and of smaller spread. The major role of these tangential stresses is to set the debris in motion (internal output). These wear phenomena do not seem to be aected by pin features. 4.2. Pin wear The mechanical and thermal stresses sustained by the pins are more continuous (constant load) even though their surfaces also show localized fatigue phenomena related to the displacement of wear areas (contact forces are not con®ned to the same asperities throughout the test). The 100Cr6 steel is mainly worn by abrasion (source output). Under light load and at the start of a test, before the temperature increase has aected its surface hardness, this abrasion (distance between scratches of the order of 10 mm) is associated with a signi®cant cracking (extensions of the order of 500 mm) where the orientation is close to perpendicular to the movement (and to the direction of the abrasion scratches, Fig. 4c).
J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423
421
Fig. 4. SEM micrographs of the worn surfaces (the arrow indicates the apparent sliding direction of the pin or the real sliding direction of the disc). Surface damage of C/SiC composite (100Cr6-C/SiC, 30 N): (a) ®bre/matrix de-cohesion and carbon ®bre pull-out; (b) inter-granular failure of C/SiC matrix with emission of several grain agglomerates. Surface damage of the pins; (c) abrasion and cracking of 100Cr6 pins (100Cr6-C/SiC, 10 N); (d) polishing and grain pull-out for alumina pins (alumina-C/SiC, 30 N).
Later, the temperature increase in the contact zone leads to a lowering of the yield stress of the 100Cr6. It is therefore more sensitive to damage involving the occurrence of a local super®cial creep of material with formation of a rear burr on the pin. The alumina is characterized by a poor capacity for deformation and a higher thermal stability, so that the mechanical and thermal stresses favour a polishing process as well as a debris formation by pull-out and grain cleavage (source output, Fig. 4d). 4.3. Debris behaviour The contact zone dynamics are heavily controlled by the features of the debris trapped between the rubbing surfaces. The heat generated in the contact zone and the high local pressures favour phenomena of adhesion and accretion of the debris stemming from the two materials in contact. Multi-layered structures consisting to a great extent of iron have been observed on the surface of the C/SiC discs that have rubbed against 100Cr6. The mechanical and thermal fatigue phenomena embrittle these deposits and cause their detachment by delamination (Fig. 5a). According to the degree of internal cohesion of these
deposits, the detachment happens either within the deposits or at the bulk composite level. This adhesion phenomenon occurring with the 100Cr6-C/SiC pair produces a higher sliding resistance (higher coecient of friction). The size of the detached ¯akes explains the temporal variation in the tangential forces (instability in the coecient of friction). Finally, this debris of large size is rapidly eliminated from the contact zone (Fig. 5c) which results in more frequent damage to the mass (high wear output). Deposits showing a granular structure characterize the surface of C/SiC discs that have rubbed against alumina (Fig. 5b). They result from a start of re-sintering of SiC grains (internal output). Nevertheless, the major part of the debris from alumina-C/SiC pairs is in the form of a ®nely ground powder, homogeneous and ¯uid (Fig. 5d), contrary to that from 100Cr6-C/SiC pairs. This bed of powder of weak adherence and low shear resistance separates the contacting bodies and can therefore limit the stresses generated by friction (provide a low coecient of friction) and thereby protect the mass (by resulting in a low wear output). In that case, the carbon ®bres (whole or pulverized) here fully play their role as a solid lubricant, facilitating the shear mechanism adaptation in the debris layer.
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J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423
Fig. 5. Aspects of the adhesive layers on C/SiC discs (arrow indicates the real sliding direction): (a) laminated and multi-layer structure of the layers after friction against 100Cr6 pins (50 N load); (b) granular structure of the layers after friction against alumina pins (120 N load). Morphology of the wear debris and diagram representing the tribological circuit taken by the debris (in and out of the contact zone); (c) with 100Cr6-C/SiC pairs, the debris is quickly ejected from the contact zone; (d) with alumina-C/SiC pairs, the debris is ground in the contact zone before being ejected.
5. Conclusion The analysis of the wear mechanisms aecting the C/SiC discs has allowed identifying the mechanism of debris formation, the role of debris entrapment and elimination, which describes the actual birth and life cycle of the contact zone. This analysis, based on a dynamic approach to the debris behaviour (source, internal and wear output), explains the dierence in behaviour between the two tested pairs. The wear on the C/SiC discs is the result of cyclical mechanical and thermal stresses (fatigue phenomena) which lead to a micro cracking of the SiC matrix and further, the emission of SiC grains. The ruining of the matrix causes the fracture and the removal of surface ®bres by load transfer. With the 100Cr6-C/SiC pair, the debris forms a heterogeneous layer, adhering strongly to the friction track. The result is a velocity adaptation both by shear in the debris mass and by fracturing of the contacting bodies that result in severe wear. On the other hand, with the alumina-C/SiC pair the debris is intensively ground owing to their prolonged stay in the contact zone. As a consequence, the velocity adaptation is mainly by a rolling and a shearing of the debris within
the powder bed resulting from detached particles acting as a protective screen (load-carrying eect), that result in low wear. Acknowledgements The composite samples were produced by CeÂramiques & Composites (Bazet, France). We want to express our gratitude for their collaboration during this study. References [1] Sliney HE. Solid lubricant materials for high temperatures Ð a review. Trib Int 1982;5:303±14. [2] Denape J, Lamon J. Sliding friction of ceramics: mechanical action of the wear debris. J Mater Sci 1990;25:3592±604. [3] Platon F, Kapelski G, Boch P, Godet M, Berthier Y. Le frottement aÁ haute tempeÂrature des ceÂramiques : lubri®cation solide. MeÂcanique MateÂriaux ElectriciteÂ, Revue du GAMI 1991;439:27± 33. [4] Belin M, Kapsa P, Viot JF, PeÂrez M. EÂtude du frottement de mateÂriaux composites ceÂramique/ceÂramique. L'Industrie CeÂramique 1985;797(9):641±4. [5] Yamamoto Y, Ura A. Wear and friction characteristics of SiC±C ceramics. ASME Wear of Materials 1991;593±596.
J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423 [6] Alexeyev N, Jahanmir S. Mechanics of friction in self-lubricating composite materials I: Mechanics of second phase deformation and motion; II: deformation of the interfacial ®lm. Wear 1993;166:41±54. [7] Lu Z, Friedrich K, Pannhorst W, Heinz J. Wear and friction of a unidirectional carbon ®ber-glass matrix composite against various counterparts. Wear 1993;162±164:1103±13. [8] Prakash B. Friction and wear caracteristics of advanced ceramic composite materials. In: Freidrich K, vol. editor. Advances in composite tribology. In: Pipes RB, series editor.
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Composite materials series, vol. 8. Elsevier Science Publishers, 1993. p. 405. [9] Jeanne JF. EÂtude de l'endommagement surfacique de mateÂriaux composites type carbone/carbone sous sollicitations tribologiques. TheÁse 3eÁme cycle, Universite de Nantes, no. ED 82-74, 1997. [10] Fournier P. Frottement sec de mateÂriaux ceÂramiques monolithiques, composites et reÂfractaires: analyse aÁ partir du concept 3eÁme corps. TheÁse 3eÁme cycle, Universite de Limoges, no. 56-97, 1997. [11] Berthier Y. Tribologie, Science carrefour. JourneÂes EuropeÂennes du Freinage, Lille, 1992, p. 490±6.
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High-speed tribological behaviour of a carbon/silicon-carbide composite J.-Y. Paris *, L. Vincent, J. Denape Laboratoire GeÂnie de Production, EÂcole Nationale d'IngeÂnieurs de Tarbes, Avenue d'Azereix, BP 1629, 65016 Tarbes Cedex, France Received 20 July 1999; received in revised form 2 November 1999; accepted 16 May 2000
Abstract Increasing attention is being paid to carbon-®bre/SiC-matrix composites (C/SiC) for their friction and wear performance because the carbon-®bre reinforcement enables a signi®cant reduction in the coecient of friction through self-lubricating properties. This study was performed by using a C/SiC composite at high-speed (15 m sÿ1). Two distinct types of tribological behaviour, as a function of the rubbing pin features (100Cr6 or alumina), were found. Analysis of both the surface degradation and velocity adaptation mechanisms has made it possible to establish the major role of debris trapped in the contact zone. The wear debris from the 100Cr6-C/SiC pair is a heterogeneous layer adhering strongly to the friction track. Consequently, the velocity adaptation is a direct eect of the shearing of the transfer layers and fracturing of bulk materials, resulting in severe wear. In contrast, with an alumina-C/SiC pair, wear particles are ®nely ground owing to the fact that they remain for a long time in the contact zone. The velocity adaptation here mainly occurs through debris rolling and shearing within the powder bed, thus resulting in reduced wear. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbon-®bre/SiC-matrix composite (C/SiC); 100Cr6 steel; Alumina; B. Friction/wear; The role of debris
1. Introduction In the last decade, there has been increasing interest in ceramic composites, in part because the use of monolithic ceramic pairs is not a viable option under dry friction conditions (ambient air) since they must be used along with solid lubricants such as graphite, sulphides, etc. [1±3]. On the other hand, the reinforcements of ceramic components (particles, whiskers and ®bres) reduce the risk of fractures and/or, through their selflubricating properties, reduce the coecient of friction. Some timely examples include composites with carbon ®bres [4±8]. However, the self-lubricating qualities of these materials varies, depending on their homogeneity (porosity, interface ®bre/matrix), on the ®brous structure of the carbon reinforcement (degree of graphitization, orientation) and the experimental friction conditions (pressure, speed, temperature and environment), as shown for composites of type C/C and C/SiC [9,10]. Somewhat abandoned to the bene®t of C/C composites, * Corresponding author. Fax: +33-5-6244-2708. E-mail addresses: [email protected] (J.-Y. Paris), [email protected] (J. Denape).
the C/SiC composites are at present showing a clear revival of interest owing to their lower sensibility to surroundings and to oxidisation. This study deals with the behaviour of a C/SiC composite, examining friction and wear at a speed of 15 m sÿ1. Studies conducted prior to this one have used speeds of either below 10 or above 30 m sÿ1. We speci®cally wanted to establish the substantial eect of the velocity adaptation mechanisms on the debris [11]. An analysis of the mechanisms of degradation of the composite C/SiC was therefore conducted while distinguishing between the different outputs of material in the contact zone: source output (debris detached from the surfaces), internal output (debris circulating between the surfaces) and wear output (debris fully eliminated from the contact zone). 2. Materials and experimental procedures 2.1. Materials The composite chosen for our study consists of a matrix of silicon carbide and of carbon ®bres. This composite C/SiC was achieved by a continuous rolling
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00124-X
418
J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423
(onto a mandrel) of the carbon ®bres pre-impregnated, through soaking, with SiC. After sintering, the tube thus produced is sliced into discs (``donuts'') of the required ®nal sample dimensions. The ®bres are thereby all oriented more or less parallel to the rubbing surface. Observation of the surface quality by SEM (raw after slicing: Ra=3.1 mm) revealed the existence of a high interstratum porosity (about 23 vol%) associated with an irregular presence of silicon carbide (Fig. 1). The volume fraction of ®bre and matrix are about 0.43 and 0.34, respectively. We also recorded generalized fracturing of surface ®bres, probably as a result of the slicing, and micro cracking of the matrix caused by a dierence in the expansion of the ®bres and the matrix during their handling. Two types of pins were used against the C/SiC: . a pin of high purity alumina (aAl2O3 of 99.7 wt.%) characterized by high mechanical and chemical stability and showing a microstructure of bimodal distribution of the grain sizes (2±5 and 10±20 mm);
. a pin of 100Cr6 (AISI 52100) martensitic steel showing high hardness (807 HV20), but also with a marked sensitivity to increases in temperature. The surface condition of these two materials corresponds to a polished state for the ceramic (Ra=0.07 mm) and a ground state for the steel (Ra=0.27 mm). 2.2. Experimental apparatus and test conditions The test apparatus was a pin-on-disc rotating tribometer. A cylindrical pin of diameter 10 mm chamfered at 45 over 1 mm (contact diameter 8 mm) was applied to a rotating disc of 80 mm diameter and 5 mm thickness. The tribometer instrumentation allowed real time collecting and visualizing of the information relative to coecient of friction (strain gauge for traction/ compression force), cumulated height wear of the samples (displacement gauge LVDT calibrated by keys), and temperature (K-type thermometer, diameter 0.6 mm). For the alumina pins, two thermocouples were glued to the surface of the pin-holder, against the pin and approximately 2 mm from the rubbing surface (one in front of the contact zone, one behind). The tests were performed at a speed of 15 m sÿ1 for 1 h (distance covered: 54 km) in the laboratory atmosphere with the applied load (dead load) ranging from 10 to 200 N (0.2±0.4 MPa). The samples were subject to a rigorous cleaning procedure for over 30 min before the tests so as to eliminate as much surface contamination as possible (grease, carbon dust retained in the coarse surface of the C/SiC). 3. Tribological behaviour 3.1. Friction behaviour
Fig. 1. (a) SEM micrograph of the surface of a C/SiC composite disc; (b) detail of a strand of carbon ®bres.
After a transitory running in period, when the coecient of friction increased rapidly (the period varying from 50 to 650 s for steel and 500 to 1000 s for alumina), the recordings showed a more regular period resulting in a slow but continuous decrease in the coef®cient of friction. However this period remained unstable during the whole test from 40 N with the pair 100Cr6-C/SiC, and from 60 N with the pair alumina-C/ SiC, with cyclic variations (a gradual increase followed by a drop in the coecient of friction) which lasted several minutes. The coecients of friction varied from 0.4 to 0.6 with steel whereas they varied from 0.04 to 0.2 with alumina, according to the loads applied (Fig. 2). The values recorded for alumina under light loads are comparable to those generally observed in a lubricated environment.
J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423
3.2. Wear The values for the wear rate of the C/SiC discs showed the same tendencies as for the coecients of friction: they were high when using 100Cr6 and increased with the load, whereas they were lower and constant when using alumina (Table 1). The short-term tests (240 s) done with the pair 100Cr6-C/SiC led to closer wear rates than those shown
419
previously. These latter results show that the surface degradation appeared very rapidly, from the start of the transitory period (Table 2), and was not limited to the removal of the natural screens (contaminant oxide layers, etc.). The continuous recording of cumulative height wear on the samples gives additional information concerning the development of degradation during the test. The general shape may be described by distinguishing two phases: . The ®rst phase corresponds to the rise in temperature of the system: a certain number of elements of the tribometer such as the pin (steel or alumina) and the pin-holder (stainless steel), dilated and led to a lifting of the loading system. As the temperature is higher for higher loads, it then takes longer to reach thermal equilibrium thereby lengthening this phase. This expansion phase masks the wear aecting the samples (ascertained visually and con®rmed experimentally). . The second phase is characterized by a linear increase in displacement. As a consequence, the cumulative wear on the two opposed materials is regular until the end of the test. This was the case for the two pairs studied. However, the slope depends on which pair, and varies with the normally applied load. 3.3. Temperature
Fig. 2. Change in coecient of friction with applied load: (a) Steel-C/ SiC pairs and (b) Alumina-C/SiC pairs.
Table 1 Change in wear rate of C/SiC disc with both applied load and pin features (1-h tests) Applied load (N)
10
30
40
50
60
90
120
200
Steel [10ÿ14 m3/(N m)] Alumina [10ÿ14 m3/(N m)]
5.7
7.8
15.1
32.4
±
±
±
±
±
0.7
±
±
1.6
2.5
2.5
1.8
Table 2 Change in wear rate of C/SiC disc with applied load (100 Cr6-C/SiC, 240 s) Applied load (N)
3.5
10
30
100
Wear rate [10ÿ14 m3/(N m)]
9
9
6
100
During the tests, the temperature evolved as the coef®cient of friction following two distinct regimes: for the ®rst a more or less rapid increase in temperature according to the load; a relatively stable development for the second. The coecient of friction and the temperature are closely connected (Fig. 3): a change in the coecient of friction is always associated with a change in the temperature, this also applies to the alumina where the thermocouples are glued to the pin-holder. The average values calculated integrating the two regimes (Table 3) still remain far below the temperatures actually existing in the contact zone. However, their increase, as a function of the load, is in agreement with the observations made during the tests: incandescent points (red/orange) appear in the contact zone, and for the highest loads (from 90 N for the set alumina-C/SiC) almost whole of the contact zone remains incandescent during the test. 4. Damage mechanisms The variations in the coecient of friction, f, are relevant to the change in the sliding resistance and aect the instantaneous power, P, (P=f.p.v where p is the applied load, and v is the slip speed) dissipated in the
420
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Fig. 3. Connected behaviour between coecient of friction and temperature (100Cr6-C/SiC, 1 h and 40 N).
Table 3 Change in average temperature with applied load (about 1 mm from the contact area in the case of steel and 2 mm for alumina) Applied load (N)
10
30
40
50
60
90
120
200
Steel ( C) Alumina ( C)
160 ±
± 65
475 ±
560 ±
± 120
± 220
± 230
± 260
contact zone. The variations in the coecient of friction also explain the change of the velocity adaptation mode in the contact zone (leading, or not, to wear). . The transitory period of the tests (establishing the coecient of friction, rise in temperature and thermal phenomena) is always very unstable. It determines the adaptation of the contact surfaces: the natural screens on the ®rst bodies are initially eliminated, then the very rough disc surface is partially levelled, the debris packing into its irregularities. . The steady period, with its slow cyclic variations of both coecient of friction and temperature, therefore corresponds to a phenomenon of stress accumulation over several hundred passes followed by a sudden relaxation, in close relationship to a displacement of the surface adaptation areas and the debris features. 4.1. Wear of the C/SiC The high frequency of passage of the pin on the C/SiC disc (77 Hz during this study) gives a cyclic succession of mechanical and thermal phenomena throughout the test. Under the pin, the C/SiC surface undergoes a sudden stress increase followed by a steep increase in the surface
temperature as a result of the large amount of energy dissipated in the contact zone. Outside the contact zone, the C/SiC surface is subject to rapid cooling in the friction track. These cycles provoke fatigue phenomena, which makes the material produce debris (source output). These cyclical normal loadings induce repeated ¯exing in the contact zone. Because of the high porosity of the composite, a redistribution of internal stresses occurs ®rst by elastic deformation of the ®bres and then by a plattering of the whole friction track. The subsequent ®bre failure leads to ®bre/matrix transfers of load, which causes local micro-mechanisms of ®bre/matrix de-cohesion, ®bre pull-out (Fig. 4a), and grain detachment from the brittle ceramic matrix (Fig. 4b), initially weaken by cracks resulting from the preparation process. Tangential stresses produced by friction cause similar degradations but more localized and of smaller spread. The major role of these tangential stresses is to set the debris in motion (internal output). These wear phenomena do not seem to be aected by pin features. 4.2. Pin wear The mechanical and thermal stresses sustained by the pins are more continuous (constant load) even though their surfaces also show localized fatigue phenomena related to the displacement of wear areas (contact forces are not con®ned to the same asperities throughout the test). The 100Cr6 steel is mainly worn by abrasion (source output). Under light load and at the start of a test, before the temperature increase has aected its surface hardness, this abrasion (distance between scratches of the order of 10 mm) is associated with a signi®cant cracking (extensions of the order of 500 mm) where the orientation is close to perpendicular to the movement (and to the direction of the abrasion scratches, Fig. 4c).
J.-Y. Paris et al. / Composites Science and Technology 61 (2001) 417±423
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Fig. 4. SEM micrographs of the worn surfaces (the arrow indicates the apparent sliding direction of the pin or the real sliding direction of the disc). Surface damage of C/SiC composite (100Cr6-C/SiC, 30 N): (a) ®bre/matrix de-cohesion and carbon ®bre pull-out; (b) inter-granular failure of C/SiC matrix with emission of several grain agglomerates. Surface damage of the pins; (c) abrasion and cracking of 100Cr6 pins (100Cr6-C/SiC, 10 N); (d) polishing and grain pull-out for alumina pins (alumina-C/SiC, 30 N).
Later, the temperature increase in the contact zone leads to a lowering of the yield stress of the 100Cr6. It is therefore more sensitive to damage involving the occurrence of a local super®cial creep of material with formation of a rear burr on the pin. The alumina is characterized by a poor capacity for deformation and a higher thermal stability, so that the mechanical and thermal stresses favour a polishing process as well as a debris formation by pull-out and grain cleavage (source output, Fig. 4d). 4.3. Debris behaviour The contact zone dynamics are heavily controlled by the features of the debris trapped between the rubbing surfaces. The heat generated in the contact zone and the high local pressures favour phenomena of adhesion and accretion of the debris stemming from the two materials in contact. Multi-layered structures consisting to a great extent of iron have been observed on the surface of the C/SiC discs that have rubbed against 100Cr6. The mechanical and thermal fatigue phenomena embrittle these deposits and cause their detachment by delamination (Fig. 5a). According to the degree of internal cohesion of these
deposits, the detachment happens either within the deposits or at the bulk composite level. This adhesion phenomenon occurring with the 100Cr6-C/SiC pair produces a higher sliding resistance (higher coecient of friction). The size of the detached ¯akes explains the temporal variation in the tangential forces (instability in the coecient of friction). Finally, this debris of large size is rapidly eliminated from the contact zone (Fig. 5c) which results in more frequent damage to the mass (high wear output). Deposits showing a granular structure characterize the surface of C/SiC discs that have rubbed against alumina (Fig. 5b). They result from a start of re-sintering of SiC grains (internal output). Nevertheless, the major part of the debris from alumina-C/SiC pairs is in the form of a ®nely ground powder, homogeneous and ¯uid (Fig. 5d), contrary to that from 100Cr6-C/SiC pairs. This bed of powder of weak adherence and low shear resistance separates the contacting bodies and can therefore limit the stresses generated by friction (provide a low coecient of friction) and thereby protect the mass (by resulting in a low wear output). In that case, the carbon ®bres (whole or pulverized) here fully play their role as a solid lubricant, facilitating the shear mechanism adaptation in the debris layer.
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Fig. 5. Aspects of the adhesive layers on C/SiC discs (arrow indicates the real sliding direction): (a) laminated and multi-layer structure of the layers after friction against 100Cr6 pins (50 N load); (b) granular structure of the layers after friction against alumina pins (120 N load). Morphology of the wear debris and diagram representing the tribological circuit taken by the debris (in and out of the contact zone); (c) with 100Cr6-C/SiC pairs, the debris is quickly ejected from the contact zone; (d) with alumina-C/SiC pairs, the debris is ground in the contact zone before being ejected.
5. Conclusion The analysis of the wear mechanisms aecting the C/SiC discs has allowed identifying the mechanism of debris formation, the role of debris entrapment and elimination, which describes the actual birth and life cycle of the contact zone. This analysis, based on a dynamic approach to the debris behaviour (source, internal and wear output), explains the dierence in behaviour between the two tested pairs. The wear on the C/SiC discs is the result of cyclical mechanical and thermal stresses (fatigue phenomena) which lead to a micro cracking of the SiC matrix and further, the emission of SiC grains. The ruining of the matrix causes the fracture and the removal of surface ®bres by load transfer. With the 100Cr6-C/SiC pair, the debris forms a heterogeneous layer, adhering strongly to the friction track. The result is a velocity adaptation both by shear in the debris mass and by fracturing of the contacting bodies that result in severe wear. On the other hand, with the alumina-C/SiC pair the debris is intensively ground owing to their prolonged stay in the contact zone. As a consequence, the velocity adaptation is mainly by a rolling and a shearing of the debris within
the powder bed resulting from detached particles acting as a protective screen (load-carrying eect), that result in low wear. Acknowledgements The composite samples were produced by CeÂramiques & Composites (Bazet, France). We want to express our gratitude for their collaboration during this study. References [1] Sliney HE. Solid lubricant materials for high temperatures Ð a review. Trib Int 1982;5:303±14. [2] Denape J, Lamon J. Sliding friction of ceramics: mechanical action of the wear debris. J Mater Sci 1990;25:3592±604. [3] Platon F, Kapelski G, Boch P, Godet M, Berthier Y. Le frottement aÁ haute tempeÂrature des ceÂramiques : lubri®cation solide. MeÂcanique MateÂriaux ElectriciteÂ, Revue du GAMI 1991;439:27± 33. [4] Belin M, Kapsa P, Viot JF, PeÂrez M. EÂtude du frottement de mateÂriaux composites ceÂramique/ceÂramique. L'Industrie CeÂramique 1985;797(9):641±4. [5] Yamamoto Y, Ura A. Wear and friction characteristics of SiC±C ceramics. ASME Wear of Materials 1991;593±596.
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Composite materials series, vol. 8. Elsevier Science Publishers, 1993. p. 405. [9] Jeanne JF. EÂtude de l'endommagement surfacique de mateÂriaux composites type carbone/carbone sous sollicitations tribologiques. TheÁse 3eÁme cycle, Universite de Nantes, no. ED 82-74, 1997. [10] Fournier P. Frottement sec de mateÂriaux ceÂramiques monolithiques, composites et reÂfractaires: analyse aÁ partir du concept 3eÁme corps. TheÁse 3eÁme cycle, Universite de Limoges, no. 56-97, 1997. [11] Berthier Y. Tribologie, Science carrefour. JourneÂes EuropeÂennes du Freinage, Lille, 1992, p. 490±6.