Effect of Braking Speeds on the Tribological Properties

of Carbon/Carbon Composites

Yu Shu1, Chen Jie1, Huang Qizhong1, Xiong Xiang1, Chang Tong1 and Li Yunping2;*

1State Key Lab for Powder Metallurgy, Central South University, Hunan Changsha, 410083, P. R. China2Institute for Materials Research, Tohoku University, Sendai 980-0813, Japan

This paper describes the tribological properties of carbon/carbon composites used as braking materials under various braking speeds, inwhich the materials with three kinds of microstructures were used: rough lamina (sample A), smooth lamina (sample B), and the mixture of roughlamina and smooth lamina (sample C), respectively. Friction tests were carried out through a laboratory dynamometer. Polarized opticalmicroscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure and worn surface. Results indicatedthat the friction coefficient of sample A reached a peak value at braking speed lower than that in both sample B and sample C. However, when thefriction coefficients reached to the peak values the temperature inside the worn surface was observed to be approximately 250�C for all of thesamples. The weight losses in all of the samples were observed to increase with increasing braking speed; however, the oxidation losses at speedof 28m�s�1 are higher than that of 30m�s�1 for all of the samples. [doi:10.2320/matertrans.M2009390]

(Received November 24, 2009; Accepted March 1, 2010; Published April 15, 2010)

Keywords: carbon/carbon composites, pyrolytic carbon, tribological property, braking speed, worn surface

1. Introduction

Carbon/carbon (C/C) composites have unique propertiessuch as high specific strength, specific heat capacity, meltingpoint, and thermal conductivity etc. In addition, the heatshock properties of C/C composites are superior to most ofthe other friction materials. The aforementioned propertiesenable the C/C composites to be used as brakes in aircraftsand to carry out three functions simultaneously: producingfriction, transferring mechanical load, and absorbing kineticenergy.1–3) Furthermore, as the aerospace brakes, C/Ccomposites have more merits such as the light weight, longservice life, low noise, and smooth operation, etc. comparedto other materials used as aerospace brakes.4,5)

In general, C/C composites are manufactured throughchemical vapor deposition (CVD) process, impregnatingresin or pitch, or the process combining these two methods.The CVD processes are divided into isothermal CVD andthermal gradient CVD processes. It has been proved thatthe microstructure of C/C composites obtained from CVDprocess are superior to both resin carbon and pitch carbonfrom the impregnating process if used for aircraft brakes.In general, the first choice for synthesizing C/C compositesused for aircraft braking materials is isothermal CVDbecause the pyrolytic carbon obtained is a single phase,while a mixed phase pyrolytic carbon exist in final productsfrom the thermal gradient CVD because of the influence ofthermal gradients during CVD processing. For C/C compo-sites used in braking process, friction and wear properties areessential, and in reality, different types of aircrafts bringvarious braking conditions, such as load weight, landingspeeds and the serve circumstances, which have a stronginfluence on the tribological properties of the used materials.In addition, the materials, containing different microstruc-tures, will also give rise to different tribological properties,so it is worthwhile to study the friction and wear performance

of C/C composites brakes with different microstructureunder different braking speeds.

In this paper, we chose three kinds of samples, in whichtwo kinds of C/C composites with rough lamina and smoothlamina structures were manufactured in isothermal CVD, andthe third one with mixed phase structure was manufacturedin thermal gradient CVD.

2. Experimental

C/C composites were manufactured from a quasi-threedimensionally needled carbon fiber preform, which wasfabricated by needling the stack of alternating layers of non-woven carbon cloth and carbon felt made from choppedfibers. Carbon fiber preform was densified by CVD processunder the deposition temperature from 1123 to 1223K in theatmosphere containing both propylene and nitrogen used asprecursor and carrier gas, respectively. The densities of thefinal C/C composites are 1.60–1.65 g/cm3. Heat treatmenttemperature at 2573K for 2 h in the argon protection wascarried out to all of the samples. The cylindrical specimenwas machined into a size of �438–237mm � H22mmbefore testing.

The microstructure of pyrolytic carbon was observedunder MEF3A polarized optical microscope and the wornsurface was analyzed under JSM-5600LV scanning electronmicroscopy. The flatness of friction film was observed andquantitatively analyzed by the KH-7700 three-dimensionaldigital microscopy. Thermal diffusivity � was measuredin JR-3 laser thermal conductivity instrument, the thermalconductivity �(W�m�1�k�1) can be obtained from thefollowing equation:

� ¼ 418:68� �� Cp � �; ð1Þ

where Cp, � denote the heat capacity, density of C/Ccomposites, respectively.

The friction and wear test of C/C composites, simulatingthe real condition of aircraft brake discs, were conducted on*Corresponding author, E-mail: lyping@imr.tohoku.ac.jp

Materials Transactions, Vol. 51, No. 5 (2010) pp. 1038 to 1043#2010 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

a home-made laboratory scale MM-1000 inertia type ofdynamometer (Fig. 1), in which the top speed is 9000 rm�s�1

and the largest torque is 15 kg�m. Ring specimens (�75–53mm � H14mm) for friction testing are coupled with onedynamic and one static disk. When the flywheel, whichrotated with the dynamic ring sample, is accelerated to acertain rotational velocity, braking is achieved through thefriction between the dynamic and static rings under a certainbraking pressure. The relationship between braking momentand braking time was recorded by computer. The frictioncoefficient is evaluated by the following expression

M ¼ �ðr1 þ r2ÞFn=2; ð2Þ

where M denotes moment; � the friction coefficient; Fn

load; r1 inner radius; r2 outer radius. The braking parametersused in current research are listed as followed:

inertia: 30 kg�m�2, braking pressure: 0.6MPa, brakingspeed: 5m�s�1, 10m�s�1, 15m�s�1, 20m�s�1, 25m�s�1,28m�s�1, and 30m�s�1.

The weight loss Lw and the linear wear Ll of sample canbe obtained directly by measuring the sample after testing,so the oxidation loss L0 could be calculated by

Lo ¼ Lw � S� �� Ll ð3Þ

where S and � denote the frictional surface area and densityof sample, respectively. The temperature of sample duringbraking process is detected in the area 1mm beneath thefrictional surface of stator sample.

3. Results and Discussions

3.1 MicrostructureSeveral factors that affect the microstructure of pyrolytic

carbon during CVD process are temperature, furnacepressure, gas residence time and the initial porosity of feltbody etc. In the current study, same felts (fiber volumeincluding Z-fiber content, preparation process and heattreatment temperature etc.) were used in various CVDprocesses, and by adjusting the process parameters afore-mentioned, C/C composites with different microstructurewere obtained in CVD process as shown in Fig. 2. In the

Fig. 2, Samples A, B were deposited in two isothermal CVDprocesses with different experimental conditions, whilesample C was from thermal gradient CVD process. It canbe observed from Fig. 2(a) and (b) that: due to the uniformtemperature distribution inside the isothermal CVD chamber,single phase structure of pyrolytic carbon can be obtainedunder the defined process parameters. Due to the widetemperature gradient in the thermal gradient CVD process,the structure of pyrolytic carbon shows a large diversity,namely, the coexistence of various structures and boundariesthat differ in microstructures could be observed (Fig. 2(c-1)and (c-2)).

Under polarized optical microscope, the Maltese Crosseshave been observed in the cross-section of all samplesindicating that the deposition were oriented concentrically tothe fiber. The microstructure of sample A was opticallyanisotropic with a high phase shift in which the extinctionangles was high as 21� and the thin and narrow columnargrowthcones, that running through the whole cross-section ofsample, was the most important characteristics of roughlaminar6,7) for its primary only one cones,8) not including thesecond or more regeneration cones. Since the Maltese Crosswas strongly disrupted by the cones, so that its appearance isrough (justifying the generic name of rough laminar, RL inshort). At the same time, for the high degree of preferredorientation and anisotropy of RL, cross-section surfaceappear rough and layering because the reflected light isstronger than that of diffuse scattering under the polarizedlight. The cross-section of sample B manifests very smoothand regular Maltese Crosses. The ringed cracks can be seenclearly after heat treatment, indicating this material, namedsmooth laminar (SL), is hard and brittle. In the samepolarized light intensity and magnification, the reflectance ofsample B is lower than that of A with its’ extinction angleswas 10, so the image appears to be more smooth.9) AS tosample C, there are two kinds of structures coexisted, whichare similar to both sample A and B, respectively. Thestructure of C-1 belongs to RL for the thin and narrowgrowthcones that grows throughout the cross-section ofsample. While in structure of C-2 the circular crack after heattreatment was observed, which is characterized as SLstructural. Therefore, in sample C mixed structure of bothRL and SL existed, which is the characteristics of pyrolyticcarbon manufactured in thermal gradient CVD process.

3.2 Tribological properties under different brakingspeeds

The changes in coefficients of friction (COF) as a functionof braking speed are showed in Fig. 3: firstly, COF of sampleA is only 0.1 at 5m�s�1 that is lower than that of B and C.With the increase braking speeds, the COF increased to apeak value of about 0.47 at a braking speed of approximately15m�s�1, and then declined toward a stable value ofapproximately 0.35 when the braking speed is above20m�s�1. Longley et al.10) suggested these phenomena areconsistent with the theory that the first low COF valuesof highly graphitic carbon is due to low surface forcesassociated with the presence of absorbed gases, not to lowshear strength, nor to the separation of surfaces by rolled-upwear products. The latter increase of COF was attributed the

Fig. 1 Schematic diagram of laboratory scale inertia type of dynamometer

used in current study. 1-inertial wheel, 2-bearing, 3-clutch, 4-specimen

holder, 5-rotor, 6-stator, 7-pressing cylinder, 8-strap, 9-electric motor, 10-

test-bed.

Effect of Braking Speeds on the Tribological Properties of Carbon/Carbon Composites 1039

increase in the real contact area with the increase of thefriction force.

The curves of sample B and C are similar for theycommonly demonstrated hard, largely non-graphitic struc-ture-SL: in the early stages of sliding (from 5m�s�1 to15m�s�1) between freshly-prepared surfaces, the COFdecrease to a very low value (0.25), but then subsequentlyincrease to a high value (0.42) at the 20m�s�1 and keep theconstant value (0.40) at higher speed. This appearance is inaccord with the study of Longley and J. K. Lancaster,11) thelow value is associated with the formation of a uniformfilm of consolidated wear debris which appears to act as aclassical thin-film solid lubricant-a film of low shear strength

on a harder substrate.12) After the formation of the debris film,the wear rate also becomes very low. Insufficient debris isnow produced to maintain the film and as it is graduallyremoved, the friction increases to a higher constant valvewhich can be preserve with the increase of the real area ofcontact.

According to the study of Fitzer,5) the friction coefficientpeak of C/C brakes appeared at the low run-in speed which iscalled ‘‘low-energy peak’’. Results show that: ‘‘low-energypeak’’ appeared at 15m�s�1 in case of sample A (RL), whilefor the materials containing SL structure (Sample B and C),the ‘‘low-energy peak’’ was observed at higher speed (atabout 20m�s�1 in both materials). The above-mentioneddifferences in the three kinds of materials are considered tobe related to the desorbing ability of water molecules in thefriction surface since the microstructures of these materialsare different.

Worn surfaces of C/C composites after braking test invarious speeds were observed as shown in Fig. 4. The frictionfilm of material A is continuous with fewer broken fibers andpores from low speed (5m�s�1) to high speed (30m�s�1),while the film of material B has obvious pores even under thehigh speed (30m�s�1), indicating the friction film has notbeen formed sufficiently. The formation of film has a greatimpact on the fictional properties of C/C brakes includingthe friction coefficient and the wear rate. The smooth andcontinuous friction films lead to the stable frictionalcoefficient and low wear. However, no obvious differencecould be observed from SEM imagine although the materialsand friction behaviors are much different as shown in Fig. 3.

Fig. 3 The friction coefficient under different braking speeds.

a

10 µm 10 µm

10 µm

c-1

10 µm

c-2

growthcones

Cracks

b

Fig. 2 The different microstructure of carbon/carbon composites.

1040 Y. Shu et al.

According to the reports of C. Pevida and J.-C. Rietsch,13,14)

the wear debris of different C/C composites or underdifferent braking energy has similar physicochemical proper-ties. Thought there was also no obvious difference as shownin the SEM micrographs in the current study, by usingthree-dimensional digital microscopy, the flatness degree offriction films for these three kinds of materials at brakingspeed of 20m�s�1 were observed to be different as shown inFig. 5, the results are tabulated in Table 1. It was observedthat the film flatness of material A is superior to that ofmaterial B and C for the lowest drop height, indicating thefriction film is relatively uniform, which is considered to bereason of no frequent wave in friction coefficients from10m�s�1 to 30m�s�1 as shown in Fig. 3.

The temperature of samples in various braking speeds,measured inside of the sample 1mm beneath the wornsurface, are shown in Fig. 6; it can be observed that,temperatures of friction surface were observed to increasewith the increasing braking speed (Fig. 6). In both samples Aand B, at higher speeds the temperatures inside the samplecould not be measured because the temperature have beenbeyond the range of the thermal couple, and only thetemperature of sample A have been detected throughoutseven speeds for its highest thermal conductivity, speciallyits highest thermal conductivity (�?) perpendicular to thefriction surface, which is thought to be also beneficial tomaintain smooth torque, to reduce material oxidation loss,and to prolong the service life of brake disc15) (see Table 2).The higher the thermal conductivity of material is, the fasterthe heat, which generated in the braking process, is trans-ferred to outside. In addition, it can also observed that nomatter what kind of materials, once the ‘‘low-energy peak’’appears, the temperature detected are observed to beapproximately 250�C which is close to the report of B. K.Yen,16) in which the ‘‘low-energy peak’’ was reported to

a-1—5 m.s-1, a-2—15 m.s-1, a-3—20 m.s-1, a-4—30 m.s-1,b-1—5 m.s-1, b-2—15 m.s-1, b-3—20 m.s-1, b-4—30 m.s-1, c-1—5 m.s-1, c-2—15 m.s-1, c-3—20 m.s-1, c-4—30 m.s-1,

Fig. 4 The morphology of the worn surface under the different braking speeds.

Table 1 Average drop height of friction film in three kinds of samples after

friction test at 20m�s�1.

A B C

Average drop height of

friction film (mm)56.76 68.38 64.73

Effect of Braking Speeds on the Tribological Properties of Carbon/Carbon Composites 1041

appear in temperatures of approximately 200�C in case ofC/C composites.

According to the report of C. Blanco,17) there are manystructural defects such as pores and dislocations in C/Ccomposites, because the matrix carbon (including pyrolyticcarbon and resin carbon) formed in CVD process is not theideal graphite crystal. In friction surfaces with numerousdefects, oxygen atoms are easily to be chemically absorbedand form the bands of C-O and C=O, and the watermolecules are physically adsorbed outside of the oxygenatoms. At lower speed, the water molecules is able to act as

friction lubricant, which leads to the low friction coefficient;with the increasing braking speed, surface temperatureincreased to a certain value and water molecules begin tobe released, and the friction coefficient will reach to apeak value for the lubrication disappearing, accordingly. Inaddition, after the heat treatment at temperature of 2300�C,structure of sample A is the closest to the ideal graphitecrystal if referred the graphitization degree of C/C compo-sites from XRD measurements (see Table 3), so it reach thepeak in a earliest stage for its least adsorption of watermolecules in the friction surface in sample A.

The weight losses of three kinds of materials include linearwear and high temperature oxidation abrasion. The weightloss can be calculated through different weight of samplebefore and after experiment, and the linear wear can bedetected by the measurement of the thickness of samplebefore and after experiment. The value of former is observedto be always higher than that of latter, and the gap isconsidered as the oxidation abrasion (eq. (3)). The weightloss and oxidation abrasion of three samples were alsoobserved to increase with the speed (Fig. 7). However, it hasto be noted that, oxidation abrasion was observed not toincrease with the speed all the time: the oxidation abrasionat 30m�s�1 is lower than that of 28m�s�1 in all the samples(see Fig. 8). That is to say the linear wear in these kinds ofthree samples increased greatly if referred eq. (3). Authorsbelieve that under the condition of high-speed and high-

(a)

(b)

(c)

Fig. 5 Flatness of worn surface of three samples, (a) A sample, (b) B

sample, and (c) C sample after friction test at 20m�s�1.

Fig. 6 The temperature under the 1mm of the worn surface at different

braking speeds.

Table 2 Thermal conductivity of three kinds of samples after heat

treatment at 2300�C for 2 h, where �? and �== represent the thermal

conductivity vertical and parallel to the friction surface.

Thermal conductivity a b c

�? (W�m�1�k�1) 47.1 19.38 23.15

�== (W�m�1�k�1) 99.8 47.15 78.09

Table 3 Graphitization degree of three kinds of sample after heat treatment

at 2300�C for 2 h.

A B C

Graphitization

degree (%)63.2 28.9 55.43

Fig. 7 The weight loss under different braking speeds.

1042 Y. Shu et al.

energy, there are more serious damaged in the frictionsurface: the old lubricant film was constantly destroyed, andthe new lubricant film has been continually formed althoughcontinuous friction films were formed during testing(Fig. 4(a-4), (b-4), and (c-4)). In this process, there is alwaysa part of ‘‘fresh’’ surface exposed that makes linear wear andoxidation abrasion relatively large. Furthermore, the oxida-tion weakened the surface and sub-surface, which in turnfacilitated removal by mechanical action at the rubbingsurface. However, with the increase of braking speed, it isconsidered that with the high temperature in the frictionsurface, the bonding energy between the graphite layers islowered, and the graphitization degree of the wear debrisparticles is improved accordingly,18,19) which is considered tolower the hardness of wear debris, which made the frictionfilm more susceptible to be deformed.20) So the lubricant filmis formed in the friction surface to reduce the oxidationabrasion effectively (Fig. 4). In addition, according to thestudy of Li and Sheehan:21) the phenomenon of that theoxidation abrasion at 30m�s�1 is lower than that of 28m�s�1,is considered to be due to chemisorptions of impurities toform lubricating films and to oxidation at a higher temper-ature, both of which is thought to reduce the oxidationabrasion under high-speed.

4. Conclusion

With the increasing braking speeds, the friction coefficientof sample A increased to the peak value at the speed of15m�s�1, and then declined toward a stable value. The

friction curves of sample B and C were similar: coefficientof friction is minimum at the speed of 15m�s�1 and increasedto a peak value at the 20m�s�1, then decreased to a constantvalue.

No matter what kinds of material, when ‘‘low-energypeak’’ appears, the temperatures under the friction surface1mm are observed to be approximately 250�C.

The weight loss of materials increased with the increasingbraking speed, while the oxidation abrasion at 30m�s�1 areless than that of 28m�s�1 for all of the samples due to the lowbond energy between the graphite layers and the hardness ofwear debris at high temperature in the friction surface.

Acknowledgements

This research was supported by National natural sciencefoundation (50702078), National key basic research supportfoundation (2006CB600901), and National High-tech R&DProgram (2009AA03Z536).

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Fig. 8 The oxidation abrasion under different braking speed.

Effect of Braking Speeds on the Tribological Properties of Carbon/Carbon Composites 1043