PERGAMON Carbon 39 (2001) 1477–1483

SiC/C multi-layered coating contributing to the antioxidation ofC/C composites and the suppression of through-thickness

cracks in the layera , b c a c*Takuya Aoki , Hiroshi Hatta , Taku Hitomi , Hiroshi Fukuda , Ichiro Shiota

aDepartment of Material Science and Technology, Faculty of Industrial Science and Technology, Science University of Tokyo, 2641,Yamazaki, Noda-shi, Chiba, 278-8510, Japan

bThe Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara-shi, Kanagawa, 229-8510, JapancDepartment of Chemical Engineering, Kogakuin University, 2665-1 Nakano-chou, Hachioji-shi, Tokyo, 192-8510, Japan

Received 25 April 2000; accepted 13 October 2000

Abstract

Silicon carbide /carbon multi-layered coatings have been formed on the surfaces of C/C composites. The multi-layeredcoatings were deposited by the CVD method at 12008C using SiCl , C H and H gases. The principal idea of the present4 3 8 2

multi-layered coatings is to suppress the through-the-thickness coating cracks by making the thickness of each SiC andcarbon layer lower than the corresponding critical thicknesses, h s. The values of h s for the SiC and carbon coatings werec c

0.2 mm and above 15 mm, respectively. The role of the SiC layers is oxidation protection and that of the carbon layers is tomechanically isolate each SiC layer. Based on theoretical and experimental discussions, the SiC/C multi-layered coating wasshown to successfully suppress the through-thickness coating cracks. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: A. Carbon/Carbon composites; B. Chemical vapor deposition; Coating; Oxidation

1. Introduction thickness coating cracks [3,4,9]. These cracks allow oxy-gen diffusion into the C/C substrate, which leads to severe

The most serious disadvantage in applications using oxidation-degradation in the substrate [2,3,10,11]. There-Carbon/Carbon composites, C/Cs, is the rapid oxidation fore, to improve the oxidation-resistance of the C/Cin high temperature environments. To improve the oxida-tion resistance of the C/Cs, ceramic coatings, such as SiC,Si N [1–4], and oxide glass coatings [5,6], have been3 4

examined and shown to be partly successful. However, dueto the cracks in the coating, oxygen still penetrates to thesubstrate C/Cs. Thus, the C/Cs have been applied only tostructures used in an inert atmosphere or to secondarystructures, to which the most serious design requirementwas other than mechanical loading [7,8].

As shown in Fig. 1, during the cooling process from thecoating treatment to room temperatures, the significantmismatch in thermal expansions between the ceramiccoating and the C/C substrate produces many through-the-

*Corresponding author. Tel.: 181-471-24-1501; fax: 181-471-23-9362. Fig. 1. Cross-sectional view of a SiC single-layered coating

E-mail address: takuya@pup.isas.ac.jp (T. Aoki). applied on the PY–C/C composite.

0008-6223/01/$ – see front matter 2001 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 00 )00276-1

1478 T. Aoki et al. / Carbon 39 (2001) 1477 –1483

composites, the suppression of the through-the-thicknesscoating cracks is definitely required.

In the present paper, a newly designed SiC/C multi-layered coating was examined for the suppression of thethrough-the-thickness coating cracks deposited on the C/Cs. The coating in this paper was applied on unidirection-ally reinforced C/Cs by a chemical vapor depositionmethod, CVD. The effect of the silicon carbide /carbon-multi-layered coating was discussed from both the ex-perimental and theoretical points of views.

2. Coating concepts

The multi-layered coatings examined in this study werecomposed of extremely thin SiC and pyrolytic carbonlayers that were alternately laminated. The main role of theSiC layers is oxidation protection and that of the pyrolyticcarbon layers is to eliminate mechanical interactionsamong the SiC layers and to isolate each SiC layer.

It has been verified that the coating cracks disappeareven if a large tensile stress arises in the coating when thethickness of the coating becomes less than a critical value.

Fig. 2. Schematic description of the expected cracking behavior inThis phenomenon is due to the reduction in the energythe SiC/C multi-layered coating.release rate during the formation of a through-the-thickness

channeling crack propagating across the coating on thesurface as discussed Section 4.1 [12–14]. The criticalthickness for a SiC coating under the present coating In addition, the self-healing capability is expected in thetreatment condition is about 0.2 mm. In addition, it was multi-layered coating. Due to the oxidation reaction ofexperimentally confirmed that a relatively thick pyrolytic SiC, SiO is formed, and due to this conversion, the2

carbon layer (|15 mm) on a C/C composite induced no volume expands by about twice. This swelling reduces thecracks. This implies that the coefficient of thermal expan- opening of the coating cracks [9–11,18]. For example, insion and the fracture toughness of the present pyrolytic the 65 mm-coating deposited at 18008C, a crack with ancarbon are relatively low and high, respectively, although opening of 0.8 mm at room temperature was observed.their exact values depend on the structures [15–17]. Thus, This crack was predicted to be sealed with SiO within2

if the pyrolytic carbon layers are inserted between the thin 300 s and 1800 s. at 15708C and 13458C, respectively [11].SiC layers (#0.2 mm) and the isolation of each SiC layer This self-healing effect is more pronounced when the crackis achieved, we can possibly obtain a thick crack-free opening is small and enhances the oxidation resistance ofmulti-layered coating. the multi-layered coating especially in the high tempera-

Even if the multi-layered coating cracks due to thermal ture range ($ about 14008C).and/or external loads, the cracks should be stronglydeflected in the carbon layers, as shown in Fig. 2. Becausethe pyrolytic carbon tends to grow with the carbon layers 3. Experimental procedureparallel to the interfaces, the resistance against the crackpropagation within and along the carbon layers should be Two kinds of C/C composites were used as the coatingextremely low [15,17]. The authors also confirmed that the substrate. The first C/C was produced by the Preformed-pyrolytic carbon prepared in this study shows a rough Yarn method [19] and supplied by the Across Co., Japanlayered structure by the S.E.M. observation. As shown in (PY–C/C). The other was fabricated by the SentanFig. 2, it is expected that the coating cracks are sig- Zairyou Co., Japan (SZ–C/C). These substrates werenificantly deflected in and along the pyrolytic carbon unidirectionaly reinforced. The reinforcing fiber of thelayers. Thus, the probability of through-the-thickness PY–C/Cs was TORAYCA-M40 (Toray Co., Japan) andcracks may be lowered. Even if the through-the-thickness the volume fraction of the fibers, V , and the heat treatmentf

cracks eventually appear, the crack passes from the surface temperature, HTT, were 50% and 20008C, respectively.to the substrate become quite long. Hence, we can expect a The SZ–C/Cs were reinforced by XN–35 (Nipponmuch lower mass loss rate due to the lower gradient of Graphite Fiber Co., Japan) and the HTT was 25008C.oxygen across the through-the-thickness cracks [9–11]. The SiC/C multi-layered coating was formed by the

T. Aoki et al. / Carbon 39 (2001) 1477 –1483 1479

Table 1 adjacent cracks, as a function of the coating thickness. AsCoating conditions for CVD–SiC and pyrolytic carbon coatings shown in this figure, 2l decreases with a decrease in the

coating thickness and at a thickness of 0.2 mm the coatingMaterials Coating Total Gascracks almost disappeared. However, a small amount oftemperature pressure composition

(8C) (Torr) (cc /min) localized coating cracks were still observed only above thematrix rich regions of the substrate C/C. In these regions,SiC1 1424 micro-cracks were observed in the substrate. These sub-CVD–SiC 1200 10 C H 1003 8strate cracks were thought to act as the source of stressH 4002

concentration causing the coating cracking. However, thesePyrolytic C 1200 10 C H 1003 8

localized cracks terminated when the cracks reached theboundaries of matrix rich regions. Thus, the coatingthickness, when the coating cracks almost disappeared,

chemical vapor deposition, CVD, using SiCl , C H and was defined as the critical thickness, h .4 3 8 c

H gases. Both the SiC and C layers were deposited at In order to analytically reconfirm h , the energy release2 c

12008C under a total gas pressure of 10 Torr. Detailed rate, G , during the formation of a through-the-thicknessch

coating conditions are shown in Table 1. The thickness of channeling crack shown in Fig. 4(a) (steady state crackthe SiC coatings was intentionally changed from 0.2 mm to propagation across the coating) was calculated using the170 mm. To identify the materials in the multi-layered finite element model shown in Fig. 5. At the criticalcoatings, X-ray diffraction and ESCA measurements were thickness, h , a few number of channeling cracks werec

performed. Based on these examinations, it was confirmed assumed to appear in the coating with sufficiently largethat the coatings were composed of stoichiometric b-SiC spacing (no interaction between the cracks). Under thisand pyrolytic carbon the regardless of coating thickness condition, G for a channel cracking per unit crackch

[20]. extension is given by the mean value of the mode-I energy.release rate, G ( y), when a crack as shown in Fig. 4(b)I

propagates up to the interface [12–14,21];

4. Results and discussion

4.1. Critical thickness for CVD–SiC single-layeredcoating

The critical thickness of the SiC single-layered coatingwas examined using the unidirectional PY–C/Cs with

3dimensions of 1531533 mm . Fig. 3 shows the variationin the observed crack spacing, 2l, the distance between

Fig. 3. Relationship between the observed crack spacing and the Fig. 4. Schematic description of a channel cracking across thethickness of the SiC single-layered coating. coating and a crack extension toward the interface.

1480 T. Aoki et al. / Carbon 39 (2001) 1477 –1483

Fig. 5. A finite element model for the calculation of the energyrelease rate during the coating cracking.

h

1]G 5 E G ( y) dy, (1)ch Ih

0

where h denotes the coating thickness. The materialproperties used in the finite element analysis, FEA, are

.shown in Table 2 [9,22–26] and the calculated G ( y)sI

using the virtual crack closure technique [27] are in Fig. 6.?Fig. 6. Calculated energy release rate, G ( y), when a crackIFig. 7 shows the calculation result of G for thech propagates from the coating surface to the interface.

temperature drop from the CVD temperature, 12008C, toroom temperature. The critical energy release rate, G , ofIC

23the SiC coating was assumed to be 30 J /m (K 53.8IC dimensions in this experiment were 2031033 mm . The]ŒMPa m), which is a typically reported value [26]. As off-axis substrates were used so as to increase the in-plane

shown in this figure, G linearly decreases with decreas-ch thermal expansion coefficient (CTE) of the substrates. Bying coating thickness and at a thickness less than 0.2 mm, this procedure, we can freely set the critical thickness, h ,cG becomes lower than G . As noted in Section 2, noch IC and then we can chose h at any value for which we canccracking was observed in the single-layered pyrolytic easily form the multi-layered coatings.carbon coating even at the thickness of 15 mm. Thus, if the In order to estimate the thermal stress in the SiC coating,pyrolytic carbon layers are inserted between the thin SiC s , deposited on the off-axis C/C substrate, the in-planeSiC

layers (#0.2 mm) and the isolation of each SiC layer is CTE of the off-axis substrate, a , was first calculatedin-plane

achieved, it is expected that a thick crack-free multi- as a function of u using the coordinate transformation lawlayered coating can be formed. [28]. Then, the s is given by;SiC

ESiC]]s 5 (a 2 a ) DT (2)4.2. Plausibility of SiC /C multi-layered coating SiC SiC in-plane1 2 nSiC

To confirm the validity of the present concept, the where E , n and a are Young’s modulus, Poisson’sSiC SiC SiC

SiC/C multi-layered coatings were deposited on the off- ratio and CTE of the SiC coating, respectively. The trendsaxis substrates cut from unidirectional SZ–C/Cs. The in a and s are shown in Fig. 9(a) as a function ofin-plane SiC

cutting angle between the coating plane and the carbon u. As shown in this figure, the thermal expansion mismatchfiber axis, u, shown in Fig. 8 was varied. The substrate is canceled at u ¯458.

Table 2aMaterial properties of CVD–SiC coating and C/C substrate used in the calculations

Materials Young’s Shear Poisson Coefficient of thermalmodulus modulus ratio expansion CTE

26(GPa) (GPa) (310 / 8C) (20–12008C)

CVD–SiC 490 0.25 4.2PY–C/C E 5170 n 50.47 a 50.15X XY X

G 57X,Y

(UD) E 515 n 50.30 a 59.0Y YZ X

SZ–C/C E 5320 n 50.47 a 50.15X XY X

G 55XY

(UD) E 57 n 50.30 a 59.0Y YZ X

a Subscripts ‘‘X’’ and ‘‘Y, Z’’ denote the longitudinal and transverse directions to the carbon fibers, respectively.

T. Aoki et al. / Carbon 39 (2001) 1477 –1483 1481

Fig. 7. Calculated energy release rate, G , for the channelingch

cracking as a function of the SiC single-layered coating thickness.

Next, let us discuss the critical thickness, h , of the SiCc

coating on the off-axis C/C substrate. Beuth [12] hasgiven h for an isotropic coating deposited on an isotropicc

substrate by setting the energy release rate, G , during thech

formation of a through-the-thickness channeling crackequal to G . His results is quite simple and given as:IC

]2EGIC Fig. 9. Dependence of the in-plane thermal stress in the coating,]]]h 5 (3)c 2 the coefficient of thermal expansion and Young’s modulus of thes pg(a, b )

off-axis substrate, and g(a, b) on the inclination angle of the]

substrate, u.where E is the plane strain modulus of the coating given2by E /(1 2 n ), s is the tensile stress in the coating, and

g(a, b) is a function of Dunders parameters, a and b [12].The Dunders parameters are modulus ratios of the coatingand substrate. If Eq. (3) is still effective for the presentaniotropic substrate, it is extremely convenient. Thus hc

based on the Beuth’s model was compared with that byFEA. In the calculation of Eq. (3), g(a,b) was determinedfrom the in-plane modulus, E , of the off-axis sub-in-plane

strate which was calculated using the anisotropic Hook’slaw [29], see Fig. 9(b), and s was set equal to s in Fig.SiC

9(a). Fig. 10 shows the calculated h s by the FEA and Eq.c

Fig. 8. Schematic drawing for the off-axis SZ–C/C substrate Fig. 10. Calculated critical thickness, h , of the SiC single-layeredc

prepared for the SiC/C multi-layered coating. coating as a function of the inclination angle, u.

1482 T. Aoki et al. / Carbon 39 (2001) 1477 –1483

(3) as a function of the inclination angle of the substrate, u. Thus in only the latter multi-layered coating did theThis figure clearly demonstrates that both results agree through-the-thickness cracks occur.reasonably well. Thus we can use Beuth’s model for a We also formed the single-layered SiC coatings on therough estimation of h . same off-axis substrates, u 536–388, with a coating thick-c

Fig. 11 shows the cross-sectional views of a SiC/C ness of 20 mm, which is equal to the total thickness of themulti-layered coating applied on the off-axis substrates. SiC layers in the multi-layered coatings. In these single-The total thickness of the coatings was about 24 mm, and layered coatings, we clearly observed the through-the-the thicknesses of each SiC and pyrolytic carbon layers thickness cracks. This example successfully demonstratedwere about 4 mm and 0.8 mm, respectively. As shown in that the SiC/C multi-layered coating is quite effective forFig. 11(a), the through-the-thickness cracks can be seen in suppressing the through-the-thickness coating cracks de-the coating on the 368 substrate. On the other hand, in the posited on C/C composites.388 coating, although slight delaminations between thelayers were observed, no through-the-thickness crack wasidentified. The difference in the cracking behavior can be 4.3. Crack deflection along SiC /C interface and inclearly explained in Fig. 10, in which the open circles and carbon layerdiagonal crosses represent the crack-free and crackedcoatings, respectively. In the 388 coating, the thickness of Even if coating cracks are induced by external and/oreach SiC layer, equals to about 4 mm, is lower than the thermal stresses, the multi-layered coating is still expectedcritical thickness (h ¯5–7 mm). On the other hand, in the to reduce the number of through-the-thickness cracks byc

368 coating, each SiC layer was thicker than h (¯3 mm). the crack deflection mechanism. To confirm this, the crackc

extension behavior was observed using a multi-layeredcoating with thicker individual layers than the criticalthicknesses. Fig. 12 shows a cross-section of a SiC/Cmulti-layered coating on the PY–C/C. Here the inclinationangle, u, was set to 08. The thicknesses of the single SiClayers in this figure are 20–40 mm, the pyrolytic carbonlayers are 5–18 mm, and the total thickness was 100 mm.The coating cracks started in the SiC layers and weredeflected at the interfaces between the SiC and pyrolyticcarbon layers. The crack opening was wider for cracksrunning in the thickness direction and narrower for thosealong the interfaces. Thus if the extremely thin pyroliticcarbon layers are laminated between the SiC layers in themulti-layered coating, the deflected cracks should be self-healed by the formation of SiO and the accompanying2

volumetric expansion in the high temperature range asdiscussed in Section 2.

Fig. 11. Cross-sectional views of the SiC/C multi-layered coat- Fig. 12. Crack deflection at the SiC/C interface and within theings applied on the off-axis substrate. pyrolytic carbon layer.

T. Aoki et al. / Carbon 39 (2001) 1477 –1483 1483

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