Materials and Design 36 (2012) 289–295

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Materials and Design

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Mechanical properties and microstructure of 3D orthogonal quartz fiberreinforced silica composites fabricated by silicasol-infiltration-sintering

Chengdong Li a, Zhaofeng Chen a, Jianxun Zhu b,⇑, Yong Liu a, Yun Jiang b, Tianru Guan a, Binbin Li a,Long Lin a

a College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR Chinab Sinoma Science & Technology Co. Ltd, Nanjing 210016, PR China

a r t i c l e i n f o

Article history:Received 29 July 2011Accepted 10 November 2011Available online 18 November 2011

Keywords:A. Ceramic matrix compositesE. MechanicalF. Microstructure

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.11.022

⇑ Corresponding author. Tel.: +86 25 52112909; faxE-mail addresses: czf_msc@nuaa.edu.cn (C. Li), jian

a b s t r a c t

3D orthogonal quartz fiber reinforced silica composites were fabricated by silicasol-infiltration-sinteringmethod at a low temperature of 450 �C, and the mechanical properties and microstructure of the compos-ites were investigated. The density of the composites was 1.71 g/cm3, the average values of flexurestrength and shear strength of the composites were 29.4 MPa and 14.8 MPa at room temperature, respec-tively. Both of the failures under flexural loading and shear loading were not catastrophic. Compared witha-quartz and a-cristobalite, the fracture toughness of 3D orthogonal quartz fiber reinforced silica com-posites has been enhanced by a wide margin. The damage features of the composites are identified, usingdigital camera and scanning electron microscopy. There was a good state without serious degradation ofquartz fibers during the preparation. The bonding between the matrix and the incorporated fibers isweak, and the matrix cracks propagate along the weak interface. Apart from this, the composite is foundto exhibit an extensive and long fiber pull-out in the fracture surface. Crack deflection and fiber pull-outcontributed to the good toughness of the composite under the loading.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Quartz fibers reinforced silica composites (QFSCs) have becomea promising candidate material in the space shuttle and otherre-entry type aerospace vehicles due to its unique combinationsof various properties, such as low density, low thermal conductiv-ity, acceptable levels of mechanical properties, excellent thermal aswell as electrical insulating properties and low dielectric constantand low loss tangents [1,2]. Continuous fiber reinforced ceramicmatrix composites (CFCCs) are fabricated at relatively high tem-peratures (typically, �1000 �C in the chemical vapor infiltration(CVI) process and even higher in the slurry impregnation/hotpressing technique) and they are used in high temperatures andoxidizing atmospheres, a feature which introduces a constraint ofthermo-chemical and thermo-mechanical compatibilities [3]. Inaddition, the high temperature process will lead to serious degra-dation of quartz fibers, therefore, it is arduous to pack matrix den-sely into quartz fiber preforms to acquire high strength compositematerials [4]. However, silicasol-infiltration-sintering (SIS) couldbe a promising method to fabricate CFCCs due to its effectiveness

ll rights reserved.

: +86 25 52112626.xun_zhu@163.com (J. Zhu).

in relatively low densification temperature (450 �C), low shrinkage,and reduced drying stresses.

Fiber reinforced textile structures can produce net shape ‘‘pre-forms’’ continuously, as well as provide a product form with im-proved damage tolerance and impact resistance at lower cost [5].In order to overcome the vulnerabilities of low interlaminar bondingstrength and interlaminar thermal conductivity in two-dimensional(2D) composites, three-dimensional (3D) woven composites havebeen pursued in the aerospace industry [6–9]. 3D orthogonal wovenfabrics are characteristic with prominent in-plane stiffness andstrength (higher than 2D weave laminates, much higher than 3Dinterlock weave composites) [10,11]. In addition, infiltration is suit-able for 3D orthogonal woven composites which contain a relativelyhigh volume fraction of reinforcement [12]. Recently, macromechanical properties of textile composites, such as their tensile,flexural and shear properties with regard to their overall fracturebehavior have been considerably researched. However, to the bestof our knowledge, few literatures concerning mechanical propertiesand failure behavior of the 3D orthogonal QFSCs have been pub-lished so far.

In this paper, 3D orthogonal QFSCs were prepared by SIS meth-od. The phase composition, flexural and shear strengths, fracturetoughness, hardness and microstructure of the as-prepared com-posites were investigated.

a =1.5 mm

So = 24 mm

B = 4 mm

W = 3mm

L = 45 mm

P

Fig. 2. Single edge notched bend (SENB) test specimen geometry.

290 C. Li et al. / Materials and Design 36 (2012) 289–295

2. Experimental details

The three dimensional fabric performs were woven by four-stepprocessing and supplied by Nanjing Institute of Glass Fiber in Chi-na. The fiber volume fraction in the preforms was 50.2%. The fibermaterial used in this study was the quartz yarn, and the matrixmaterial was silica which was derived from silicasol. As shown inFig. 1, the composites were manufactured by SIS method, includingvacuum infiltration and sintering process. Firstly, the 3D orthogo-nal preforms were vacuum impregnated using colloidal silica solu-tion precursor (35 vol.% silica) for 0.5 h, and then the containerpressure was increased to 10 atm and maintained for 1 h. Afterthat, the preform was dried at 80 �C for 1 h and 110 �C for 1 h,respectively. Then the dried preforms were heated in an oven at450 �C for 2 h in order to remove the coupling agent and boundwater. The whole processes were repeated 15 times to enhancethe density of the composites. The density of the sample was deter-mined by the water displacement method.

Both of the bending and shear tests were measured at SANSCMT5105 electronic universal testing machine. Three-point bend-ing tests were carried out using rectangular samples, accordingto ASTM C1341-06 standard [13]. The flexure sample size was40 mm � 5 mm � 3.5 mm, while the span and the velocity of thecrosshead were 30 mm and 0.3 mm/min, respectively. Shearstrength was measured by the iosipescu shear testing method(according to ASTM D5379-05 [14]), meanwhile, the compositepanels were cut into V-notched beam specimens. The shear samplesize was 80 mm � 18 mm � 3.5 mm, and the crosshead speed wasset at 0.3 mm/min.

The fracture toughness of 3D orthogonal QFSCs was studied bysingle-edge-notched-beam (SENB) method. The SENB sampleswith dimensions of 3 mm � 4 mm � 45 mm (see Fig. 2) weretested in three-point loading with a 24 mm span at a cross-headspeed of 0.03 mm/min. A preliminary notch 1.5 mm deep was cutin the package by a 150–300 lm thick diamond wafering saw.The standard Rockwell B-scale test, featuring a 1.588 mm diameterspherical steel indenter, was used to characterize the hardness. Aminor load and a major load of 10 kg and 100 kg, respectively, wereimposed on the sample. The hardness value (HRB) was directlydetermined by the depth of indentation beyond the minor load.The phase composition of the as-prepared sample was determinedby X-ray diffraction (XRD, Rigaku D/Max-B) using Ni-filtered Cu Karadiation at a scanning rate of 4�/s and scanning from 20� to 70� of

Preform

One Cycle

Valve

Fig. 1. Preparation process o

2h. The fracture surface of the sample was observed by digital cam-era and scanning electron microscopy (SEM, JEOL JSM-6360).

3. Results and discussion

3.1. Density-cycle curve

As shown in Fig. 3, after 15 cycles, the density of the materialsincreased from 0.53 g/m3 to 1.71 g/m3, but almost stopped increas-ing after 11 cycles. The reason was that, as cycle times increase,infiltration paths of the slurry had being jammed which would leadto the size of the silica sol particle get too big to infiltrate into thecomposites fully, so the infiltration efficiency was debased [15].The relative density of the 3D orthogonal QFSCs is about 0.77.

3.2. Phase composition

Fig. 4 shows XRD pattern of as-received composites. A wide andhigh peak was observed in the as-received composites, suggestingan amorphous state of the quartz fibers and matrix material. Theamorphous state of quartz fibers gave less degradation, thus con-tributing to good reinforcement ability for the composites [4].

3.3. Flexural loading

The density of the composites was 1.71 g/cm3 while the hard-ness of the composite is about 55HRB. Fig. 5 shows the typical flex-ural failure behavior of 3D orthogonal woven composites at roomtemperature. The average value of flexure strength of the compos-ites was 29.4 MPa at room temperature. The stress–displacement

Oven

Preform

Silicasol

Vacuometer

f 3D orthogonal QFSCs.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0.6

0.8

1.0

1.2

1.4

1.6

1.8D

ensi

ty, g

/m3

Cycle

Fig. 3. Density-cycle curve of 3D orthogonal QFSCs prepared by silicasol-infiltra-tion-sintering.

20 40 60 800

200

400

600

800

1000

1200

1400

Inte

nsity

(a.

u.)

2 theta / ( o )

Fig. 4. XRD pattern of the surfaces of as-received composites.

0.00 0.05 0.10 0.15 0.20 0.255

10

15

20

25

30

35

Stre

ss, M

Pa

Displacement, mm

A1

B1

A2

B2A3

B3

Fig. 5. Flexural stress–displacement curve.

Matrix

Layers

P

Fig. 6. Schematic diagram of specimen under bending.

C. Li et al. / Materials and Design 36 (2012) 289–295 291

curve (see Fig. 5) could be divided into two stages. At I stage, themechanical behavior of the composites was linear elastic. As theincrease of load, the initial cracks propagated first in the matrixas the fibers were stronger and showed higher strain to failure.When the matrix crack approached the fiber matrix interface, thestress concentration at the interface did not induce fiber failurebut initial fiber matrix debonding. At II stage, the material showed

a distinct and sudden stress drops (shown as A1B1, A2B2 and A3B3)with the increase in displacement. In this case, the fiber bundlesunderwent significant bending, as then the fibers were pulled outof the matrix. Consequently, the fiber bundles failed when themaximum load arrived, causing the unstable fracture of thecomposites.

The step-like decreasing after the maximum stress was due to thefracture of different layers in the 3D orthogonal composites (seeFig. 6). Fig. 6 shows the schematic diagram of specimen under bend-ing. When the strength was measured, P was loaded perpendicularto the surface of the sample. When P was loaded, the stress evolvedfrom compression stress on the upper surface of the sample to ten-sion stress on the bottom surface of the sample. The whole tows ofthe bottom surface carried the force nearly simultaneously. Withthe value of stress increasing near to maximum flexural stress, thetension stress reached the tensile strength of the bottom tows. Thenthe sample began to fracture. The other tows was broken layer-by-layer rapidly, then the sample was fractured. It indicated a good duc-tility of the composites, which did not fracture completely after theflexural strength test [16]. The efficiency of the fiber/matrix (F/M)interface is to transfer the additional stress thrown onto the fibers.The best interphase materials might be those with a layered struc-ture, the layers being parallel to the fiber surface and weakly bondedto one another, and the whole interphase strongly adherent to thefiber [17,18]. There are two main types of bonding at an interface:mechanical bonding or chemical bonding [19]. Mechanical bondingresults from thermally induced residual stresses, while chemicalbonding arises from chemical reaction during processing or thermalshrinkage during cooling. In this study, the composites were pre-pared at a low temperature, the residual stresses of the compositeswas relative low. In addition, the strong chemical reactions betweenthe constituents or detrimental change in the fiber microstructurecould be avoided. Therefore, the fiber matrix bond strength wasweak. For CFCCs, the mechanical properties are affected by the fi-bers, matrix and F/M interfaces. Higher strength of reinforcing fibers,higher density of the matrix and relatively weak interfacial bondingall contributed to desired mechanical properties. The 3D orthogonalQFSCs material with high fiber content (50.2%) reflects a good rein-forcement of fiber. Since the density of the composites was en-hanced by multiple infiltration and dry process, the silica matrixcould sufficiently permeate into the quartz perform. The load-trans-ferring ability of the silica matrix was sufficiently taken effect be-cause of a relatively high density (1.71 g/cm3). Thus the 3Dorthogonal QFSCs displayed a relatively good bending failurebehavior.

3.4. Shear loading

Shear strength (s) of the composites was calculated by the fol-lowing equation:

s ¼ P=hx ð1Þ

0.00 0.05 0.10 0.15 0.20 0.255

10

15

20

25

30

35

Loa

d, N

Displacement, mm

Stable behavior

Unstable behavior

Fig. 8. Load displacement diagrams from precracked beam tests.

Table 1Coefficients for the polynomial g (a/W) for three-point flexure.

S0/W

5 6 7 8 10

A0 1.9109 1.9230 1.9322 1.9381 1.9472A1 �5.1552 �5.1389 �5.1007 �5.0947 �5.0247A2 12.6880 12.6194 12.3621 12.3861 11.8954A3 �19.5736 �19.5510 �19.0071 �19.2142 �18.0635A4 15.9377 15.9841 15.4677 15.7747 14.5986A5 �5.1454 �5.1736 �4.9913 �5.1270 �4.6896

Table 2Fracture toughness of several kinds of ceramic materials.

Ceramic materials Fracture toughness (MPa m1/2)

a-Quartz (SiO2) 0.715a-Cristobalite (SiO2) 0.7103D orthogonal QFSCs

(measurement in this study)3.3905

292 C. Li et al. / Materials and Design 36 (2012) 289–295

where P is the maximum fracture load (N), h and x are the heightand the minimum distance between V-notched of the sample,respectively. The typical shear stress–displacement curve and mac-roscopic fracture profile are shown in Fig. 7. The average value ofshear strength of the composites was 14.8 MPa at room tempera-ture. It was observed from Fig. 7a that the mechanical behaviorwas initially linear elastic. Then, a nonlinear region was observed,reflecting matrix damage which induced significantly compliance,and residual displacement. Finally, the fiber failed, initiating atthe maximum stress, causing the unstable fracture of the compos-ites. The stress drop was very gradual after the maximum stresspoint at room temperature. Fig. 7b showed the macrograph of frac-ture surface which exhibited curving fracture path and rugged frac-ture morphology. The results indicated that the 3D orthogonalQFSCs displayed a good shear resistant.

3.5. Fracture toughness

Fig. 8 shows the load displacement diagrams from precrackedbeam tests. The maximum load of QFSCs was 34.0 N. Precrackedbeam tests can be either stable or unstable. As shown in Fig. 8,the load did not decrease suddenly but gently. Different from sta-ble behavior, the load did not increase slowly but linearly in initialstage. We call this curve pop-in stable behavior. In general, unsta-ble tests may result in greater fracture toughness values than thosefrom tests with stable crack extension [20,21]. The examination ofthe specimens fractured in the tests revealed a fracture crack thatpropagated from the points where ‘‘additional’’ stress concentra-tors were present [22]. This observation also confirms the assump-tion that the fracture of a loaded ceramic specimen starts from asmall crack ahead of a machined notch root [23]. The total ab-sorbed energy was the area under the force–displacement curve.In this context, it is interesting to mention that KIpb magnitudesare influenced by the sharpness of a notch root rather than by itsshape. In this study, fracture toughness (KIpb) of the compositeswas calculated by the following equation (according to ASTM C1421-01b standard [24]):

KIpb ¼ gPmaxS0 � 10�6

BW3=2

" #3½a=W�1=2

2½1� a=W�3=2

" #ð2Þ

g ¼ gða=WÞ ¼ A0 þ A1ða=WÞ þ A2ða=WÞ2 þ A3ða=WÞ3

þ A4ða=WÞ4 þ A5ða=WÞ5

where Pmax is the maximum force (break force) as determined inFig. 8, S0 is the span, B is the side to side dimension of the test

(a)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8024681012141618

Stre

ss,M

pa

Displacement,mm

Fig. 7. Shear stress–displacement curve and macroscopic fracture profil

specimen perpendicular to the crack length, W is the top to bottomdimension of the test specimen parallel to the crack length, a is thecrack depth, coefficients for g are shown in Table 1.

The results of fracture toughness of several kinds of ceramicmaterials are summarized in Table 2 [25]. The fracture toughnessof 3D orthogonal QFSCs is more than four times that of monolithica-quartz (fracture toughness: 0.715 MPa m1/2) or a-cristobalite(fracture toughness: 0.710 MPa m1/2). Therefore, the fiber toughen-

(b)

0.9 1.0

e. (a) Stress–displacement curve and (b) cross section photograph.

Crack deflection

Fiber

Matrix

Crack blockage

Fig. 9. Crack blockage and crack deflection.

Crack

Singly deflected

Doubly deflected

Fiber

Penetrated

Fiber

Fiber

Fiber fracture

Matrix crack

Matrix

Fiber

Crack deflection

Residual stress

Debond surfaces

Microcrack

Fig. 10. Crack bridging mechanism in fiber-reinforced composites.

C. Li et al. / Materials and Design 36 (2012) 289–295 293

ing effect of 3D orthogonal QFSCs has been reflected in the presentstudy. Fig. 9 clearly reveals that the crack propagation is eitherblocked or deflected by the dark regions. When the energy of crackpropagation is low, crack will be blocked by quartz fiber. Mean-while, a portion of energy will be absorbed through crack deflec-

(a)

(c)

Matrix crack

Fig. 11. Fracture surfaces of the 3D orthogonal QFSCs under bending. (a) Fiber/m

tion if the energy of crack propagation is high. Therefore, theenergy of crack could be decreased efficiently. In addition, residualstresses could transmit from silica matrix to quartz fibers throughcrack propagation. The improvement in fracture toughness wasdue not only to crack blockage or deflection by uniform dispersionof in situ formed quartz fiber with high Young’s modulus, but alsothe residual stresses transformation between quartz fibers and sil-ica matrix.

3.6. Microstructure

It is known that the properties of CFCCs are closely related totheir structure. However, the structure depends on the phase,chemical composition, microstructure and the preparation methodof the materials. Therefore, the development of new materials suchas 3D orthogonal QFSCs becomes complex owing to the existenceof a variety of microstructure. Although both constituents (fibersand matrix) of fiber-reinforced ceramics are brittle, the compositesdisplay quasi-ductile deformation due to mechanisms like crackdeflection, crack bridging, or fiber pull-out [19,26–28]. The quartzfiber can prevent cracks extension and enable the crack to bridge,change directions, and branch out. When the quartz fibers rupture,they can display their adaptability and prevent any disastrous out-come. Fig. 10 shows various events and processes of crack bridgingmechanism in fiber-reinforced composites. It is notable that thecrack extension process essentially involves matrix microcracking,the F/M debonding, fiber fracture and fiber pull-out. Examinationof the failure process at the microstructural level indicates thatthe first step is matrix cracking. When a crack further propagatesthrough the matrix and intercepts the fiber, two competing effectshave to be considered. On the one hand, the matrix crack can prop-agate through the fiber and trigger a brittle failure of the compos-ite. On the other hand, the matrix crack can be deflected along theF/M interface and continue to propagate along it in a benign man-

(b)

(d)atrix interface, (b) fiber and matrix, (c) matrix crack and (d) fiber pull-out.

294 C. Li et al. / Materials and Design 36 (2012) 289–295

ner [29]. Since the matrix crack deflection at the F/M interfacecould results in the F/M interface debonding, matrix crack deflec-tion mechanism is a prerequisite for the activation of varioustoughening mechanisms such as fiber pull-out. There are at leastthree possible crack paths for a matrix crack at the F/M interfacein the composite [30]. The simplest possible failure paths: crackdeflection on one side of the interface (singly deflected crack);crack deflection on both sides (doubly deflected crack); and crackpenetration across the interface. If the interface is weak enoughfor the matrix crack to be deflected along the interface, the fibersremain intact and the composite can be tough. If the interface istoo strong, the matrix crack penetrates into the fibers and the com-posite is brittle like a monolithic ceramic [31]. In this study, thebonding between the matrix and the incorporated fibers typicallyis weak, thus the matrix microcracks propagate along the weakinterface. The fibers ultimately fracture at a point out of the matrixcrack plane and then were pulled out of the matrix and hence thecomposite failed with continued loading. Microcracking proceedsuntil the matrix is completely disintegrated. A sequence of pro-cesses is taken into account: matrix microcracking—local fiber fail-ure—evolution of bundle damage—coalescence into ‘‘bundlecracks’’—linkage of bundle cracks, that is, ‘‘fiber pull-out’’.

Generally speaking, after the maximum value of the load isreached, the subsequent extension degree of a ceramic matrixcomposite is strongly dependent on the nature of the F/M interface,and the microstructure of the composite is consistent with itsmechanical properties. Li et al. [32] suggested that a relativelyweak bond between interfaces was beneficial to mechanical prop-erties of the composites, since cracks might be kept in the stratifiedstructure of interphase layer to consume more energy. It is wellknown that interfacial bonding strength can be evaluated by themorphology of fracture surfaces [33]. Fig. 11 shows the SEM micro-graphs of the fracture surfaces of the 3D orthogonal QFSCs underbending. Fig. 11a shows a clear gap between the quartz fiber andmatrix, which indicate the weak fiber matrix bond strength. Asseen from Fig. 11b, it can be seen that the fiber surface and the ma-trix particle are compatible, and a lot of silica particles adhere tothe fiber surface and increase the surface roughness. As shown inFig. 11c, the debonding between the quartz fiber and the matrix oc-curs when the matrix crack propagates along the weak interface.An extensive fiber pull-out indicates a relatively weak F/M interfa-cial bonding, while little fiber pull-out and short pull-out lengthindicate a strong F/M interfacial bonding. In this study, an exten-sive and long fiber pull-out in the fracture surface of the 3D orthog-onal QFSCs (see Fig. 11d) exhibited the effectiveness of fiberreinforcement. These facts show a good state without serious deg-radation of quartz fibers during the preparation. The fiber pull-outmechanism is also supported by the stress–displacement curveshown in Fig. 5. Therefore, the F/M interfacial bonding in the com-posites were desirable, the 3D orthogonal woven composites had agood toughness.

4. Conclusion

3D orthogonal quartz fibers reinforced silica composites weresuccessfully fabricated by silicasol-infiltration-sintering method.The average values of flexure strength and the shear strength ofthe composites were 29.4 and 14.8 MPa at room temperature,respectively. Due to a relatively low preparation temperature of450 �C, the fiber matrix bond strength was weak. The matrix crackcould deflect along the F/M interface and propagate along it in abenign manner. The composites under the loading displayed agood toughness due to the crack deflection and fiber pull-outmechanism.

Acknowledgment

This work was supported by the Basic Research Project of Sci-ence and Technology of Jiangsu Province (No. BK2009002) andFunding of Jiangsu Innovation Program for Graduate Education(No. CXLX11_0188). The author thank Dr Wangping Wu for thehelp of English grammar.

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