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Ceramics International 40 (2014) 9087–9094www.elsevier.com/locate/ceramint

Thermal shock properties of 3D-C/SiC composites prepared via polymerinfiltration pyrolysis (PIP)

Xiang Yangn, Hai-feng Hu, Yu-di Zhang, Zhao-hui Chen

Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, China

Received 20 December 2013; received in revised form 25 January 2014; accepted 25 January 2014Available online 1 February 2014

Abstract

SiC ceramic matrix composites reinforced by three-dimensional braided carbon fibers were prepared via polymer infiltration pyrolysis (PIP).The thermal shock properties of C/SiC composites were investigated in air. 3D-C/SiC specimens were thermally shocked for five cycles between1500 1C and 100 1C. The thermal shock resistance was characterized by mass variation and residual flexural strength properties. Scanningelectron microscope was used to examine the change of the surface morphology and micro-structural evolution of the composites. In addition, thephase evolution of the surfaces was identified by XRD, EDS and XPS. It was found that the strength of 3D-C/SiC composites decreased afterthermal shock tests. The phase composition of 3D-C/SiC composites was mainly β-SiC and carbon. After thermal shock tests, more oxides weregenerated and more O atoms were detected when there was no coating on the surface of 3D-C/SiC. The matrix cracking offered the channelsthrough which oxygen could penetrate into the composites and consume the fibers during thermal shock tests. CVD-SiC coating played animportant role in enhancing the resistance to the thermal shock of the composites.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Oxidation; Polymer infiltration pyrolysis (PIP); C/SiC composites; Thermal shock

1. Introduction

In view of their low specific weight, excellent resistance tohigh temperature environment, carbon fiber–reinforced siliconcarbide composites (C/SiC) represented an interesting materialfor high-temperature components of space propulsion systems.They were widely used as structural materials in differentfields such as advanced engines and so on [1–3].

Polymer infiltration pyrolysis (PIP) was a major manufac-turing process of C/SiC composites. Compared with chemicalvapor infiltration (CVI), PIP had many advantages such aslower component fabrication time to reduce costs significantlyand lower fabrication temperature [4,5].

However, carbon fibers could be easily oxidized if thecomposites were exposed to high-temperature in oxidizing

10.1016/j.ceramint.2014.01.12214 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ86 731 84576269; fax: þ86 731 84576578.ss: shmily_0427@163.com (X. Yang).

environments [6]. The oxidation of fibers led to the degrada-tion of mechanical properties, and then shortened the lifetimeof the composites.When C/SiC composites were used in thermo-mechanical

environments, the thermal shock/transient thermal conditions(a sudden change in temperature) were often inevitable. Mean-while, owing to the thermal expansion mismatch between Cfibers and SiC matrix, the additional thermal stresses could beintroduced. Moreover, many factors, such as the type of fiber/matrix, fiber architecture, interfacial characteristics, and matrix-forming processes, also affected their thermal shock behaviors[7–10].The thermal shock resistance was evaluated by measuring

the residual strength or modulus, or observing the damagewithin the composites. It was found by Yin [11] that thethermal shock generated several matrix cracks in a 3D-C/SiC.The flexural strength degraded gradually until the cracks weresaturated in the matrix. Mei [12,13] found that the thermal

X. Yang et al. / Ceramics International 40 (2014) 9087–90949088

strain of C/SiC was closely related to the cyclic temperature.In addition, the oxygen in the atmosphere could aggravate thedamage.

The thermal shock behaviors of a two-dimensional (2D)carbon fiber–reinforced silicon carbide composite with amultilayered self-healing coating (2D-C/SiC) were investigatedbetween 300 1C and 900 1C in air, which were studied byZhang [14]. The thermal shock generated several matrix cracksin a 3D-C/SiC via CVI process [11]. Thermal shock behaviorof angle-ply and woven dense ceramic–matrix composites wasstudied by Kastritseas [15]. However, studies on thermal shockbehaviors of 3D-C/SiC composites prepared via PIP processwere rarely reported.

The thermal shock behaviors of a 3D-C/SiC compositeswere investigated between 1500 1C and 100 1C in air in thepresent study. The specimens were cooled through boiling water.The thermal shock resistance of 3D-C/SiC was characterizedby residual flexural strength properties and mass variation.In addition, the microstructures of the thermally shocked speci-mens were examined to reveal the damage caused by thethermal shock. In the meantime, the thermal shock resistance of3D-C/SiC composites with CVD-SiC coating was characterizedby mass variation and residual flexural strength properties.

2. Experimental procedures

2.1. Specimen preparation

Three-dimensional braided carbon fibers (T-300, ex-PANcarbon fiber, Toray) were used as the reinforcement [16].Polycarbosilane (PCS) with molecular weight 1742 and softenpoint 175 1C was synthesized in our laboratory. Xylene wasused as the solvent for PCS.

The C/SiC composites denoted as raw sample were preparedusing 9–12 cycles of infiltration of PCS–xylene solution andsubsequently pyrolysed at 1200 1C under an inert atmosphere [3].

2.2. CVD-SiC coating preparation

Methyltrichlorosilane (MTS, CH3SiCl3), with a molar ratioof 10 between H2 and MTS, was carried by bubbling hydrogenin gas phase. Ar atmosphere as the dilute gas was used to slowdown the chemical reaction rate during deposition. Thedeposition temperature was controlled in 1100 1C for 5 h at3 kPa [17].

2.3. Procedures for thermal shocks tests

The substrate and coated samples were heated at 1500 1C inair, and then cooled through boiling water. The thermal shockexperiments were conducted by alternating the specimensquickly between 1500 1C in static air and 100 1C in water.Each specimen was held in a tube furnace, which waspreheated to 1500 1C for 10 min and then cooled to 100 1Cin water for 10 min. After the temperature of the specimendropped to 100 1C in water for 10 min, it was immediately sentback to the furnace, temperature of which was held at 1500 1C,

for the next heating–cooling cycle. The time to move thespecimen from the furnace to the boiling water was approxi-mately 5 s. Such a heating–cooling cycle indicated that thespecimens were thermally shocked once. In order to reveal thethermal shock resistance of the composites, at least threespecimens were investigated at prescribed number of fivecycles.

2.4. Samples characterization

The samples were weighed after thermal shock tests by anelectronic balance with a sensitivity of 70.001 g. Three-pointbending tests were used to evaluate the flexural strength ofC/SiC composites with the span/height ratio of 15 and acrosshead speed of 0.5 mm/min before and after tests.The s3b was calculated as follows:

s3b ¼3PL

2BH2 ð1Þ

where P is the load at a point of deflection of a load–displacement curve in test, L is the outer support span, B is thespecimen width, and H is the specimen thickness. The sampleswere cut and polished to 80 mm in length, 5 mm in width and4 mm in thickness. The length and width directions wereparallel to warp and weft directions, respectively.The microstructures of the samples before and after thermal

shock tests and fracture surfaces of the specimens after three-point bending tests were examined on the scanning electronmicroscopy (SEM, JSM-5600LV) combined with energy disper-sive spectroscopy (EDS).The phase and composition of samples before tests were

identified by X-ray diffraction (XRD, D8 Advance, Cu-kαradiation, 20–901, 2θ range, 0.011 wide scanning steps, and1 s/step acquisition time).The elements of samples after tests were identified by XPS

(K-Alpha 1063, 12,000 V, 72 W, 0.5 eV, and 10�9 mBar).

3. Results and discussion

3.1. Residual flexural strength properties and mass variation

The degradation of the flexural strength properties caused bythe thermal shock cycles is determined by measuring theretained strength after thermally quenching the samples from1500 1C to 100 1C.Fig. 1 shows the load–displacement curves of 3D-C/SiC un-

shocked and thermally shocked for five cycles. Table 1 showsthe mass variation of the materials before and after tests. Themass loss of the composites is caused by the oxidization ofcarbon fibers.The strength decreased for the shocked 3D-C/SiC compo-

sites. Compared to uncoated C/SiC composites, CVD-SiCcoated C/SiC samples show higher strength, which means thatCVD-SiC coating is able to improve the mechanical propertiesof the composites effectively.Fig. 1 shows that thermal shock has great effects on the

degradation of mechanical properties of the 3D-C/SiC. The

Fig. 1. Load–displacement curves of 3D-C/SiC composites before and afterthermal shock tests.

Table 1The mass variation of the material after thermal shock tests.

Samples Before tests(g)

After tests(g)

Mass retention rate(%)

3D-C/SiC 3.359 2.94 87.53.447 2.988 86.73.384 2.973 87.9Average 87.4

CVD-SiC coated 3D-C/SiC

4.17 4.112 98.63.965 3.938 99.34.105 4.075 99.3Average 99.1

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degradation of the mechanical properties is closely related tothe evolution of the damage in the composites. The degrada-tion of the mechanical properties is closely related to theevolution of the damage in the composites [18,19].

In thermal shock test, the surface of the composites wascooled to the lower temperature rapidly (5 s in the presentinvestigation), whereas the interior of the composites remainedat higher temperature. The temperature gradient created tensilestress at the surface and compressive stress in the interior. Thestress gradient could result in cracking and peeling off of thesurface. Moreover, SiC matrix had a greater thermal expansioncoefficient (CTE) (4.6� 10�6/K) than the longitudinal CTE ofthe carbon fiber (0.5� 10�6/K) [14]. The sudden change oftemperature could produce tensile stress in the matrix. Withincreasing the number of thermal shock cycles, the tensile stressin the matrix was accumulated and could reach the critical valuesufficient to create matrix cracks in the composites.

Meanwhile, the number and dimension of the matrix cracksincreased as the number of thermal cycles was increased [15].In addition, the radial CTE of the fiber (7.0� 10�6/K) wasgreater than that of the matrix [14]. Tensile stress wasproduced on the fiber/matrix interface during the successivequenching operations from 1500 1C to 100 1C. The repeatedtensile stresses in the interfaces of fiber (bundles)/matrix couldweaken the interface bonding, which led to delamination anddebonding of the fiber/matrix interface.

3.2. Fracture and surface morphology

Fig. 2 shows several typical fracture morphologies of 3D-C/SiC specimens. Extensive pullout of fibers can be found in 3D-C/SiC composites, as shown in Fig. 2(a) and (c). Extensivepullout of fibers can also be found in the shocked CVD-SiCcoated 3D-C/SiC composites, as shown in Fig. 2(d).Little pullout of fibers can be found in the shocked 3D-C/

SiC composites without coating, as shown in Fig. 2(b). Thedebonding between the fiber bundles and the matrix can beseen on the fractured surfaces, which suggests lower bondingstrength of the fiber/matrix interface. Also, a large section ofthe fibers in the near-surface zone vanished for the specimenthermally shocked for five cycles, as indicated in Fig. 2(b).With increasing number of cycles, the carbon fibers in thenear-surface zone would be further oxidized, even completelybe vanished, as is evident in Fig. 2(b). The cone-like endindicates that the carbon fibers are consumed due to oxidation.Meanwhile, it can be found from Fig. 2 that the fibers in the

center of the fracture surface are not oxidized. Previous resultsshowed that the oxidization always occurs preferentially in thenear-surface zone under a cracked surface [20]. Therefore, theenhanced consumption of the carbon fibers in the near-surfacezone suggest that more cracks and (or) larger cracks formwhen the composites are exposed to thermal shock tests.The amounts of the vanished carbon fibers in the near-surface

zone agree well with the mass variation, shown in Table 1.A greater number of disappearing fibers correspond to larger massloss. It can be primarily concluded that the oxidation of the fiberscontributed to the mass loss in the composites.Fig. 3 shows the surface morphologies of 3D-C/SiC

composites. 3D-C/SiC composites are mainly composed ofcarbon fibers, pores and SiC. Pores existed in the compositesby polymer precursor pyrolysis. It can be seen that there arevisible denude regions and matrix cracks (Fig. 3(a)).Fig. 3(b) shows SEM photograph of 3D-C/SiC composites

after tests. The microstructure of the composites is changed.Some SiC matrix is lost and still some fibers oxidized in thesamples.Dome-like columnar grains, which are a typical form of

CVD-SiC, are found on the as-received material in Fig. 3(c).The characteristic of the dome-like morphology faded withthermal shock tests (Fig. 3(d)). From Fig. 3, we know that thedimensions of the grains shown in Fig. 3(d) became smallerthan those on the un-shocked specimen (Fig. 3(c)). After beingshocked, some dome-like morphology completely disappeared,as seen in Fig. 3(d). These observations agree well with theresults on fractured surfaces as shown in Fig. 2.

3.3. Micro-structural observations and discussion

As stated above, the degradation of flexural strength propertiesand mass variation are closely related to the micro-structuralchanges of the composites during the thermal shock tests.The crystallographic structure of 3D-C/SiC is studied by the

XRD method. The XRD patterns are shown in Fig. 4.

Fig. 2. Fractured micrographs of 3D-C/SiC composites before and after thermal shock tests: (a) 3D-C/SiC composites before tests, (b) 3D-C/SiC composites aftertests, (c) CVD-SiC coated 3D-C/SiC composites before tests, and (d) CVD-SiC coated 3D-C/SiC composites after tests.

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The phase composition of 3D-C/SiC composites is mainlyβ-SiC and carbon (Fig. 4), and the diffraction peak of β-SiCis broad since SiC formed by the PIP process had lowcrystallinity.

The phase composition of coated 3D-C/SiC composites isβ-SiC. Three obvious XRD peaks at about the 2θ values of35.78, 60.48 and 71.88 (d¼0.251, 0.154 and 0.131 nm) aredetected, which correspond to the (111), (220) and (311)reflections of β-SiC according to the standard JCPDS cards(SiC: 49-1428) [21]. Therefore, CVD-SiC coating is β-SiC phase.

In order to discover the mechanism in the morphologicalchange, the composition of the specimen surface is examinedby EDS (Figs. 5 and 6). The composition of the surface ismainly composed of Si and O for the thermal shockedspecimens. This result indicates that a greater amount of SiCis oxidized and transformed to SiO2 after thermal shock tests.

The XPS is also used to testify the element of 3D-C/SiCcomposites after thermal shock tests, which revealed thecomposition of the most external surface of the products.Fig. 7 is the XPS patterns of 3D-C/SiC composites afterthermal shock tests. Data from the XPS survey of thecomposites indicate that the major peaks in the spectra aredue to the C1s (�284 ev), O1s (�532 ev), and Si2s

(�153 ev) as well as Si2p (�100 ev) photoelectrons. MoreO atoms are detected when there was no coating on the surfaceof 3D-C/SiC (Fig. 7(a)), suggesting that the coating could playan important role in enhancing the resistance to the thermalshock of the composites.After thermal shock tests, more oxides are generated.

Furthermore, the oxidizing gas could penetrate into the speci-mens and react with carbon fibers. Accordingly, non-uniformoxidized carbon fibers and consumption of fibers could be seenin Fig. 5(b). The carbon fibers in the near-surface zone arecompletely consumed. Such morphology is produced by theescapement of a large amount of gas phases beneath thesurface. These observations agree well with the results onsurfaces as shown in Fig. 3.During thermal shock tests, 3D-C/SiC composites react with

oxygen and vapor as follows [22,23]:

2SiCðsÞþ3O2-2SiO2ðsÞþ2COðgÞ ð2Þ

CðsÞþO2ðgÞ-CO2ðgÞ ð3Þ

SiCðsÞþ3H2OðgÞ-SiO2ðsÞþCOðgÞþ3H2ðgÞ ð4Þ

SiO2ðsÞþ2H2OðgÞ-SiðOHÞ4ðgÞ ð5Þ

Fig. 3. Surface micrographs of 3D-C/SiC composites before and after thermal shock tests: (a) 3D-C/SiC composites before tests, (b) 3D-C/SiC composites aftertests, (c) CVD-SiC coated 3D-C/SiC composites before tests, and (d) CVD-SiC coated 3D-C/SiC composites after tests.

X. Yang et al. / Ceramics International 40 (2014) 9087–9094 9091

CðsÞþH2OðgÞ-COðgÞþH2ðgÞ ð6Þ

SiO2ðsÞþH2OðgÞ-SiOðOHÞ2ðgÞ ð7Þ

SiO2ðsÞþ3H2OðgÞ-SiOðOHÞ6ðgÞ ð8ÞAccording to reactions (2) and (4), oxidation of SiC result in

the formation of a thin SiO2 layer on the coating surface,leading to weight gaining of 3D-C/SiC. By reactions (5), (7)and (8), some SiO2 film is later destroyed in the boiling waterdue to the volatilization of Si(OH)4,which is shown in Fig. 2(b).The through-thickness cracks or holes are apt to appear in thethin coating, which provide diffusion channel for oxygen inair and vapor in boiling water to attack 3D-C/SiC composites.So, obviously they are the main reasons for the weight loss of3D-C/SiC composites.

CVD-SiC coating shows a better protection in the thermalshock tests. Little fibers are oxidized during the tests. Thoughcracking and peeling off of the coating can be found on thespecimen, little oxygen in air could diffuse into the compositethrough such damage and reacted with the carbon fibers. Theseobservations agree well with the results on surfaces as shownin Figs. 1 and 3.

Concerning the micro-structural changes of uncoated 3D-C/SiCcomposites, as shown in Figs. 2 and 5, evident discrepancy can be

found in comparison with CVD SiC coated 3D-C/SiC composites.After the thermal shocks, obvious pores are observed on thesurface of uncoated 3D-C/SiC. That means the uncoated 3D-C/SiC composites show worse thermal shock property. Thus, theCVD-SiC coating exhibits better thermal shock resistance.

4. Conclusions

The thermal shock behaviors of 3D-C/SiC composites viapolymer infiltration pyrolysis (PIP) were investigated between1500 1Cand 100 1C in the present study:

(1)

The strength decreased for the shocked 3D-C/SiC compo-sites. The degradation in the mechanical properties wasclosely related to the damage produced by the thermalshock tests. Compared to the uncoated C/SiC composites,CVD-SiC coated C/SiC samples showed higher strength,which meant that SiC coating was able to improve themechanical properties of the samples effectively.

(2)

The phase evolution of the surfaces was identified by XRD,EDS and XPS. The phase composition of 3D-C/SiC compo-sites was mainly β-SiC and carbon, while that of coated3D-C/SiC composites was β-SiC. After thermal shock tests,more oxides were generated, which meant SiC was oxidized

Fig. 4. XRD of 3D-C/SiC composites before thermal shock tests: (a) CVD-SiC coated 3D-C/SiC composites and (b) 3D-C/SiC composites.

Fig. 5. EDS of 3D-C/SiC composites before and after thermal shock tests: (a) 3D-C/SiC composites before tests and (b) 3D-C/SiC composites after tests.

X. Yang et al. / Ceramics International 40 (2014) 9087–90949092

Fig. 6. EDS of CVD-SiC coated 3D-C/SiC composites before and after thermal shock tests: (a) CVD-SiC coated 3D-C/SiC composites before tests and (b) CVD-SiC coated 3D-C/SiC composites after tests.

Fig. 7. XPS survey spectra of 3D-C/SiC composites after thermal shock tests: (a) 3D-C/SiC composites and (b) CVD-SiC coated 3D-C/SiC composites.

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and transformed to SiO2. More O atoms were detected whenthere was no coating on the surface of 3D-C/SiC.

(3)

Owing to the accumulated thermal stress during thethermal shock, the bonding strengths of fiber/matrix orcoating/composites interfaces were weakened. The matrixcracking offered the channels through which oxygen couldpenetrate into the composites and consume the fibers afterthermal shock tests. The CVD-SiC played an importantrole in enhancing the resistance to the thermal shock.

Acknowledgments

The authors are grateful to Aid Program for InnovativeGroup of National University of Defense Technology and AidProgram for Science and Technology Innovative ResearchTeam in Higher Educational Institutions of Hunan Province.

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