Composites: Part B 47 (2013) 320–325

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Composites: Part B

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Improved ablation resistance of carbon–phenolic composites by introducingzirconium diboride particles

Yaxi Chen a,⇑, Ping Chen a, Changqing Hong b, Baoxi Zhang b, David Hui c

a School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116024, PR Chinab Key Laboratory of Science and Technology for Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR Chinac Dept. of Mechanical Engineering, University of New Orleans, New Orleans, LA 70124, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 September 2012Received in revised form 19 October 2012Accepted 1 November 2012Available online 29 November 2012

Keywords:A. Polymer–matrix composites (PMCs)B. Thermal propertiesD. Thermal analysisE. Thermosetting resinAblation resistance

1359-8368/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compositesb.2012.11.007

⇑ Corresponding author. Tel./fax: +86 451 8640301E-mail address: shimlyhs@126.com (Y. Chen).

Carbon–phenolic (C–Ph) composites are well fabricated to meet the requirements of thermal protectionsystem by introducing ZrB2 particles. TG analysis demonstrates that the existence of ZrB2 particles couldobviously aggrandize the char yield of phenolic, although it does not enhance the thermal stability ofphenolic. What is more, the employed method of introducing ZrB2 could notably improve ablation resis-tance and insulation performance of C–Ph composites, which is mainly owing to the formation of ZrO2

and B2O3. As depicted in the microstructure, the ablation rate of matrix is evidently higher than carbonfibers in the C–Ph composites. However, the ablation rate of carbon fibers is identical to matrix in Z C–Phcomposites.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

A spacecraft, entering or traveling through an atmosphere atvery high speed when typically subjected to a high heat flux, re-quires a thermal protection system (TPS) to maintain a relatively‘‘cold’’ temperature, so lots of thermal protection materials havebeen extensively investigated to protect space vehicle against theaerodynamic heating encountered in hypersonic flight [1]. Theablation materials represent one of the traditional approaches tothermal protection systems which have been vastly explored andinvestigated [2,3]. The mechanism of ablation materials is that aquantity of energy is excellently absorbed by removed material.

The carbon–phenolic composites (C–Ph) are considered exten-sively to be efficient ablative thermal protection materials [4,5],owing to their excellent ablative resistant properties. Ablativeresistance of C–Ph composites plays a very important role in aero-space application when subject to high temperatures. Many effortshave been made to evidently improve this performance of C–Phcomposites in recent years. Boron modified phenolic is synthesizedfrom boric acid, phenol, and formaldehyde, and it is widely used asmatrix of C–Ph ablation composites, because of its good heat resis-tance, mechanical properties, electric properties and absorbance ofneutron radiation [6,7]. C–Ph composites, containing 30–45 wt%

ll rights reserved.

6.

vapor grown carbon fibers, exhibit extremely good erosion resis-tance and display less weight loss compared with the composites(composed of woven ex-rayon carbon fibers, carbon black fillersand phenolic) [8]. Furthermore, it appears to be a far better insula-tor. The C–Ph composites, treated with polyhedral oligomericsilsequioxanes, emerge a better ablative performance [9]. H3PO4�coated carbon fiber–phenolic composites can undergo strongerthermomechanical influence during ablation, and give a lower ero-sion rate to retard the ablation process [10]. The composites, mak-ing of three-dimension reticulated SiC ceramic, carbon fibers andboron-modified phenolic, have less linear ablation rate comparedwith pure boron modified phenolic or carbon fiber/boron-modifiedphenolic composites [11]. Nanosilica powder modifiedrayon-based carbon–fabric/phenolic composites reveal improvedablation resistance, reduced thermal conductivity and higher in-ter-laminar shear strength at a controlled quantity [12]. In general,these methods are frequently utilized to improve the ablativeperformance of C–Ph composites by modification carbon fibers orphenolic, interface treatment and addition of other compounds.

Zirconium diboride (ZrB2) has high melting point (P3000 �C),which oxidizes to ZrO2 and B2O3 [13]. Some previous works havebeen well reported that the formation of protective ZrO2 coatingcould markedly improve ablation resistance of carbon composites[14]. In addition, ZrB2 and B4C particles are applied as oxidationinhibitors to improve significantly ablation performance of bulkC–C composites [15]. However, the effects of ZrB2 on the ablationperformance of C–Ph composites are still unclear.

Y. Chen et al. / Composites: Part B 47 (2013) 320–325 321

In this article, we extensive review the current state of thisexciting field, and emphasize that the effects of introducing ZrB2

particles on the ablation and insulation performance of C–Ph com-posites. It is well fabricated by impregnating method that phenolicresin and ZrB2 particles fill the pores of the carbon fiber fabric. Theablation performance of introducing ZrB2 particles modified car-bon–phenolic (Z C–Ph) composites is reasonable tested by oxy-gen–acetylene, and the temperatures of the ablated surface andthe back surface are real-time monitored. The influence of ZrB2

on thermal stability of phenolic is investigated extensively. In addi-tion, the mechanisms of improving ablation resistance are deeplydiscussed.

18

2. Experimental

For the purpose of proper demonstration, the proposed three-dimensional carbon fibers fabric was impregnated in mixture.The three-dimensional carbon fibers matrix was defined as40 � 40 � 40 mm (with the density q = 0.185 g/cm3). The mixturecontained ethanol, phenolic and ZrB2 powders at the mass rationof 1:1:0.2. It was the optimum to manufacture Z C–Ph compositeswhich has implied in our previous work. The three-dimensionalcarbon fibers fabric was put into the mixture for 20 min. Thenwe left the fabric over 24 h to evaporate the solvent. The heatingcure was done in 30 min, 90 min and 180 min in an oven at80 �C, 110 �C and 150 �C, respectively. In addition, the heating ratewas approximately 1 �C/min, and sample cooling was completed atthe room temperature. The cured fabric (0.515 g/cm3) was dividedinto smaller sample with the size of £25 � 20 mm for oxygen–acetylene testing, and the C–Ph composites (0.491 g/cm3) was alsodecollate to sub-sample of £25 � 20 mm. This mixture containsethanol and phenolic at the mass ration of 1:1.

Thermogravimetry (TG) analysis was carried out by TGA/SDTA851e (Switzerland). The samples were placed in alumina cru-cibles, and heated from normal temperature (21 �C) to 1000 �Cwith increased heating rate of 5 �C/min in N2 atmosphere. Thephase composition was analyzed by an X-ray diffraction device(Cu Ka radiation, D/max-RB, Japan). Ablation performance wastested by oxygen–acetylene. Scheme of the ablation experimentsmedium is shown in Fig. 1. The flow rates of oxygen and acetylenewere 500 l/h and 400 l/h, respectively. The oxygen–acetylene gunwith 2 mm dimension was perpendicular to the surface of thespecimen, and the distance from gun to the surface of the specimenwas 50 mm. The ablation surface temperatures were monitored bythe infrared temperature measurement system (IR-AHU, Unitedstates). The back surface temperatures of samples were measuredwith the experimental K-type thermocouple in ablation process.Microstructure of C–Ph composites was analyzed by SEM devicebefore and after ablation. Then we obtained energy dispersivespectroscopy (EDS) for chemical analysis.

Fig. 1. The scheme of the ablation experiments medium.

3. Results and discussion

3.1. Thermogravimetry (TG) and X-ray diffraction analysis

The thermal stabilities of cured phenolic and the cured mixture,containing phenolic and ZrB2 (mass ration 1:0.2), are explored bythermal gravimetric analysis. Thermal gravimetric curves are pre-sented in Fig. 2.

As well known, the phenolic has pyrolysis phenomenon whenthe temperature exceeds 300 �C, and the weight loss below300 �C is not mainly owing to the pyrolysis of phenolic but thepost-cured process [16]. In Fig. 2, we could find that the mixtureand phenolic all have the thermal degradation phenomenon whenthe temperature goes up to 300 �C. More and more thermal degra-dation byproducts are found when the temperature continues tocreep from 480 �C to 650 �C. It is clear that ZrB2 particle does notchange the temperature distribution of phenolic pyrolysis, andthe thermal stability of phenolic does not increase significantly.The char yield of phenolic and phenolic containing ZrB2 at1000 �C are 67.3% and 73.4%, respectively. If calculated weight losswas owing to the pyrolysis of phenolic, and ZrB2 weight was con-sidered to be constant, the char yield of the mixture at 1000 �Cshould be 72.7%. This value is lower than the measured value ofphenolic contains ZrB2. Therefore, the weight gain reactions shouldbe occurred in this condition.

Fig. 3 reveals that the XRD patterns of the products after TGanalysis. The XRD spectrum for the mixture demonstrates the for-mation of ZrO2, which results from the reactions between ZrB2 andthe pyrolysis products of phenolic in N2 atmosphere. It is wellknown that the main pyrolysis products of phenolic include hydro-gen, methane, slight amount of ethane, water, small amount of car-bon dioxide, carbon monoxide and porous amorphous carbon (thechar from phenolic) [16]. Two reactions of forming ZrO2 are givenby:

ZrB2ðsÞ þ 5COðgÞ ¼ ZrO2ðsÞ þ B2O3ðgÞ þ 5CðsÞ ð1Þ2ZrB2ðsÞ þ 5CO2ðgÞ ¼ 2ZrO2ðsÞ þ 2B2O3ðgÞ þ 5CðsÞ ð2Þ

The byproduct of B2O3 is not detected by XRD due to its evapo-ration under N2 atmosphere [17–19]. Therefore, the increased charyield of the mixture contributes to the formation of ZrO2(s) andC(s). As shown in Fig. 3, broad peaks (a and b) are observed at 2hangles of about 24�and 44�, respectively. They are characteristicsof amorphous carbon from phenolic [20]. However, these broadpeaks are not obvious observed in the mixture, which are veryweak compared with those of ZrB2.

0 200 400 600 800 10006

8

10

12

14

16

Wei

ght

(mg)

Temperature (oC)

Phenolic +ZrB2

Phenolic

Fig. 2. TG curves under N2 atmosphere.

Fig. 3. XRD patterns of the products.

322 Y. Chen et al. / Composites: Part B 47 (2013) 320–325

3.2. Ablation property

The samples of Z C–Ph and C–Ph composites are shown in Fig. 4.The ablation process lasts 160 s, and the maximum ablation depthof sample surface is 0.56 mm, when the white ZrO2 layer isstripped. Therefore, the linear ablation rate is 0.0035 mm/s. Themaximum ablation depth of C–Ph composites is 2.65 mm, thusthe linear ablation rate of sample is 0.0166 mm/s. The linear abla-tion rate of Z C–Ph composites reduced by 79% compared with C–Ph composites. From all above, we can easily discover that theablation resistance is well improved by introducing ZrB2 particlesinto C–Ph composites.

As depicted in Fig. 4b, green colored gas is found in ablation testof Z C–Ph composites. This green colored gas is expected to be bor-ia gas [15]. In Fig. 4c, white ZrO2 layer is found on the ablated sur-face of Z C–Ph composites. Evident improving ablation resistanceof Z C–Ph composites contributes to the formation of ZrO2 andB2O3, which are the oxidation of ZrB2 and the above-mentionedreaction between ZrB2 and CO or the reaction between ZrB2 and

Fig. 4. Sample surface of oxygen–acetylene ablation: (a) Z C–Ph composites; (b) ablationablation of C–Ph composites; (f) the ablated C–Ph composites.

CO2. In addition, the increased amount of char is one of factors thatcan improve the ablation resistance [21]. When the temperature isbeneath 1200 �C, liquid B2O3 (melting point 450 �C) filling in por-ous char acts as a barrier to inhibit the diffusion of oxygen[22,15]. Simultaneously, the formed solid ZrO2 (high melting point)has ability of increasing the strength of the char [3,22,23]. Whenthe temperature is above 1200 �C, because of the rapid evaporationof B2O3, only ZrO2 layer enhances the ablation resistance [15].

3.3. The back surface and ablated surface temperatures

The temperature curves of ablating and back surface of Z C–Phand C–Ph composites are illustrated in Fig. 5. As implied in Fig. 5a,their ablating surface temperatures are identical to each other until35th second. Nevertheless, the ablating surface temperature of ZC–Ph composites is higher than C–Ph composites after 35th sec-ond. We could find that back surface temperature of Z C–Ph com-posites is lower than C–Ph composites in the ablation process inFig. 5b. The temperature of Z C–Ph composites creeps slowly until25th second. On the other side, the temperature increases rapidlywhen the time is over 25th second. The temperature of Z C–Phcomposites is lower than C–Ph composites about 100 �C from63th second to the end.

We observe that Z C–Ph composites perform a better heat insu-lation performance compared with C–Ph composites. For the most,the addition of ZrB2 distinctly increases the grain boundary ther-mal resistance, which would reduce markedly the thermal conduc-tivity of composites. The addition of nanosilica could reduce thethermal conductivity of carbon–phenolic composites, which is wellreported in article [12]. Moreover, ZrO2 has a low thermal conduc-tivity (2 W m�1 K�1), so the thermal barrier ZrO2 layer on the sur-face can slow down the advancement of heat front, and produce asteep temperature gradient field [24]. What is more, the endother-mic evaporation of B2O3 might absorb part of energy. Simulta-neously, since gaseous B2O3 increases the velocity of escapingflux, more incoming convective heat flux is blocked by outcomingflow of gaseous B2O3 and pyrolysis gas of phenolic [21]. In addition,the increased amount of char can improve heat insulation perfor-

of Z C–Ph composites; (c) the ablated Z C–Ph composites; (d) C–Ph composites; (e)

0 20 40 60 80 100 120 140 160 1801000

1200

1400

1600

1800

2000

2200

C-Ph

Z C-Ph

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

700

Tem

pera

ture

(o C

)

Tem

pera

ture

(o C

)

Time (s)Time (s)

C-Ph

Z C-Ph

(a) (b)

Fig. 5. Surface temperatures of Z C–Ph composites and C–Ph composites: (a) the ablated surface and (b) the back surface.

Y. Chen et al. / Composites: Part B 47 (2013) 320–325 323

mance of Z C–Ph composites in virtue of ZrB2 particles existence[3,21].

3.4. Microstructure characterization

Carbon fiber and carbonized phenolic are carbon material. Car-bon material is susceptible to oxidation at temperature more than450 �C [25]. We shall mainly focus on ablation by oxidation in theoxygen–acetylene test. In C–Ph composites, the phenolic matrixoccupies the voids between the carbon fiber, and surrounds the fi-bers. The fiber length is large compared with its diameter, thus theablation of fibers should be mainly due to radial recession [25]. Theoxidation reaction in composites is assumed to be the matrixrecession and reduction of fiber radius [25].

In order to investigate ablation zone of C–Ph composites, we ob-serve the microstructure of the C–Ph composites and the ablated

(a)

(b) (

Fig. 6. SEM micrographs for C–Ph composites: (a) the C–Ph composites; (b) th

C–Ph composites (Fig. 6). In Fig. 6b, only a lot of carbon fibers areexisted on the surface of ablated C–Ph composites. As depicted inFig. 6c, there is lots of char in-between carbon fibers beneath theablated surface, and the content of char increases gradually withdepth. All these imply that the depths of matrix and fibers recessare different, and the recession rate of matrix is higher than thatof carbon fibers. Moreover, these mean that the ablation occursnot in surface, but in volume, and the ablation of fiber is not dueto radial recession.

From Fig. 6c, there are amounts of pores which result from thepyrolysis of phenolic resin matrix, pits and cracks in the char. How-ever, radius of carbon fibers with fewer pores and pits is not obvi-ous reduced. The carbonized phenolic has more defects thancarbon fibers, and the contact area of char with oxygen is large.Oxygen molecule can penetrate deeply inside the matrix with reac-tion, thus ablation by the oxidation of carbonized matrix is acceler-

c)

e ablated C–Ph composites and (c) the side section of the ablated surface.

PhenolicCarbon fiber

Pyrolysis gases

Amorphous carbon

Pyrolysis zone of phenolic

Pore

Fig. 7. The ablation scheme of C–Ph composites.

324 Y. Chen et al. / Composites: Part B 47 (2013) 320–325

ated. Furthermore, the strength of char is weak because of the de-fects. Part of char is removed by external shear forces [3].

In C–Ph composites, the thermosetting phenolic resin, as thematrix, holds the fibers firmly in place and binds the carbon fibers

(a) (

(c)

(e)

Fig. 8. SEM images for Z C–Ph composites: (a) the surface of Z C–Ph composites; (b) thDetail view of (b); (e) the cross-section of the ablated surface and (f) detail view of (e).

together. In oxygen–acetylene test, the carbonized phenolic is sub-ject to oxidation, and there is only carbon fiber left on the ablatedsurface. Without the matrix, the carbon fibers on the ablated sur-face are not bonded together and easy to be stripped off by the highspeed stream. This is the reason for the ablation of carbon fiber.

The ablation mechanism of C–Ph composites is illustrated inFig. 7. Phenolic pyrolysis could release gas and leave amorphousporous char in above-mentioned temperature from 480 �C to650 �C. We continue to heat up the C–Ph composites, the mainpyrolysis zone proceeds into the materials below the surface[3,21,26]. However, the reaction of oxygen with carbon fibersand char is existed in the ablating surface.

In order to know whether the ablation of Z C–Ph composites oc-curs in volume or surface, the microstructures of Z C–Ph compos-ites are analyzed. Fig. 8 represents SEM images of Z C–Phcomposites and ablated Z C–Ph composites. We obtain that theablation rate of carbon fibers is close to matrix in Fig 8b. FromFig. 8c and d, we observe carbon fiber radius is obvious reduced,and ZrO2 particles are existed on amorphous carbon. However,B2O3 is not detected owing to its rapid evaporation at high temper-

b)

(d)

(f)

e surface of the ablated Z C–Ph composites; (c) EDS analysis of ablated surface; (d)

Phenolic ZrB2Carbon fiber ZrO2

Pyrolysis gases

ZrO2 layer

B2O3

Amorphous carbon

Pyrolysis zone of phenolic

Pore

Fig. 9. The ablation scheme of Z C–Ph composites.

Y. Chen et al. / Composites: Part B 47 (2013) 320–325 325

ature (Fig. 8d). All these imply that the ablation of Z C–Ph compos-ites occurs not in volume, but in surface. In addition, carbon fibermaybe ablate mainly due to radial recession. The amounts of thepores decrease gradually from the ablated surface into the interior(Fig. 8e and f), which implies that the amounts of the channels forthe oxygen to diffuse into the interior reduce significantly.

From the SEM analysis, we know that the C–Ph composites andZ C–Ph composites reveal different ablation behavior. The ablationmechanism of Z C–Ph composites is implied in Fig. 9. Phenolicpyrolyzes intensively between 480 �C and 650 �C, and ZrB2 hasnot evident oxidized phenomenon. When the temperature rangesfrom 650 �C to 800 �C, CO and CO2 from pyrolysis reaction reactswith ZrB2 in the ablation surface [16]. On the same time, the reac-tion of ZrB2 and O2 could be happened [19]. More liquid B2O3 andsolid ZrO2 are formed when the temperature varies from 800 �C to1200 �C. The oxidation resistance is reduced due to liquid B2O3

evaporating rapidly, and a solid ZrO2 layer is formed on the ablat-ing surface when the temperature is beyond 1200 �C.

4. Conclusions

Z C–Ph composites with low density are well fabricated byimpregnating method. TG analysis illustrates that introducingZrB2 could obviously increase the char yield of phenolic, althoughthe thermal stability of phenolic is not enhanced. Ablation perfor-mance is reasonable tested by oxygen–acetylene, and the real-timetemperatures of ablated surface and back surface are well mea-sured during ablation process. Results indicate that the employedmethod of introducing ZrB2 into C–Ph composites could pro-foundly improve the ablation performance. The linear ablation rateof Z C–Ph composite reduces by 79% compared with C–Ph compos-ites. This novel improvement mainly results from the formation ofZrO2 and B2O3. The ablating surface temperature of Z C–Ph com-posites is evidently higher than C–Ph composites during ablationprocess. Nevertheless, the back surface temperature of Z C–Phcomposites is obviously lower than C–Ph composites about100 �C. As depicted in the microstructure, ablation rate of matrixis higher than carbon fibers in the C–Ph composites. However,the ablation rate of carbon fibers is identical to matrix in Z C–Phcomposites. This work provides an effective way to prominently

improve the ablation performance of C–Ph composites, and itmaybe become a backbone of thermal structure in aerospace.

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