Improved ablation resistance of carbon–phenolic composites by introducing zirconium diboride particles

Composites: Part B 47 (2013) 320–325

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Improved ablation resistance of carbon–phenolic composites by introducing zirconium 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 China Key Laboratory of Science and Technology for Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China c Dept. of Mechanical Engineering, University of New Orleans, New Orleans, LA 70124, United States b

a r t i c l e

i n f o

Article history: Received 13 September 2012 Received in revised form 19 October 2012 Accepted 1 November 2012 Available online 29 November 2012 Keywords: A. Polymer–matrix composites (PMCs) B. Thermal properties D. Thermal analysis E. Thermosetting resin Ablation resistance

a b s t r a c t Carbon–phenolic (C–Ph) composites are well fabricated to meet the requirements of thermal protection system by introducing ZrB2 particles. TG analysis demonstrates that the existence of ZrB2 particles could obviously aggrandize the char yield of phenolic, although it does not enhance the thermal stability of phenolic. What is more, the employed method of introducing ZrB2 could notably improve ablation resistance 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 carbon fibers in the C–Ph composites. However, the ablation rate of carbon fibers is identical to matrix in Z C–Ph composites. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction A spacecraft, entering or traveling through an atmosphere at very high speed when typically subjected to a high heat flux, requires a thermal protection system (TPS) to maintain a relatively ‘‘cold’’ temperature, so lots of thermal protection materials have been extensively investigated to protect space vehicle against the aerodynamic heating encountered in hypersonic flight [1]. The ablation materials represent one of the traditional approaches to thermal protection systems which have been vastly explored and investigated [2,3]. The mechanism of ablation materials is that a quantity of energy is excellently absorbed by removed material. The carbon–phenolic composites (C–Ph) are considered extensively to be efficient ablative thermal protection materials [4,5], owing to their excellent ablative resistant properties. Ablative resistance of C–Ph composites plays a very important role in aerospace application when subject to high temperatures. Many efforts have been made to evidently improve this performance of C–Ph composites in recent years. Boron modified phenolic is synthesized from boric acid, phenol, and formaldehyde, and it is widely used as matrix of C–Ph ablation composites, because of its good heat resistance, mechanical properties, electric properties and absorbance of neutron radiation [6,7]. C–Ph composites, containing 30–45 wt%

⇑ Corresponding author. Tel./fax: +86 451 86403016. E-mail address: [email protected] (Y. Chen). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.11.007

vapor grown carbon fibers, exhibit extremely good erosion resistance and display less weight loss compared with the composites (composed of woven ex-rayon carbon fibers, carbon black fillers and phenolic) [8]. Furthermore, it appears to be a far better insulator. The C–Ph composites, treated with polyhedral oligomeric silsequioxanes, emerge a better ablative performance [9]. H3PO4 coated carbon fiber–phenolic composites can undergo stronger thermomechanical influence during ablation, and give a lower erosion rate to retard the ablation process [10]. The composites, making of three-dimension reticulated SiC ceramic, carbon fibers and boron-modified phenolic, have less linear ablation rate compared with pure boron modified phenolic or carbon fiber/boron-modified phenolic composites [11]. Nanosilica powder modified rayon-based carbon–fabric/phenolic composites reveal improved ablation resistance, reduced thermal conductivity and higher inter-laminar shear strength at a controlled quantity [12]. In general, these methods are frequently utilized to improve the ablative performance of C–Ph composites by modification carbon fibers or phenolic, 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 have been well reported that the formation of protective ZrO2 coating could markedly improve ablation resistance of carbon composites [14]. In addition, ZrB2 and B4C particles are applied as oxidation inhibitors to improve significantly ablation performance of bulk C–C composites [15]. However, the effects of ZrB2 on the ablation performance of C–Ph composites are still unclear.

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In this article, we extensive review the current state of this exciting field, and emphasize that the effects of introducing ZrB2 particles on the ablation and insulation performance of C–Ph composites. It is well fabricated by impregnating method that phenolic resin and ZrB2 particles fill the pores of the carbon fiber fabric. The ablation performance of introducing ZrB2 particles modified carbon–phenolic (Z C–Ph) composites is reasonable tested by oxygen–acetylene, and the temperatures of the ablated surface and the back surface are real-time monitored. The influence of ZrB2 on thermal stability of phenolic is investigated extensively. In addition, the mechanisms of improving ablation resistance are deeply discussed. 2. Experimental For the purpose of proper demonstration, the proposed threedimensional carbon fibers fabric was impregnated in mixture. The three-dimensional carbon fibers matrix was defined as 40  40  40 mm (with the density q = 0.185 g/cm3). The mixture contained ethanol, phenolic and ZrB2 powders at the mass ration of 1:1:0.2. It was the optimum to manufacture Z C–Ph composites which has implied in our previous work. The three-dimensional carbon fibers fabric was put into the mixture for 20 min. Then we left the fabric over 24 h to evaporate the solvent. The heating cure was done in 30 min, 90 min and 180 min in an oven at 80 °C, 110 °C and 150 °C, respectively. In addition, the heating rate was approximately 1 °C/min, and sample cooling was completed at the room temperature. The cured fabric (0.515 g/cm3) was divided into smaller sample with the size of £25  20 mm for oxygen– acetylene testing, and the C–Ph composites (0.491 g/cm3) was also decollate to sub-sample of £25  20 mm. This mixture contains ethanol 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 crucibles, and heated from normal temperature (21 °C) to 1000 °C with increased heating rate of 5 °C/min in N2 atmosphere. The phase composition was analyzed by an X-ray diffraction device (Cu Ka radiation, D/max-RB, Japan). Ablation performance was tested by oxygen–acetylene. Scheme of the ablation experiments medium is shown in Fig. 1. The flow rates of oxygen and acetylene were 500 l/h and 400 l/h, respectively. The oxygen–acetylene gun with 2 mm dimension was perpendicular to the surface of the specimen, and the distance from gun to the surface of the specimen was 50 mm. The ablation surface temperatures were monitored by the infrared temperature measurement system (IR-AHU, United states). The back surface temperatures of samples were measured with the experimental K-type thermocouple in ablation process. Microstructure of C–Ph composites was analyzed by SEM device before and after ablation. Then we obtained energy dispersive spectroscopy (EDS) for chemical analysis.

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 by thermal gravimetric analysis. Thermal gravimetric curves are presented in Fig. 2. As well known, the phenolic has pyrolysis phenomenon when the temperature exceeds 300 °C, and the weight loss below 300 °C is not mainly owing to the pyrolysis of phenolic but the post-cured process [16]. In Fig. 2, we could find that the mixture and phenolic all have the thermal degradation phenomenon when the temperature goes up to 300 °C. More and more thermal degradation byproducts are found when the temperature continues to creep from 480 °C to 650 °C. It is clear that ZrB2 particle does not change the temperature distribution of phenolic pyrolysis, and the thermal stability of phenolic does not increase significantly. The char yield of phenolic and phenolic containing ZrB2 at 1000 °C are 67.3% and 73.4%, respectively. If calculated weight loss was owing to the pyrolysis of phenolic, and ZrB2 weight was considered to be constant, the char yield of the mixture at 1000 °C should be 72.7%. This value is lower than the measured value of phenolic contains ZrB2. Therefore, the weight gain reactions should be occurred in this condition. Fig. 3 reveals that the XRD patterns of the products after TG analysis. The XRD spectrum for the mixture demonstrates the formation of ZrO2, which results from the reactions between ZrB2 and the pyrolysis products of phenolic in N2 atmosphere. It is well known that the main pyrolysis products of phenolic include hydrogen, methane, slight amount of ethane, water, small amount of carbon dioxide, carbon monoxide and porous amorphous carbon (the char from phenolic) [16]. Two reactions of forming ZrO2 are given by:

ZrB2 ðsÞ þ 5COðgÞ ¼ ZrO2 ðsÞ þ B2 O3 ðgÞ þ 5CðsÞ

ð1Þ

2ZrB2 ðsÞ þ 5CO2 ðgÞ ¼ 2ZrO2 ðsÞ þ 2B2 O3 ðgÞ þ 5CðsÞ

ð2Þ

The byproduct of B2O3 is not detected by XRD due to its evaporation under N2 atmosphere [17–19]. Therefore, the increased char yield of the mixture contributes to the formation of ZrO2(s) and C(s). As shown in Fig. 3, broad peaks (a and b) are observed at 2h angles of about 24°and 44°, respectively. They are characteristics of amorphous carbon from phenolic [20]. However, these broad peaks are not obvious observed in the mixture, which are very weak compared with those of ZrB2.

18

Weight (mg)

16 14

Phenolic

12 10 8

Phenolic +ZrB2 6 0

200

400

600

800

Temperature (oC) Fig. 1. The scheme of the ablation experiments medium.

Fig. 2. TG curves under N2 atmosphere.

1000

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CO2. In addition, the increased amount of char is one of factors that can improve the ablation resistance [21]. When the temperature is beneath 1200 °C, liquid B2O3 (melting point 450 °C) filling in porous 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]. When the temperature is above 1200 °C, because of the rapid evaporation of B2O3, only ZrO2 layer enhances the ablation resistance [15].

3.3. The back surface and ablated surface temperatures

Fig. 3. XRD patterns of the products.

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 depth of sample surface is 0.56 mm, when the white ZrO2 layer is stripped. Therefore, the linear ablation rate is 0.0035 mm/s. The maximum ablation depth of C–Ph composites is 2.65 mm, thus the linear ablation rate of sample is 0.0166 mm/s. The linear ablation rate of Z C–Ph composites reduced by 79% compared with C– Ph composites. From all above, we can easily discover that the ablation resistance is well improved by introducing ZrB2 particles into C–Ph composites. As depicted in Fig. 4b, green colored gas is found in ablation test of Z C–Ph composites. This green colored gas is expected to be boria gas [15]. In Fig. 4c, white ZrO2 layer is found on the ablated surface of Z C–Ph composites. Evident improving ablation resistance of Z C–Ph composites contributes to the formation of ZrO2 and B2O3, which are the oxidation of ZrB2 and the above-mentioned reaction between ZrB2 and CO or the reaction between ZrB2 and

The temperature curves of ablating and back surface of Z C–Ph and C–Ph composites are illustrated in Fig. 5. As implied in Fig. 5a, their ablating surface temperatures are identical to each other until 35th second. Nevertheless, the ablating surface temperature of Z C–Ph composites is higher than C–Ph composites after 35th second. We could find that back surface temperature of Z C–Ph composites is lower than C–Ph composites in the ablation process in Fig. 5b. The temperature of Z C–Ph composites creeps slowly until 25th second. On the other side, the temperature increases rapidly when the time is over 25th second. The temperature of Z C–Ph composites is lower than C–Ph composites about 100 °C from 63th second to the end. We observe that Z C–Ph composites perform a better heat insulation performance compared with C–Ph composites. For the most, the addition of ZrB2 distinctly increases the grain boundary thermal resistance, which would reduce markedly the thermal conductivity of composites. The addition of nanosilica could reduce the thermal conductivity of carbon–phenolic composites, which is well reported in article [12]. Moreover, ZrO2 has a low thermal conductivity (2 W m1 K1), so the thermal barrier ZrO2 layer on the surface can slow down the advancement of heat front, and produce a steep temperature gradient field [24]. What is more, the endothermic evaporation of B2O3 might absorb part of energy. Simultaneously, since gaseous B2O3 increases the velocity of escaping flux, more incoming convective heat flux is blocked by outcoming flow of gaseous B2O3 and pyrolysis gas of phenolic [21]. In addition, the increased amount of char can improve heat insulation perfor-

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

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2200

700

(a)

(b)

600

Z C-Ph

Temperature ( C)

1800

500

C-Ph

o

o

Temperature ( C)

2000

1600

C-Ph

1400

400 300 200

Z C-Ph 1200 1000

100 0 0

20

40

60

80

100

120

140

160

0

180

Time (s)

20

40

60

80

100

120

140

160

Time (s)

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

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. Carbon material is susceptible to oxidation at temperature more than 450 °C [25]. We shall mainly focus on ablation by oxidation in the oxygen–acetylene test. In C–Ph composites, the phenolic matrix occupies the voids between the carbon fiber, and surrounds the fibers. The fiber length is large compared with its diameter, thus the ablation of fibers should be mainly due to radial recession [25]. The oxidation reaction in composites is assumed to be the matrix recession and reduction of fiber radius [25]. In order to investigate ablation zone of C–Ph composites, we observe the microstructure of the C–Ph composites and the ablated

C–Ph composites (Fig. 6). In Fig. 6b, only a lot of carbon fibers are existed on the surface of ablated C–Ph composites. As depicted in Fig. 6c, there is lots of char in-between carbon fibers beneath the ablated surface, and the content of char increases gradually with depth. All these imply that the depths of matrix and fibers recess are different, and the recession rate of matrix is higher than that of carbon fibers. Moreover, these mean that the ablation occurs not in surface, but in volume, and the ablation of fiber is not due to radial recession. From Fig. 6c, there are amounts of pores which result from the pyrolysis of phenolic resin matrix, pits and cracks in the char. However, radius of carbon fibers with fewer pores and pits is not obvious reduced. The carbonized phenolic has more defects than carbon fibers, and the contact area of char with oxygen is large. Oxygen molecule can penetrate deeply inside the matrix with reaction, thus ablation by the oxidation of carbonized matrix is acceler-

(a)

(b)

(c)

Fig. 6. SEM micrographs for C–Ph composites: (a) the C–Ph composites; (b) the ablated C–Ph composites and (c) the side section of the ablated surface.

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Carbon fiber Phenolic Pore

Pyrolysis gases

Pyrolysis zone of phenolic

Amorphous carbon

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

ated. Furthermore, the strength of char is weak because of the defects. Part of char is removed by external shear forces [3]. In C–Ph composites, the thermosetting phenolic resin, as the matrix, holds the fibers firmly in place and binds the carbon fibers

(a)

together. In oxygen–acetylene test, the carbonized phenolic is subject to oxidation, and there is only carbon fiber left on the ablated surface. Without the matrix, the carbon fibers on the ablated surface are not bonded together and easy to be stripped off by the high speed stream. This is the reason for the ablation of carbon fiber. The ablation mechanism of C–Ph composites is illustrated in Fig. 7. Phenolic pyrolysis could release gas and leave amorphous porous char in above-mentioned temperature from 480 °C to 650 °C. We continue to heat up the C–Ph composites, the main pyrolysis zone proceeds into the materials below the surface [3,21,26]. However, the reaction of oxygen with carbon fibers and char is existed in the ablating surface. In order to know whether the ablation of Z C–Ph composites occurs in volume or surface, the microstructures of Z C–Ph composites are analyzed. Fig. 8 represents SEM images of Z C–Ph composites and ablated Z C–Ph composites. We obtain that the ablation rate of carbon fibers is close to matrix in Fig 8b. From Fig. 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)

(c) (d)

(e)

(f)

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

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

Carbon fiber Phenolic ZrB2

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

ZrO2 ZrO2 layer Pore

B2O3

Pyrolysis gases Pyrolysis zone of phenolic

325

Amorphous carbon

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

ature (Fig. 8d). All these imply that the ablation of Z C–Ph composites occurs not in volume, but in surface. In addition, carbon fiber maybe ablate mainly due to radial recession. The amounts of the pores decrease gradually from the ablated surface into the interior (Fig. 8e and f), which implies that the amounts of the channels for the oxygen to diffuse into the interior reduce significantly. From the SEM analysis, we know that the C–Ph composites and Z C–Ph composites reveal different ablation behavior. The ablation mechanism of Z C–Ph composites is implied in Fig. 9. Phenolic pyrolyzes intensively between 480 °C and 650 °C, and ZrB2 has not evident oxidized phenomenon. When the temperature ranges from 650 °C to 800 °C, CO and CO2 from pyrolysis reaction reacts with ZrB2 in the ablation surface [16]. On the same time, the reaction of ZrB2 and O2 could be happened [19]. More liquid B2O3 and solid ZrO2 are formed when the temperature varies from 800 °C to 1200 °C. The oxidation resistance is reduced due to liquid B2O3 evaporating rapidly, and a solid ZrO2 layer is formed on the ablating surface when the temperature is beyond 1200 °C.

4. Conclusions Z C–Ph composites with low density are well fabricated by impregnating method. TG analysis illustrates that introducing ZrB2 could obviously increase the char yield of phenolic, although the thermal stability of phenolic is not enhanced. Ablation performance is reasonable tested by oxygen–acetylene, and the real-time temperatures of ablated surface and back surface are well measured during ablation process. Results indicate that the employed method of introducing ZrB2 into C–Ph composites could profoundly improve the ablation performance. The linear ablation rate of Z C–Ph composite reduces by 79% compared with C–Ph composites. This novel improvement mainly results from the formation of ZrO2 and B2O3. The ablating surface temperature of Z C–Ph composites is evidently higher than C–Ph composites during ablation process. Nevertheless, the back surface temperature of Z C–Ph composites is obviously lower than C–Ph composites about 100 °C. As depicted in the microstructure, ablation rate of matrix is higher than carbon fibers in the C–Ph composites. However, the ablation rate of carbon fibers is identical to matrix in Z C–Ph composites. This work provides an effective way to prominently

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