Improved Ablation Resistance of Carbonв€“Phenolic Composites by Introducing Zirconium Diboride Particles

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 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 b 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 article info abstract

Article history: Carbon–phenolic (C–Ph) composites are well fabricated to meet the requirements of thermal protection Received 13 September 2012 system by introducing ZrB2 particles. TG analysis demonstrates that the existence of ZrB2 particles could Received in revised form 19 October 2012 obviously aggrandize the char yield of phenolic, although it does not enhance the thermal stability of Accepted 1 November 2012 phenolic. What is more, the employed method of introducing ZrB could notably improve ablation resis- Available online 29 November 2012 2 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 carbon Keywords: ﬁbers in the C–Ph composites. However, the ablation rate of carbon ﬁbers is identical to matrix in Z C–Ph A. Polymer–matrix composites (PMCs) composites. B. Thermal properties D. Thermal analysis Ó 2012 Elsevier Ltd. All rights reserved. E. Thermosetting resin Ablation resistance

1. Introduction vapor grown carbon fibers, exhibit extremely good erosion resistance and display less weight loss compared with the composites A spacecraft, entering or traveling through an atmosphere at (composed of woven ex-rayon carbon fibers, carbon black fillers very high speed when typically subjected to a high heat flux, re- and phenolic) [8]. Furthermore, it appears to be a far better insula- quires a thermal protection system (TPS) to maintain a relatively tor. The C–Ph composites, treated with polyhedral oligomeric

‘‘cold’’ temperature, so lots of thermal protection materials have silsequioxanes, emerge a better ablative performance [9].H3PO4 been extensively investigated to protect space vehicle against the coated carbon fiber–phenolic composites can undergo stronger aerodynamic heating encountered in hypersonic flight [1]. The thermomechanical influence during ablation, and give a lower ero- ablation materials represent one of the traditional approaches to sion rate to retard the ablation process [10]. The composites, mak- thermal protection systems which have been vastly explored and ing of three-dimension reticulated SiC ceramic, carbon fibers and investigated [2,3]. The mechanism of ablation materials is that a boron-modified phenolic, have less linear ablation rate compared quantity of energy is excellently absorbed by removed material. with pure boron modified phenolic or carbon fiber/boron-modified The carbon–phenolic composites (C–Ph) are considered exten- phenolic composites [11]. Nanosilica powder modified sively to be efficient ablative thermal protection materials [4,5], rayon-based carbon–fabric/phenolic composites reveal improved owing to their excellent ablative resistant properties. Ablative ablation resistance, reduced thermal conductivity and higher in- resistance of C–Ph composites plays a very important role in aero- ter-laminar shear strength at a controlled quantity [12]. In general, space application when subject to high temperatures. Many efforts these methods are frequently utilized to improve the ablative have been made to evidently improve this performance of C–Ph performance of C–Ph composites by modification carbon fibers or composites in recent years. Boron modified phenolic is synthesized phenolic, interface treatment and addition of other compounds. from boric acid, phenol, and formaldehyde, and it is widely used as Zirconium diboride (ZrB2) has high melting point (P3000 °C), matrix of C–Ph ablation composites, because of its good heat resis- which oxidizes to ZrO2 and B2O3 [13]. Some previous works have tance, mechanical properties, electric properties and absorbance of been well reported that the formation of protective ZrO2 coating neutron radiation [6,7]. C–Ph composites, containing 30–45 wt% 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

⇑ Corresponding author. Tel./fax: +86 451 86403016. C–C composites [15]. However, the effects of ZrB2 on the ablation E-mail address: [email protected] (Y. Chen). performance of C–Ph composites are still unclear.

In this article, we extensive review the current state of this 3. Results and discussion exciting field, and emphasize that the effects of introducing ZrB2 particles on the ablation and insulation performance of C–Ph com- 3.1. Thermogravimetry (TG) and X-ray diffraction analysis posites. It is well fabricated by impregnating method that phenolic resin and ZrB2 particles fill the pores of the carbon fiber fabric. The The thermal stabilities of cured phenolic and the cured mixture, ablation performance of introducing ZrB2 particles modified car- containing phenolic and ZrB2 (mass ration 1:0.2), are explored by bon–phenolic (Z C–Ph) composites is reasonable tested by oxy- thermal gravimetric analysis. Thermal gravimetric curves are pre- gen–acetylene, and the temperatures of the ablated surface and sented in Fig. 2. the back surface are real-time monitored. The influence of ZrB2 As well known, the phenolic has pyrolysis phenomenon when on thermal stability of phenolic is investigated extensively. In addi- the temperature exceeds 300 °C, and the weight loss below tion, the mechanisms of improving ablation resistance are deeply 300 °C is not mainly owing to the pyrolysis of phenolic but the discussed. post-cured process [16].InFig. 2, we could find that the mixture and phenolic all have the thermal degradation phenomenon when 2. Experimental the temperature goes up to 300 °C. More and more thermal degradation byproducts are found when the temperature continues to For the purpose of proper demonstration, the proposed three- creep from 480 °C to 650 °C. It is clear that ZrB2 particle does not dimensional carbon fibers fabric was impregnated in mixture. change the temperature distribution of phenolic pyrolysis, and The three-dimensional carbon fibers matrix was defined as the thermal stability of phenolic does not increase significantly. 40 40 40 mm (with the density q = 0.185 g/cm3). The mixture The char yield of phenolic and phenolic containing ZrB2 at 1000 °C are 67.3% and 73.4%, respectively. If calculated weight loss 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 was owing to the pyrolysis of phenolic, and ZrB2 weight was con- which has implied in our previous work. The three-dimensional sidered to be constant, the char yield of the mixture at 1000 °C carbon fibers fabric was put into the mixture for 20 min. Then should be 72.7%. This value is lower than the measured value of we left the fabric over 24 h to evaporate the solvent. The heating phenolic contains ZrB2. Therefore, the weight gain reactions should cure was done in 30 min, 90 min and 180 min in an oven at be occurred in this condition. 80 °C, 110 °C and 150 °C, respectively. In addition, the heating rate Fig. 3 reveals that the XRD patterns of the products after TG was approximately 1 °C/min, and sample cooling was completed at analysis. The XRD spectrum for the mixture demonstrates the for- the room temperature. The cured fabric (0.515 g/cm3) was divided mation of ZrO2, which results from the reactions between ZrB2 and into smaller sample with the size of £25 20 mm for oxygen– the pyrolysis products of phenolic in N2 atmosphere. It is well acetylene testing, and the C–Ph composites (0.491 g/cm3) was also known that the main pyrolysis products of phenolic include hydro- decollate to sub-sample of £25 20 mm. This mixture contains gen, methane, slight amount of ethane, water, small amount of car- ethanol and phenolic at the mass ration of 1:1. bon dioxide, carbon monoxide and porous amorphous carbon (the Thermogravimetry (TG) analysis was carried out by TGA/ char from phenolic) [16]. Two reactions of forming ZrO2 are given SDTA851e (Switzerland). The samples were placed in alumina cru- by: cibles, and heated from normal temperature (21 °C) to 1000 °C ZrB2ðsÞþ5COðgÞ¼ZrO2ðsÞþB2O3ðgÞþ5CðsÞð1Þ with increased heating rate of 5 °C/min in N2 atmosphere. The phase composition was analyzed by an X-ray diffraction device 2ZrB2ðsÞþ5CO2ðgÞ¼2ZrO2ðsÞþ2B2O3ðgÞþ5CðsÞð2Þ (Cu Ka radiation, D/max-RB, Japan). Ablation performance was The byproduct of B2O3 is not detected by XRD due to its evapo- tested by oxygen–acetylene. Scheme of the ablation experiments ration under N2 atmosphere [17–19]. Therefore, the increased char medium is shown in Fig. 1. The flow rates of oxygen and acetylene yield of the mixture contributes to the formation of ZrO2(s) and were 500 l/h and 400 l/h, respectively. The oxygen–acetylene gun C(s). As shown in Fig. 3, broad peaks (a and b) are observed at 2h with 2 mm dimension was perpendicular to the surface of the angles of about 24°and 44°, respectively. They are characteristics specimen, and the distance from gun to the surface of the specimen of amorphous carbon from phenolic [20]. However, these broad was 50 mm. The ablation surface temperatures were monitored by peaks are not obvious observed in the mixture, which are very the infrared temperature measurement system (IR-AHU, United weak compared with those of ZrB2. 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 18 spectroscopy (EDS) for chemical analysis. 16

14 Phenolic

12 Weight (mg) 10

8 Phenolic +ZrB2 6 0 200 400 600 800 1000 Temperature (oC)

Fig. 1. The scheme of the ablation experiments medium. Fig. 2. TG curves under N2 atmosphere. 322 Y. Chen et al. / Composites: Part B 47 (2013) 320–325

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

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 com- Fig. 3. XRD patterns of the products. posites is lower than C–Ph composites in the ablation process in Fig. 5b. The temperature of Z C–Ph composites creeps slowly until 3.2. Ablation property 25th second. On the other side, the temperature increases rapidly when the time is over 25th second. The temperature of Z C–Ph The samples of Z C–Ph and C–Ph composites are shown in Fig. 4. composites is lower than C–Ph composites about 100 °C from The ablation process lasts 160 s, and the maximum ablation depth 63th second to the end. of sample surface is 0.56 mm, when the white ZrO2 layer is We observe that Z C–Ph composites perform a better heat insu- stripped. Therefore, the linear ablation rate is 0.0035 mm/s. The lation performance compared with C–Ph composites. For the most, maximum ablation depth of C–Ph composites is 2.65 mm, thus the addition of ZrB2 distinctly increases the grain boundary ther- the linear ablation rate of sample is 0.0166 mm/s. The linear abla- mal resistance, which would reduce markedly the thermal conduc- tion rate of Z C–Ph composites reduced by 79% compared with C– tivity of composites. The addition of nanosilica could reduce the Ph composites. From all above, we can easily discover that the thermal conductivity of carbon–phenolic composites, which is well ablation resistance is well improved by introducing ZrB2 particles reported in article [12]. Moreover, ZrO2 has a low thermal conduc- 1 1 into C–Ph composites. tivity (2 W m K ), so the thermal barrier ZrO2 layer on the sur- As depicted in Fig. 4b, green colored gas is found in ablation test face can slow down the advancement of heat front, and produce a of Z C–Ph composites. This green colored gas is expected to be bor- steep temperature gradient field [24]. What is more, the endother- ia gas [15].InFig. 4c, white ZrO2 layer is found on the ablated sur- mic evaporation of B2O3 might absorb part of energy. Simulta- face of Z C–Ph composites. Evident improving ablation resistance neously, since gaseous B2O3 increases the velocity of escaping of Z C–Ph composites contributes to the formation of ZrO2 and flux, more incoming convective heat flux is blocked by outcoming

B2O3, which are the oxidation of ZrB2 and the above-mentioned ﬂow of gaseous B2O3 and pyrolysis gas of phenolic [21]. In addition, reaction between ZrB2 and CO or the reaction between ZrB2 and 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. Y. Chen et al. / Composites: Part B 47 (2013) 320–325 323

2200 700 (a) (b) 2000 600 Z C-Ph 500 C-Ph C) C) 1800 o o 400 1600 C-Ph 300 1400 Temperature ( Temperature ( 200 Z C-Ph 1200 100

1000 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 Time (s) 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 C–Ph composites (Fig. 6). In Fig. 6b, only a lot of carbon fibers are [3,21]. 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 3.4. Microstructure characterization ablated surface, and the content of char increases gradually with depth. All these imply that the depths of matrix and fibers recess Carbon fiber and carbonized phenolic are carbon material. Car- are different, and the recession rate of matrix is higher than that bon material is susceptible to oxidation at temperature more than of carbon fibers. Moreover, these mean that the ablation occurs 450 °C [25]. We shall mainly focus on ablation by oxidation in the not in surface, but in volume, and the ablation of fiber is not due oxygen–acetylene test. In C–Ph composites, the phenolic matrix to radial recession. occupies the voids between the carbon fiber, and surrounds the fi- From Fig. 6c, there are amounts of pores which result from the bers. The fiber length is large compared with its diameter, thus the pyrolysis of phenolic resin matrix, pits and cracks in the char. How- ablation of fibers should be mainly due to radial recession [25]. The ever, radius of carbon fibers with fewer pores and pits is not obvi- oxidation reaction in composites is assumed to be the matrix ous reduced. The carbonized phenolic has more defects than recession and reduction of fiber radius [25]. carbon fibers, and the contact area of char with oxygen is large. In order to investigate ablation zone of C–Ph composites, we ob- Oxygen molecule can penetrate deeply inside the matrix with reac- serve the microstructure of the C–Ph composites and the ablated tion, 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. 324 Y. Chen et al. / Composites: Part B 47 (2013) 320–325

Carbon fiber Phenolic Pore 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. Pyrolysis The ablation mechanism of C–Ph composites is illustrated in gases Fig. 7. Phenolic pyrolysis could release gas and leave amorphous porous char in above-mentioned temperature from 480 °Cto 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 Pyrolysis zone of Amorphous and char is existed in the ablating surface. phenolic carbon In order to know whether the ablation of Z C–Ph composites occurs in volume or surface, the microstructures of Z C–Ph compos- Fig. 7. The ablation scheme of C–Ph composites. ites are analyzed. Fig. 8 represents SEM images of Z C–Ph composites and ablated Z C–Ph composites. We obtain that the ated. Furthermore, the strength of char is weak because of the de- ablation rate of carbon fibers is close to matrix in Fig 8b. From fects. Part of char is removed by external shear forces [3]. Fig. 8c and d, we observe carbon fiber radius is obvious reduced, In C–Ph composites, the thermosetting phenolic resin, as the and ZrO2 particles are existed on amorphous carbon. However, matrix, holds the fibers firmly in place and binds the carbon fibers B2O3 is not detected owing to its rapid evaporation at high temper-

(a) (b)

(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) 320–325 325

ZrO layer improve the ablation performance of C–Ph composites, and it Carbon fiber Phenolic ZrB2 ZrO2 2 maybe become a backbone of thermal structure in aerospace. Pore References

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