Availableonlineatwww.sciencedirect.com Science Direct CERAMICS INTERNATIONAL ELSEVIER Ceramics International 36(2010)1463-1466 www.elsevier.com/locate/ceramint Short communication A hot-pressing reaction technique for SiC coating of carbon/carbon composites Fu Qian-Gang", Xue Hui, Wu Heng, Li He-Jun, Li Ke-Zhi, Tao Jun C/C Composites Research Center, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, PR China Received 16 September 2009: received in revised form 31 October 2009; accepted 12 December 2009 Available online 28 January 2010 Abstract A hot-pressing reactive sintering(HPRS)technique was explored to prepare SiC coating for protecting carbon/carbon(C/C)composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC ating obtained by HPRs has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt %o after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a Sic coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-Sic coating C 2010 Elsevier Ltd and Techna Group s.r. l. All rights reserved. Keywords: A. Hot pressing: A Sintering: D SiC 1. Introduction densification occur in single step. This technique can be economical due to low cost starting powders and relatively low Oxidation resistance is a key requirement for carbon/carbon sintering temperature, and it also leads to the possibility of / C)composites for applications in an oxygen-containing densifying the materials without additives [10, 11]. As far as the environment at high temperature [1-3]. To prevent C/C authors know, no literature has been published about using composites from oxidation, SiC coating was widely used HPRS technique to prepare ceramic coatings for C/C due to its excellent oxidation resistance and good compatibility composites. Though HPRS technique is not practical to with C/C composites [4, 5]. Presently, Sic can be coated on the prepare coatings for complex shaped C/C composites, it can surface of C/C composites by several methods, such as pack be applicable to regular shaped ones by designing suitable cementation [6], chemical vapor deposition(CVD)[7] and moulds. In the present work, HPRS technique was proposed to laser-induced chemical decomposition (LICD)[8]. Among prepare SiC coating for C/C composites. The microstructures these methods, pack cementation was usually used for and oxidation protective ability of the coatings were providing a strong interface bonding between SiC coating investigated, and the effect of Sic coating on the flexural and C/C composites [9]. However, cracks will be formed property of the coated C/C composites was also studied inevitably in this coating during the cooling process from high temperature to room temperature due to the mismatch of 2. Experimental thermal expansion between SiC and C/C composites, which offer entrance channels for oxygen and result in the failure of The substrates were cut from bulk 2D C/C composites with a the coating [5,7] density of 1.75 g/cm. Powder compositions for the HPRS Hot-pressing reactive sintering(HPRS)is a technique in which both the chemical reactions of the starting materials and process were 65-80 wt% Si and 20-35 wt. graphite. These powders were mixed by a blender for 2 h C/C specimens were packed by these mixtures in a graphite crucible, and were pre aphite pressing head. The pressi Eomoil address f +862988494197;fax:+862988495764 controlled by 250 kPa. Then the graphite crucible was heated g@nwpu.edu.cn(F.Qian-Gang) to 1873-2073 K and held at that temperature for 2 h under 2-8842/$36.00 2010 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.1016 1-ceramint.2010.01002
Short communication A hot-pressing reaction technique for SiC coating of carbon/carbon composites Fu Qian-Gang *, Xue Hui, Wu Heng, Li He-Jun, Li Ke-Zhi, Tao Jun C/C Composites Research Center, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China Received 16 September 2009; received in revised form 31 October 2009; accepted 12 December 2009 Available online 28 January 2010 Abstract A hot-pressing reactive sintering (HPRS) technique was explored to prepare SiC coating for protecting carbon/carbon (C/C) composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC coating obtained by HPRS has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt.% after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a SiC coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating. # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Hot pressing; A. Sintering; D. SiC 1. Introduction Oxidation resistance is a key requirement for carbon/carbon (C/C) composites for applications in an oxygen-containing environment at high temperature [1–3]. To prevent C/C composites from oxidation, SiC coating was widely used due to its excellent oxidation resistance and good compatibility with C/C composites [4,5]. Presently, SiC can be coated on the surface of C/C composites by several methods, such as pack cementation [6], chemical vapor deposition (CVD) [7] and laser-induced chemical decomposition (LICD) [8]. Among these methods, pack cementation was usually used for providing a strong interface bonding between SiC coating and C/C composites [9]. However, cracks will be formed inevitably in this coating during the cooling process from high temperature to room temperature due to the mismatch of thermal expansion between SiC and C/C composites, which offer entrance channels for oxygen and result in the failure of the coating [5,7]. Hot-pressing reactive sintering (HPRS) is a technique in which both the chemical reactions of the starting materials and densification occur in single step. This technique can be economical due to low cost starting powders and relatively low sintering temperature, and it also leads to the possibility of densifying the materials without additives [10,11]. As far as the authors know, no literature has been published about using HPRS technique to prepare ceramic coatings for C/C composites. Though HPRS technique is not practical to prepare coatings for complex shaped C/C composites, it can be applicable to regular shaped ones by designing suitable moulds. In the present work, HPRS technique was proposed to prepare SiC coating for C/C composites. The microstructures and oxidation protective ability of the coatings were investigated, and the effect of SiC coating on the flexural property of the coated C/C composites was also studied. 2. Experimental The substrates were cut from bulk 2D C/C composites with a density of 1.75 g/cm3 . Powder compositions for the HPRS process were 65–80 wt.% Si and 20–35 wt.% graphite. These powders were mixed by a blender for 2 h. C/C specimens were packed by these mixtures in a graphite crucible, and were pressed by a graphite pressing head. The pressure was controlled by 250 kPa. Then the graphite crucible was heated to 1873–2073 K and held at that temperature for 2 h under www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 36 (2010) 1463–1466 * Corresponding author. Tel.: +86 29 88494197; fax: +86 29 88495764. E-mail address: fuqiangang@nwpu.edu.cn (F. Qian-Gang). 0272-8842/$36.00 # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2010.01.002
F. Qian-Gang et al /Ceramics International 36(2010)1463-1466 b:阝-sic Pressing head Pack powders C/C composites Fig. 1. Sketch of preparing SiC coating for C/C composites by HPRs slight argon flow. During this process, Si will melt and react with C/C composites to form SiC coating. The sketch of Fig. 2. XRD patterns of the surface of the C/C samples with coatings prepared preparing SiC coating for C/C composites by HPRS is shown in by PRS (a) and HPRS (b) Fig. 1. For comparison, another kind of Sic coating was prepared by pressure-less reactive sintering(PRS) with the same starting powders and heat-treatment temperature. During the preparation of coating, melted Si will react with C/C To investigate the thermal stress resistance of the coatings, to form SiC coating. Under pressure, some of liquid Si in the the thermal cycling test between 1773 K and room temperature pack powders was congregated to the surface of the samples was performed. The samples were weighed at room tempera- and left in the SiC coating, which is advantageous to relax the ture by electronic balance with a sensitivity of +0. I mg during stress at the end of the cracks and heal up the cracks in the thermal cycling tests. To evaluate the mechanical properties of coating the samples, three-point bending tests were carried out in a Fig. 3(a) displays SEM image of the coating prepared by servohydraulic machine of 8871 (INSTRON CO, Ltd, USA). PRS. This coating has a porous structure with some The span was 40 mm and the crosshead speed was 0.5 mm/min. microcracks, resulted from bigger coefficient of thermal Five samples for each kind of sample were tested and the final expansion of Sic coating than that of C/C composites Due flexural properties were obtained by computing the average to its loose structure, this coating might provide a poor values of five samples. oxidation protective ability for C/C composites. From Fig. 3(b) The morphologies and crystalline structures of the coatings the as-received coating prepared by HPRS possesses a dense were analyzed by JSM-6460 scanning electron microscopy and crack-free structure With pressure, the coatings will have (SEM) and Rigaku D/max-3C X-ray diffraction (XRD) compressive stress, which can effectively make up for the by the shrinkage of SiC coating during 3. Results and discussion cooling process from high temperature to room temperature The thermal stress in SiC coatings can be relaxed by the Fig. 2 shows XRD patterns of the surface of the coated introduction of pressure samples. From Fig. 2(a), the diffraction peaks of graphite and SEM images of the cross-section of the SiC coated samples cubic B-Sic are detected from the surface of the coating are shown in Fig 4 From Fig 4(a), there are large numbers of obtained by PRS. Graphite is corresponding to the C/c holes in the SiC coating prepared by PRS, owing to the difficult ubstrate, and B-SiC comes from the coating. Fig. 2(b)displays sintering of SiC ceramic without pressure From Fig 4(b), Sic that a new phase of Si was generated in the coating by HPRS. coating prepared by HPRs is denser than that without pressure, Fig. 3. SEM images of the surface of the coatings prepared by PRS (a) and HPRS(b)
slight argon flow. During this process, Si will melt and react with C/C composites to form SiC coating. The sketch of preparing SiC coating for C/C composites by HPRS is shown in Fig. 1. For comparison, another kind of SiC coating was prepared by pressure-less reactive sintering (PRS) with the same starting powders and heat-treatment temperature. To investigate the thermal stress resistance of the coatings, the thermal cycling test between 1773 K and room temperature was performed. The samples were weighed at room temperature by electronic balance with a sensitivity of 0.1 mg during thermal cycling tests. To evaluate the mechanical properties of the samples, three-point bending tests were carried out in a servohydraulic machine of 8871 (INSTRON CO., Ltd., USA). The span was 40 mm and the crosshead speed was 0.5 mm/min. Five samples for each kind of sample were tested and the final flexural properties were obtained by computing the average values of five samples. The morphologies and crystalline structures of the coatings were analyzed by JSM-6460 scanning electron microscopy (SEM) and Rigaku D/max-3C X-ray diffraction (XRD). 3. Results and discussion Fig. 2 shows XRD patterns of the surface of the coated samples. From Fig. 2(a), the diffraction peaks of graphite and cubic b-SiC are detected from the surface of the coating obtained by PRS. Graphite is corresponding to the C/C substrate, and b-SiC comes from the coating. Fig. 2(b) displays that a new phase of Si was generated in the coating by HPRS. During the preparation of coating, melted Si will react with C/C to form SiC coating. Under pressure, some of liquid Si in the pack powders was congregated to the surface of the samples and left in the SiC coating, which is advantageous to relax the stress at the end of the cracks and heal up the cracks in the coating. Fig. 3(a) displays SEM image of the coating prepared by PRS. This coating has a porous structure with some microcracks, resulted from bigger coefficient of thermal expansion of SiC coating than that of C/C composites. Due to its loose structure, this coating might provide a poor oxidation protective ability for C/C composites. From Fig. 3(b), the as-received coating prepared by HPRS possesses a dense and crack-free structure. With pressure, the coatings will have compressive stress, which can effectively make up for the tensile stress induced by the shrinkage of SiC coating during the cooling process from high temperature to room temperature. The thermal stress in SiC coatings can be relaxed by the introduction of pressure. SEM images of the cross-section of the SiC coated samples are shown in Fig. 4. From Fig. 4(a), there are large numbers of holes in the SiC coating prepared by PRS, owing to the difficult sintering of SiC ceramic without pressure. From Fig. 4(b), SiC coating prepared by HPRS is denser than that without pressure, Fig. 1. Sketch of preparing SiC coating for C/C composites by HPRS. Fig. 3. SEM images of the surface of the coatings prepared by PRS (a) and HPRS (b). Fig. 2. XRD patterns of the surface of the C/C samples with coatings prepared by PRS (a) and HPRS (b). 1464 F. Qian-Gang et al. / Ceramics International 36 (2010) 1463–1466
F. Qian-Gang et al /Ceramics Intemational 36(2010)1463-1466 65 b Fig. 4. SEM images of the cross-section of the samples with SiC coatings prepared by PRS(a) and HPRS (b) and the coating materials are existent at the edges of the holes in oxidation resistance, and the mass loss of the C/C sample 'C composites near the coating. At the processing tempera- with this coating is only 1. 84 wt. after 15-time thermal ture, the silicon powders in the original pack mixtures melted cycles. The Sic coating obtained by HPRS has a dense and and possessed strong infiltration ability. The liquid Si would crack-free structure, so it has a better oxidation protective nfiltrate deeply to the inside of the C/C composites through the ability for C/C composites holes and cracks in these composites and react with C/C, which The flexural property parameters of C/C and coated C/C is advantageous to the bonding between coating and C/c composites are shown in Table 1. The flexural strength of C/C, PRS-SiC coated C/C and HPRS-SiC coated C/C are 89+7 Fig 5 shows the percent mass losses of the samples Bare Dare C/C composites, the Sic coated C/C composites have hermal cycling between 1773 K and room temperature. C/C composite specimen has a poor oxidation resistance, and higher strength and modulus for the reason that SiC coating can its mass loss percentage is up to 70 wt. after thermal cycling eliminate some defects in C/C composites near their surface for only 10 times. With a SiC coating prepared by PRS, the These defects consist of microcracks resulted from the sample loses mass for about 17.6 wt. after 15-time thermal machining of C/C and holes left in C/C substrate during their cycles. Compared with the SiC coating by PRS, the SiC coating preparation. More defects will be eliminated by the stronger prepared by HPRS exhibits a better thermal shock and infiltration ability of Si under pressure. So, the sample with a HPRS-SiC coating exhibits higher mechanical property than that with a PRS-SiC coating Fig. 6 shows the load-displacement curves of the samples. It can be seen that the bare C/C sample exhibited a bit pseudo- C+PRS-SIC C+HPRS-SiC plastic fracture behavior. With a PRS-SiC coating, the toughness of the sample was reduced evidently. While the HPRS-SiC coated sample experienced a bit of brittle fracture Fig. 7 shows the fracture surface of the samples after flexural tests. From Fig. 7(a), carbon fibers were pulled out from C/C+HPRS-SiC 0246810121416 Thermal shock time/time →CC+ PRS SIC Fig. 5. Percent mass losses of samples during thermal cycling between 1773 K Table Flexural properties of the as-tested samples. Strength/MPa Modulus/GPa 89±7 11±1 C/C + PRS-Sic 105±10 12±1 Displacement/mm C/C+ HPRS-SiC 140±12 27士3 Fig. 6. The load-displacement curves of the samples
and the coating materials are existent at the edges of the holes in C/C composites near the coating. At the processing temperature, the silicon powders in the original pack mixtures melted and possessed strong infiltration ability. The liquid Si would infiltrate deeply to the inside of the C/C composites through the holes and cracks in these composites and react with C/C, which is advantageous to the bonding between coating and C/C composites. Fig. 5 shows the percent mass losses of the samples during thermal cycling between 1773 K and room temperature. Bare C/C composite specimen has a poor oxidation resistance, and its mass loss percentage is up to 70 wt.% after thermal cycling for only 10 times. With a SiC coating prepared by PRS, the sample loses mass for about 17.6 wt.% after 15-time thermal cycles. Compared with the SiC coating by PRS, the SiC coating prepared by HPRS exhibits a better thermal shock and oxidation resistance, and the mass loss of the C/C sample with this coating is only 1.84 wt.% after 15-time thermal cycles. The SiC coating obtained by HPRS has a dense and crack-free structure, so it has a better oxidation protective ability for C/C composites. The flexural property parameters of C/C and coated C/C composites are shown in Table 1. The flexural strength of C/C, PRS-SiC coated C/C and HPRS-SiC coated C/C are 89 7, 105 10 and 140 12 MPa, respectively. Compared to the bare C/C composites, the SiC coated C/C composites have higher strength and modulus for the reason that SiC coating can eliminate some defects in C/C composites near their surface. These defects consist of microcracks resulted from the machining of C/C and holes left in C/C substrate during their preparation. More defects will be eliminated by the stronger infiltration ability of Si under pressure. So, the sample with a HPRS-SiC coating exhibits higher mechanical property than that with a PRS-SiC coating. Fig. 6 shows the load-displacement curves of the samples. It can be seen that the bare C/C sample exhibited a bit pseudoplastic fracture behavior. With a PRS-SiC coating, the toughness of the sample was reduced evidently. While the HPRS-SiC coated sample experienced a bit of brittle fracture mode. Fig. 7 shows the fracture surface of the samples after flexural tests. From Fig. 7(a), carbon fibers were pulled out from Table 1 Flexural properties of the as-tested samples. Samples Strength/MPa Modulus/GPa C/C 89 7 11 1 C/C + PRS-SiC 105 10 12 1 C/C + HPRS-SiC 140 12 27 3 Fig. 4. SEM images of the cross-section of the samples with SiC coatings prepared by PRS (a) and HPRS (b). Fig. 5. Percent mass losses of samples during thermal cycling between 1773 K and room temperature. Fig. 6. The load-displacement curves of the samples. F. Qian-Gang et al. / Ceramics International 36 (2010) 1463–1466 1465
F. Qian-Gang et al /Ceramics International 36(2010)1463-1466 Fig. 7. Fracture surface micrographs of the samples after flexural tests. (a)C/C and(b) HPRS-SiC coated C/C pyrocarbon evidently, which is advantageous to the ductility of Fund of State Key Laboratory of Solidification Processing the sample. From Fig. 7(b), the HPRS-SiC coated C/C sample (NWPU), China( Grant No. KP200913) had an even fracture surface. accordant with its brittle fracture characteristic. It is well-known that defects. such as holes and References cracks, are generally existent at the interface between carbon fiber and carbon matrix. By HPRS-SiC coating, these defects [11 N.S.Jacobson, D.M. Curry, Oxidation microstructure studies of reinforced will be eliminated partially, and the interface bonding between carbon/carbon, Carbon 44(2006)1142-1 150 ber and matrix will be improved largely. Therefore, the [2] J.F. Huang, H.J. Li, X.B. Xiong, X.R. Zeng K.Z. Li, YWFu, Progress on the oxidation protective coating of carbon-carbon composites, New fracture mode of the C/C sample changed from a pseudo-plastic Carbon Materials 20(2005)373-379 fracture behavior to a brittle one after being coated with a [3] W.M. Lu, DD L Chung, Oxidation protection of carbon materials by acid HPRS-SiC coating 4. Conclusions W hosphate impregnation, Carbon 40(2002)1249-12<>y/ Surface and tion through coating cracks of SiC-protected carbon/carb Coatings Technology 203(2008)372-383 [5 J.F. Huang, X.R. Zeng. H. Li, X.B. Xiong. Y.w. Fu, Influence of the HPRS technique can be applied to obtain a dense and crack- preparation are on the phase, microstructure and anti-oxidation free SiC coating for C/C composites. The HPRS-SiC coating es, Carbon42(2004)1517-1521 can infiltrate into C/C substrate deeply, resulting in excellent [6] C.A. A Cairo, M.L.A. Graca, C.R.M. Silva,JC.Bressiani,Functionally thermal shock and oxidation resistance between 1773 K and radient ceramic coating for C-C antioxidation protection, Journal of the European Ceramic Society 21(2001)325-329 room temperature. The flexural property of the HPRS-SiC [7) B Wang, K.Z. Li, H.J. Li, Q.G. Fu, X Wang, Sic coating prepared by a coated C/C sample is higher than those of the bare C/C and the ique of pack cementation and CVD on carbon/carbor PRS-SiC coated C/C sample. The fracture mode of the C/C posites, Journal of Inorganic Materials 22(2007)737-741 ample changes from a pseudo-plastic behavior to a brittle one [8] S. Lloyd. N. Avery, M. Pal, A novel laser technique for oxidation-resistar after being coated with a HPrs-Sic coating ting of carbon-carbon composite, Carbon 39(2001)991-999 [9 Q.G. Fu, H.. Li, X.H. Shi, K.Z. Li, G D. Sun, Silicon carbide coating to protect carbon/carbon composites against oxidation, Scripta Materialia 52 Acknowledgements 2005)923-927 [10] D. Veljovic. B Jokic, R. Petrovic, E. Palcevskis, A. Dindune, I.N. Mihai This work has been supported by the National Natural escu,D. Janackovic, Processing of dense nanostructured HAP ceramics by Science Foundation of China under Grant No. 50802075 and nd hot pressing, Ceramics International 35(2009)1407-1413 [11 LJ. Huang, L Geng, A B. Li, F.Y. Yang, H.X. Peng, In situ TiBw/Ti-6Al- the""Project under Grant No. 08040, and supported by 4V composites with novel reinforcement architecture fabricated by rea NPU Foundation for Fundamental Research and the Research tion hot pressing, Scripta Materialia 60(2009)996-999
pyrocarbon evidently, which is advantageous to the ductility of the sample. From Fig. 7(b), the HPRS-SiC coated C/C sample had an even fracture surface, accordant with its brittle fracture characteristic. It is well-known that, defects, such as holes and cracks, are generally existent at the interface between carbon fiber and carbon matrix. By HPRS-SiC coating, these defects will be eliminated partially, and the interface bonding between fiber and matrix will be improved largely. Therefore, the fracture mode of the C/C sample changed from a pseudo-plastic fracture behavior to a brittle one after being coated with a HPRS-SiC coating. 4. Conclusions HPRS technique can be applied to obtain a dense and crackfree SiC coating for C/C composites. The HPRS-SiC coating can infiltrate into C/C substrate deeply, resulting in excellent thermal shock and oxidation resistance between 1773 K and room temperature. The flexural property of the HPRS-SiC coated C/C sample is higher than those of the bare C/C and the PRS-SiC coated C/C sample. The fracture mode of the C/C sample changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating. Acknowledgements This work has been supported by the National Natural Science Foundation of China under Grant No. 50802075, and the ‘‘111’’ Project under Grant No.08040, and supported by NPU Foundation for Fundamental Research and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. KP200913). References [1] N.S. Jacobson, D.M. Curry, Oxidation microstructure studies of reinforced carbon/carbon, Carbon 44 (2006) 1142–1150. [2] J.F. Huang, H.J. Li, X.B. Xiong, X.R. Zeng, K.Z. Li, Y.W. Fu, Progress on the oxidation protective coating of carbon-carbon composites, New Carbon Materials 20 (2005) 373–379. [3] W.M. Lu, D.D.L. Chung, Oxidation protection of carbon materials by acid phosphate impregnation, Carbon 40 (2002) 1249–1254. [4] N.S. Jacobson, D.J. Roth, R.W. Rauser, J.D. Cawley, D.M. Curry, Oxidation through coating cracks of SiC-protected carbon/carbon, Surface and Coatings Technology 203 (2008) 372–383. [5] J.F. Huang, X.R. Zeng, H.J. Li, X.B. Xiong, Y.W. Fu, Influence of the preparation temperature on the phase, microstructure and anti-oxidation property of a SiC coating for C/C composites, Carbon 42 (2004) 1517–1521. [6] C.A.A Cairo, M.L.A. Graca, C.R.M. Silva, J.C. Bressiani, Functionally gradient ceramic coating for C–C antioxidation protection, Journal of the European Ceramic Society 21 (2001) 325–329. [7] B. Wang, K.Z. Li, H.J. Li, Q.G. Fu, X. Wang, SiC coating prepared by a two-step technique of pack cementation and CVD on carbon/carbon composites, Journal of Inorganic Materials 22 (2007) 737–741. [8] S. Lloyd, N. Avery, M. Pal, A novel laser technique for oxidation-resistant coating of carbon–carbon composite, Carbon 39 (2001) 991–999. [9] Q.G. Fu, H.J. Li, X.H. Shi, K.Z. Li, G.D. Sun, Silicon carbide coating to protect carbon/carbon composites against oxidation, Scripta Materialia 52 (2005) 923–927. [10] D. Veljovic, B. Jokic, R. Petrovic, E. Palcevskis, A. Dindune, I.N. Mihailescu, D. Janackovic, Processing of dense nanostructured HAP ceramics by sintering and hot pressing, Ceramics International 35 (2009) 1407–1413. [11] L.J. Huang, L. Geng, A.B. Li, F.Y. Yang, H.X. Peng, In situ TiBw/Ti–6Al– 4V composites with novel reinforcement architecture fabricated by reaction hot pressing, Scripta Materialia 60 (2009) 996–999. Fig. 7. Fracture surface micrographs of the samples after flexural tests. (a) C/C and (b) HPRS-SiC coated C/C. 1466 F. Qian-Gang et al. / Ceramics International 36 (2010) 1463–1466