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SiC composites ablated by oxyacetylene torch at 1800 °C
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
journal homepage: www.elsevier.com/locate/jmatprotec
Effects of ablation at different regions in three-dimensional orthogonal C/SiC composites ablated by oxyacetylene torch at 1800 ◦ C Bo Yan a , Zhaofeng Chen a,∗ , Jianxun Zhu a,b , Jianzhong Zhang b , Yun Jiang b a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Sinoma Science & Technology Co., Ltd., Nanjing 210012, PR China
a r t i c l e
i n f o
Article history:
a b s t r a c t C/SiC composites were prepared by polycarbosilane infiltration pyrolysis and ablated by
Received 10 November 2007
oxyacetylene flame at 1800 ◦ C for 180 s. Morphology and microstructure of C/SiC have been
Received in revised form
studied by scanning electron microscopy/energy dispersive spectroscopy, and X-ray diffrac-
20 July 2008
tion analysis. Two concentric ring regions appeared on the surface of the ablated C/SiC. In
Accepted 3 August 2008
the centre region of the ablated C/SiC was composed of irregular SiC particles with a lot of pores. The pores were resulted from (i) gas expansion in closed pores during ablation, and (ii) from the release of SiO gas produced by the oxidation of SiC with sufficiently low oxygen
Keywords:
partial pressure. The results indicated the ablation was due to a combination of oxidation,
C/SiC composites
mechanical erosion and recrystallization of the surface SiC.
Ablation mechanism
© 2008 Elsevier B.V. All rights reserved.
Oxyacetylene torch
1.
Introduction
Ablation is an erosive phenomenon with a removal of material by a combination of thermomechanical, thermochemical, and thermophysical factors arising from high temperature, pressure, and velocity of combustion flame (D’Aleio and Parker, 1971). Future engine components, in particular combustion chambers and expansion nozzles which are subjected to high thermal loads, need new materials which possess outstanding thermomechanical and thermochemical properties (Beyer et al., 1999). In view of their low specific weight, their excellent resistance to ablation as well as cost effective production, C/SiC composites represent an interesting group of materials for high-temperature components for space propulsion systems (Khan, 1996; Beyer et al., 2005; Mital and Murthy, 2001; Hald et al., 2003). Three-dimensional (3D) woven composites, especially those with orthogonal configurations, have been
∗
Corresponding author. Tel.: +86 25 52112901; fax: +86 25 52112626. E-mail address: zhaofeng [email protected] (Z. Chen). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.08.002
pursued in the aerospace industry primarily to improve the vulnerability of 2D composites to delamination under impact and subsequent local buckling failure under in-plane compression (Tan et al., 1997; Chiu and Cheng, 2003; Aitharaju and Averil, 1999). Before the practical application of C/SiC in these cases, investigation on the evolution of morphology and microstructure during ablation is required. The main experimental methods are oxyacetylene flame ablation, plasma arc ablation, and kerosene–liquid oxygen flame ablation. Oxyacetylene flame ablation method is the simplest and easiest to conduct with the lowest cost. Oxyacetylene flame testing is often used to simulate the rocket engine exhaust flame (Song et al., 2003; Kanevce et al., 1999). So far, ablation research mainly concentrate on optimization of testing environment, C/C composites and 2D C/SiC composites, while studies on ablation morphology and microstructure of three-dimensional C/SiC
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
3439
composites prepared by polycarbosilane infiltration pyrolysis (PIP) are rarely reported (Beyer et al., 1999; Patterson et al., 2000; Lee and Joo, 2004a,b; Tang et al., 2006; Pan et al., 2006). The present paper is to investigate the effects of ablation at different regions in three-dimensional orthogonal C/SiC composites ablated by oxyacetylene torch at 1800 ◦ C.
2.
Experimental
2.1.
Composites fabrication
Fig. 1 shows experimental flow diagram for three-dimensional orthogonal C/SiC composites prepared by PIP. The carbon fiber utilized is M30 (from Toray, Japan), and each yarn contained 3000 fibers. The threedimensional orthogonal preforms were supplied by Nanjing Institute of Glass Fiber, China, with a porosity of 56%. The composite was fabricated by PIP. The infiltration solution was composed of polycarbosilane (PCS) as preceramic and divinylbenzene (DVB) as solvent (mass ratio 1:0.4). Carbon fiber preforms were immersed into the PCS/DVB mixture solution (65 ◦ C, 2 MPa, 30 min) and then dried at 150 ◦ C for 6 h. The ◦
prepregs were pyrolysed at 1200 C in N2 with the heating program of 15 ◦ C/min, holding at 1200 ◦ C for 0.5 h and then naturally cooling inside furnace. The infiltration-pyrolysis process was repeated six times. 2.2.
Ablation experiment
Ablation experiment was conducted using oxyacetylene torch. The sample size of C/SiC composites was 27 mm × 20 mm × 18 mm. The flow rates of O2 and C2 H2 were 0.24 and 0.18 L/s, respectively. The specimen was placed vertically to the flame direction in air. The distance between the nozzle tip of the oxyacetylene gun and the front surface of the specimen was 30 mm and the inner diameter of the tip was 2.5 mm. The surface temperature of the samples was about 1800 ◦ C monitored with an optical pyrometer. The ablation period was 180 s.
2.3.
Sample characterization
The ablation morphology, microstructure and elemental composition of 3D orthogonal C/SiC composites were examined by a scanning electron microscope (SEM, FEI CO., Quanta200). Elemental composition was carried out using energy dispersive spectroscopy (EDS). The phases of the composite was characterized by X-ray diffraction analysis (XRD, Rigaku D/Max-B) using Ni-filtered Cu K␣ radiation at a scanning rate of 0.5◦ /s and scanning from 10◦ to 70◦ of 2.
3.
Results and discussion
Fig. 2 shows photograph of the as-ablated C/SiC composites. Two concentric ring regions (e.g., centre ablation region and outer ablation region) can be observed on the surface of the
Fig. 2 – Photograph of the as-ablated C/SiC composites (c, centre; o, outer).
ablated sample. The centre ablation region covers a gray coating. Some macroscopic pores were observed in this region, which can be interpreted as the few erosion of SiC by the gas flow under high temperature (about 1800 ◦ C). The outer ablation region covers a dust-colour ring. The macroscopic appearance of the region besides the ablation region (outside ablation region) did not change. There is no obvious ablation crater on the surface of three-dimensional orthogonal C/SiC composites, indicating the relative homogenous ablation front. Fig. 3 shows XRD patterns of C/SiC before and after ablation. The peaks (at 2 = 36◦ , 42◦ , 60◦ ) were assigned to (1 1 1), (2 0 0), (2 2 0) planes of -SiC according to the standard JCPDS cards (29-1129). The peaks (at 2 = 27◦ , 54◦ ) were assigned to (0 0 3), (0 0 6) planes of graphite (carbon fiber) according to the standard JCPDS cards (26-1079). A small peak at 2 = 26◦ was also found, corresponding to the (0 0 2) diffraction peak of crystalline graphite because some residual carbon existed in the PCS pyrolysis products (Soraru et al., 1990; Lewis et al., 1994; Zhu et al., 2005). The peaks of SiC, SiO2 and residual carbon in ablated composite became stronger than un-ablated composite whereas the peaks of carbon fiber decreased. It indicated that the carbon fibers had been oxidized, the surface SiC and the residual carbon in composite matrix had recrystallized during ablation, part of surface SiC have been oxidized to form SiO2 . The crystal particle size of SiC can be calculated from the Scherrer equation (Klug and Alexander, 1974): D=
0.89 ˇ cos
Fig. 1 – Experimental flow diagram for three-dimensional orthogonal C/SiC composites prepared by PIP.
(1)
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
Fig. 5 – EDS pattern of a white spot in Fig. 4(b). Fig. 3 – XRD patterns of C/SiC composites before and after ablation (a, carbon fiber; r, residual carbon; b, -SiC; c, SiO2 ).
where is the wavelength of the characteristic X-rays, is the Bragg angle, and ˇ is the calibrated width of the half-height of diffraction peaks. The average size of SiC crystals before and after ablation calculated by (1) was 80–100 and 90–120 nm, respectively.
3.1.
Centre ablation region
Fig. 4 shows SEM micrographs of the centre ablation region before and after ablation. It indicated the ablation surface was covered by irregular SiC particles with an average particle size of approximately 10 m. Some micropores were observed between adjacent particles. White spots were also observed on the surface of the irregular particles. Fig. 5 shows EDS pattern of a white spot in Fig. 4(b). It indicated that the white spots were composed of Si and O. Due to its poor oxidation resistance (Buckley, 1988), the carbon fibers on the surface in Fig. 4(a) were oxidized completely after ablation, and the surface of this region was completely covered by SiC. The high temperature caused the recrystallization of the SiC matrix.
Recrystallization of the SiC matrix will consume the thermal energy, which is beneficial to reduced ablation. Meanwhile, the recrystallization of the SiC will improve the hardness of the composite matrix. The coatings were evenly distributed through out the composite of this region and no cracks were observed. Micropores were distributed uniformly between SiC particles and on the surface of individual SiC particles. Formation mechanism of these micropores could be a combinative effect of the expansion of gases inside the closed pores in the sample and the impact of high speed oxyacetylene torch during ablation. The micropores could also be caused by releasing of SiO gas produced by the oxidation of SiC with sufficiently low oxygen partial pressure in the composite (Fritze et al., 1998; Lü et al., 2003). SiC(s) + O2 (g) → SiO(g) + CO(g)
(2)
These micropores are the diffusion passages of oxidizing gases at high temperature. Doubtlessly, the presence of micropores will debase the oxidation resistance of composite. The hems of the micropores were more easily oxidized due to their extensive contact area to thermal oxidizing atmosphere, and it obeyed the following oxidation reaction (Han et al.,
Fig. 4 – SEM micrographs of centre ablation region: (a) before ablation and (b) after ablation.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
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Fig. 6 – SEM micrographs of the outer ablation region of the as-ablated C/SiC: (a) outer ablation region; (b) zone I in (a); (c) zone II in (a); and (d) zone III in (a).
2008): 2SiC(s) + 3O2 (g) → 2SiO2 (l) + 2CO(g)
(3)
Parts of SiO2 were eroded by the high speed airflow, the remaining SiO2 formed spots, adhering to the ablation surface. In centre ablation region, ablation was due to a combination of oxidation, recrystallization of SiC and erosion factors from high temperature, pressure, and velocity of combustion oxyacetylene flame.
3.2.
Outer ablation region
Fig. 6 shows SEM micrographs of the outer ablation region of the as-ablated C/SiC. Fig. 6(a) shows SEM micrograph of outer ablation region of three-dimensional orthogonal C/SiC composites after ablation. There were three zones in this region, e.g., the zone I where the fibers were perpendicular to the surface, the zone II where the fibers were parallel to the surface and the zone III was the SiC matrix. The surfaces were eroded uniformly, and there were no obvious macroscopic pits among the bundles in the ablation tests, indicating the relative homogenous ablation front. Fig. 6(b) shows the zone where the carbon fibers were perpendicular to the surface in outer ablation region. The SiC coating on the composite
surface had been cleared away by the mechanism erosion of the high speed oxyacetylene torch. Parts of SiC were oxidized forming SiO2 with a small volume expansion. The SiO2 not only shielded the inter-bundle pores on the erosion surface from the further attack by the thermal oxidizing atmosphere, but also acted as a barrier to oxygen diffusion due to its low oxygen permeability, leading to the homogeneous erosion (Tang et al., 2006). Due to its poorer oxidation resistance than SiC, the carbon fibers perpendicular to the surface of ablation were eroded, forming the depression structure comparing with the matrix. In zone II (see Fig. 6(c)), the carbon fibers were parallel to the surface in outer ablation region. The fibers were oxidized and fractured into several parts. The lower temperature related to the temperature at the central ablation region led to the mild oxidation in this region. The mismatch of the coefficients of thermal expansion between the fibers and the SiC matrix led to the fracture of fibers. The zone III (see Fig. 6(d)) was the gap among carbon fiber bundles and was filled by the SiC matrix. The surface SiC particles of zone III were in cataclastic shapes, with a particle size less than 10 m. Parts of SiC particles were oxidized by the thermal oxidizing atmosphere formed SiO2 . The formation of this structure could be attributed to the mechanical erosion by high speed oxyacetylene torch after the cataclase of SiC particles induced by the drastic change in temperature
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
Fig. 7 – SEM micrographs of the outside ablation region: (a) low magnification and (b) large magnification of II in (a)
in a short time. Meanwhile, because of the lower temperature here, recrystallization of SiC seems to be insignificant. It can be found that the surface roughness increased during ablation, which will accelerates the mechanism erosion. The increment of surface roughness increases the contact area to thermal oxidizing atmosphere, leads to heat accumulating on composite surface, which intensifies the ablation degree. In outer ablation region, ablation mechanisms mainly were oxidation and mechanism erosion. The ablation mechanism of SiC matrix was different the ablation mechanism of the SiC matrix with additives such as high melting point compound. In ZrB2 –SiC system, the residual oxide was glassy silica-based islands and locally tiny zirconia crystals embedded in the glass after ablation at about 2200 ◦ C (Han et al., 2008). The erosion rate obviously decreases with the HfC addition, indicating that the HfC addition has a positive effect for improvement of the ablation properties of the C/C–1ZrB2 –2SiC system. The effect of ZrB2 and HfC on the ablation behavior was attributed to the formation of the high melting point solid ZrO2 and HfO2 (Tang et al., 2007).
3.3.
Outside ablation region
Fig. 7 shows SEM micrographs of the outside ablation region. The surface of this region also contains three zones, e.g., zone I where the fibers were perpendicular to the surface, zone II where fibers were parallel to the surface and zone III was SiC coating. In zone I, the surface was covered uniformly by spherical SiC grains with an average particle size of approximately 5 m, close to the diameter of the carbon fiber. In zone II, SiC particles agglomerated to form a strip structure along the fiber direction. The strips were nearly 5–10 m wide and 100 m long (see Fig. 7(b)). Oxyacetylene torch test in this region annealed at comparatively lower temperature than centre ablation region in atmosphere. Because of poor wettability between SiC and carbon fibers, recrystallized SiC strips-like formed. Zone III was SiC matrix among carbon fiber yarns. Non-compactness of SiC matrix in this zone caused pits with size close to 260 m × 670 m (see Fig. 7(a)). The white colour resulted from the oxidation of SiC to SiO2 . The
surface of depression structure or convex structure could be easily oxidized due to the high specific surface area with thermal oxidizing atmosphere. Cracks with an average width less than 5 m appeared on the surface of SiC coating. The formation mechanism of surface cracks was the mismatch of coefficient of thermal expansion between SiC matrix and carbon fibers. The crack propagation and deflection were also observed, which had prevented the quick extension of cracks, exhibited high strength of fiber reinforced composite (Carrère et al., 2000). However, the cracks are also diffusion passages of oxidizing gases at high temperature. In the outside ablation region, light oxidation and recrystallization of surface SiC were main erosion mechanism.
4.
Conclusions
(1) The surface of the C/SiC composites was covered by a porous SiC coating with residual carbon after ablation by oxyacetylene torch at 1800 ◦ C. The surface SiC and the residual carbon within C/SiC composites had recrystallized during ablation. (2) There were a lot of micropores on the surface of the ablation region, which resulted from (i) combinative effect of the expansion of gases inside the closed pores in the sample and the impact of high speed oxyacetylene torch during ablation, and (ii) the release of SiO gas produced by the oxidation of SiC with sufficiently low oxygen partial pressure in the composites. (3) In outer ablation region, the carbon fibers were eroded and the SiC matrix was oxidized to SiO2 by the oxyacetylene torch. The increase of the surface roughness of the outer ablation region during ablation accelerated the mechanical erosion.
Acknowledgement This work has been supported by the national natural science foundation of china (50571045/E011002) and the innovation fund of Nanjing University of Aeronautics and Astronautics.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
references
Aitharaju, V.R., Averil, R.C., 1999. Three-dimensional properties of woven-fabric composites. Compos. Sci. Technol. 59 (12), 1901–1911. Beyer, S., Knabe, H., Strobel, F., 1999. Development and testing of C/SiC components for liquid rocket propulsion applications. AIAA99, 2896. Beyer, S., Schmidt, S., Peres, P., Bouchez, M., 2005. Advanced ceramic matrix composite materials for current and future propulsion system applications. AIAA2005, 3644. Buckley, J.D., 1988. Carbon–carbon: an overview. Am. Ceram. Soc. Bull. 67, 364–368. Carrère, N., Martin, E., Lamonb, J., 2000. The influence of the interphase and associated interfaces on the deflection of matrix cracks in ceramic matrix composites. Compos. A: Appl. Sci. Manuf. 31, 1179–1190. Chiu, C.H., Cheng, C.C., 2003. Weaving method of 3D woven preforms for advanced composite materials. Text Res. J. 73 (1), 37–41. D’Aleio, G.F., Parker, J.A., 1971. Ablative Plastics. Marcel Dekker, New York, pp. 1–39. Fritze, H., Jojic, J., Witke, T., Ruscher, Weber, S., Scherrer, S., Weisz, R., Schultrich, B., Borchardt, G., 1998. Mullite based oxidation protection for SiC–C/C composites in air at temperatures up to 1900 K. J. Eur. Ceram. Soc. 18 (16), 2351–2364. Hald, H., Weihs, H., Reimer, T., Ullmann, T., 2003. Development of hot CMC structures for space reentry vehicles via flight experiments. AIAA2003, 2696. Han, J.C., Hu, P., Zhang, X.H., Meng, S.H., Han, W.B., 2008. Oxidation-resistant ZrB2 –SiC composites at 2200 ◦ C. Compos. Sci. Technol. 68 (3–4), 799–806. Kanevce, L.P., Kanevce, G.H., Angelevski, Z.Z., 1999. Comparison of two-kinds of experiments for estimation of thermal properties of ablative composite. In: Proceedings of Third International Conference on Inverse Problems in Engineering: Theory and Practice. ASME, Port Ludlow, Washington, USA, pp. 473–480. Khan, M.B., 1996. An investigation of the ablation behavior of advanced ultrahigh-temperature EPDM/epoxy insulation composites. Polym. Plast. Technol. Eng. 35 (1), 187–206. Klug, H.P., Alexander, L.E., 1974. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials. Wiley, New York, pp. 687–690.
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Lee, Y.J., Joo, H.J., 2004a. Ablation characteristics of carbon fiber reinforced carbon (CFRC) composites in the presence of silicon carbide (SiC) coating. Surf. Coat. Technol. 180–181, 286–289. Lee, Y.J., Joo, H.J., 2004b. Investigation on ablation behavior of CFRC composites prepared at different pressure. Compos. A: Appl. Sci. Manuf. 35 (11), 1285–1290. Lewis, J.A., Cima, M.J., Rhine, W.E., 1994. Direct observation of preceramic and organic binder decomposition in 2D model microstructures. J. Am. Ceram. Soc. 77 (7), 1839–1845. Lü, Z.L., Li, S.B., Gao, J.Q., Jin, Z.H., Li, H.J., 2003. Effects of mullite coating on the anti-oxidation of silicon carbide at high temperature. Rare Metal Mater. Eng. 3 (7), 534–537. Mital, S.K., Murthy, P.L.N., 2001. Characterizing the properties of a C/SiC composite using micromechanics analysis. AIAA2001, 1363. Pan, Y.S., Xu, Y.D., Chen, Z.F., Cheng, L.F., Zhang, L.T., Xiong, D.S., 2006. Ablation properties analysis of 2D C/SiC composites. Ordnance Mater. Sci. Eng. 29 (1), 17–20 (a Chinese Academic Journal). Patterson, M., Leslie, C., Fehrenbacher, L., Koenig, J., Johns, M., 2000. Development and testing of refractory, functionally graded CMCs for high thermal flux applications. AIAA2000, 5148. Song, G.M., Zhou, Y., Wang, Y.J., 2003. Effect of carbide particles on the ablation properties of tungsten composites. Mater. Charact. 50 (4–5), 293–303. Soraru, G.D., Babonneau, F., Mackenzie, J.D., 1990. Structural evolutions from polycarbosilane to SiC ceramic. J. Mater. Sci. 25, 3886–3893. Tan, P., Tong, L., Steven, G.P., 1997. Modeling for predicting the mechanical properties of textile composites: a review. Compos. A: Appl. Sci. Manuf. 28 (11), 903–922. Tang, S.F., Deng, J.Y., Liu, W.C., Yang, K., 2006. Mechanical and ablation properties of 2D-carbon/carbon composites pre-infiltrated with a SiC filler. Carbon 44 (14), 2877–2882. Tang, S.F., Deng, J.Y., Wang, S.J., Liu, W.C., Yang, K., 2007. Ablation behaviors of ultra-high temperature ceramic composites. Mater. Sci. Eng. A 465 (1–2), 1–7. Zhu, S.M., Ding, S.Q., Xi, H.A., Wang, R.D., 2005. Low-temperature fabrication of porous SiC ceramics by preceramic polymer reaction bonding. Mater. Lett. 59, 595–597.
journal homepage: www.elsevier.com/locate/jmatprotec
Effects of ablation at different regions in three-dimensional orthogonal C/SiC composites ablated by oxyacetylene torch at 1800 ◦ C Bo Yan a , Zhaofeng Chen a,∗ , Jianxun Zhu a,b , Jianzhong Zhang b , Yun Jiang b a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Sinoma Science & Technology Co., Ltd., Nanjing 210012, PR China
a r t i c l e
i n f o
Article history:
a b s t r a c t C/SiC composites were prepared by polycarbosilane infiltration pyrolysis and ablated by
Received 10 November 2007
oxyacetylene flame at 1800 ◦ C for 180 s. Morphology and microstructure of C/SiC have been
Received in revised form
studied by scanning electron microscopy/energy dispersive spectroscopy, and X-ray diffrac-
20 July 2008
tion analysis. Two concentric ring regions appeared on the surface of the ablated C/SiC. In
Accepted 3 August 2008
the centre region of the ablated C/SiC was composed of irregular SiC particles with a lot of pores. The pores were resulted from (i) gas expansion in closed pores during ablation, and (ii) from the release of SiO gas produced by the oxidation of SiC with sufficiently low oxygen
Keywords:
partial pressure. The results indicated the ablation was due to a combination of oxidation,
C/SiC composites
mechanical erosion and recrystallization of the surface SiC.
Ablation mechanism
© 2008 Elsevier B.V. All rights reserved.
Oxyacetylene torch
1.
Introduction
Ablation is an erosive phenomenon with a removal of material by a combination of thermomechanical, thermochemical, and thermophysical factors arising from high temperature, pressure, and velocity of combustion flame (D’Aleio and Parker, 1971). Future engine components, in particular combustion chambers and expansion nozzles which are subjected to high thermal loads, need new materials which possess outstanding thermomechanical and thermochemical properties (Beyer et al., 1999). In view of their low specific weight, their excellent resistance to ablation as well as cost effective production, C/SiC composites represent an interesting group of materials for high-temperature components for space propulsion systems (Khan, 1996; Beyer et al., 2005; Mital and Murthy, 2001; Hald et al., 2003). Three-dimensional (3D) woven composites, especially those with orthogonal configurations, have been
∗
Corresponding author. Tel.: +86 25 52112901; fax: +86 25 52112626. E-mail address: zhaofeng [email protected] (Z. Chen). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.08.002
pursued in the aerospace industry primarily to improve the vulnerability of 2D composites to delamination under impact and subsequent local buckling failure under in-plane compression (Tan et al., 1997; Chiu and Cheng, 2003; Aitharaju and Averil, 1999). Before the practical application of C/SiC in these cases, investigation on the evolution of morphology and microstructure during ablation is required. The main experimental methods are oxyacetylene flame ablation, plasma arc ablation, and kerosene–liquid oxygen flame ablation. Oxyacetylene flame ablation method is the simplest and easiest to conduct with the lowest cost. Oxyacetylene flame testing is often used to simulate the rocket engine exhaust flame (Song et al., 2003; Kanevce et al., 1999). So far, ablation research mainly concentrate on optimization of testing environment, C/C composites and 2D C/SiC composites, while studies on ablation morphology and microstructure of three-dimensional C/SiC
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
3439
composites prepared by polycarbosilane infiltration pyrolysis (PIP) are rarely reported (Beyer et al., 1999; Patterson et al., 2000; Lee and Joo, 2004a,b; Tang et al., 2006; Pan et al., 2006). The present paper is to investigate the effects of ablation at different regions in three-dimensional orthogonal C/SiC composites ablated by oxyacetylene torch at 1800 ◦ C.
2.
Experimental
2.1.
Composites fabrication
Fig. 1 shows experimental flow diagram for three-dimensional orthogonal C/SiC composites prepared by PIP. The carbon fiber utilized is M30 (from Toray, Japan), and each yarn contained 3000 fibers. The threedimensional orthogonal preforms were supplied by Nanjing Institute of Glass Fiber, China, with a porosity of 56%. The composite was fabricated by PIP. The infiltration solution was composed of polycarbosilane (PCS) as preceramic and divinylbenzene (DVB) as solvent (mass ratio 1:0.4). Carbon fiber preforms were immersed into the PCS/DVB mixture solution (65 ◦ C, 2 MPa, 30 min) and then dried at 150 ◦ C for 6 h. The ◦
prepregs were pyrolysed at 1200 C in N2 with the heating program of 15 ◦ C/min, holding at 1200 ◦ C for 0.5 h and then naturally cooling inside furnace. The infiltration-pyrolysis process was repeated six times. 2.2.
Ablation experiment
Ablation experiment was conducted using oxyacetylene torch. The sample size of C/SiC composites was 27 mm × 20 mm × 18 mm. The flow rates of O2 and C2 H2 were 0.24 and 0.18 L/s, respectively. The specimen was placed vertically to the flame direction in air. The distance between the nozzle tip of the oxyacetylene gun and the front surface of the specimen was 30 mm and the inner diameter of the tip was 2.5 mm. The surface temperature of the samples was about 1800 ◦ C monitored with an optical pyrometer. The ablation period was 180 s.
2.3.
Sample characterization
The ablation morphology, microstructure and elemental composition of 3D orthogonal C/SiC composites were examined by a scanning electron microscope (SEM, FEI CO., Quanta200). Elemental composition was carried out using energy dispersive spectroscopy (EDS). The phases of the composite was characterized by X-ray diffraction analysis (XRD, Rigaku D/Max-B) using Ni-filtered Cu K␣ radiation at a scanning rate of 0.5◦ /s and scanning from 10◦ to 70◦ of 2.
3.
Results and discussion
Fig. 2 shows photograph of the as-ablated C/SiC composites. Two concentric ring regions (e.g., centre ablation region and outer ablation region) can be observed on the surface of the
Fig. 2 – Photograph of the as-ablated C/SiC composites (c, centre; o, outer).
ablated sample. The centre ablation region covers a gray coating. Some macroscopic pores were observed in this region, which can be interpreted as the few erosion of SiC by the gas flow under high temperature (about 1800 ◦ C). The outer ablation region covers a dust-colour ring. The macroscopic appearance of the region besides the ablation region (outside ablation region) did not change. There is no obvious ablation crater on the surface of three-dimensional orthogonal C/SiC composites, indicating the relative homogenous ablation front. Fig. 3 shows XRD patterns of C/SiC before and after ablation. The peaks (at 2 = 36◦ , 42◦ , 60◦ ) were assigned to (1 1 1), (2 0 0), (2 2 0) planes of -SiC according to the standard JCPDS cards (29-1129). The peaks (at 2 = 27◦ , 54◦ ) were assigned to (0 0 3), (0 0 6) planes of graphite (carbon fiber) according to the standard JCPDS cards (26-1079). A small peak at 2 = 26◦ was also found, corresponding to the (0 0 2) diffraction peak of crystalline graphite because some residual carbon existed in the PCS pyrolysis products (Soraru et al., 1990; Lewis et al., 1994; Zhu et al., 2005). The peaks of SiC, SiO2 and residual carbon in ablated composite became stronger than un-ablated composite whereas the peaks of carbon fiber decreased. It indicated that the carbon fibers had been oxidized, the surface SiC and the residual carbon in composite matrix had recrystallized during ablation, part of surface SiC have been oxidized to form SiO2 . The crystal particle size of SiC can be calculated from the Scherrer equation (Klug and Alexander, 1974): D=
0.89 ˇ cos
Fig. 1 – Experimental flow diagram for three-dimensional orthogonal C/SiC composites prepared by PIP.
(1)
3440
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3438–3443
Fig. 5 – EDS pattern of a white spot in Fig. 4(b). Fig. 3 – XRD patterns of C/SiC composites before and after ablation (a, carbon fiber; r, residual carbon; b, -SiC; c, SiO2 ).
where is the wavelength of the characteristic X-rays, is the Bragg angle, and ˇ is the calibrated width of the half-height of diffraction peaks. The average size of SiC crystals before and after ablation calculated by (1) was 80–100 and 90–120 nm, respectively.
3.1.
Centre ablation region
Fig. 4 shows SEM micrographs of the centre ablation region before and after ablation. It indicated the ablation surface was covered by irregular SiC particles with an average particle size of approximately 10 m. Some micropores were observed between adjacent particles. White spots were also observed on the surface of the irregular particles. Fig. 5 shows EDS pattern of a white spot in Fig. 4(b). It indicated that the white spots were composed of Si and O. Due to its poor oxidation resistance (Buckley, 1988), the carbon fibers on the surface in Fig. 4(a) were oxidized completely after ablation, and the surface of this region was completely covered by SiC. The high temperature caused the recrystallization of the SiC matrix.
Recrystallization of the SiC matrix will consume the thermal energy, which is beneficial to reduced ablation. Meanwhile, the recrystallization of the SiC will improve the hardness of the composite matrix. The coatings were evenly distributed through out the composite of this region and no cracks were observed. Micropores were distributed uniformly between SiC particles and on the surface of individual SiC particles. Formation mechanism of these micropores could be a combinative effect of the expansion of gases inside the closed pores in the sample and the impact of high speed oxyacetylene torch during ablation. The micropores could also be caused by releasing of SiO gas produced by the oxidation of SiC with sufficiently low oxygen partial pressure in the composite (Fritze et al., 1998; Lü et al., 2003). SiC(s) + O2 (g) → SiO(g) + CO(g)
(2)
These micropores are the diffusion passages of oxidizing gases at high temperature. Doubtlessly, the presence of micropores will debase the oxidation resistance of composite. The hems of the micropores were more easily oxidized due to their extensive contact area to thermal oxidizing atmosphere, and it obeyed the following oxidation reaction (Han et al.,
Fig. 4 – SEM micrographs of centre ablation region: (a) before ablation and (b) after ablation.
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Fig. 6 – SEM micrographs of the outer ablation region of the as-ablated C/SiC: (a) outer ablation region; (b) zone I in (a); (c) zone II in (a); and (d) zone III in (a).
2008): 2SiC(s) + 3O2 (g) → 2SiO2 (l) + 2CO(g)
(3)
Parts of SiO2 were eroded by the high speed airflow, the remaining SiO2 formed spots, adhering to the ablation surface. In centre ablation region, ablation was due to a combination of oxidation, recrystallization of SiC and erosion factors from high temperature, pressure, and velocity of combustion oxyacetylene flame.
3.2.
Outer ablation region
Fig. 6 shows SEM micrographs of the outer ablation region of the as-ablated C/SiC. Fig. 6(a) shows SEM micrograph of outer ablation region of three-dimensional orthogonal C/SiC composites after ablation. There were three zones in this region, e.g., the zone I where the fibers were perpendicular to the surface, the zone II where the fibers were parallel to the surface and the zone III was the SiC matrix. The surfaces were eroded uniformly, and there were no obvious macroscopic pits among the bundles in the ablation tests, indicating the relative homogenous ablation front. Fig. 6(b) shows the zone where the carbon fibers were perpendicular to the surface in outer ablation region. The SiC coating on the composite
surface had been cleared away by the mechanism erosion of the high speed oxyacetylene torch. Parts of SiC were oxidized forming SiO2 with a small volume expansion. The SiO2 not only shielded the inter-bundle pores on the erosion surface from the further attack by the thermal oxidizing atmosphere, but also acted as a barrier to oxygen diffusion due to its low oxygen permeability, leading to the homogeneous erosion (Tang et al., 2006). Due to its poorer oxidation resistance than SiC, the carbon fibers perpendicular to the surface of ablation were eroded, forming the depression structure comparing with the matrix. In zone II (see Fig. 6(c)), the carbon fibers were parallel to the surface in outer ablation region. The fibers were oxidized and fractured into several parts. The lower temperature related to the temperature at the central ablation region led to the mild oxidation in this region. The mismatch of the coefficients of thermal expansion between the fibers and the SiC matrix led to the fracture of fibers. The zone III (see Fig. 6(d)) was the gap among carbon fiber bundles and was filled by the SiC matrix. The surface SiC particles of zone III were in cataclastic shapes, with a particle size less than 10 m. Parts of SiC particles were oxidized by the thermal oxidizing atmosphere formed SiO2 . The formation of this structure could be attributed to the mechanical erosion by high speed oxyacetylene torch after the cataclase of SiC particles induced by the drastic change in temperature
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Fig. 7 – SEM micrographs of the outside ablation region: (a) low magnification and (b) large magnification of II in (a)
in a short time. Meanwhile, because of the lower temperature here, recrystallization of SiC seems to be insignificant. It can be found that the surface roughness increased during ablation, which will accelerates the mechanism erosion. The increment of surface roughness increases the contact area to thermal oxidizing atmosphere, leads to heat accumulating on composite surface, which intensifies the ablation degree. In outer ablation region, ablation mechanisms mainly were oxidation and mechanism erosion. The ablation mechanism of SiC matrix was different the ablation mechanism of the SiC matrix with additives such as high melting point compound. In ZrB2 –SiC system, the residual oxide was glassy silica-based islands and locally tiny zirconia crystals embedded in the glass after ablation at about 2200 ◦ C (Han et al., 2008). The erosion rate obviously decreases with the HfC addition, indicating that the HfC addition has a positive effect for improvement of the ablation properties of the C/C–1ZrB2 –2SiC system. The effect of ZrB2 and HfC on the ablation behavior was attributed to the formation of the high melting point solid ZrO2 and HfO2 (Tang et al., 2007).
3.3.
Outside ablation region
Fig. 7 shows SEM micrographs of the outside ablation region. The surface of this region also contains three zones, e.g., zone I where the fibers were perpendicular to the surface, zone II where fibers were parallel to the surface and zone III was SiC coating. In zone I, the surface was covered uniformly by spherical SiC grains with an average particle size of approximately 5 m, close to the diameter of the carbon fiber. In zone II, SiC particles agglomerated to form a strip structure along the fiber direction. The strips were nearly 5–10 m wide and 100 m long (see Fig. 7(b)). Oxyacetylene torch test in this region annealed at comparatively lower temperature than centre ablation region in atmosphere. Because of poor wettability between SiC and carbon fibers, recrystallized SiC strips-like formed. Zone III was SiC matrix among carbon fiber yarns. Non-compactness of SiC matrix in this zone caused pits with size close to 260 m × 670 m (see Fig. 7(a)). The white colour resulted from the oxidation of SiC to SiO2 . The
surface of depression structure or convex structure could be easily oxidized due to the high specific surface area with thermal oxidizing atmosphere. Cracks with an average width less than 5 m appeared on the surface of SiC coating. The formation mechanism of surface cracks was the mismatch of coefficient of thermal expansion between SiC matrix and carbon fibers. The crack propagation and deflection were also observed, which had prevented the quick extension of cracks, exhibited high strength of fiber reinforced composite (Carrère et al., 2000). However, the cracks are also diffusion passages of oxidizing gases at high temperature. In the outside ablation region, light oxidation and recrystallization of surface SiC were main erosion mechanism.
4.
Conclusions
(1) The surface of the C/SiC composites was covered by a porous SiC coating with residual carbon after ablation by oxyacetylene torch at 1800 ◦ C. The surface SiC and the residual carbon within C/SiC composites had recrystallized during ablation. (2) There were a lot of micropores on the surface of the ablation region, which resulted from (i) combinative effect of the expansion of gases inside the closed pores in the sample and the impact of high speed oxyacetylene torch during ablation, and (ii) the release of SiO gas produced by the oxidation of SiC with sufficiently low oxygen partial pressure in the composites. (3) In outer ablation region, the carbon fibers were eroded and the SiC matrix was oxidized to SiO2 by the oxyacetylene torch. The increase of the surface roughness of the outer ablation region during ablation accelerated the mechanical erosion.
Acknowledgement This work has been supported by the national natural science foundation of china (50571045/E011002) and the innovation fund of Nanjing University of Aeronautics and Astronautics.
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