High-Strength Reaction-Sintered Silicon Carbide for Large-Scale Mirrors - Effect of surface oxide layer on bending strength -

Transcription

1 Advances in Science and Technology Vol. 63 (2010) pp Online available since 2010/Oct/27 at (2010) Trans Tech Publications, Switzerland doi: / High-Strength Reaction-Sintered Silicon Carbide for Large-Scale Mirrors - Effect of surface oxide layer on bending strength - Shoko Suyama 1, a and Yoshiyasu Itoh 1, b 1 Power and Industrial Systems R&D Center, TOSHIBA Corporation 2-4 Suehiro-cho, Tsurumi-ku, Yokohama, , Japan a shoko.suyama@toshiba.co.jp, b yoshiyasu.ito@toshiba.co.jp Keywords: reaction-sintering, silicon carbide, full dense, high-strength, surface oxide layer, good shape capability, low processing temperature, lightweight, mirror substrate Abstract. Reaction-sintered silicon carbide of 800 MPa class bending strength had been newly developed. The developed silicon carbide showed good rigidity, high thermal conductivity, and high density, like a conventional sintered silicon carbide. The developed silicon carbide is one of the most attractive materials for large-scale ceramic structures because of its low processing temperature, good shape capability, low-cost processing and high purity. We had fabricated some lightweight space mirrors, such as a high-strength reaction-sintered silicon carbide mirror of 650 mm in diameter. In this study, experiments were conducted to investigate the effect of annealing on the bending strength of high-strength reaction-sintered silicon carbide. The annealing heat treatments were carried out at 1073 K, 1273 K, and 1473 K in an air atmosphere. The maximum bending strength of 1091 MPa at room temperature was achieved by the annealing heat-treatment at 1273 K for 10 h in air. We confirmed that annealing heat treatment was effective to improve the bending strength of reaction-sintered silicon carbide by inducing compressive residual stress at the surface oxide layer. Introduction Silicon carbide (SiC) is a highly suitable material for various telescope mirrors because of its high stiffness, low density, low coefficient of thermal expansion, high thermal conductivity and superior thermal stability. From this point of view, sintered silicon carbide (S-SiC) has been considered to show very attractive characteristics. After more than ten years of development and characterization, S-SiC telescope mirrors have been manufactured by BOOSTEC, S.A. [1, 2]. Also, a ceramic matrix composite (C/SiC) manufactured by ECM has remarkable mechanical properties and lightweight performance [3, 4]. Densification of the surface layer of the C/SiC substrate by use of CVD-SiC etc. is necessary for its application to space mirrors. A rapid process of fabricating lightweight SiC optical mirrors has been developed by POCO [5]. The chemically converted SiC from graphite is clad with CVD-SiC to yield low surface roughness values after final polishing. On the other hand, we have newly developed a high-strength reaction-sintered silicon carbide for various telescope mirrors [6, 7], which is an attractive material for lightweight optical mirrors, having a bending strength over two times higher than the other SiC-based materials. The polished surface has no pores and is suited to the visible region as well as the infrared region without CVD-SiC coating. Also, the fabrication process, with low sintering temperature and small sintering shrinkage, is also suited to the fabrication of large-scale objects, such as telescope mirrors. For a long time, SiC has been recognized as an excellent material for high-performance optical applications because it offers many advantages in comparison with other commonly used materials, such as glass (Zerodure) and beryllium. Sintered SiC shows extremely high specific rigidity, E/ρ (E: Young s modulus, ρ: density), and high thermal stability, α/λ (α: coefficient of thermal expansion, λ: thermal conductivity). A comparison of these properties between high-strength reaction-sintered silicon carbide and other common optical materials is shown in Table 1. It is clear that high-strength reaction-sintered silicon carbide has two times higher bending strength than conventional sintered SiC (S-SiC) [6, 7]. Thus, we have considered that high-strength reaction-sintered silicon carbide All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, USA-10/03/15,05:59:58)

2 Advances in Science and Technology Vol would be suitable for developing even more lightweight optical telescopes in future. The bending strength of high-strength reaction-sintered silicon carbide showed the world s highest value (>800 MPa) at present, and the specific strength is as large as that of FRP (fiber-reinforced plastic) and beryllium. Also, the basic material properties of high-strength reaction-sintered silicon carbide are shown in Table 1, together with those of other representative mirror materials. The good mechanical and thermal properties of high-strength reaction-sintered silicon carbide, with excellent homogeneity, make it a promising material for various telescope mirror substrates. Moreover, the well-balanced performances of high-strength reaction-sintered silicon carbide make it cost-effective in comparison with beryllium which is toxic, the ultra-lightweight ULE/Zerodure and the conventional sintered SiC. We have been developing the mirrors as shown in Fig. 1. In light of the issues described above, high-strength reaction-sintered silicon carbide is considered to be an excellent candidate for large space mirrors in future, and we have developed high-strength reaction-sintered silicon carbide products, such as mirrors and their structural components, by optimizing the processing parameters from three points of view: The first point was to realize a lightweight mirror by reducing the thickness of the green body; the second point was to increase the mirror size; and the last point was to achieve material homogeneity for maintaining reliability of the mirror substrate. Figure 1 shows the development of large high-strength reaction-sintered silicon carbide mirror. At the beginning of development, the applications were limited to small mirrors, such as oval mirrors of dimensions 160 mm 220 mm, as shown in Fig. 1. Also, we developed a flat mirror with a diameter of 250 mm. The weight of kg corresponds to an areal density of 21.4 kg/m 2. Various characteristics of high-strength reaction-sintered silicon carbide, such as mechanical and thermal properties, could be obtained using the flat mirror. At the second stage of development, we attempted to fabricate a flat mirror with a diameter of 400 mm and a rib height of 80 mm. The weight of 6.01 kg corresponds to an areal density of 47.8 kg/m 2. At the third stage of development, a high-strength reaction-sintered silicon carbide mirror substrate with a diameter of 650 mm was developed as a demonstration model for space telescope applications. The weight of 10.4 kg corresponds to an areal density of 21.4kg/m 2. Various properties, such as the density, bending strength, coefficient of thermal expansion, Young s modulus, Poisson s ratio, and fracture toughness, were measured using test pieces cut from the fabricated mirror substrates. From the results, we confirmed the material homogeneity of each high-strength reaction-sintered silicon carbide mirror. The most important factors that limit the applications of silicon carbide-based ceramics are their low strength and brittleness. Much attention has been paid to design starting compositions in order to improve the strength and fracture toughness. Recently, the possibility of annealing heat-treatment of silicon carbide ceramics has been investigated to improve the bending strength and fracture toughness. Annealing heat-treatment have been carried out at different temperatures and holding times to evaluate the possibility of a further improvement of the material properties. Attention has been focused on the micro-structural modifications induced by heat-treatment, such as the formation of elongated grains, reduction and modification of secondary amorphous and crystalline phases. The mechanical properties of the annealed materials were explained on the basis of micro-structural changes and compared to those of the as-sintered materials. Micro-structural changes of the grain boundary phase were found to be the main factor affecting such improvements [13, 14, 15]. On the other hand, it is well-known that the strength of silicon carbide is strongly affected by the surface finish. In case of telescope mirror applications, there is no problem with regard to the surface finish of the mirror-polished area. However, the strength tends to be reduced at non-polished areas, such as the back faces of mirrors and other structural components. From this viewpoint, we investigated the effect of annealing heat-treatment on the bending strength of high-strength reaction-sintered silicon carbide, focusing on the strengthening mechanism by annealing due to compressive residual stress. In this paper, we describe the well-defined and cost efficient reaction-sintered silicon carbide technology, which presents high growth potential for the future. In particular, we confirmed the effect of annealing heat treatment on the bending strength of high-strength reaction-sintered silicon carbide.

3 376 12th INTERNATIONAL CERAMICS CONGRESS PART B Table 1 Material characteristic of newly developed high-strength reaction-sintered silicon carbide and other mirror materials. Density, g/cm 3 Highstrength RS-SiC 3.03 Be 1.84 Zerodure 2.52 S-SiC 3.10 Young s modulus, GPa Bending strength, MPa Coefficient of thermal expansion, ppm/k Thermal conductivity, W/mK Specific heat capacity, J/K/kg Specific rigidity, E /ρ Thermal stability, α / λ mm in diameter 250mm in diameter 400mm in diameter 650mm in diameter Fig. 1 History of development of large high-strength reaction-sintered silicon carbide mirrors, and 650 mm diameter high-strength reaction-sintered silicon carbide mirror that can be fabricated.

4 Advances in Science and Technology Vol Experiments A flowchart of fabrication process of a high-strength reaction-sintered silicon carbide mirror is shown in Fig. 2 [6, 7]. Silicon carbide (SiC) and carbon (C) powders are used as the starting materials. The carbon powder, SiC powder, and some dispersant are mixed and spray-dried. The green body is formed by cold isostatic pressing and is machined to form the prescribed mirror shape. The machined green body is reaction-sintered at about 1700 K in vacuum by contact with molten silicon. The green body infiltrated with the molten silicon turns to β SiC by the reaction with the original carbon powder, and non-reacted silicon remains as a residual material in the reaction-sintered SiC. The reaction-sintered SiC is mechanically finished to form the final shape, such as a mirror substrate. It has been already confirmed that high-strength reaction-sintered silicon carbide has no open pores [6, 7]. This means that high-strength reaction-sintered silicon carbide substrates without a CVD-SiC coating can provide a good optical surface for visible applications. Also, since the porosity is nearly zero, there is almost no shrinkage (less than 0.5%) during the reaction-sintering process, which makes it easy to fabricate large-scale structures. Also, the sintering temperature of high-strength reaction-sintered silicon carbide is about 1700 K in a vacuum atmosphere. This sintering temperature is significantly lower than that required to achieve high density in conventional sintered SiC, which is about K [6, 7]. Experiments were conducted to investigate the effect of annealing on the bending strength of high-strength reaction-sintered silicon carbide. Starting SiC powder with a particle size of 1 μm and C powder with a particle size of 0.3 μm were used in this experiment. The composition of the starting material (C/SiC ratio) was 0.5. Other conditions for fabricating high-strength reaction-sintered silicon carbide were the same as described above. The cross-sections of the specimens were examined by a scanning electron micrograph (HITACH/S-5000) after annealing heat treatment. The annealing heat treatments were carried out at 1073 K, 1273 K, and 1473 K for 1 hour in an air atmosphere. Four-point bending specimens, which were 4 mm wide, 3 mm thick, and 40 mm long, were tested at room temperature (296 K) and a displacement speed of 0.5 mm/min. The volume fraction of residual Si in the specimens was calculated from the measured density of the sintered body and the theoretical densities of Si and SiC. The mean size of residual Si was measured by the mercury intrusion method following heat treatment [8]. Briefly, the sintered body was heated to 1873 K in a vacuum atmosphere to vaporize the residual Si, and the mean pore size in the specimens was measured using the mercury intrusion method. SiC powder + Si C powder SiC+C SiC+C Si/SiC Mixing & palletizing Cold iso-static pressing Green machining Reaction sintering (before) Reaction sintering (after) Machining Fig. 2 Fabrication process of high-strength reaction-sintered silicon carbide.

5 378 12th INTERNATIONAL CERAMICS CONGRESS PART B Effect of annealing heat-treatment on bending strength Microstructure of oxide layer induced by annealing. Figure 3 shows typical electron micrograph of high-strength reaction-sintered silicon carbide of 800 MPa class bending strength. We selected the conditions for the annealing heat-treatment without any change of microstructure. Figure 4 shows SEM images of cross-sections of the oxide layer formed at the surface of high-strength reaction-sintered silicon carbide after annealing at 1273 K for 10 h in air. The dense oxide layer can be clearly observed. The thickness of the oxide layer formed on the SiC particles was about 40 nm. A thicker oxide layer of about 100 nm in thickness was formed on the residual Si area. These tendencies were almost the same as in the other annealed specimens. The thickness of oxide layer increased with increasing annealing temperature. The thin oxide layer was found to be composed of silicon and oxygen by Auger electron spectroscopy (AES) analysis. It was also confirmed from X-ray diffraction patterns that the thin oxide layer did not show the clear peak of cristobalite (SiO 2 ). It was assumed that the thin oxide layer of high-strength reaction-sintered silicon carbide consists of the amorphous phase after annealing at 1273 K for 10 h in air. Fig. 3 Typical transmission electron micrograph of high-strength reaction-sintered silicon carbide. Thickness of oxide layer Thickness of oxide layer 1μm 100nm Fig. 4 Scanning electron micrograph observation of the oxide layer formed at the surface of high-strength reaction-sintered silicon carbide after annealing heat treatment (annealing temperature: 1273 K, holding time: 10 h).

6 Advances in Science and Technology Vol Residual stress analyses induced by forming oxide layer. An infinite high-strength reaction-sintered silicon carbide plate sandwiched between oxide layers was used as the analysis model of residual stress. The model was assumed to be a perfect elastic body. We used the model to determine the residual stress generated by the temperature difference when the three-layered model described above was heated uniformly at the annealing temperature and then cooled down to room temperature. The analysis of residual stress induced by annealing was carried out using the analytical solution of an infinite plate based on strain suppression and Timoshenko s beam theory [16]. In this analysis, the thermal expansion in the thickness direction was not taken into account. The residual stress distributions obtained by this method showed good agreement with the results of finite element method analysis in the case where the thickness of each layers was sufficiently thin in comparison with the other dimensions of width and length. The material constants used for the residual stress analysis were measured by using specimens machined from high-strength reaction-sintered silicon carbide and sintered SiO 2. Thermal expansion coefficients (SiO 2 : /K, SiC: /K) were measured using specimens of 5 mm in diameter and 15 mm in length. Mechanical properties, such as Young s modulus (SiO 2 : 72 GPa, SiC: 362 GPa) and Poisson s ratio (SiO 2 : 0.17, SiC: 0.18), were measured in a bending test with strain gages using specimens of 1 mm in thickness, 5 mm in width, and 50 mm in length. Figure 5(a), (b) shows the analytical results of the effect of the annealing temperature and the thickness of the oxide layer on residual stress. Compressive residual stress is induced at the surface oxide layer, and tensile residual stress is induced at the high-strength reaction-sintered silicon carbide substrate by the annealing heat treatment, because the thermal expansion coefficient of the oxide layer is smaller than that of the high-strength reaction-sintered silicon carbide. The compressive residual stress induced at the oxide layer tends to become higher with decreasing thickness of the oxide layer. In contrast, the tensile residual stress induced at the substrate becomes lower with decreasing thickness of the oxide layer. Both the compressive and the tensile residual stresses become higher with increasing annealing temperature. If it is possible to effectively use the compressive residual stress induced at the oxide layer, it should be possible to improve the bending strength of high-strength reaction-sintered silicon carbide. Improvement of bending strength by annealing heat treatment. Figure 6 shows the effect of annealing temperature on the four-point bending strength of high-strength reaction-sintered silicon carbide. The experimental results are plotted in the figure, and black circles show the mean values at each annealing temperature. The maximum bending strength of high-strength reaction-sintered silicon carbide was obtained by the annealing heat-treatment at 1273 K for 10 h in air. However, the four-point bending strength of high-strength reaction-sintered silicon carbide reduced with increasing annealing temperature above 1273 K. The solid curve in Fig. 6 is the four-point bending strength of high-strength reaction-sintered silicon carbide estimated by taking into consideration the compressive residual stress induced at the oxide layer. The thickness of oxide layer was measured by the SEM observation, as shown in Fig. 4, to estimate the bending strength. By comparing the experimental results and the estimated ones, the improvement of bending strength by annealing can be explained by the effect of compressive residual stress. On the other hand, the experimental results are not in agreement with the estimated ones above 1273 K. Though there is a tendency for the surface roughness of annealed high-strength reaction-sintered silicon carbide to increase with increasing annealing temperature, we would like to give convincing evidence of the strength reduction in the near future. A Weibull plot of the four-point bending strength of high-strength reaction-sintered silicon carbide is shown in Fig. 7 in comparison with the high-strength reaction-sintered silicon carbide annealed at 1273 K for 10 h in air. The high-strength reaction-sintered silicon carbide showed a mean four-point bending strength of 807 MPa (m = 9.6) at room temperature. As shown in Fig. 6, we confirmed that the mean four-point bending strength was improved to 1091 MPa (m = 9.6) at room temperature by the annealing heat-treatment.

7 380 12th INTERNATIONAL CERAMICS CONGRESS PART B 500 Residual stress, -σ o /MPa σ o Substrate Oxide σ s Thickness of oxide layer, t/mm 1573K 1473K 1373K 1273K 1173K 1073K (a) K 1473K 1373K 1273K 1173K 1073K 50 σ o Substrate Oxide σ s Thickness of oxide layer, t/mm (b) Fig. 5 Effect of annealing temperature and the thickness of oxide layer on residual stress analyzed by the analytical solution of an infinite plate based on strain suppression and Timoshenko s beam theory. (a) Compressive residual stress induced surface oxide layer (b) Tensile residual stress induced at substrate of high-strength reaction-sintered SiC

8 Advances in Science and Technology Vol Bending strength, σ B /MPa Estimated curve Holding time:10h Annealing temperature, T/K Fig. 6 Effect of annealing temperature on bending strength at room temperature, and estimated curve based on the effect of compressive residual stress induced by annealing treatment. Fracture probability (%) MPa(m=9.6) Annealing 1091MPa(m=10.4) point bending strength (MPa) Fig. 7 Weibull plots of bending strength of high-strength reaction-sintered silicon carbide in comparison with high-strength reaction-sintered silicon carbide with annealing heat treatment (annealing temperature: 1273 K, holding time: 10 h).

9 382 12th INTERNATIONAL CERAMICS CONGRESS PART B Conclusions Newly developed high-strength reaction-sintered silicon carbide, which has two times higher strength than conventional sintered SiC, is one of the most promising candidate materials for the lightweight substrates of optical mirrors, because of its fully dense structure, low weight, small sintering shrinkage (0.5 %), good shape capability, and low processing temperature. In order to improve the performance of lightweight high-strength reaction-sintered silicon carbide mirrors, experiments were conducted to investigate the effect of annealing on the bending strength of high-strength reaction-sintered silicon carbide. The annealing heat treatments were carried out at 1073 K, 1273 K, and 1473 K in an air atmosphere. The maximum bending strength of high-strength reaction-sintered silicon carbide was obtained by the annealing heat-treatment at 1273 K for 10 h in air. We confirmed that annealing heat treatment was effective to improve the bending strength of reaction-sintered silicon carbide by inducing compressive residual stress at the surface oxide layer. Annealing heat-treatment was found to be effective in improving the strength of the non-polished surfaces of high-strength reaction-sintered silicon carbide by inducing compressive residual stress. References [1] J. Breysse, D. Castel, B. Laviron, D. Logut, M. Bougoin, Proc. of 5 th ICSO, Toulouse, S5 (2004). [2] M. Bougoin, P. Deny, Proc. of SPIE, Vol.5494, Bellingham (2004) p.9. [3] M. Krödel, J. Lichtsheindl, Proc. of SPIE, Vol.5494, Bellingham (2004) p.19. [4] M. Krödel, Proc. of SPIE, Vol.5494, Bellingham (2004) p.297. [5] J. S. Johnson, K. Grobsky, D. J. Bray, Proc. of SPIE, Vol.4771 (2002) p.243. [6] K. Tsuno, H. Irikado, K. Hamada, K. Ohno, J. Ishida, S. Suyama, Y. Itoh, N. Ebizuka, H. Eto, Y. Dai, Proc. of 5 th ICSO, Toulouse, S5 (2004) p.681. [7] K. Tsuno, H. Irikado, K. Ohno, J. Ishida, S. Suyama, Y. Itoh, N. Ebizuka, H. Eto, Y. Dai, W. Lin, T. Suzuki, H. Omori, Y. Yui, T. Kimura, Y. Tange, Proc. of SPIE, Asian-Pacific RS (2004). [8] S. Suyama, T. Kameda, Y. Itoh, Diamond and related materials, 12, (2003) p [13] J. Korous, M. C. Chu, M. Nakatani, K. Andoh, J. Am. Ceram. Soc., 83-11(2000) p [14] D. Sciti, S. Guicciardi, A. Bellosi, J. Euro. Ceram. Soc., 21(2001) p.621. [15] G. Zhan, M. Mitomo, H. Tanaka, J. Am. Ceram. Soc. 83-6(2000) p [16] Y. Itoh, M. Takahashi, H. Takano, Fusion Eng. and Design, 31(1996) p.279.

10 12th INTERNATIONAL CERAMICS CONGRESS PART B / High-Strength Reaction-Sintered Silicon Carbide for Large-Scale Mirrors - Effect of Surface Oxide Layer on Bending Strength / DOI References [8] S. Suyama, T. Kameda, Y. Itoh, Diamond and related materials, 12, (2003) p doi: /s (03)