Polymer impregnation and pyrolysis (PIP) is one of the most attractive fabrication processes for silicon carbide (SiC) composites due to the shape flexibility, mass ...
Materials Transactions, Vol. 45, No. 2 (2004) pp. 307 to 310 #2004 The Japan Institute of Metals
Effect of Heat Treatment on Microstructure and Mechanical Properties of Stoichiometric SiC/SiC Composites*1 Takashi Nozawa, Kazumi Ozawa*2 , Yutai Katoh*3 and Akira Kohyama Institute of Advanced Energy, Kyoto University, Kyoto 611-0011, Japan Polymer impregnation and pyrolysis (PIP) is one of the most attractive fabrication processes for silicon carbide (SiC) composites due to the shape flexibility, mass production and relatively low cost. In particular, advanced PIP SiC/SiC composite with high-crystallinity and nearstoichiometric composition is expected to have superior thermo-mechanical properties including good oxidation resistance, due to the reduction of impurities and well-organized crystal structure. Additionally, by applying a thin carbon interphase by chemical vapor infiltration (CVI), control of the crack propagation and oxidation resistance are also achieved. In this study, a CVI + PIP hybrid process based on the recently developed stoichiometric PIP process was performed. Specifically, matrix crystallization was enhanced by heat treatment in Ar, and its effect on microstructures and mechanical properties were evaluated. Stoichiometric SiC/SiC composites exhibit superior flexural strength up to 1573 K in Ar and 1273 K in air. This is because the stoichiometric composition in PIP-SiC matrix reduces inner oxidation by impurities. Also, the thin pyrolytic carbon interphase tailored by CVI process effectively controls crack propagation at fiber and matrix interphase. In a similar manner, the microstructure of the stoichiometric PIP-SiC matrix, constructed by the mixture of amorphous SiC and highly crystalline SiC, was stable against the high-temperature heat exposure up to 1773 K in Ar. In particular, stoichiometric SiC/SiC composites heated at 1773 K in Ar provides superior stability of mechanical properties up to 1573 K even in air atmosphere, although extensive crystallization, in the case of heat treatment at 1973 K in Ar, caused brittle composite fracture. (Received November 4, 2003; Accepted December 17, 2003) Keywords: silicon carbide, ceramic matrix composites, hybrid process, polymer impregnation and pyrolysis process, chemical vapor infiltration process, heat treatment, flexural properties
1.
Introduction
In the development of ceramic matrix composites (CMCs), oxidation protection is critical for high-temperature, oxidative environments. Silicon carbide (SiC) and its composite (SiC/SiC) are considered to have inherent oxidation resistance and are considered for various applications such as energy and space industries.1,2) For near short application, it is very important to identify the oxidation mechanism; passive and active oxidation and passive/active transition under the practical environments, and to explore the way to improve thermo-mechanical properties for structural applications. In recent years, various approaches such as improvements of constituents, process optimization, thermal and environmental barrier coatings (TBC/EBC) have been pointed out to address this issue.3–6) From the viewpoint of development of composite constituents, tailoring the high-crystallinity and near-stoichiometry composition is essential for the improvement of oxidation resistance. Actually, Hi-NicalonTM Type-S and TyrannoTM -SA fibers, recently developed highly crystalline and near-stoichiometric SiC fibers, exhibited excellent thermo-mechanical properties.3,4) In particular, the latter one is known to have superior oxidation resistance up to 2273 K in Ar and 1773 K in air. From these facts, this approach should be beneficial to SiC matrix. Chemical vapor infiltration (CVI) technique is one of the novel processes to endow the high-purity and high-crystallinity -SiC matrix. Additionally, CVI process has a big *1This Paper was Presented at the Autumn Meeting of the Japan Institute of
Metals, held in Sapporo, on October 11, 2003 Student, Kyoto University *3Present address: Metals and Ceramics Division, Oak Ridge National Laboratory, TN 37831-6138, USA *2Graduate
advantage to form thin pyrolytic carbon (PyC) fiber/matrix (F/M) interphase. Thinner PyC is revealed to have good oxidation resistance, and good interfacial bonding and friction in SiC/SiC composite system.5,7) This should also be an important advantage in materials tailoring. However, CVI SiC/SiC had large pores among lay-up fabric sheets becoming a main diffusion path for oxygen. This is an obvious disadvantage in the high-temperature application. The other disadvantages are less shape flexibility and mass producibility due to the restriction of equipments and difficulty in uniform gas and temperature control in the preform body. On the contrary, recently developed advanced polymer impregnation and pyrolysis (PIP) process with stoichiometric composition offers several advantages in shape variety; nearnet shaping, matrix densification, application as composite joint and low cost.8) However, amorphous based SiC matrix is not appropriate in the high-temperature applications. Therefore, improvement of crystallinity in SiC matrix is fundamental issue in stoichiometric PIP process. In this study, hybrid process with CVI and stoichiometric PIP was designed and fundamental properties of this composite were evaluated in order to optimize the PIP process. Specifically, matrix crystallinity was controlled by the heat treatment and its effect on microstructures and mechanical properties were investigated. 2.
Experimental
2.1 Materials A stoichiometric SiC/SiC composite was fabricated at Ube Industries, Co. Ltd. in the following steps. First, reinforced fiber bundles using Si-Al-C fibers (TyrannoTM SA, Ube Industries, Co. Ltd.) were woven into the orthogonal
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three-directional fabrics (X:Y:Z = 1.0:1.0:0.2). Second, 100 nm PyC and 500 nm outer SiC shells were formed by the CVI method to tailor the multi-function F/M interphase. Third, 10 repeated PIP cycles were carried out with pyrolysis at 1573 K. The source of the PIP-SiC matrix was the copolymer composed of the mixture of polycarbosilane (PCS) and polymethylsilane (PMS) dissolved in 50 mass% of toluene solution. This process resulted in near-stoichiometry and high-purity PIP-SiC matrix (C/Si = 1:02 1:04, O2 : 1 2 mass%). In order to improve crystallinity of stoichiometric PIP-SiC matrix, SiC/SiC composite plates, which were cut from bulk composite plate, were annealed at 1773 and 1973 K in flowing commercial argon gas of 2 l/min for 1 h, respectively. In the following parts, heat-treated composites are described as HT1773 and HT1973, respectively, with the as-received composite symbolized as NHT. 2.2 Flexural test Three-point flexural tests at room- and elevated-temperatures were conducted as specified in ASTM C1341. Specimen size was chosen to be L 25.0 W 4.0 T 2.0 mm and an 18.0 mm support span was used. Flexural tests were controlled by the displacement with the constant rate of 0.5 mm/min (strain rate: 3:0 10 4 s 1 ). In high-temperature tests, two atmospheres: air and commercial argon with total pressure of 10 Pa (Ar: 99.998 vol%, N2 :
