J. Aum Ceram. Sac, 85 [7 1815-22(2002) ournal Developing Interfacial Carbon-Boron-Silicon Coatings for Silicon Nitride- Fiber-Reinforced Composites for Improved Oxidation Resistance Kiyoshi Sato, f Hiroki Morozumi, Osamu Funayama, Hiroshi Kaya, and Takeshi Isoda Tonen General Sekiya, Iruma-gun, Saitama 356-8500, Japan C-B-Si coatings were formed on a Sin fiber using chemical deposition(CVD). That work has demonst vapor deposition and embedded in a Si-N-C matrix using oxidation resistance on single-filament layer was anticipated to form borosilicate glass containing only-a microcomposite oxygen-diffusion s. Two types of C-B-Si coatings were A Bn and oxide layer has been investigated elsewhere as a tested on the fiber-matrix interface, and they improved the ubstitute for the carbon layer. h-Bn has a layered crystal multilayered:a crystalline sublayer composed of B-Si-C was between its crystal layers. A BN layer is useful for controlling SiC-fiber-reinforced Sic and SiC-fiber-reinforced glass. 2The second coating was a graphitelike carbon layer containing a oxidation starting temperature of BN is only 100 K higher than small amount of boron and silicon. The carbon(sub)layer of both coatings weakened the fiber-matrix bonding, giving the carbon, but BN is expected to form B, O,, which seals th composites a high flexural strength(l1 GPa). The composites ygen-diffusion passes. A composite with a Bn layer shows retained 60%0-70% of their initial strength, even after oxida oxidation resistance to >1100 K, but it experiences embrittlement tion at 1523 K for 100 h. The mechanism for improved oxidation at intermediate temperatures, 900-1 100 K, because of the active resistance was discussed through the microstructure of the xidation of E nterface, morphology of the fracture surface, and oxygen An interfacial oxide coating offers the considerable advantage distribution on a cross section of the oxidized composite. of causing no oxidation degradation. However, an oxide interface is not adequate for a non-oxide fiber, because the oxide layer diffuses oxygen to the fiber. Recently, all-oxide composites . Introduction have been actively studied because of their stability under an oxidizing atmosphere. -However, among the disadvantages of F ER-REINFORCED ceramic composites show promise for over. all-oxide composites is high-temperature creep above -1500 K, which results from an intrinsic property of ordinal oxides. Thus, the bonding between fiber and matrix is important to giving a the problem of improving the oxidation resistance of ceramic composite strength and toughness, because weak bonding prevents a matrix crack from propagating to a fiber. A carbon layer has composites has remained unsolved. The present study focused on a non-oxide composite, because the carbon layer oxidizes above -700 K in air, limiting the the high strength of such a composite at high temperature improves oxidation resistance of the composite , Two approaches have oxidation resistance. A C-B-Si coating was produced using CVD been investigated as a solution to this problem:()preventin ea at oxidation of the carbon layer; and(ii)replacing the carbon lay the C-B-Si layer would form a borosilicate glass, sealing the with an oxidation-resistant layer oxygen-diffusion passes, as had the BN,and boron-containing Various studies have investigated preventing oxidation of the carbon layers, and(ii) controlling the B: Si ratio in the C-B-Si carbon layer. The addition of boron in a matrix or an interface layer would change the softening temperature and viscosity of the layer, to form a B2O3 or borosilicate-glass seal at low tempera- borosilicate glass, improving the performance of the seal. The tures, has been studied. The oxygen-diffusion passes that must be ternary system C-B-Si was selected to form a carbon-rich phase, ealed are the matrix cracks and interfacial gaps resulting from the which would cause fiber-matrix debonding. A Si-B-N layer could oxidation loss of the carbon layer. The addition of a boride powde function as C-B-Si if a h-BN sublayer was formed, but a nitride to the matrix partially prevents oxidatio coating was more difficult than a carbon coating to fabricate usi Jacques et al. lo have produced an interface layer of 20 mol% CVD. a polymer-impregnation and pyrolysis(PIP)method- boron-containing carbon on a SiC fiber using chemical vapor vas u sed in the present study to fabricate the composite. The advantage of the PIP method was that it could make use of shaping techniques from the industrial process for fiber-reinforced plastics T.A. Parthasarathry-contributing editor However, improving the oxidation resistance of PlP composites was difficult because of open pores and matrix cracks, formed by pyrolysis shrinkage of the polymer, that diffused oxygen into th composite. The present method of providing oxidation resistance Manuscript No. 188719. Received February 25, 2000; approved September 10, to PIP composites should be effective on other porous composite orted by the Ministry of Economy, Trade, and Industry, and work conducted such as reaction-bonded composites and pore-free composites such as chemical vapor infiltrated composites dence should be addressed. Now with Advanced This paper first describes the properties of a C-B-Si coating on Materials International Co, Fuji, Shizuoka 416-0946, Japan. Kanagawa 234-0192, Japan. echnical Center, Nissan Motor Co, Ltd, Atsugi, Si3N4 fiber. Results for the investigation of composites with C-B-Si interfacial layer are described in terms of the fiber-matrix Formerly Tonen Corporation interface microstructure, mechanical properties, and oxidation 1815Developing Interfacial Carbon-Boron-Silicon Coatings for Silicon Nitride-Fiber-Reinforced Composites for Improved Oxidation Resistance Kiyoshi Sato,* ,† Hiroki Morozumi,‡ Osamu Funayama,* Hiroshi Kaya, and Takeshi Isoda Tonen General Sekiya,§ Iruma-gun, Saitama 356-8500, Japan C-B-Si coatings were formed on a Si3N4 fiber using chemical vapor deposition and embedded in a Si-N-C matrix using polymer impregnation and pyrolysis. The boron-containing layer was anticipated to form borosilicate glass and seal oxygen-diffusion passes. Two types of C-B-Si coatings were tested on the fiber–matrix interface, and they improved the oxidation resistance of the composite. The first coating was multilayered: a crystalline sublayer composed of B-Si-C was sandwiched between two graphitelike carbon sublayers. The second coating was a graphitelike carbon layer containing a small amount of boron and silicon. The carbon (sub)layer of both coatings weakened the fiber–matrix bonding, giving the composites a high flexural strength (1.1 GPa). The composites retained 60%–70% of their initial strength, even after oxida￾tion at 1523 K for 100 h. The mechanism for improved oxidation resistance was discussed through the microstructure of the interface, morphology of the fracture surface, and oxygen distribution on a cross section of the oxidized composite. I. Introduction FIBER-REINFORCED ceramic composites show promise for over￾coming the brittleness of monolithic ceramics.1–3 Control of the bonding between fiber and matrix is important to giving a composite strength and toughness, because weak bonding prevents a matrix crack from propagating to a fiber.4,5 A carbon layer has been applied at the fiber–matrix interface for this purpose,1–5 but the carbon layer oxidizes above
700 K in air, limiting the oxidation resistance of the composite.6,7 Two approaches have been investigated as a solution to this problem: (i) preventing oxidation of the carbon layer; and (ii) replacing the carbon layer with an oxidation-resistant layer. Various studies have investigated preventing oxidation of the carbon layer. The addition of boron in a matrix or an interface layer, to form a B2O3 or borosilicate-glass seal at low tempera￾tures, has been studied. The oxygen-diffusion passes that must be sealed are the matrix cracks and interfacial gaps resulting from the oxidation loss of the carbon layer. The addition of a boride powder to the matrix partially prevents oxidation of the carbon interface.8,9 Jacques et al.10 have produced an interface layer of 20 mol% boron-containing carbon on a SiC fiber using chemical vapor deposition (CVD). That work has demonstrated the advantage of oxidation resistance on single-filament-reinforced composites only—a microcomposite. A BN and oxide layer has been investigated elsewhere as a substitute for the carbon layer. h-BN has a layered crystal structure, similar to graphite, which causes slip and debonding between its crystal layers. A BN layer is useful for controlling adhesion between the fiber and matrix in many composites, such as SiC-fiber-reinforced SiC11 and SiC-fiber-reinforced glass.12 The oxidation starting temperature of BN is only 100 K higher than carbon, but BN is expected to form B2O3, which seals the oxygen-diffusion passes. A composite with a BN layer shows oxidation resistance to 1100 K, but it experiences embrittlement at intermediate temperatures, 900–1100 K, because of the active oxidation of BN.13,14 An interfacial oxide coating offers the considerable advantage of causing no oxidation degradation.15–17 However, an oxide interface is not adequate for a non-oxide fiber, because the oxide layer diffuses oxygen to the fiber.18 Recently, all-oxide composites have been actively studied because of their stability under an oxidizing atmosphere.19–22 However, among the disadvantages of all-oxide composites is high-temperature creep above
1500 K, which results from an intrinsic property of ordinal oxides. Thus, the problem of improving the oxidation resistance of ceramic composites has remained unsolved. The present study focused on a non-oxide composite, because the high strength of such a composite at high temperature improves oxidation resistance. A C-B-Si coating was produced using CVD and applied at the fiber–matrix interface. We anticipated that (i) the C-B-Si layer would form a borosilicate glass, sealing the oxygen-diffusion passes, as had the BN11,12 and boron-containing carbon10 layers, and (ii) controlling the B:Si ratio in the C-B-Si layer would change the softening temperature and viscosity of the borosilicate glass, improving the performance of the seal. The ternary system C-B-Si was selected to form a carbon-rich phase, which would cause fiber–matrix debonding. A Si-B-N layer could function as C-B-Si if a h-BN sublayer was formed, but a nitride coating was more difficult than a carbon coating to fabricate using CVD. A polymer-impregnation and pyrolysis (PIP) method23–26 was used in the present study to fabricate the composite. The advantage of the PIP method was that it could make use of shaping techniques from the industrial process for fiber-reinforced plastics. However, improving the oxidation resistance of PIP composites was difficult because of open pores and matrix cracks, formed by pyrolysis shrinkage of the polymer, that diffused oxygen into the composite. The present method of providing oxidation resistance to PIP composites should be effective on other porous composites, such as reaction-bonded composites and pore-free composites, such as chemical vapor infiltrated composites. This paper first describes the properties of a C-B-Si coating on a Si3N4 fiber. Results for the investigation of composites with a C-B-Si interfacial layer are described in terms of the fiber–matrix interface microstructure, mechanical properties, and oxidation T. A. Parthasarathy—contributing editor Manuscript No. 188719. Received February 25, 2000; approved September 10, 2001. Supported by the Ministry of Economy, Trade, and Industry, and work conducted by the Petroleum Energy Center. *Member, American Ceramic Society. † Author to whom correspondence should be addressed. Now with Advanced Materials International Co., Fuji, Shizuoka 416-0946, Japan. ‡ Present address: Nissan Technical Center, Nissan Motor Co., Ltd., Atsugi, Kanagawa 234-0192, Japan. § Formerly Tonen Corporation. J. Am. Ceram. Soc., 85 [7] 1815–22 (2002) 1815 journal