July 2002 C-B-Si Coatings for S,N/Fiber-Reinforced Composites for Improved Oxidation Resistance l821 and/or B.C, however, the crystal phase could not be identified supplied from the outside. More et al showed a borosilicate- because of the obscurity of the electron diffraction pattern of L2 -matrix interface of th LI was the first sublayer of the coating, but its composition wa oxidized SiC/BN/SiC composite. The interface-layer morphology uncertain. LI was estimated to be a graphitelike carbon, because of was similar to that of the present oxidized composite(Fig. 4(c)) the similarity of its TEM image and electron diffraction pattern to These similarities also suggested the formation of borosilicate those for L3 glass at the interfa In the case of coating Il, TEM observation and eds analysis o the interface showed that the fiber-matrix interface of this com posite was a monolayer 20 nm thick, consisting mainly of carbon, with a small amount of silicon, and having a weak crystal Newly developed C-B-Si interfacial coatings were orientation parallel to the fiber surface. AES analysis of the coated the fiber-matrix interfaces of a Si,Na fiber-reinforced fiber and the fracture surface of the interface revealed that the to improve the oxidation resistance of the composite, The Si3N4 coating structure remained even after the PIP process, as in coating fiber was coated with the C-B-Si layer using CVD and embedded I. When the information of the AES analysis was added to the in the Si-N-C matrix by a PIP process. Two types of C-B-Si TEM observation, the interface was determined to be a graphit coatings enhanced the oxidation resistance of the PlP composites, like carbon laver containing a small amount of boron and silico although the matrix had many cracks, resulting from pyrolysis with an outer carbon-rich sublayer. shrinkage of the precursor, that allowed the easy permeation of The fracture of the fiber-matrix interface of composites I and Il oxygen. The first coating, coating I, formed a multilayered proceeded on the matrix side of the interface layer, which fiber-matrix interface, which consisted of three sublayers: a corresponded to the outer sublayer of the fiber coating. The crystalline sublayer containing boron, silicon, and carbon was debonding on the outer surface of the coating was necessary for the sandwiched between two graphitelike carbon layers. The second coati ing to be the pip at the inner part of the fiber coating, the fiber coating was lost from face, which consisted of a graphitelike carbon layer containing a the fiber surface after cyclic impregnation of PIP. small amount of boron and silicon. Debonding between the fiber and the matrix occurred at the carbon(sub)layer for both of th composites and gave the composites a flexural strength as high as (2) Mechanism for Improving Oxidation Resistance 1. 1 GPa. The composites retained 77%(coating I)and 60% (coating If) of their original strength, even after oxidation at 1523 The high strength of the composites was obtained by weakened K for 360 ks. Coating I was also effective in the improvement of iber-matrix bonding by the carbon sublayer for the monolayered the oxidation resistance of a SiC-fiber-reinforced composite terface(composite ID) and the multilayered interface(composite D), as shown by AES analysis of the fracture surfaces. The carbon The mechanism by which oxidation resistance was improved sublayer prevented the propagation of a matrix crack through the hypothesized as follows. The carbon(sub )layer was easily oxi- fiber as a conventional carbon interface I-5 dized near the surface of the composite. Simultaneously, the The mechanism for d oxidation resistance boron- and silicon-containing(sub)layer (the center crystall ivolves the microstructure of the interface, the morphology of the sublayer for coating I and the graphitelike carbon layer itself fo fracture surface, and the oxygen distribution on a cross section of coating ID) formed borosilicate glass, the periphery of which sealed the oxidized composite. The carbon sublayer adjacent to the the matrix cracks. As a result, the 0. 1-0.3 nm wide periphery of surface of the sample oxidizes easily if the sample is exposed the coml pne fiber and the matrix with the borosilicate glass,but the of the c-B-Si interface over a conventional carbon interface is the inside of the composite was unoxidized and showed many long formation of borosilicate glass. The center crystalline sublayer pullout fibers on its fracture surface supplies boron and silicon in the case of coating I, and the No borosilicate layers, the existence of which would have verified boron-containing graphitelike carbon layer supplies boron in the the mechanism of oxidation resistance, were directly detected at the case of coating Il Boron-rich borosilicate glass melts at K 4 interface by microanalytical techniques, such as TEM and EDS near the starting temperature for the oxidation of carbon. Borosil because of equipment limitations for the detection of boron. However, cate glass seals the matrix cracks and the fiber-matrix gaps the proposed mechanism adequately explained the morphology of the resulting from oxidation loss of the carbon sublayers and prevents fracture surfaces and the oxygen concentration distributions of the the permeation of oxygen into the composite cross sections of the oxidized composites If few matrix cracks exist in a composite, oxygen permeation is stopped within the thin oxygen-sealing layer around the composite References However, a composite fabricated using the PIP process has many E. Fitzer and R. Gadow."Fiber-Reinforced Silicon Carbide. Am. Cera. Soc natrix cracks. Therefore, full suppression of oxygen permeation Ball,652]326-35(1986) equires a thick oxygen-sealing layer around the composite. In the oxygen-sealing layer, the borosilicate glass bonds matrix crac M. Prewo, "Fiber-Reinforced Ceramics: New Opportunities for Composite esul flat fracture Irface at the pe Materials," Am Ceran. Soc. Bull., 68[2] 395-400(19 On the other hand, all the C-B-Si interface layers remain unoxi R.J.Kerans, R. S. Hay, N. J. Pagano, and T, A. Parthasarathy,""The Role of the dized at the inside of the composite, causing much fiber pullout on Fiber-Matrix Interface in Ceramic Composites, Am. Ceram. Soc. Bull., 68 [2] 4212(89 No borosilicate glass, the direct evidence of an antioxidation J.J. Brennan,"Eflects of Bischoff, O. Sbaizero, M. Rhule, A G. Evans, D B. Marshall, and on the Properties of Fiber-Reinforced Ceramics, mechanism, was detected using TEM or EDS in the present J. Am. Ceram Soc., 73 6]1691-99(1990) samples because of detection limitations of the equipment. How K. M. Prewo, "Fatigue and Stress Rupture of Silicon Carbide-Fiber-Reinforced ever, the phenomenon is well supported by the hypothesis that L. Filipuzzi, G. Camus R. Naslain, and J, Th ""Oxidation Mechanisms and borosilicate glass seals the oxygen-diffusion passes. In an attempt Kinetics of ID-SiC/C/SiC Composite Materials: L, An Experimental Approach, to determine whether borosilicate glass and B2O3 could form by the oxidation of the other boron-containing interfacial layer. T. E. Steyer, F. w. Zok, and D. P. Walls, "Stress Rupture of an Enhanced Sheldon et al.>conducted thermodynamic calculations on the Soc, 81 [8]2140-46(1998) system consisting of a SiC fiber, a Bn interface, and an SiC sS. Zhu, M. Mizuno, Y. Nagano, J. Cao, Y. Kagawa, and H. Kaya, "Creep and matrix. Lee et al. 7 observed fiber-matrix interfaces for the same system. Those researchers showed that B,O3 and/or borosilicate eroc819269-7198 S. Jacques, A. Guette, F. Langlais, R. Naslain, and S. Goujard,"High- glass formed when the amount of oxygen was high or oxygen was Temperature Lifetime in Air of SiC/C(BySiC Microcomposites Prepared byand/or B4C; however, the crystal phase could not be identified because of the obscurity of the electron diffraction pattern of L2. L1 was the first sublayer of the coating, but its composition was uncertain. L1 was estimated to be a graphitelike carbon, because of the similarity of its TEM image and electron diffraction pattern to those for L3. In the case of coating II, TEM observation and EDS analysis of the interface showed that the fiber–matrix interface of this com￾posite was a monolayer 20 nm thick, consisting mainly of carbon, with a small amount of silicon, and having a weak crystal orientation parallel to the fiber surface. AES analysis of the coated fiber and the fracture surface of the interface revealed that the coating structure remained even after the PIP process, as in coating I. When the information of the AES analysis was added to the TEM observation, the interface was determined to be a graphite￾like carbon layer containing a small amount of boron and silicon with an outer carbon-rich sublayer. The fracture of the fiber–matrix interface of composites I and II proceeded on the matrix side of the interface layer, which corresponded to the outer sublayer of the fiber coating. The debonding on the outer surface of the coating was necessary for the coating to be applied on the PIP composite. If debonding occurred at the inner part of the fiber coating, the fiber coating was lost from the fiber surface after cyclic impregnation of PIP.33 (2) Mechanism for Improving Oxidation Resistance The high strength of the composites was obtained by weakened fiber–matrix bonding by the carbon sublayer for the monolayered interface (composite II) and the multilayered interface (composite I), as shown by AES analysis of the fracture surfaces. The carbon sublayer prevented the propagation of a matrix crack through the fiber as a conventional carbon interface.1–5 The mechanism for improved oxidation resistance apparently involves the microstructure of the interface, the morphology of the fracture surface, and the oxygen distribution on a cross section of the oxidized composite. The carbon sublayer adjacent to the surface of the sample oxidizes easily if the sample is exposed under an oxidizing atmosphere at high temperature. The advantage of the C-B-Si interface over a conventional carbon interface is the formation of borosilicate glass. The center crystalline sublayer supplies boron and silicon in the case of coating I, and the boron-containing graphitelike carbon layer supplies boron in the case of coating II. Boron-rich borosilicate glass melts at
700 K,34 near the starting temperature for the oxidation of carbon. Borosili￾cate glass seals the matrix cracks and the fiber–matrix gaps resulting from oxidation loss of the carbon sublayers and prevents the permeation of oxygen into the composite. If few matrix cracks exist in a composite, oxygen permeation is stopped within the thin oxygen-sealing layer around the composite. However, a composite fabricated using the PIP process has many matrix cracks. Therefore, full suppression of oxygen permeation requires a thick oxygen-sealing layer around the composite. In the oxygen-sealing layer, the borosilicate glass bonds matrix cracks and fiber–matrix interfaces. This hard bonding of the interfaces results in a flat fracture surface at the periphery of the composite. On the other hand, all the C-B-Si interface layers remain unoxi￾dized at the inside of the composite, causing much fiber pullout on the fracture surface. No borosilicate glass, the direct evidence of an antioxidation mechanism, was detected using TEM or EDS in the present samples because of detection limitations of the equipment. How￾ever, the phenomenon is well supported by the hypothesis that borosilicate glass seals the oxygen-diffusion passes. In an attempt to determine whether borosilicate glass and B2O3 could form by the oxidation of the other boron-containing interfacial layer, Sheldon et al.35 conducted thermodynamic calculations on the system consisting of a SiC fiber, a BN interface, and an SiC matrix. Lee et al.17 observed fiber–matrix interfaces for the same system. Those researchers showed that B2O3 and/or borosilicate glass formed when the amount of oxygen was high or oxygen was supplied from the outside. More et al.36 showed a borosilicate￾glass layer with bubbles on the fiber–matrix interface of the oxidized SiC/BN/SiC composite. The interface-layer morphology was similar to that of the present oxidized composite (Fig. 4(c)). These similarities also suggested the formation of borosilicate glass at the interface. V. Conclusions Newly developed C-B-Si interfacial coatings were applied at the fiber–matrix interfaces of a Si3N4-fiber-reinforced composite to improve the oxidation resistance of the composite. The Si3N4 fiber was coated with the C-B-Si layer using CVD and embedded in the Si-N-C matrix by a PIP process. Two types of C-B-Si coatings enhanced the oxidation resistance of the PIP composites, although the matrix had many cracks, resulting from pyrolysis shrinkage of the precursor, that allowed the easy permeation of oxygen. The first coating, coating I, formed a multilayered fiber–matrix interface, which consisted of three sublayers: a crystalline sublayer containing boron, silicon, and carbon was sandwiched between two graphitelike carbon layers. The second coating, coating II, formed a morphologically monolayered inter￾face, which consisted of a graphitelike carbon layer containing a small amount of boron and silicon. Debonding between the fiber and the matrix occurred at the carbon (sub)layer for both of the composites and gave the composites a flexural strength as high as 1.1 GPa. The composites retained 77% (coating I) and 60% (coating II) of their original strength, even after oxidation at 1523 K for 360 ks. Coating I was also effective in the improvement of the oxidation resistance of a SiC-fiber-reinforced composite. The mechanism by which oxidation resistance was improved is hypothesized as follows. The carbon (sub)layer was easily oxi￾dized near the surface of the composite. Simultaneously, the boron- and silicon-containing (sub)layer (the center crystalline sublayer for coating I and the graphitelike carbon layer itself for coating II) formed borosilicate glass, the periphery of which sealed the matrix cracks. As a result, the 0.1–0.3 nm wide periphery of the composite showed brittle fracture, caused by the hard bond between the fiber and the matrix with the borosilicate glass, but the inside of the composite was unoxidized and showed many long pullout fibers on its fracture surface. No borosilicate layers, the existence of which would have verified the mechanism of oxidation resistance, were directly detected at the interface by microanalytical techniques, such as TEM and EDS, because of equipment limitations for the detection of boron. However, the proposed mechanism adequately explained the morphology of the fracture surfaces and the oxygen concentration distributions of the cross sections of the oxidized composites. References 1 E. Fitzer and R. Gadow, “Fiber-Reinforced Silicon Carbide,” Am. Ceram. Soc. Bull., 65 [2] 326–35 (1986). 2 K. M. Prewo, J. J. Brennan, and G. K. Layden, “Fiber-Reinforced Glasses and Glass-Ceramics for High-Performance Applications,” Am. Ceram. Soc. Bull., 65 [2] 305–13 (1986). 3 K. M. Prewo, “Fiber-Reinforced Ceramics: New Opportunities for Composite Materials,” Am. Ceram. Soc. Bull., 68 [2] 395–400 (1989). 4 R. J. Kerans, R. S. Hay, N. J. Pagano, and T. A. Parthasarathy, “The Role of the Fiber–Matrix Interface in Ceramic Composites,” Am. Ceram. Soc. Bull., 68 [2] 429–42 (1989). 5 H. C. Cao, E. Bischoff, O. Sbaizero, M. Rhule, A. G. Evans, D. B. Marshall, and J. J. Brennan, “Effects of Interfaces on the Properties of Fiber-Reinforced Ceramics,” J. Am. Ceram. Soc., 73 [6] 1691–99 (1990). 6 K. M. Prewo, “Fatigue and Stress Rupture of Silicon Carbide-Fiber-Reinforced Glass-Ceramics,” J. Mater. Sci., 22, 2695–701 (1987). 7 L. Filipuzzi, G. Camus R. Naslain, and J. Thebault, “Oxidation Mechanisms and Kinetics of 1D-SiC/C/SiC Composite Materials: I, An Experimental Approach,” J. Am. Ceram. Soc., 77 [2] 459–66 (1994). 8 T. E. Steyer, F. W. Zok, and D. P. Walls, “Stress Rupture of an Enhanced Nicalon/Silicon Carbide Composite at Intermediate Temperatures,” J. Am. Ceram. Soc., 81 [8] 2140–46 (1998). 9 S. Zhu, M. Mizuno, Y. Nagano, J. Cao, Y. Kagawa, and H. Kaya, “Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature,” J. Am. Ceram. Soc., 81 [9] 2269–77 (1998). 10S. Jacques, A. Guette, F. Langlais, R. Naslain, and S. Goujard, “High￾Temperature Lifetime in Air of SiC/C(B)/SiC Microcomposites Prepared by July 2002 C-B-Si Coatings for S3N4-Fiber-Reinforced Composites for Improved Oxidation Resistance 1821