December 2000 eramic Composites with Multilayer Interfa layer axes aligned parallel and the crack normal to the stress those of composites reinforced with ceramic-grade Nicalon Mac- direction. Naslain and co-workers have been able to synthesize roscopic tensile testing also indicated that multilayer interface materials in which cracking occurs within the interface coating by material has properties equivalent to composites having a single- use of a fiber treatment to increase bonding between the carbon carbon- layer interface. Oxidation protection based on the multi- layer and the fibers. , -The stronger bonding shifts the weakest layer C/Sic concept was not apparent from the materials and zone to either within a carbon layer or to the carbon-SiC layer testing reported here. However, fairly thick carbon- interface layers interfaces, resulting in greater energy absorption and higher were utilized (0. 1-0.2 um), and it has been demonstrated that macroscopic tensile strength. Naslain et al have also improved the thinner layers(<0. 1 um) are required to obtain protective sealing composition and structure of the alternating layers through the use by the growth of silica. In addition, because untreated fibers were of pulsed chemical vapor deposition. o,2 used, debonding occurred between the first carbon layer and the The shapes of the tensile curves for the ceramic-grade Nicalon fiber, and thus the material behaved not very differently from nicomposite specimens with single-layer carbon interface(MC omposites having single-carbon-layer interfaces 22)are reproduced in the multilayer interface specimens, although with a possible reduction in ultimate load of 20%-30%. Either the relative thinness of the individual carbon layers or their irregular nature may be the cause of the lower ultimate load because they Acknowledgments rovide a less compliant layer to accommodate interface rough- ness.3,29 This effect is seen in the flexural-strength measurements We would like to acknowledge the useful of P F. Tortorelli, H. T, Lin, of Lowden and Stinton2and in the tensile test results of Naslain, 4 and R.A. Lowden. J C. McLaughlin performed much of the experimental infiltration in which there is limited measured displacement in the mechanical-property testing of materials with a relatively thin carbon interface layer. This finding may be supported by the References results from the minicomposite samples, in which the multilayer interface sample having the largest total thickness of carbon had R. A. Lowden and K. I Effect of Fiber Coatings on Interfa the maximum tensile strength (Table m) The results of the tensile strength measurements of the plate posites. Edited by C. G. Pantano and E. J. H. Chen. Materials Researc samples are consistent with those of Naslain and co-workers The plateau exhibited in their tensile curves after matrix cracking, which is absent in the curves generated in this work. may be th 5 essed by Cvl,C两hmB28(190 carbon thickness of 0.5 um)as compared with 0.25 um for the R. Naslain,"The Concep.? result of their substantially thicker interface layer(having a total Ceramic Composites, Scr. M ater,3l{81079-84(1994) Layered Interphases in SiC/SiC Pp. 23-39 in naterial prepared in this work. The thicker layer yields a lower High-Temperature Ceramic-Matrix Composites ll. Edited by A G. Evans and R. interfacial shear stress, allowing for greater slippage of the fibers American Ceramic Society. Westerville. OH. 1995 SE. Gourb aces in SiC/C CVD the matrix. The strengths of the composites prepared by Naslain ers, "Mater. Sci. Forum, 207(209J237-40(1996)- C) Mu. laye as Fibre C and co-workers from untreated ceramic-grade Nicalon fibers are higher than those reported here, although the present composites Oxidation Protection and Improved Mechanical Behavior; pp. 530-31 in prepared from Hi-Nicalon with single and multilayer interfaces iennial Conference on Carbon. American Carbon Society, University Park, had similar tensile behavior and ultimate strength PA,1997 7F, Rebillat, J. Lamon, R. Naslain, E. Lara-Curzio, M.K. Ferber, and T.M Because of their relatively large thickness, the multilayer Besmann, "Properties of Multilayered Interphases in SiC/SiC Chemical-Vapor- interfaces prepared in this study appeared to afford no benefit in Infiltrated Composites with"Weak and"Strong'Interfaces, ".Am. Ceram. Soc.,81 oxidation; all specimens experienced severe degradation of me- 192315-26(1998 chanical properties (i. e, low strength and strain tolerance). It is nd J. w. Bohlen, "Fiber Coatings for Ceramic Matrix Composites, Ceram. Eng. Sci. Proc., 13 [7-8]23-56(1992). likely that this condition resulted from the consumption of the and F. Heurtevent, "Method of manufacturing a carbon coating adjacent to the fibers via oxidation, followed by Composite Material with Lamellar Interphase Between Reinforced Fibers and Matrix silica formation that filled the resulting void space. 3,29The and Material Obtained, International Pat, No. wO 95/09136, Societe European relatively thick gap in the interlayers left by the oxidized carbon Propulsion, 19 may not have allowed sufficient rapid sealing of the exposed w.S. Steffier, "Multilayer Fiber Coating Comprising Altermate Fugitive Carbon surface because the layers were of the order of 0.1 to 0. 2 um thick and Ceramic Coating Material for Toughened Ceramic Composite Materials, U.S Pat.No.5455106,1995 (Fig. 6). The modeling and experimental efforts of Filipuzzi and G. Camus, R. Naslain, and J. Thebault, "Oxidation Mechanisms and co-workers, 2 and of Tortorelli and co-workers 4, 30 indicated Kinetics of 1-D-SiC/C/SiC Composite Materials: I, An Experimental Approach, that thicknesses of less than 0 I um are required. 12L. Filipuzzi and R. Naslain, "Oxidation Mechanisms and Kinetics of I-D-SiC/ C/SiC Composite Materials: Il, Modeling, J. Am. Ceran Soc., 77[21 467-80 V. Conclusions IR. D. James, R. A. Lowden, and K. L. More, "The Effects of Oxidation an In minicomposites, multilayer interfaces result in mechanical properties that may be equivalent to those of single-layer carbon interfaces. Properties are highly influenced by the total thickness Applications. Edited by M. D. Sacks. American Ceramic Society, Westerville, OH, of the carbon interface material, probably in accordance with the IP F. Tortorelli, S Nijhawan, L. Riester, and R. A Lowden, "Influence of Fiber need for a compliant interna surface roughness. Although there is some evidence for crack R. H. Jones, C. H. Henager Jr, and C. F, Windisch Jr,"High Temperature deflection within the multilayer interfacial coating, cracks are Corrosion and Crack Growth of SiC-SiC at Variable Oxygen Pressures, Mater. Sci edominantly directed along the interface between the fiber and rst carbon interface layer because of poor bonding between these Ibw. J. Lackey, S. Vaidyaraman, and K. L. More, "Laminated C-SiC Matrix materials. Thus, for the composites of this study, the additional Composites Produced by CVI, J. Am. Ceram Soc., 80[1]113-16( 1997) 17L. L. Snead, D. Steiner, and S J. Zinkle, "Measurement of the Effect ers largely provided no mechanical advantage, Gradual decay Ceramic Composite Interfacial Strength, J. Nucl. Mater, 191[1941 of the tensile load was observed after peak strength f for minicom- 556-70(1992) posites with presumably low interfacial shear stres K. L More E. Lara-Curzio H It has been demonstrated that the FCVi process can successfully deposit alternating layers of carbon and SiC Tensile strengths and Conference and Exposition on Co c=“ Materials and strain tolerances of composites with Hi-Nicalor Structures(Cocoa Beach, FL, January 1996)layer axes aligned parallel and the crack normal to the stress direction. Naslain and co-workers have been able to synthesize materials in which cracking occurs within the interface coating by use of a fiber treatment to increase bonding between the carbon layer and the fibers.4,7,27 The stronger bonding shifts the weakest zone to either within a carbon layer or to the carbon–SiC layer interfaces, resulting in greater energy absorption and higher macroscopic tensile strength. Naslain et al. have also improved the composition and structure of the alternating layers through the use of pulsed chemical vapor deposition.6,28 The shapes of the tensile curves for the ceramic-grade Nicalon minicomposite specimens with single-layer carbon interface (MC- 22) are reproduced in the multilayer interface specimens, although with a possible reduction in ultimate load of 20%–30%. Either the relative thinness of the individual carbon layers or their irregular nature may be the cause of the lower ultimate load because they provide a less compliant layer to accommodate interface rough￾ness.3,29 This effect is seen in the flexural-strength measurements of Lowden and Stinton29 and in the tensile test results of Naslain,4 in which there is limited measured displacement in the mechanical-property testing of materials with a relatively thin carbon interface layer. This finding may be supported by the results from the minicomposite samples, in which the multilayer interface sample having the largest total thickness of carbon had the maximum tensile strength (Table II). The results of the tensile strength measurements of the plate samples are consistent with those of Naslain and co-workers.4,27 The plateau exhibited in their tensile curves after matrix cracking, which is absent in the curves generated in this work, may be the result of their substantially thicker interface layer (having a total carbon thickness of 0.5 mm) as compared with 0.25 mm for the material prepared in this work. The thicker layer yields a lower interfacial shear stress, allowing for greater slippage of the fibers in the matrix. The strengths of the composites prepared by Naslain and co-workers from untreated ceramic-grade Nicalon fibers are higher than those reported here, although the present composites prepared from Hi-Nicalon with single and multilayer interfaces had similar tensile behavior and ultimate strength. Because of their relatively large thickness, the multilayer interfaces prepared in this study appeared to afford no benefit in oxidation; all specimens experienced severe degradation of me￾chanical properties (i.e., low strength and strain tolerance). It is likely that this condition resulted from the consumption of the carbon coating adjacent to the fibers via oxidation, followed by silica formation that filled the resulting void space.13,29 The relatively thick gap in the interlayers left by the oxidized carbon may not have allowed sufficient rapid sealing of the exposed surface because the layers were of the order of 0.1 to 0.2 mm thick (Fig. 6). The modeling and experimental efforts of Filipuzzi and co-workers11,12 and of Tortorelli and co-workers14,30 indicated that thicknesses of less than 0.1 mm are required. V. Conclusions In minicomposites, multilayer interfaces result in mechanical properties that may be equivalent to those of single-layer carbon interfaces. Properties are highly influenced by the total thickness of the carbon interface material, probably in accordance with the need for a compliant interlayer material to accommodate fiber￾surface roughness. Although there is some evidence for crack deflection within the multilayer interfacial coating, cracks are predominantly directed along the interface between the fiber and first carbon interface layer because of poor bonding between these materials. Thus, for the composites of this study, the additional layers largely provided no mechanical advantage. Gradual decay of the tensile load was observed after peak strength for minicom￾posites with presumably low interfacial shear stress. It has been demonstrated that the FCVI process can successfully deposit alternating layers of carbon and SiC. Tensile strengths and strain tolerances of composites with Hi-Nicalon were superior to those of composites reinforced with ceramic-grade Nicalon. Mac￾roscopic tensile testing also indicated that multilayer interface material has properties equivalent to composites having a single￾carbon-layer interface. Oxidation protection based on the multi￾layer C/SiC concept was not apparent from the materials and testing reported here. However, fairly thick carbon-interface layers were utilized (0.1–0.2 mm), and it has been demonstrated that thinner layers (,0.1 mm) are required to obtain protective sealing by the growth of silica. In addition, because untreated fibers were used, debonding occurred between the first carbon layer and the fiber, and thus the material behaved not very differently from composites having single-carbon-layer interfaces. Acknowledgments We would like to acknowledge the useful comments of P. F. Tortorelli, H. T. Lin, and R. A. Lowden. J. C. McLaughlin performed much of the experimental infiltration work, and T. S. Geer prepared the metallographic samples. References 1 R. A. Lowden and K. L. More, “The Effect of Fiber Coatings on Interfacial Shear Strength and the Mechanical Behavior of Ceramic Composites”; pp. 205–14 in Materials Research Society Symposium Proceedings, Vol. 170, Interfaces in Com￾posites. Edited by C. G. Pantano and E. J. H. Chen. Materials Research Society, Pittsburgh, PA, 1990. 2 R. Naslain, “Fiber-Matrix Interphases and Interfaces in Ceramic-Matrix Compos￾ites Processed by CVI,” Compos. Interfaces, 1 [3] 253–86 (1993). 3 R. J. Kerans, “Issues in the Control of Fiber-Matrix Interface Properties in Ceramic Composites,” Scr. Metall. Mater., 31 [8] 1079–84 (1994). 4 R. Naslain, “The Concept of Layered Interphases in SiC/SiC”; pp. 23–39 in High-Temperature Ceramic-Matrix Composites II. Edited by A. G. Evans and R. Naslain. American Ceramic Society, Westerville, OH, 1995. 5 F. Gourbilleau, G. Nouet, and M. Ducarroir, “Interfaces in SiC/C CVD Multilay￾ers,” Mater. Sci. Forum, 207 [209] 237–40 (1996). 6 F. Heurtevent, R. Pailler, and X. Bourrat, “(PyC/SiC) Multilayer as Fibre Coating: Oxidation Protection and Improved Mechanical Behavior”; pp. 530–31 in Carbon ’97, 23rd Biennial Conference on Carbon. American Carbon Society, University Park, PA, 1997. 7 F. Rebillat, J. Lamon, R. Naslain, E. Lara-Curzio, M. K. Ferber, and T. M. Besmann, “Properties of Multilayered Interphases in SiC/SiC Chemical-Vapor￾Infiltrated Composites with ‘Weak’ and ‘Strong’ Interfaces,” J. Am. Ceram. Soc., 81 [9] 2315–26 (1998). 8 H. W. Carpenter and J. W. Bohlen, “Fiber Coatings for Ceramic Matrix Composites,” Ceram. Eng. Sci. Proc., 13 [7–8] 23–56 (1992). 9 S. Goujard, P. Dupel, R. Pailler, and F. Heurtevent, “Method of Manufacturing a Composite Material with Lamellar Interphase Between Reinforced Fibers and Matrix, and Material Obtained,” International Pat. No. WO 95/09136, Societe European Propulsion, 1995. 10W. S. Steffier, “Multilayer Fiber Coating Comprising Alternate Fugitive Carbon and Ceramic Coating Material for Toughened Ceramic Composite Materials,” U.S. Pat. No. 5455106, 1995. 11L. Filipuzzi, G. Camus, R. Naslain, and J. Thebault, “Oxidation Mechanisms and Kinetics of 1-D-SiC/C/SiC Composite Materials: I, An Experimental Approach,” J. Am. Ceram. Soc., 77 [2] 459–66 (1994). 12L. Filipuzzi and R. Naslain, “Oxidation Mechanisms and Kinetics of 1-D-SiC/ C/SiC Composite Materials: II, Modeling,” J. Am. Ceram. Soc., 77 [2] 467–80 (1994). 13R. D. James, R. A. Lowden, and K. L. More, “The Effects of Oxidation and Combustion Environments on the Properties of Nicalon/SiC Composites”; pp. 925–35 in Ceramic Transactions, Vol. 19, Advanced Composite Materials: Processing, Microstructures, Bulk and Interfacial Properties, Characterization Methods, and Applications. Edited by M. D. Sacks. American Ceramic Society, Westerville, OH, 1991. 14P. F. Tortorelli, S. Nijhawan, L. Riester, and R. A. Lowden, “Influence of Fiber Coatings on the Oxidation of Fiber-Reinforced SiC Composites,” Ceram. Eng. Sci. Proc., 14 [7–8] 358–66 (1993). 15R. H. Jones, C. H. Henager Jr., and C. F. Windisch Jr., “High Temperature Corrosion and Crack Growth of SiC–SiC at Variable Oxygen Pressures,” Mater. Sci. Eng. A, 198, 103–12 (1995). 16W. J. Lackey, S. Vaidyaraman, and K. L. More, “Laminated C-SiC Matrix Composites Produced by CVI,” J. Am. Ceram. Soc., 80 [1] 113–16 (1997). 17L. L. Snead, D. Steiner, and S. J. Zinkle, “Measurement of the Effect of Radiation Damage to Ceramic Composite Interfacial Strength,” J. Nucl. Mater., 191 [194] 556–70 (1992). 18K. L. More, E. Lara-Curzio, H. T. Lin, P. J. Tortorelli, R. Shinavsky, and W. S. Steffier, “Multilayered SiC Fiber Coatings in NicalonTM Fiber-Reinforced Ceramic Matrix Composites”; unpublished poster presented at the 20th Annual Cocoa Beach Conference and Exposition on Composites, Advanced Ceramics, Materials and Structures (Cocoa Beach, FL, January 1996). December 2000 Ceramic Composites with Multilayer Interface Coatings 3019