ournal In. Ceran. Soc, 82[1]1415-55(1999) Fiber Effects on minicomposite mechanical Properties for Several Silicon Carbide Fiber-Chemically Vapor-Infiltrated Silicon Carbide Matrix Systems Gregory N Morscher, t Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio Julian Martinez-Fernandez Departamento de Fisica de la Materia Condensada, University of Seville, Seville, Spain Several different types of SiC fiber tows were coated with fiber tests while mimicking, to some degree, the larger-scale BN and composited using chemically vapor- infiltrated Sic macrocomposite tensile behavior. Single-fiber tests have been to form single-tow minicomposites. The types of SiC fib used to determine the interfacial properties for a Nicalon fiber/ included NicalonM, Hi-Nicalon'M, and the new SyiramieT CVI-SiC matrix system, a CVD-fiber/glass-matrix system, 0 were determined from unload-reload tensile hysteresis- studies, the model of Marshall 2 for fiber pullout was used to loop tests. The ultimate stress and strain properties also determine the interfacial properties of the fiber/matrix system. were determined for the minicomposites. The ultimate Also, the effect of fiber roughness on fiber sliding was deter- strengths of the newer Hi-Nicalon and Sylramic fibers were mined for the CVD-SiC/glass matrix system superior to that of Nicalon minicomposites with similar Because macrocomposite are composed of several tows in iber volume fractions. The Sylramic minicomposites had the loading direction, the minicomposite represents a subele- strength, respectively, because of the high modulus of the of minicomposites is very similar to that of macrocomposite, fiber and the rough surface of this fiber type. The apparent because the interfacial and elastic properties of the constituents interfacial shear strength increased as the stress increased are the same. The matrix and fibers also have different flaw for the Sylramic minicomposites, which also was attributed distributions, as in a macrocomposite, which account for, in to the surface roughness of this fiber accordance with the interfacial properties, the stress-dependent cracking behavior and fiber failure prior to ultimate failure L. Introduction The aspect of a macrocomposite that is foreign to minicom- posites are 90 plies, which are often low-stress crack-initiation composites to be used at temperatures >1200%C, fibers with(tunnel cracking)intersect load-bearing tows. Therefore,the better creep and rupture properties than ceramic-grade(CG absolute stress-strain behavior of minicomposites is expected to differ from macrocomposite, because of the differences in able' Fiber development has been underway, and several ven- crack morphology that result from differences in matrix-flaw dors are or will be offering higher-use-temperature SiC-based distributions and fiber architecture fibers. 2-4 Currently, SylramicTM and Hi-Nicalon TM fibers are substantially more expensive than CG NicalonTM. It is es of fiber and similar interphases and matrices and then that. as these fibers find greater use ca In this study, minicomposites were fabricated with different their cost will also de- tensile tested at room temperature, to determine the effect of crease. There is a current need to assess the effect of different fiber properties on the SIiC /BN,SiCm system. Even though the fibers for a given composite system in a cost-effective manner. absolute stress-strain behaviors of minicomposites are ex- One way to accomplish this is to fabricate and test single-tow pected to differ from macrocomposite that have been made minicomposites. Minicomposite fabrication and testing are es- with the same constituents the relative difference in stress. pecially practical for ceramic composite systems that use strain behavior of the different fiber-type minicomposites is chemically vapor infiltrated(CVI) interphases and matrices expected to translate to macrocomposite stress-strain behavior such as in many SiC fiber/BN interphase/SiC matrix systems This test approach has already been used to determine roor Il. Experimental Procedure temperature interfacial and ultimate tensile properties,6 and high-temperature stress rupture, and cyclic stress properties Single-fiber-tow composites were processed in the same way in ambient air for several fiber/interphase/matrix composite as that described by Morscher. A single-fiber tow was coate with a BN interphase, and then the coated tow was infiltrated The minicomposite test incorporates the analysis of single with SiC. The three different minicomposites are listed in Table I, with pertinent physical rties of the composite components. Note that the Bn interphase for Nicalon-fiber ites was processed by a different vendor than the K.T. Faber--contributing editor Hi-Nicalon-fiber and Sylramic-fiber minicomposites. There are two reasons for this. The nicalon fibers were coated for an earlier study and at the time when the Hi-Nicalon and syl- ramic fibers were coated. the vendor who coated the nicalon lo. 190649. Received October 6, 1997, approved April 24, 1998. fibers was not in the fiber-coating business anymore. also A-Lewis Research Center. Cleveland OH Nicalon could not be coated with the BN coating applied to the 44135-3127 Hi-Nicalon or Sylramic fibers because the BNFiber Effects on Minicomposite Mechanical Properties for Several Silicon Carbide Fiber–Chemically Vapor-Infiltrated Silicon Carbide Matrix Systems Gregory N. Morscher*,† Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio Julian Martinez-Fernandez Departamento de Fisica de la Materia Condensada, University of Seville, Seville, Spain Several different types of SiC fiber tows were coated with BN and composited using chemically vapor-infiltrated SiC to form single-tow minicomposites. The types of SiC fiber included Nicalon™, Hi-Nicalon™, and the new Sylramic™ polycrystalline SiC fiber. The interfacial shear stresses were determined from unload–reload tensile hysteresis￾loop tests. The ultimate stress and strain properties also were determined for the minicomposites. The ultimate strengths of the newer Hi-Nicalon and Sylramic fibers were superior to that of Nicalon minicomposites with similar fiber volume fractions. The Sylramic minicomposites had the lowest strain to failure and highest interfacial shear strength, respectively, because of the high modulus of the fiber and the rough surface of this fiber type. The apparent interfacial shear strength increased as the stress increased for the Sylramic minicomposites, which also was attributed to the surface roughness of this fiber. I. Introduction I T IS evident that, in order for SiC-fiber-reinforced ceramic composites to be used at temperatures >1200°C, fibers with better creep and rupture properties than ceramic-grade (CG) Nicalon™ (Nippon Carbon, Tokyo, Japan) need to be avail￾able.1 Fiber development has been underway, and several ven￾dors are or will be offering higher-use-temperature SiC-based fibers.2–4 Currently, Sylramic™ and Hi-Nicalon™ fibers are substantially more expensive than CG Nicalon™. It is expected that, as these fibers find greater use, their cost will also de￾crease. There is a current need to assess the effect of different fibers for a given composite system in a cost-effective manner. One way to accomplish this is to fabricate and test single-tow minicomposites.5 Minicomposite fabrication and testing are es￾pecially practical for ceramic composite systems that use chemically vapor infiltrated (CVI) interphases and matrices such as in many SiC fiber/BN interphase/SiC matrix systems. This test approach has already been used to determine room￾temperature interfacial and ultimate tensile properties5,6 and high-temperature stress rupture7,8 and cyclic stress8 properties in ambient air for several fiber/interphase/matrix composite systems. The minicomposite test incorporates the analysis of single￾fiber tests while mimicking, to some degree, the larger-scale macrocomposite tensile behavior. Single-fiber tests have been used to determine the interfacial properties for a Nicalon fiber/ CVI-SiC matrix system,9 a CVD-fiber/glass-matrix system,10 and a CVD-SiC fiber/CVI-SiC matrix system.11 For all these studies, the model of Marshall12 for fiber pullout was used to determine the interfacial properties of the fiber/matrix system. Also, the effect of fiber roughness on fiber sliding was deter￾mined for the CVD-SiC/glass matrix system.10 Because macrocomposites are composed of several tows in the loading direction, the minicomposite represents a subele￾ment of the macrocomposite. The tensile stress–strain behavior of minicomposites is very similar to that of macrocomposites,5 because the interfacial and elastic properties of the constituents are the same. The matrix and fibers also have different flaw distributions, as in a macrocomposite, which account for, in accordance with the interfacial properties, the stress-dependent cracking behavior and fiber failure prior to ultimate failure. The aspect of a macrocomposite that is foreign to minicom￾posites are 90° plies, which are often low-stress crack-initiation sites13 and non-through-thickness cracking that may or may not (tunnel cracking14) intersect load-bearing tows. Therefore, the absolute stress–strain behavior of minicomposites is expected to differ from macrocomposites, because of the differences in crack morphology that result from differences in matrix-flaw distributions and fiber architecture. In this study, minicomposites were fabricated with different types of fiber and similar interphases and matrices and then tensile tested at room temperature, to determine the effect of fiber properties on the SiCf /BNi /SiCm system. Even though the absolute stress–strain behaviors of minicomposites are ex￾pected to differ from macrocomposites that have been made with the same constituents, the relative difference in stress– strain behavior of the different fiber-type minicomposites is expected to translate to macrocomposite stress–strain behavior. II. Experimental Procedure Single-fiber-tow composites were processed in the same way as that described by Morscher.7 A single-fiber tow was coated with a BN interphase, and then the coated tow was infiltrated with SiC. The three different minicomposites are listed in Table I, with pertinent physical properties of the composite components. Note that the BN interphase for Nicalon-fiber minicomposites was processed by a different vendor than the Hi-Nicalon-fiber and Sylramic-fiber minicomposites. There are two reasons for this. The Nicalon fibers were coated for an earlier study, and at the time when the Hi-Nicalon and Syl￾ramic fibers were coated, the vendor who coated the Nicalon fibers was not in the fiber-coating business anymore. Also, Nicalon could not be coated with the BN coating applied to the Hi-Nicalon or Sylramic fibers because the BN processing tem￾K. T. Faber—contributing editor Manuscript No. 190649. Received October 6, 1997; approved April 24, 1998. *Member, American Ceramic Society. † Resident Research Associate at NASA–Lewis Research Center, Cleveland, OH 44135–3127. J. Am. Ceram. Soc., 82 [1] 145–55 (1999) Journal 145