J. An ceran.Soc,83[2]3014-2002000) urna Ceramic Composites with Multilayer Interface Coatings Theodore M. Besmann, Elizabeth R Kupp, Edgar Lara-Curzio, and Karren L. More Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6063 Silicon carbide matrix composites have been fabricated from contact with the ma cause the carbon layer is ither ceramie -grade Nicalon'Mor Hi-NicalonM fibers coated progressive oxida silica grows into the annul e left with an interface material consisting of six alternating carbon by the oxidized carbon, and the fiber becomes well bonded to the and silicon carbide lavers. Initial efforts involved the use of matrix, resulting in a material with little strain tolerance 11-15 chemical vapor infiltration to produce minicomposites (single Thin layers of carbon leave significantly more narrow channels tows of fibers). In subsequent work, forced-flow thermal- s they are oxidized than do thicker layers. The portion of the gradient chemical vapor infiltration was used to produce a carbon layer that is removed near the exposed surface is quickly single composite plate with a multilayer interface from replaced with silica growing from the Sic layers and fiber; the eramic-grade Nicalon fabric and two plates from Hi-Nicalon narrow entrance is sealed before a significant fraction of the abric, one with a single carbon layer and one with a multilayer carbon is lost down the axial length of the layer. Thus, the strongly interface. Tensile testing of the minicomposites and of speci- bonded length is relatively short and, if there is not extensive mens cut from the plates revealed typical composite cracking, the carbon interface layer is retained in much of the specimens to 950 C air for 100 h resulted in large losses in composite. Filipuzzi and co-workers, 12 and Tortorelli and co- and strengths for the as-processed samples. Exposure of tensile strength and strain tolerance regardless of the interface coat- temperatures(>800 C in 100 kPa of dr xygen) and thin carbor ing. The results demonstrate that foreed-nlow thermal-gradient layers(<100 nm)allow this mechanism to operate, thus poten- chemical vapor infiltration can be used to prepare multilayer interface material. The results also verified that relatively tially protecting the integrity of the composite. An additional thick(>100 nm) single or multiple carbon layers are suscep benefit of multilayer interfaces may be in their ability to cause cracks to follow a tortuous, energy-absorbing path within the tible to oxidation that causes the loss of composite properties. interface layers. 4 ackey and co-workers have advanced that concept to produce multilayer matrices Reducing the thickness of the individual carbon layers in SiC-matrix composites has other advantages. For applications in A CONTINUING issue in the development of ceramic fiber high-radiation environments, such as nuclear fusion reactors, the ceramic matrix composites is the need for an interface presence of carbon can lead to dimensional instability, Carbon material that both controls the friction and bonding between the tends to change dimensions anisotropically during irradiation, fibers and the matrix and protects the fibers from the detrimental ausing fiber decohesion and loss of stress transfer between fibers effects of matrix infiltration and environmental attack. -'In non and the matrix, thus causing loss of composite behavior, The use oxide systems the interface materials that have yielded the best of thinner layers of carbon may reduce this effect perties are graphitic carbon and hexagonal BN. Recentl In the work of Naslain and Rebillat, composites were fabri- multilayers of these materials have been considered as interfaces cated from ceramic-grade Nicalon coated with one to four pairs of or increasing oxidation resistance C/SiC coatings that had a total thickness of 0.5 um. The work The concept of multilayer interfaces consisting of unbonded consisted of preparing both minicomposites, which consist of a layers was first described by Carpenter and Bohlen. They depos- single tow of fibers infiltrated with the fiber coating and matrix, ited multiple layers of Sic on the surface of a large-diameter Sic and larger, two-dimensionally reinforced tensile specimens. Me lament and oxide layers in the outer region, which would be in chanical properties were measured by fiber pushout and tensile contact with an oxide matrix. Work on alternating layers of carbon testing. The mechanical properties of the as-fabricated multilayer nd Sic (C/Sic layers) for small-diameter-fiber SiC-matrix mate interface composites did not appear to be different from those of rials was begun in 1990 at the University of Bordeaux. Steffier samples with a single-carbon-layer interface. Microscopic exami- was also an early developer of this concept for SiC-based fibers. nation revealed that crack deflection occurred at the interface etween the fiber and the first carbon layer in composites prepared of the multilayer interface should be better than that of a single with as-received Nicalon, regardless of the number of interface arbon layer because significantly thinner individual carbon layers layers. They determined that a fiber-surface treatment would be an be used. 4, l,2 Under oxidative conditions, thick(100 nm) required to increase the adhesion between the first carbon layer single carbon layers are removed by oxidation, and silica grows and the fiber. Cracks would then be directed into the weak from both the matrix and the fiber to fill the void volume. The multilayer interface material result is an initially weakened material in which fibers make poor More et al. studied ceramic-matrix composites with multi- layer C/SiC-interface layers prepared by Hyper-Therm, Inc.(Hun- ton Beach, CA). The work involved the characterization of 12 pairs of C/SiC coatings with the SiC film thickness increasing R. Naslain--contributing editor rom the fiber to the matrix side of the interface. The total thickness of these interfaces was almost 3 um. More et al, in luscript No. 189173. Receive closest to the fiber controlled the mechanical behavior of the composite for composites in which untreated fibers were utilized and Technology Devel 需 Stress-rupture testing(four-point bending) at 950C in air at did LC, for the u.s. Department of Energy under not show any improvement in oxidation resistance over a compos- Member, American Ceramic Society te with a single 0.25-pm carbon interface layer. 3014Ceramic Composites with Multilayer Interface Coatings Theodore M. Besmann,* Elizabeth R. Kupp,* Edgar Lara-Curzio,* and Karren L. More Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6063 Silicon carbide matrix composites have been fabricated from either ceramic-grade NicalonTM or Hi-NicalonTM fibers coated with an interface material consisting of six alternating carbon and silicon carbide layers. Initial efforts involved the use of chemical vapor infiltration to produce minicomposites (single tows of fibers). In subsequent work, forced-flow thermal￾gradient chemical vapor infiltration was used to produce a single composite plate with a multilayer interface from ceramic-grade Nicalon fabric and two plates from Hi-Nicalon fabric, one with a single carbon layer and one with a multilayer interface. Tensile testing of the minicomposites and of speci￾mens cut from the plates revealed typical composite behavior and strengths for the as-processed samples. Exposure of tensile specimens to 950°C air for 100 h resulted in large losses in strength and strain tolerance regardless of the interface coat￾ing. The results demonstrate that forced-flow thermal-gradient chemical vapor infiltration can be used to prepare multilayer interface material. The results also verified that relatively thick (>100 nm) single or multiple carbon layers are suscep￾tible to oxidation that causes the loss of composite properties. I. Introduction A CONTINUING issue in the development of ceramic fiber– ceramic matrix composites is the need for an interface material that both controls the friction and bonding between the fibers and the matrix and protects the fibers from the detrimental effects of matrix infiltration and environmental attack.1–3 In non oxide systems the interface materials that have yielded the best properties are graphitic carbon and hexagonal BN.1 Recently, multilayers of these materials have been considered as interfaces for improving properties or increasing oxidation resistance.4–7 The concept of multilayer interfaces consisting of unbonded layers was first described by Carpenter and Bohlen.8 They depos￾ited multiple layers of SiC on the surface of a large-diameter SiC filament and oxide layers in the outer region, which would be in contact with an oxide matrix. Work on alternating layers of carbon and SiC (C/SiC layers) for small-diameter-fiber SiC-matrix mate￾rials was begun in 1990 at the University of Bordeaux.9 Steffier10 was also an early developer of this concept for SiC-based fibers. Naslain and his co-workers noted that the oxidation resistance of the multilayer interface should be better than that of a single carbon layer because significantly thinner individual carbon layers can be used.4,11,12 Under oxidative conditions, thick ($100 nm) single carbon layers are removed by oxidation, and silica grows from both the matrix and the fiber to fill the void volume. The result is an initially weakened material in which fibers make poor contact with the matrix because the carbon layer is absent. After progressive oxidation the silica grows into the annular space left by the oxidized carbon, and the fiber becomes well bonded to the matrix, resulting in a material with little strain tolerance.11–15 Thin layers of carbon leave significantly more narrow channels as they are oxidized than do thicker layers. The portion of the carbon layer that is removed near the exposed surface is quickly replaced with silica growing from the SiC layers and fiber; the narrow entrance is sealed before a significant fraction of the carbon is lost down the axial length of the layer. Thus, the strongly bonded length is relatively short and, if there is not extensive cracking, the carbon interface layer is retained in much of the composite. Filipuzzi and co-workers11,12 and Tortorelli and co￾workers14 have shown theoretically and experimentally that high temperatures (.800°C in 100 kPa of dry oxygen) and thin carbon layers (,100 nm) allow this mechanism to operate, thus poten￾tially protecting the integrity of the composite. An additional benefit of multilayer interfaces may be in their ability to cause cracks to follow a tortuous, energy-absorbing path within the interface layers.4 Lackey and co-workers16 have advanced that concept to produce multilayer matrices. Reducing the thickness of the individual carbon layers in SiC-matrix composites has other advantages. For applications in high-radiation environments, such as nuclear fusion reactors, the presence of carbon can lead to dimensional instability. Carbon tends to change dimensions anisotropically during irradiation, causing fiber decohesion and loss of stress transfer between fibers and the matrix, thus causing loss of composite behavior.17 The use of thinner layers of carbon may reduce this effect. In the work of Naslain4 and Rebillat,7 composites were fabri￾cated from ceramic-grade Nicalon coated with one to four pairs of C/SiC coatings that had a total thickness of 0.5 mm. The work consisted of preparing both minicomposites, which consist of a single tow of fibers infiltrated with the fiber coating and matrix, and larger, two-dimensionally reinforced tensile specimens. Me￾chanical properties were measured by fiber pushout and tensile testing. The mechanical properties of the as-fabricated multilayer interface composites did not appear to be different from those of samples with a single-carbon-layer interface. Microscopic exami￾nation revealed that crack deflection occurred at the interface between the fiber and the first carbon layer in composites prepared with as-received Nicalon, regardless of the number of interface layers. They determined that a fiber-surface treatment would be required to increase the adhesion between the first carbon layer and the fiber. Cracks would then be directed into the weaker multilayer interface material. More et al.18 studied ceramic-matrix composites with multi￾layer C/SiC-interface layers prepared by Hyper-Therm, Inc. (Hun￾tington Beach, CA).10 The work involved the characterization of 12 pairs of C/SiC coatings with the SiC film thickness increasing from the fiber to the matrix side of the interface. The total thickness of these interfaces was almost 3 mm. More et al., 18 in agreement with Naslain4 and Rebillat,7 concluded that the coating closest to the fiber controlled the mechanical behavior of the composite for composites in which untreated fibers were utilized. Stress-rupture testing (four-point bending) at 950°C in air at did not show any improvement in oxidation resistance over a compos￾ite with a single 0.25-mm carbon interface layer. R. Naslain—contributing editor Manuscript No. 189173. Received August 25, 1999; approved July 17, 2000. This research was sponsored by the Office of Fossil Energy, Advanced Research and Technology Development Materials Program, U.S. Department of Energy, and performed by Oak Ridge National Laboratory, which is operated by UT-Battelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. *Member, American Ceramic Society. J. Am. Ceram. Soc., 83 [12] 3014–20 (2000) 3014 journal