journal J.Am. Ceram.Soc,812]329-3602000) Fugitive Interfacial Carbon Coatings for Oxide/Oxide Composites Kristin A. Keller, * T Tai-lI Mah, * T Triplicane A. Parthasarathy, f and Charles M. Cooke? Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLN, Wright-Patterson AFB The effectiveness of fugitive interfacial carbon coatings in prevents fiber/matrix reaction during processing an NextelM 720-based composites was investigated. Dense defines th of the interfacial gap. In the case of an idealized >90%) matrix( calcium aluminosilicate,0°and±45°)com p, the fiber would be unconstrained after carbon ites and porous matrix (mullite/alumina, eight-harness removal, with no load transfer from the matrix to the fiber(Fig atin fabric) composites were fabricated and tensile tested in I(a)). The fibers would easily pull out from the fractured matrix two control conditions(uncoated or carbon-coated) and with and the corresponding pull-out lengths would be extremely long the carbon removed ( fugitive interface). Results indicated that In reality, however, load transfer occurs by the combined effects arbon removal in dense matrix composites did not signifi- of mechanical interlocking, intermittent fiber/matrix bonding, and antly change unidirectional composite strength, even after roughness of both the fiber and matrix. Figure 1(b) illustrates this long-term exposure at 1000C. For porous matrix composites, scenario, where a crack is shown approaching a rough fiber/matrix composite strength was independent of the fiber/matrix inter- interface. As the crack approaches the interface, matrix strain face, even after exposure at 1150%C for 500 h in air. results in sliding at the fiber/matrix interface, leading to point contacts between the fiber and matrix where mechanical interlock- ing and possible bonding occurs. Upon further stressing, the fiber L. Introduction Qr is well-known that the properties of ceramic composites can be experienced ahead of the crack tip results in the loading of the fiber otinv The primary approach toward this control has been could be activated resulting in the failure of the fiber. Pull-out lengths in this case would be much shorter and would depend on provide interfaces that ultimately increase the strain-to-failure of urface roughnesses. The viability of this concept has been the composites through crack deflection and fiber pullout, A demonstrated previously variety of coatings have been developed for use in ceramic-matri One of the most important factors associated with these coatings composites(CMCs)- with vary ing degrees of success, however, is the thickness of the carbon, which defines the gap. If the BN and carbon-4 remain the most widely used interface too wide. there would be little interaction between fiber and materials matrix. Presently, there is littile quantitative information as to an Carbon forms a weak interface between the fiber and the matrix optimal thickness. It is expected that the optimal thickness is in a composite, as first demonstrated in the NicalonM(Nippon tem-specific because of several factors, such as coefficient of Carbon Co., Tokyo, Japan) fiber/glass matrix composites, where thermal expansion mismatch and roughness effects of the fiber and matrIX the carbon layer is formed in situ. 4 The problem inherent with Composites with a fugitive gap at the interface also can be these Nicalon-based composites is the oxidation of the in situ thought of as one extreme in a range of composites wsa rated carbon layer at elevated temperatures, which results in a rapid illing of the interface with a SiO, reaction product from the ifferent distributions in the matrix. If the pores are concer fiber. This strongly bonded interface, in turn, leads to cata- at the interface, the interface is a gap. If the pores are distributed but still very close to the interface, the interface has a porous strophic failure of the specimens. However, work completed by interlayer. 7, io If the pores are distributed uniformly over the entire Plucknett et al.- indicated that Nicalon/calcium aluminosilicate (CAS)composite strength could be retained if the in situ carbon matrix, the composite is a porous matrix composite. Note that the was removed at a temperature below SiO, formation(450C) overall porosity in these composites varies significantly because of ave not This suggests that, for composites containing oxidatively stable the difference in the volume of th porous material. There fibers and matrices, there is a possibility of using a gap at the been any studies aimed at understanding the relative merits of interface to protect the fiber and achieve good strength. This these variations in composite design. In this work, we attempt to principle can be used to build composites with fugitive coat- compare the strength of a porous matrix composite with that of one The fugitive coating concept relies on the retention of a carbon Another objective of the present work is to examine the benefits and effectiveness of using fugitive coatings in both dense matrix coating during composite processing and its subsequent removal and porous matrix through oxidation. This removal leaves an unbonded fiber/matrix osites. both uncoated fibers and carbon- coated fibers embedded in dense and porous matrices are used as interface and, for most choices of coating thickness and coefficient control composites. The effect of fugitive gap thickness on the p. The carbon primarily modulus and strength of both unidirectional and +45 composites is studied. Finally, the relative merits of dense matrix composites th fugitive coatings and porous matrix composites are discussed F. Zok--contributing edito in light of the presented results I. Experimental Procedures () Dense Matrix Com rted by the Air Fe Laboratory under No.F3361596 Fiber tows(Nextel M 720, 3M, St. Paul, MN) were nominally coated with either a 0.04 or a 0.02 um thick carbon coa UES, Inc, Dayton, (Synterials, Inc Herndon, VA). These coatings were depositedFugitive Interfacial Carbon Coatings for Oxide/Oxide Composites Kristin A. Keller,* ,† Tai-Il Mah,* ,† Triplicane A. Parthasarathy,* ,† and Charles M. Cooke† Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLN, Wright–Patterson AFB, Ohio 45433 The effectiveness of fugitive interfacial carbon coatings in Nextel™ 720-based composites was investigated. Dense (>90%) matrix (calcium aluminosilicate, 0° and 645°) com￾posites and porous matrix (mullite/alumina, eight-harness satin fabric) composites were fabricated and tensile tested in two control conditions (uncoated or carbon-coated) and with the carbon removed (fugitive interface). Results indicated that carbon removal in dense matrix composites did not signifi￾cantly change unidirectional composite strength, even after long-term exposure at 1000°C. For porous matrix composites, composite strength was independent of the fiber/matrix inter￾face, even after exposure at 1150°C for 500 h in air. I. Introduction I T IS well-known that the properties of ceramic composites can be optimized if control over the fiber/matrix interface can be gained.1–4 The primary approach toward this control has been through the application of fiber coatings, which provide weakly bonded interfaces that ultimately increase the strain-to-failure of the composites through crack deflection and fiber pullout. A variety of coatings have been developed for use in ceramic-matrix composites (CMCs)5–17 with varying degrees of success; however, BN18–21 and carbon22–24 remain the most widely used interface materials. Carbon forms a weak interface between the fiber and the matrix in a composite, as first demonstrated in the Nicalon™ (Nippon Carbon Co., Tokyo, Japan) fiber/glass matrix composites, where the carbon layer is formed in situ. 23,25 The problem inherent with these Nicalon-based composites is the oxidation of the in situ carbon layer at elevated temperatures, which results in a rapid filling of the interface with a SiO2 reaction product from the fiber.26 This strongly bonded interface, in turn, leads to cata￾strophic failure of the specimens. However, work completed by Plucknett et al.27 indicated that Nicalon/calcium aluminosilicate (CAS) composite strength could be retained if the in situ carbon was removed at a temperature below SiO2 formation (;450°C). This suggests that, for composites containing oxidatively stable fibers and matrices, there is a possibility of using a gap at the interface to protect the fiber and achieve good strength. This principle can be used to build composites with fugitive coat￾ings.13,28 The fugitive coating concept relies on the retention of a carbon coating during composite processing and its subsequent removal through oxidation. This removal leaves an unbonded fiber/matrix interface and, for most choices of coating thickness and coefficient of thermal expansion mismatch, a gap. The carbon primarily prevents possible fiber/matrix reaction during processing and defines the width of the interfacial gap. In the case of an idealized smooth, straight gap, the fiber would be unconstrained after carbon removal, with no load transfer from the matrix to the fiber (Fig. 1(a)). The fibers would easily pull out from the fractured matrix and the corresponding pull-out lengths would be extremely long. In reality, however, load transfer occurs by the combined effects of mechanical interlocking, intermittent fiber/matrix bonding, and roughness of both the fiber and matrix. Figure 1(b) illustrates this scenario, where a crack is shown approaching a rough fiber/matrix interface. As the crack approaches the interface, matrix strain results in sliding at the fiber/matrix interface, leading to point contacts between the fiber and matrix where mechanical interlock￾ing and possible bonding occurs. Upon further stressing, the fiber is prevented from slipping due to the roughness. The stress experienced ahead of the crack tip results in the loading of the fiber in an isolated region between the contact points. Flaws in the fiber could be activated, resulting in the failure of the fiber. Pull-out lengths in this case would be much shorter and would depend on surface roughnesses. The viability of this concept has been demonstrated previously.28 One of the most important factors associated with these coatings is the thickness of the carbon, which defines the gap. If the gap is too wide, there would be little interaction between fiber and matrix. Presently, there is little quantitative information as to an optimal thickness. It is expected that the optimal thickness is system-specific because of several factors, such as coefficient of thermal expansion mismatch and roughness effects of the fiber and matrix. Composites with a fugitive gap at the interface also can be thought of as one extreme in a range of composites with pores of different distributions in the matrix. If the pores are concentrated at the interface, the interface is a gap. If the pores are distributed but still very close to the interface, the interface has a porous interlayer.7,10 If the pores are distributed uniformly over the entire matrix, the composite is a porous matrix composite.29 Note that the overall porosity in these composites varies significantly because of the difference in the volume of the porous material. There have not been any studies aimed at understanding the relative merits of these variations in composite design. In this work, we attempt to compare the strength of a porous matrix composite with that of one with a fugitive gap. Another objective of the present work is to examine the benefits and effectiveness of using fugitive coatings in both dense matrix and porous matrix composites. Both uncoated fibers and carbon￾coated fibers embedded in dense and porous matrices are used as control composites. The effect of fugitive gap thickness on the modulus and strength of both unidirectional and 645° composites is studied. Finally, the relative merits of dense matrix composites with fugitive coatings and porous matrix composites are discussed in light of the presented results. II. Experimental Procedures (1) Dense Matrix Composites Fiber tows (Nextel™ 720, 3M, St. Paul, MN) were nominally coated with either a 0.04 or a 0.02 mm thick carbon coating (Synterials, Inc., Herndon, VA). These coatings were deposited F. Zok—contributing editor Manuscript No. 189762. Received November 5, 1998; approved July 15, 1999. Supported by the Air Force Research Laboratory under Contract No. F33615-96- C-5258. *Member, American Ceramic Society. † UES, Inc., Dayton, OH 45432. J. Am. Ceram. Soc., 83 [2] 329–36 (2000) 329