J.Am. Ceram.So,88间61521-1528(2005 DO:10.l111551-2916.2005.00303.x urna Toughening of Mullite/Cordierite Laminated Composites by Transformation Weakening of B-Cristobalite Interphases Waltraud M. Kriven*,**, f and Sang-Jin Lee* f Department of Materials Science and Engineering. University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 An interesting concept for achieving graceful failure in oxide panied by a volume increase of (+)3.0% at 950C or(+)4.9% composites is discussed. It is based on crack deflection in a weak temperature. It is postulated that"transformation interphase between a matrix and reinforcement (e.g. fiber), weakening "of ceramic interphases can lead to overall toughen- around a fibrous core in a fibrous monolith, or in an interphase ing of a ceramic matrix composite. The interphases can separate in a laminated composite. The interphase can be phase trans- laminates, fibrous monolithic cores, or be placed at fiber/matrix formation weakened by a crystallographic unit cell, volume con- interfaces In thermally induced transformations, all interphases traction, and or shape change. Mullite/cordierite laminates with re pre-transformed before the approach of a crack, with some B-a-cristobalite, transformation-weakened interphase were consequent loss of overall strength of the material. In the ideal, investigated for interphase debonding behavior. The laminates shear stress-induced case, an oncoming crack induces a trans- were fabricated by stacking alternate tape-cast, green sheets of formation in its immediate environment, with strength only chemically doped p-cristobalite, which was synthesized by an minimally reduced throughout the bulk. Maximum toughening organic steric entrapment method, and a mullite/cordierite ma- is achieved, since the propagating crack needs to do work in trix mixture. The laminate showed fracture behavior dependir order to overcome the nucleation barrier and cause transforma na critical particle size effect. The grain size of polycrystalline tion, and onset of the other synergistic toughening mechanisms B-cristobalite was controlled by annealing time at 1300C.a(e.g. crack formation)occurs. t-pressed laminated composite, annealed for 10 h at 1300oC. The terminology of"transformation weakening "was first had an average grain size of -4 um and a 3-point flexure troduced by Kriven to describe the deleterious effect of strength of 131 MPa. Its work of fracture was 2.4 k 5.5% volume contraction in enstatite(Mgo- SiO] or MgSio non-catastrophic fracture behavior was demonstrated as it transforms from orthorhombic protoenstatite to monocline dentation response indicated crack deflection along the cristobal- ic clinoenstatite at 865C on quenching. Elastic tensor calcula lite debonding inte With nnealing time, the tions of the enstatite transformation strain 19,20 indicated an trength decreased due to the formation of internal macrocrack anisotro lume contraction, with a maximum 16.5% in the cristobalite layer, which occurred spontaneously during in the [c] axis of the product phase with respect to the parent thermally induced transformation crystal lattice. This gives rise to intragranular microcracks ori- ented perpendicular to the [c] monoclinic axis." Intragranular due to thermally induced transformation have been observed-in clinoenstatite grains which were grown HE brittleness and unreliability of ceramics remain difficult beyond a critical particle size of 7 un and unsolved problems. Attempts to impart"graceful fail a distinction can be drawn between thermally versus stress- ure"analogous to ductility in metals have been partially suc- induced transformations. Displacive transformations can be as- cessful with the use of composites. These are ceramic matrices sociated with a critical particle size in dense ceramics reinforced with fibers, particulates, platelets, or whisker-shaped tudies in zirconia transformation toughening, it is known that second phases. It is now well established that toughening results exceeding their critical particle size transform from debonding at the interface between matrix and reinforce usly on cooling through their transformation temperature. Op- ment phase. 2-7This is a crucial step leading to crack energy timally aged grains can be metastably retained down to room issipation, such as frictional sliding at the interface, while temperature, but can be induced to transform through the ac- transferring load-bearing forces on to fibers.-fo-i7using crack an interphase transformation-weakened composite, the differ- tion of applied shear or tensile stresses In the context of deflection mechanisms to operate in laminates The aim of this work was to introduce a new mechanism of ence between a shear- versus thermally-induced mechanism is nterfacial debonding in oxide ceramics, which has a small mis that in the latter, almost all of the interphase coating grains have atch of thermal expansion coefficient. It is postulated that an Iready transformed at room temperature, while in the former overall increase in toughness can be achieved by inducing a most of the grains are ideally at their optimum critical particle by a large, but negative volume change on cooling, or significant the critical resolved shear stress needed for transformation, 8- unit cell shape change. The proposed mechanism is schemat- The shear stress-induced transformation may be an effective ically illustrated in Fig. 1, and is based on the analogy with toughening mechanism in fully dense bodies. In the as-fabricat transformation toughening in zirconia(Zro,) which is accom- ed state, the composite has maximum bulk strength, and spe- cifically, transverse strength in directions perpendicular to fiber D. Marshalk-contributing editor lengths. Should a matrix crack approach the fiber, however, it induces transformation weakening in the interphase, but only in the immediate area of the crack. rather than in all of the inter uscript No 11138 Received June 25, 2004; approved December 14, 2004 mally induced transformation orted by the U.s. Air Force Office of Scientific Research, under Grant number In this paper, we demonstrate the feasibility of transforma on weakening as a viable debonding mechanism in ceramic matrix composites. A model system was chosen based on the res: Depa kent of Materials Science and Engincering Mokpo National cubic(B)to tetragonal(a) transformation in cristobalite(SiO2) The lattice correspondence operating in the cubic-tetragonalToughening of Mullite/Cordierite Laminated Composites by Transformation Weakening of b-Cristobalite Interphases Waltraud M. Kriven* , **,w and Sang-Jin Lee* ,z Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 An interesting concept for achieving graceful failure in oxide composites is discussed. It is based on crack deflection in a weak interphase between a matrix and reinforcement (e.g. fiber), around a fibrous core in a fibrous monolith, or in an interphase in a laminated composite. The interphase can be phase trans￾formation weakened by a crystallographic unit cell, volume con￾traction, and/or shape change. Mullite/cordierite laminates with a b-a-cristobalite, transformation-weakened interphase were investigated for interphase debonding behavior. The laminates were fabricated by stacking alternate, tape-cast, green sheets of chemically doped b-cristobalite, which was synthesized by an organic steric entrapment method, and a mullite/cordierite ma￾trix mixture. The laminate showed fracture behavior depending on a critical particle size effect. The grain size of polycrystalline b-cristobalite was controlled by annealing time at 13001C. A hot-pressed laminated composite, annealed for 10 h at 13001C, had an average grain size of B4 lm and a 3-point flexure strength of 131 MPa. Its work of fracture was 2.4 kJ/m2 but non-catastrophic fracture behavior was demonstrated. The in￾dentation response indicated crack deflection along the cristoba￾lite debonding interphase. With increasing annealing time, the strength decreased due to the formation of internal macrocracks in the cristobalite layer, which occurred spontaneously during thermally induced transformation. I. Introduction THE brittleness and unreliability of ceramics remain difficult and unsolved problems. Attempts to impart ‘‘graceful fail￾ure’’ analogous to ductility in metals have been partially suc￾cessful with the use of composites. These are ceramic matrices reinforced with fibers, particulates, platelets, or whisker-shaped second phases.1 It is now well established that toughening results from debonding at the interface between matrix and reinforce￾ment phase.2–7 This is a crucial step leading to crack energy dissipation, such as frictional sliding at the interface, while transferring load-bearing forces on to fibers,7–13 or causing crack deflection mechanisms to operate in laminates.14–17 The aim of this work was to introduce a new mechanism of interfacial debonding in oxide ceramics, which has a small mis￾match of thermal expansion coefficient. It is postulated that an overall increase in toughness can be achieved by inducing a phase transformation in an oxide coating which is accompanied by a large, but negative volume change on cooling, or significant unit cell shape change.18 The proposed mechanism is schemat￾ically illustrated in Fig. 1, and is based on the analogy with transformation toughening in zirconia (ZrO2) which is accom￾panied by a volume increase of (1) 3.0% at 9501C or (1) 4.9% at room temperature. It is postulated that ‘‘transformation weakening’’ of ceramic interphases can lead to overall toughen￾ing of a ceramic matrix composite. The interphases can separate laminates, fibrous monolithic cores, or be placed at fiber/matrix interfaces. In thermally induced transformations, all interphases are pre-transformed before the approach of a crack, with some consequent loss of overall strength of the material. In the ideal, shear stress-induced case, an oncoming crack induces a trans￾formation in its immediate environment, with strength only minimally reduced throughout the bulk. Maximum toughening is achieved, since the propagating crack needs to do work in order to overcome the nucleation barrier and cause transforma￾tion, and onset of the other synergistic toughening mechanisms (e.g. crack formation) occurs. The terminology of ‘‘transformation weakening’’ was first in￾troduced by Kriven19 to describe the deleterious effect of the
5.5% volume contraction in enstatite (MgO SiO2 or MgSiO3) as it transforms from orthorhombic protoenstatite to monoclin￾ic clinoenstatite at 8651C on quenching. Elastic tensor calcula￾tions of the enstatite transformation strain19,20 indicated an anisotropic volume contraction, with a maximum of
16.5% in the [c] axis of the product phase with respect to the parent crystal lattice. This gives rise to intragranular microcracks ori￾ented perpendicular to the [c] monoclinic axis.20 Intragranular microcracks due to thermally induced transformation have been previously observed21 in clinoenstatite grains which were grown beyond a critical particle size of 7 mm.22 A distinction can be drawn between thermally versus stress￾induced transformations. Displacive transformations can be as￾sociated with a critical particle size in dense ceramics.22–25 From studies in zirconia transformation toughening, it is known that grains exceeding their critical particle size transform spontane￾ously on cooling through their transformation temperature. Op￾timally aged grains can be metastably retained down to room temperature, but can be induced to transform through the ac￾tion of applied shear or tensile stresses.23,26–28 In the context of an interphase transformation-weakened composite, the differ￾ence between a shear- versus thermally-induced mechanism is that in the latter, almost all of the interphase coating grains have already transformed at room temperature, while in the former, most of the grains are ideally at their optimum critical particle size, ready to be stress-induced by a propagating crack or by the critical resolved shear stress needed for transformation.28–33 The shear stress-induced transformation may be an effective toughening mechanism in fully dense bodies. In the as-fabricat￾ed state, the composite has maximum bulk strength, and spe￾cifically, transverse strength in directions perpendicular to fiber lengths. Should a matrix crack approach the fiber, however, it induces transformation weakening in the interphase, but only in the immediate area of the crack, rather than in all of the inter￾phases throughout the bulk of the material, as occurs in ther￾mally induced transformation. In this paper, we demonstrate the feasibility of transforma￾tion weakening as a viable debonding mechanism in ceramic matrix composites. A model system was chosen based on the cubic (b) to tetragonal (a) transformation in cristobalite (SiO2). The lattice correspondence operating in the cubic-tetragonal 1521 Journal J. Am. Ceram. Soc., 88 [6] 1521–1528 (2005) DOI: 10.1111/j.1551-2916.2005.00303.x D. Marshall—contributing editor Supported by the U.S. Air Force Office of Scientific Research, under Grant number AFOSR-F49620-93-1-0227. *Member, American Ceramic Society. **Fellow, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: kriven@uiuc.edu z Present address: Department of Materials Science and Engineering, Mokpo National University, Muan 534-729, Korea. Manuscript No. 11138. Received June 25, 2004; approved December 14, 2004.