Journal J. An. Ceran. Soc, 80 [10] 2171-87(1997) Fibrous monolithic ceramics Desiderio Kovar, * t Bruce H. King, *. Rodney W. Trice, *and John W.Halloran Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136 Fibrous monolithic ceramics are an example of a laminate Si3N4) into fibrouscells' separated by boron nitride(BN) in which a controlled, three-dimensional structure has been 'cell boundaries results in monolithic ceramics with wood- introduced on a submillimeter scale. This unique structure like fibrous structures. which are called fibrous monolithic allows this all-ceramic material to fail in a nonbrittle man- ceramics "3 Fibrous monoliths are fabricated using a coextru- ner. Materials have been fabricated and tested with a va- sion process, 5 to produce green filaments. The filaments then structure of the constituent phases and the architecture in Among the many materials that have been manufactured using hich they are arranged are discussed. The elastic proper- icon nitride-boron nitride(Si3Na-BI Cxisting models. These models also can be extendedipasg brous monoliths are the most promising ties of these materials can be effectively predicted In this article we examine the structure of Si. N-BN fibrous orientation and architecture. However the mechanisms cell-cell boundary features to the nanometer scale of the BN that govern the energy absorption capacity of fibrous cell boundaries. We also show how the elastic properties and monoliths are unique, and experimental results do not fol- strength vary with the architecture of the cells, and how this low existing models. Energy dissipation occurs through two can be described using laminate theory. We present the fracture dominant mechanisms--delamination of the weak inter- behavior in some detail, relating the strength and fracture en- chases and then frictional sliding after cracking occurs ergy to fracture of the SisNa cells and crack deflection within The properties of the constituent phases that maximize en the bn cell boundaries ergy absorption are discussed IL. Structure of SiaN-BN Fibrous Monoliths . Introduction () Submillimeter Structure looK AND gORDon first introduced the idea that crack propagation in brittle materials could be controlled by in- Figure 1. constructed from low-magnification scanning elec- ron microscopy(SEM) micrographs of polished sections corporating a fabric of microstructural features that change the shows three-dimensional representations of the submillimeter crack path. More recently, Clegg demonstrated that, by ar- structure of two architectures of fibrous monoliths. The poly ranging layers of a strong phase and separating them with weak crystalline SisN, cells appear in dark contrast, and the continu- ous Bn cell boundaries appear in bright contrast. The cross brittle manner. Another way to accomplish this is to generalize section of Fig. 1(a) shows the Si Na cells as flattened hexagons the idea of a laminate by adding a three-dimensional structure with an aspect ratio of -2. The cells are -200 um wide; there- f crack-modifying features. The division of silicon nitride fore, there are several hundred B-Si3N4 grains through the thickness of each Sis N, cell. For the uniaxially aligned archi- tecture shown in Fig. 1(a), the SigNa cells run continuous down the length of the specimen. Figure 1(b) illustrates the D.J. Greer--Contributing editor [0790] architecture, where uniaxially aligned layers are rotated 90 between lamina. The architecture of fibrous monoliths is altered easily by changing the stacking sequence of filament layers. Much of our work has focused on the [0/45/90] archi- Manuscript No. 19182 Received February 24, 1%b a ed June Research tecture, which has isotropic elastic properties in the plane of the jects Agency under Contract No. No014-95-0302. tNow with the University The cell boundaries are typically 15-25 um thick layers of Now with Sandia National Laboratory polycrystalline BN consisting of many well-aligned BN grains eature 2471Fibrous Monolithic Ceramics Desiderio Kovar,*,† Bruce H. King,*,‡ Rodney W. Trice,* and John W. Halloran* Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136 Fibrous monolithic ceramics are an example of a laminate in which a controlled, three-dimensional structure has been introduced on a submillimeter scale. This unique structure allows this all-ceramic material to fail in a nonbrittle man￾ner. Materials have been fabricated and tested with a va￾riety of architectures. The influence on mechanical prop￾erties at room temperature and at high temperature of the structure of the constituent phases and the architecture in which they are arranged are discussed. The elastic proper￾ties of these materials can be effectively predicted using existing models. These models also can be extended to pre￾dict the strength of fibrous monoliths with an arbitrary orientation and architecture. However, the mechanisms that govern the energy absorption capacity of fibrous monoliths are unique, and experimental results do not fol￾low existing models. Energy dissipation occurs through two dominant mechanisms—delamination of the weak inter￾phases and then frictional sliding after cracking occurs. The properties of the constituent phases that maximize en￾ergy absorption are discussed. I. Introduction COOK AND GORDON1 first introduced the idea that crack propagation in brittle materials could be controlled by in￾corporating a fabric of microstructural features that change the crack path. More recently, Clegg2 demonstrated that, by ar￾ranging layers of a strong phase and separating them with weak interphases, brittle ceramics could be made to fail in a non￾brittle manner. Another way to accomplish this is to generalize the idea of a laminate by adding a three-dimensional structure of crack-modifying features. The division of silicon nitride (Si3N4) into fibrous ‘‘cells’’ separated by boron nitride (BN) ‘‘cell boundaries’’ results in monolithic ceramics with wood￾like fibrous structures, which are called ‘‘fibrous monolithic ceramics.’’3 Fibrous monoliths are fabricated using a coextru￾sion process4,5 to produce green filaments. The filaments then are arranged using methods similar to those used to manufac￾ture textiles, creating analogs of many composite architectures. Among the many materials that have been manufactured using this technique,6–9 silicon nitride–boron nitride (Si3N4–BN) fi￾brous monoliths are the most promising. In this article, we examine the structure of Si3N4–BN fibrous monoliths from the submillimeter scale of the crack-deflecting cell–cell boundary features to the nanometer scale of the BN cell boundaries. We also show how the elastic properties and strength vary with the architecture of the cells, and how this can be described using laminate theory. We present the fracture behavior in some detail, relating the strength and fracture en￾ergy to fracture of the Si3N4 cells and crack deflection within the BN cell boundaries. II. Structure of Si3N4–BN Fibrous Monoliths (1) Submillimeter Structure Figure 1, constructed from low-magnification scanning elec￾tron microscopy (SEM) micrographs of polished sections, shows three-dimensional representations of the submillimeter structure of two architectures of fibrous monoliths. The poly￾crystalline Si3N4 cells appear in dark contrast, and the continu￾ous BN cell boundaries appear in bright contrast. The cross section of Fig. 1(a) shows the Si3N4 cells as flattened hexagons with an aspect ratio of ∼2. The cells are ∼200 mm wide; there￾fore, there are several hundred b-Si3N4 grains through the thickness of each Si3N4 cell. For the uniaxially aligned archi￾tecture shown in Fig. 1(a), the Si3N4 cells run continuously down the length of the specimen. Figure 1(b) illustrates the [0/90] architecture, where uniaxially aligned layers are rotated 90° between lamina. The architecture of fibrous monoliths is altered easily by changing the stacking sequence of filament layers. Much of our work has focused on the [0/±45/90] archi￾tecture, which has isotropic elastic properties in the plane of the lamina. The cell boundaries are typically 15–25 mm thick layers of polycrystalline BN consisting of many well-aligned BN grains. D. J. Green–Contributing editor Manuscript No. 191182. Received February 24, 1997; approved June 6, 1997. Supported by U.S. Office of Naval Research and Defense Advanced Research Projects Agency under Contract No. N0014-95-0302. *Member, American Ceramic Society. † Now with the University of Texas at Austin. ‡ Now with Sandia National Laboratory. J. Am. Ceram. Soc., 80 [10] 2471–87 (1997) Journal 2471