《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_j.1151-2916.1997.tb03148.x[1]

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 2471

Fibrous 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

Journal of the American Ceramic Society-Kovar et al. Vol. 80. No. 10 Fig. 2. SEM micrograph of a fracture surface showing a BN cell boundary between two Si,N, cells viewed edge on. Glassy Phase 20m Fig. 1. Low-magnification SEM composites illustrating three sec- (Si,N, cells run continuously down the length of 5um separated by BN cell boundaries)and a(b)[/90] architectu of cells are stacked with a 90 rotation between the fractue surface prateletike mor phology of the bn irais as wetl The BN grain alignment is obvious by visual examination and as the discontinuous glassy phase are visible confirmed by X-ray diffractometry (XRD). 0 It is crucial that the(0001)cleavage planes be oriented parallel to the Si3N4 interface; otherwise, cracks do not deflect in the BN inter- boundary, looking down onto the fracture surface. The platey hase. This grain alignment occurs during the coextrusion step of green fabrication, during which the BN platelets are plane oriented parallel to the cell boundary. In this secondary- (2) Microstructure at Scale of the grains darker regions are BN platelets and the brighter areas are yttria aluminosilicate glass The microstructure within the Si3N4 cells is quite conven- lon-milled samples of fibrous monoliths were prepared for tional for this particular grade of Si, N4 densified with 6 wt% ansmission electron microscopy(TEM)using techniques de- ular, grains within a matrix of a glassy, grain-boundary revealed by TE Sewhere. 12 The major features of the BN rocracks between the (0001) basal planes of BN platelets. These are shown in Fig. 4 ditions, we find B-Si Na grains 0.2-1.5 um wide, with aspect The inset diffraction pattern indicates the foil plane to be Junctions. Figure 2 is an SEM micrograph of a fracture s efie, Som G,, te how each BN grain has exfoliated along its basal ratios of 2-10. The grain-boundary phase is glassy, present as (20).No the usual thin film between grains and in pockets at Si3 N4 grain planes into many layers. A higher magnification view of a BN in shown in Fig. 5 reveals a finer pattern of microcrack showing a Bn cell boundary between two Si3 N4 cells. Visual Some layers are divided as fine as 50 nm. (A unit graphi inspection suggests that many of the B-Si3N4 grains in the cell layer in the BN crystal structure has a thickness of are oriented with their [0001] long axes aligned along the cell nm. direction. This texture has been confirmed by XRD. o Note A similar microcrack structure has been described by also the obvious orientation of the Bn platelets in the cell Mrozowski 3 in graphite that has a crystalline structure similar boundary to BN 14 Sinclair and Simmons is have attributed these basal Figure 3 is an SEM micrograph of the fractured BN-rich cell plane cracks that they observed using TEM to the thermal

The BN grain alignment is obvious by visual examination and confirmed by X-ray diffractometry (XRD).10 It is crucial that the (0001) cleavage planes be oriented parallel to the Si3N4 interface; otherwise, cracks do not deflect in the BN inter￾phase.11 This grain alignment occurs during the coextrusion step of green fabrication, during which the BN platelets are oriented by the flow field in the extrusion die. (2) Microstructure at Scale of the Grains The microstructure within the Si3N4 cells is quite conven￾tional for this particular grade of Si3N4 densified with 6 wt% Y2O3 and 2 wt% Al2O3. This grade of Si3N4 consists of acic￾ular b-Si3N4 grains within a matrix of a glassy, grain-boundary phase. For our particular raw materials and densification con￾ditions, we find b-Si3N4 grains 0.2–1.5 mm wide, with aspect ratios of 2–10. The grain-boundary phase is glassy, present as the usual thin film between grains and in pockets at Si3N4 grain junctions. Figure 2 is an SEM micrograph of a fracture surface, showing a BN cell boundary between two Si3N4 cells. Visual inspection suggests that many of the b-Si3N4 grains in the cell are oriented with their [0001] long axes aligned along the cell direction. This texture has been confirmed by XRD.10 Note also the obvious orientation of the BN platelets in the cell boundary. Figure 3 is an SEM micrograph of the fractured BN-rich cell boundary, looking down onto the fracture surface. The platey features are the BN grains, which lie with their (0001) basal plane oriented parallel to the cell boundary. In this secondary￾electron micrograph, there are two distinct contrast areas. The darker regions are BN platelets and the brighter areas are yttria aluminosilicate glass. Ion-milled samples of fibrous monoliths were prepared for transmission electron microscopy (TEM) using techniques de￾scribed in detail elsewhere.12 The major features of the BN revealed by TEM are extensive microcracks between the (0001) basal planes of BN platelets. These are shown in Fig. 4. The inset diffraction pattern indicates the foil plane to be (2110). Note how each BN grain has exfoliated along its basal planes into many layers. A higher magnification view of a BN grain shown in Fig. 5 reveals a finer pattern of microcracking. Some layers are divided as fine as 50 nm. (A unit ‘‘graphine’’ layer in the BN crystal structure has a thickness of c0 4 0.66 nm.) A similar microcrack structure has been described by Mrozowski13 in graphite that has a crystalline structure similar to BN.14 Sinclair and Simmons15 have attributed these basal plane cracks that they observed using TEM to the thermal Fig. 3. SEM micrograph of the BN cell boundary looking down onto the fracture surface. Plateletlike morphology of the BN grains as well as the discontinuous glassy phase are visible. Fig. 1. Low-magnification SEM composites illustrating three sec￾tions of a fibrous monolith with a (a) uniaxially aligned architecture (Si3N4 cells run continuously down the length of the specimen and are separated by BN cell boundaries) and a (b) [0/90] architecture (layers of cells are stacked with a 90° rotation between lamina). Fig. 2. SEM micrograph of a fracture surface showing a BN cell boundary between two Si3N4 cells viewed edge on. 2472 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10

October 1997 Fibrous Monolithic Ceramics 2473 Boron Nitride Boron Nitride CTO00IDirection Glassy Phase Cracks 250nm Fig. 4. TEM micrograph showing extensive microcracks between the(0001)basal planes of the BN platelets. Also note the presence of Fig. 5. Higher-magnification view of a single bN platelet showing a glassy phase between the bn platelets expansion anisotropy between the a-axis and c-axis of graph- te.5In the basal plane, the coefficient of thermal expansion (CTE)of BN is slightly negative through 800C, about -2 10-6rC. 16 Perpendicular to the basal plane, the CTE is very large and positive, about +40 x 10-b/oC. As the composite Glassy Phase Silicon Nitride Doled from the hot-pressing ter ature(1750°C, the BN contracts perpendicular to the basal plane (i.e, in the [0001] direction), while there is a small expansion within the plane. Hf the surrounding Si3 Na grains or glassy phase constrain the BN platelets, large tensile stresses are developed perpendicular to the basal plane upon cooling. This acts to separate the BN platelet into layers along the basal plane direction. Further- more, shear stresses developed parallel to the basal plane shea the surfaces of the platelets relative to each other. The BN platelets labeled A and B in Fig. 5 clearly once existed as a single platelet before they were split and translated relative to one another during cooling A representative TEM micrograph of a Sia NBN interface shown in Fig. 6. There is no cracking between the Si3N4 and Boron Nitride the BN Rather there seems to be excellent adhesion between the two phases. a thin layer of glass is observed between the two phases in some places a glassy phase also is found residing in pockets in the BN cell boundary. No glass-forming compounds were added to the Bn powders; therefore, this glass must be residual liquid in- truded into the cell boundary from the neighboring Si N, cells 250nm during hot pressing. Figure 4 shows a large pocket of glass between exfoliated layers of BN. The selected-area electron diffraction pattern in Fig. 4 shows amorphous rings from the Fig. 6. Bright-field TEM image of a typical interface between the hase exists in pockets between booklets of bn grains. The composition of the glass in Si3N4 cells and BN cell-boundary glassy phases was determined with energy dispersive spectros- borate. The Y: Al ratio of the glass in the Bn cell boundaries copy(EDS). EDS spectra of the glassy phase between the bn is similar to the composition of the glass between Si3N4 grains platelets show the presence of yttrium, aluminum, silicon, oxy- Because of the presence of silicon, aluminum, and yttrium, it is gen, and nitrogen. Boron could not be detected by this EDS clear that the sintering-aid glass is being either drawn or forced spectrometer; therefore, we do not know if the glass contains into the bn during hot pressing

expansion anisotropy between the a-axis and c-axis of graph￾ite.15 In the basal plane, the coefficient of thermal expansion (CTE) of BN is slightly negative through 800°C, about −2 × 10−6/°C.16 Perpendicular to the basal plane, the CTE is very large and positive, about +40 × 10−6/°C.17 As the composite is cooled from the hot-pressing temperature (1750°C), the BN contracts perpendicular to the basal plane (i.e., in the [0001] direction), while there is a small expansion within the plane. If the surrounding Si3N4 grains or glassy phase constrain the BN platelets, large tensile stresses are developed perpendicular to the basal plane upon cooling. This acts to separate the BN platelet into layers along the basal plane direction. Further￾more, shear stresses developed parallel to the basal plane shear the surfaces of the platelets relative to each other. The BN platelets labeled A and B in Fig. 5 clearly once existed as a single platelet before they were split and translated relative to one another during cooling. A representative TEM micrograph of a Si3N4–BN interface is shown in Fig. 6. There is no cracking between the Si3N4 and the BN. Rather, there seems to be excellent adhesion between the two phases. A thin layer of glass is observed between the two phases in some places. A glassy phase also is found residing in pockets in the BN cell boundary. No glass-forming compounds were added to the BN powders; therefore, this glass must be residual liquid in￾truded into the cell boundary from the neighboring Si3N4 cells during hot pressing. Figure 4 shows a large pocket of glass between exfoliated layers of BN. The selected-area electron diffraction pattern in Fig. 4 shows amorphous rings from the glass with diffraction spots identified with BN. The glassy phase exists in pockets between booklets of BN grains. The composition of the glass in Si3N4 cells and BN cell-boundary glassy phases was determined with energy dispersive spectros￾copy (EDS). EDS spectra of the glassy phase between the BN platelets show the presence of yttrium, aluminum, silicon, oxy￾gen, and nitrogen. Boron could not be detected by this EDS spectrometer; therefore, we do not know if the glass contains borate. The Y:A1 ratio of the glass in the BN cell boundaries is similar to the composition of the glass between Si3N4 grains. Because of the presence of silicon, aluminum, and yttrium, it is clear that the sintering-aid glass is being either drawn or forced into the BN during hot pressing. Fig. 4. TEM micrograph showing extensive microcracks between the (0001) basal planes of the BN platelets. Also note the presence of a glassy phase between the BN platelets. Fig. 5. Higher-magnification view of a single BN platelet showing fine-scale pattern of microcracking. Fig. 6. Bright-field TEM image of a typical interface between the Si3N4 and the BN. October 1997 Fibrous Monolithic Ceramics 2473

Joumal of the American Ceramic Sociery-Kovar et al. Vol. 80. No. 10 Ill. Mechanical Properties of Si3NBN facial sliding resistance. Particular emphas Fibrous Monoliths veloping a methodology to predict the properties, stre materials as Fibrous monoliths are novel materials. therefore. it is a function of architecture. Because fibrous monoliths are in- essary to identify the micromechanical properties that infl tended for use in applications where stresses are primarily gen- ring properties. These include the fracture erated because of bending, we focus on flexural properties tance of cells the interfacial fracture resistance. and the In some respects, fibrous monoliths are similar to ceramic- Panel A. Processing of fibrous monoliths Schematic illustrations of the steps used to fabricate binder, is 83 vol% sinterable Si,N-6 wt%Y203-2 wt% SigNa-BN fibrous monoliths are shown in Fig. Al. We start Al,O3(6Yn2Al-Si3 N4)and 17 wt% BN by mixing conventional ceramic powders in a polyme Sheets of uniaxially aligned green filaments are produced Starck and Co. Newton. MA. or SN-e- by winding the filaments around a mandrel and fixing them into place with a spray adhesive. Fibrous monolith speci New York, NY), consist primarily of equiaxed ax-SiaN4 par- mens are assembled from these sheets. Typically, 25 sheets ticles, nominally 0.5 um in diameter, with a BET specifi are used to produce a specimen. The uniaxially aligned surface area of m/g. The BN powder is a well- chitecture is produced by stacking the sheets without Advanced Ceramics Corp, Cleveland, OH) moplastic; therefore, after stacking, the assembly is molded The thermoplastic extrudable compound is made by into a solid block at temperatures between 1000 and 150C ing ceramic powder with thermoplastic polymers in a heated at a pressure of 2 MPa Shaped objects can be formed using mixer. The solids loading for the cell materials (Sis N conventional compression-molding dies. The filaments, Y2O3, and Al,O3)is 52 vol% ceramic, whereas the cladding which initially have a round cross section, deform during (BN)contains 50 vol% ceramic. After it is mixed, the Siy N4 this warm-pressing operation, filling the interstitial spaces compound is compression molded into a 20 mm diameter etween the filaments and producing flattened hexagon rod. A similar BN compound is compression molded into a shaped cells cylindrical shell, I mm thick, with a 20 mm inner diameter The thermoplastic binder is removed by heating slowly to The bn shell is fitted around the Si3N4 rod to make a 700C in a nitrogen atmosphere Hot pressing at 1750C fo for a piston-style extruder. The feedrod is then 2 h produces a density of 3.05 g/cm3,-98% of the estimated through a heated extrusion die to create 220 um er green filaments with the same Si3 N4 core and BN ng as the feedrod. The flexible green filament is col- onal ZrO,(contamination from the milling media a spool. The ceramic composition, excluding the Extrude feedrod into fine filament feedrod Si NA-filled polymer Form extrusion feedrod heated di spoo上 Hot-press to densify rolysis to remove polymer. binder Laminate sheets of filament to form solid billet Fig. Al. Schematic illustrations showing processing route to fabricate fibrous monoliths

III. Mechanical Properties of Si3N4–BN Fibrous Monoliths Fibrous monoliths are novel materials; therefore, it is nec￾essary to identify the micromechanical properties that influence the engineering properties. These include the fracture resis￾tance of cells, the interfacial fracture resistance, and the inter￾facial sliding resistance. Particular emphasis is placed on de￾veloping a methodology to predict the elastic properties, strength, and energy absorption capability of these materials as a function of architecture. Because fibrous monoliths are in￾tended for use in applications where stresses are primarily gen￾erated because of bending, we focus on flexural properties. In some respects, fibrous monoliths are similar to ceramic￾Panel A. Processing of Fibrous Monoliths Schematic illustrations of the steps used to fabricate Si3N4–BN fibrous monoliths are shown in Fig. A1. We start by mixing conventional ceramic powders in a polymer binder system. The commercial Si3N4 powders (M11, H. C. Starck and Co., Newton, MA, or SN-E-10, Ube Industries, New York, NY), consist primarily of equiaxed a-Si3N4 par￾ticles, nominally 0.5 mm in diameter, with a BET specific surface area of 9–13 m2 /g. The BN powder is a well￾crystallized, hexagonal BN powder consisting of platey par￾ticles 7–10 mm in diameter and 0.1–0.3 mm thick (HCP-BN, Advanced Ceramics Corp., Cleveland, OH). The thermoplastic extrudable compound is made by mix￾ing ceramic powder with thermoplastic polymers in a heated mixer. The solids loading for the cell materials (Si3N4, Y2O3, and Al2O3) is 52 vol% ceramic, whereas the cladding (BN) contains 50 vol% ceramic. After it is mixed, the Si3N4 compound is compression molded into a 20 mm diameter rod. A similar BN compound is compression molded into a cylindrical shell, 1 mm thick, with a 20 mm inner diameter. The BN shell is fitted around the Si3N4 rod to make a feedrod for a piston-style extruder. The feedrod is then forced through a heated extrusion die to create 220 mm diameter green filaments with the same Si3N4 core and BN cladding as the feedrod. The flexible green filament is col￾lected on a spool. The ceramic composition, excluding the binder, is 83 vol% sinterable Si3N4–6 wt% Y2O3–2 wt% Al2O3 (6Y/2Al–Si3N4) and 17 wt% BN. Sheets of uniaxially aligned green filaments are produced by winding the filaments around a mandrel and fixing them into place with a spray adhesive. Fibrous monolith speci￾mens are assembled from these sheets. Typically, 25 sheets are used to produce a specimen. The uniaxially aligned architecture is produced by stacking the sheets without ro￾tation, whereas, for the [0/±45/90] architecture, the filament direction is rotated between layers. The filaments are ther￾moplastic; therefore, after stacking, the assembly is molded into a solid block at temperatures between 100° and 150°C at a pressure of 2 MPa. Shaped objects can be formed using conventional compression-molding dies. The filaments, which initially have a round cross section, deform during this warm-pressing operation, filling the interstitial spaces between the filaments and producing flattened hexagon￾shaped cells. The thermoplastic binder is removed by heating slowly to 700°C in a nitrogen atmosphere. Hot pressing at 1750°C for 2 h produces a density of 3.05 g/cm3 , ∼98% of the estimated theoretical density for this composition. XRD shows the presence of b-Si3N4, hexagonal BN, and a trace of tetrag￾onal ZrO2 (contamination from the milling media). Fig. A1. Schematic illustrations showing processing route to fabricate fibrous monoliths. 2474 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10

October 1997 Fibrous monolithic ceramics 247: matrix-fiber-reinforced composites. For example, we find that point bend test Charalambides et al. 21 This test is the elastic properties of fibrous monoliths can be predicted with specimen, and then loading it in tor pi the elastic behar four-point bend ination occurs. The steady-state einforced laminates. But the fra d necessary delamination crack and the of fibrous monoliths is quite different, because these materials specimen dimer used to compute the interfacial ontain neither strong fibers nor a weak matrix. The failure fracture resistar mechanisms and associated dissipative mechanisms that are important in fiber-reinforced composites, I9 do not occur r in (2) Elastic Properties fibrous monoliths; therefore, those theories are not applicable To predict the elastic response of fibrous monolithic ceram- Instead, we find that the fracture process that occurs in fibrous s with multiaxial architectures, it is necessary to first under monoliths can be described by existing theories for the fracture stand the elastic behavior of uniaxially aligned fibrous mono- count for the unique structure of fibrous monoliths on to ac- liths along principal directions. These predictions are made of two-dimensional layered materials after modificatio using appropriate micromechanical models and a knowledge of the elastic properties of the constituent materials. Once these () Experimental Procedure predictions are made, laminate theory is used to predict the Elastic properties of both the fibrous monolithic ceramics off-axis elastic properties for uniaxially aligned materials and and monolithic ceramics were measured using the impulse- the elastic moduli for fibrous monolithic ceramics with multi- excitation technique using a commercially available tester axial architectures. The predictions are verified by measuring Grindo-sonic Model MK4x, J. w. Lemmon, St Louis, MO) he elastic moduli in many test coupons ccording to ASTM E 494-92a 20 In this test, the specimen is We assume that uniaxial fibrous monolithic ceramics pos- excited using a small driver, and the resonant frequency is sess a plane of isotropy perpendicular to the axis of the fibrous measured using a piezoelectric transducer. The modulus is then texture. The additional assumption that out-of-plane stresses calculated from the resonant frequency, the specimen dimen- can be ignored reduces the number of required elastic constants sions, and the specimen density. Youngs modulus was deter to four and allows the use of classical laminate theory to predict mined using bars with dimensions 3 mm x 4 mm x 45 mm, and he properties at an arbitrary angle for materials with uniaxial shear modulus was determined on plates 3 mm x 20 mm x 45 architectures and the moduli for materials with biaxial archi- mm. Bars with a uniaxially aligned architecture were machined tectures 22 These four elastic constants are calculated in terms parallel or perpendicular to the fibrous texture to determine of the engineering properties E1, E2, G12, and v12.We express Young's modulus in the I and 2 directions(E and E2, respec these properties for each architecture in terms of the composi- of the BN (EBN) and sin angle(0) with respect to the I direction to determine E(0). The (EsN) constituent materials shear modulus was determined using plates machined with the (A) Elastic Properties along Principal Directions: Uni- ng axis parallel to the direction of interest. For biaxial archi- axial Architecture: All elastic property predictions for fibrous tectures, one layer was designated the 0o layer, and the axis of monolithic ceramics are made from the elastic moduli of the Strength measurements at room temperature and at elevated Si3N4 of 320 GPa is used. This value is obtained from mea- emperature were performed using a computer-controlled surements performed on bars of monolithic Si3 Na of the same screw-driven, testing machine(Model 4483, Instron Corp, composition as that of the fibrous monolithic cells and hot- Canton, MA)operated in displacement control. The crosshead pressed under the same conditions and onsistent with val displacement rate was 0.5 mm/min for all tests. Specimens cited in the literature. 23 It is difficult to measure the elastic were tested in four-point flexure with an inner span of 20 mm properties of bulk BN. Similar to fabricated graphite, 4 the and an outer span of 40 mm. For elevated-temperature tests, the elastic properties of bulk BN vary greatly with fabrication tech- and an outer strowds allowed to stabilize for 10 min prior nique. Furthermore, the high degree of internal damping makes to testing. The energy absorption capability of a specimen was measurement using the impulse-excitation technique difficult characterized by the work-of-fracture(WOF), which was com- Only a few examples of successful modulus measurements on outed by taking the total area under the load-displacement BN are known in the literature. Two of the more commonl ng by twice the cross-sectional area of the reported values are 19.6 GPa26 and 22 GPa.27 However, these specimen values should be used with caution because the microstructure Interfacial fracture resistance was determined using a four- of the BN present in hot-pressed fibrous monolithic ceramics is Panel B. Material Combinations Although this article focuses on fibrous monoliths made Table bl. Material Combinations that have been used om SiaN4 and BN, fibrous monoliths have been fabricated to Fabricate Fibrous monoliths using many different material combinations. Some ex- Cell boundary Reference amples of all-ceramic fibrous monoliths and metal-ceramic fibrous monoliths that have been successfully fabricated are All-ceramic fibrous monoliths presented below. The usual limitations to processing of composite materials also apply to fibrous monoliths: i.e. the Hb2 constituent materials must be phase compatible. In addition, the constituent materials must be compatible with the poly C(graphite) mer binders that are used in the extrusion process ALO -ZrO Ceramic-metal fibrous monoliths ALo Advanced Ceramic Research, Tucson, AZ

matrix–fiber-reinforced composites. For example, we find that the elastic properties of fibrous monoliths can be predicted with existing theories used for predicting the elastic behavior of traditional fiber-reinforced laminates. But the fracture behavior of fibrous monoliths is quite different, because these materials contain neither strong fibers nor a weak matrix. The failure mechanisms and associated dissipative mechanisms that are important in fiber-reinforced composites18,19 do not occur in fibrous monoliths; therefore, those theories are not applicable. Instead, we find that the fracture process that occurs in fibrous monoliths can be described by existing theories for the fracture of two-dimensional layered materials after modification to ac￾count for the unique structure of fibrous monoliths. (1) Experimental Procedure Elastic properties of both the fibrous monolithic ceramics and monolithic ceramics were measured using the impulse￾excitation technique using a commercially available tester (Grindo-sonic Model MK4x, J. W. Lemmon, St. Louis, MO) according to ASTM E 494-92a.20 In this test, the specimen is excited using a small driver, and the resonant frequency is measured using a piezoelectric transducer. The modulus is then calculated from the resonant frequency, the specimen dimen￾sions, and the specimen density. Young’s modulus was deter￾mined using bars with dimensions 3 mm × 4 mm × 45 mm, and shear modulus was determined on plates 3 mm × 20 mm × 45 mm. Bars with a uniaxially aligned architecture were machined parallel or perpendicular to the fibrous texture to determine Young’s modulus in the 1 and 2 directions (E1 and E2, respec￾tively). Young’s modulus also was measured as a function of angle (u) with respect to the 1 direction to determine E(u). The shear modulus was determined using plates machined with the long axis parallel to the direction of interest. For biaxial archi￾tectures, one layer was designated the 0° layer, and the axis of the bar was machined parallel to this layer. Strength measurements at room temperature and at elevated temperature were performed using a computer-controlled, screw-driven, testing machine (Model 4483, Instron Corp., Canton, MA) operated in displacement control. The crosshead displacement rate was 0.5 mm/min for all tests. Specimens were tested in four-point flexure with an inner span of 20 mm and an outer span of 40 mm. For elevated-temperature tests, the furnace temperature was allowed to stabilize for 10 min prior to testing. The energy absorption capability of a specimen was characterized by the work-of-fracture (WOF), which was com￾puted by taking the total area under the load–displacement curve and dividing by twice the cross-sectional area of the specimen. Interfacial fracture resistance was determined using a four￾point bend test developed by Charalambides et al.21 This test is performed by first notching a specimen, and then loading it in four-point bending until delamination occurs. The steady-state load necessary to propagate the delamination crack and the specimen dimensions are then used to compute the interfacial fracture resistance. (2) Elastic Properties To predict the elastic response of fibrous monolithic ceram￾ics with multiaxial architectures, it is necessary to first under￾stand the elastic behavior of uniaxially aligned fibrous mono￾liths along principal directions. These predictions are made using appropriate micromechanical models and a knowledge of the elastic properties of the constituent materials. Once these predictions are made, laminate theory is used to predict the off-axis elastic properties for uniaxially aligned materials and the elastic moduli for fibrous monolithic ceramics with multi￾axial architectures. The predictions are verified by measuring the elastic moduli in many test coupons. We assume that uniaxial fibrous monolithic ceramics pos￾sess a plane of isotropy perpendicular to the axis of the fibrous texture. The additional assumption that out-of-plane stresses can be ignored reduces the number of required elastic constants to four and allows the use of classical laminate theory to predict the properties at an arbitrary angle for materials with uniaxial architectures and the moduli for materials with biaxial archi￾tectures.22 These four elastic constants are calculated in terms of the engineering properties E1, E2, G12, and n12. We express these properties for each architecture in terms of the composi￾tion (VBN) and the elastic properties of the BN (EBN) and Si3N4 (ESN) constituent materials. (A) Elastic Properties along Principal Directions: Uni￾axial Architecture: All elastic property predictions for fibrous monolithic ceramics are made from the elastic moduli of the constituent Si3N4 and BN. A value for the Young’s modulus of Si3N4 of 320 GPa is used. This value is obtained from mea￾surements performed on bars of monolithic Si3N4 of the same composition as that of the fibrous monolithic cells and hot￾pressed under the same conditions and is consistent with values cited in the literature.23 It is difficult to measure the elastic properties of bulk BN. Similar to fabricated graphite,24 the elastic properties of bulk BN vary greatly with fabrication tech￾nique. Furthermore, the high degree of internal damping makes measurement using the impulse-excitation technique difficult. Only a few examples of successful modulus measurements on BN are known in the literature.25 Two of the more commonly reported values are 19.6 GPa26 and 22 GPa.27 However, these values should be used with caution, because the microstructure of the BN present in hot-pressed fibrous monolithic ceramics is Panel B. Material Combinations Although this article focuses on fibrous monoliths made from Si3N4 and BN, fibrous monoliths have been fabricated using many different material combinations. Some ex￾amples of all-ceramic fibrous monoliths and metal–ceramic fibrous monoliths that have been successfully fabricated are presented below. The usual limitations to processing of composite materials also apply to fibrous monoliths; i.e., the constituent materials must be phase compatible. In addition, the constituent materials must be compatible with the poly￾mer binders that are used in the extrusion process. Table BI. Material Combinations that have been Used to Fabricate Fibrous Monoliths Cell Cell boundary Reference All-ceramic fibrous monoliths ZrB2 BN † HfB2 BN † SiC BN 8, 53 SiC C (graphite) 7, 53 Al2O3 C (graphite) 54 Al2O3 Al2TiO5 6 Al2O3 Al2O3–ZrO2 6 Ceramic–metal fibrous monoliths Al2O3 Fe–Ni 55 Al2O3 Fe 55 Al2O3 Ni 56 † Advanced Ceramic Research, Tucson, AZ. October 1997 Fibrous Monolithic Ceramics 2475