MAERIAL ELSEVIER Materials Science and Engineering A195( 1995)263-268 Fibrous monoliths: non-brittle fracture from powder-processed ceramIcs G. Hilmas, A Brady, U. Abdali, G. Zywicki, J. Halloran Department of Materials Science and Engineering, 2168 H.H. Dow Building, University of Michigan, Ann Arbor, MI48100-2l36,USA Abstract Fibrous monoliths are non brittle cerami ated from conventional powders using new methods that create a fibrous mesostructure to obtain an in situ composite C BN fibrous monoliths fracture like wood, with graceful failure occurring by shear delamination. Examples are Si3N4-BN monoliths, which have flexural strengths around 460 MPa between room temperature and 1000'C and retain about half of their load ng ability after fracture initiates, which apparent fracture energies up to 4800 J m-2. (SiC-alloy)-BN fibrous monoliths have flex rengths around 400 MPa and exhibit graceful failure between room temperature and 1200C Ceramic-metal fibrous monoliths also have graceful failure. Results are presented for (yttria-zirconia)-Ni fibrous monoliths, which fail by tensile cracking, with crack bridging by ductile metal ligaments. About 25 vol %o Ni imparts a fracture Keywords: Fibres; Monoliths; Fracture; Powders; Ceramics 1. Introduction extruded to form"green fibers", the precursor for the woody texture of fibrous monoliths. The Si3N4-BN Ceramics have no useful ductility, but their brittle- fibrous monoliths are fabricated from silicon nitride ness can be ameliorated so they can sustain load after polymer green fibers. The fibers are subsequently dip the onset of fracture and avod catastrophic crack coated with a shurry of boron nitride powder. The propagation, becoming effectively non-brittle Ceramic coated green fibers are layed up uniaxially, molded into fiber reinforced ceramic matrix composites were the a solid block, and then hot pressed, creating a mono- first practical examples of non-brittle ceramics, but lithic ceramic containing cells" of polycrystalline their application has been limited by high cost. Similar Si3 N4, the remnant of the green fibers, separated by behavior can be obtained with less expensive powder- thin " cell boundaries"of BN, the remnant of the processed ceramics if there are planes of weakness in coating. The cell-cell boundary mesostructure of the composite providing shear delamination in the fibrous monoliths creates the woody texture and ceramic during fracture. Clegg et al. 1 demonstrated enables graceful failure to be achieved. The SiC-BN very tough Sic-graphite laminated ceramics where the fibrous monoliths are made in a similar fashion using a graphite layers delaminate during flexural failure. high strength "BS80"SiC alloy for the cells. This Bs80 Based on the "fibrous monolith "microstructure design consists of 80 vol. B-SiC 20 vol. 3AIN: 1Al2O3 of Coblenz [2 we have previously reported a variety of which produces a two-phase alloy with the 6H and 2H non-brittle ceramics [3-7, with typical strengths polytypes [8, 9]. These boron-nitride-containing fibrous around 200 MPa. In this paper we present new results monoliths are non-brittle because of delamination of on more refined versions of these systems, with signifi- the weak BN cell boundaries, promoting shear crack- cantly improved strength and fracture energy ing and serving to deflect tensile cracks Fibrous monolithic(FM)ceramics have a wood-like A second variety of fibrous monolith employs texture, created by extrusion. A mixture of ductile metal cell boundaries. With as little as 7 vol% powder with thermoplastic polymers can be nickel, for example, alumina can exhibit graceful failure
ELSEVIER Materials Science and Engineering A195 (1995) 263-268 MATERIALS SCIENCE & ENGINEERING A Fibrous monoliths: non-brittle fracture from powder-processed ceramics G. Hilmas, A. Brady, U. Abdali, G. Zywicki, J. Halloran Department of Materials Science and Engineering, 2168 H.H. Dow Building, University of Michigan, Ann Arbor, MI48109-2136, USA Abstract Fibrous monoliths are non-brittle ceramics, fabricated from conventional powders using new methods that create a fibrous mesostructure to obtain an in situ composite. Ceramic-BN fibrous monoliths fracture like wood, with graceful failure occurring by shear delamination. Examples are Si3N4-BN fibrous monoliths, which have flexural strengths around 460 MPa between room temperature and 1000 °C and retain about half of their load-bearing ability after fracture initiates, which apparent fracture energies up to 4800 J m-2. (SiC-alloy)-BN fibrous monoliths have flexural strengths around 400 MPa and exhibit graceful failure between room temperature and 1200 °C. Ceramic-metal fibrous monoliths also have graceful failure. Results are presented for (yttria-zirconia)-Ni fibrous monoliths, which fail by tensile cracking, with crack bridging by ductile metal ligaments. About 25 vol.% Ni imparts a fracture energy of 2200 J m- 2, with a strength of 574 MPa. Keywords: Fibres; Monoliths; Fracture; Powders; Ceramics 1. Introduction Ceramics have no useful ductility, but their brittleness can be ameliorated so they can sustain load after the onset of fracture and avod catastrophic crack propagation, becoming effectively non-brittle. Ceramic fiber reinforced ceramic matrix composites were the first practical examples of non-brittle ceramics, but their application has been limited by high cost. Similar behavior can be obtained with less expensive powderprocessed ceramics if there are planes of weakness in the composite providing shear delamination in the ceramic during fracture. Clegg et al. [1] demonstrated very tough SiC-graphite laminated ceramics where the graphite layers delaminate during fiexural failure. Based on the "fibrous monolith" microstructure design of Coblenz [2], we have previously reported a variety of non-brittle ceramics [3-7], with typical strengths around 200 MPa. In this paper we present new results on more refined versions of these systems, with significantly improved strength and fracture energy. Fibrous monolithic (FM) ceramics have a wood-like fibrous texture, created by extrusion. A mixture of ceramic powder with thermoplastic polymers can be 0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD1 0921-5093(94)06525-X extruded to form "green fibers", the precursor for the woody texture of fibrous monoliths. The Si3N4-BN fibrous monoliths are fabricated from silicon nitridepolymer green fibers. The fibers are subsequently dip coated with a slurry of boron nitride powder. The coated green fibers are layed up uniaxially, molded into a solid block, and then hot pressed, creating a monolithic ceramic containing "cells" of polycrystalline Si3N4, the remnant of the green fibers, separated by thin "cell boundaries" of BN, the remnant of the coating. The cell-cell boundary mesostructure of fibrous monoliths creates the woody texture and enables graceful failure to be achieved. The SiC-BN fibrous monoliths are made in a similar fashion using a high strength "BS80" SiC alloy for the cells. This BS80 consists of 80 vol.% r-sic+20 vol.% 3A1N:lAI203, which produces a two-phase alloy with the 6H and 2H polytypes [8,9]. These boron-nitride-containing fibrous monoliths are non-brittle because of delamination of the weak BN cell boundaries, promoting shear cracking and serving to deflect tensile cracks. A second variety of fibrous monolith employs ductile metal cell boundaries. With as little as 7 vol.% nickel, for example, alumina can exhibit graceful failure
264 G. Hilmas et al./ Materials Science and Engineering A195(1995)263-268 [6 because of a combination of bridging of tensile consolidated by hot pressing at 1250C for 1 h at 2 cracks by Ni ligaments and shear cracking at the MPa. After machining test bars, the zirconia-Ni speci alumina-Ni interface. For the oxide-metal fibrous mens were annealed at 1100C in an N2+5%H, monoliths,oxide green fibers are dip coated with a atmosphere for 3 h. The density was 6.16 g cm 3, with slurry of Nio, which forms nickel on reduction pore-free zirconia cells and microstructural evidence for porosity in the Ni cell boundaries. The Ni content is 25 vol % by stereological point count analysis on a 2. Experimental procedure polished surface. 2. 1. Fabrication fibrous monoliths 2. 2. Mechanical property characterization The silicon nitride and silicon carbide green fibers Flexural strength was measured at room tempera were produced by melt spinning a blend of 51 vol. ture in four-point bending with a 20 mm inner span (EEA, DPDA NT, Union Carbide Chemicals and driven testimo Malar(Instron model 4483, Instron was compounded with molten polymer, then extruded min Specimens were machined to dimensions of 3 through a 320 um die using a laboratory scale fiber mm x 4 mm x 47 mm and chamfered with a 1000 grit extrusion machine( Bradford University Research Ltd resin-bonded diamond wheel, with the machining Bradford, UK). The silicon carbide green fibers were direction along the axis of the bar. Each hot-pressed made from 78.39 wt %B-SiC powder(B-10 B-SiC, billet produced only five flexural bars, so typically just Hermann C. Starck, Inc, Newton, MA),11.81 wt% one bar was available for each testing temperature aiN powder( Keramont Advanced Ceramic Products, Results are reported as load vS deflection. The Tucson, AZ)and 9.80 wt. Al,O, powder(A-16 SG, flexural stress values are correct until the first load Alcoa, Pittsburgh, PA). The composition of the silicon drop. Beyond that point the bar delaminates, so nitride was 88 wt. Si3 N4 powder(E-10 silicon nitride stresses cannot be calculated from beam theory. We powder, Ube Industries, Tokyo, Japan) with 9 wt% report the peak flexural stress as the apparent flexural Y2O3+3 wt% AL2O3. The green fibers were coated strength of the bar. The specimen continues to support with a 10-20 um layer of boron nitride by dip coating some load to deflections so large that the bar is visibly the fiber in a slurry of bn powder(HCP grade, bent. The test is terminated when the load drops to Advanced Ceramics Corp, Cleveland, OH) 50N. The work of fracture is calculated from the The coated green fibers were arranged into a uni- cross-sectional area of the bar and the area under the xial alignment in a graphite hot pressing die and warm load-deflection curve in the non-elastic region pressed at about 150C. The warm pressing collapses between the first load drop until the load falls to 50 N the green fibers and molds the material into a mono- Young s modulus, determined by the impulse excita- lithic block of green ceramic. The polymer binder was tion technique(Grindo-sonic MK4, J W. Lemmons removed by heating at 30C h-I to 400C in an Inc, St Louis, MO)on bars excited in a flexural mode, actively pumped vacuum Silicon nitride-BN materials was 290#6 GPa for the Si3N4-BN,374+6 GPa for were hot pressed at 1750'C for 1 h at 25 MPa, achiev- the BS 80 SiC-BN, and 21 1 GPa for the zirconia-Ni ing essentially full density at 3. 2 g cm-3. The volume fraction of BN was 12 vol %, as determined by quanti- tative metallography. The bs80 silicon carbide-Bn 3. Results and discussion materials were hot pressed at 2100C for 1 h at 25 MPa, to achieve a density of 3. 1 g cm"3, close to the 3. 1. Structure of the fibrous monoliths rule-of-mixtures theoretical density for this composite Volume fraction of BN was 20 vol % The polytypes in Fig. 1 shows the cell-cell boundary structure of a the BS80 SiC were determined by X-ray diffraction to BS80 SiC-BN fibrous monolith, where the axially be primarily 6H with about 20% 2H and a trace of aligned Sic cells appear grey and the bn cell bound residual 3C aries are in bright contrast. The end-on view(Fig. 1(a) A zirconia-Ni fibrous monolith was prepared by shows that the polycrystalline Sic cells are about 250 extruding green fibers with zirconia-3 moL% yttria um across, with a roughly hexagonal shape created powder(HSY-3.0, Tosoh USA, Atlanta, GA). These during molding of the soft green fibers. The BN cell were prepared as a fibrous monolith with Nio as the boundaries are about 10-20 um wide. Fig. 1(b)is the cell boundary material. After binder burn-out the Nio side view of the same specimen, showing long Sic cells was reduced to nickel metal, and the ceramic was gradually weaving into the plane of polish. The Si,3N4
264 G. Hilmas et al. / Materials Science and Engineering A195 (1995) 263-268 [6] because of a combination of bridging of tensile cracks by Ni ligaments and shear cracking at the alumina-Ni interface. For the oxide-metal fibrous monoliths, oxide green fibers are dip coated with a slurry of NiO, which forms nickel on reduction. 2. Experimental procedure consolidated by hot pressing at 1250 °C for 1 h at 25 MPa. After machining test bars, the zirconia-Ni specimens were annealed at 1100 °C in an N2+5% H 2 atmosphere for 3 h. The density was 6.16 g cm -3, with pore-free zirconia cells and microstructural evidence for porosity in the Ni cell boundaries. The Ni content is 25 vol.%, by stereological point count analysis on a polished surface. 2.1. Fabrication of fibrous monoliths 2.2. Mechanical property characterization The silicon nitride and silicon carbide green fibers were produced by melt spinning a blend of 51 vol.% ceramic powder mixture in ethylene ethylacrylate resin (EEA, DPDA 6182 NT, Union Carbide Chemicals and Plastics Company, Inc., Danbury, CT). The powder was compounded with molten polymer, then extruded through a 320 /zm die using a laboratory scale fiber extrusion machine (Bradford University Research Ltd., Bradford, UK). The silicon carbide green fibers were made from 78.39 wt.% r-sic powder (B-10 r-sic, Hermann C. Starck, Inc., Newton, MA), 11.81 wt.% AIN powder (Keramont Advanced Ceramic Products, Tucson, AZ) and 9.80 wt.% A1203 powder (A-16 SG, Alcoa, Pittsburgh, PA). The composition of the silicon nitride was 88 wt.% Si3N 4 powder (E-10 silicon nitride powder, Ube Industries, Tokyo, Japan) with 9 wt.% Y203 + 3 wt.% A1203. The green fibers were coated with a 10-20/zm layer of boron nitride by dip coating the fiber in a slurry of BN powder (HCP grade, Advanced Ceramics Corp., Cleveland, OH). The coated green fibers were arranged into a uniaxial alignment in a graphite hot pressing die and warm pressed at about 150 °C. The warm pressing collapses the green fibers and molds the material into a monolithic block of green ceramic. The polymer binder was removed by heating at 30 °C h -~ to 400 °C in an actively pumped vacuum. Silicon nitride-BN materials were hot pressed at 1750 °C for 1 h at 25 MPa, achieving essentially full density at 3.2 g cm-3. The volume fraction of BN was 12 vol.%, as determined by quantitative metallography. The BS80 silicon carbide-BN materials were hot pressed at 2100 °C for 1 h at 25 MPa, to achieve a density of 3.1 g cm -3, close to the rule-of-mixtures theoretical density for this composite. Volume fraction of BN was 20 vol.%. The polytypes in the BS80 SiC were determined by X-ray diffraction to be primarily 6H with about 20% 2H and a trace of residual 3C. A zirconia-Ni fibrous monolith was prepared by extruding green fibers with zirconia-3 mol.% yttria powder (HSY-3.0, Tosoh USA, Atlanta, GA). These were prepared as a fibrous monolith with NiO as the cell boundary material. After binder burn-out the NiO was reduced to nickel metal, and the ceramic was Flexural strength was measured at room temperature in four-point bending with a 20 mm inner span and 40 mm outer span. Testing was done on a screwdriven testing machine (Instron model 4483, Instron Corp., Canton, MA) at a cross-head speed of 0.05 mm min-1. Specimens were machined to dimensions of 3 mm x 4 mm x 47 mm and chamfered with a 1000 grit resin-bonded diamond wheel, with the machining direction along the axis of the bar. Each hot-pressed billet produced only five flexural bars, so typically just one bar was available for each testing temperature. Results are reported as load vs. deflection. The flexural stress values are correct until the first load drop. Beyond that point the bar delaminates, so stresses cannot be calculated from beam theory. We report the peak flexural stress as the apparent flexural strength of the bar. The specimen continues to support some load to deflections so large that the bar is visibly bent. The test is terminated when the load drops to 50 N. The work of fracture is calculated from the cross-sectional area of the bar and the area under the load-deflection curve in the non-elastic region between the first load drop until the load fails to 50 N. Young's modulus, determined by the impulse excitation technique (Grindo-sonic MK4, J.W. Lemmons, Inc., St. Louis, MO) on bars excited in a flexural mode, was 290 _+ 6 GPa for the Si3N4-BN, 374_+ 6 GPa for the BS 80 SiC-BN, and 211 GPa for the zirconia-Ni. 3. Results and discussion 3.1. Structure of the fibrous monoliths Fig. 1 shows the cell-cell boundary structure of a BS80 SiC-BN fibrous monolith, where the axially aligned SiC cells appear grey and the BN cell boundaries are in bright contrast. The end-on view (Fig. l(a)) shows that the polycrystalline SiC cells are about 250 /~m across, with a roughly hexagonal shape created during molding of the soft green fibers. The BN cell boundaries are about 10-20 pm wide. Fig. l(b) is the side view of the same specimen, showing long SiC cells gradually weaving into the plane of polish. The Si3N 4-
G. Hilmas et al. Materials Science and Engineering A195(1995)263-268 OUnn la)s bH b Fig. 1. Cell-cell boundary structure of a BS80 Sic-BN fibrous monolith, with boron nitride cell boundaries in bright contrast: Fig. 2. Cell-cell boundary structure of an(yttria-zirconia)S-Ni (a]end-on view of axially aligned SiC cells; (b)side view of cells. fibrous monolith, with nickel cell boundaries in bright contrast (a)end-on view of axially aligned zirconia cells; b side view of BN system has a quite similar cell-cell boundary structure. Within the bN cell boundaries there are flexure at room ter densely packed bN platelets 2-10 um wide and abor 2 um thick. The microstructure within the cells are designed with particularly low shear strengths, so characteristic of the matrix material. The silicon nitride they will fail by a combination of shear and tension. In consists of elongated 0.5 um B-silicon nitride grains. flexural loading, FM ceramics can display non-cata- The BS80 Sic consists primarily of elongated 6H Sic strophic or " graceful"failure because the specimen exhibits a shear-dominated failure mode. The shear The zirconia-Ni material is shown in Fig 2a)where cracking makes the specimen more compliant in the axially aligned zirconia cells are dark and the ni retaining a pility to cell boundaries are in bright contrast. The zirconia load. Fibrous monoliths can also display a graceful cells are about 300 um across, and the Ni cell bound failure mode which initiates in tension with subsequent aries are about 20 um wide. Fig. 2(b)is the side shear delamination. The relationships between flexural failure mode have been discussed elsewhere [4, 7.The longer than they appear, as the green fiber alignment ductile Ni-containing systems have quite different was irregular, so cells weave in and out of the plane of behavior, with tensile cracking The load-deflection curves for the Si3N2-BN syS 2. Mechanical behavior of fibrous monoliths in tem are shown in Fig 3 for three samples tested at fe room temperature,800℃C,and1000°C. The room temperature bar reached a peak load corresponding to Conventional ceramics have high shear strength, so an apparent flexural strength of 484 MPa, followed by they exhibit only tensile initiated failure when loaded in a sharp load drop due to shear delamination near the
G. Hilmas et al. / Materials Science and Engineering A 195 (1995) 263-268 265 Fig. 1. Cell-cell boundary structure of a BS80 SiC-BN fibrous monolith, with boron nitride cell boundaries in bright contrast: (a) end-on view of axially aligned SiC cells; (b) side view of cells. BN system has a quite similar cell-cell boundary structure. Within the BN cell boundaries there are densely packed BN platelets 2-10/~m wide and about 2 ,urn thick. The microstructure within the cells is characteristic of the matrix material. The silicon nitride consists of elongated 0.5 /~m r-silicon nitride grains. The BS80 SiC consists primarily of elongated 6H SiC grains, about 4/~m wide. The zirconia-Ni material is shown in Fig. 2(a) where the axially aligned zirconia cells are dark and the Ni cell boundaries are in bright contrast. The zirconia cells are about 300/~m across, and the Ni cell boundaries are about 20/~m wide. Fig. 2(b) is the side view of the same specimen. The zirconia cells are actual much longer than they appear, as the green fiber alignment was irregular, so cells weave in and out of the plane of polish. 3.2. Mechanical behavior of fibrous monoliths in flexure Conventional ceramics have high shear strength, so they exhibit only tensile initiated failure when loaded in Fig. 2. Cell-cell boundary structure of an (yttria-zirconia)S-Ni fibrous monolith, with nickel cell boundaries in bright contrast: (a) end-on view of axially aligned zirconia cells; (b) side view of cells. flexure at room temperature. In contrast, FM ceramics are designed with particularly low shear strengths, so they will fail by a combination of shear and tension. In flexural loading, FM ceramics can display non-catastrophic or "graceful" failure because the specimen exhibits a shear-dominated failure mode. The shear cracking makes the specimen more compliant in flexure, while still retaining ability to bear significant load. Fibrous monoliths can also display a graceful failure mode which initiates in tension with subsequent shear delamination. The relationships between flexural loading conditions, specimen span:depth ratio, and failure mode have been discussed elsewhere [4,7]. The ductile Ni-containing systems have quite different behavior, with tensile cracking [6]. The load-deflection curves for the Si3N3-BN system are shown in Fig. 3 for three samples tested at room temperature, 800 °C, and 1000 °C. The room temperature bar reached a peak load corresponding to an apparent flexural strength of 484 MPa, followed by a sharp load drop due to shear delamination near the
G Himas et al. Materials Science and Engineering A195 (1995)263-268 800- ture work is 2936 J m-2. The strength drop for the 4802J/m2800c453MPa fibrous monolith is probably due to weakening of the 600 000°C:44lMP silicon nitride, since the strength of simple monolithic samples(with no BN)dropped from 800 MPa at roor temperature to about 610 MPa at 1000C T400 After testing, the specimen is intact but noticeably bent and splintered, as shown in Fig 4(a), a low mag- 200 nification photograph of the fractured region of the specimen. Visible in this photograph are prominent splinters on the tensile side and large shear cracks near 000.20406081.01.21.4 compression side, where the sample is beginning to Displacement(mm) buckle. Delaminated cell boundaries and fractured load-displacement curves for silicon cells are barely visible in this photograph. Details of the fibrous monoliths tested at room tempera- fracture path in the bent region of the sample appear in . C in air. The peak flexural strength and Fig. 4(b), showing several delaminated BN cell bound work of fracture values are annotated aries with fractured and displaced silicon nitride cells Typical damage remote from the bent region is shown in Fig 4(c), where many of the BN cell boundaries have midplane of the bar. After this event, the specimen delaminated, and a few cells have fractured retained about 60% of its peak load with load rising to The load-deflection curves for the BS80 SiC-BN 70%of peak on further displacement. Subsequent load system are shown in Fig. 5 for three samples tested at drops were associated with further delamination and room temperature, 1000C, and 1200C. The room tensile cracking between fragments. The work of temperature bar reached a peak load corresponding to fracture, figured from the area under the load-deflec- an apparent flexural strength of 421 MPa. The speci tion curve after the first load drop, is 4800 m 2. The men then exhibited a sharp load drop due to a tensile behavior is similar at 800C, where the strength is 463 initiation event, with subsequent shear delamination. MPa and the fracture work is 3600 J m -2, and at. After this event, the ten retained about 25% of 1000C, where the strength is 441 MPa and the frac- peak load, with load gradually falling with further nn Fig 4. Detail of fracture path in silicon nitride-BN flexural bars after testing at room temperature: (a)side view; (b)near bent region of bar; (c)remote from bent region
266 G. Hilmas et al. / Materials Science and Engineering A195 (1995) 263-268 800 ' ' 600- Z 400- O 200- 0 , 0.0 0.2 Fig. 3. Flexural silicon , ~ , I , I , I , I , I R : 484 MPa 4802J/m 2 800°C: 463 MPa / 3600 J/m 2 ./ IO00°C: 441 MPa ' i ' i ' i , i ' i ' i 0.4 0.6 0.8 1.0 1.2 1.4 Displacement (mm) load-displacement curves for nitride-boron nitride fibrous monoliths tested at room temperature, 800 °C and 1000 ° C in air. The peak flexural strength and work of fracture values are annotated. midplane of the bar. After this event, the specimen retained about 60% of its peak load with load rising to 70% of peak on further displacement. Subsequent load drops were associated with further delamination and tensile cracking between fragments. The work of fracture, figured from the area under the load-deflection curve after the first load drop, is 4800 J m -2. The behavior is similar at 800 °C, where the strength is 463 MPa and the fracture work is 3600 J m -2, and at 1000 °C, where the strength is 441 MPa and the fracture work is 2936 J m -2. The strength drop for the fibrous monolith is probably due to weakening of the silicon nitride, since the strength of simple monolithic samples (with no BN) dropped from 800 MPa at room temperature to about 610 MPa at 1000 °C. After testing, the specimen is intact but noticeably bent and splintered, as shown in Fig. 4(a), a low magnification photograph of the fractured region of the specimen. Visible in this photograph are prominent splinters on the tensile side and large shear cracks near the compression side, where the sample is beginning to buckle. Delaminated cell boundaries and fractured cells are barely visible in this photograph. Details of the fracture path in the bent region of the sample appear in Fig. 4(b), showing several delaminated BN cell boundaries with fractured and displaced silicon nitride cells. Typical damage remote from the bent region is shown in Fig. 4(c), where many of the BN cell boundaries have delaminated, and a few cells have fractured. The load-deflection curves for the BS80 SiC-BN system are shown in Fig. 5 for three samples tested at room temperature, 1000 °C, and 1200 °C. The room temperature bar reached a peak load corresponding to an apparent flexural strength of 421 MPa. The specimen then exhibited a sharp load drop due to a tensile initiation event, with subsequent shear delamination. After this event, the specimen retained about 25% of peak load, with load gradually falling with further 2 mm Fig. 4. Detail of fracture path in silicon nitride-BN flexur.~l bars after testing at room temperature: (a) side view; (b) near bent region of bar; (c) remote from bent region
G. Hilmas et al. Materials Science and Engineering A195(1995)263- 267 cross-head displacement. After testing, the specimen is 4.Conclusions intact but bent and delaminated as shown in Fig. 6.In contrast with the silicon nitride version, the BS80 Sic FM ceramics can be fabricated as ceramic-ceramic fibrous monolith has little splintering. The work of or ceramic-metal versions with non-brittle fracture fracture, calculated from the area under the load- behavior when tested in flexure. The ceramic-ceramic deflection curve after the first load drop, is about 1300 materials, such as Si, Na-BN and Sic-BN, have a J m". There is more load retention at 1000C, where mesostructure of Si, N, or Sic polycrystalline cells and the strength is 370 MPa and the fracture work is 1500 BN cell boundaries. These materials undergo delam J m, and the sample maintains about 35% of peak nation of low shear strength cell boundaries to provide load. At 1200C, the strength is 417 MPa and the graceful failure with significant retention of load- fracture work is about 1300 J m-2 bearing ability after fracture begins. The ceramic Only one specimen was available for the (yttria- metal fibrous monoliths, such as(3 mol% yttria-zirco- Zirconia brous monolith, so the results are very nia -Ni, are non-brittle as a result of a combination of preliminary. The load-deflection curve is shown in Fig. crack bridging by ductile Ni cell boundaries and h of 574 MPa. about 40% of delamination at the zirconia-Ni interface the peak load is retained immediately after first load drop, with gradually dropping load retention on further testing. However, the sample retained about 10% of the peak load up to 1 mm deflection, and the specimen was intact when the test was stopped. The work of fracture Acknowledgments was 2200 J m-2. The fracture surface had occasional Ni cell boundaries bridging the crack, but mostly The authors are grateful for research support by the featured localized delamination around a central Advanced Research Projects Agency and Office of tensile crack in the center of the span Naval Research under Contract N00014-93-1-0302 700 MPaP. 370 MPa 417 MP 600 z 574 MPa 22m2 40% 0.8 Displacement (mm) Displacement (mm) Fig. 5. Flexural load-displacement curves for BS80 silicon Fig. 7. Flexural load-displacement curves for zirconia-Ni carbide-boron nitride fibrous monoliths tested at room tem- fibrous monoliths tested at room temperature, showing peak perature, 1000'C and 1200C in air. The peak flexural strength strength of 574 MPa, 40% retained load immediately after first ated load drop, and work of fracture of 2200 J m-2 SiC/BN Room Temperature Fig 6 Side view of BS80 SiC-BN flexural bars after testing at room temperature
G. Hilmas et al. / Materials Science and Engineering A 195 (1995) 263-268 267 cross-head displacement. After testing, the specimen is intact but bent and delaminated, as shown in Fig. 6. In contrast with the silicon nitride version, the BSS0 SiC fibrous monolith has little splintering. The work of fracture, calculated from the area under the loaddeflection curve after the first load drop, is about 1300 J m -2. There is more load retention at 1000 °C, where the strength is 370 MPa and the fracture work is 1500 J m -2, and the sample maintains about 35% of peak load. At 1200 °C, the strength is 417 MPa and the fracture work is about 1300 J m -2. Only one specimen was available for the (yttriazirconia)-Ni fibrous monolith, so the results are very preliminary. The load-deflection curve is shown in Fig. 7, showing peak strength of 574 MPa. About 40% of the peak load is retained immediately after first load drop, with gradually dropping load retention on further testing. However, the sample retained about 10% of the peak load up to 1 mm deflection, and the specimen was intact when the test was stopped. The work of fracture was 2200 J m -2. The fracture surface had occasional Ni cell boundaries bridging the crack, but mostly featured localized delamination around a central tensile crack in the center of the span. 4. Conclusions FM ceramics can be fabricated as ceramic-ceramic or ceramic-metal versions with non-brittle fracture behavior when tested in flexure. The ceramic-ceramic materials, such as Si3N4-BN and SiC-BN, have a mesostructure of Si3N 4 or SiC polycrystalline cells and BN cell boundaries. These materials undergo delamination of low shear strength cell boundaries to provide graceful failure with significant retention of loadbearing ability after fracture begins. The ceramicmetal fibrous monoliths, such as (3 mol.% yttria-zirconia)-Ni, are non-brittle as a result of a combination of crack bridging by ductile Ni cell boundaries and delamination at the zirconia-Ni interface. Acknowledgments The authors are grateful for research support by the Advanced Research Projects Agency and Office of Naval Research under Contract N00014-93-1-0302. 800 , I , I , I i I i • Room Temp. 1000°C 1200°C 700- 421 MPa 370 MPa 417 MPa 000 soo- 1/ /1 ,oo- IA/I 200 O/.-" / ( , , . , 0.0 0.2 0.4 0.6 0'.8 1.0 Displacement (mm) Fig. 5. Flexural load-displacement curves for BS80 silicon carbide-boron nitride fibrous monoliths tested at room temperature, 1000 °C and 1200 °C in air. The peak flexurai strength values are annotated. 800 i I i I i I i I i 600- A~ 574 MPa ~ m 2 N 400- 22 0 J/ 0 200- 0 ' I ' I ' I ' I 0.0 0.2 0.4 0.6 0.8 1.0 Displacement (mm) Fig. 7. Flexural load-displacement curves for zirconia-Ni fbrous monoliths tested at room temperature, showing peak strength of 574 MPa, 40% retained load immediately after first load drop, and work of fracture of 2200 J m -2. Fig. 6. Side view of BS80 SiC-BN flexural bars after testing at room temperature. i 2 mm
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