《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_fibrous molithic-16

e scripta MATERIALIA PERGAMON cripta Materialia 46(2002)747-751 www.actamat-journals Interfacial sliding stress in Si3N4/BN fibrous monoliths D Singh K.C. Goretta , J.W. Richardson Jr. ,A.R. de arellano-Lopez Energy Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4838, US.A Departmento de Fisica de la Materia Condensada, Universidad de sevilla, 41080 Sevilla, spain Received 9 January 2002; accepted 25 February 2002 Abstract e Pushout tests of Si Na cells in Sia N/BN fibrous monoliths yielded values for debond and sliding stresses of 45+8 nd 25+7 MPa, respectively. The sliding stress was consistent with estimates of residual stresses and the interfacial friction coeficient. o 2002 Acta Materialia Inc. Published by elsevier Science Ltd. All rights reserved Keywords: Interfa ace: Stress: Fibrous mon 1. Introduction from duplex filaments that consist of a cell phase surrounded by a sheath that forms a continuous Ceramic fibrous monoliths generally consist of cell boundary. In flexure, fibrous monoliths ex- strong ceramic cells that are surrounded by a hibit graceful failure, with substantial sliding of weaker cell boundary. They are produced from the cells [1-10]. In many applications, fibrous ceramic powders by conventional fabrication tech- monoliths may offer low-cost alternatives to con- niques, such as extrusion or dip-coating [1, 2]. Parts ventional continuous-fiber ceramic composites are often fabricated as laminates that are laid up Several compositions of ceramics and cermets have been processed successfully in fibrous monoli thic form [4-6]. Fibrous monoliths consisting of k supported by the Defence Advanced Research Pro Si3N4 cells and a continuous BN cell boundary ts Agency through an Interagency agreement with the U have achieved excellent mechanical properties, and Department of Energy, under Contract W-31-109-Eng-38, and therefore have been studied most thoroughly by North Atlantic Treaty Organization Grant PST. Flexural strength can exceed 700 MPa and work- 977016. The submitted manuscript has been created by the of-fracture values, while typically 3-6 kJ/m, can University of Chicago as Operator of Argonne Nationa Laboratory (Argonne") under Contract no. W-31-109-Eng- exceed 10 kJ/m[3-71 As with other ceramic composites, fracture 38 with the US Department of Energy. The US Government characteristics strongly depend on interfacial retains for itself, and others acting on its behalf, a paid-up nonexclusive. irrevocable worldwide license in said article to properties. bn does not bond well to Si3 N4, but in reproduce, prepare derivative works, distribute copies to the the Si N,/BN fibrous monoliths now produced public, and perform publicly and display publicly, by or on commercially by Advanced Ceramics Research behalf of the government Corresponding author. Tel. +1-630-252-7761: fax: +1-630. (Tucson, AZ), sintering aids added to the Si,N induce moderate bonding between the two princi E-mail address: ken. goretta@aoard af mil(KC Coretta pal phases. An interfacial fracture energy at room 1359-6462/02/S- see front matter 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PI:S1359-6462(02)00068-4

Interfacial sliding stress in Si3N4/BN fibrous monoliths q D. Singh a , K.C. Goretta a,*, J.W. Richardson Jr. a , A.R. de Arellano-Lopez
b a Energy Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4838, USA b Departmento de Fisica de la Materia Condensada, Universidad de Sevilla, 41080 Sevilla, Spain Received 9 January 2002; accepted 25 February 2002 Abstract Pushout tests of Si3N4 cells in Si3N4/BN fibrous monoliths yielded values for debond and sliding stresses of 45
8 and 25
7 MPa, respectively. The sliding stress was consistent with estimates of residual stresses and the interfacial friction coefficient.
2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Interface; Stress; Fibrous monolith; Composite 1. Introduction Ceramic fibrous monoliths generally consist of strong ceramic cells that are surrounded by a weaker cell boundary. They are produced from ceramic powders by conventional fabrication tech￾niques, such as extrusion or dip-coating [1,2]. Parts are often fabricated as laminates that are laid up from duplex filaments that consist of a cell phase surrounded by a sheath that forms a continuous cell boundary. In flexure, fibrous monoliths ex￾hibit graceful failure, with substantial sliding of the cells [1–10]. In many applications, fibrous monoliths may offer low-cost alternatives to con￾ventional continuous-fiber ceramic composites. Several compositions of ceramics and cermets have been processed successfully in fibrous monoli￾thic form [4–6]. Fibrous monoliths consisting of Si3N4 cells and a continuous BN cell boundary have achieved excellent mechanical properties, and therefore have been studied most thoroughly. Flexural strength can exceed 700 MPa and work￾of-fracture values, while typically 3–6 kJ/m2, can exceed 10 kJ/m2 [3–7]. As with other ceramic composites, fracture characteristics strongly depend on interfacial properties. BN does not bond well to Si3N4, but in the Si3N4/BN fibrous monoliths now produced commercially by Advanced Ceramics Research (Tucson, AZ), sintering aids added to the Si3N4 induce moderate bonding between the two princi￾pal phases. An interfacial fracture energy at room Scripta Materialia 46 (2002) 747–751 www.actamat-journals.com qWork supported by the Defence Advanced Research Pro￾jects Agency through an Interagency agreement with the US Department of Energy, under Contract W-31-109-Eng-38, and by North Atlantic Treaty Organization Grant PST.CLG. 977016. The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (‘‘Argonne’’) under Contract no. W-31-109-Eng- 38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. * Corresponding author. Tel.: +1-630-252-7761; fax: +1-630- 252-3604. E-mail address: ken.goretta@aoard.af.mil (K.C. Goretta). 1359-6462/02/$ - see front matter
2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 0 6 8 - 4

748 D. Singh et al. Scripta Materialia 46(2002)747-75 temperature of 37 J/m- has been reported for these Specimens for the pushout tests were cut from fibrous monoliths [11]. During recent studies of the billet with a slow-speed diamond-blade saw their in-plane fracture, attempts have been made All samples were polished with I um diamond to determine the interfacial sliding stress between paste. Final thicknesses ranged from 0.44 to l1l boundary. However, measure- mm. Tests were conducted at a constant displace ments of cell pullout lengths and crack mouth ment rate of 0.05 mm/min in an Instron Model pening displacements yielded different values of 4505 universal tester(Canton, MA). The proce the sliding stress ts [12] dures followed those used for previous ceramic cea We report here on direct measurements by composites [14-17]. Specimens were mounted on pushout of the debond and sliding stresses. a holder so that the individual cell to be tested In support of these studies, we also report on straddled an 8-mm-wide slot. An x-y microposi neutron-diffraction measurements to characterize tioning stage was then used to locate the cell im- crystallographic texture and estimate the residual mediately beneath a flat pushout tool. To obtain are useful to assess the measured value of ts and to 30 cells were pushed out. Pushed cells and the support additional thermomechanical studies of general microstructure were examined by scanning hese composites [13] electron microscopy (SEM) In support studies, crystallographic texture was assessed and residual strains were measured at the 2. Experimental procedures Intense Pulsed Neutron Source at Argonne Na- tional Laboratory. Time-of-flight neutron powder Specimens, fabricated by Advanced Ceramic difraction data were gathered and the residual Research, were made from Si N,/BN green fila- strains were calculated [18]. With time-of-flight ments [2], 320-330 um in diameter, that were difiraction, entire difraction patterns were ac- produced by melt coextrusion of a blend of 52 cumulated at all scattering angles. The fibrous vol% ceramic powder mixture in an ethylene-based monolith and a strain-free Si3 N4 specimen were copolymer binder [6]. The coextruded filaments examined. BN absorbs neutrons well; therefore contained 82 vol% Si3 N4 core(E-10, Ube In- reliable strain data in the Si3 N4/Bn could be ac- dustries, Tokyo)and 18 vol% BN sheath(HCp quired for the Si3n4 only. These data required Grade, Advanced Ceramics Corporation, Cleve- compensation for a systematic offset of appar land ) The Si3 N4 was a sinterable composition, 92 ent lattice spacings for the BN-containing fibrous wt% commercial Si, N4 powder, 6 wt %Y2O3, and monolith. This offset arose from an apparent up 2 wt% Al,O stream shift of the centroid of scattering in the Sheets of uniaxially aligned green filaments sample, which induced an underestimation of lat- were produced by a winding on a cylindrical tice spacings in the fibrous monolith [191 mandrel. The filaments were held in place with a spray adhesive that, upon drying, allowed removal from the mandrel of unidirectional sheets of green 3 Results and discussion fibrous monolith. The sheets were then stacked to fabricate a unidirectional laminate [2, 3]. The lam The microstructure of the unidirectional Si3 N/ inate was warm-pressed at 160C to produce a BN fibrous monolith specimen was typical of what olid green panel, which was then subjected to a has been reported for these materials [5-13 binder pyrolysis step that consisted of slow heating Highly dense Si3N4 cells, with cross sections in flowing N, to 600C over a period of 42h. that approximated flattened hexagons, were sur The Si3 N,/BN panels were then hot-pressed at rounded by a Bn cell boundary that was 10 um 1740C for I h under 28 MPa pressure, which thick. The interface between the two phases was yielded a 3-mm-thick billet with a density that was moderately rough in places along both transverse 98% of theoretical and longitudinal directions(Fig. 1). The plate-like

temperature of 37 J/m2 has been reported for these fibrous monoliths [11]. During recent studies of their in-plane fracture, attempts have been made to determine the interfacial sliding stress between the cell and cell boundary. However, measure￾ments of cell pullout lengths and crack mouth opening displacements yielded different values of the sliding stress ss [12]. We report here on direct measurements by cell pushout of the debond and sliding stresses. In support of these studies, we also report on neutron-diffraction measurements to characterize crystallographic texture and estimate the residual strains in Si3N4/BN fibrous monoliths. These data are useful to assess the measured value of ss and to support additional thermomechanical studies of these composites [13]. 2. Experimental procedures Specimens, fabricated by Advanced Ceramic Research, were made from Si3N4/BN green fila￾ments [2], 320–330 lm in diameter, that were produced by melt coextrusion of a blend of 52 vol% ceramic powder mixture in an ethylene-based copolymer binder [6]. The coextruded filaments contained 82 vol% Si3N4 core (E-10, Ube In￾dustries, Tokyo) and 18 vol% BN sheath (HCP Grade, Advanced Ceramics Corporation, Cleve￾land). The Si3N4 was a sinterable composition, 92 wt.% commercial Si3N4 powder, 6 wt.% Y2O3, and 2 wt.% Al2O3. Sheets of uniaxially aligned green filaments were produced by a winding on a cylindrical mandrel. The filaments were held in place with a spray adhesive that, upon drying, allowed removal from the mandrel of unidirectional sheets of green fibrous monolith. The sheets were then stacked to fabricate a unidirectional laminate [2,3]. The lam￾inate was warm-pressed at 160 C to produce a solid green panel, which was then subjected to a binder pyrolysis step that consisted of slow heating in flowing N2 to 600 C over a period of 42 h. The Si3N4/BN panels were then hot-pressed at 1740 C for 1 h under 28 MPa pressure, which yielded a 3-mm-thick billet with a density that was >98% of theoretical. Specimens for the pushout tests were cut from the billet with a slow-speed diamond-blade saw. All samples were polished with 1 lm diamond paste. Final thicknesses ranged from 0.44 to 1.11 mm. Tests were conducted at a constant displace￾ment rate of 0.05 mm/min in an Instron Model 4505 universal tester (Canton, MA). The proce￾dures followed those used for previous ceramic composites [14–17]. Specimens were mounted on a holder so that the individual cell to be tested straddled an 8-mm-wide slot. An x–y microposi￾tioning stage was then used to locate the cell im￾mediately beneath a flat pushout tool. To obtain representative values for the interfacial properties, 30 cells were pushed out. Pushed cells and the general microstructure were examined by scanning electron microscopy (SEM). In support studies, crystallographic texture was assessed and residual strains were measured at the Intense Pulsed Neutron Source at Argonne Na￾tional Laboratory. Time-of-flight neutron powder diffraction data were gathered and the residual strains were calculated [18]. With time-of-flight diffraction, entire diffraction patterns were ac￾cumulated at all scattering angles. The fibrous monolith and a strain-free Si3N4 specimen were examined. BN absorbs neutrons well; therefore reliable strain data in the Si3N4/BN could be ac￾quired for the Si3N4 only. These data required compensation for a systematic offset of appar￾ent lattice spacings for the BN-containing fibrous monolith. This offset arose from an apparent up￾stream shift of the centroid of scattering in the sample, which induced an underestimation of lat￾tice spacings in the fibrous monolith [19]. 3. Results and discussion The microstructure of the unidirectional Si3N4/ BN fibrous monolith specimen was typical of what has been reported for these materials [5–13]. Highly dense Si3N4 cells, with cross sections that approximated flattened hexagons, were sur￾rounded by a BN cell boundary that was 10 lm thick. The interface between the two phases was moderately rough in places along both transverse and longitudinal directions (Fig. 1). The plate-like 748 D. Singh et al. / Scripta Materialia 46 (2002) 747–751

D. Singh et al. Scripta Materialia 46 (2002)747-751 100m 100m SEM photomicrographs of undirectional Si, N4/BN fibrous monolith: (a) transverse cross section, showing distorted hexagonal lls, and (b) longitudinal cross section, showing some roughness along each cell bn grains appeared to exhibit a strong fiber enon at the interface. Subsequently, the load in- texture, with their c axis perpendicular to the creased and then gradually dropped, indicating longitudinal (i.e, extrusion) direction. Neutron frictional sliding and pushout of the cell(Fig 4) diffraction confirmed the strong texture of the Machine compliance was not evaluated BN cell-boundary phase. The [00 l] peaks were strongly enhanced parallel to the hot-pressing di he maximum(Pmax)and debonded load values s) during pushout were used to determine (td) rection. The Si3 N4 grains exhibited only modest and frictional sliding stresses(Is) texture(Fig. 2) The minimum dimension of the transverse cell [=P/wt, cross section was >100 um, well suited to pushout where w is the perimeter of the cell and t is the sting. Each of the cells that was tested was dis- sample thickness. Based on 30 pushed-out placed evenly(Fig. 3(a)). In most cases, sliding the average debonding stress was 45+8 MPa and occurred along the Si3N4-BN interface(Fig 3(b)). the average sliding stress was 25+7 MPa. In a typical load-displacement plot, there was a Pushout tests overestimate t values because of linear increase in load, followed by a sharp drop, the Poisson stress generated by the compressive presumed to correspond to a debonding phenom- load. Given the relatively low value of 25 MPa a) B(221) β(210) (011 BN(100) 3.43.0262.21.8 Fig. 2. Neutron-diffraction data for Si3 N4/BN fibrous monolith:(a) hot-pressed surface and(b)along longitudinal direction of cells

BN grains appeared to exhibit a strong fiber texture, with their c axis perpendicular to the longitudinal (i.e., extrusion) direction. Neutron diffraction confirmed the strong texture of the BN cell-boundary phase. The [0 0 1] peaks were strongly enhanced parallel to the hot-pressing di￾rection. The Si3N4 grains exhibited only modest texture (Fig. 2). The minimum dimension of the transverse cell cross section was >100 lm, well suited to pushout testing. Each of the cells that was tested was dis￾placed evenly (Fig. 3(a)). In most cases, sliding occurred along the Si3N4–BN interface (Fig. 3(b)). In a typical load–displacement plot, there was a linear increase in load, followed by a sharp drop, presumed to correspond to a debonding phenom￾enon at the interface. Subsequently, the load in￾creased and then gradually dropped, indicating frictional sliding and pushout of the cell (Fig. 4). Machine compliance was not evaluated. The maximum (Pmax) and debonded load values (Ps) during pushout were used to determine (sd) and frictional sliding stresses (ss) s ¼ P=wt; ð1Þ where w is the perimeter of the cell and t is the sample thickness. Based on 30 pushed-out cells, the average debonding stress was 45
8 MPa and the average sliding stress was 25
7 MPa. Pushout tests overestimate s values because of the Poisson stress generated by the compressive load. Given the relatively low value of 25 MPa, Fig. 1. SEM photomicrographs of undirectional Si3N4/BN fibrous monolith: (a) transverse cross section, showing distorted hexagonal Si3N4 cells, and (b) longitudinal cross section, showing some roughness along each cell. Fig. 2. Neutron-diffraction data for Si3N4/BN fibrous monolith: (a) hot-pressed surface and (b) along longitudinal direction of cells. D. Singh et al. / Scripta Materialia 46 (2002) 747–751 749

D Singh et al. Scripta Materialia 46(2002)747-75 a) BN Sina 100ym 65 5 HI Fig 3 SEM photomicrographs of (a)back-scattered electron image of representative pushed-out Si, N cell and(b) secondary-electron image of sliding along a Si3 Ng-BN interface. range of values reported for ot er ceramIc com- Because of diffusion of the Y,O3 and Al,O P sintering additives to the Si3 N4 [ll], there was di rect bonding between the Si3N4 and bn phases Therefore, the debond stress was significantly greater than the sliding stress. The principal con- tributors to the sliding stress could have been Coulomb friction or surface roughness and con comitant asperity interactions. Longitudinal sec- tioning revealed some roughness along the si3N4 Displacement(mm) BN cell boundary, instances of direct Si3 N,/Si,N Fig.4. Representative pushout data, with loads for calculating contact during pushout would appear to have been minimal Inscribing of the softer bn by the SigN during sliding was the far more likely eve etty has analyzed frictional sliding by a shear the extent of overestimation in ts is likely to have lag model [21] and Bright et al. [16] have noted that been small in these tests. Most importantly, the ts in ceramic composites the frictional sliding stress tr value obtained agrees well with that calculated can be related directly to the clamping residual from crack mouth openings in Si3 N4/BN in which stress by all cells had fractured. In those specimens, resis tance to fracture, which should have been entirely from frictional sliding, was calculated to be 23-33 where u is the coefficient of friction and Go is the MPa [12]. A value of 25 MPa for frictional slid- clamping stress. Because of the difficulties associ- ing is, for example, near that found in pushout ted in compensating for absorption of neutrons tests of BN-coated SiC fibers in a ZrSiO4 matrix by the bn phase, neutron-diffraction data yielded 15 MPa)[15] and that found in pullout of heat- approximate residual strains only. Average values treated C-coated SiC fibers in a lithium aluminum of strains were obtained from top and side mea- silicate glass ceramic (30 MPa)[20]. The sliding surements of a Si3 N4/BN plate.(Because of the stress measured matched other measurements on flattened shape of the cells, the top and side Si3 N4/BN fibrous monoliths and was within the stresses would not be expected to be identical, but

the extent of overestimation in ss is likely to have been small in these tests. Most importantly, the ss value obtained agrees well with that calculated from crack mouth openings in Si3N4/BN in which all cells had fractured. In those specimens, resis￾tance to fracture, which should have been entirely from frictional sliding, was calculated to be 23–33 MPa [12]. A value of 25 MPa for frictional slid￾ing is, for example, near that found in pushout tests of BN-coated SiC fibers in a ZrSiO4 matrix (15 MPa) [15] and that found in pullout of heat￾treated C-coated SiC fibers in a lithium aluminum silicate glass ceramic (30 MPa) [20]. The sliding stress measured matched other measurements on Si3N4/BN fibrous monoliths and was within the range of values reported for other ceramic com￾posites. Because of diffusion of the Y2O3 and Al2O3 sintering additives to the Si3N4 [11], there was di￾rect bonding between the Si3N4 and BN phases. Therefore, the debond stress was significantly greater than the sliding stress. The principal con￾tributors to the sliding stress could have been Coulomb friction or surface roughness and con￾comitant asperity interactions. Longitudinal sec￾tioning revealed some roughness along the Si3N4 cells, but given the thickness and density of the BN cell boundary, instances of direct Si3N4/Si3N4 contact during pushout would appear to have been minimal. Inscribing of the softer BN by the Si3N4 during sliding was the far more likely event. Shetty has analyzed frictional sliding by a shear￾lag model [21] and Bright et al. [16] have noted that in ceramic composites the frictional sliding stress sf can be related directly to the clamping residual stress by sf ¼ lr0; ð2Þ where l is the coefficient of friction and r0 is the clamping stress. Because of the difficulties associ￾ated in compensating for absorption of neutrons by the BN phase, neutron-diffraction data yielded approximate residual strains only. Average values of strains were obtained from top and side mea￾surements of a Si3N4/BN plate. (Because of the flattened shape of the cells, the top and side stresses would not be expected to be identical, but Fig. 4. Representative pushout data, with loads for calculating sd and ss indicated. Fig. 3. SEM photomicrographs of (a) back-scattered electron image of representative pushed-out Si3N4 cell and (b) secondary-electron image of sliding along a Si3N4–BN interface. 750 D. Singh et al. / Scripta Materialia 46 (2002) 747–751

D. Singh et al. Scripta Materialia 46(2002)747-751 they would be expected to be similar and far Orlova, and V.V. Shpeizman. This work was different from the direction along the length of supported by the Defense Advanced Research the cells. )A residual compressive strain of a4- Projects Agency through an Interagency agree- 5x 10-4 was obtained for the Si N4. Taking the ment with the US Department of Energy, under elastic modulus of the Si, N4 to be a300 GPa, Contract W-31-109-Eng-38, and by North Atlan which is reasonable for a moderately textured hot- tic Treaty Organization Grant PST CLG. 977016. pressed material [22, 23], a clamping stress on the cells of a120-150 MPa would be expected. The coefficient of friction between hexagonal bN and references (r=o=0.2×120-150MPa=2430MPa)is thus consistent with the residual-stress state and [l Coblenz ws, US patent 4772 524, 1988 2 Popovic D, Halloran Jw, Hilmas Ge, Brady GA, Somas the likely coefficient of friction S. Bard A et al.. US Patent 5 645781. 1997 The load-displacement data from the pushout 3 Hilmas G, Brady GA, Abdali U, Zywicki G, Halloran tests yielded approximate work-of-fracture values J. Mater Sci Eng A 1995: 195: 26 of 0.5-1 kJ/m. These values are roughly one order [4 Popovich D, Danko GA, Hilmas GE, Stufile K, King BH, ady GA, et al. Ceram Eng Sci Proc 1996: 17(3): 278 of magnitude lower than what has been reported in 5 Kovar D, King BH, Trice RW, Halloran JW.J Am Ceram flexure of Si,,/BN fibrous monoliths [3-7. The Soc1997;80:2471 apparent discrepancy is a consequence of the type [6] Danko GA, Hilmas GE, Halloran Jw,King B Ceram Eng of testing. Si3 N//BN fibrous monoliths are very Sci proc1997;18(3):607 tough in flexure because of splintering of the hard [7 Tlustochowitz M, Singh D, Ellingson WA, Goretta KC, ali m. Sutaria m. Ceram Trans 2000: 103- 245 cells and substantial interaction between broken 18 Singh D, Cruse TA, Hermanson DJ, Goretta KC, Zok cells and their neighbors. In tension, Si3 N4/BN fi- W, McNulty JC. Ceram Eng Sci Proc 2000: 21(3): 59 brous monoliths are far less tough and cell pullout [9] Finch JL, Staehler JM, Zawada LP, Ellingson WA, Sun lengths tend to be rather short [12] JG, Deemer CM. Ceram Eng Sci Proc 1999: 20(3): 341 [10] Guo H, Huang Y, Wang C-A. J Mater Sci 1999:34: 2455 [ll] Trice RW, Halloran JW. J Am Ceram Soc 1999: 82: 2502. [12] McNulty JC, Zok Fw, Begley MR. J Am Ceram Soc 4. Conclusions 2001:84:367 [13] He MY, Singh D, MeNulty JC, Zok FW. J Comp Sci unidirectional SisN/BN fibrous monolith yielded [4 Mandell)l Grande de t siang Ir-s, MGarry F), Special an average debond stress td of 45+8 MPa for for Testing Materials: 1986. p. 89 cell sliding. Bonding between the two phases Am Ceram Soc 1989- 72: 1864 promoted by diffusion during processing of Y2O3 [16] Bright JD, Shetty DK, Griffin CW, Limaye SY. J Am and Al,O3 sintering additives from the Si3 N4 to the 198972:1891 BN(9). The sliding stress ts for the cells was 25+7 7 Singh D. singh JP Majumdar s, Kupperman DS, Cowdin MPa. estimates of residual stresses from neutron [8 Majumdar S, Kupperman D, Singh J J Am Ceram Soc diffraction data and studies of crack mouth open- ing of fully fractured Si3 N4/BN fibrous monoliths [19] Carpenter JM, Mildner DFR, Richardson JW. Nucl Instr [10] were consistent with this value of ts Meth a1991:301:305 [20]Thouless MD, Sbaizero O. Sigl LS, Evans AG. J Am Ceram Soc 1989: 72: 525. 21 Shetty DK. J Am Ceram Soc 1988: 71: C-107. Acknowledgements 22] In: Ceramic Source, vol 6. Westerville: American Ceramic ociety: 1990. We thank Dr. Mark Rigali of Advanced Ce- [23] Smirnov BI, Burenkov YuA, Kardashev BK, Singh D ramics Research for providing the test material Coretta KC, de Arellano-Lopez AR. Phys So 2001:43:2094. and Dr. A. Erdimer for providing data. Helpful (24) Erdermir A. Argonne National Laboratory, unpublished discussions were held with Drs. B.l. Smirnov tA data. 2001

they would be expected to be similar and far different from the direction along the length of the cells.) A residual compressive strain of 4– 5  104 was obtained for the Si3N4. Taking the elastic modulus of the Si3N4 to be 300 GPa, which is reasonable for a moderately textured hot￾pressed material [22,23], a clamping stress on the cells of 120–150 MPa would be expected. The coefficient of friction between hexagonal BN and Si3N4 is 0.2 [24]. A sliding stress of 25 MPa (sf ¼ lr0 ¼ 0:2  120–150 MPa ¼ 24–30 MPa) is thus consistent with the residual-stress state and the likely coefficient of friction. The load–displacement data from the pushout tests yielded approximate work-of-fracture values of 0.5–1 kJ/m2. These values are roughly one order of magnitude lower than what has been reported in flexure of Si3N4/BN fibrous monoliths [3–7]. The apparent discrepancy is a consequence of the type of testing. Si3N4/BN fibrous monoliths are very tough in flexure because of splintering of the hard cells and substantial interaction between broken cells and their neighbors. In tension, Si3N4/BN fi- brous monoliths are far less tough and cell pullout lengths tend to be rather short [12]. 4. Conclusions Pushout tests of Si3N4 cells in a commercial unidirectional Si3N4/BN fibrous monolith yielded an average debond stress sd of 45
8 MPa for cell sliding. Bonding between the two phases was promoted by diffusion during processing of Y2O3 and Al2O3 sintering additives from the Si3N4 to the BN [9]. The sliding stress ss for the cells was 25
7 MPa. Estimates of residual stresses from neutron￾diffraction data and studies of crack mouth open￾ing of fully fractured Si3N4/BN fibrous monoliths [10] were consistent with this value of ss. Acknowledgements We thank Dr. Mark Rigali of Advanced Ce￾ramics Research for providing the test materials and Dr. A. Erdimer for providing data. Helpful discussions were held with Drs. B.I. Smirnov, T.A. Orlova, and V.V. Shpeizman. This work was supported by the Defense Advanced Research Projects Agency through an Interagency agree￾ment with the US Department of Energy, under Contract W-31-109-Eng-38, and by North Atlan￾tic Treaty Organization Grant PST.CLG.977016. References [1] Coblenz WS, US patent 4 772 524, 1988. [2] Popovic’ D, Halloran JW, Hilmas GE, Brady GA, Somas S, Bard A, et al., US Patent 5 645 781, 1997. [3] Hilmas G, Brady GA, Abdali U, Zywicki G, Halloran J. Mater Sci Eng A 1995;195:263. [4] Popovich D, Danko GA, Hilmas GE, Stuffle K, King BH, Brady GA, et al. Ceram Eng Sci Proc 1996;17(3):278. [5] Kovar D, King BH, Trice RW, Halloran JW. J Am Ceram Soc 1997;80:2471. [6] Danko GA, Hilmas GE, Halloran JW, King B. Ceram Eng Sci Proc 1997;18(3):607. [7] Tlustochowitz M, Singh D, Ellingson WA, Goretta KC, Rigali M, Sutaria M. Ceram Trans 2000;103:245. [8] Singh D, Cruse TA, Hermanson DJ, Goretta KC, Zok FW, McNulty JC. Ceram Eng Sci Proc 2000;21(3):597. [9] Finch JL, Staehler JM, Zawada LP, Ellingson WA, Sun JG, Deemer CM. Ceram Eng Sci Proc 1999;20(3):341. [10] Guo H, Huang Y, Wang C-A. J Mater Sci 1999;34:2455. [11] Trice RW, Halloran JW. J Am Ceram Soc 1999;82:2502. [12] McNulty JC, Zok FW, Begley MR. J Am Ceram Soc 2001;84:367. [13] He MY, Singh D, McNulty JC, Zok FW. J Comp Sci Technol, submitted for publication. [14] Mandell JF, Grande DH, Tsiang T-S, McGarry FJ, Special Technical Publication 893, Philadelphia: American Society for Testing Materials; 1986. p. 89. [15] Singh R. J Am Ceram Soc 1989;72:1864. [16] Bright JD, Shetty DK, Griffin CW, Limaye SY. J Am Ceram Soc 1989;72:1891. [17] Singh D, Singh JP, Majumdar S, Kupperman DS, Cowdin E, Bhatt RT. J Am Ceram Soc 1994;77:2561. [18] Majumdar S, Kupperman D, Singh J. J Am Ceram Soc 1988;71:858. [19] Carpenter JM, Mildner DFR, Richardson JW. Nucl Instr Meth A 1991;301:305. [20] Thouless MD, Sbaizero O, Sigl LS, Evans AG. J Am Ceram Soc 1989;72:525. [21] Shetty DK. J Am Ceram Soc 1988;71:C-107. [22] In: Ceramic Source, vol. 6. Westerville: American Ceramic Society; 1990. [23] Smirnov BI, Burenkov YuA, Kardashev BK, Singh D, Goretta KC, de Arellano-Lopez AR. Phys Sol State
2001;43:2094. [24] Erdermir A. Argonne National Laboratory, unpublished data, 2001. D. Singh et al. / Scripta Materialia 46 (2002) 747–751 751