E驅≈3S al of the euro Plastic deformation of silicon nitride/boron nitride fibrous monoliths A.R. de arellano-Lopeza,*,S. Lopez-Pomberoa, A Dominguez-Rodriguez a J. L. Routbort b d. singh b K.c. goretta Departamento de Fisica de la Materia Condensada, Universidad de sevilla, PO Box 1065, 41080 Seville, spain Energy Technology Division, Argonne National Laboratory, Argonne, IL 60349-4838, USA Received 27 November 1999: received in revised form 31 May 2000: accepted 1l June 2000 High-temperature compressive creep of unidirectional Si3 N4/BN fibrous monoliths has been investigated at 1300-1500oC in an inert tmosphere. The results were then compared to those for deformation of the Si3 N4 and bn base materials. Plasticity of the fibrous monoliths was limited to very low stresses when the Si3N4 cells were oriented perpendicular to the stress axis because the bn cell steady-state deformation controlled by deformation of the Si3 N4 cells was achieved. C 2001 Elsevier Science Ltd. All rights reserved 9 boundaries failed, followed by failure of the Si3 N4 cells. In the fibrous monolith in which cells were oriented parallel to the stress ax Keywords: Creep; BN; Fibrous monoliths; Mechanical properties; Si3N4 1. Introduction creep.,0 Fracture testing of Si3 N4/BN fibrous monoliths in air at temperatures up to 1400C has indicated that Ceramic fibrous monoliths generally consist of strong shear failures become dominant at 1100C and above ceramic cells that are surrounded by a weaker cell and that creep of the Si3 n4 phase becomes significant at boundary(see schematic diagram in Fig. la). These 21400 C. The creep rates have not been quantified monoliths are produced from ceramic powders by con We report here on compressive creep of Si3 N4/BN ventional fabrication techniques, such as extrusion 1.2 fibrous monoliths that were tested in inert atmosphe They exhibit graceful failure; in flexure, they splinter. -5 For comparison, Si3 N4 and BN specimens with compo- In many applications, fibrous monoliths may offer a sitions similar to those of the components of the fibrous low-cost alternative to conventional continuouS-fiber monolith phase were also tested ceramic composites. Several compositions of ceramics and cermets have een processed successfully in rous mon 2. Experimental details The most thoroughly investigated fibrous monolith consists of Si3 N4 cells and a continuous bn cell 2. 1. Specimen fabrication boundary. 3- Through appropriate selection of initial powders, and extrusion and hot-pressing parameters The fibrous monoliths were produced by Advanced strong, very tough Si3 N4/bn products have been ramics Research of Tucson, AZ. They were fabri produced. The high toughness is due primarily to cated from A325-Hm-diameter Si3 N4/BN coextruded crack deflection along the weaker bn phase green filaments that were produced by melt coextrusion Fracture of Si3 N4/BN fibrous monoliths has been of a blend of N52 vol. ceramic powder mixture in an studied extensively at both room and high tempe- ethylene-based copolymer binder. The coextruded ratures 3-I However, very little effort has focused on filaments contained nominally 85 vol. core Si3N4 material(E-10, Ube Industries, Tokyo) and 15 vol% Corresponding author. Tel. +34-95-4552891; ext 96; fax: +34- n cladding(HCP Grade, Advanced Ceramics Cor- poration, Cleveland ) The Si3N4 was of a sinterable E-mail address: ramirez(@ cica. es(A R. de Arellano-Lopez) composition, 92 wt. commercial Si3 N4 powder, 6 0955-2219/01/S. see front matter C 2001 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00175
Plastic deformation of silicon nitride/boron nitride ®brous monoliths A.R. de Arellano-LoÂpez a,*, S. LoÂpez-Pombero a , A. DomõÂnguez-RodrõÂguez a , J.L. Routbort b, D. Singh b, K.C. Goretta b a Departamento de Fisica de la Materia Condensada, Universidad de Sevilla, PO Box 1065, 41080 Seville, Spain bEnergy Technology Division, Argonne National Laboratory, Argonne, IL 60349-4838, USA Received 27 November 1999; received in revised form 31 May 2000; accepted 11 June 2000 Abstract High-temperature compressive creep of unidirectional Si3N4/BN ®brous monoliths has been investigated at 1300±1500C in an inert atmosphere. The results were then compared to those for deformation of the Si3N4 and BN base materials. Plasticity of the ®brous monoliths was limited to very low stresses when the Si3N4 cells were oriented perpendicular to the stress axis because the BN cell boundaries failed, followed by failure of the Si3N4 cells. In the ®brous monolith in which cells were oriented parallel to the stress axis, steady-state deformation controlled by deformation of the Si3N4 cells was achieved. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Creep; BN; Fibrous monoliths; Mechanical properties; Si3N4 1. Introduction Ceramic ®brous monoliths generally consist of strong ceramic cells that are surrounded by a weaker cell boundary (see schematic diagram in Fig. 1a). These monoliths are produced from ceramic powders by conventional fabrication techniques, such as extrusion.1,2 They exhibit graceful failure; in ¯exure, they splinter.1ÿ5 In many applications, ®brous monoliths may oer a low-cost alternative to conventional continuous-®ber ceramic composites. Several compositions of ceramics and cermets have been processed successfully in ®brous monolithic form.4 The most thoroughly investigated ®brous monolith consists of Si3N4 cells and a continuous BN cell boundary.3ÿ5 Through appropriate selection of initial powders, and extrusion and hot-pressing parameters, strong, very tough Si3N4/BN products have been produced.2ÿ7 The high toughness is due primarily to crack de¯ection along the weaker BN phase. Fracture of Si3N4/BN ®brous monoliths has been studied extensively at both room and high temperatures.3ÿ11 However, very little eort has focused on creep.9,10 Fracture testing of Si3N4/BN ®brous monoliths in air at temperatures up to 1400C has indicated that shear failures become dominant at 1100C and above and that creep of the Si3N4 phase becomes signi®cant at 1400C.11 The creep rates have not been quanti®ed. We report here on compressive creep of Si3N4/BN ®brous monoliths that were tested in inert atmosphere. For comparison, Si3N4 and BN specimens with compositions similar to those of the components of the ®brous monolith phase were also tested. 2. Experimental details 2.1. Specimen fabrication The ®brous monoliths were produced by Advanced Ceramics Research of Tucson, AZ. They were fabricated from 325-mm-diameter Si3N4/BN coextruded green ®laments5 that were produced by melt coextrusion of a blend of 52 vol.% ceramic powder mixture in an ethylene-based copolymer binder.11 The coextruded ®laments contained nominally 85 vol.% core Si3N4 material (E-10, Ube Industries, Tokyo) and 15 vol.% BN cladding (HCP Grade, Advanced Ceramics Corporation, Cleveland). The Si3N4 was of a sinterable composition, 92 wt.% commercial Si3N4 powder, 6 0955-2219/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00175-8 Journal of the European Ceramic Society 21 (2001) 245±250 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +34-95-4552891; ext. 96; fax: +34- 95-4612097. E-mail address: ramirez@cica.es (A.R. de Arellano-LoÂpez)
A.R. de Arellano-Lopez et al. Journal of the European Ceramic Society 21(2001)245-250 fibrous-monolith billets with densities >98% of their theoretical density. During hot pressing, some of the oxide additives to the sian diffused into the BN 11, 12 The final structure of the material is documented in the scanning electron photomicrograph montage in Fig. Ib By similar techniques, monolithic Si3N4 and bn bi lets were also hot pressed from extruded filaments. Each monolithic ceramic also contained Y2O3 and AlO3 in phases of the Si3 N4/BN fibrous monoliths. 12/o- the approximate concentrations that are foun BN cell boundary 2. 2. Creep tests and microstructural analyses Right parallelepiped specimens≈3×3×5 mm were cut from the fibrous-monolith and Si3 N4 billets with a slow- speed diamond-blade saw Si3 N4 and BN specimens were prepared; the Bn was cut parallel(BNpara) and per- pendicular(BNperp) to the extrusion direction. Fibrous monoliths were also cut parallel(FMpara) and perpen 0.2mm dicular(FMperp) to the axis of the filaments. All com pression surfaces were polished to be flat and parallel Both constant-strain-rate(CSR) and constant-load (CL) tests were conducted. The purpose of the CSr tests was to assess the likely ranges of plasticity tests, each specimen was compressed at constant between Si3 N4 platens in an Instron Model 1125 tester. 10, 14 The atmosphere was static N2, the tempera- ture was 1300-1500oC, and the engineering strain rates ()were≈l×10-6-5×10-6s-1. All csr tests wer pleted within 10 h. For most tests, the specimens were unloaded and reloaded and at least two data point were taken for apparent steady-state stress(o) CL tests were conducted in a dead-load prototype Fig. 1.(a) Schematic diagram of the structure of the fibrous apparatus that has been described. A static Ar atmo liths:(b) sEM phote sphere was used, and temperatures from 1400 to 1500C were selected. Based on previous high-temperature fracture studies, I creep of Si3 N4/BN fibrous monoliths wt%Y2O3, and 2 wt. Al2O3. The oxides were added should be relatively rapid in this temperature range CL to promote densification testing allowed for measurement of a wide range of Single-filament-thick sheets of uniaxially aligned stresses(10-200 MPa), and calculation of creep para- green filaments were produced by a winding operation meters from load and temperature changes. that placed the coextruded filaments side by side on a The microstructures of the undeformed and deformed cylindrical mandrel. The filaments were held in place specimens were analyzed by optical and scanning elec with a spray adhesive that, upon drying, allowed tron microscopy (SEM). The bn grains were platelike: removal of the unidirectional sheets of green fibrous the Si3 N4 grains exhibited large aspect ratios, with aver monolith from the mandrel. The sheets were stacked age dimensions of alx4 um. o Morphological changes fabricate unidirectional, 0 specimens. Although of the cells were studied with an image analyzer(Zeis Advanced Ceramics Research can produce complex Videoplan). Microstructural evolution during deforma- laminated architectures, only 0 specimens were exam tion was correlated to deformation mechanisms ined. The laminates were cut into the desired preform and warm pressed at 160C to produce a solid green panel Simple, rectangular, flat panels were fabricated and 3. Results and discussion objected to a binder pyrolysis step that consisted of slow heating in flowing N2 to 600C over a period of 42 h. The CSR tests were performed on BNpara and BNperp panels were then uniaxially hot pressed at 1740oC for I h samples. As seen in Fig. 2, apparent steady-state stresses under 28 MPa pressure. This procedure yielded were higher for the BNpara samples than for the
wt.% Y2O3, and 2 wt.% Al2O3. The oxides were added to promote densi®cation. Single-®lament-thick sheets of uniaxially aligned green ®laments were produced by a winding operation that placed the coextruded ®laments side by side on a cylindrical mandrel. The ®laments were held in place with a spray adhesive that, upon drying, allowed removal of the unidirectional sheets of green ®brous monolith from the mandrel. The sheets were stacked to fabricate unidirectional, 0 specimens.5 Although Advanced Ceramics Research can produce complex laminated architectures, only 0 specimens were examined. The laminates were cut into the desired preform and warm pressed at 160C to produce a solid green panel. Simple, rectangular, ¯at panels were fabricated and subjected to a binder pyrolysis step that consisted of slow heating in ¯owing N2 to 600C over a period of 42 h. The panels were then uniaxially hot pressed at 1740C for 1 h under 28 MPa pressure. This procedure yielded ®brous-monolith billets with densities >98% of their theoretical density. During hot pressing, some of the oxide additives to the Si3N4 diused into the BN.11,12 The ®nal structure of the material is documented in the scanning electron photomicrograph montage in Fig. 1b. By similar techniques, monolithic Si3N4 and BN billets were also hot pressed from extruded ®laments. Each monolithic ceramic also contained Y2O3 and Al2O3 in the approximate concentrations that are found in the phases of the Si3N4/BN ®brous monoliths.12,13 2.2. Creep tests and microstructural analyses Right parallelepiped specimens 335 mm were cut from the ®brous-monolith and Si3N4 billets with a slowspeed diamond-blade saw. Si3N4 and BN specimens were also prepared; the BN was cut parallel (BNpara) and perpendicular (BNperp) to the extrusion direction. Fibrous monoliths were also cut parallel (FMpara) and perpendicular (FMperp) to the axis of the ®laments. All compression surfaces were polished to be ¯at and parallel. Both constant-strain-rate (CSR) and constant-load (CL) tests were conducted. The purpose of the CSR tests was to assess the likely ranges of plasticity. In these tests, each specimen was compressed at constant velocity between Si3N4 platens in an Instron Model 1125 universal tester.10,14 The atmosphere was static N2, the temperature was 1300±1500C, and the engineering strain rates (" : ) were 110ÿ6 ±510ÿ6 sÿ1 . All CSR tests were completed within 10 h. For most tests, the specimens were unloaded and reloaded and at least two data points were taken for apparent steady-state stress (). CL tests were conducted in a dead-load prototype apparatus that has been described.15 A static Ar atmosphere was used, and temperatures from 1400 to 1500C were selected. Based on previous high-temperature fracture studies,11 creep of Si3N4/BN ®brous monoliths should be relatively rapid in this temperature range. CL testing allowed for measurement of a wide range of stresses (10±200 MPa), and calculation of creep parameters from load and temperature changes. The microstructures of the undeformed and deformed specimens were analyzed by optical and scanning electron microscopy (SEM). The BN grains were platelike; the Si3N4 grains exhibited large aspect ratios, with average dimensions of 14 mm.10 Morphological changes of the cells were studied with an image analyzer (Zeiss Videoplan). Microstructural evolution during deformation was correlated to deformation mechanisms. 3. Results and discussion CSR tests were performed on BNpara and BNperp samples. As seen in Fig. 2, apparent steady-state stresses were higher for the BNpara samples than for the Fig. 1. (a) Schematic diagram of the structure of the ®brous monoliths; (b) SEM photomicrographs showing the structure of the materials in this study. 246 A.R. de Arellano-LoÂpez et al. / Journal of the European Ceramic Society 21 (2001) 245±250
ar de ipez et al. Journal of the European Ceramic Society 21(2001)245-250 BNperp samples. Apparent stresses were essentially A summary of all of the creep tests that were per- independent of temperature and strain rate. SEM formed on the FM and Si3N4 samples is shown as a log- revealed, however, that the deformation of the bn log plot in Fig. 4. The data points in this figure corre- samples was dominated by sequential fracture(Fig 3). spond to the range in which apparent steady states were As has also been observed in creep testing of high-tem- measured. below the minimum stress for each material perature Bi-based cuprate superconductors, which con- no deformation was recor above the maximum sist of platelike grains, apparent steady-state responses values, the high stresses led to accelerating deformation can be obtained, but deformation actually proceeds by (i.e. tertiary creep). For comparison, the Bn results bending of the grains with concomitant microcracking have also been included. Over the experimental tem- between grains. The finding of creep fracture, rather perature range, BN is substantially softer than the Si3N4 than steady-state creep, in these CSr tests was not sur- and FM samples and does not deform in the steady prising. Torsional creep experiments achieved steady state, as noted above state deformation only at temperatures >2000 C 17 The The FMperp samples did not undergo significant fact that the fracture stress was approximately indepen- plastic deformation under the testing conditions. At dent of temperature over the narrow range used in these 1400C, only two data points were recorded, at 12 and tests is expected 40 O0A44 10 0LA 10 1000 6.6 7 7,4 Fig. 4. CL tests on Si3N4 monoliths and FMs. The symbols are: O Fig. 2. Apparent steady-state stresses from CSR tests on extruded SiN 4-1500°C;△, AMpara-1400°C;▲ BNpara AMpara-1500°c口, FMperp-1400°C. The slope of the line is I BNperp,=2-10-6s-1;▲ BNperp,g=1×10-6s-1 he shaded region indicates range of BN response. (a) Jo JI pm Fig 3. SEM photomicrographs, showing fracture of BN from deformation:(a)low-magnification view and (b) high-magnification view, in which cleavage and bending of bN gra
BNperp samples. Apparent stresses were essentially independent of temperature and strain rate. SEM revealed, however, that the deformation of the BN samples was dominated by sequential fracture (Fig. 3). As has also been observed in creep testing of high-temperature Bi-based cuprate superconductors, which consist of platelike grains, apparent steady-state responses can be obtained, but deformation actually proceeds by bending of the grains with concomitant microcracking between grains.16 The ®nding of creep fracture, rather than steady-state creep, in these CSR tests was not surprising. Torsional creep experiments achieved steadystate deformation only at temperatures >2000C.17 The fact that the fracture stress was approximately independent of temperature over the narrow range used in these tests is expected. A summary of all of the creep tests that were performed on the FM and Si3N4 samples is shown as a log± log plot in Fig. 4. The data points in this ®gure correspond to the range in which apparent steady states were measured. Below the minimum stress for each material, no deformation was recorded; above the maximum values, the high stresses led to accelerating deformation (i.e. tertiary creep). For comparison, the BN results have also been included. Over the experimental temperature range, BN is substantially softer than the Si3N4 and FM samples and does not deform in the steadystate, as noted above. The FMperp samples did not undergo signi®cant plastic deformation under the testing conditions. At 1400C, only two data points were recorded, at 12 and Fig. 2. Apparent steady-state stresses from CSR tests on extruded BN: *, BNpara, " : =210ÿ6 sÿ1 ; ~, BNpara, " : =110ÿ6 sÿ1 ; *, BNperp, " : =2ÿ10ÿ6 sÿ1 ; ~ BNperp, " : =110ÿ6 sÿ1 . Fig. 3. SEM photomicrographs, showing fracture of BN from deformation: (a) low-magni®cation view and (b) high-magni®cation view, in which cleavage and bending of BN grains are evident. Fig. 4. CL tests on Si3N4 monoliths and FMs. The symbols are: *, Si3N4 Ð 1400C; *, Si3N4 Ð 1500C; ~, FMpara Ð 1400C; ~, FMpara Ð 1500C; &, FMperp Ð 1400C. The slope of the line is 1; the shaded region indicates range of BN response. A.R. de Arellano-LoÂpez et al. / Journal of the European Ceramic Society 21 (2001) 245±250 247
A.R. de Arellano-Lopez et al. Journal of the European Ceramic Society 21(2001)245-250 0 MPa, before fracture occurred at only 0. 7% strain. Dr o/l This orientation was extremely brittle. The cracked BN matrix could not prevent the sliding of the Si3 N4 cells, where D, S, and I refer to the diameter, area, and length and fracture occurred, primarily by propagation of a of a cell and the subscripts o and f refer to the initial single intercell crack, along a diagonal of the section and final dimensions, respectively. Based on constant perpendicular to the cells(Fig. 5). On the other hand, cell volume, the final average diameter of the cells was the FMpara and Si3 N4 samples deformed A10% with- calculated to be 109.2 um. The measured value was 110.0+1.7 um, in excellent agreement with the calcu The FMpara samples did not deform in steady state lated value under lower stresses. For those stresses, the only sig- For analysis of the creep data, a classical semi- nificant feature was cracking within the Bn matri phenomenological equation was use until the load was fully transferred to the si3N4 cells. At 1400C, the first apparent steady-state E=Aoe-Q/rT under 75 MPa; at 1500oC, the lowest stress that yielded a steady-state was 57 MPa. Once a critical stress was urpassed and the load could be transferred to the cells, where e is the strain rate, o is stress, T is absolute tem- they deformed plastically and controlled the deformation. perature, n and Q are adjustable parameters, A is a The monolithic Si3N4 deformed in steady state at constant, and R is the gas constant 1500C at stresses as low as 13 MPa At higher stresses, Fig 4 indicates that the dependence of strain rate on the agreement between the absolute creep rates of the stress is approximately linear (nal); thus, a more- FMpara and Si3 N4 monolith was outstanding, because detailed understanding can be achieved by plotting the for both types of samples, the creep rate was controlled data on a linear-linear graph(Fig. 7). The regression by the deformation of the polycrystalline Si3 N4 lines are reasonable: both were selected to intersect the This conclusion was confirmed by studying the evo- origin. The slopes of the regression lines at T1=1400oC lution of the sections of the cells of the FMpara samples during creep. The equivalent diameter of the unde formed cells was 103.0-+2.3 um. A 3940-Hm-long sample as deformed to a final length of 3500 um. A mor phological study was performed on a section of this sample(Fig. 6a). SEM photomicrographs revealed intercell cracks; however, the sample did not fail. The cracks within the bn phase were due to buckling of the cells, mostly those nearest the surface(Fig. 6b) If one assumes a constant cell volume Sr= So(lo/l1) 出mr 200m 1 mm Fig. 5. SEM photomicrograph of deformed FMperp sample, showing Fig. 6. SEM photomicrographs of deformed FMpara sample:(a)sec- formation of damage that led to failure; in the figure the compression tions of cells and(b) buckling of cells that were near the surface; in the axis was horizontal figure the compression axis was horizontal
40 MPa, before fracture occurred at only 0.7% strain. This orientation was extremely brittle. The cracked BN matrix could not prevent the sliding of the Si3N4 cells, and fracture occurred, primarily by propagation of a single intercell crack, along a diagonal of the section perpendicular to the cells (Fig. 5). On the other hand, the FMpara and Si3N4 samples deformed 10% without fracture. The FMpara samples did not deform in steady state under lower stresses. For those stresses, the only signi®cant feature was cracking within the BN matrix, until the load was fully transferred to the Si3N4 cells. At 1400C, the ®rst apparent steady-state was measured under 75 MPa; at 1500C, the lowest stress that yielded a steady-state was 57 MPa. Once a critical stress was surpassed and the load could be transferred to the cells, they deformed plastically and controlled the deformation. The monolithic Si3N4 deformed in steady state at 1500C at stresses as low as 13 MPa. At higher stresses, the agreement between the absolute creep rates of the FMpara and Si3N4 monolith was outstanding, because for both types of samples, the creep rate was controlled by the deformation of the polycrystalline Si3N4. This conclusion was con®rmed by studying the evolution of the sections of the cells of the FMpara samples during creep. The equivalent diameter of the undeformed cells was 103.02.3 mm. A 3940-mm-long sample was deformed to a ®nal length of 3500 mm. A morphological study was performed on a section of this sample (Fig. 6a). SEM photomicrographs revealed intercell cracks; however, the sample did not fail. The cracks within the BN phase were due to buckling of the cells, mostly those nearest the surface (Fig. 6b). If one assumes a constant cell volume, Sf So lo=lf 1 and Df Do lo=lf p ; where D, S, and l refer to the diameter, area, and length of a cell and the subscripts o and f refer to the initial and ®nal dimensions, respectively. Based on constant cell volume, the ®nal average diameter of the cells was calculated to be 109.2 mm. The measured value was 110.01.7 mm, in excellent agreement with the calculated value. For analysis of the creep data, a classical semiphenomenological equation was used,18 " : An eÿQ=RT ; 2 where " : is the strain rate, is stress, T is absolute temperature, n and Q are adjustable parameters, A is a constant, and R is the gas constant. Fig. 4 indicates that the dependence of strain rate on stress is approximately linear (n 1); thus, a moredetailed understanding can be achieved by plotting the data on a linear-linear graph (Fig. 7). The regression lines are reasonable; both were selected to intersect the origin. The slopes of the regression lines at T1=1400C Fig. 5. SEM photomicrograph of deformed FMperp sample, showing formation of damage that led to failure; in the ®gure the compression axis was horizontal. Fig. 6. SEM photomicrographs of deformed FMpara sample: (a) sections of cells and (b) buckling of cells that were near the surface; in the ®gure the compression axis was horizontal. 248 A.R. de Arellano-LoÂpez et al. / Journal of the European Ceramic Society 21 (2001) 245±250
ar de Arellano-Lopez et al / Journal of the European Ceramic Society 21(2001)245-250 B1=7.6710x10-10 MPa- s-)and T2=1500c boundary phase 9-24 Several authors have suggested (B2=7.7903x10-9MPa-ls-I)can be used to obtain an that compressive creep occurs by grain-boundary sliding estimate of the activation energy as follows probably accommodated by solution/reprecipitation of the Si3 N4 phase and viscous flow of the boundary e-O/RT1 BI RTI -O/RT2 B It was determined that the polycrystalline Si3N4 phase controls the creep-rate of the parallel-orientation FMs Eq(3) yields Q=570+150 kJ /mol. A specific experi This conclusion is similar to that obtained from creep of ment to determine g directly was performed by tem- unidirectional Sic-fiber/mullite composites. 25 In gen- perature jumps between 1400 and 1500C on a FMpara eral, creep of unidirectional FMs is likely to be domi- sample, under a constant stress of 105 MPa(Fig 8). The nated by the weaker, cell-boundary phase when esult was 0=625+40 kJ/mol, which is in fair agree- compression is perpendicular to the cells and dominated ment with the o value calculated when all of the data by the stronger cells when compression is parallel to the points are considered These values for n and 0, are in excellent agreeme vith previous work on Si3 N4 that contained a grain 4. Conclusions 10×10°「T+TT Compressive creep of a unidirectional Si3 N4/BN fibrous monoliths and its individual constituents. bN 1300-1500%C. When the cells in the fibrous monolith were oriented perpendicular to the stress axis, steady- state deformation was limited to very low stresses. For U0.5×10 this orientation fracture of the bn matrix occurred followed by fracture of the Si3 N4 cells. When the cells in the fibrous monolith were oriented parallel to the stress axis, steady-state deformation and permanent strains to 10% were achieved. In this orientation, plastic deforma- tion was controlled by deformation of the Si3 N4 cell 050100150200 Acknowledgements Fig. 7. CL creep results on a linear-linear graph; O, A=1400C and le thank M. rigali and M. Sutaria for supplying the ●,△=1500°C test materials. This work was supported in Sevilla by the Spanish Ministerio de educacion, CICYT Project MAT97-1007-C02, and at Argonne National Labora tory by the Defense Advanced Research Projects Agency, through a Department of Energy Interagency Agreement, under Contract W-31-109-Eng-38. J. L.R. is grateful to IBERDROLA for providing funds during his stay at the References atent 4, 772, 524, 20 September 1988 as, G. E, Brady, G.A. Somas, S, Bard, A and Zywicki, G. Process for preparing tex- tured ceramic composites. US Patent. 5, 645. 781, 8 July dy, G. A, Abdali, U, Zywicki. G. and Hallora 105/RT cessed ceramics. Mater. Sci. Eng 1995. A195, 263-268 4. Kovar, D, King. B. H. Trice, R. w. and Halloran, J. W. Fig 8. CL test to determine 0=625+40 kJ/mol for FMpara sample Fibrous monolithic ceramics. J. Am. Ceram Soc., 1997. 80. 2471-
(B1=7.671010ÿ10 MPaÿ1 sÿ1 ) and T2=1500C (B2=7.790310ÿ9 MPaÿ1 sÿ1 ) can be used to obtain an estimate of the activation energy as follows: eÿQ=RT1 eÿQ=RT2 B1 B2 ) Q RT1T2 T1 ÿ T2 ln B1 B2 : 3 Eq. (3) yields Q=570150 kJ/mol. A speci®c experiment to determine Q directly was performed by temperature jumps between 1400 and 1500C on a FMpara sample, under a constant stress of 105 MPa (Fig. 8). The result was Q=62540 kJ/mol, which is in fair agreement with the Q value calculated when all of the data points are considered. These values for n and Q, are in excellent agreement with previous work on Si3N4 that contained a grainboundary phase.19ÿ24 Several authors have suggested that compressive creep occurs by grain-boundary sliding probably accommodated by solution/reprecipitation of the Si3N4 phase and viscous ¯ow of the boundary phase.19ÿ24 It was determined that the polycrystalline Si3N4 phase controls the creep-rate of the parallel-orientation FMs. This conclusion is similar to that obtained from creep of unidirectional SiC-®ber/mullite composites.25 In general, creep of unidirectional FMs is likely to be dominated by the weaker, cell-boundary phase when compression is perpendicular to the cells and dominated by the stronger cells when compression is parallel to the cells. 4. Conclusions Compressive creep of a unidirectional Si3N4/BN ®brous monoliths and its individual constituents, BN and Si3N4, was investigated in inert atmosphere at 1300±1500C. When the cells in the ®brous monolith were oriented perpendicular to the stress axis, steadystate deformation was limited to very low stresses. For this orientation, fracture of the BN matrix occurred, followed by fracture of the Si3N4 cells. When the cells in the ®brous monolith were oriented parallel to the stress axis, steady-state deformation and permanent strains to 10% were achieved. In this orientation, plastic deformation was controlled by deformation of the Si3N4 cells. Acknowledgements We thank M. Rigali and M. Sutaria for supplying the test materials. This work was supported in Sevilla by the Spanish Ministerio de EducacioÂn, CICYT Project MAT97-1007-C02, and at Argonne National Laboratory by the Defense Advanced Research Projects Agency, through a Department of Energy Interagency Agreement, under Contract W-31-109-Eng-38. J.L.R. is grateful to IBERDROLA for providing funds during his stay at the University of Sevilla. References 1. Coblenz, W. S. Fibrous monolithic ceramic and method for production. US Patent 4,772,524, 20 September 1988. 2. Popovich, D., Halloran, J. W., Hilmas, G. E., Brady, G. A., Somas, S., Bard, A. and Zywicki, G. Process for preparing textured ceramic composites. US Patent. 5,645,781, 8 July 1997. 3. Hilmas, G., Brady, G. A., Abdali, U., Zywicki, G. and Halloran, J., Fibrous monoliths: non-brittle fracture from powder-processed ceramics. Mater. Sci. Eng., 1995, A195, 263±268. 4. Kovar, D., King, B. H., Trice, R. W. and Halloran, J. W., Fibrous monolithic ceramics. J. Am. Ceram. Soc., 1997, 80, 2471± 2487. Fig. 7. CL creep results on a linear±linear graph; *,~=1400C and *,~=1500C. Fig. 8. CL test to determine Q=62540 kJ/mol for FMpara sample; stress=105 MPa. A.R. de Arellano-LoÂpez et al. / Journal of the European Ceramic Society 21 (2001) 245±250 249
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