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 thewt.% 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 exam￾ined. 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 di€used 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 bil￾lets 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 slow￾speed diamond-blade saw. Si3N4 and BN specimens were also prepared; the BN was cut parallel (BNpara) and per￾pendicular (BNperp) to the extrusion direction. Fibrous monoliths were also cut parallel (FMpara) and perpen￾dicular (FMperp) to the axis of the ®laments. All com￾pression 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 tempera￾ture was 1300±1500C, and the engineering strain rates (" : ) were 110ÿ6 ±510ÿ6 sÿ1 . All CSR tests were com￾pleted 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 atmo￾sphere 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 para￾meters from load and temperature changes. The microstructures of the undeformed and deformed specimens were analyzed by optical and scanning elec￾tron microscopy (SEM). The BN grains were platelike; the Si3N4 grains exhibited large aspect ratios, with aver￾age dimensions of 14 mm.10 Morphological changes of the cells were studied with an image analyzer (Zeiss Videoplan). Microstructural evolution during deforma￾tion 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 mono￾liths; (b) SEM photomicrographs showing the structure of the materi￾als in this study. 246 A.R. de Arellano-LoÂpez et al. / Journal of the European Ceramic Society 21 (2001) 245±250