Y.H. Koh et al. Journal of the European Ceramic Society 24(2004)2339-2347 uniaxially aligned 250 micron cells of composition commercially available tester Grindo-sonic model Si3 N4(E-10, Ube Industries, Tokyo, Japan) with 6 MK4x, J W. Lemmon, St, Louis, MO, USA). 20 wt%Y2O3(99.9%, Johnson Matthey Electronics, MA USA)and 2 wt. Al,O3(HP-DBM, Reynolds, Bauxite, AK, USA), separated by 15-25 micron boron nitride 3. Results (HCP, Advanced Ceramics Corp Cleveland, OH, USA cell boundaries. Further details on the fabrication of 3. 1. Microstructure and mechanical properties before fibrous monoliths are described elsewhere. 4 For com thermal shock parison, monolithic Si3 N4 with 6 wt %Y2O3 and 2 wt% Al2O3 as sintering aids was also fabricated. The The typical microstructure of fibrous monolithic green billets were hot-pressed at 1740C under an Si3 N4/BN ceramic(FM)is shown in Fig. 1. Low mag applied pressure of 25 MPa for 2 h in a flowing N2 nification SEM micrographs of polished sections, shows atmosphere. The density of the specimens was measured three-dimensional representations of the sub-millimeter using the Archimedes method and the theoretical density structure of fibrous monoliths. The polycrystalline sili- of the specimens was estimated by the rule of mixture con nitride cells appear in dark contrast, while the con- tinuous boron nitride cell boundaries appear in bright 2.2. Specimen preparation contrast. The Si3 N4 cells are surrounded by the cell boundaries consisting of bn particles bonded with The thermal shock resistance was determined by yttriumaluminosilicate. measuring the retention of the flexural strength of The mechanical properties of monolithic Si3 N4 and water-quenched specimen. Specimens were machined FM samples are summarized in Table l. For FM, the nto a bar shape with dimensions of 3x4x45 mm and measured density (p) was slightly higher than theoretical ground with a 600-grit diamond wheel. The tensile side value(based on 82.5 vol. Si3 N4 cells and 17.5 vol% of the specimens was polished using diamond paste BN cell boundaries for fibrous monoliths), implying full down to 3 um, and subsequently chamfered to minimize densification of both Si3 N4 cell and BN cell boundary machining flaws. Also, the side surfaces of each speci- materials occurred. Elastic modulus (E)and flexural men were polished down to 30 um strength(MOR) of FM were slightly lower than those of monolithic Si3 N4, while apparent WoF increased 23. Thermal shock test remarkably due to the noncatastrophic failure through extensive crack interactions along the weak Bn cell Thermal shock test was carried out in a vertical tube boundaries furnace at temperatures between 800C and 1400C in The typical flexural responses of monolithic Si3 N4 and laboratory air. The furnace was heated at a heating rate FM are shown in Fig. 2. As expected, monolithic Sign of 10 C/min and maintained at exposure temperatures. showed higher strength but negligible apparent wo Polished specimens, suspended at the end of a platinum wire, were inserted into the hot-zone from the top and were soaked for 30 min to induce the homogeneous temperature distribution. After exposure, the specimens were quickly dropped into the water bath with a capa city of 5000 cc. The temperature of water bath did not increase notably after dropping the specimen 2.4. Mechanical test and characterization The flexural strength after thermal shock test was measured at room temperature by a four-point flexural configuration at a cross-head speed of 0.5 mm/min, and inner- and outer-spans of 20 and 40 mm, respectively The load versus crosshead deflection response and the work of fracture, calculated by determining the area under the load-crosshead deflection curve and dividing 250μm it by twice the cross-sectional area of the sample, are reported. Also, crack propagation during flexural Fig. I. Low magnification SEM micrographs of polished strength test after thermal shock was observed by an shows three-dimensional representations of the submillimeter of fibrous monoliths. The polycrystalline silicon nitride cells optical microscope and an SEM microscope. Elastic dark contrast and the continuous boron nitride cell boundaries are in moduli were measured by the impulse technique using a bright contrast. Courtesy of Bruce King)uniaxially aligned 250 micron cells of composition Si3N4 (E-10, Ube Industries, Tokyo, Japan) with 6 wt.% Y2O3 (99.9%, Johnson Matthey Electronics, MA, USA) and 2 wt.% Al2O3 (HP-DBM, Reynolds, Bauxite, AK, USA), separated by 1525 micron boron nitride (HCP, Advanced Ceramics Corp., Cleveland, OH, USA) cell boundaries. Further details on the fabrication of fibrous monoliths are described elsewhere.1,4 For com￾parison, monolithic Si3N4 with 6 wt.% Y2O3 and 2 wt.% Al2O3 as sintering aids was also fabricated. The green billets were hot-pressed at 1740
C under an applied pressure of 25 MPa for 2 h in a flowing N2 atmosphere. The density of the specimens was measured using the Archimedes method and the theoretical density of the specimens was estimated by the rule of mixture. 2.2. Specimen preparation The thermal shock resistance was determined by measuring the retention of the flexural strength of water-quenched specimen. Specimens were machined into a bar shape with dimensions of 3445 mm and ground with a 600-grit diamond wheel. The tensile side of the specimens was polished using diamond paste down to 3 mm, and subsequently chamfered to minimize machining flaws. Also, the side surfaces of each speci￾men were polished down to 30 mm. 2.3. Thermal shock test Thermal shock test was carried out in a vertical tube furnace at temperatures between 800
C and 1400
C in laboratory air. The furnace was heated at a heating rate of 10
C/min and maintained at exposure temperatures. Polished specimens, suspended at the end of a platinum wire, were inserted into the hot-zone from the top and were soaked for 30 min to induce the homogeneous temperature distribution. After exposure, the specimens were quickly dropped into the water bath with a capa￾city of 5000 cc. The temperature of water bath did not increase notably after dropping the specimen. 2.4. Mechanical test and characterization The flexural strength after thermal shock test was measured at room temperature by a four-point flexural configuration at a cross-head speed of 0.5 mm/min, and inner- and outer-spans of 20 and 40 mm, respectively. The load versus crosshead deflection response and the work of fracture, calculated by determining the area under the load–crosshead deflection curve and dividing it by twice the cross-sectional area of the sample, are reported. Also, crack propagation during flexural strength test after thermal shock was observed by an optical microscope and an SEM microscope. Elastic moduli were measured by the impulse technique using a commercially available tester (Grindo-sonic model MK4x, J. W. Lemmon, St, Louis, MO, USA).20 3. Results 3.1. Microstructure and mechanical properties before thermal shock The typical microstructure of fibrous monolithic Si3N4/BN ceramic (FM) is shown in Fig. 1. Low mag￾nification SEM micrographs of polished sections, shows three-dimensional representations of the sub-millimeter structure of fibrous monoliths. The polycrystalline sili￾con nitride cells appear in dark contrast, while the con￾tinuous boron nitride cell boundaries appear in bright contrast. The Si3N4 cells are surrounded by the cell boundaries consisting of BN particles bonded with yttriumaluminosilicate. The mechanical properties of monolithic Si3N4 and FM samples are summarized in Table 1. For FM, the measured density (
) was slightly higher than theoretical value (based on 82.5 vol.% Si3N4 cells and 17.5 vol.% BN cell boundaries for fibrous monoliths), implying full densification of both Si3N4 cell and BN cell boundary materials occurred. Elastic modulus (E) and flexural strength (MOR) of FM were slightly lower than those of monolithic Si3N4, while apparent WOF increased remarkably due to the noncatastrophic failure through extensive crack interactions along the weak BN cell boundaries. The typical flexural responses of monolithic Si3N4 and FM are shown in Fig. 2. As expected, monolithic Si3N4 showed higher strength but negligible apparent WOF Fig. 1. Low magnification SEM micrographs of polished sections, shows three-dimensional representations of the submillimeter structure of fibrous monoliths. The polycrystalline silicon nitride cells appear in dark contrast and the continuous boron nitride cell boundaries are in bright contrast. (Courtesy of Bruce King). 2340 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 2339–2347