April 1998 Crack Deflection and Propagation in Layered silicon Nitride/Boron Nitride Ceramics Bulk si n I00 1.2 I N, in Interphase (% Crosshead Displacement(mm Fig. 1. You (Eof the ic. measured the impulse ensile stress(o), plotted versus crosshead displace. ine is the rule- of- odulus. The value ment, for specimens containing 10, 25, 50, and 80 vol% Si,N4 in the en taken from Kovar et al. interphase four-point bending tent in the interphase in Fig. 1. The value of E seems to increase from hexagonal BN(HCP, Advanced Ceramics Corp, Cleve- land, OH), Si3N4, water, and ethanol. Individual billets were linearly as the Si3N4 content in the interphase increases, and E follows the voigt rule of mixtures 13 The e value measured manufactured using interphases made from 0, 10, 20, 50, and from the load-deflection plots followed a similar trend, and 80 vol% SiaNa(the remainder was BN). After coating, the moduli measured using both techniques agreed within 6% sheets were dried, stacked, and pressed at a temperature of 130C under a pressure of 6.9 kPa to mold them into a solid (2) Strength and Energy Absorption billet Four-point flexural tests were performed using a screw- After forming the billet, the polymer binder was pyrolyzed driven machine operated in displacement control( Model 4483 by heating it slowly in a flowing nitrogen atmosphere. The Instron, Danvers, MA). All tests were performed using a fully heating rates were60°C/htol50°C,2°C/hto250°C,4°C/hto articulating testing jig with free-rolling pins using an outer span 370°C,andl8°C/hto700°C. A slow heating rate was neces of 40 mm and an inner span of 20 mm. Data were collected ary to minimize bloating and cracking during pyrolysis, which using a computerized data-acquisition system at a rate of 5 can result in distortion of the layers. After pyrolysis, the billets points per second. Strength and WOF were measured on un- were placed in a BN-coated graphite die and hot pressed at notched specimens at a crosshead displacement rate of 0.5 1750 C for 2 h under a pressure of 25 MPa. Specimens for mm/min. Prior to testing, the specimens were polished to a 3 flexural tests were cut and ground from the billets to um finish using resin-bonded diamond wheels (TBw, Furlong, 4mm×50mm PA)on the tensile surface and on one side surface. The edges of the specimen on the tensile surface were also chamfered IlL Results Tests were interrupted when the specimen fractured com pletely, the retained load dropped below 5 N, or the crosshead After hot pressing, the thicknesses of the layers were mea- displacement exceeded I mr hichever came first. The sured on a polished surface of representative specimens using strength of the specimens was calculated using standard elastic- optical microscopy. The layer thicknesses were 116 34 um beam equations, whereas the WOF value was calculated by and 36+ 18 um for the SiaN4 layers and the BN-containing dividing the total area under the load-deflection curve by twice he cross-sectional area of the specimen. For specimens that cated that all of the Si3Na transformed to B-SisNa during hot fractured catastrophically, the WOF value was reported as zero pressing Hexagonal BN and a very small amount of tetragonal The nominal stressft on the tensile surface for representative irconia(t-ZrO2)were also detected The 0, contamination specimens is plotted versus crosshead deflection for unnotched resulted from the media used during the ball milling of the specimens in Fig. 2. In general, the load remains linear up to the peak load for all the materials. After the peak load, some of ( Young's Modulus the specimens continue to retain load at specimen deflections as large as I mm. the greatest degree of load retention is The Youngs modulus of the specimens was measured using observed in the materials with the lowest SiN, content in the n impulse-excitation technique(Grindo-Sonic MK4x,JW interphase; no load retention is observed following the peak Lemmens, St. Louis, MO), according to ASTM Method C load when the SiaN4 content in the interphase exceeds 25% 1259-94. To verify that these results were valid for layered The nominal strength and wOF are plotted in Fig. 3 as a ceramics, the stiffness of selected specimens were also mea- function of the Sia Na content in the interphase. Although there sured from the slope of load-deflection curves taken in the is scatter in the nominal strengths, there does not seem to be a elastic regime in four-point bending Specimen deflection at systematic change in strength with increasing Si3, content in the center of the span was monitored using a linearly variable the interphase. However, the WoF value decreases precipi displacement transducer(LVDT) and corrected for the compli tously as the SiaN4 content in the interphase increases. The ance of the machine, which had been determined previously slight decrease in strength and WOF for the specimens that The Youngs modulus (E), determined using the pulsed excitation technique, is plotted as a function of the Si3 N4 con- nd that the stress state is American Society for Standards and Testing, Philadelphia, PA when cracking occurfrom hexagonal BN (HCP, Advanced Ceramics Corp., Cleve￾land, OH), Si3N4, water, and ethanol. Individual billets were manufactured using interphases made from 0, 10, 20, 50, and 80 vol% Si3N4 (the remainder was BN). After coating, the sheets were dried, stacked, and pressed at a temperature of 130°C under a pressure of 6.9 kPa to mold them into a solid billet. After forming the billet, the polymer binder was pyrolyzed by heating it slowly in a flowing nitrogen atmosphere. The heating rates were 60°C/h to 150°C, 2°C/h to 250°C, 4°C/h to 370°C, and 18°C/h to 700°C. A slow heating rate was neces￾sary to minimize bloating and cracking during pyrolysis, which can result in distortion of the layers. After pyrolysis, the billets were placed in a BN-coated graphite die and hot pressed at 1750°C for 2 h under a pressure of 25 MPa. Specimens for flexural tests were cut and ground from the billets to nominal dimensions of 3 mm × 4 mm × 50 mm. III. Results After hot pressing, the thicknesses of the layers were mea￾sured on a polished surface of representative specimens using optical microscopy. The layer thicknesses were 116 ± 34 mm and 36 ± 18 mm for the Si3N4 layers and the BN-containing interphases, respectively. X-ray diffractometry (XRD) indi￾cated that all of the Si3N4 transformed to b-Si3N4 during hot pressing. Hexagonal BN and a very small amount of tetragonal zirconia (t-ZrO2) were also detected. The ZrO2 contamination resulted from the media used during the ball milling of the powders. (1) Young’s Modulus The Young’s modulus of the specimens was measured using an impulse-excitation technique (Grindo-Sonic MK4x, J. W. Lemmens, St. Louis, MO), according to ASTM Method C 1259-94.¶ To verify that these results were valid for layered ceramics, the stiffness of selected specimens were also mea￾sured from the slope of load–deflection curves taken in the elastic regime in four-point bending. Specimen deflection at the center of the span was monitored using a linearly variable displacement transducer (LVDT) and corrected for the compli￾ance of the machine, which had been determined previously.12 The Young’s modulus (E), determined using the pulsed￾excitation technique, is plotted as a function of the Si3N4 con￾tent in the interphase in Fig. 1. The value of E seems to increase linearly as the Si3N4 content in the interphase increases, and E follows the Voigt rule of mixtures.13 The E value measured from the load–deflection plots followed a similar trend, and moduli measured using both techniques agreed within 6%. (2) Strength and Energy Absorption Four-point flexural tests were performed using a screw￾driven machine operated in displacement control (Model 4483, Instron, Danvers, MA). All tests were performed using a fully articulating testing jig with free-rolling pins using an outer span of 40 mm and an inner span of 20 mm. Data were collected using a computerized data-acquisition system at a rate of 5 points per second. Strength and WOF were measured on un￾notched specimens at a crosshead displacement rate of 0.5 mm/min. Prior to testing, the specimens were polished to a 3 mm finish using resin-bonded diamond wheels (TBW, Furlong, PA) on the tensile surface and on one side surface. The edges of the specimen on the tensile surface were also chamfered. Tests were interrupted when the specimen fractured com￾pletely, the retained load dropped below 5 N, or the crosshead displacement exceeded 1 mm, whichever came first. The strength of the specimens was calculated using standard elastic￾beam equations, whereas the WOF value was calculated by dividing the total area under the load–deflection curve by twice the cross-sectional area of the specimen. For specimens that fractured catastrophically, the WOF value was reported as zero. The nominal stress†† on the tensile surface for representative specimens is plotted versus crosshead deflection for unnotched specimens in Fig. 2. In general, the load remains linear up to the peak load for all the materials. After the peak load, some of the specimens continue to retain load at specimen deflections as large as 1 mm. The greatest degree of load retention is observed in the materials with the lowest Si3N4 content in the interphase; no load retention is observed following the peak load when the Si3N4 content in the interphase exceeds 25%. The nominal strength and WOF are plotted in Fig. 3 as a function of the Si3N4 content in the interphase. Although there is scatter in the nominal strengths, there does not seem to be a systematic change in strength with increasing Si3N4 content in the interphase. However, the WOF value decreases precipi￾tously as the Si3N4 content in the interphase increases. The slight decrease in strength and WOF for the specimens that ¶ American Society for Standards and Testing, Philadelphia, PA. ††The nominal stress is calculated using standard elastic-beam theory, assuming elastic isotropy. It is recognized that the true stress is dependent on the local micro￾structure (i.e., the stiffer Si3N4 bears higher stress) and that the stress state is altered when cracking occurs anywhere in the beam. Fig. 1. Young’s modulus (E) of the layered ceramic, measured using the impulse-excitation technique, plotted versus Si3N4 content in the interphase; the solid line is the rule-of-mixtures modulus. The value for bulk Si3N4 has been taken from Kovar et al.11 Fig. 2. Nominal tensile stress (s), plotted versus crosshead displace￾ment, for specimens containing 10, 25, 50, and 80 vol% Si3N4 in the interphase tested in four-point bending. April 1998 Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics 1005