D. Rodeghiero et al/ Materials Science and Engineering 4244(1998)11-21 2. 4. Characterization The density of all hot-pressed composites was deter mined by calculating the mass and volume of each X-ray diffraction(XRD) of both the precursor and pellet after cutting into a prismatic configuration. In heat treated powders as well as the sintered pellets wa addition, the relative or percentage density of each performed using a Scintag Pad X diffractometer with material was calculated by dividing the measured den CuKa radiation. Reduced metal-ceramic powders were sity by the theoretical density based on composite com- encapsulated in 1.0 mm diameter glass capillaries under position. The resulting number, always slightly argon to prevent reoxidation of the very small metallic < 100%, was used as an indicator of residual porosity particles present at this stage of processing In the case of the metal-ceramic composites, electrical Polished, sintered microstructures were investigated resistivity of the hot-pressed pellets was also measured with both optical and scanning electron microscopy with the use of a standard two-probe ohmmeter, in (SEM). The optical microscope used was an inverted order to analyze percolation of the metallic phase. Only PME Olympus instrument while the scanning electron approximate measurements needed to be performed microscope employed was a Leica 440 Stereoscan ma- since the quantitative difference between percolated and chine with both secondary and backscattered electron non-percolated readings was several orders of magni- imaging capabilities. The SEM was also occasionally tude sed to image composite fracture surfaces The elastic constants of the isotropic metal-ceramic composites were determined using an ultrasonic tech- nique previously reported [15]. a similar approach was adopted for the SiC/a-Al2O3 materials; however, due to the uniaxial nature of the hot-pressing employed, the Sic whiskers and platelets had a tendency to lie perpen- dicular to the hot-pressing direction, resulting in an- isotropic composites of the transverse isotropy' type symmetry [16]. This required the incorporation of a quipment was needed but measurements had to be performed in several additional directions including at platelet alignment. The exact mathematics of this ex tended analysis will not be presented here, but the reader is referred to the derivations of Neighbours and Schacher for more insight [17] fracture toughness testing was performed by cutting 25m the sintered composites into beams, machining chevron notches into the beams, and then breaking the speci mens to complete failure on an Instron Model 1125 mechanical testing instrument equipped with a three point bending fixture. A linear variable displacement transducer(RDP-Electrosense, model RDP D5/10G8) was used to measure beam load-point displacement while a piezoelectric transducer (Kistler Instrument, model 9301A) recorded specimen load under constant displacement rate conditions. Data in the form of load/ displacement curves were collected with the use of a computerized data acquisition system. The beam di- mensions used throughout the testing were 1.9 x 1.9 8.0 mm. and the chevron notches were machined with a Well wire saw (Ahlburg Technical Equipment, model 3242) using 220 um diameter diamond impreg nated steel wire. The included angle of the chevron notch was maintained at x94o while all other sample and notch dimensions were in compliance with the Fig. 5. Optical micrographs of the microstructure of a 20 /80 vol% SiC(whisker)/a-Al,O, composite;(a) pellet face perpendicular to hot. work by Wu[18]. A much more thorough explanation ressing direction and(b) pellet face parallel to hot-pressing direction; of the toughness testing apparatus and process used is SiC=light contrast, a-Al, O,=dark contrast currently in preparation [191E.D. Rodeghiero et al. / Materials Science and Engineering A244 (1998) 11–21 15 2.4. Characterization X-ray diffraction (XRD) of both the precursor and heat treated powders as well as the sintered pellets was performed using a Scintag Pad X diffractometer with CuKa radiation. Reduced metal–ceramic powders were encapsulated in 1.0 mm diameter glass capillaries under argon to prevent reoxidation of the very small metallic particles present at this stage of processing. Polished, sintered microstructures were investigated with both optical and scanning electron microscopy (SEM). The optical microscope used was an inverted PME Olympus instrument while the scanning electron microscope employed was a Leica 440 Stereoscan ma￾chine with both secondary and backscattered electron imaging capabilities. The SEM was also occasionally used to image composite fracture surfaces. The density of all hot-pressed composites was deter￾mined by calculating the mass and volume of each pellet after cutting into a prismatic configuration. In addition, the relative or percentage density of each material was calculated by dividing the measured den￾sity by the theoretical density based on composite com￾position. The resulting number, always slightly B100%, was used as an indicator of residual porosity. In the case of the metal–ceramic composites, electrical resistivity of the hot-pressed pellets was also measured, with the use of a standard two-probe ohmmeter, in order to analyze percolation of the metallic phase. Only approximate measurements needed to be performed since the quantitative difference between percolated and non-percolated readings was several orders of magni￾tude. The elastic constants of the isotropic metal–ceramic composites were determined using an ultrasonic tech￾nique previously reported [15]. A similar approach was adopted for the SiC/a-Al2O3 materials; however, due to the uniaxial nature of the hot-pressing employed, the SiC whiskers and platelets had a tendency to lie perpen￾dicular to the hot-pressing direction, resulting in an￾isotropic composites of the ‘transverse isotropy’ type symmetry [16]. This required the incorporation of a much more complex acoustic analysis. (No additional equipment was needed but measurements had to be performed in several additional directions including at least one direction oblique to the plane of whisker/ platelet alignment.) The exact mathematics of this ex￾tended analysis will not be presented here, but the reader is referred to the derivations of Neighbours and Schacher for more insight [17]. Fracture toughness testing was performed by cutting the sintered composites into beams, machining chevron notches into the beams, and then breaking the speci￾mens to complete failure on an Instron Model 1125 mechanical testing instrument equipped with a three￾point bending fixture. A linear variable displacement transducer (RDP-Electrosense, model RDP D5/10G8) was used to measure beam load-point displacement while a piezoelectric transducer (Kistler Instrument, model 9301A) recorded specimen load under constant displacement rate conditions. Data in the form of load/ displacement curves were collected with the use of a computerized data acquisition system. The beam di￾mensions used throughout the testing were 1.9×1.9× 8.0 mm3 , and the chevron notches were machined with a Well® wire saw (Ahlburg Technical Equipment, model 3242) using 220 mm diameter diamond impreg￾nated steel wire. The included angle of the chevron notch was maintained at :94° while all other sample and notch dimensions were in compliance with the work by Wu [18]. A much more thorough explanation of the toughness testing apparatus and process used is currently in preparation [19]. Fig. 5. Optical micrographs of the microstructure of a 20/80 vol.% SiC(whisker)/a-Al2O3 composite; (a) pellet face perpendicular to hot￾pressing direction and (b) pellet face parallel to hot-pressing direction; (SiC=light contrast, a-Al2O3=dark contrast)