Journal of the American Ceramic Sociery Kim and Kriven Vol. 87. No. 5 mm Fig. 3. Optical micrographs of green pellets: (a)25 filament green pellet, (b) 150 filament green pellet. constant green interphase thickness of 0.073 mm, 10, 30, and 50 The microstructures of the monofilament rod. the multifila vol% graphite powder was added to the AlPO4 interphase to make ment rod, and the green pellet were examined by optical a more porous and hence weaker interphase layer after sintering. microscopy(Model SMZ-2T, Nikon, Tokyo, Japan), Scanning To make three -layer monofilament rods. the al electron microscopy(Model S-4700, Hitachi, Osaka, Japai quence for the first extrusion was inner mullite rod-AIPO was used to analyze the microstructures of sintered and me- nterphase laver-outer mullite laver: 93 two-laver or three-layer chanically tested, fibrous monolithic composites. Flexural onofilament rods were arranged into a cylindrical mold and then strengths were measured with a screw-driven machine(Model second extruded into a die with an orifice of 2 mm diameter, to 4502, Instron Corp, Canton, MA)in a three-point bend testing numbers of two-layer and three-layer monofilament rods were calculating the area under the lono ample was determined by ake two-layer and three-layer multifilament rods. The same The work of fracture for each sample was determined by randomly mixed nged into the cylindrical mold, and re- The strength and work of fracture data for each composite were extruded into a die of 2.0 mm orifice diameter, to make mixed 50% determined after testing three to five samples. The supportin two-layer: 50% three-layer multifilament rods. The aim of the span was 30 mm, the crosshead speed was 0. I mm/min, and the mixed multifilament rods was to interdisperse, on a small, micro- sample size was 3 mm(h)x 4 mm()X 40 mm(L) structural scale, regions of high strength with regions of high toughness The multifilament rods were cut into 47 mm lengths: 55 II. Results and Discussion ultifilament rods were ged into a molding die and warm ressed at 150%C and 34.5 MPa. The binder was then removed To change the number of filaments in a given area of fibrous from the pressed pellet. The heat treatment cycle for the binder monolithic composite by changing the number of filaments in a removal from the composites without graphite was as follows: heat two-layer multifilament rod, the first extrusions were made from25°to250° C at a ramp rate ofo.05°C/min, heat from250°to through dies having 1.5., and 4.0 mm diameter orifices. figure 450%C at a ramp rate of 0. 1 C/min, heat from 450 to 650 C at a I shows the monofilament rods which were passed through the 4.0 ramp rate of 0.3C/min, maintain at 650C for 2 h, and subse mm(Fig. 1(a))and 1.5 mm(Fig. 1(b) diameter orifices, respec- lently cool down to room temperature with a ramp rate of tively. The porous and weak, AlPOa interphase layer was well then sintered at 1600%C for 10 h. The binder removal cvcle for the monofilament rods which were first extruded through the 4.0, 2.0 composite with graphite in its interphase was the same as that of and 1.5 mm diameter orifices, respectively, were arranged into a the composite without graphite except for maintaining at 550Cfor cylindrical mold of 23 mm diameter and then extruded again 2 h. After removal of binder, the pellet was CIPed at 413.7 MPa through an orifice of 2 mm diameter to make 25, 93, and 150 d then the graphite was removed from the composites using a multifilament rods, respectively. Figure 2 comprises optical m heating cycle as follows: from 25 to 550 C heat at a ramp rate of crographs of the 25(Fig. 2(a))and 150(Fig. 2(b)multifilament 88°C/min, from 550% to80° C heat at a ramp rate of o.o8°C/min rod samples. The population densities of the 25, 93, and 150 d from800°to900° C heat at a ramp rate of o.l°c/ min and ples were 7, 27, and 43 filaments/mm maintain at 900C for 2 h. The binder- and graphite-free pellet was sintered at 1600C for 10 h Table Ill. Effects of Amount of Graphite in the Green Table Il. Effects of Green Interphase Thickness on the AlPO4 Interphase on the Strength and work of Strength and work of fracture of sintered fibrous Fracture of Sintered Fibrous Monolithic Composite Monolithic Composites Amount of graphite in the green Work of interphase(vol% Strength(MPa) fracture(kJ/m") Green interphase thickness Work of fracture Strength(MPa) 162±10 0.26±0.0 10 0.45±0.02 102±100.69± 0.06 0.58±0.05 Green interphase thickness =0.073 mmconstant green interphase thickness of 0.073 mm, 10, 30, and 50 vol% graphite powder was added to the AlPO4 interphase to make a more porous and hence weaker interphase layer after sintering. To make three-layer monofilament rods, the alignment se￾quence for the first extrusion was inner mullite rod–AlPO4 interphase layer–outer mullite layer; 93 two-layer or three-layer monofilament rods were arranged into a cylindrical mold and then second extruded into a die with an orifice of 2 mm diameter, to make two-layer and three-layer multifilament rods. The same numbers of two-layer and three-layer monofilament rods were randomly mixed, arranged into the cylindrical mold, and re￾extruded into a die of 2.0 mm orifice diameter, to make mixed 50% two-layer:50% three-layer multifilament rods. The aim of the mixed multifilament rods was to interdisperse, on a small, micro￾structural scale, regions of high strength with regions of high toughness. The multifilament rods were cut into 47 mm lengths; 55 multifilament rods were arranged into a molding die and warm pressed at 150°C and 34.5 MPa. The binder was then removed from the pressed pellet. The heat treatment cycle for the binder removal from the composites without graphite was as follows: heat from 25° to 250°C at a ramp rate of 0.05°C/min, heat from 250° to 450°C at a ramp rate of 0.1°C/min, heat from 450° to 650°C at a ramp rate of 0.3°C/min, maintain at 650°C for 2 h, and subse￾quently cool down to room temperature with a ramp rate of 0.5°C/min. The binder-free body was CIPed at 413.7 MPa, and then sintered at 1600°C for 10 h. The binder removal cycle for the composite with graphite in its interphase was the same as that of the composite without graphite except for maintaining at 550°C for 2 h. After removal of binder, the pellet was CIPed at 413.7 MPa, and then the graphite was removed from the composites using a heating cycle as follows: from 25° to 550°C heat at a ramp rate of 8.8°C/min, from 550° to 800°C heat at a ramp rate of 0.08°C/min, and from 800° to 900°C heat at a ramp rate of 0.1°C/min and maintain at 900°C for 2 h. The binder- and graphite-free pellet was sintered at 1600°C for 10 h. The microstructures of the monofilament rod, the multifila￾ment rod, and the green pellet were examined by optical microscopy (Model SMZ-2T, Nikon, Tokyo, Japan). Scanning electron microscopy (Model S-4700, Hitachi, Osaka, Japan) was used to analyze the microstructures of sintered and me￾chanically tested, fibrous monolithic composites. Flexural strengths were measured with a screw-driven machine (Model 4502, Instron Corp., Canton, MA) in a three-point bend testing. The work of fracture for each sample was determined by calculating the area under the load versus displacement curve. The strength and work of fracture data for each composite were determined after testing three to five samples. The supporting span was 30 mm, the crosshead speed was 0.1 mm/min, and the sample size was 3 mm (H) 4 mm (W) 40 mm (L). III. Results and Discussion To change the number of filaments in a given area of fibrous monolithic composite by changing the number of filaments in a two-layer multifilament rod, the first extrusions were made through dies having 1.5, 2.0, and 4.0 mm diameter orifices. Figure 1 shows the monofilament rods which were passed through the 4.0 mm (Fig. 1(a)) and 1.5 mm (Fig. 1(b)) diameter orifices, respec￾tively. The porous and weak, AlPO4 interphase layer was well coated around the mullite center rod. The 25, 93, and 150 monofilament rods which were first extruded through the 4.0, 2.0, and 1.5 mm diameter orifices, respectively, were arranged into a cylindrical mold of 23 mm diameter and then extruded again through an orifice of 2 mm diameter to make 25, 93, and 150 multifilament rods, respectively. Figure 2 comprises optical mi￾crographs of the 25 (Fig. 2(a)) and 150 (Fig. 2(b)) multifilament rod samples. The population densities of the 25, 93, and 150 multifilament rod samples were 7, 27, and 43 filaments/mm2 , Fig. 3. Optical micrographs of green pellets: (a) 25 filament green pellet, (b) 150 filament green pellet. Table II. Effects of Green Interphase Thickness on the Strength and Work of Fracture of Sintered Fibrous Monolithic Composites Green interphase thickness (mm) Strength (MPa) Work of fracture (kJ/m2 ) 0.33 76  5 0.45  0.02 0.19 41  2 0.49  0.05 0.073 162  10 0.26  0.03 Table III. Effects of Amount of Graphite in the Green AlPO4 Interphase on the Strength and Work of Fracture of Sintered Fibrous Monolithic Composites† Amount of graphite in the green interphase (vol%) Strength (MPa) Work of fracture (kJ/m2 ) 0 162  10 0.26  0.03 10 109  6 0.61  0.02 30 102  10 0.69  0.06 50 77  5 0.58  0.05 † Green interphase thickness 0.073 mm. 796 Journal of the American Ceramic Society—Kim and Kriven Vol. 87, No. 5