mullite (3al2o3·2sio2)–aluminum phosphate (alpo4), oxide, fibrous monolithic composites

Mullite (3Al2O3�2SiO2)–Aluminum Phosphate (AlPO4), Oxide,Fibrous Monolithic Composites

Dong-Kyu Kim and Waltraud M. Kriven**Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801

Mullite–AlPO4 fibrous monolithic composites were fabricatedby a co-extrusion technique using ethylene vinyl acetate (EVA)as a binder. Processing routes such as mixing formulation,extrusion sequence, binder removal cycle, pressing, and sin-tering procedures are described. An effort to make toughercomposites was conducted by modifying the microstructures ofthe composites. Different kinds of monolithic composites werefabricated by changing the number of filaments, and thecomposition and thickness of interphase layers, and theirmicrostructural and mechanical properties were character-ized. To make the interphase more porous and to facilitatedebonding and fiber pullout in the composite, graphite wasadded as a fugitive “space filler” into the interphase materialand then removed. A fibrous monolithic composite with asintered interphase thickness of 5–10 �m and an interphasecomposition of 50 vol% graphite and 50 vol% AlPO4 had athree-point bend strength and a work of fracture of 129 � 2MPa and 0.86 � 0.05 kJ/m2, respectively. This corresponded to42% of the strength but 162% of the work of fracture whencompared with the values for a single-phase mullite. Two-layer, mixed 50% two-layer:50% three-layer, and three-layerfibrous monoliths were fabricated and their microstructuraland mechanical properties were studied. The difference in thesintering behaviors of the two-layer and three-layer compos-ites is described.

I. Introduction

SEVERAL different approaches have been reported for tougheningmechanisms of ceramics. These include transformation tough-

ening,1–3 crack deflection toughening,4–6 microcrack toughen-ing,7–10 and fiber-reinforced, ceramic-matrix composites(CMCs).11–15

Alternative tough ceramics to fiber-reinforced ceramic compos-ites can be simple, powder-processed, laminated composites, orfibrous monolithic composites. Following the research of Clegg etal.,16 different kinds of laminated ceramic composites were fabri-cated by tape casting,17–19 slip casting,20 electrophoretic deposi-tion,21 die pressing,22 rolling,16,23 co-extrusion,24 and sequentialcentrifuging.25 “Functionally graded,” laminated ceramics can bemade by controlling the composition, layer thickness, and stackingsequence, etc.17 As another type of ceramic material which alsoshows “graceful failure,” the fibrous monolithic composite wasdeveloped.19,26,27

Fibrous monoliths are sintered (or hot-pressed) monolithicceramics with a distinct fibrous texture, consisting of cells of a

primary phase, separated by cell boundaries of a tailored, second-ary phase. The cells are not ceramic fibers, but rather polycrystal-line ceramic domains. The cell boundary phases can be weakinterphases, microcracked zones, ductile-phase filaments, or inter-phases with different physical properties.28 Coblenz26 first intro-duced the concept of fibrous monolithic composites. Some fibrousmonolithic systems that have already been studied are the follow-ing:27–37 SiC/BN, SiC/graphite, Si3N4/BN, Al2O3/graphite, Al2O3/Al2TiO5, Al2O3/Al2O3–ZrO2, ZrB2/BN, HfB2/BN, TiB2/BN,Al2O3/Ni, Al2O3/Ni-20Cr, and Y-ZrO2/Ni, TiO2/MgSiO3, andAl2O3/Al2O3 platelets.

Mullite is an attractive, high-temperature, structural materialdue to its excellent strength and creep resistance at high temper-ature, good thermal stability, low thermal conductivity, as well asits chemical inertness.38 Aluminum phosphate (AlPO4) is ther-mally stable (mp �2000°C39), chemically inert, electrically neu-tral, and highly covalent. Because of these properties, AlPO4 is anattractive candidate material for high-temperature applications.40

Mullite and aluminum phosphate are chemically compatible witheach other after sintering at 1600°C for 10 h.41

In this study, oxide fibrous monolithic composites were fabri-cated by a co-extrusion technique, using mullite as a matrix andaluminum phosphate as an interphase material.

A study to optimize the microstructure of the two-layer, fibrousmonolithic composites was conducted by controlling the compo-sition and thickness of the AlPO4 interphase, as well as the numberof filaments in a given area. Fibrous monolithic composites withdifferent microstructural architectures of two layers, mixed 50%two layers:50% three layers, and three layers were fabricated, andtheir microstructures and room-temperature mechanical propertieswere investigated.

II. Experimental Procedure

Mullite powder (Kyoritsu, KM 101) was the matrix material.The AlPO4 interphase powder was synthesized by the organic,steric entrapment method,42–46 using aluminum nitrate nonahy-drate (Al(NO3)3�9H2O, Aldrich Chemical, Inc., 98%) and ammo-nium phosphate diabasic ((NH4)2�HPO4, Fisher Scientific) as theAl and P sources, respectively. A mixture of Elvax 210, 220, and250 ethylene vinyl acetate (EVA) copolymers (Dupont, Wilming-ton, DE) was used as a binder phase. The molecular weightincreased from Elvax 210, 220, to 250 with values of 44 010,

R. J. Kerans—contributing editor

Manuscript No. 186580. Received October 24, 2002; approved December 29,2003.

**Fellow, American Ceramic Society.

Table I. Ceramic Powders and Polymer MixingFormulations for Extrusion†

Powder

Binder (Elvax)‡

Plasticizer (DP)§ Lubricant (SA)¶210 220 250

MulliteInner 52 2.4 4.8 40.8 – –Outer 50 34 4 2 7.5 2.5

AlPO4 40 9.6 24 14.4 9 3†All ingredients are in volume percent. ‡Elvax � ethylene vinyl acetate copolymer

(Dupont). §DP � dioctyl phthalate (99%, Aldrich). ¶SA � stearic acid (95%,Aldrich).

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58 433, and 112 000 g, respectively. The reason for using amixture of three different EVA grades as a binder was to facilitatethe binder removal process. Dioctyl phthalate (Aldrich, 99%) andstearic acid (Aldrich, 95%) were used as a plasticizer and alubricant, respectively.

Ceramic powder, binder, plasticizer, and lubricant were mixedusing a computer-controlled, high-shear mixer (Model 2100, C. W.Brabender, NJ). The mixtures were made following the formula-tions in Table I. In the case of mullite, to facilitate the extrusion ofthe outer layer so as to have lower viscosity, a lower amount ofsolids content of 50 vol% and a higher amount of lower-molecular-weight EVA binder were used; 40 vol% of powder loading wasused for mixing the AlPO4 interphase. The Brabender mixingtemperature for the mullite inner rod and AlPO4 interphase layerwas 150°C. The mixing temperature for the mullite outer layer was120°C because of its lower viscosity.

The inner mullite rods were extruded into cylindrical shapeshaving diameters of 11, 13.5, 16, and 20 mm, depending ondifferent subsequent configurations. The extrusion rate was 50mm/min. The Brabender-mixed, AlPO4, interphase formulations

and outer mullite layer formulations were warm pressed into halftubular shapes, with the aid of a thickness-controllable mold,47,48

at a temperature of 150°C and a pressure of �34.5 MPa. Thecenter rod and AlPO4 interphase, half tubular shapes were ar-ranged into a cylindrical mold which was heated to 90°C, having23 mm diameter and 150 mm length, and extruded into a die of 1.5,2.0, or 4.0 mm diameter, depending on subsequent applications.This is called the “first extrusion” and the extrudate is referred toas a two-layer monofilament rod. Because the “warm” extrusionwas made at 90°C, the “seamless” half tubular shapes were bondedaround the center rod after the first extrusion; 25, 93, or 150two-layer monofilament rods were arranged into a cylindricalmold which was heated to 90°C and extruded again into a die of 2mm diameter. This is called the “second extrusion” and theextrudate is called a two-layer multifilament rod. To study theeffects of decreasing the thickness of the porous and weak AlPO4

interphase layer on the strength and work of fracture of fibrousmonolithic composites, monofilament rods with green interphasethicknesses of 0.33, 0.19, and 0.073 mm were extruded and usedto make fibrous monolithic composites. For the composites with a

Fig. 1. Optical micrographs of mullite–AlPO4 monofilament rods (M: matrix (mullite); I: interphase (AlPO4)): (a) 25 monofilament rod which was firstextruded through a die with an orifice diameter of 4.0 mm; (b) 150 monofilament rod which was first extruded through a die with an orifice diameter of 1.5mm.

Fig. 2. Optical micrographs of the mullite–AlPO4 multifilament rod (M: matrix (mullite); I: interphase (AlPO4)): (a) 25 multifilament rod, (b) 150multifilament rod.

May 2004 Mullite (3Al2O3�2SiO2)–Aluminum Phosphate (AlPO4), Oxide, Fibrous Monolithic Composites 795

constant green interphase thickness of 0.073 mm, 10, 30, and 50vol% graphite powder was added to the AlPO4 interphase to makea 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-layermonofilament rods were arranged into a cylindrical mold and thensecond extruded into a die with an orifice of 2 mm diameter, tomake two-layer and three-layer multifilament rods. The samenumbers of two-layer and three-layer monofilament rods wererandomly 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 themixed multifilament rods was to interdisperse, on a small, micro-structural scale, regions of high strength with regions of hightoughness.

The multifilament rods were cut into 47 mm lengths; 55multifilament rods were arranged into a molding die and warmpressed at 150°C and 34.5 MPa. The binder was then removedfrom the pressed pellet. The heat treatment cycle for the binderremoval from the composites without graphite was as follows: heatfrom 25° to 250°C at a ramp rate of 0.05°C/min, heat from 250° to450°C at a ramp rate of 0.1°C/min, heat from 450° to 650°C at aramp 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 of0.5°C/min. The binder-free body was CIPed at 413.7 MPa, andthen sintered at 1600°C for 10 h. The binder removal cycle for thecomposite with graphite in its interphase was the same as that ofthe composite without graphite except for maintaining at 550°C for2 h. After removal of binder, the pellet was CIPed at 413.7 MPa,and then the graphite was removed from the composites using aheating cycle as follows: from 25° to 550°C heat at a ramp rate of8.8°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 andmaintain at 900°C for 2 h. The binder- and graphite-free pellet wassintered at 1600°C for 10 h.

The microstructures of the monofilament rod, the multifila-ment rod, and the green pellet were examined by opticalmicroscopy (Model SMZ-2T, Nikon, Tokyo, Japan). Scanningelectron microscopy (Model S-4700, Hitachi, Osaka, Japan)was used to analyze the microstructures of sintered and me-chanically tested, fibrous monolithic composites. Flexuralstrengths were measured with a screw-driven machine (Model4502, Instron Corp., Canton, MA) in a three-point bend testing.The work of fracture for each sample was determined bycalculating the area under the load versus displacement curve.The strength and work of fracture data for each composite weredetermined after testing three to five samples. The supportingspan was 30 mm, the crosshead speed was 0.1 mm/min, and thesample 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 fibrousmonolithic composite by changing the number of filaments in atwo-layer multifilament rod, the first extrusions were madethrough dies having 1.5, 2.0, and 4.0 mm diameter orifices. Figure1 shows the monofilament rods which were passed through the 4.0mm (Fig. 1(a)) and 1.5 mm (Fig. 1(b)) diameter orifices, respec-tively. The porous and weak, AlPO4 interphase layer was wellcoated around the mullite center rod. The 25, 93, and 150monofilament rods which were first extruded through the 4.0, 2.0,and 1.5 mm diameter orifices, respectively, were arranged into acylindrical mold of 23 mm diameter and then extruded againthrough an orifice of 2 mm diameter to make 25, 93, and 150multifilament rods, respectively. Figure 2 comprises optical mi-crographs of the 25 (Fig. 2(a)) and 150 (Fig. 2(b)) multifilamentrod samples. The population densities of the 25, 93, and 150multifilament 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 theStrength 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 � 0.020.19 41 � 2 0.49 � 0.050.073 162 � 10 0.26 � 0.03

Table III. Effects of Amount of Graphite in the GreenAlPO4 Interphase on the Strength and Work of

Fracture of Sintered Fibrous Monolithic Composites†

Amount of graphite in the greeninterphase (vol%) Strength (MPa)

Work offracture (kJ/m2)

0 162 � 10 0.26 � 0.0310 109 � 6 0.61 � 0.0230 102 � 10 0.69 � 0.0650 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

respectively. The green, bend bar samples were made by stacking55 multifilament rods into a rectangular die and pressing at 34.5MPa. The optical micrographs of the green rectangular samples areshown in Fig. 3. It was estimated that 289 and 1774 filaments werealigned in the 7.5 mm � 5.5 mm area for the 25 and 150multifilament green samples, respectively. The 25, 93, and 150multifilament sintered bars were tested in three-point bending.They had bend strengths of 6 � 2, 76 � 5, and 4 � 1 MPa,respectively, with works of fracture of 0.10 � 0.02, 0.45 � 0.02,and 0.03 � 0.01 kJ/m2, respectively.

To make fibrous monolithic composites having different thick-nesses of AlPO4 interphase layer, monofilament rods with greeninterphase thicknesses of 0.33, 0.19, and 0.073 mm were extruded.Table II summarizes the effects of interphase thickness on thethree-point bend strength and the work of fracture of the sinteredcomposites. Even though the fibrous monolithic composite with anAlPO4 interphase thickness of 0.073 mm had the highest strengthof 162 � 10 MPa, it showed brittle fracture and had the lowestwork of fracture of 0.26 � 0.03 kJ/mm2. To increase the work offracture of this composite by making a more porous interphase andfacilitating debonding and fiber pullout, 10, 30, and 50 vol% ofgraphite powder were added to the green interphase. Table IIIrepresents the results of mechanical testing of these composites.The strengths of the sintered composites decreased with increasingamounts of graphite in the green interphase. However the works offracture of the composites were increased after adding graphitepowder to the interphase. The fibrous monolithic composite with30 vol% graphite in the interphase had the highest work of fractureof 0.69 � 0.06 kJ/m2. The thin interphase was fully densified bydiffusion of matrix after a relatively short period. In the case of thethick interphase, there was insufficient matrix powder diffusion

into the interphase to cause full densification. Intermediate thick-ness has a suitable interphase strength for debonding at roomtemperature. However, the dependence of the matrix diffusion intothe interphase on interphase thickness can be changed, dependingon the application temperature of the composite.

Fig. 4. SEM micrographs of the sintered, mullite–AlPO4 fibrous monolithic composite. (The thickness of the interphase was 5–10 �m after sintering. Thecomposition of the green interphase was 50 vol% graphite:50 vol% AlPO4, and the graphite was removed after heat treatment.)

Fig. 5. Load–displacement curves for the three different kinds ofmullite–AlPO4 fibrous monolithic composites fabricated.


Further efforts to make tougher fibrous monolithic compositesincluded making a composite with a thinner interphase and a greeninterphase composition of 50 vol% graphite and 50 vol% AlPO4.The microstructures of such composites after sintering at 1600°Cfor 10 h are seen in Fig. 4. The interphase thickness of thatcomposite was 5–10 �m after sintering. The microstructures werevery uniform and homogeneous. The strength and work of fractureof that composite were 129 � 2 MPa and 0.86 � 0.05 kJ/m2,respectively. To compare values, a single mullite pellet was made,sintered at 1600°C for 10 h, and then tested in three-point bending.The strength and work of fracture of single-phase mullite were308 � 11 MPa and 0.53 � 0.01 kJ/m2, respectively. Thus,compared with mullite, the fibrous monolith just described had42% of single-phase mullite strength, and 162% of the work offracture of pure mullite.

To increase the overall strength of the composite, 10 and 30vol% mullite powders were added to the aluminum phosphateinterphase. The green interphase thickness of the composite thenwas 0.33 mm. Figure 5 compares the load versus displacementcurves for the three different kinds of composites. Composites withpure AlPO4 and 10 vol% mullite added to the interphase compo-sition showed apparent nonbrittle fracture, with bending strengths

of 76 � 5 and 83 � 15 MPa, respectively, and works of fractureof 0.45 � 0.02 and 0.46 � 0.03 kJ/m2, respectively. Thecomposite with 30-vol%-mullite-added interphase compositionshowed brittle fracture and had a bending strength of 106 � 5 MPaand a work of fracture of 0.17 � 0.03 kJ/m2.

The microstructures of the composites consisted of two-layer,mixed 50% two-layer:50% three-layer, and three-layer textures.Figure 6 presents optical micrographs of the two-layer (Fig. 6(a))and three-layer, first-extruded, monofilament rods (Fig. 6(b)).Figure 7 presents optical micrographs of three-layer (Fig. 7(a)) andmixed 50% two-layer:50% three-layer multifilament rods (Fig.7(b)) resulting from the second extrusion. The three-layer multi-filament rod contained about 93 three-layer textures in a circle of2.1 mm diameter. The 50 vol% two-layer and 50 vol% three-layermonofilament rods were randomly mixed and extruded a secondtime to make a mixed 50% two-layer:50% three-layer multifila-ment rod. The mixed 50% two-layer:50% three-layer multifila-ment rod possessed an interlocking texture of the mullite matrixand AlPO4 interphase. Figure 8 displays optical micrographs ofthree-layer (Fig. 8(a)) and mixed 50% two-layer:50% three-layer(Fig. 8(b)) green bodies.

Fig. 6. Optical micrographs of the monofilament rods composed of mullite and AlPO4 (M: matrix (mullite); I: interphase (AlPO4)): (a) two-layer structure,(b) three-layer structure.

Fig. 7. Optical microgrphs of multifilament rods: (a) three-layer structure, (b) mixed 50% two-layer:50% three-layer structure.


Fig. 8. Optical micrographs of the green pellets: (a) three-layer structure, (b) mixed 50% two-layer:50% three-layer structure.

Fig. 9. SEM micrographs of the sintered, two-layer, mullite–AlPO4 fibrous monolithic composite (M: matrix (mullite); I: interphase (AlPO4); arrowsindicate the circumferential cracks formed around mullite center rods).

Fig. 10. SEM micrographs of the sintered, three-layer, mullite–AlPO4 fibrous monolithic composite (M: matrix (mullite); I: interphase (AlPO4)).


The SEM micrographs of the sintered two-layer composite arepresented in Fig. 9. The dense mullite matrix, as well as the porousand weak AlPO4 interphase, sintered to a uniform microstructure.The higher-magnification SEM micrograph shows the microstruc-ture of the porous AlPO4 interphase layer. The arrows in Fig. 9(a)indicate microcracks formed along the mullite–AlPO4 interfacesafter sintering. Figures 10(a) and (b) are SEM micrographs of thethree-layer mullite–AlPO4 fibrous monolithic composite. Thethree-layer structure of the mullite inner rod–AlPO4 interphaselayer–mullite outer layer can clearly be seen. There is no formationof interface microcracks in the three-layer composite.

The difference in the sintering behaviors of the two-layer andthree-layer fibrous monolithic composites is schematically ex-plained in Fig. 11. In the two-layer fibrous monolithic composite(Fig. 11(a)), the isolated, highly sinterable, inner mullite matrixrods underwent sintering shrinkage, but the interconnected, poorlysinterable interphase layers did not densify significantly. Becauseof this sinterability difference, circumferential shrinkage crackswere formed around the mullite inner rods, as seen in Fig. 9(a), andthe two-layer composite had little sintering shrinkage. In the caseof the three-layer structure (Fig. 11(b)), the isolated, mullite innerrods shrunk, but the AlPO4 interphase layers did not shrink asmuch. However, the interconnected, mullite outer layer shrunkconsiderably and produced compressive stresses in the AlPO4

interphase layers. Thus, the overall sample size was reducedduring sintering and the circumferential cracks did not formaround the mullite inner rods as can be seen in Fig. 10. TheSEM micrograph of the sintered, mixed 50% two-layer:50%three-layer fibrous monolithic composite is presented in Fig.12. The mullite matrix and AlPO4 interphase layer formed aninterlocking microstructure.

The three-point bending strengths for the two-layer, mixed 50%two-layer:50% three-layer, and three-layer structures were mea-sured as 76 � 5, 123 � 12, and 176 � 7 MPa, respectively, havingcorresponding works of fracture of 0.45 � 0.02, 0.30 � 0.05, and0.25 � 0.01 kJ/m2, respectively. Figure 13 shows load versusdisplacement curves corresponding to the two-layer, mixed 50%two-layer:50% three-layer, and three-layer fibrous monolithic

composites. These materials showed apparent nonbrittle, interme-diate, and brittle fracture behavior, respectively. The side andcross- sectional views of a fractured two-layer fibrous monolithiccomposite are seen in Fig. 14. The side view shows that the crackfollowed a tortuous path due to deflection. The cross-sectionalview confirms that the crack propagates along the weak AlPO4

interphase. The SEM micrographs of Fig. 15 display the fracturesurfaces of the two-layer fibrous monolithic composite, and giveevidence for somewhat extensive fiber pullout and hence a roughfracture surface. This kind of extensive fiber pullout, apparentnonbrittle fracture, and low strength behavior in the two-layerfibrous monolithic composite is expected from the circumferentialshrinkage cracks around the mullite center rods as depicted in Fig.9(a) and as discussed using Fig. 11(a). Crack propagation in thethree-layer fibrous monolithic composite is seen in Fig. 16. Thecrack propagates not only along the weak AlPO4 interphase, butalso across the strong mullite matrix phase. Fracture surfaces of thethree-layer fibrous monolithic composites are seen in Fig. 17. Thethree-layer structure was maintained throughout the composite, butthere was very little fiber pullout, at room temperature, leading toa relatively smooth fracture surface. These poor fiber pullout,brittle fracture, and higher strength behaviors of the three-layerfibrous monolithic composite could be predicted from the com-pressional stresses acting on the interphase layer during sintering,as can be seen from Fig. 10 and as illustrated in Fig. 11(b).

Fig. 11. Schematic of the sintering behavior of the two-layer andthree-layer fibrous monolithic composite.

Fig. 12. SEM micrograph of the sintered, mixed 50% two-layer:50%three-layer, mullite–AlPO4 fibrous monolithic composite.

Fig. 13. Load–displacement curves for the two-layer, mixed 50% two-layer:50% three-layer, and three-layer mullite–AlPO4 fibrous monolithiccomposite.


Fracture surfaces of the mixed 50% two-layer:50% three-layerfibrous monolithic composites are presented in Fig. 18. The shapeof pulled-out fibers was irregular, and the fracture surface wasrough.

While the room-temperature mechanical properties of the three-layer, oxide, fibrous monolithic microstructure produced were notsignificantly tough, they may still exhibit significant pullout and

toughening at elevated temperatures. Further work needs to bedone on such composites to explore their high-temperature me-chanical behavior.

IV. Summary

Mullite–AlPO4, oxide fibrous monolithic composites havingdifferent microstructures, interphase thicknesses, and composi-tions were made by a co-extrusion technique. Of the sinteredmullite–AlPO4 fibrous monolithic composites made with 25, 93,and 150 multifilament rods, the 93 multifilament compositeexhibited apparent nonbrittle fracture and had the highest three-point bending strength of 76 � 5 MPa. Of the sintered compositeshaving different interphase thicknesses, the composite with 0.073mm green interphase thickness had the highest three-point bendstrength of 162 � 10 MPa, but underwent brittle fracture, havinga work of fracture of 0.26 � 0.03 kJ/m2. By adding 30 vol%graphite to the fibrous monolithic composite with a green inter-phase thickness of 0.073 mm, the work of fracture increased to0.69 � 0.06 kJ/m2. A fibrous monolithic composite with sinteredinterphase thickness of 5–10 �m and interphase compositioncontaining 50 vol% graphite showed pseudoductile fracture behav-ior, and had a bend strength of 129 � 2 MPa and a work of fractureof 0.86 � 0.05 kJ/m2. The composites with pure AlPO4 and 10-and 30-vol%-mullite-added interphase compositions were fabri-cated. As the amount of mullite in the interphase increased, the

Fig. 14. SEM micrographs of the fractured two-layer fibrous monolithic composite (M: matrix (mullite); I: interphase (AlPO4)): (a) side view, (b)cross-sectional view.

Fig. 15. SEM micrographs of the fracture surface of the two-layer fibrous monolithic composite.

Fig. 16. SEM micrograph showing crack propagation in the three-layercomposite.


three-point bending strength of the composite was increased.However, the work of fracture of the composites did not show anyclear dependence on the composition of the composites.

Two-layer, mixed 50% two-layer:50% three-layer, and three-layer mullite–AlPO4 fibrous monolithic composites were fabri-cated. They exhibited apparent nonbrittle, intermediate, and brittlefracture behaviors, respectively. The two-layer composite had thelowest three-point bending strength of 76 � 5 MPa and the highestwork of fracture of 0.45 � 0.02 kJ/m2. The two-layer mullite–AlPO4 fibrous monolithic composite showed evidence of exten-sive pullout and had a corresponding rough fracture surface. Thereasons for this behavior were attributed to the formation ofcircumferential debonding cracks around the inner mullite rods,resulting from differential sintering shrinkages between the porousAlPO4 interphase and the isolated inner mullite rods. The low levelof pullout and brittle fracture of the three-layer fibrous monolithiccomposite were attributed to the formation of compressive stressesin the AlPO4 interphase layer, resulting from the high sinteringshrinkage of the interconnected outer mullite layer. The fracturesurface of the mixed 50% two-layer:50% three-layer fibrousmonolithic composite showed irregularly shaped pullout.

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Fig. 17. SEM microgrphs of the fracture surface of the three-layer fibrous monolithic composite.

Fig. 18. SEM micrographs of the fracture surface of the mixed 50% two-layer:50% three-layer fibrous monolithic composite.


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mullite (3al2o3·2sio2)–aluminum phosphate (alpo4), oxide, fibrous monolithic composites

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