JAm. Ceran. Scc,871794-80302004) ournal Mullite(3Al2O3 2Sio2)-Aluminum Phosphate(AlPO4), Oxide, Fibrous monolithic Composites Dong-Kyu Kim and waltraud m. Riven Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 Mullite-AlPO, fibrous monolithic composites were fabricated primary phase, separated by cell boundaries of a tailored, second- by a co-extrusion technique using ethylene vinyl acetate(EVA) ary phase. The cells are not ceramic fibers, but rather polycrystal as a binder. Processing routes such as mixing formulation line ceramic domains. The cell boundary phases can be weak extrusion sequence, binder removal cycle, pressing, and sin- interphases, microcracked zones, ductile-phase filaments, or inter- tering procedures are described. An effort to make tougher phases with different physical properties. 8 Coblenz first intro- composites was conducted by modifying the microstructures of duced the concept of fibrous monolithic composites. Some fibre the composites. Different kinds of monolithic composites were mono 37 Sic/BN, SiC/graphite, Si N,/BN, AL2O/graphite, Al 0y/ monolithic systems that have already been studied are the follow- fabricated by changing the number of filaments, and the composition and thickness of interphase layers, and their Al,TiOs, Al,O3/ Al2O3-ZrO2, ZrB,/BN, HfB,/BN, TiB,/BN, microstructural and mechanical properties were character- AL,O3/Ni, AL,O3/Ni-20Cr, and Y-ZrO2/Ni, TiO,/MgSiO3, and ized. To make the interphase more porous and to facilitate AlO3/Al2O3 platelets. debonding and fiber pullout in the composite, graphite was Mullite is an attractive, high-temperature, structural material added as a fugitive"space filler"into the interphase material due to its excellent strength and creep resistance at high and then removed. A fibrous monolithic composite with a ature, good thermal stability, low thermal conductivity, as sintered interphase thickness of 5-10 um and an interphase its chemical inertness. Aluminum phosphate(AlPO4)is composition of 50 vol% graphite and 50 vol% AIPO, had a mally stable(mp -2000oC ), chemically inert, electrically neu- three-point bend strength and a work of fracture of 129+2 tral, and highly covalent. Because of these properties, AlPO4 is an MPa and 0.86+0.05 kJ/m, respectively. This corresponded to attractive candidate material for high-temperature applications. o 42% of the strength but 162% of the work of fracture when Mullite and aluminum phosphate are chemically compatible with compared with the values for a single-phase mullite. Two- each other after sintering at 1600%C for 10 h 4 layer, mixed 50% two-layer: 50% three-layer, and three-layer In this study, oxide fibrous monolithic composites were fabri fibrous monoliths were fabricated and their microstructural cated by a co-extrusion technique, using mullite as a matrix and and mechanical properties were studied. The difference in the minum phosphate as an interphase materi sintering behaviors of the two-layer and three-layer compos- o,. A study to optimize the microstructure of the two-layer, fibrous onolithic composites was conducted by controlling the compo- sition and thickness of the AlPO4 interphase, as well as the number of filaments in a given area. Fibrous monolithic composites with different microstructural architectures of two layers, mixed 50% L. Introduction two layers 50% three layers, and three layers were fabricated, and everal diff aches have been reported for toughening their microstructures and room-temperature mechanical properties chanisms es. These include transformation tough were investigated toughening, microcrack toughen- and fiber-reinforced, ceramic-matrix composites IL. Experimental Procedure (CMCs ites can be simple, powder-processed, laminated composites, or The AlPOa interphase powder was synthesized by the organic al, different kinds of laminated ceramic composites were fabri- drate(Al(NO3)3. 9H,O, Aldrich Chemical, Inc, 98%)and ammo- cated by tape casting, 7-l9 ing, electrophoretic depo nium phosphate diabasic((NHA), HPO,, Fisher Scientific)as the tion,die pressing,rolling, -co-extrusion, and sequential Al and P sources, respectively. A mixture of Elvax 210, 220, and entrifuging. "Functionally graded, laminated ceramics can be 250 ethylene vinyl acetate(EVA)copolymers(Dupont, Wilming made by controlling the composition, layer thickness, and stacking ton, DE) was used as a binder phase. The molecular weight equence, etc. As another type of ceramic material which also increased from Elvax 210. 220. to 250 with values of 44 010 shows"graceful failure, the fibrous monolithic composite was developed.9,26,27 Fibrous monoliths are sintered (or hot-pressed) lithic Table l. Ceramic Powders and polymer mixing ceramics with a distinct fibrous texture consisting Formulations for extrusion' Binder(Elvax Powder 210 220 250 Plasticizer(DP) Lubricant(SA) R. J. Kerans--contributing editor Mullite ner52244.840.8 Outer 50 34 4 7.5 AlPO4 9.624 script No. 186580. Received October 24, 2002, approved December 29 percent. Elvax ethylene Fellow, American Ceramic Society.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 fabricated by 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 tougher composites was conducted by modifying the microstructures of the composites. Different kinds of monolithic composites were fabricated by changing the number of filaments, and the composition and thickness of interphase layers, and their microstructural and mechanical properties were character￾ized. To make the interphase more porous and to facilitate debonding and fiber pullout in the composite, graphite was added as a fugitive “space filler” into the interphase material and then removed. A fibrous monolithic composite with a sintered interphase thickness of 5–10
m and an interphase composition of 50 vol% graphite and 50 vol% AlPO4 had a three-point bend strength and a work of fracture of 129 2 MPa and 0.86 0.05 kJ/m2 , respectively. This corresponded to 42% of the strength but 162% of the work of fracture when compared with the values for a single-phase mullite. Two￾layer, mixed 50% two-layer:50% three-layer, and three-layer fibrous monoliths were fabricated and their microstructural and mechanical properties were studied. The difference in the sintering behaviors of the two-layer and three-layer compos￾ites is described. I. Introduction SEVERAL different approaches have been reported for toughening mechanisms 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, or fibrous monolithic composites. Following the research of Clegg et al., 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 sequential centrifuging.25 “Functionally graded,” laminated ceramics can be made by controlling the composition, layer thickness, and stacking sequence, etc.17 As another type of ceramic material which also shows “graceful failure,” the fibrous monolithic composite was developed.19,26,27 Fibrous monoliths are sintered (or hot-pressed) monolithic ceramics 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 weak interphases, microcracked zones, ductile-phase filaments, or inter￾phases with different physical properties.28 Coblenz26 first intro￾duced the concept of fibrous monolithic composites. Some fibrous monolithic 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, and Al2O3/Al2O3 platelets. Mullite is an attractive, high-temperature, structural material due to its excellent strength and creep resistance at high temper￾ature, good thermal stability, low thermal conductivity, as well as its 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 an attractive candidate material for high-temperature applications.40 Mullite and aluminum phosphate are chemically compatible with each 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 and aluminum phosphate as an interphase material. A study to optimize the microstructure of the two-layer, fibrous monolithic composites was conducted by controlling the compo￾sition and thickness of the AlPO4 interphase, as well as the number of filaments in a given area. Fibrous monolithic composites with different microstructural architectures of two layers, mixed 50% two layers:50% three layers, and three layers were fabricated, and their microstructures and room-temperature mechanical properties were 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 the Al and P sources, respectively. A mixture of Elvax 210, 220, and 250 ethylene vinyl acetate (EVA) copolymers (Dupont, Wilming￾ton, DE) was used as a binder phase. The molecular weight increased 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 Mixing Formulations for Extrusion† Powder Binder (Elvax)‡ Plasticizer (DP)§ Lubricant (SA)¶ 210 220 250 Mullite Inner 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). J. Am. Ceram. Soc., 87 [5] 794–803 (2004) 794 journal