AERIALS SHENGE ENGINEERIG SEVIER Materials Science and Engineering A 380(2004)237-244 www.elsevier.com/locate/msea Fibrous monoliths of mullite-AlPO4 and alumina/YAG-alumina platelets Dong-Kyu Kim, Waltraud M. Kriven* Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA Received 30 May 2003 received in revised form 23 March 2004 Abstract Two-layer, fibrous monolithic composites consisting of mullite-aluminum phosphateAlPO4)and 50 vol. alumina: 50 vol. YAG in situ composite matrix-alumina platelet interphase components, were fabricated by a co-extrusion technique. The four powders were characterized for particle size, specific surface area, and SEM analysis. The mixing formulations for extruding the powders were developed using ethylene vinylacetate copolymer as a binder. The variation in the mixing torque, in a Brabender mixer, as a function of temperature was measured The binder removal behavior of the mullite-AIPO4 fibrous monolithic composite was studied by thermogravimetric analysis (TGA). The AlPO4 and alumina platelet interphase layers formed a porous and less porous interphase region, respectively, after sintering. The sintered mullite-AlPO4 two-layer fibrous monolithic showed non-brittle fracture behavior. Its 3-point bend strength and work of fracture were 76+ MPa and 0.45+0.02 kJ/m", respectively O 2004 Elsevier B v. All rights reserved Keywords: Fibrous monolithic composite; Mullite-aluminum phosphate(AlPO4): 50 vol. Alumina: 50 vol. YAG in situ composite-alumina platelet, Sintering, Mechanical property 1. Introduction ing from 1200 to 1400C[12, 15-18]. Mullite is known to be an excellent creep resistant material. Lessing et al. 191 To overcome the brittleness of ceramics, several different reported the creep rate of mullite to be, approximately, an approaches have been adopted. After the work of Clegg et al. order of magnitude less than that of Al2O3 [1] different kinds of laminated ceramic composites were YAG (yttrium aluminum garnet, Y3AlsO12)and alumina made to increase toughness [2-6]. Fibrous monolithic com- are chemically compatible with each other and can constitute osites are an alternative approach to the design of tough structurally sound composites [20-22]. YAG-alumina eutec ceramics, without the incorporation of ceramic fibers into a tic materials have been made by induction heating [23-25 composite. This concept was first introduced by Coblenz[7]. The alumina-YAG eutectic composite has a flexural strength Fibrous monolithic composites are sintered (or hot-pressed) of 360-500 MPa. This strength is maintained nearly con- monolithic ceramics with a distinct fibrous texture, consist- stantly over the range from room temperature to 1700C ing of cells of a primary phase, separated by cell boundaries [25-28]. The creep resistance of the alumina-YAG eutec of a tailored secondary phase[8. Halloran et al. [8-ll con- tic composite is better than that of polycrystalline YAG and ducted extensive research on non-oxide, fibrous monolithic that of a-axis sapphire fiber [28]. An alumina-YAG eutectic composites of SiC/C and Si3 N4/BN composite has excellent high temperature strength and creep Mullite is an attractive structural material due to its excel- resistance, but its processing route is complicated and ex lent strength and creep resistance at high temperatures, low pensive. Therefore, as an alternative route to the benefits of thermal expansion and conductivity, good thermal stability, a eutectic composite, an alternative, cheap, alumind-YAG in and chemical inertness [12-14. The reported bend strengths situ matrix composite was developed in our laboratory [291 of mullite at room temperature lie between 250-400 MPa, Aluminum phosphate(AlPO4) is chemically compatible and these strengths are maintained up to temperatures rang- with mullite and has a density corresponding to 61% of the- strength of1.5±02 after sintering at 1600C for 10h, resulting in AlPO4 act- Corresponding author. Tel. +12173335258; fax: +1217 2736. ing as a chemically stable, porous, and weak interphase E-mail address: w-kriven staff uiuc. edu(WM. Riven material [30]- Alumina platelets were aligned in random 0921-5093/s-see front matter 2004 Elsevier B V. All rights reserved doi:10.1016 j.msea200403.083Materials Science and Engineering A 380 (2004) 237–244 Fibrous monoliths of mullite-AlPO4 and alumina/YAG-alumina platelets Dong-Kyu Kim, Waltraud M. Kriven∗ Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA Received 30 May 2003; received in revised form 23 March 2004 Abstract Two-layer, fibrous monolithic composites consisting of mullite-aluminum phosphate (AlPO4) and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet interphase components, were fabricated by a co-extrusion technique. The four powders were characterized for particle size, specific surface area, and SEM analysis. The mixing formulations for extruding the powders were developed using ethylene vinylacetate copolymer as a binder. The variation in the mixing torque, in a Brabender mixer, as a function of temperature was measured. The binder removal behavior of the mullite-AlPO4 fibrous monolithic composite was studied by thermogravimetric analysis (TGA). The AlPO4 and alumina platelet interphase layers formed a porous and less porous interphase region, respectively, after sintering. The sintered mullite-AlPO4 two-layer fibrous monolithic showed non-brittle fracture behavior. Its 3-point bend strength and work of fracture were 76 ± 5 MPa and 0.45 ± 0.02 kJ/m2, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Fibrous monolithic composite; Mullite-aluminum phosphate(AlPO4); 50 vol.% Alumina:50 vol.% YAG in situ composite-alumina platelet; Sintering; Mechanical property 1. Introduction To overcome the brittleness of ceramics, several different approaches have been adopted. After the work of Clegg et al. [1], different kinds of laminated ceramic composites were made to increase toughness [2–6]. Fibrous monolithic com￾posites are an alternative approach to the design of tough ceramics, without the incorporation of ceramic fibers into a composite. This concept was first introduced by Coblenz [7]. Fibrous monolithic composites are sintered (or hot–pressed) monolithic ceramics with a distinct fibrous texture, consist￾ing of cells of a primary phase, separated by cell boundaries of a tailored secondary phase [8]. Halloran et al. [8–11] con￾ducted extensive research on non-oxide, fibrous monolithic composites of SiC/C and Si3N4/BN. Mullite is an attractive structural material due to its excel￾lent strength and creep resistance at high temperatures, low thermal expansion and conductivity, good thermal stability, and chemical inertness [12–14]. The reported bend strengths of mullite at room temperature lie between 250–400 MPa, and these strengths are maintained up to temperatures rang- ∗ Corresponding author. Tel.: +1 217 333 5258; fax: +1 217 333 2736. E-mail address: w-kriven@staff.uiuc.edu (W.M. Kriven). ing from 1200 to 1400 ◦C [12,15–18]. Mullite is known to be an excellent creep resistant material. Lessing et al. [19] reported the creep rate of mullite to be, approximately, an order of magnitude less than that of Al2O3. YAG (yttrium aluminum garnet, Y3Al5O12) and alumina are chemically compatible with each other and can constitute structurally sound composites[20–22]. YAG–alumina eutec￾tic materials have been made by induction heating [23–25]. The alumina–YAG eutectic composite has a flexural strength of 360–500 MPa. This strength is maintained nearly con￾stantly over the range from room temperature to 1700 ◦C [25–28]. The creep resistance of the alumina–YAG eutec￾tic composite is better than that of polycrystalline YAG and that of a-axis sapphire fiber [28]. An alumina–YAG eutectic composite has excellent high temperature strength and creep resistance, but its processing route is complicated and ex￾pensive. Therefore, as an alternative route to the benefits of a eutectic composite, an alternative, cheap, alumina–YAG in situ matrix composite was developed in our laboratory [29]. Aluminum phosphate (AlPO4) is chemically compatible with mullite and has a density corresponding to 61% of the￾oretical value. It has a bending strength of 1.5 ± 0.2 MPa after sintering at 1600 ◦C for 10 h, resulting in AlPO4 act￾ing as a chemically stable, porous, and weak interphase material [30]. Alumina platelets were aligned in random, 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.03.083