E驅≈3S ournal of the European Ceramic Society 20(2000)551-559 Microstructure and properties of monazite (LaPO4)coated saphikon fiber / alumina matrix composites KK. Chawlaa, *H. Liu.J. Janczak-Rusch. s. sambasivand a Department of Materials and Mechancial Engineering, University of Alabama at Birmingham, 254 BEC, 1530 Third Ave. South, Birmingham, AL35294. Sumitomo, Sitix, Albuquerque, NM 87131, USA FEMPA Thun, Feuerwerkstrasses 39, CH-3602 Diibendorf. Switzerland BIRL, Northwestern University, Evanston, IL 60201, US.A Accepted 10 August 1999 Abstract The objective of this research was to engineer a weak interfacial bond in single crystal a-alumina( Saphikon) fiber/polycrystalline alumina matrix composites by incorporating a monazite (lanthanum phosphate, LaPO4) coating onto Saphikon fibers via soh-gel dip process. Uniaxial hot pressing was used to densify LaPOa-coated AL,O3 fiber in an AlO3 matrix composites. Characterization of the composites was done by optical microscopy, SEM(scanning electron microscopy), EDS(energy dispersive spectrometer). dentation tests, three-point bend and fiber pushout tests. The results showed that the Saphikon fiber/monazite interface was weaker than the polycrystalline alumina/monazite interface Crack deflection, interfacial debonding and fiber pullout occurred at this interface. This was attributed to the fact that the Saphikon fiber /monazite interface was smoother than the monazite/poly- crystalline alumina matrix interface. Monazite coating obtained by sol-gel dip coating method withstood high fabrication tem- peratures(1400C)and was conducive to the toughness properties of the composites. 2000 Elsevier Science Ltd. All rights Keywords: Al,O, fibers; AL2O3 matrix; Composites: Interfaces: LaPOa 1. Introduction between fiber and matrix plays a crucial role in determining the strength and toughness of the composite. 3 For Ceramic matrix composites(CMCs)consisting of instance, in a composite consisting of Al2O3 fiber in a nonoxide fiber/nonoxide matrix, nonoxide fiber/oxide SiO2-based matrix, a strong interfacial bond(chemical matrix or oxide fiber/nonoxide matrix are susceptible to bond) causes the failure mode to be similar to that of oxidation in oxidizing environments at high tempera- monolithic ceramics(brittle). 4 In some simple eutectic tures, causing loss of strength and rapid decrease in type oxide systems, such as Al,O -SnO: and Al2O3- toughness. The degradation of properties of CMCs at ZrO2, no chemical reactions would be expected and elevated temperatures may be due to oxidation of the these composites are relatively stable, moreover, a weak fiber, matrix, and/ or interface, thermal expansion- interface can change the failure mode from brittle to induced residual stresses, and matrix microcracking. 2 non-brittle. 4 In Al2O3/ Al2O3 system, very stron g lonIc Thus, for high temperature applications, in air, an and or covalent bonding leads to low toughness. It thus oxide/oxide composite system would be desirable becomes necessary to apply an interface engineering because of its inherent stability at high temperatures and in approach to increase the toughness by adding a suitable oxidizing atmospheres. In all composites, the interface interphase material between the fiber and matrix. 5 Morgan and Marshall6-s investigated a number of interphase materials including simple metal oxides and Corresponding author. Tel :+1-205-934-8450; fax:+1-205-934. mixed oxides for oxide/oxide composite systems.More significantly, for Al2O3/Al2O3 composites, they found E-mail address; kchawla(@uab.edu(KK. Chawla). that lanthanum phosphate, LaPO4(monazite)was a 0955-2219/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(99)00253-8Microstructure and properties of monazite (LaPO4) coated saphikon ®ber/alumina matrix composites K.K. Chawlaa,*, H. Liub, J. Janczak-Ruschc , S. Sambasivand a Department of Materials and Mechancial Engineering, University of Alabama at Birmingham, 254 BEC, 1530 Third Ave. South, Birmingham, AL 35294- 4461, USA bSumitomo, Sitix, Albuquerque, NM 87131, USA c EMPA Thun, Feuerwerkstrasses 39, CH-3602 DuÈbendorf, Switzerland dBIRL, Northwestern University, Evanston, IL 60201, USA Accepted 10 August 1999 Abstract The objective of this research was to engineer a weak interfacial bond in single crystal a-alumina (Saphikon) ®ber/polycrystalline alumina matrix composites by incorporating a monazite (lanthanum phosphate, LaPO4) coating onto Saphikon ®bers via sol±gel dip process. Uniaxial hot pressing was used to densify LaPO4-coated Al2O3 ®ber in an Al2O3 matrix composites. Characterization of the composites was done by optical microscopy, SEM (scanning electron microscopy), EDS (energy dispersive spectrometer), indentation tests, three-point bend and ®ber pushout tests. The results showed that the Saphikon ®ber/monazite interface was weaker than the polycrystalline alumina/monazite interface. Crack de¯ection, interfacial debonding and ®ber pullout occurred at this interface. This was attributed to the fact that the Saphikon ®ber/monazite interface was smoother than the monazite/poly￾crystalline alumina matrix interface. Monazite coating obtained by sol±gel dip coating method withstood high fabrication tem￾peratures (1400C) and was conducive to the toughness properties of the composites. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Al2O3 ®bers; Al2O3 matrix; Composites; Interfaces; LaPO4 1. Introduction Ceramic matrix composites (CMCs) consisting of nonoxide ®ber/nonoxide matrix, nonoxide ®ber/oxide matrix or oxide ®ber/nonoxide matrix are susceptible to oxidation in oxidizing environments at high tempera￾tures, causing loss of strength and rapid decrease in toughness.1 The degradation of properties of CMCs at elevated temperatures may be due to oxidation of the ®ber, matrix, and/or interface, thermal expansion￾induced residual stresses, and matrix microcracking.2 Thus, for high temperature applications, in air, an oxide/oxide composite system would be desirable because of its inherent stability at high temperatures and in oxidizing atmospheres. In all composites, the interface between ®ber and matrix plays a crucial role in determining the strength and toughness of the composite.3 For instance, in a composite consisting of Al2O3 ®ber in a SiO2-based matrix, a strong interfacial bond (chemical bond) causes the failure mode to be similar to that of monolithic ceramics (brittle).4 In some simple eutectic type oxide systems, such as Al2O3±SnO2 and Al2O3± ZrO2, no chemical reactions would be expected and these composites are relatively stable, moreover, a weak interface can change the failure mode from brittle to non-brittle.4 In Al2O3/Al2O3 system, very strong ionic and/or covalent bonding leads to low toughness. It thus becomes necessary to apply an interface engineering approach to increase the toughness by adding a suitable interphase material between the ®ber and matrix.5 Morgan and Marshall6±8 investigated a number of interphase materials including simple metal oxides and mixed oxides for oxide/oxide composite systems. More signi®cantly, for Al2O3/Al2O3 composites, they found that lanthanum phosphate, LaPO4 (monazite) was a 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(99)00253-8 Journal of the European Ceramic Society 20 (2000) 551±559 * Corresponding author. Tel.: +1-205-934-8450; fax: +1-205-934- 8485 E-mail address; kchawla@uab.edu (K.K. Chawla)