Journal of the European Ceramic Society 19(1999)2421-2426 C1999 Elsevier Science Ltd Printed in Great Britain. All rights reserved Oxide Composites of Al2O3 and LaPO4 J B. Davis, * D.B. Marshall and P E. D. morgan Rockwell Science Center. 1049 Camino dos rios Thousand oaks. Ca 91360. USA abstract fibers and matrix, the porous strongly to the fibers. However, because of the low Some properties of oxide composites based on A12O3 elastic stiffness and fracture toughness resulting and LapO,(La-monazite) are examined. A com- from the porosity, cracks do not readily pass from posite consisting of woven Al203 fibers with a porous the matrix to the fibers, and tensile loading causes matrix of A12O3 and LapO+ is shown to be damage extensive damage in the matrix before failure of the tolerant and notch insensitive. The feasibility of fibers. Failure ultimately involves pullout of fibers achieving fiber sliding and pullout in a composite mainly in tows, and the fracture strength is relatively with a fully dense matrix is investigated using a insensitive to the presence of stress concentrators small hot-pressed composite of sapphire fibers and such as notches and holes. The high temperature LaPO, matrix. C 1999 Elsevier Science Ltd. All limitations of such composites are expected to come rights reserved. from several sources: microstructural stability of fine Keywords: La PO4, composites, fibres, mechanical diffusion of the matrix and fibers. matrix, and inter- grained fibers, coarsening of the properties, Al,O In fully dense systems, two classes of oxides that allow debonding have recently been identified. One relies on layered crystal structures such as micas, I 1 Introduction B-aluminas, 2 magnetoplumbite d per ovskites, 4 with intrinsically weak cleavage planes Damage-tolerant, nonlinear behavior in ceramic The other is a group of refractory mixed oxides composites requires weak surfaces or phases that (rare-earth orthophosphates with the monazite 5-18 decouple fracture processes in the matrix and the or xenotime structures, and several tungstates20 fiber reinforcements. This can be achieved in com- and vanadates), which have been shown to form posites with a weak interphase separating strong weak interfaces with other oxides such as Al_O3, fibers from a strong matrix, such as in glass-matrix/ YAG, ZrO, and mullite. In the case of La-mon SiC-fiber composites with a weak carbon inter- azite (LaPO4), long-term stability with sapphire phase.-3It can also be achieved in composites fibers has been demonstrated at 1600C, thus consisting of strong fibers in a porous, weak matrix, indicating the prospect of fabricating dense-matrix such as in carbon-matrix composites Until fairly composites with better stability than the porous recently, all ceramic composites relied on either matrix systems presently available. However, most carbon or BN for the weak phase. As a result, they studies of these systems have involved model spe suffered from limited oxidation resistance cimen configurations designed to assess interfacial During the past 5 years, several analogous oxide debonding. With the exception of some multi systems have been identified, with potential for layered composites, fully dense composites have long-term stability in oxidizing environments. not been formed, mainly because of the difficulty of Composites with weak porous matrices of Sio,/ processing such composites without degrading the Al2O3, 6 Al2O3, Al2O3/mullite,,9 and AlPO4 oxide fibers that are presently available have been reported with room temperature In this paper we present some properties of an mechanical properties very similar to carbon-carbon oxide composite that combines a porous matrix composites. These oxide composites were fabri- and a weak fiber-matrix interface to give enhanced cated by infiltration of slurries into preforms of nonlinear behavior and potentially improved sta woven fibers(small-diameter, polycrystalline alu- bility at high temperatures. The composite consists mina or mullite fibers) followed by sintering In the of polycrystalline Al2O3 fibers in a porous, two absence of a separate weak interphase between the phase matrix of LaPO4 and Al2O3. We also report some preliminary experiments to test the ability of LaPO4 to facilitate fiber debonding and pullout in To whom correspondance should be addressed. fully dense sysOxide Composites of Al2O3 and LaPO4 J. B. Davis,* D. B. Marshall and P. E. D. Morgan Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360, USA Abstract Some properties of oxide composites based on Al2O3 and LaPO4 (La-monazite) are examined. A com￾posite consisting of woven Al2O3 ®bers with a porous matrix of Al2O3 and LaPO4 is shown to be damage tolerant and notch insensitive. The feasibility of achieving ®ber sliding and pullout in a composite with a fully dense matrix is investigated using a small hot-pressed composite of sapphire ®bers and LaPO4 matrix. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: LaPO4, composites, ®bres, mechanical properties, Al2O3. 1 Introduction Damage-tolerant, nonlinear behavior in ceramic composites requires weak surfaces or phases that decouple fracture processes in the matrix and the ®ber reinforcements. This can be achieved in com￾posites with a weak interphase separating strong ®bers from a strong matrix, such as in glass-matrix/ SiC-®ber composites with a weak carbon inter￾phase.1±3 It can also be achieved in composites consisting of strong ®bers in a porous, weak matrix, such as in carbon±matrix composites.4,5 Until fairly recently, all ceramic composites relied on either carbon or BN for the weak phase. As a result, they su€ered from limited oxidation resistance. During the past 5 years, several analogous oxide systems have been identi®ed, with potential for long-term stability in oxidizing environments. Composites with weak porous matrices of SiO2/ Al2O3, 6 Al2O3, 7 Al2O3/mullite,8,9 and AlPO4 10 have been reported with room temperature mechanical properties very similar to carbon±carbon composites. These oxide composites were fabri￾cated by in®ltration of slurries into preforms of woven ®bers (small-diameter, polycrystalline alu￾mina or mullite ®bers) followed by sintering. In the absence of a separate weak interphase between the ®bers and matrix, the porous matrix bonds strongly to the ®bers. However, because of the low elastic sti€ness and fracture toughness resulting from the porosity, cracks do not readily pass from the matrix to the ®bers, and tensile loading causes extensive damage in the matrix before failure of the ®bers. Failure ultimately involves pullout of ®bers, mainly in tows, and the fracture strength is relatively insensitive to the presence of stress concentrators such as notches and holes.9 The high temperature limitations of such composites are expected to come from several sources: microstructural stability of ®ne grained ®bers, coarsening of the matrix, and inter￾di€usion of the matrix and ®bers. In fully dense systems, two classes of oxides that allow debonding have recently been identi®ed. One relies on layered crystal structures such as micas,11 -aluminas,12 magnetoplumbites12,13 and per￾ovskites,14 with intrinsically weak cleavage planes. The other is a group of refractory mixed oxides (rare-earth orthophosphates with the monazite15±18 or xenotime19 structures, and several tungstates20 and vanadates21), which have been shown to form weak interfaces with other oxides such as Al2O3, YAG, ZrO2, and mullite. In the case of La-mon￾azite (LaPO4), long-term stability with sapphire ®bers has been demonstrated at 1600C,18 thus indicating the prospect of fabricating dense-matrix composites with better stability than the porous matrix systems presently available. However, most studies of these systems have involved model spe￾cimen con®gurations designed to assess interfacial debonding. With the exception of some multi￾layered composites,17 fully dense composites have not been formed, mainly because of the diculty of processing such composites without degrading the oxide ®bers that are presently available. In this paper we present some properties of an oxide composite that combines a porous matrix and a weak ®ber±matrix interface to give enhanced nonlinear behavior and potentially improved sta￾bility at high temperatures. The composite consists of polycrystalline Al2O3 ®bers in a porous, two￾phase matrix of LaPO4 and Al2O3. We also report some preliminary experiments to test the ability of LaPO4 to facilitate ®ber debonding and pullout in fully dense systems. Journal of the European Ceramic Society 19 (1999) 2421±2426 # 1999 Elsevier Science Ltd Printed in Great Britain. All rights reserved PII: S0955-2219(99)00112-0 0955-2219/99/$ - see front matter 2421 *To whom correspondance should be addressed