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 sys
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 Some properties of oxide composites based on Al2O3 and LaPO4 (La-monazite) are examined. A composite 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 composites with a weak interphase separating strong ®bers from a strong matrix, such as in glass-matrix/ SiC-®ber composites with a weak carbon interphase.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 suered 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 fabricated by in®ltration of slurries into preforms of woven ®bers (small-diameter, polycrystalline alumina 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 stiness 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 interdiusion 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 perovskites,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-monazite (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 specimen con®gurations designed to assess interfacial debonding. With the exception of some multilayered composites,17 fully dense composites have not been formed, mainly because of the diculty 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 stability at high temperatures. The composite consists of polycrystalline Al2O3 ®bers in a porous, twophase 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
2422 B. Davies et al 2 Experiments this temperature). Approximately 200 fibers of length 40 mm were coated individually with a thick A porous matrix composite was fabricated by layer of rhabdophane(hydrated LaPO4) powder infiltrating woven fabric of polycrystalline alumina and stacked together along a diameter of a 50 mm fibers(3M company, Nextel 610, 8-harness satin cylindrical graphite die. The remainder of the die weave) with a slurry of Al_O3/LaPO4. The slurry was filled with Al_O3 powder to prevent direct contained dispersed a-alumina powder(Sumitomo, contact between the graphite die and the LaPO4 AKP50)and solution precursor for LaPO4 Indivi The compact was hot pressed at 1400C for 1 h,to dual layers of fabric, 15 x 15 cm, were infiltrated d, form an alumina disk with a small embedded sap- stacked (10 layers), dried while vacuum bagging phire/LaPO4 composite, of cross-section dimen- and warm pressing, then sintered at 1100C in air sions m2 x 2 mm, along its diameter. for 1 h. The resulting composite plate was 2 mm The test specimen shown schematically in Fig. 2 hickness. Double-edge-notched tensile test speci- was cut from the hot pressed disc and loaded in mens, with dimensions 15 x I cm aligned parallel four-point bending with the embedded composite to the weave direction, were cut from the plate in tension. The hole drilled adjacent to the embed using a diamond saw(Fig. 1). The saw cuts that ded composite served to reduce the gradient of formed the notches were of 150 um width. The tensile bending stress across the composite and to specimens were loaded in tension using wedge grips allow cracking of the ligament of material between and tabs glued to the ends. The gauge section the hole and the edge of the specimen without extension was measured using a clip gauge attached fracturing the entire beam 12.5 mm above and below the mid plane, which contained the notche A composite with a fully dense LaPo 3 Results nd sapphire fiber reinforcement was fabricated by hot pressing. Since the sole purpose of this com- 3.1 Porous Al2O3/LaPO4 matrix composites posite was to test whether debonding and fiber The microstructure of the AlO3/LaPO4 matrix pullout occur in this system under tensile loading composite is shown in the scanning electron parallel to the fibers, the simplest possible fabrica- microscope(SEM) image of Fig. 3. The matrix tion route, making use of available hot pressing consists of a two-phase mixture of AlO3 and facilities, was used. Sapphire fibers were chosen to LaPO4 grains, both with dimensions smaller than allow hot pressing at sufficiently high temperature approximately 0.5 um, with fine-scale porosit to densify the matrix(1400oC)without damaging between the grains. The alumina grains in the the fibers(polycrystalline fibers would degrade at matrix are always separated from the fibers by a thin layer of LaPO4. The composites consisted of approximately 40 vol% of fibers(20% in each of the 0 and 90 orientations), 40 vol% matrix(AlO3 and LaPO4 in the ratio approximately 2: 1)and 20 vol% porosity(as estimated from weight measure- ments) The tensile of a test with notch depth greater than half of the test section (a/w=0.54)is shown in Fig. 4. The net section stress during testing (load divided by the remaining cross-sectional area between the ends of the not- gage ches)is plotted as a function of the extension mea- sured from the clip gauge. Also shown along the top border is the average strain calculated from the Fig. 1. Double-edge-notched tensile test specimen of compo- site consisting of woven AlO3 fibers and porous Al2O3- Fig. 2. Test specimen(Al2O3) with embedded composite of
2 Experiments A porous matrix composite was fabricated by in®ltrating woven fabric of polycrystalline alumina ®bers (3M company, Nextel 610, 8-harness satin weave) with a slurry of Al2O3/LaPO4. The slurry contained dispersed -alumina powder (Sumitomo, AKP50) and solution precursor for LaPO4. Individual layers of fabric, 15 15 cm2 , were in®ltrated, stacked (10 layers), dried while vacuum bagging and warm pressing, then sintered at 1100C in air for 1 h. The resulting composite plate was 2 mm thickness. Double-edge-notched tensile test specimens, with dimensions 15 1 cm aligned parallel to the weave direction, were cut from the plate using a diamond saw (Fig. 1). The saw cuts that formed the notches were of 150 m width. The specimens were loaded in tension using wedge grips and tabs glued to the ends. The gauge section extension was measured using a clip gauge attached 12.5 mm above and below the mid plane, which contained the notches. A composite with a fully dense LaPO4 matrix and sapphire ®ber reinforcement was fabricated by hot pressing. Since the sole purpose of this composite was to test whether debonding and ®ber pullout occur in this system under tensile loading parallel to the ®bers, the simplest possible fabrication route, making use of available hot pressing facilities, was used. Sapphire ®bers were chosen to allow hot pressing at suciently high temperature to densify the matrix (1400C) without damaging the ®bers (polycrystalline ®bers would degrade at this temperature). Approximately 200 ®bers of length 40 mm were coated individually with a thick layer of rhabdophane (hydrated LaPO4) powder and stacked together along a diameter of a 50 mm cylindrical graphite die. The remainder of the die was ®lled with Al2O3 powder to prevent direct contact between the graphite die and the LaPO4. The compact was hot pressed at 1400C for 1 h, to form an alumina disk with a small embedded sapphire/LaPO4 composite, of cross-section dimensions 2 2 mm, along its diameter. The test specimen shown schematically in Fig. 2 was cut from the hot pressed disc and loaded in four-point bending with the embedded composite in tension. The hole drilled adjacent to the embedded composite served to reduce the gradient of tensile bending stress across the composite and to allow cracking of the ligament of material between the hole and the edge of the specimen without fracturing the entire beam. 3 Results 3.1 Porous Al2O3/LaPO4 matrix composites The microstructure of the Al2O3/LaPO4 matrix composite is shown in the scanning electron microscope (SEM) image of Fig. 3. The matrix consists of a two-phase mixture of Al2O3 and LaPO4 grains, both with dimensions smaller than approximately 0.5 m, with ®ne-scale porosity between the grains. The alumina grains in the matrix are always separated from the ®bers by a thin layer of LaPO4. The composites consisted of approximately 40 vol% of ®bers (20% in each of the 0 and 90 orientations), 40 vol% matrix (Al2O3 and LaPO4 in the ratio approximately 2:1) and 20 vol% porosity (as estimated from weight measurements). The tensile response of a test specimen with notch depth greater than half of the test section (a=w 054) is shown in Fig. 4. The net section stress during testing (load divided by the remaining cross-sectional area between the ends of the notches) is plotted as a function of the extension measured from the clip gauge. Also shown along the top border is the average strain calculated from the Fig. 1. Double-edge-notched tensile test specimen of composite consisting of woven Al2O3 ®bers and porous Al2O3± LaPO4 matrix. Fig. 2. Test specimen (Al2O3) with embedded composite of sapphire ®bers and LaPO4 matrix. 2422 J. B. Davies et al.
Oxide composites of Al203 and LaPO4 2423 extension measurements; however, the strain is very ing occurred in the LapOa matrix, and pieces of the nonuniform within the gauge area. The stress-strain matrix pulled out with the fibers as in Fig 8(a) curve shows substantial nonlinearity in the loading higher magnification micrographs such as Fig 8(b) portion and continuously decreasing load beyond show that debonding and sliding occurred at the the peak. Test specimens without notches showed fiber-matrix interface [scratch marks due to fiber similar stress-strain curves, with strengths, in the sliding can be seen in the LaPOa matrix in Fig range 200 to 220 MPa 8(b), in the hole left behind after pulling out the The nonlinear response is associated with exten- fiber ive pullout of fibers, as seen from the separated ends of the test specimen in Fig. 5. Failure of the specimen occurred by separation of the matrix 4 Discussion along an approximately planar section between the ends of the two notches and pullout of individual Under tensile loading parallel to the 0 or 90 weave fibers over lengths up to ml cm from both sides of direction, the AlO3/LaPO4 matrix composite the fracture plane. Small but finite loads were sup- shows nonlinear stress-strain response, with the ported by these bridging fibers at crack opening nonlinear component of strain at the peak load displacements of several mm. Examples of intact being several times the linear elastic strain extra- pecimens loaded beyond the peak in the stress- polated from low loads. Failure is non-cata strain curve are shown in Fig. 6: at a crack opening strophic, with fiber pullout over distances of l of Nl mm in Fig. 6(a),(showing bridging fibers) cm. Strengths, in the range 200-250 MPa, are and at a crack opening of 3 mm in Fig. 6(b)(in- unaffected by the presence of severe notches. These situ micrograph with net section stress 8 MPa) characteristics differ from the responses reported 3.2 Fully dense LaPO4-matrix composite The microstructure of the hot pressed LaPO4-matrix composite is shown in the polished cross-section of 0.01 0.02 Fig. 7. The matrix contains a few isolated pores but is mostly fully dense LaPO4. There is no apparent degradation of the sapphire fibers or reaction with the matrix Micrographs of the specimen depicted in Fig. 2 after loading to failure are shown in Fig. 8. Pullout of the sapphire fibers occurred over lengths of sev eral mm, with most fibers pulling out from one side of the fracture surface. Although extensive crack. Fig 4. Tensile test results from co 1g. Fig. 3. Microstructure of composite from Fig. 1(secondary Fig. 5. Separated test specimen from test in Fig. 4
extension measurements; however, the strain is very nonuniform within the gauge area. The stress±strain curve shows substantial nonlinearity in the loading portion and continuously decreasing load beyond the peak. Test specimens without notches showed similar stress±strain curves, with strengths, in the range 200 to 220 MPa. The nonlinear response is associated with extensive pullout of ®bers, as seen from the separated ends of the test specimen in Fig. 5. Failure of the specimen occurred by separation of the matrix along an approximately planar section between the ends of the two notches and pullout of individual ®bers over lengths up to 1 cm from both sides of the fracture plane. Small but ®nite loads were supported by these bridging ®bers at crack opening displacements of several mm. Examples of intact specimens loaded beyond the peak in the stress± strain curve are shown in Fig. 6: at a crack opening of 1 mm in Fig. 6(a), (showing bridging ®bers) and at a crack opening of 3 mm in Fig. 6(b) (insitu micrograph with net section stress 8 MPa). 3.2 Fully dense LaPO4-matrix composite The microstructure of the hot pressed LaPO4-matrix composite is shown in the polished cross-section of Fig. 7. The matrix contains a few isolated pores, but is mostly fully dense LaPO4. There is no apparent degradation of the sapphire ®bers or reaction with the matrix. Micrographs of the specimen depicted in Fig. 2 after loading to failure are shown in Fig. 8. Pullout of the sapphire ®bers occurred over lengths of several mm, with most ®bers pulling out from one side of the fracture surface. Although extensive cracking occurred in the LaPO4 matrix, and pieces of the matrix pulled out with the ®bers as in Fig. 8(a), higher magni®cation micrographs such as Fig. 8(b) show that debonding and sliding occurred at the ®ber±matrix interface [scratch marks due to ®ber sliding can be seen in the LaPO4 matrix in Fig. 8(b), in the hole left behind after pulling out the ®ber]. 4 Discussion Under tensile loading parallel to the 0 or 90 weave direction, the Al2O3/LaPO4 matrix composite shows nonlinear stress±strain response, with the nonlinear component of strain at the peak load being several times the linear elastic strain extrapolated from low loads. Failure is non-catastrophic, with ®ber pullout over distances of 1 cm. Strengths, in the range 200±250 MPa, are unaected by the presence of severe notches. These characteristics dier from the responses reported Fig. 4. Tensile test results from composite of Fig. 1. Fig. 5. Separated test specimen from test in Fig. 4. Fig. 3. Microstructure of composite from Fig. 1 (secondary electron image). Oxide composites of Al2O3 and LaPO4 2423
2424 B. Davies et al load (a) Fig. 6.(a) Bridging fiber tows in test loaded as in on.(b)In-situ optical micrograph from test as in Fig. 4 at net section stress f MPa 50m ApOA Fig 8.(a) Fractured test piece of LaPO4 matrix composite. (b) Higher magnification of region of fracture surface from same specimen as(a). Dark circular area is fracture surface of sap- phire fiber. Smooth light region is LaPO4 coating in cylindrical hole (going into page from right to left)remaining after pull out of the other half of the fractured fiber sapphire weave direction. However, the large nonlinear response has not been observed in tensile loading parallel to the 0 or 90 weave directions. In this case, the composites may show a very small non Fig.7. Cross-section of hot-pressed LaPOa-matrix composite linearity before the peak load(nonlinear compo- ( secondary electron image). nent of strain less than 10% of the total strain) and a jagged fracture surface associated with some pullout of fiber tows. However, failure is cata or other porous matrix composites in systems strophic and the extensive pullout of individual where strong local bonding between matrix and fibers, such as in Fig. 5, is not observed. Therefore, fibers would be expected(SiO2/Al2O3,6 Al2O3/ it is evident that, while the porous nature of the mullite, 8 .9AlPO4 ) The strengths of such compo- matrix in the Al2O3/ LaPO4 composite most likely sites under loading parallel to the 0 or 90 direc- contributes to the nonlinear response, the presence of tions have been reported in the same range, while the weakly bonded LaPO phase at the fiber-matrix notch-insensitivity along with nonlinear stress- interface greatly enhances the damage-tolerant strain response and noncatastrophic failure have behavior of the composite been reported under flexural loading and under Although porous-matrix composites may be tensile loading in a direction at 45 to the 0/90 adequate for many applications, certain uses bene
for other porous matrix composites in systems where strong local bonding between matrix and ®bers would be expected (SiO2/Al2O3, 6 Al2O3/ mullite,8,9 AlPO4 10). The strengths of such composites under loading parallel to the 0 or 90 directions have been reported in the same range, while notch-insensitivity along with nonlinear stressstrain response and noncatastrophic failure have been reported under ¯exural loading and under tensile loading in a direction at 45 to the 0/90 weave direction.9 However, the large nonlinear response has not been observed in tensile loading parallel to the 0 or 90 weave directions. In this case, the composites may show a very small nonlinearity before the peak load (nonlinear component of strain less than 10% of the total strain) and a jagged fracture surface associated with some pullout of ®ber tows. However, failure is catastrophic and the extensive pullout of individual ®bers, such as in Fig. 5, is not observed. Therefore, it is evident that, while the porous nature of the matrix in the Al2O3/LaPO4 composite most likely contributes to the nonlinear response, the presence of the weakly bonded LaPO4 phase at the ®ber±matrix interface greatly enhances the damage-tolerant behavior of the composite. Although porous-matrix composites may be adequate for many applications, certain uses beneFig. 7. Cross-section of hot-pressed LaPO4-matrix composite (secondary electron image). Fig. 6. (a) Bridging ®ber tows in test specimen loaded as in Fig. 4, interrupted before complete separation. (b) In-situ optical micrograph from test as in Fig. 4 at net section stress of 8 MPa. Fig. 8. (a) Fractured test piece of LaPO4 matrix composite. (b) Higher magni®cation of region of fracture surface from same specimen as (a). Dark circular area is fracture surface of sapphire ®ber. Smooth light region is LaPO4 coating in cylindrical hole (going into page from right to left) remaining after pullout of the other half of the fractured ®ber. 2424 J. B. Davies et al
Oxide composites of Al203 and LaPO4 2425 fit from, or require a fully dense matrix (e.g, Acknowledgements extremely corrosive environments, need for her metic seal). Previous studies of LaPO/AlO3 Funding for this work was provided by the U. S interfaces have demonstrated that debonding Air Force Office of Scientific Research under con occurs in a variety of cracking configurations tract F49620-96-C-0026 monitored by Dr. A fully del i8 However, damage- Pechenik(work on fully dense matrix composites) tolerant behavior requires sliding and pullout of and the U.S. Office of Naval Research under con- fibers in addition to debonding Fiber sliding after tract N00014-95-C-0057 monitored by Dr. S debonding would be expected to be more difficult Fishman(work on porous matrix composite) in fully dense systems than in composites with porous matrices, because the higher stiffness of the matrix would be less accommodating for the misfit References caused by the sliding motion of any irregularities at the interface 22 I. Prewo, K. M. and Brennan, J. J, High strength silicon Fiber sliding was demonstrated previously in carbon fiber reinforced glass matrix composites. J. Mater pushout experiments involving isolated sapphire 2. Brennan.Jj and Prewo,K.M. Silicon carbide fiber fibers with LaPOa coatings in a fully dense Al2O reinforced glass-ceramic matrix composites exhil matrix.The interface in these experiments con high strength and toughness. Mater. Sci, 1982, tained irregularities in the form of grain boundary 2371-2383 3. Sun, E. Y, Nutt, S.R. and Brennan, J.J., Fiber coatings grooves and cusps with height 50 nm. Sliding for SiC-fiber-reinforced BMAs glass-ceramic composites. occurred without permanent deformation, imply J.Am. Ceran.Soc,1997,80(1),264. ing that the misfitting asperities were accom- 4. Turner, K.R., Speck, J.S. and Evans, A. G, Mechanisms of deformation and failure in carbon-matrix composites modated by elastic strains during sliding. The subject to tensile and shear loading. J. Am. Ceram. Soc. LaPOa-matrix composite examined in the present 1995,78(7),1841-1848 study was prepared using the same starting mate 5. Buckley, J. D, Carbon-carbon, an overview. Am. Ceram rials and nominally identical processing conditions Soc.Bull,1988,67(2),364368 6. Harrison. M.G. Millard. M. L. and szweda. A as the composite used in the previous pushout reinforced ceramic matrix composite member and experiments. 5 The results in Figs. 7 and 8 demon d method for making. U.S. Patent No. 5306554: UK Patent No. 2 strate the feasibility of achieving fiber pullout in 230259(1994) 7. Tu, w.C., Lange, F. F and Evans, A G, Concept for a fully dense systems amage-tolerant ceramic composite withstrong'inter The development of oxide composites with fully faces. J. Am. Ceram Soc., 1996. 79(2). 417-424 dense matrices is presently limited by our ability to 8. Levi, C. G, Yang, J.Y., Dalgleish, B J, Zok, F. w. and densify the matrix under conditions that do not Evans, A.G., Processing and performance of an all-oxid ceramic composite. J. Am. Ceram. Soc., 1998, 81(8) degrade the fibers. Processing temperatures for 2077-2086 composites containing polycrystalline Al2O3 and 9. Lange. F. F, Tu. w-C. and Evans. A. G. Processing of mullite fibers are limited to 1200-1300C(lower amage-tolerant. oxidation-resistant ceramic-matrix com- if pressure is used to aid densification). Such sites by a precursor infiltration and pyrolysis method Mater.Sci.Eng.,1995,A195,145-1 composites require development of either higher 10. Keith, w.P. and Kedward, K. T, Shear damage mechan temperature fibers(e.g. eutectic or single crystal isms in a woven. nicalon reinforced ceramic matrix com- posite.J. Am. Ceram Soc., 1997, 80(2), 357-364 fibers)or methods, currently being examined, for I1. Cooper, R. F. and Hall, P. C,Reactions be promoting densification of the matrix at lower thetic mica and simple oxide compounds with cation temperatures to oxidation-resistant ceramic composite. J. Am. Ce 12. Morgan, P. E. D and Marshall, D. B, Functional 5 Conclusions 13. Cinibulk netoplumbite compounds as a fiber pating for oxide- oxide composites. Ceram. Eng. and S An oxide composite consisting of woven Al2O3 fibers and a porous matrix of Al2O3 and LaPO 14. Petuskey, WT(private communication) was found to exhibit much greater nonlinear 15. Morgan, P. E D. and Marshall, D. B, Ceramic compo- sites of monazite and alumina. J. Am. Ceram. Soc. 1995. response and notch insensitivity than other porous 78(6),1553-1563 matrix composites. The enhanced properties were 16. Morgan, P.E. D. Marshall,DB and Housley,R. M attributed to weak bonding between the fibers and High temperature stability of monazite-alumina the LapOa phase, which allowed extensive fiber ites. J. Mat Sci. Eng王D, and Housley, R. M pullout. Debonding in multilayered composites of zirconia and The feasibility of achieving fiber pullout in fully LaPO4.J.Am. Ceran.Soc.,1997,80(7),1677-1683 R. M dense Al]O3-LaPO4 composites was demonstrated aI. D P. E Cheung, J. T, High temperature stability of the AlO3 using a hot pressed composite with sapphire fibers LaPOa system J. Am. Ceram Soc., 1998, 81(4), 951-956
®t from, or require a fully dense matrix (e.g., extremely corrosive environments, need for hermetic seal). Previous studies of LaPO4/Al2O3 interfaces have demonstrated that debonding occurs in a variety of cracking con®gurations in fully dense systems.15±18 However, damagetolerant behavior requires sliding and pullout of ®bers in addition to debonding. Fiber sliding after debonding would be expected to be more dicult in fully dense systems than in composites with porous matrices, because the higher stiness of the matrix would be less accommodating for the mis®t caused by the sliding motion of any irregularities at the interface.22 Fiber sliding was demonstrated previously in pushout experiments involving isolated sapphire ®bers with LaPO4 coatings in a fully dense Al2O3 matrix.15 The interface in these experiments contained irregularities in the form of grain boundary grooves and cusps with height 50 nm. Sliding occurred without permanent deformation, implying that the mis®tting asperities were accommodated by elastic strains during sliding. The LaPO4-matrix composite examined in the present study was prepared using the same starting materials and nominally identical processing conditions as the composite used in the previous pushout experiments.15 The results in Figs. 7 and 8 demonstrate the feasibility of achieving ®ber pullout in fully dense systems. The development of oxide composites with fully dense matrices is presently limited by our ability to densify the matrix under conditions that do not degrade the ®bers. Processing temperatures for composites containing polycrystalline Al2O3 and mullite ®bers are limited to 1200±1300C (lower if pressure is used to aid densi®cation). Such composites require development of either higher temperature ®bers (e.g. eutectic or single crystal ®bers) or methods, currently being examined, for promoting densi®cation of the matrix at lower temperatures. 5 Conclusions An oxide composite consisting of woven Al2O3 ®bers and a porous matrix of Al2O3 and LaPO4 was found to exhibit much greater nonlinear response and notch insensitivity than other porous matrix composites. The enhanced properties were attributed to weak bonding between the ®bers and the LaPO4 phase, which allowed extensive ®ber pullout. The feasibility of achieving ®ber pullout in fully dense Al2O3±LaPO4 composites was demonstrated using a hot pressed composite with sapphire ®bers. Acknowledgements Funding for this work was provided by the U.S. Air Force Oce of Scienti®c Research under contract F49620-96-C-0026 monitored by Dr. A. Pechenik (work on fully dense matrix composites) and the U.S. Oce of Naval Research under contract N00014-95-C-0057 monitored by Dr. S. 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