ournal JAm. Ceram.So,86[21305-1602003) Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite Interphases Janet B. Davis, Randall S. Hay, *f David B. Marshall, *f Peter E D Morgan, and Ali sayir*, s Rockwell Scientific. Thousand Oaks. California 91360 Air Force Research Laboratory, Materials Directorate, Wright-Patterson AFB, Ohio 45433 NASA-Glenn Research Center/Case Western Reserve University, Cleveland, Ohio 44 135 Room-temperature debonding and sliding of fibers coated witl sliding occur in fiber pushout tests with model composites con- La-monazite is assessed using a composite with a polycrystal sisting of LaPOa-coated single crystal fibers of Al2O, and line alumina matrix and fibers of several different single Y3AlsO12 (YAG)in polycrystalline Al2O3 matrices crystal (mullite and sapphire)and directionally solidified Damage-tolerant behavior in ceramic composites requires slid- eutectic(Al,O3/Y3AlsO1 and Al,O3Y-ZrO2) compositions. and pullout of fibers in addition to interfacial debonding These fibers provide a range of residual stresses and interfacial Recent calculations suggest that such pullout would be strongly roughnesses. Sliding occurred over a debond crack at the suppressed in fully dense oxide composites by misfit stresses fiber-coating interface when the sliding displacement and generated during sliding of fibers with rough interfaces or with surface roughness were relatively small. At large sliding minor fluctuations in diameter. For given strain mismatch, these displacements with relatively rough interfaces, the monazite misfit stresses are expected(assuming elastic accommodation)to coatings were deformed extensively by fracture, dislocations, be larger in composites with oxide interphases than in composites nd occasional twinning whereas the fibers were undamaged with turbostratic carbon or boron nitride interphases, which have Dense, fine-grained areas (10 nm grain size) resembling re- low transverse elastic modulus. However the misfit stresses could crystallized microstructures were also observed in the most heavily deformed regions of the coatings. Frictional heating such microstructures at low temperature are discussed, and a different thermal expansion coefficients th matrix and fibers of during sliding is assessed. Potential mechanisms for forming esidual thermal stresses in systems In this study, we investigate the debonding and sliding behavior radiation damage. The ability of La-monazite to undergo both of four La-monazite coated fibers le-crystal alumina and debonding and plastie deformation relatively easily at low mullite, directionally solidified eutectics of Al,O/YAG, and temperatures may enable its use as a composite interface. A,,/Y-ZrO2), chosen to provide different residual stress states and interface morphology. The coated fibers were surrounded with a matrix of polycrystalline Al,O3. Debonding and sliding were assessed using indentation fracture and pushout techniques. Dam R ARE-EARTH orthophosphates(monazite and xenotime)are of e in the coating, including plastic deformation, was identified by rest for fiber-matrix interphases that enable interfacial scanning and transmission electron microscopy(SEM and TEM) debonding and damage tolerance in oxide ites.- They are refractory materials(LaPO4 melting point, 2070C), comp Il. Experimental Procedure ible in high-temperature oxidizing environments with many oxides Four different single crystal or directionally solidified eutectic for future development as fibers and matrixes. They are also oxide fibers, grown at NASA Glenn by a laser-heated float zone GPa). Studies of several combinations of oxides and rare-earth rhabdophane(hydrated LaPO4). The coated fibers were embedded phosphates (LaPO4-Al2O3, LaPO2-ZrO2, CePO2-ZrO2, YPO4- in a-alumina powder(AKP50, Sumitomo Chemicals, Tokyo, AL2O3, and NdPO2-Al2O3)have shown that the oxide-phosphate apan)and hot pressed in graphite dies for I h at 1400.C Uncoated interfacial bond is sufficiently weak that debonding occurs when fibers were included in the same specimen for reference. The fibers ever a crack approaches an interface from within the phos- were arranged in rows within the one hot-pressed disk, with separation between fibers "2 mm, thus ensuring identical process- AL, O, system. Other studies have shown that debonding and onditions for all fibers. In an earlie dy.' the same rhabdophane slurry yielded pure La-monazite, with no excess um or pensive spectroscopy(EDS)analysis of the monazite or by reaction R. Naslain -contributing editor of the monazite with sapphire fibers after long-term heat treatment The fibers had different surface textures and coefficients, thus allowing assessment of the ef No. 187143. Received March I l morphology and residual stress on debonding of nisms. The fibers were as follows (1) Directionally solidified Al,O,/ZrO, eutectic fibe by the U.S. Air Force Office tific Research, under Contract No. F4 Contract No. NCC3-372 and Space Administration (NASA), under two-phase microstructure of alumina and cubic zirconia(stabilized with Y2O3). Dimensions of the individual phases were -0.5 um Member, American Ceramic Society. The starting compos sIton o f the feed rod was 60.8 mol% Al,O3: ckwell Scientific Air Force Research Laboratory, Materials Directorate, Wright-Patterson AFB 39.2 mol% ZrO2(9.5 mol%Y,O3) with purity levels 99.995%OI better. X-ray diffractometry(XRD) and SEM/TEM analysisInfluence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite Interphases Janet B. Davis,† Randall S. Hay,* ,‡ David B. Marshall,* ,† Peter E. D. Morgan,† and Ali Sayir* ,§ Rockwell Scientific, Thousand Oaks, California 91360 Air Force Research Laboratory, Materials Directorate, Wright-Patterson AFB, Ohio 45433 NASA–Glenn Research Center/Case Western Reserve University, Cleveland, Ohio 44135 Room-temperature debonding and sliding of fibers coated with La-monazite is assessed using a composite with a polycrystal￾line alumina matrix and fibers of several different single crystal (mullite and sapphire) and directionally solidified eutectic (Al2O3/Y3Al5O12 and Al2O3/Y-ZrO2) compositions. These fibers provide a range of residual stresses and interfacial roughnesses. Sliding occurred over a debond crack at the fiber-coating interface when the sliding displacement and surface roughness were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations, and occasional twinning, whereas the fibers were undamaged. Dense, fine-grained areas (10 nm grain size) resembling re￾crystallized microstructures were also observed in the most heavily deformed regions of the coatings. Frictional heating during sliding is assessed. Potential mechanisms for forming such microstructures at low temperature are discussed, and a parallel is drawn with the known resistance of monazite to radiation damage. The ability of La-monazite to undergo both debonding and plastic deformation relatively easily at low temperatures may enable its use as a composite interface. I. Introduction R ARE-EARTH orthophosphates (monazite and xenotime) are of interest for fiber-matrix interphases that enable interfacial debonding and damage tolerance in oxide composites.1–11 They are refractory materials (LaPO4 melting point, 2070°C),12 compat￾ible in high-temperature oxidizing environments with many oxides that are either currently available as reinforcing fibers or of interest for future development as fibers and matrixes. They are also relatively soft for such refractory materials (LaPO4 hardness, 5GPa).1 Studies of several combinations of oxides and rare-earth phosphates (LaPO4–Al2O3, LaPO4–ZrO2, CePO4–ZrO2, YPO4– Al2O3, and NdPO4–Al2O3) have shown that the oxide-phosphate interfacial bond is sufficiently weak that debonding occurs when￾ever a crack approaches an interface from within the phos￾phate.1,13–15 The most detailed studies have involved the LaPO4– Al2O3 system. Other studies have shown that debonding and sliding occur in fiber pushout tests with model composites con￾sisting of LaPO4-coated single crystal fibers of Al2O3 and Y3Al5O12 (YAG) in polycrystalline Al2O3 matrices.1,16 Damage-tolerant behavior in ceramic composites requires slid￾ing and pullout of fibers in addition to interfacial debonding. Recent calculations suggest that such pullout would be strongly suppressed in fully dense oxide composites by misfit stresses generated during sliding of fibers with rough interfaces or with minor fluctuations in diameter.17 For given strain mismatch, these misfit stresses are expected (assuming elastic accommodation) to be larger in composites with oxide interphases than in composites with turbostratic carbon or boron nitride interphases, which have low transverse elastic modulus. However, the misfit stresses could potentially be reduced by plastic deformation of the interphase. The higher elastic modulus in oxide interphases also causes larger residual thermal stresses in systems with matrix and fibers of different thermal expansion coefficients. In this study, we investigate the debonding and sliding behavior of four La-monazite coated fibers (single-crystal alumina and mullite, directionally solidified eutectics of Al2O3/YAG, and Al2O3/Y-ZrO2), chosen to provide different residual stress states and interface morphology. The coated fibers were surrounded with a matrix of polycrystalline Al2O3. Debonding and sliding were assessed using indentation fracture and pushout techniques. Dam￾age in the coating, including plastic deformation, was identified by scanning and transmission electron microscopy (SEM and TEM). II. Experimental Procedure Four different single crystal or directionally solidified eutectic oxide fibers, grown at NASA Glenn by a laser-heated float zone technique,18,19 were coated with LaPO4 by dipping in a slurry of rhabdophane (hydrated LaPO4). The coated fibers were embedded in
-alumina powder (AKP50, Sumitomo Chemicals, Tokyo, Japan) and hot pressed in graphite dies for 1 h at 1400°C. Uncoated fibers were included in the same specimen for reference. The fibers were arranged in rows within the one hot-pressed disk, with separation between fibers 2 mm, thus ensuring identical process￾ing conditions for all fibers. In an earlier study,3 the same rhabdophane slurry yielded pure La-monazite, with no excess lanthanum or phosphorus being detectable either by energy dis￾persive spectroscopy (EDS) analysis of the monazite or by reaction of the monazite with sapphire fibers after long-term heat treatment (200 h at 1600°C). The fibers had different surface textures and thermal expansion coefficients, thus allowing assessment of the effects of interfacial morphology and residual stress on debonding and sliding mecha￾nisms. The fibers were as follows: (1) Directionally solidified Al2O3/ZrO2 eutectic fibers with a two-phase microstructure of alumina and cubic zirconia (stabilized with Y2O3).20 Dimensions of the individual phases were 0.5 m. The starting composition of the feed rod was 60.8 mol% Al2O3; 39.2 mol% ZrO2 (9.5 mol% Y2O3) with purity levels 99.995% or better. X-ray diffractometry (XRD) and SEM/TEM analysis did R. Naslain—contributing editor Manuscript No. 187143. Received March 11, 2002; approved October 1, 2002. Funding for this work at Rockwell was provided by the U.S. Air Force Office of Scientific Research, under Contract Nos. F49620-96-C-0026 and F49620-00-C-0010. Work at NASA on development of new directionally solidified fibers were supported by the U.S. Air Force Office of Scientific Research, under Contract No. F49620-00- 1-0048 and the National Aeronautics and Space Administration (NASA), under Contract No. NCC3-372. *Member, American Ceramic Society. † Rockwell Scientific. ‡ Air Force Research Laboratory, Materials Directorate, Wright-Patterson AFB. § NASA–Glenn Research Center/Case Western Reserve University. J. Am. Ceram. Soc., 86 [2] 305–16 (2003) 305 journal