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 analysis
Influence 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 polycrystalline 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 recrystallized 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 compatible 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 whenever a crack approaches an interface from within the phosphate.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 consisting of LaPO4-coated single crystal fibers of Al2O3 and Y3Al5O12 (YAG) in polycrystalline Al2O3 matrices.1,16 Damage-tolerant behavior in ceramic composites requires sliding 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. Damage 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 processing 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 dispersive 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 mechanisms. 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
Journal of the American Ceramic SocieryDavis et al. Vol 86. No. 2 not show any evidence for a third phase, indicating that all the of --200 um Thermal mismatch during cooling of the composite Y,O, was in solid solution in the zro,. The surfaces caused tensile radial stresses normal to the fiber surface(Table D) were rough on the scale of the microstructure(Fig. I(a)). The fiber (2) Directionally solidified Al,Oy/YAG eutectic fibers, wi diameters were "100 um with fluctuations of - 2 um over lengths two-phase microstructure of dimensions 0.5 um and surface roughness on the scale of the microstructure(Fig. I(b). The fiber diameters were 100 um, with fluctuations of <I um over lengths of -l mm. Thermal mismatch stresses were of the same sign as for the Al,O,/ZrO, fibers, but were smaller in magnitude able D) (3) Mullite single-crystal fibers formed from a so high-purity(99.99%)p na powder (CERAC Milwaukee, Wn)and 99.99% pure SiO,(Alfa Products, Ward Hil MA), which gave 2: 1 mullite as described in Ref. 19. In the as-grown condition, the fibers had smooth surfaces but relative large fluctuations in diameter(50 +5 um, Fig. I(c)) with perio 100 um. Thermal mismatch caused large compressive radial stress in the coating and at the fiber-coating and coating-matrix interfaces, with tensile circumferential stress in the coating and matrix(Table 1). (4) Sapphire fibers, which had smooth surfaces(as-grown) and relatively uniform diameter (100 I um). These wer included for comparison with previous studies of this system. ,3 2um All residual stresses except the circumferential(and axial)tension in the coating are small The hot-pressed disk was cut into slices(thickness.3-2 mm) normal to the fibers. The surfaces of the slices were polished using diamond paste and some of the polished slices were thermally etched. The thicker slices were used for indentation cracking experiments, which involved placing Vickers indentations(10 kg AL, O,ZrO,(eutectic) load) in the polycrystalline alumina matrix near the fibers. The indenter was oriented so that one of the median/radial cracks grew toward the fiber to test for interfacial debonding. The thinner slices ■UU■ were used for fiber pushout experiments, which involved loadin a flat punch( truncated Vickers indenter)onto the end of each fiber while the slice was supported in a fixture with a gap beneath the fiber. Some specimens were fractured after the pushout test to separate the debonded interface. The indented and pushed out specimens were examined by optical microscopy and sEM Specimens used for fiber pushout were also sectioned parallel and perpendicular to the fiber axes and examined by TEM(Model CM20 FEG operating at 200 kV, Phillips, Eindhoven, Nether- lands) to allow identification of damage within the LapOa coatin caused by debonding and sliding. Four Al,O YAG fibers were examined in the parallel section; one mullite and one Al,O/ZrO2 were examined in the axial section. The TEM foils were prepared by impregnating the specimens with epoxy, tripod polishing to a thickness of 10 um, followed by ion milling(Model 691 operating at 4.5 kv, Gatan, Pleasanton, CA) (I) Microstructural Observations All the coated fibers were LapOa and a fully dense matrix of polycrystalline Al,O3- Defor- mation during hot pressing caused the coating thickness to be YAG/AL O3(eutectic larger along the sides of the fibers(-5 um) than at the top and bottom(-I um). No reactions were observed between the LaPO a and any of the fibers, although a few isolated elongated La- magnetoplumbite (LaAl1O1g)grains were observed along the coating-matrix interface (perhaps the result of alkali or divalent impurities in the matrix, which are known to assist formation of rare-earth magetoplumbite-like structures). Despite the presence Mullite (single crystal) of substantial tensile residual stresses in all the LapOa coatings (300-400 MPa, Table D), no evidence of cracking was detected by SEM examination of polished or thermally etched cross sections(although fine-scale through-thickness coating cracks were observed in thin TEM foils of other similar composites). The 1. SEM micrographs of fiber surfaces: (a)Al,, /ZrO2 eutectic fiber, grain sizes were -0.5-1 um in the monazite and -2-10 um in the (b)Al2O3/YAG eutectic fiber, and (c)mullite single crystal fiber alumina matrix
not show any evidence for a third phase, indicating that all the Y2O3 was in solid solution in the ZrO2. The surfaces of the fibers were rough on the scale of the microstructure (Fig. 1(a)). The fiber diameters were 100 m with fluctuations of 2 m over lengths of 200 m. Thermal mismatch during cooling of the composite caused tensile radial stresses normal to the fiber surface (Table I). (2) Directionally solidified Al2O3/YAG eutectic fibers,21 with a two-phase microstructure of dimensions 0.5 m and surface roughness on the scale of the microstructure (Fig. 1(b)). The fiber diameters were 100 m, with fluctuations of 1 m over lengths of 1 mm. Thermal mismatch stresses were of the same sign as for the Al2O3/ZrO2 fibers, but were smaller in magnitude (Table I). (3) Mullite single-crystal fibers formed from a source rod of high-purity (99.99%) polycrystalline alumina powder (CERAC, Milwaukee, WI) and 99.99% pure SiO2 (Alfa Products, Ward Hill, MA), which gave 2:1 mullite as described in Ref. 19. In the as-grown condition, the fibers had smooth surfaces but relatively large fluctuations in diameter (50 5 m, Fig. 1(c)) with period 100 m. Thermal mismatch caused large compressive radial stress in the coating and at the fiber-coating and coating-matrix interfaces, with tensile circumferential stress in the coating and matrix (Table I). (4) Sapphire fibers, which had smooth surfaces (as-grown) and relatively uniform diameter (100 1 m). These were included for comparison with previous studies of this system.1,3 All residual stresses except the circumferential (and axial) tension in the coating are small. The hot-pressed disk was cut into slices (thickness 0.3–2 mm) normal to the fibers. The surfaces of the slices were polished using diamond paste and some of the polished slices were thermally etched. The thicker slices were used for indentation cracking experiments, which involved placing Vickers indentations (10 kg load) in the polycrystalline alumina matrix near the fibers. The indenter was oriented so that one of the median/radial cracks grew toward the fiber to test for interfacial debonding. The thinner slices were used for fiber pushout experiments, which involved loading a flat punch (truncated Vickers indenter) onto the end of each fiber, while the slice was supported in a fixture with a gap beneath the fiber. Some specimens were fractured after the pushout test to separate the debonded interface. The indented and pushed out specimens were examined by optical microscopy and SEM. Specimens used for fiber pushout were also sectioned parallel and perpendicular to the fiber axes and examined by TEM (Model CM20 FEG operating at 200 kV, Phillips, Eindhoven, Netherlands) to allow identification of damage within the LaPO4 coating caused by debonding and sliding. Four Al2O3/YAG fibers were examined in the parallel section; one mullite and one Al2O3/ZrO2 were examined in the axial section. The TEM foils were prepared by impregnating the specimens with epoxy, tripod polishing to a thickness of 10 m, followed by ion milling (Model 691 operating at 4.5 kV, Gatan, Pleasanton, CA).26 III. Results (1) Microstructural Observations All the coated fibers were surrounded with a continuous layer of LaPO4 and a fully dense matrix of polycrystalline Al2O3. Deformation during hot pressing caused the coating thickness to be larger along the sides of the fibers (5 m) than at the top and bottom (1 m). No reactions were observed between the LaPO4 and any of the fibers, although a few isolated elongated Lamagnetoplumbite (LaAl11O19) grains were observed along the coating-matrix interface (perhaps the result of alkali or divalent impurities in the matrix, which are known to assist formation of rare-earth magetoplumbite-like structures3 ). Despite the presence of substantial tensile residual stresses in all the LaPO4 coatings (300–400 MPa, Table I), no evidence of cracking was detected by SEM examination of polished or thermally etched cross sections (although fine-scale through-thickness coating cracks were observed in thin TEM foils of other similar composites). The grain sizes were 0.5–1 m in the monazite and 2–10 m in the alumina matrix. Fig. 1. SEM micrographs of fiber surfaces: (a) Al2O3/ZrO2 eutectic fiber, (b) Al2O3/YAG eutectic fiber, and (c) mullite single crystal fiber. 306 Journal of the American Ceramic Society—Davis et al. Vol. 86, No. 2
February 2003 Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monacite 307 Table 1. Representative Residual Stresses in Composites of Monazite-Coated Fibers in a Polycrystalline Al, O3 Matrix Residual stress(MPa Stress component AL,O,/YAG AL,,/ZrO2 Radial (coating/fiber) Radial(matrix/coating) Circumferential(coating) 300 290 Axial(fiber) 1160 240 Values in this table are intended only as rough guide for relative stresses. They were calculated using a lowing Young's moduli and thermal Al2 YAG(350GPa8.5×10-6°c- YAG/AI fiber LaPo YAG/Al2 O3 fiber ● 2um 2um LaPo Fiber YAG/AlO3 Fig. 2. SEM micrographs showing ions of indentation cracks with Al, O,/YAG eutectic fibers:(a)uncoated fiber in alumina matrix(indentation located below region shown);(b) fiber coated with LaPO (indentation located out of field of views, as indicated in(d)); (c)same fiber as in(b) but showing egion further along the debonded interface(arrows indicate magnitude of sliding displacement across debond crack); (d) schematic showing locations of(b)
Fig. 2. SEM micrographs showing interactions of indentation cracks with Al2O3/YAG eutectic fibers: (a) uncoated fiber in alumina matrix (indentation located below region shown); (b) fiber coated with LaPO4 (indentation located out of field of views, as indicated in (d)); (c) same fiber as in (b) but showing region further along the debonded interface (arrows indicate magnitude of sliding displacement across debond crack); (d) schematic showing locations of (b) and (c). Table I. Representative† Residual Stresses in Composites of Monazite-Coated Fibers in a Polycrystalline Al2O3 Matrix Stress component Residual stress (MPa) Sapphire Mullite Al2O3/YAG Al2O3/ZrO2 Radial (coating/fiber) 15 720 130 240 Radial (matrix/coating) 25 630 140 240 Circumferential (coating) 300 420 290 280 Axial (fiber) 7 1160 240 420 † Values in this table are intended only as rough guide for relative stresses. They were calculated using a coaxial cylinder analysis,22 assuming a temperature change of T 1000°C, coating thickness of 2 m, zero volume fraction of fibers, and the following Young’s moduli and thermal expansion coefficients (nominal isotropic, temperature-independent values): polycrystalline Al2O3 (400 GPa, 8 106 °C1 ); sapphire (400 GPa, 8 106 °C1 ); mullite (200 GPa, 4 106 °C1 ); Al2O3/ZrO2 (300 GPa, 9 106 °C1 ); and Al2O3/YAG (350 GPa, 8.5 106 °C1 ).23–25 February 2003 Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite 307
Journal of the American Ceramic SocieryDavis et al. VoL 86. No. 2 ( Interfacial Debonding LaPO, interface(-4.5 J/m). It is noteworthy that the fibers were cted all the fibers from penetration of protected from cracking even when the contact area of the Vickers indentation cracks, whereas uncoated fibers were always pene indentation was close enough to the fiber to overlap the coating AL,O,/YAG, AL, O,ZrO, and (Fig 3(b). In that case, the residual stress from the indentatio mullite fibers in Figs. 2-4. The indentation cracks generally (compressive normal to the interface, tensile on the prospective xtended from the matrix into the lapo, coatings then arrested crack plane into the fiber) would tend to inhibit debonding and nd caused debonding at the coating/fiber interface. In a few cases favor fiber penetration. with the AL,O,/Zro, fibers, debonding occurred at both interfaces The interfacial roughnesses for both of the eutectic fibers were (matrix/ coating and coating/fiber). The former response was ob- similar to the surface roughnesses of the as-formed fibers, with served previously with coated sapphire fibers and was shown to be amplitude"100-300 nm and period 500 nm(Figs. 2(a) and consistent with the debond criterion of He and Hutchinson- and 3(a)). This roughness amplitude is greater than that of the inter the measured fracture toughnesses of the fibers, coating, and faces at the single-crystal mullite and sapphire fibers. The initially interface.Although the fracture toughnesses of the YAG/ApoA smooth single crystal fibers developed cusps during hot pressing d mullite/LaPO4 interfaces have not been measured, the present where grain boundaries of the monazite coating intersected the observations suggest that they are similar to that of the alumina/ fiber surface. Measurements of the cusp profiles on sapphire fibers Indentation ALO / ZrO. lber Al2O3 Al2O3/Zro2 Al2o 2 um 20 um Fig. 3. SEM micrographs showing interactions of indentation cracks with Al,O, eutectic fibers:(a)uncoated fiber in alumina matrix(indentation located below region shown);(b) fiber coated with LapO4(indentation located at top of field of view) LaPo (b) Al2O3 LaPO Mullite Mullite 21 Fig. 4. SEM micrographs showing interaction of indentation crack with single-crystal mullite fiber(coated with LaPO4, in alumina matrix):(a) of indentation crack with interface and debonding(indentation located above region shown), (b) same fiber as in(a) but showing region further to the right along the debonded interface (arrows indicate magnitude of sliding displacement across debond crack)
(2) Interfacial Debonding The LaPO4 coatings protected all the fibers from penetration of indentation cracks, whereas uncoated fibers were always penetrated. Examples are shown for the Al2O3/YAG, Al2O3/ZrO2 and mullite fibers in Figs. 2–4. The indentation cracks generally extended from the matrix into the LaPO4 coatings then arrested and caused debonding at the coating/fiber interface. In a few cases with the Al2O3/ZrO2 fibers, debonding occurred at both interfaces (matrix/coating and coating/fiber). The former response was observed previously with coated sapphire fibers and was shown to be consistent with the debond criterion of He and Hutchinson27 and the measured fracture toughnesses of the fibers, coating, and interface.1 Although the fracture toughnesses of the YAG/LaPO4 and mullite/LaPO4 interfaces have not been measured, the present observations suggest that they are similar to that of the alumina/ LaPO4 interface (4.5 J/m2 ). It is noteworthy that the fibers were protected from cracking even when the contact area of the Vickers indentation was close enough to the fiber to overlap the coating (Fig. 3(b)). In that case, the residual stress from the indentation (compressive normal to the interface, tensile on the prospective crack plane into the fiber) would tend to inhibit debonding and favor fiber penetration. The interfacial roughnesses for both of the eutectic fibers were similar to the surface roughnesses of the as-formed fibers, with amplitude 100–300 nm and period 500 nm (Figs. 2(a) and 3(a)). This roughness amplitude is greater than that of the interfaces at the single-crystal mullite and sapphire fibers. The initially smooth single crystal fibers developed cusps during hot pressing where grain boundaries of the monazite coating intersected the fiber surface. Measurements of the cusp profiles on sapphire fibers Fig. 3. SEM micrographs showing interactions of indentation cracks with Al2O3/ZrO2 eutectic fibers: (a) uncoated fiber in alumina matrix (indentation located below region shown); (b) fiber coated with LaPO4 (indentation located at top of field of view). Fig. 4. SEM micrographs showing interaction of indentation crack with single-crystal mullite fiber (coated with LaPO4, in alumina matrix): (a) intersection of indentation crack with interface and debonding (indentation located above region shown); (b) same fiber as in (a) but showing region further to the right along the debonded interface (arrows indicate magnitude of sliding displacement across debond crack). 308 Journal of the American Ceramic Society—Davis et al. Vol. 86, No. 2
February 2003 Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monacite 309 by atomic force microscopy (ATM) have been reported else- fiber surface. In some areas, this smeared layer was overlaid with where28,29 The cusp heights were typically 50 nm, and the a less dense, coarser grained agglomeration of angular monazite ular distortions of the surface were small(=20%). The cusps or particles of diameter 50-100 m(Fig. 7), suggestive f cata- the mullite surfaces were very similar clastic flow, a process involving repeated microfracture and Some insight into the effect of interfacial roughness on fiber fine-particle transport. Similar features(intense deformation sliding and pullout can be gained from the observations of Figs. fine crystallites, and agglomerates of angular particles)were 2-4. As the debond grows around the circumference of the fiber, observed in monazite debris (irregularly shaped balls, -100-500 the loading on the crack tip due to the indentation stress field is nm diameter) in the debond crack. initially mostly shear(although the loading eventually changes to C) Mullite Fibers: Sliding of the mullite fibers occurred tension if the crack grows sufficiently ) Because fiber pullout also predominantly at the fiber-coating interface SEM observations of involves shear loading of a debond crack, the initial region of separated interfaces showed plastic deformation of the LaPOa growth of the deflected cracks in Figs 2-4 should be representa- coating where the varying fiber diameter caused compression tive of the behavior during the corresponding stage of pullout. the coating during sliding, as depicted in region B of Fig. 9.(Note In all cases. the initial debond crack followed the fiber-matrn that the sliding displacements are smaller than the period of the interface, even when the interface was rough. For the mullite fibers diameter fluctuations and larger than the spacing of cusps associ- (Fig. 4)the sliding displacement of the debond crack surfaces ated with grain boundaries in the LaPO, coating )Where sliding the coating-fiber interface(region A in Fi is smaller than the average spacing between the interfacial cusps 9), the separated interface was similar to that of the sapphire fiber, (-600 nm). Sliding caused separation of the debonded surfaces to with grain-boundary cusps, clean separation, and no damage in the ccommodate their misfit (Fig. 4(b)), despite the constrained fiber or the coating configuration with large residual compressive normal stress(-700 MPa, Table D). The misfit was apparently accommodated by elastic diameter fluctuation by TEM was difficult, because only limited strains. with no irreversible deformation of the mullite fiber or the areas were observed. Nevertheless. some trends are evident. LaPO4 coating discernable by SEM. In contrast, sliding of the Deformation was distributed, ofte eutectic fibers caused extensive damage in the LaPO coating(Fig. entire coating thickness(Figs. 10 and 11), rather than being 2(c), without discernable damage in the fibers. The dama localized in a thin layer adjacent to the fiber as for the Al,O3/YAG included cracks across the full width of the coating. aligned at 45 to the interface on planes of maximum tension, similar to undeformed, whereas in others plastic deformation was confined to enVious o pservations of cracking in layers of LaPOa sandwiched an isolated grain(Fig. 10). The region of Fig. 10 was thought to between polycrystalline Al 2O3. More intense local damage is have experienced tension during sliding(as in Region A, Fig 9) evident at individual asperities, as in Fig. 2(c). The damage although the correlation with fiber diameter is uncertain because included cracking and fine lamellar features, which could be some of the fiber adjacent to the debond crack was removed during racks or twins ion milling. Extensive microcracking was distributed throughout the coating, often at x+45 to the fiber surface(Figs. 10 and 11) 3) Fiber pushout In regions of coating inferred to have been compressed during sliding(as in region B of Fig. 9), almost the entire coating was All the fibers debonded during the pushout experiments. Sliding microcracked and plastically deformed (Fig. 11). Extensive dislo- curred unstably over distances of "5-10 um at a critical load between 10-20 N. The average shear stress (load divided by fiber cation plasticity was evident, with variations in density from grai urface area)at the critical load was 130 10 MPa for the to grain. Some grains were twinned parallel to the fiber/matrix sapphire fibers, 200+ 20 MPa for the mullite fibers, 190+20 interface(Fig. ID), the orientation of maximum shear stress due to MPa for the Al2O3/YAG fibers, and 255 t 30 MPa for the pushout of the fiber. Microcracking at -45 to the fiber surface AL,O/ZrO, fibers was extensive, with crack spacings as small as "50 nm and ar (A) Sapphire Fibers: Sliding of the sapphire fiber occurred abundance of planar segments consistent with cleavage on, (100), at the fiber-coating interface, as reported previously. Grain (010), and (001), as reported previously. There was some tendency for cracks oriented normal to the maximum tensile stress boundary cusps were observed al he separated interfaces by SEM and AFM, although no damage was visible in either the fiber (northwest to southeast in Fig. 11)to be longer and have greater opening displacements than other cracks; however, the trends are subjective and the possibility of a sample preparation artifact B)Al2O,YAG and AlyOyzrO, Fibers: Extensive wear cannot be ruled out tracks were observed in the ApoA coatings on both eutectic fibers, indicating that sliding involved plastic deformation (Fig. 5). Sliding occurred mostly adjacent to the fiber-matrix interfac although smeared fragments of the LaPO coating remained on the (1) Effects of Residual Stress ber surface. In some regions(such as Fig. 5), sliding occurred near the matrix-coating interface The residual stresses noted in Table I might be expected to TEM observations from a typical specimen containing influence interfacial debonding. Therefore, it is necessary to ut Al,O3/YAG fiber are shown in Figs. 6-8. Slidi establish whether the fracture behavior in the model experiments along a debond crack between the ApoA coating and the fiber eported here is representative of that in real composites, given the most regions, a thin layer of the lapA coating within -100-300 differences in residual stress states and crack orientation nm of the fiber was heavily deforme In the analysis of He et al, 2 the presence of residual stresses igs 6-8). The intensity of shifts the debond criterion by an amount that depends on the deformation decreased with distance from the debond crack, with ions more than-500 nm from the fiber being undeformed. The parameters mp and nd Deformation in the ApoA consisted of tangled dislocation K lamellar features resembling twins, microcracks, and regions of densely packed fine crystallites(diameter as small as 10 nm) that where o and od are the residual stresses normal to potential crack resemble recrystallized microstructures(Fig. 6). The density of paths along the interface or into the fiber, K is the applied stress dislocations varied from grain to grain and generally decreased intensity factor for the incident crack, and a is a defect size. For a with distance from the debond crack. The nano-crystalline regions crack approaching the fiber on a radial plane, as in the indentation were within -50-100 nm of the debond crack. In one region, there cracking experiments of Section Il(2), the residual stresses o and was no deformation on the monazite side of the debond crack, but ca (radial and hoop stresses at the fiber surface) are equal, so the a thin layer of dense nano-crystalline monazite was smeared on the debond condition is not affected by the residual stresses
by atomic force microscopy (ATM) have been reported elsewhere.28,29 The cusp heights were typically 50 nm, and the angular distortions of the surface were small (20°). The cusps on the mullite surfaces were very similar. Some insight into the effect of interfacial roughness on fiber sliding and pullout can be gained from the observations of Figs. 2–4. As the debond grows around the circumference of the fiber, the loading on the crack tip due to the indentation stress field is initially mostly shear (although the loading eventually changes to tension if the crack grows sufficiently). Because fiber pullout also involves shear loading of a debond crack, the initial region of growth of the deflected cracks in Figs. 2–4 should be representative of the behavior during the corresponding stage of pullout. In all cases, the initial debond crack followed the fiber-matrix interface, even when the interface was rough. For the mullite fibers (Fig. 4) the sliding displacement of the debond crack surfaces (250 nm, i.e., opening displacement of initial indentation crack) is smaller than the average spacing between the interfacial cusps (600 nm). Sliding caused separation of the debonded surfaces to accommodate their misfit (Fig. 4(b)), despite the constrained configuration with large residual compressive normal stress (700 MPa; Table I). The misfit was apparently accommodated by elastic strains, with no irreversible deformation of the mullite fiber or the LaPO4 coating discernable by SEM. In contrast, sliding of the eutectic fibers caused extensive damage in the LaPO4 coating (Fig. 2 (c)), without discernable damage in the fibers. The damage included cracks across the full width of the coating, aligned at 45° to the interface on planes of maximum tension, similar to previous observations of cracking in layers of LaPO4 sandwiched between polycrystalline Al2O3. 1 More intense local damage is evident at individual asperities, as in Fig. 2(c). The damage included cracking and fine lamellar features, which could be cracks or twins. (3) Fiber Pushout All the fibers debonded during the pushout experiments. Sliding occurred unstably over distances of 5–10 m at a critical load between 10–20 N. The average shear stress (load divided by fiber surface area) at the critical load was 130 10 MPa for the sapphire fibers, 200 20 MPa for the mullite fibers, 190 20 MPa for the Al2O3/YAG fibers, and 255 30 MPa for the Al2O3/ZrO2 fibers. (A) Sapphire Fibers: Sliding of the sapphire fiber occurred at the fiber-coating interface, as reported previously.1 Grainboundary cusps were observed along the separated interfaces by SEM and AFM, although no damage was visible in either the fiber or the coating. (B) Al2O3/YAG and Al2O3/ZrO2 Fibers: Extensive wear tracks were observed in the LaPO4 coatings on both eutectic fibers, indicating that sliding involved plastic deformation (Fig. 5). Sliding occurred mostly adjacent to the fiber-matrix interface, although smeared fragments of the LaPO4 coating remained on the fiber surface. In some regions (such as Fig. 5), sliding occurred near the matrix-coating interface. TEM observations from a typical specimen containing a pushed out Al2O3/YAG fiber are shown in Figs. 6–8. Sliding occurred along a debond crack between the LaPO4 coating and the fiber. In most regions, a thin layer of the LaPO4 coating within 100–300 nm of the fiber was heavily deformed (Figs. 6–8). The intensity of deformation decreased with distance from the debond crack, with regions more than 500 nm from the fiber being undeformed. The Al2O3/YAG fiber was also undamaged. Deformation in the LaPO4 consisted of tangled dislocations, lamellar features resembling twins, microcracks, and regions of densely packed fine crystallites (diameter as small as 10 nm) that resemble recrystallized microstructures (Fig. 6). The density of dislocations varied from grain to grain and generally decreased with distance from the debond crack. The nano-crystalline regions were within 50–100 nm of the debond crack. In one region, there was no deformation on the monazite side of the debond crack, but a thin layer of dense nano-crystalline monazite was smeared on the fiber surface. In some areas, this smeared layer was overlaid with a less dense, coarser grained agglomeration of angular monazite particles of diameter 50–100 nm (Fig. 7), suggestive of cataclastic flow, a process involving repeated microfracture and fine-particle transport.30 Similar features (intense deformation, fine crystallites, and agglomerates of angular particles) were observed in monazite debris (irregularly shaped balls, 100–500 nm diameter) in the debond crack. (C) Mullite Fibers: Sliding of the mullite fibers occurred predominantly at the fiber-coating interface. SEM observations of separated interfaces showed plastic deformation of the LaPO4 coating where the varying fiber diameter caused compression of the coating during sliding, as depicted in region B of Fig. 9. (Note that the sliding displacements are smaller than the period of the diameter fluctuations and larger than the spacing of cusps associated with grain boundaries in the LaPO4 coating.) Where sliding caused tension across the coating-fiber interface (region A in Fig. 9), the separated interface was similar to that of the sapphire fiber, with grain-boundary cusps, clean separation, and no damage in the fiber or the coating. Direct correlation of the changes in coating damage with fiber diameter fluctuation by TEM was difficult, because only limited areas were observed. Nevertheless, some trends are evident. Deformation was distributed, often nonuniformly, through the entire coating thickness (Figs. 10 and 11), rather than being localized in a thin layer adjacent to the fiber as for the Al2O3/YAG fiber. In some places, the monazite adjacent to the fiber was undeformed, whereas in others plastic deformation was confined to an isolated grain (Fig. 10). The region of Fig. 10 was thought to have experienced tension during sliding (as in Region A, Fig. 9), although the correlation with fiber diameter is uncertain because some of the fiber adjacent to the debond crack was removed during ion milling. Extensive microcracking was distributed throughout the coating, often at 45° to the fiber surface (Figs. 10 and 11). In regions of coating inferred to have been compressed during sliding (as in region B of Fig. 9), almost the entire coating was microcracked and plastically deformed (Fig. 11). Extensive dislocation plasticity was evident, with variations in density from grain to grain. Some grains were twinned parallel to the fiber/matrix interface (Fig. 11), the orientation of maximum shear stress due to pushout of the fiber. Microcracking at 45° to the fiber surface was extensive, with crack spacings as small as 50 nm and an abundance of planar segments consistent with cleavage on, (100), (010), and (001), as reported previously.31 There was some tendency for cracks oriented normal to the maximum tensile stress (northwest to southeast in Fig. 11) to be longer and have greater opening displacements than other cracks; however, the trends are subjective and the possibility of a sample preparation artifact cannot be ruled out. IV. Discussion (1) Effects of Residual Stress The residual stresses noted in Table I might be expected to influence interfacial debonding. Therefore, it is necessary to establish whether the fracture behavior in the model experiments reported here is representative of that in real composites, given the differences in residual stress states and crack orientations. In the analysis of He et al., 32 the presence of residual stresses shifts the debond criterion by an amount that depends on the parameters p and d: p pa1/ 2 K , d da1/ 2 K (1) where p and d are the residual stresses normal to potential crack paths along the interface or into the fiber, K is the applied stress intensity factor for the incident crack, and a is a defect size. For a crack approaching the fiber on a radial plane, as in the indentation cracking experiments of Section II(2), the residual stresses p and d (radial and hoop stresses at the fiber surface) are equal, so the debond condition is not affected by the residual stresses. February 2003 Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite 309