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 matrixnot 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, Nether￾lands) 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. Defor￾mation 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 La￾magnetoplumbite (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