December 1999 Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites 4 achieved. The final composite was received in the form of and heat-treated at 130.C for 5 min, followed by another 9 rectangular plates of 3 mm nominal thickness. The porosity of min at 180C. The samples were cut with a diamond saw, the composite was in the range 5%10% mechanically polished up to 100 um thick, and dimpled until hey perforated IlL. Experimental Techniques The in situ fiber strength at different temperatures was de termined for the Al,,-matrix composites from the size of the Parallelepiped-shaped samples(15 mm x 15 mm x 3 mm) mirror region on the fiber fracture surfaces. 13, 16, 7 To this ene were cut from the center of the composite plates and heat- tensile dog-bone-shaped samples were machined from th treated I h in air in an electric-resistance furnace. The heat plates. The central portion of each sample had a uniform width reatment temperatures for the AlO-matrix composite were of 4 mm and a length of 15 mm. The samples were tested in 800°,10001200°,andl400°C. In addition, one sample was tension, In aIr,at25°,800°,1000,and1200°C. All of the heat-treated at 1200.C for I h in an argon atmosphere, to samples were held at the test temperature for l h before tes ucidate the influence of the environment on the oxidation more details about the mechanical tests are found elsewhere processes. The ShC-N-matrix composite samples were heat The fracture surfaces of the broken samples were examined by treated at800°and1200° for 1 h in ai scanning electron microscopy(SEM; Model No 6300, JEOL All of the samples for the transmission electron microscopy and the fracture surfaces of approximately 130 fibers, selected (TEM) studies were prepared from the center of the parallel at random, were analyzed in each sample. Previous studie do, such a way that the fiber bundles were perpendicular to the values of in situ fiber strength, enough to obtain reliable epiped-shaped samples, by cutting square slices(600 um thick are surface. The slices were polished to 180 um thick and mounted with wax on an alumina rod 3 mm in diameter. The amples were dimpled down to 20 um and, finally, thinned by IV. Interface Degradation enter was perforated. The milled samples were examined by ()Ay OrAtrix Composite TEM (Model No 2000FX Il, JEOL, Tokyo, Japan). Com As shown in previous studies, O, II the porous Al2O3 matrix sitional analysis of the microstructural features was performed of the Al2O3-matrix composite filled the space between the by energy-dispersive spectroscopy, using probe sizes of-10- fibers, which were surrounded by the thick polycrystalline SiC 20nm. coating deposited by CVD. The thin BN layer(100 nm)be In order to ascertain whether the silicon oxides found in tween the fiber and the Sic external coating could be resolved les were actually silica or silicates, an SiO2 stan- only at higher magnification, by TEM (Fig. I(A)). No reaction dard was prepared, mixing equal volumes of SiO2 powder(0.8 zone was detected at the SiC/BN interface, in agreement with um average grain size, 99.9% purity) with a thermosetting he results of previous investigations. 8, 9 In contrast, a very resin(Araldite AT, Ciba Specialty Chemicals Corp, East Lan- thin region, of-10 nm, with slightly different contrast was sing, MI). The mixture was compacted in an aluminum tube detected at the BN/fiber interface. The energy spectra, obtained (A) ms8 08 0 00 0.5 1.2 S8N Fig. 1. (A) TEM photograph showing the BN coating between the nicalon fiber and the Sic external coating(carbon-rich interlayer region at ACMNMNiLYYMYW BN/fiber interface is marked with arrows);(B) energy spectrum corresponding to BN coating, (C)energy spectrum for BN/fiber interlayer.achieved. The final composite was received in the form of rectangular plates of 3 mm nominal thickness. The porosity of the composite was in the range 5%–10%. III. Experimental Techniques Parallelepiped-shaped samples (15 mm × 15 mm × 3 mm) were cut from the center of the composite plates and heat￾treated 1 h in air in an electric-resistance furnace. The heat￾treatment temperatures for the Al2O3-matrix composite were 800°, 1000°, 1200°, and 1400°C. In addition, one sample was heat-treated at 1200°C for 1 h in an argon atmosphere, to elucidate the influence of the environment on the oxidation processes. The Si–C–N-matrix composite samples were heat￾treated at 800° and 1200°C for 1 h in air. All of the samples for the transmission electron microscopy (TEM) studies were prepared from the center of the parallel￾epiped-shaped samples, by cutting square slices (600 mm thick) in such a way that the fiber bundles were perpendicular to the square surface. The slices were polished to 180 mm thick and mounted with wax on an alumina rod 3 mm in diameter. The samples were dimpled down to 20 mm and, finally, thinned by ion milling (milling conditions: 5 kV, 2.5 mA, 15°) until the center was perforated. The milled samples were examined by TEM (Model No. 2000FX II, JEOL, Tokyo, Japan). Compo￾sitional analysis of the microstructural features was performed by energy-dispersive spectroscopy, using probe sizes of ∼10– 20 nm. In order to ascertain whether the silicon oxides found in several samples were actually silica or silicates, an SiO2 stan￾dard was prepared, mixing equal volumes of SiO2 powder (0.8 mm average grain size, 99.9% purity) with a thermosetting resin (Araldite AT, Ciba Specialty Chemicals Corp., East Lan￾sing, MI). The mixture was compacted in an aluminum tube and heat-treated at 130°C for 5 min, followed by another 90 min at 180°C. The samples were cut with a diamond saw, mechanically polished up to 100 mm thick, and dimpled until they perforated. The in situ fiber strength at different temperatures was de￾termined for the Al2O3-matrix composites from the size of the mirror region on the fiber fracture surfaces.13,16,17 To this end, tensile dog-bone-shaped samples were machined from the plates. The central portion of each sample had a uniform width of 4 mm and a length of 15 mm. The samples were tested in tension, in air, at 25°, 800°, 1000°, and 1200°C. All of the samples were held at the test temperature for 1 h before testing. More details about the mechanical tests are found elsewhere.13 The fracture surfaces of the broken samples were examined by scanning electron microscopy (SEM; Model No. 6300, JEOL), and the fracture surfaces of approximately 130 fibers, selected at random, were analyzed in each sample. Previous studies demonstrated that this number is enough to obtain reliable values of in situ fiber strength.13,16 IV. Interface Degradation (1) Al2O3-Matrix Composite As shown in previous studies,10,11 the porous Al2O3 matrix of the Al2O3-matrix composite filled the space between the fibers, which were surrounded by the thick polycrystalline SiC coating deposited by CVD. The thin BN layer (∼100 nm) be￾tween the fiber and the SiC external coating could be resolved only at higher magnification, by TEM (Fig. 1(A)). No reaction zone was detected at the SiC/BN interface, in agreement with the results of previous investigations.18,19 In contrast, a very thin region, of ∼10 nm, with slightly different contrast was detected at the BN/fiber interface. The energy spectra, obtained Fig. 1. (A) TEM photograph showing the BN coating between the Nicalon fiber and the SiC external coating (carbon-rich interlayer region at BN/fiber interface is marked with arrows); (B) energy spectrum corresponding to BN coating; (C) energy spectrum for BN/fiber interlayer. December 1999 Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites 3495