10 Vol. 80. No. 3 Il. Experimental Procedure load was applied using a constant crosshead speed of 0.5 The materials studied were Si-C-O Nicalon-fiber-reinforced mm/min. To elucidate the effect of applied stresses on the xidation behavior, an annealing experiment also was con- BMAS glass ceramics. The fibers were dual coated with Sic ducted at 600%C in air for 500 h with the flexural strength of the coating was applied to the fibers by Cvd using a proprietary annealed sample measured at room temperature precursor(3M Co., St. Paul, MN) chosen to give an approxi The fracture surfaces of composites that failed during the mate composition of 40 at. boron, 40 at.% atigue tests or fractured at room temperature after the fatigue 20 at. carbon. The bn coating exhibited a turbostratic struc tests were examined using a high-resolution scanning electron ture and was comprised of nanoscale crystallites. The oxygen croscopy(SEM)microscope(Model $4100, Hitachi, To content in the Sic and coating layers was measured apan)that had a field-emission gun and was equipped fo by Auger spectroscopy to be <3 at. % Composite panels energy-dispersive spectroscopy(EDS )capable of light-element (100 mm X 100 mm) were fabricated by hot pressing a layup detection Compositional analysis of the fracture surfaces was of0°/90° plies at1450° for 5 min under a pressure of69MPa conducted using scanning auger microscopy(SAM)(Model 0, PHI Electronics, Eden Prairie, MN). The areas that were ture of 1200C for 24 h to crystallize the BMAS matrix to the reduce carbon contamination. The beam current was in the barium osumilite phase(BaMg2Al3 (Si._ O3o). The proce- range of 27-47 nA. During the experiments, an accelerating dures have been described in detail in Prew et al. 16 The final voltage of 5-10 kV was used to locate the microstructural composite contained-50 vol%fibers features, then a lower accelerating voltage of 2 kv was used for Static-fatigue experiments were conducted at 600 and 950 quantification analysis to reduce specimen charging. A pure BN using four-poI lexural tests, with the outer ply of compound was used as a standard for quantitative analysi fibers parallel to the stress axis(0 plies). The maximum tensile The interfacial microstructures of composites before and after fatigue tests were examined using conventional transmis- MPa, above the matrix-cracking stress of the composite, which sion electron microscopy(TEM). For fatigued samples, TEM was-160 MPa at room temperature and 140 MPa at 1100C. S thin foils were prepared from areas near the tensile surface, The dimensions of the bend bars were 6 mm x 55 mm x with disks 3 mm in diameter cut from the tensile surface 2.5 mm. All the flexural bars contained twelve plies through the ound and polished 20 um off the tensile surface side, and then dimpled from the back side. The foil was argon-ion milled In the following text, the ply on the tensile surface(0% ply)is to perforation and carbon coated to prevent electrostatic charg ing effects. Specimens were examined using a microscope 90 ply. A pneumatic-type loading system was used to apply Model EM400T, Phillips Electronic Instruments, Mahwah, the load to the test bars through an alumina pushrod. The test N) that was operated at 100 kv. The microscope had a field- outer spans of 6.35 and 40 mm, respectively. The test bars were spectrometry(EDAX)Model 9100, EDAX, Mahwah, NJ)and eld in the fixture with a small load (outer fiber tensile stress of allel electron-energy-loss spectrometry(PEELS)(Model <15 MPa) and heated to the desired test temperature. The 7. Gatan, Pleasanton, CA). Interfacial chemistry was ana- sample was then allowed to equilibrate for at least 30 min lyzed using EDS and PEELS, and the microscope was operated before increasing the applied load to the selected level. The in the scanning TEM (STEM) mode with the smallest probe applied load was held constant until the test bar failed. At that size(<2 nm) oint, sensors interrupted the furnace power supply circuit so that the bend bars were able to cool to room temperature within 20 min(under applied load), minimizing damage and oxidation II. Results after 500 h, the experiment was terminated and the retained ( Static-Fatigue Behavior room-temperature strength of the sample was evaluated using a The results of static-fatigue experiments indicated that no four-point flexural test. During the room-temperature tests, the distinguishable difference existed in the stress dependence of Four-Point Flexure in air 600 500 之400 300 200 101001000 Timc to Failure(hour) Fig. 1. Stress dependence of the composite lifetime at(o)950 and(B)600C. ( Arrows indicate that the specimens did not fail after 500 h at the610 Journal of the American Ceramic Society— Sun et al. Vol. 80, No. 3 II. Experimental Procedure load was applied using a constant crosshead speed of 0.5 mm/min. To elucidate the effect of applied stresses on the The materials studied were Si-C-O Nicalon-fiber-reinforced oxidation behavior, an annealing experiment also was con- BMAS glass ceramics. The fibers were dual coated with SiC ducted at 600C in air for 500 h with the flexural strength of the over BN using chemical vapor deposition (CVD). The BN annealed sample measured at room temperature. coating was applied to the fibers by CVD using a proprietary The fracture surfaces of composites that failed during the precursor (3M Co., St. Paul, MN) chosen to give an approxi- fatigue tests or fractured at room temperature after the fatigue mate composition of 40 at.% boron, 40 at.% nitrogen, and tests were examined using a high-resolution scanning electron 20 at.% carbon. The BN coating exhibited a turbostratic struc- microscopy (SEM) microscope (Model S4100, Hitachi, Tokyo, ture and was comprised of nanoscale crystallites.8 The oxygen Japan) that had a field-emission gun and was equipped for content in the SiC and BN coating layers was measured energy-dispersive spectroscopy (EDS) capable of light-element by Auger spectroscopy to be 3 at.%. Composite panels detection. Compositional analysis of the fracture surfaces was (100 mm  100 mm) were fabricated by hot pressing a layup conducted using scanning Auger microscopy (SAM) (Model of 0/90 plies at 1450C for 5 min under a pressure of 6.9 MPa 660, PHI Electronics, Eden Prairie, MN). The areas that were in argon with a graphite die. After hot pressing, the composite studied were sputtered by the electron beam for 1–15 min to panels were cut into bars and heat treated in argon at a tempera- reduce carbon contamination. The beam current was in the ture of 1200C for 24 h to crystallize the BMAS matrix to the range of 27–47 nA. During the experiments, an accelerating barium osumilite phase (BaMg2Al3(Si9Al3O30)). The proce- voltage of 5–10 kV was used to locate the microstructural dures have been described in detail in Prewo et al.16 The final features, then a lower accelerating voltage of 2 kV was used for composite contained
50 vol% fibers. Static-fatigue experiments were conducted at 600 and 950C quantification analysis to reduce specimen charging. A pure BN compound was used as a standard for quantitative analysis. in air using four-point flexural tests, with the outer ply of fibers parallel to the stress axis (0 plies). The maximum tensile The interfacial microstructures of composites before and stresses applied in the bend bars were in the range of 250–450 after fatigue tests were examined using conventional transmis￾MPa, above the matrix-cracking stress of the composite, which sion electron microscopy (TEM). For fatigued samples, TEM was
160 MPa at room temperature and 140 MPa at 1100C. thin foils were prepared from areas near the tensile surface, 15 The dimensions of the bend bars were 6 mm  55 mm  with disks 3 mm in diameter cut from the tensile surface, 2.5 mm. All the flexural bars contained twelve plies through the ground and polished 20 m off the tensile surface side, and thickness, with 0 plies on the tensile and compressive surfaces. then dimpled from the back side. The foil was argon-ion milled In the following text, the ply on the tensile surface (0 ply) is to perforation and carbon coated to prevent electrostatic charg￾called the “top ply” and the 90 ply beneath it is called the “first ing effects. Specimens were examined using a microscope 90 ply.” A pneumatic-type loading system was used to apply (Model EM400T, Phillips Electronic Instruments, Mahwah, the load to the test bars through an alumina pushrod. The test NJ) that was operated at 100 kV. The microscope had a field- fixtures were fabricated from sintered -SiC with inner and emission gun and was equipped for energy-dispersive X-ray outer spans of 6.35 and 40 mm, respectively. The test bars were spectrometry (EDAX) (Model 9100, EDAX, Mahwah, NJ) and held in the fixture with a small load (outer fiber tensile stress of parallel electron-energy-loss spectrometry (PEELS) (Model 15 MPa) and heated to the desired test temperature. The 607, Gatan, Pleasanton, CA). Interfacial chemistry was ana￾sample was then allowed to equilibrate for at least 30 min lyzed using EDS and PEELS, and the microscope was operated before increasing the applied load to the selected level. The in the scanning TEM (STEM) mode with the smallest probe applied load was held constant until the test bar failed. At that size (2 nm). point, sensors interrupted the furnace power supply circuit so that the bend bars were able to cool to room temperature within III. Results 20 min (under applied load), minimizing damage and oxidation of the fracture surface. However, if the specimen did not fail (1) Static-Fatigue Behavior after 500 h, the experiment was terminated and the retained room-temperature strength of the sample was evaluated using a The results of static-fatigue experiments indicated that no four-point flexural test. During the room-temperature tests, the distinguishable difference existed in the stress dependence of Fig. 1. Stress dependence of the composite lifetime at (
) 950 and () 600C. (Arrows indicate that the specimens did not fail after 500 h at the applied stress level and, therefore, the tests were terminated.)