Journal of the American Ceramic Sociery-Morscher Vol. 80. No 8 CVI SiC Nicalon Fiber Sio2 5.0 kV: 38. 0k1. 00pm eek::e CVI Sic Sio2 Hi-Nicalon Fiber 4 5.0kV X30. 0K 1a8pm mediate temperatures for BN-interphase minicomposites(a) usually on the fiber or(b) sometimes on the matrix surrounding the fiber. Figure 8(a)is a 3MBN-Nic minicomposite that ruptured after 42 h at 700C, Fig. 8(b)is a 3MBN-HN minicomposite that ruptured after 12.5 h at 700C. correspond to individual fiber failure were easily observed for evident that Hi-Nicalon surface flaws grow and dominate under all the C-Nic samples(room temperature and 700C tested), these stressed-oxidation conditions vith the exception of the sample that was tested at 700C for For the PBN-HN sample that ruptured after 98 h at 950C 2.3 h(Fig. 5(b)). Only 16 fiber fracture surfaces could be individual fiber failure did not occur, because of the strong obtained for this sample, because the long pulled-out fibers bonding that occurred from the glass formation between the were moving in the SEM microscope and many of the fibers fiber and matrix. Therefore, the fiber strengths were not deter during the fracture event. There were a few internal flaws. which caused fiber failure for room-temperature fracture(Fig 11(a); however, surface flaws usually caused fiber failure at IV. Discussion room temperature and always caused fiber failure at 700C Fracture mirrors that corresponded to individual fiber failure fr 1) Comparisons to Known Fiber and Composite posites that were tested at room temperature lower temperature Stress-Rupture Data (700C), and higher temperature(1200oC). Internal flaws were Yun and DiCarlo& have shown that the fast-fracture strength detected 70% of the time for individual fiber tensile tests on and stress-rupture data of Nicalon 9,20 and Hi-Nicalon%, 20 fi- as-produced Hi-Nicalon fibers. This is similar to what was bers can be related using a Larson-Miller21 2 approach. The found in this study for room-temperature fracture of PBN-HN Larson-Miller approach is empirical; yet, it has been used suc- minicomposites(50%, see Fig. 11(b). However, there were cessfully to quantify stress-rupture properties of metals and fewer internal fiber flaws for the minicomposite that was tested ceramics. The Larson-Miller parameter, q, relates the time- at 700C for 50 h(22%)and no internal flaws for the mini- temperature condition, where time-dependent rupture occurs composite that was tested at 1200@C for 3 h(Fig. 11(b)). It is for a given stresscorrespond to individual fiber failure were easily observed for all the C-Nic samples (room temperature and 700°C tested), with the exception of the sample that was tested at 700°C for 2.3 h (Fig. 5(b)). Only 16 fiber fracture surfaces could be obtained for this sample, because the long pulled-out fibers were moving in the SEM microscope and many of the fibers had fracture surfaces with a ‘‘lip;’’ i.e., they failed in bending during the fracture event. There were a few internal flaws, which caused fiber failure for room-temperature fracture (Fig. 11(a)); however, surface flaws usually caused fiber failure at room temperature and always caused fiber failure at 700°C. Fracture mirrors that corresponded to individual fiber failure from PBN-HN minicomposites were observed for minicom￾posites that were tested at room temperature, lower temperature (700°C), and higher temperature (1200°C). Internal flaws were detected 70% of the time for individual fiber tensile tests on as-produced Hi-Nicalon fibers.17 This is similar to what was found in this study for room-temperature fracture of PBN-HN minicomposites (50%, see Fig. 11(b)). However, there were fewer internal fiber flaws for the minicomposite that was tested at 700°C for 50 h (22%) and no internal flaws for the mini￾composite that was tested at 1200°C for 3 h (Fig. 11(b)). It is evident that Hi-Nicalon surface flaws grow and dominate under these stressed-oxidation conditions. For the PBN-HN sample that ruptured after 98 h at 950°C, individual fiber failure did not occur, because of the strong bonding that occurred from the glass formation between the fiber and matrix. Therefore, the fiber strengths were not deter￾mined for this sample. IV. Discussion (1) Comparisons to Known Fiber and Composite Stress-Rupture Data Yun and DiCarlo18 have shown that the fast-fracture strength and stress-rupture data of Nicalon19,20 and Hi-Nicalon19,20 fi￾bers can be related using a Larson–Miller21,22 approach. The Larson–Miller approach is empirical; yet, it has been used suc￾cessfully to quantify stress-rupture properties of metals and ceramics. The Larson–Miller parameter, q, relates the time– temperature condition, where time-dependent rupture occurs for a given stress: Fig. 8. Typical examples of a glass layer formed at intermediate temperatures for BN-interphase minicomposites (a) usually on the fiber or (b) sometimes on the matrix surrounding the fiber. Figure 8(a) is a 3MBN-Nic minicomposite that ruptured after 42 h at 700°C; Fig. 8(b) is a 3MBN-HN minicomposite that ruptured after 12.5 h at 700°C. 2036 Journal of the American Ceramic Society—Morscher Vol. 80, No. 8