August 1997 Tensile Stress Rupture of Sic/siCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2033 L1000 5v Time at 700oC, hr. ◇3 MBN Hi-Nic ○PBNH|Nc 700c 1200c 0.1 Time at 700oC, hr. Fig. 4. Minicomposite stress-rupture data for(a) Nicalon-fiber minicomposites and(b) Hi-Nicalon-fiber minicomposites. temperature conditions(700oC), the fracture surfaces appear to for an outside fiber of the PBn minicomposite, where the BN be the same as room-temperature fracture surfaces, with some thickness could be 3 um. The thinner BN interphase regions pullout and a jagged matrix contour(Fig. 6(a)). For more- also were depleted of BN. For most of the lower-temperature severe conditions(950oC after >20 h), the fracture surface is (700 and 950oC for times of <20 h)minicomposites that were planar(Fig. 6(b), with significant oxide formation between the tested, the interphase region was vacant or had thin silica fiber and matrix(Fig. 7(a), where most of the fibers appear to (SiO2) layers on the fiber and/or matrix that were only 0.1-0.2 be fused to the matrix. For this sample, there are at least 15 um thick( Fig 8) BN was present between the fiber and the fibers that are pulled out and appear to fracture independent of matrix of one PBN-HN sample(700 C for 50 h) the fiber-fused-to-matrix region. For the most severe test con- For slightly more-severe test conditions(950C for time t> dition(1200oC), a planar fracture surface was again evident; 98 h), glass is clearly evident between the fiber and matrix and however, the fibers had failed at slightly above and slightl spreads out from the fiber/matrix interphase region into the below the matrix fracture surface(Fig. 6(c) matrix crack(Fig. 7(a)). EDS was performed on this glass, and The most surprising observation is shown in Fig. 7(b). Frac- only silicon and oxygen could be detected--no boron. There is ture had occurred in the hot zone after 98 h at 950C ( Fig. 7(a)) little glass formation on the SiC at this temperature, as is evi- The sample was subsequently broken at a distance of-20 mm denced by the texture of the SiC matrix from the fracture surface while handling the minicomposite For the most-severe test conditions(1200 C), glass is evi- after it was removed from the test rig. The temperature that ent everywhere on the matrix surface and between the fiber orresponded to this region of the furnace(as determined from and the matrix( Fig. 9(a). Again, EDS analysis indicates only the furnace le), where this handling fracture occurred silicon and oxygen in the glass. Fiber fracture occurs very near an 500C. The interphase area that was previously the matrix crack plane; however, there is the appearance ofinterphase minicomposite types. Again, for the mildest time/ temperature conditions (700°C), the fracture surfaces appear to be the same as room-temperature fracture surfaces, with some pullout and a jagged matrix contour (Fig. 6(a)). For more￾severe conditions (950°C after >20 h), the fracture surface is planar (Fig. 6(b)), with significant oxide formation between the fiber and matrix (Fig. 7(a)), where most of the fibers appear to be fused to the matrix. For this sample, there are at least 15 fibers that are pulled out and appear to fracture independent of the fiber-fused-to-matrix region. For the most severe test con￾dition (1200°C), a planar fracture surface was again evident; however, the fibers had failed at slightly above and slightly below the matrix fracture surface (Fig. 6(c)). The most surprising observation is shown in Fig. 7(b). Frac￾ture had occurred in the hot zone after 98 h at 950°C (Fig. 7(a)). The sample was subsequently broken at a distance of ∼20 mm from the fracture surface while handling the minicomposite after it was removed from the test rig. The temperature that corresponded to this region of the furnace (as determined from the furnace profile), where this handling fracture occurred, was no more than 500°C. The interphase area that was previously filled by BN was vacant. The fiber that is shown in Fig. 7(b) is for an outside fiber of the PBN minicomposite, where the BN thickness could be 3 mm. The thinner BN interphase regions also were depleted of BN. For most of the lower-temperature (700° and 950°C for times of <20 h) minicomposites that were tested, the interphase region was vacant or had thin silica (SiO2) layers on the fiber and/or matrix that were only 0.1–0.2 mm thick (Fig. 8). BN was present between the fiber and the matrix of one PBN-HN sample (700°C for 50 h). For slightly more-severe test conditions (950°C for time t $ 98 h), glass is clearly evident between the fiber and matrix and spreads out from the fiber/matrix interphase region into the matrix crack (Fig. 7(a)). EDS was performed on this glass, and only silicon and oxygen could be detected—no boron. There is little glass formation on the SiC at this temperature, as is evi￾denced by the texture of the SiC matrix. For the most-severe test conditions (1200°C), glass is evi￾dent everywhere on the matrix surface and between the fiber and the matrix (Fig. 9(a)). Again, EDS analysis indicates only silicon and oxygen in the glass. Fiber fracture occurs very near the matrix crack plane; however, there is the appearance of Fig. 4. Minicomposite stress-rupture data for (a) Nicalon-fiber minicomposites and (b) Hi-Nicalon-fiber minicomposites. August 1997 Tensile Stress Rupture of SiCf/SiCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air 2033