S. Mall, J. L Ryba/ Composites Science and Technology 68(2008)274-28 continued through the specimen virtually unimpeded in the condition at a given temperature, and likewise relatively intermediate temperature range tests while in the 400C more at 950C than at 400C in a given test environment. and 950C tests the fibers diverted the cracks for a short However, a different damage mechanism of the Bn inter- period before the entire tow failed phase is present at 750C. The bn interphase near the Fig.9 shows the fracture surface of the representative fiber/ matrix or fiber/fiber locations appears to have been specimens from all six stress rupture test environments at oxidized by the moisture under humid test environment. intermediate magnification(1000x). At this magnification, This occurred most probably due to the oxidation of Bn it can be seen that there is exterior and interior fiber deb- to form the boria(B2O3), which reacted with SiC to form onding, and fiber pullout and fracture along different a borosilicate melt. It then solidified where boron was lea planes at 400C(Fig. 9a and d) and 950C(Fig. 9c ched out from the condensed phase resulting in a glass with and f), i.e., below and above the intermediate range. There a very high SiO, content. This fused fibers together caused is very minimal fiber pullout with the fracture appearing the embrittlement(Fig. 10b and e), i.e., failure on the same lmost completely planar at 750C, i.e., within the inter- plane with no fiber pull-out, again a typical feature of fiber/ mediate range(Fig. 9b and e) matrix embrittlement. The planar fracture surface at At higher magnification(8000x) shown in Fig. 10 the 750C, as elaborated earlier, was due to the bonding details of degradation of the BN interphase material can together of the fibers and matrix by the silicate glass, so be observed. At 400C and 950C, the bn interphase that as the weakest fiber in a tow failed, the rest of the shows the evidence of fracture or recession(marked by tow also failed with it. These microscopic analyses thus arrows as few such examples). This is relatively more prom- clearly suggests that the tested CMC system experienced inent in the steam environment than the laboratory air test more embrittlement of fiber/ matrix interphase in the inter a)br. Monotonic Test-T 「(b)m118h 了(①曙12.31h 10 LmI Fig 12. Damage progression with time in 950C steam stress rupture tests-(a)5min, (b)1. 18 h,(c)4.2 h and (d)12.3 hcontinued through the specimen virtually unimpeded in the intermediate temperature range tests while in the 400 C and 950 C tests the fibers diverted the cracks for a short period before the entire tow failed. Fig. 9 shows the fracture surface of the representative specimens from all six stress rupture test environments at intermediate magnification (1000·). At this magnification, it can be seen that there is exterior and interior fiber deb￾onding, and fiber pullout and fracture along different planes at 400 C (Fig. 9a and d) and 950 C (Fig. 9c and f), i.e., below and above the intermediate range. There is very minimal fiber pullout with the fracture appearing almost completely planar at 750 C, i.e., within the inter￾mediate range (Fig. 9b and e). At higher magnification (8000·) shown in Fig. 10 the details of degradation of the BN interphase material can be observed. At 400 C and 950 C, the BN interphase shows the evidence of fracture or recession (marked by arrows as few such examples). This is relatively more prom￾inent in the steam environment than the laboratory air test condition at a given temperature, and likewise relatively more at 950 C than at 400 C in a given test environment. However, a different damage mechanism of the BN inter￾phase is present at 750 C. The BN interphase near the fiber/matrix or fiber/fiber locations appears to have been oxidized by the moisture under humid test environment. This occurred most probably due to the oxidation of BN to form the boria (B2O3), which reacted with SiC to form a borosilicate melt. It then solidified where boron was lea￾ched out from the condensed phase resulting in a glass with a very high SiO2 content. This fused fibers together caused the embrittlement (Fig. 10b and e), i.e., failure on the same plane with no fiber pull-out, again a typical feature of fiber/ matrix embrittlement. The planar fracture surface at 750 C, as elaborated earlier, was due to the bonding together of the fibers and matrix by the silicate glass, so that as the weakest fiber in a tow failed, the rest of the tow also failed with it. These microscopic analyses thus clearly suggests that the tested CMC system experienced more embrittlement of fiber/matrix interphase in the inter￾Fig. 12. Damage progression with time in 950 C steam stress rupture tests – (a) 5 min, (b) 1.18 h, (c) 4.2 h and (d) 12.3 h. S. Mall, J.L. Ryba / Composites Science and Technology 68 (2008) 274–282 281