October 1998 Stress Concentration Due to Fiber-Matrix Fusion in Ceramic-Matrix Composite. 2599 CDS sc5-0 fiber wath coaling induced by oxidizing to form a layer of Sio2. All oxidation experiments were performed in air. All anneals took place at 1 100C for times ranging from 20 to 120 h, with the exception of one set that was oxidized at 1200%C for 50 h. A set of control Carhnn interphase burned out 5]"C air specimens also was tested to establish a baseline, to which the tensile performance of the sealed microcomposites could be Crack induced in costinE compared. These included as-received SCS-0 fibers, as- assembled microcomposites( which were not subjected to oxi- dation and, hence, not fused), and bare SCS-0 fibers(which were oxidized by exposure to air at 1@C for 68 h). Sets of monofilaments and microcomposites that were exposed to 1 100C air for 100 h were treated for I h in a room-temperature Scction of ocating removed from fiber 50% HF solution to remove the SiO, oxidation product. Fol lowing acid treatment, there was no visible evidence of a SiO layer, and the sections of Sic sheath moved freely along the length of the SiC monofilament. IV. Microcomposite Results and Discussion As-received scs-0 fiber Figure 6A reports the nominal tensile failure stress for single SCS-0 fiber specimens. The as-received SCS-0 fibers failed at a nominal fiber stress between 2.5 and 3 GPa, in agreement with independent data obtained previously. Bare fibers that were subjected to 1100C air for 68 h failed at stresses between for oxidized SCS-0 monofilaments. Removal of the SiO, scale by etching in HF restored the strength of the fibers to values comparable to those of the as-received fibers. This indicated Fig. 4. Schematic of the procedure used to prepare microcomposit tensile specimens suitable for experimental evaluation of fusion- site that the temperatures and times used during oxidation did not induced stress concentration cause significant fiber degradation As indicated in Fig. 6(B), the failure stress for microcom- resembled)ranged between 1.5 and 2.5 GPa. This decrease in ite specimens that were not fused via oxidation(i.e,as- strength compared to as-received fibers was expected, consid- SCS-O fiber SiC sheath manual threading of a SiC sheath with an-2 um radial toler ance on to a pristine SCS-0 monofilament Fa gins for the as-assembled microcomposites were ra located along the length of the fiber; i. e, they were ne the final location of sic sheath Microcomposites that were fused to the sheath with a layer of SiO, following oxidation exhibited failure at a nominal fiber tress centered at -700 MPa. all observed failure origins in the sealed microcomposites occurred where the SiC monofilament entered the SiC sheath( Fig. 7). Two of the five specimens oxidized for 20 h at 1.C had strengths approaching that for In bare and oxidized SCS-0. but failure was observed where the fiber entered the sheath. For the remaining three 20 h speci- Fig. 5. SEM micrograph of an as-assembled microcomposite mens failure occurred at a fiber nominal stress <1 GPa. similar to that for microcomposites oxidized for longer times Figure 8 depicts SEM micrographs of the mating fiber from he fractured microcomposite specimen in Fig. 7. The SiO scale on the fiber and matrix was-I um in thickness. Where the location of the initial failure site for microstructural evalu- the fiber enters the sheath, a SiO2 scale had formed with a ation a challenge. Fourth, direct experimental confirmation of radius of curvature of -2 um for the majority of the fiber models most relevant to true composites was not possible. circumference Thus, the experimental strategy was to test a microcomposite The fibers typically were displaced to one side of the model that contained features similar to those in a SiO2-fused d, for the shorter oxidation times(<120 h), were sealed bulk composite system and also was accessible to FEM and over a portion of their circumference. For example m en that was oxidized in air for 40 h at 1 100%C. the sheath composite sy stem allowed confidence when inferring the com- siles testing reveal asegrignof ibsr that was n side the sheath Tensile testing of the microcomposite specimens Fig 9).A""of SiO, that formed at the edge of the sheath formed at room temperature on a test frame(Instron Corp was observed to extend over approximately one-sixth the fiber Danvers, MA)with pneumatic rubber grips. The single fila circumference. Behind the SiO, lip, the portion of the Sio microcomposite and control specimens were mounted to interlayer that bridged the fiber and sheath had become ex mene tabs using epoxy to aid in the gripping process. The specimen gauge length(distance between paper tabs) was 2.5 cm. All specimens were pulled at a rate of 1 x 10- m/s, and the ad at failure was electronically recorded Sealing between the Sic monofilament and SiC sheath was one case, fiber failure occurred inside the sheath -200 Hm from the end forthe location of the initial failure site for microstructural evalu￾ation a challenge. Fourth, direct experimental confirmation of models most relevant to true composites was not possible. Thus, the experimental strategy was to test a microcomposite model that contained features similar to those in a SiO2-fused bulk composite system and also was accessible to FEM and experimental analysis. Confirmation of FEM for the micro￾composite system allowed confidence when inferring the com￾posite behavior from the results of FEM for actual composites. Tensile testing of the microcomposite specimens was per￾formed at room temperature on a test frame (Instron Corp., Danvers, MA) with pneumatic rubber grips. The single fila￾ment microcomposite and control specimens were mounted to paper tabs using epoxy to aid in the gripping process. The specimen gauge length (distance between paper tabs) was 2.5 cm. All specimens were pulled at a rate of 1 × 10−5 m/s, and the load at failure was electronically recorded. Sealing between the SiC monofilament and SiC sheath was induced by oxidizing to form a layer of SiO2. All oxidation experiments were performed in air. All anneals took place at 1100°C for times ranging from 20 to 120 h, with the exception of one set that was oxidized at 1200°C for 50 h. A set of control specimens also was tested to establish a baseline, to which the tensile performance of the sealed microcomposites could be compared. These included as-received SCS-0 fibers, as￾assembled microcomposites (which were not subjected to oxi￾dation and, hence, not fused), and bare SCS-0 fibers (which were oxidized by exposure to air at 1100°C for 68 h). Sets of monofilaments and microcomposites that were exposed to 1100°C air for 100 h were treated for 1 h in a room-temperature 50% HF solution to remove the SiO2 oxidation product. Fol￾lowing acid treatment, there was no visible evidence of a SiO2 layer, and the sections of SiC sheath moved freely along the length of the SiC monofilament. IV. Microcomposite Results and Discussion Figure 6A reports the nominal tensile failure stress for single SCS-0 fiber specimens. The as-received SCS-0 fibers failed at a nominal fiber stress between 2.5 and 3 GPa, in agreement with independent data obtained previously.9 Bare fibers that were subjected to 1100°C air for 68 h failed at stresses between 1.4 and 2 GPa, also in agreement with data previously obtained for oxidized SCS-0 monofilaments.9 Removal of the SiO2 scale by etching in HF restored the strength of the fibers to values comparable to those of the as-received fibers. This indicated that the temperatures and times used during oxidation did not cause significant fiber degradation. As indicated in Fig. 6(B), the failure stress for microcom￾posite specimens that were not fused via oxidation (i.e., as￾assembled) ranged between 1.5 and 2.5 GPa. This decrease in strength compared to as-received fibers was expected, consid￾ering the likelihood of handling damage associated with the manual threading of a SiC sheath with an ∼2 mm radial toler￾ance on to a pristine SCS-0 monofilament. Failure origins for the as-assembled microcomposites were randomly located along the length of the fiber; i.e., they were not correlated with the final location of SiC sheath. Microcomposites that were fused to the sheath with a layer of SiO2 following oxidation exhibited failure at a nominal fiber stress centered at ∼700 MPa. All observed failure origins in the sealed microcomposites occurred where the SiC monofilament entered the SiC sheath (Fig. 7).‡ Two of the five specimens oxidized for 20 h at 1100°C had strengths approaching that for bare and oxidized SCS-0, but failure was observed where the fiber entered the sheath. For the remaining three 20 h speci￾mens, failure occurred at a fiber nominal stress <1 GPa, similar to that for microcomposites oxidized for longer times. Figure 8 depicts SEM micrographs of the mating fiber from the fractured microcomposite specimen in Fig. 7. The SiO2 scale on the fiber and matrix was ∼1 mm in thickness. Where the fiber enters the sheath, a SiO2 scale had formed with a radius of curvature of ∼2 mm for the majority of the fiber circumference. The fibers typically were displaced to one side of the sheath and, for the shorter oxidation times (<120 h), were sealed only over a portion of their circumference. For example, in one specimen that was oxidized in air for 40 h at 1100°C, the sheath was dislodged from its original position on the fiber after ten￾sile testing to reveal a region of fiber that was inside the sheath (Fig. 9). A ‘‘lip’’ of SiO2 that formed at the edge of the sheath was observed to extend over approximately one-sixth the fiber circumference. Behind the SiO2 lip, the portion of the SiO2 interlayer that bridged the fiber and sheath had become ex- ‡ In one case, fiber failure occurred inside the sheath ∼200 mm from the end for reasons that are unclear. Fig. 4. Schematic of the procedure used to prepare microcomposite tensile specimens suitable for experimental evaluation of fusion￾induced stress concentration. Fig. 5. SEM micrograph of an as-assembled microcomposite specimen. October 1998 Stress Concentration Due to Fiber–Matrix Fusion in Ceramic-Matrix Composites 2599