718 Communications of the American Ceramic Sociery Vol 81. No. 3 For tensile tests, the ends of the minicomposite spec, The 14 were imbedded in aluminum blind rivets tensile tests were conducted using an in-house developed ma- hine at a constant loading rate of I um/s. Self-aligning cou plers were incorporated into the load train to minimize spurious nding loading. Fracture surfaces were characterized by scan- ning electron microscopy(SEM, Hitachi S-800). A transmis- sion electron microscope (TEM, Hitachi HF2000) equipped structure and composition of the multilayered oxide interphase region before and after oxidation A TEM image of the multilayered oxide interphase region of 0020.4060.81121.4 an as-prepared minicomposite specimen 1)shows the esence of the am ous SiO,. monoclinic ZrO, and amor Displacement (mm) phous SiO2 layers. Interestingly, another thin layer enriched with Zr(30 nm) was observed adjacent to the fiber surface Fig. 2. Tensile behavior of as-prepared minicomposites The incorporation of this contamination layer was attributed to It appeared that a small amount of residual ZrClg in the chlo- sile tests were stopped after a total displacement of l mm. The rinator might have been transported to the fiber surface during maximum loads observed for these specimens, 90 and IoN first sio, laver took place. The average thickness of the first containing a carbon interphase coating prepared in our lab- CVD SIO, layer was-100 nm, whereas the second CVD SiO oratory. The fracture surface of one of the tensile speci- was-50 nm thick. The Zro, layer was -200 to 400 mens is shown in Fig 3. Significant crack deflection was ob- thick, depending on axial location. The thickness of the mul- served within the multilayer interphase region. Although tilayered oxide interphase was relatively uniform in the radial exact deflection locations were difficult to determine from direction of the fiber tow but was not uniform along the lens these SEM images, it appeared that crack deflection and of the specimens because of reagent depletion along the direc- fiber pull-out occurred preferentially at the inner Sioz/zrOz tion of gas flow. In particular, the thickness of the SiO, layers interface. It was fairly difficult to prepare TEM specimens tended to decrease along the flow direction in the range of -50 showing the entire interphase region near the fracture locations, to 200 nm. The Zro, layer had a rough growth morpholog because the inner SiO,/ZrO, interface tended to become com- Consequently, the inner SiO, /ZrO, interface was relativ ly separated during TEM specimen preparation. The av- smooth while the outer ZrO,/SiO, interface was highly irregu length of fiber pull-out was on the order of tens of mi- lar and interlocked Otherwise the multilayered interphase re- crometers ion in its as-prepared conditions was mostly free of impurities der to assess the effects of oxidation on the or microcracks behavior of the minicomposites two specimens were loaded to Figure 2 shows the load-displacement curves of two as- 80N to induce matrix cracking. The precracked specimens red minicomposite specimens. Both specimens exhibited en remo ed from the tensile machine and exposed to graceful composite behavior with evidence of multiple matrix ambient air at 960 C for l and 10 h using a small furnace. After he oxidation treatments, the specimens were retested As cracking. Ihe specimens remained in one piece when the ten- shown in Fig. 4, the specimens still exhibited composite be- havior after the oxidation treatments although their maximum load capability (80 N) was somewhat decreased in compar sImens It appeared that the curvature of the tail end of the load-displacement curves became steeper with the longer oxidation treatment. The SEM fracture surfaces and TEM image of the specimen exposed to the 10 h oxidation treatment are shown in Figs. 5 and 6. From these images, it was apparent that crack deflection occurred mostly at the inner SiO,/ZrO, interface and to a less extent at the outer ZrO,/SiO interface as well. The preferred location for fiber pull-out mained the inner SiO,/ZrO, interface At least for 8 h of Sic infiltration at 900-950C and 10 h of oxidation treatment at 960C, the SiO /ZrO2/SiO2 interphase exhibited excellent chemical stability as evidenced by the pres ervation of their sharp interface boundaries( Fig. 6). No diffu- sion of Si and Zr within the multilayered structure, as well as no carbon as an impurity phase in the interphase region, was Fiber V. Discussion and conclusions The observed mechanical behavior and oxidation resistance of the minicomposites containing the multilayered oxide inter- Fig. 1. TEM image of the multilayered oxide interphase of an as hase are encouraging. It seems that one of the plausible rea prepared minicomposite specimen sons for the observed composite behavior may be the signifFor tensile tests, the ends of the minicomposite specimens were imbedded in aluminum blind rivets using epoxy. The tensile tests were conducted using an in-house developed ma￾chine at a constant loading rate of 1 mm/s. Self-aligning cou￾plers were incorporated into the load train to minimize spurious bending loading. Fracture surfaces were characterized by scan￾ning electron microscopy (SEM, Hitachi S-800). A transmis￾sion electron microscope (TEM, Hitachi HF2000) equipped with a field emission gun (<1.5 nm probe size) and an energy dispersive spectrometer (EDS) was used to characterize the structure and composition of the multilayered oxide interphase region before and after oxidation. III. Results A TEM image of the multilayered oxide interphase region of an as-prepared minicomposite specimen (Fig. 1) shows the presence of the amorphous SiO2, monoclinic ZrO2, and amor￾phous SiO2 layers. Interestingly, another thin layer enriched with Zr (∼30 nm) was observed adjacent to the fiber surface. The incorporation of this contamination layer was attributed to the presence of the internal Zr chlorinator in the CVD chamber. It appeared that a small amount of residual ZrCl4 in the chlo￾rinator might have been transported to the fiber surface during the initial heating sequence, just before the deposition of the first SiO2 layer took place. The average thickness of the first CVD SiO2 layer was ∼100 nm, whereas the second CVD SiO2 layer was ∼50 nm thick. The ZrO2 layer was ∼200 to 400 nm thick, depending on axial location. The thickness of the mul￾tilayered oxide interphase was relatively uniform in the radial direction of the fiber tow, but was not uniform along the length of the specimens because of reagent depletion along the direc￾tion of gas flow. In particular, the thickness of the SiO2 layers tended to decrease along the flow direction in the range of ∼50 to 200 nm. The ZrO2 layer had a rough growth morphology. Consequently, the inner SiO2/ZrO2 interface was relatively smooth while the outer ZrO2/SiO2 interface was highly irregu￾lar and interlocked. Otherwise the multilayered interphase re￾gion in its as-prepared conditions was mostly free of impurities or microcracks. Figure 2 shows the load–displacement curves of two as￾prepared minicomposite specimens. Both specimens exhibited graceful composite behavior with evidence of multiple matrix cracking. The specimens remained in one piece when the ten￾sile tests were stopped after a total displacement of 1 mm. The maximum loads observed for these specimens, 90 and 110 N, compared favorably to those of Nicalon/SiC minicomposites containing a carbon interphase coating prepared in our lab￾oratory.4 The fracture surface of one of the tensile speci￾mens is shown in Fig. 3. Significant crack deflection was ob￾served within the multilayer interphase region. Although exact deflection locations were difficult to determine from these SEM images, it appeared that crack deflection and fiber pull-out occurred preferentially at the inner SiO2/ZrO2 interface. It was fairly difficult to prepare TEM specimens showing the entire interphase region near the fracture locations, because the inner SiO2/ZrO2 interface tended to become com￾pletely separated during TEM specimen preparation. The av￾erage length of fiber pull-out was on the order of tens of mi￾crometers. In order to assess the effects of oxidation on the mechanical behavior of the minicomposites, two specimens were loaded to 80 N to induce matrix cracking. The precracked specimens were then removed from the tensile machine and exposed to ambient air at 960°C for 1 and 10 h using a small furnace. After the oxidation treatments, the specimens were retested. As shown in Fig. 4, the specimens still exhibited composite be￾havior after the oxidation treatments although their maximum load capability (∼80 N) was somewhat decreased in compari￾son to the as-prepared specimens. It appeared that the curvature of the tail end of the load–displacement curves became steeper with the longer oxidation treatment. The SEM fracture surfaces and TEM image of the specimen exposed to the 10 h oxidation treatment are shown in Figs. 5 and 6. From these images, it was apparent that crack deflection occurred mostly at the inner SiO2/ZrO2 interface, and to a less extent at the outer ZrO2/SiO2 interface as well. The preferred location for fiber pull-out re￾mained the inner SiO2/ZrO2 interface. At least for 8 h of SiC infiltration at 900–950°C and 10 h of oxidation treatment at 960°C, the SiO2/ZrO2/SiO2 interphase exhibited excellent chemical stability as evidenced by the pres￾ervation of their sharp interface boundaries (Fig. 6). No diffu￾sion of Si and Zr within the multilayered structure, as well as no carbon as an impurity phase in the interphase region, was observed within the detection limits of EDS. IV. Discussion and Conclusions The observed mechanical behavior and oxidation resistance of the minicomposites containing the multilayered oxide inter￾phase are encouraging. It seems that one of the plausible rea￾sons for the observed composite behavior may be the signifi￾Fig. 1. TEM image of the multilayered oxide interphase of an as￾prepared minicomposite specimen. Fig. 2. Tensile behavior of as-prepared minicomposites. 718 Communications of the American Ceramic Society Vol. 81, No. 3