Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

Room and high temperature strength

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3.2. Room and high temperature strength

The compressive strength was measured at room temperature, 1400, and 1550 °C in atmospheric air for the ZS, ZSC, and ZSS composites. Fig. 4 shows representative room temperature stress–strain curves for the compositions, strain rates, and surface finishing studied. Note that due to the large differences measured in strength, the stress–strain curves are plotted in a semilogarithmic scale.

No strain rate dependence was observed for the room temperature strength in the strain rate range of 2 × 10−5 s−1 to 2 × 10−4 s−1, as can be seen in the stress–strain curves of ZS and ZSC materials (Fig. 4). This is typical of defect-controlled strength situation in which defects act as stress concentrators nucleating cracks and failure. Average compression strength of 3.1 ± 0.2 GPa were measured for ZS versus the 0.88 ± 0.08 GPa strength obtained for the ZS unpolished samples. For ZSC the average compression strength of 560 ± 80 MPa was measured for the finely polished samples and average compression strength of 630 ± 50 MPa was obtained for the ZSC unpolished samples, implying that internal instead of external defects are responsible for fracture. It is clear that ZS materials have exceptionally high intrinsic strength, which is limited by fabrication or finishing defects. In the case of the ZSC material the carbon regions act as flaws for crack nucleation and the finish of the external surface is not a limiting factor in strength.

Fig. 5A shows a general view of the fracture surface microstructure of a ZS sample that failed at 3.2 GPa. It is clear from the figure that the ZrB2 failure is transgranular, while SiC grains are pulled out at the fracture surface and therefore remain intact. In Fig. 5B a higher magnification detail of the fracture surface is presented, where presence of intergranular small cracks can be noticed. It is interesting to note that the stress–strain curves are not linear at high stresses (Fig. 4), indicating that the microstructure is changing due to the generation of internal defects until they reach the critical size for failure.

Typical stress–strain curves of ZSS along directions parallel and perpendicular to the fibers are also shown in Fig. 4. Measured strengths were 330 ± 150 and 240 ± 30 MPa for samples loaded parallel and perpendicular to the fibers, respectively. The compression strength is controlled by the penny-shaped cracks formed during the sintering step and therefore is higher when the fibers are parallel to the compression direction, as in that case the cracks are perpendicular to the applied load.

Representative high temperature stress–displacement curves are shown in Fig. 6 for materials tested at 1400, and 1550 °C. Fig. 7 summarizes the results obtained, and shows the average compressive strength as a function of temperature. Surface defects are seen to play an important role in limiting the compressive strength of ZS materials up to 1400 °C, while at 1550 °C the measured strength is similar for polished and unpolished samples, implying that oxidation effects are dominant at these temperatures. In general, the ZS composite shows higher compressive strength than the carbon containing ZSC. This is attributed to both the weak carbon bonding to the other phases, evidenced as grain pullout in the SEM observations, and the burnout of carbon at high temperature in air. This creates porosity and also produces channels through which air can enter, oxidizing the ZrB2 phase not only on the surface but also inside the sample.

Plastic deformation is clear in ZS and ZSC materials at 1400, and 1550 °C. The ZS materials can undergo substantially more strain than ZSC materials, because grains start loosing contact points earlier in ZSC due to the initial porosity associated to carbon burnout. Plastic deformation must be associated to grain boundary sliding, because of the high intrinsic strength and low diffusion coefficient of ZrB2. Grain boundary sliding is however difficult by the lack of intergranular phases and the presence of smaller SiC grains the boundaries and triple points.

The extensive cracking in ZSS causes very poor high temperature behavior. At high temperature, cracks cause extensive oxidation through the sample and elimination of the fibers’ coating (Fig. 8). The high temperature strength is therefore very low compared to ZS and ZSC composites.

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