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to the oxidation-induced phase change of the zirconium oxynitride to monoclinic zirconia.

8.3.2 Degradation by Moisture

Lifetimes that are predicted from different fatigue tests will vary. Slow crack growth has been reported by Kawakubo and Komeya [8.36] to accelerate under cyclic conditions, especially of the tension—compression type cycle at room temperature for sintered silicon nitride. They also reported a plateau at about 70–90% of the stress intensity factor, when crack velocity was plotted vs. KI. Three regions in the data were observed, very similar to that reported for glasses as shown in Fig. 8.1. As the materials studied had a glassy grain boundary phase, the fatigue mechanism was assumed to be the same as that reported for glassy materials [8.13] (i.e., stress corrosion cracking due to moisture in the air). Fett et al. [8.37] reported that at 1200°C, the lifetimes for cyclic loads were higher than for static loads. Tajima et al. [8.38] reported that a gas pressure sintered silicon nitride was resistant to slow crack growth up to 900°C, but then was susceptible to slow crack growth at 1000°C because of the softening of the glassy grain boundary phase. A higher fatigue resistance was reported for higher frequencies of the load cycle due to the viscoelastic nature of the glassy grain boundary phase.

8.3.3 Degradation by Other Atmospheres

Carbides and Nitrides

Clark [8.39] reported that Nicalon™ SiC fibers when aged in nitrogen or humid air at 1200°C for 2 hr, lost about one-half of their tensile strength. A more gradual strength decrease was observed for fibers that were exposed to hot argon. Although the time dependence of strength loss for the different aging environments was similar, the mechanisms causing strength loss were quite different. For exposure to nitrogen, Clark attributed the strength loss to crack propagation from existing

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Properties and Corrosion

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flaws; for exposure to argon, he attributed the loss to grain growth and porosity; and for exposure to humid air, he attributed the strength loss to fiber coalescence at the silica surface, to poor adherence of the surface silica layer, to a cracked crystalline silica surface layer, and to bubbles at the silica/fiber interface. Clark also pointed out that thermal stability should not be based solely upon weight change data, because for this fiber, the weight gain produced by oxidation to silica was offset by weight loss due to CO evolution.

Siliconized, boron-doped, and aluminum-doped SiC samples were exposed to gaseous environments containing mixtures of predominantly N2, H2, and CO, representative of metallurgical heat-treatment atmospheres at 1300°C for up to 1000 hr by Butt et al. [8.40]. They reported significant strength losses for all three materials for times less than 100 hr when exposed to a gas mixture containing about 40% nitrogen. At longer exposure times, no additional strength loss occurred. The aluminum-doped SiC, unlike the other two, exhibited a slight strength increase after 1000 hr when exposed to a gas mixture containing 98.2% nitrogen. The strength losses were attributed primarily to pitting that was related to the presence of transition metal impurities.

It has been shown by Li and Langley [8.41] that ceramic fibers composed of Si–C–N–O experienced various degrees of strength degradation when aged in atmospheres of various hot gases. The rate of strength loss experienced by fibers aged in these hot gases was related to the rate of diffusion of the gases formed by decomposition. The gases of decomposition (N2, CO, and SiO) diffused through the fiber porosity and any surface boundary layers present. The diffusion of these product gases can be controlled by aging the fibers in atmospheres of these gases. Thus, greater strength loss was exhibited when fibers were aged in argon compared to aging in nitrogen. This effect can be seen by examining the data of Table 8.2.

Zirconia-Containing Materials

Brinkman et al. [8.42] studied the effects of a diesel engine environment upon the strength of two commercial zirconias

Copyright © 2004 by Marcel Dekker, Inc.