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TABLE 8.2 Effects of Aging in Various Atmospheres upon Strength of Si–C–N–O Fibers [8.41]

Source: Ref. 8.41.

partially stabilized with magnesia. The combustion environment at temperatures between 500 and 900°C contained Fe, Zn, Ca, Mg, and P contaminants from the fuel along with water vapor. The average flexural strength decreased by about 32% after exposure for 100 hr for the material rated as thermally shock resistant, and decreased by only 9% for the one rated as maximum strength. When the surface reaction products were removed from the thermally shock resistant material before strength testing, the strength decreased 22%. Both materials when exposed to air for 100 hr at 700 and 750°C exhibited decreases in strength of 6–8%, indicating a much more significant effect of the actual diesel engine environment. They found that the strength decreased as the amount of monoclinic zirconia increased. Thus the primary mechanism of degradation was attributed to localized increases in the monoclinic content.

8.3.4 Degradation by Molten Salts

Carbides and Nitrides

The strength loss of α-SiC and siliconized-SiC tubes exposed to a combustion flame into which a sodium silicate/water solution was injected was evaluated by Butt and Mecholsky [8.43]. The corrosive exposure was for times up to 373 hr, at temperatures 900–1050°C, with an oxygen partial pressure of about 4 kPa. Strength losses exceeded 50% for the α-SiC, and were 25–45% for the siliconized-SiC. Strength tests were conducted on C-ring

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samples after most of the reaction products were removed. Those samples for which the reaction products were not removed prior to strength testing exhibited no significant loss of strength, although an increase in scatter of the data was reported. Surface or corrosion pits were identified as the fracture origin for both types of SiC. In addition, the α-SiC exhibited grain boundary attack, whereas the siliconized-SiC exhibited oxidation of the silicon matrix and attack of the large SiC grains.

In a study of the effects of molten salt upon the mechanical properties of silicon nitride, Bourne and Tressler [8.44] reported that hot-pressed silicon nitride exhibited a more severe degradation in flexural fracture strength than did reaction sintered silicon nitride, although the weight loss of the hotpressed material was less than that of the sintered one as reported by Tressler et al. [8.45] in a previous study. Their strength data are shown in Fig. 8.4. The exposure to a eutectic mixture of NaCl and Na2SO4 was more severe than to molten NaCl alone for the hot-pressed material, whereas for the reaction sintered material the effect was about the same. The differences between these two materials were attributed to the diffusion of contaminants along grain boundaries in the hotpressed material and penetration of contaminants into pores of the reaction sintered material. This was based upon the observation that the grain boundaries of the hot-pressed material were more severely affected than those of the reaction sintered material, which did not contain an oxide grain boundary phase. The lowered fracture strengths resulted from an increase in the critical flaw size and a decrease in the critical stress intensity factor. The slight increase in fracture strengths at 1200°C was a result of a slight increase in the critical stress intensity factor. The NaCl/Na2SO4 eutectic mixture, being more oxidizing than the NaCl melt, caused a greater increase in the critical flaw size.

In the application of ceramics to turbine engines, the static fatigue life is of prime importance. Compared to the other types of mechanical testing in corrosive environments, little work has been reported on the long time exposure effects to

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FIGURE 8.4 Fracture strength vs. molten salt composition. HP=hot pressed, RS=reaction sintered, AR=as-received, NC=NaCl, and EU=eutectic mixture of NaCl+Na2SO4. (From Ref. 8.44.)

static fatigue life. Swab and Leatherman [8.46] reported that, at stresses between 300 and 500 MPa, there was a significant decrease in the time-to-failure for Si3N4 containing magnesia exposed to Na2SO4 at 1000°C. At stresses above 500 MPa and below 300 MPa, little change in the time-to-failure was noted. Because molten salt was not replenished during the test, corrosion pits were unable to grow to a size sufficient to decrease the time-to-failure at stress levels between 300 and 500 MPa. Although the decrease in room temperature strength for a yttria-containing silicon nitride after exposure to sodium sulfate was about 35%, it retained a greater strength than the magnesia-containing material (549 MPa for the Y-containing material vs. 300 MPa for the Mg-containing material) [8.47]. Fox and Smialek [8.48] tested sintered silicon nitride in a

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simulated gas turbine rig, where the corrosive environment was continued throughout the 1000°C/40 hr of the test. Roomtemperature MOR fracture origins were located at pits in 17 of 22 samples. Pit formation was attributed to gas evolution during the oxidation of the silicon nitride and subsequent reaction of the silica with sodium sulfate-forming a low viscosity sodium silicate liquid. Fracture stresses were on the order of 300 MPa after exposure.

Boronand carbon-doped injected molded sintered α-SiC sprayed with thin films of Na2SO4 and Na2CO3 were exposed to several gas mixtures at 1000°C for 48 hr by Smialek and Jacobson [8.49]. The gas mixtures used were 0.1%SO2 in oxygen and 0.1%CO2 in oxygen in combination with the sulfate or carbonate thin films, respectively. The sulfate-covered sample was also exposed to pure air. Strength degradation was most severe in the sulfate/SO2 exposure (49% loss in strength), intermediate in the sulfate/air exposure (38% loss in strength), and least severe in the carbonate/CO2 exposure. The latter exposure caused a statistically insignificant decrease in strength when analyzed by Student’s t-test.* The primary mode of degradation was the formation of pits that varied in size and frequency depending upon the corrosion conditions.

The size of the pits correlated quite well with the strength degradation (i.e., larger pits caused greater strength loss). Jacobson and Smialek [8.50] attributed this pit formation to the disruption of the silica scale by the evolution of gases and bubble formation.

Zirconia-Containing Materials

Although a considerable amount of scatter existed in the data of Swab and Leatherman [8.46], they concluded that Ce-TZP survived 500 hr at 1000°C in contact with Na2SO4 at stress levels below 200 MPa. At stress levels greater than 250 MPa,

* The application of the Student’s t-test can be found in any elementary statistics book.

Copyright © 2004 by Marcel Dekker, Inc.