4y

250°C/24 hrs.

, 250°C/24 hrs.

in dry N2,

250°C/168 hrs.

in

vacuum

RESULTS AND DISCUSSION OF RESULTS The results of the initial characterization studies (Task I) are compiled in Table 5 and the results of the initial flexure strength measurements at 25°C in dry N2 and 1050°C in 10% water vapor are shown schematically in Figure 1. SEM photographs of polished and etched sections of each of the eight materials and one laboratory prepared Zr02 disc are shown in Figures 2 through 9. The results of the flexure strength (MOR) measurements for the five materials studied in Task II are presented in Table 6. The linear correlation of the measured MOR with temperature, based on a best fit computer analysis, is presented in Figures 10 through 14.

The flexure strength of all the transformation toughened ceramics evaluated decreased with increasing temperature. Similar results have been reported by a number of other investigators. 1~2 Except for the AZ301 material, the transformation toughened ceramics had an average strength of approximately 200 MPa at 1050°C. The AZ301 material had an average room temperature strength in excess of 1200 MPa and an average strength of nearly 600 MPa at 1050°C. The average strength measured in dry N2 was higher than the average strength measured in N2 with 10% H20 for a majority of matrix test conditions evaluated. In addition, the average strength measured at the rapid loading rate was higher than the average strength measured at the slow loading rate. These results suggest these materials are susceptable to environmental degradation.

For most of the fractured specimens it was observed that fracture originated at the tensile surface. Failure appeared to be due to cracks, large pores (voids), and other defects located at the surface of the tested specimens. A small number (10 to 15%) of the fractured specimens failed from an internal imperfection (large pores, cracks, and inhomogeneities).

Using the flexure data the dynamic fatigue constants (n and A) for these materials were calculated. A summary of these data for Tasks II and III are compiled in Table 7. Typical Ln-Ln plots based on these calculations are shown in Figures 15 to 18.

Forty percent of the n values were found to be statistically significant. The n values were designated as statistically significant if the difference between average MOR, for the two loading rates, were different from zero at the 95% level of confidence. The dynamic fatigue analysis data indicated subcritical crack growth in the MS and CTZP materials due to water corrosion at 25°C and 250°C. The data also showed that the MS and Z201 materials exhibit slow crack growth at 25°C in dry N2. The slow crack growth in dry N2 environments may be due to minute quantities of water trapped at the crack tip. The occurrence of slow crack growth at temperatures above 250°C could not be verified for these two materials. A statistically significant negative n value was found for the MS material at 1050°C in 10% H20. The significance of a negative n is not understood, however it may be due to the closing of surface cracks resulting from a volume expansion during phase transformation.

The effects of aging on the eight commercial materials was evaluated by XRD analysis and MOR testing. The results are compiled in Table 8. A comparison of the photomicrographs of the unaged material and the aged material indicated no change. The results of the XRD analysis for the 2Y, 3Y, and 4Y Zr02 discs, prior to and after aging, are presented in Table 9. As shown in Table 9, the 2Y and 3Y Zr02 materials appear to be susceptible to the effects of aging treatments in 10% H20 and dry N2. It was also observed that the 2Y Zr02 was subject to premature t->-m transformation after aging in vacuum. To confirm

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