4y

0(t)

0(t)

0(c/t)

0(t)

MP - Major Zr02 crystal phase: (m)- monocl inic; (t) - tetragonal;(c)- cubic.

these results, infrared and Raman spectroscopy were used to analyze the 2Y, 3Y, and 4Y Zr02 discs before and after aging. The results of these spectroscopic evaluations are shown in Figures 19 through 28.

Aging treatments were found to cause some degree of strength reduction in several of the materials studied. Reduction in flexure strength was most pronounced in the XS241 and Z201 materials. The XS241 material aged for 120 hours at 250°C showed a strength reduction of 35% and the Z201 a reduction of 17% after aging at 300°C for 25 hours. The TS material showed a decrease in strength in all of the environments evaluated. In contrast, the aging treatment for XS121 resulted in an increase in the strength. It is believed that the effect of aging on strength is related to the thickness of the surface transformation zone. A surface transformation layer less than 100 ym results in compressive surface . stresses which enhance strength. A surface transformation layer greater than 100 ym results in the generation of microcracks which reduce strength.

Premature t -»■ m transformation was found to occur during aging at both the 10% water vapor and dry N2 environments for the MS, TS, XS121, and XS241 materials. In addition, it was found that the MS and TS materials are susceptible to the effects of aging treatments at the two temperatures evaluated (250°C and 800°C). The amount of transformation for the MS and TS materials was relatively equivalent at both aging temperatures. This result had not been previously reported. It would appear that MS and TS materials are subject to t^-m transformation when heat treated above 250°C.

The XRD data, as well as the IR and Raman spectra for the 2Y, 3Y, and 4Y Zr02 materials (TZP) clearly demonstrated that the 2Y and 3Y materials are susceptible to the effects of aging, both in the presence of H20 vapor and in dry

N2. The 2Y material also demonstrated significant transformation to the monoclinic phase as a result of the vacuum aging treatment. The 4Y material was not affected by the different aging treatments and retained the tetragonal crystal structure after exposure to all aging environments. These results appear to be consistent with results published in the literature.4-11

For the tetragonal zirconia poly-crystal (TZP) materials it has been reported that the 2Y Zr02 is virtually all tetragonal (t) and the 6Y ZrOj is nearly all cubic.

The 3Y and 4Y materials are a mixture of tetragonal and cubic (c) phases. The tetragonal grains contain 1.8Yand the cubic grains contain 6.9 Y. The majority of large grains (above 1 ym) are cubic. The amount of tetragonal phase present is a function of the mole percent yttria stabilizer. As the mole percent Y203 increases, the critical size for aging degradation increases. At 2 mole percent the material shows aging effects unless the grain size is less than 0.2 ym. At 5 mole percent aging effects occur if the grain size is above 0.7 ym.'

The specific mechanisms responsible for the aging phenomena are not well understood. However, several theories have been proposed to account for the aging degradation of Y203 stabilized TZP materials. High surface stresses due to the anisotropy of the thermal expansion coefficients, formation of zirconium or yttrium hydroxide, or other gas-solid reactions at the surface have all been proposed as the cause of premature t-*m transformation. Our examinations of the aged TZP materials (XRD, XPS, FTIR, and Raman) did not detect the presence of OH- or the formation of OH compounds.

The combination of XRD, FTIR, and Raman analysis proved to be a particularly effective means to evaluate the crystal phase changes on the surface of the Zr02 candidate materials. The data obtained

»•: MM it 250-C In Or, », for 24 Hn. »C: Au« it 2S0*C In 10« ufa M m

»•: MM it 250-C In Or, », for 24 Hn. »C: Au« it 2S0*C In 10« ufa M m r d

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Figure 19. ATR FTIR spectra for Z-2Y.

• A: Aged it 250'C In 101 H,0 far 24 Hi-t .8: Ag.d tt 250*C In Ory for 24 Hrj»

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Figure 19. ATR FTIR spectra for Z-2Y.

• A: Aged it 250'C In 101 H,0 far 24 Hi-t .8: Ag.d tt 250*C In Ory for 24 Hrj»

Figure 21. ATR FTIR spectra for Z-4Y.

■ A: A9ad.1t 2S0*C In Or, R, for 2« Mr«. • I: Agad <t 250'C la 101 MÍO for SO Mr«. -C: As flacat««4

■ A: A9ad.1t 2S0*C In Or, R, for 2« Mr«. • I: Agad <t 250'C la 101 MÍO for SO Mr«. -C: As flacat««4

Figure 20. ATR FTIR spectra for Z-3Y.

Figure 20. ATR FTIR spectra for Z-3Y.

RAMAN SHIFT [1/cm]

Figure 22. Two percent yttria stabilized zirconia (as-received)-

RAMAN SHIFT [1/cm]

Figure 22. Two percent yttria stabilized zirconia (as-received)-

Figure 23. Two percent yttria stabilized zirconia, aged 24 hours in 10% water vapor at 250°C.

RAMAN SHIFT [1/Cffl]

Figure 23. Two percent yttria stabilized zirconia, aged 24 hours in 10% water vapor at 250°C.

Figure 24. Two percent yttria stabilized zirconia, aged 24 hours in dry nitrogen at 250°C.

RAMAN SHIFT [1/cffl]

Figure 24. Two percent yttria stabilized zirconia, aged 24 hours in dry nitrogen at 250°C.

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