Stability of Tetragonal Zirconia in Molten Fluoride Salts

The phase stability of tetragonal zirconia specimens in various molten salts was studied by a corrosion test. The corrosion test was examined using 3 mol% Y_2O_3-partially stabilized ZrO_2 ( 3 Y-PSZ) with high thermal phase stability. XRD results indicated that the phase transformation of 3 Y-PSZ from tetragonal (t) to monoclinic (m) phase ( t→m transformation) was observed only in molten fluoride salts. When 3 Y-PSZ was dipped into molten Na_2SiF_6 salt, more than 90% tetragonal phase on the surface of 3 Y-PSZ transformed into monoclinic one in 5 min-dipping. The corrosion depth accompanied by the t→m transformation reached 300 μm for 200 min-dipping. From the results of corrosion tests with alkaline metal fluorides, the rate of t→m transformation on the surface of a sintered body and the rate of depth increase from surface to inside depended on the ionic radius of univalent cations. In the same test with PbF_2\u27 the sintered body was heavily attacked, but no remarkable t→m transformation was observed on the surface of the sintered body. Raman spectra for the surface of test samples gave dispersed bands characteristic of the tetragonal phase. The result indicated that Pb restrains tetragonal particles. The corrosion depth for 3 Y-PSZ-Al_2O_3 was the same as that for 3 Y-PSZ. EPMA results indicated that the region enriched with Si or Na was equivalent to the t→m transformation range over 90% by XRD analysis. Remarkable t→m transformation is considered to be influenced by these cations. The activation energy was measured to estimate the t→m transformation rate. The activation energies of 3 Y-PSZ, 3 Y-PSZ-Al_2O_3 and 3 Y-PSZ-Cr_2O_3 were 12, 44 and 154 kJ/mol, respectively. The t→m transformation depth of 3 Y-PSZ-Cr_2O_3 was the smallest among the three 3 Y-PSZ complexes. 3 Y-PSZ- Cr_2O_3 had high phase stability in molten fluoride salts.rights: 社団法人日本セラミックス協会rights: 本文データは学協会の許諾に基づきCiNiiから複製したものであるrelation: IsVersionOf: http://ci.nii.ac.jp/naid/110002316770