Stability of tetragonal zirconia in Molten fluoride salts

Abstract

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% Y2O3-partially stabilized ZrO2(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 Na2SiF6 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→y m transformation reached 300μm 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 rations. In the same test with PbF2, 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-Al2O3 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 rates The activation energies of 3 Y-PSZ, 3 Y-PSZ-Al2O3 and 3 Y-PSZ-Cr2O3 were 12, 44 and 154kJ/mol, respectively. The t→m transformation depth of 3 Y-PSZ-Cr2O3 was the smallest among the three 3 Y-PSZ complexes. 3 Y-PSZ-Cr2O3 had high phase stability in molten fluoride salts.