Transmission electron microscopy observations of t′-phase after laser sealing of yttria partially stabilized zirconia

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The version of the zirconia-rich portion of the zirconia-yttria phase diagram as reported by Scott [1] has until now been considered the most reliable; in this diagram both the equilibrium tetragonal (t) phase and non-equilibrium tetragonal (t') phase, also called non-transformable (sometimes designated as secondary tetragonal) [2] have been reported. The significant differences between these tetragonal phases are that t-phase contains less yttria, corresponding to the phase diagram, while the t'-phase contains approximately the same amount of yttria as the starting material. Moreover, the tetragonality (c/a ratio) of t'-phase is less than that of t-phase and approaches unity; t'-phase is produced by athermal transformation and t-phase is produced by diffusional transformation. Upon decreasing the annealing temperature to below about 1100 °C, the t-phase transforms martensitically to monoclinic (m) phase. Transmission electron microscopy (TEM) investightions have been used to evaluate plasma-sprayed layers and laser-sealed layers of 6 wt % yttria stabilized zirconia (YSZ) and 8 w t % YSZ [3]. Z rO2-6wt% Y203 plasma-sprayed layers were found to have equiaxed grains approximately 500 nm in size, consisting entirely of tetragonal variants in the form of plates of width approximately 0.1 p~m. Thecell size was changed to about 1.75/xm after laser sealing and consisted entirely of tetragonal variants approximately 0.08/xm wide. On the other hand, ZrO; -8 wt% Y203 plasma-sprayed layers [3] consisted of very fine grains of approximately 20 rim, of cubic structure, tetragonal or cubic grains of size 0.1-0.5 tzm and large grains of size 1-5/xm consisting entirely of tetragonal variants of width about 200 nm. After laser sealing, all cells were found to be in the range 1-3/zm and consisted entirely of approximately 150 nm tetragonal variants. In the work reported here, specimens of 8.5 wt % yttria-partially stabilized zirconia (YPSZ) plasmasprayed layers of thickness about 0.5 mm were remelted by CO2 laser radiation at a power of 0.8-1.7 kW at a constant beam diameter of 5 mm and different traverse speeds (5 .3-370mms-i) . Overlapped tracks of approximately 10% overlap were produced. The microstructures of the upper surface and transverse section of the layers were evaluated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) for