Phase formation and microstructure in reactively sputter-deposited zirconia and yttria-stabilized zirconia coatings.

摘要

Knowledge of the phase formation and stability in zirconia (ZrO 2) and yttria-stabilized zirconia (YSZ) coatings is fundamentally important for improving the properties and performance of these materials for engineering applications. This investigation demonstrated that reactive bias sputter deposition can modify the crystal structure and phase stabilities in zirconia and yttria stabilized zirconia coatings.; The research described in this dissertation developed fundamental microstructure-processing relationships for zirconia-yttria (0, 2, and 4.5 mol% Y2O 3) coatings produced by reactive sputtering deposition. Relationships have been developed between the microstructure (crystal structure, texture, phase formation, thermal stability, and growth morphology) and major processing variables (substrate bias, Y2O3 content, and post-deposition annealing). This research used analytical techniques including room/high temperature X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution electron microscopy (HREM).; It was found that phase formation and crystallographic texture in ZrO 2 coating were each strongly dependent on the level of applied negative substrate bias and the amount of Y2O3 alloying. The results showed that the crystal structure of ZrO2 coating changed from random equilibrium monoclinic to random metastable tetragonal and finally to strong (111) oriented tetragonal crystalline when the substrate bias was varied from 0 to 850 V. Furthermore, a highly (111) preferred orientation of tetragonal and cubic zirconia was found in 2 and 4.5 mol% Y2O 3 coatings, respectively, and each of these coatings was grown by reactive sputtering with an applied substrate bias of --400V.; XRD and TEM analyses revealed that biased sputtering could effectively decrease crystallite size in the as-deposited coatings, which resulted in room temperature stabilization of the metastable tetragonal phase. In-situ high temperature XRD showed that the kinetics of tetragonal-to-monoclinic phase transformation was strongly dependent on the tetragonal grain size, which was a function of substrate bias and post-deposition annealing. The transformation fraction of metastable tetragonal phase, the desired phase for transformation toughening, can be controlled by a combination of the substrate bias and annealing temperature.