Zirconia–alumina transformation-toughening nanolaminates were fabricated by reactive sputter deposition. The average crystallite size and volume fraction of each zirconia polymorph were determined by x-ray diffraction. The volume fraction of tetragonal zirconia, the phase necessary for transformation toughening, was found to strongly depend upon the zirconia layer thickness. An end-point thermodynamics model involving hemispherical cap zirconia crystallites was developed to explain this phenomenon. In excellent agreement with experimental results, the model predicts that unity volume fraction of tetragonal zirconia is produced in the nanolaminate when the zirconia layer thickness is less than the radius at which a growing zirconia crystallite spontaneously transforms to the monoclinic phase.

Composite systems containing zirconia have been used extensively as transformation-toughening materials based on a stress induced martensitic transformation of the metastable tetragonal phase of zirconia to the monoclinic phase. Recently it has been shown that tetragonal zirconia can be stabilized in zirconia-alumina nanolaminates grown by reactive sputter deposition, when the zirconia layer is less than 6 nm thick. Cross-section high resolution transmission electron microscopy (HRTEM) of these nanolaminates revealed localized tetragonal-to-monoclinic transformation caused by sample preparation. In this study, quantitative HRTEM is used to analyze the zirconia nanocrystallite transformation in situ, by controlled exposure of the sample to the electron beam of the microscope.The irradiation conditions used in this study to induce the zirconia transformation are summarized in Table 1. The mildest irradiation condition corresponds to normal imaging illumination used in this study to obtain high resolution images. Under these normal illumination conditions, the first condenser lens (CI) is used to form a 0.1 μm sized probe which is over focused on the sample by the second condenser lens (C2).

High resolution electron microscopy is employed to study the crystallography and morphology of zirconia nanocrystallites in zirconia-alumina nanolaminates and zirconia films. Unity volume fraction of tetragonal zirconia formed when the zirconia layer thickness was less than 6.2 nm, a theoretically predicted critical size for tetragonal-to-monoclinic zirconia (t -> m-ZrO2) transformation. In thicker layers, monoclinic zirconia formed, accompanied by renucleation and void formation which caused roughness to the zirconia nanolayers. The average position of the voids in the layers was 6.3 nm from the growth interface, coinciding with the critical dimension for t -> m-ZrO2 transformation.