TRANSMISSION ELECTRON-MICROSCOPY STUDY OF ZIRCONIA-ALUMINA NANOLAMINATES GROWN BY REACTIVE SPUTTER-DEPOSITION - PART I - ZIRCONIA NANOCRYSTALLITE GROWTH-MORPHOLOGY

Pure zirconia films and zirconia-alumina nanolaminate films grown by r eactive sputter deposition are studied by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EE LS). The phase composition and morphology associated with zirconia cry stallite growth are investigated by examining films containing zirconi a layers of varying thickness. These studies, performed at room temper ature, suggest that the zirconia crystallites initially grow in the te tragonal phase to a critical size of 6.0 +/- 0.2 nm, in agreement with a value of 6.2 nm predicted by end-point thermodynamics. Past the cri tical size, incorporation of additional zirconia molecules into the zi rconia layers is accomplished predominantly by transformation of the g rowing crystallites to the monoclinic phase, and less frequently by de position of amorphous zirconia. Transformation to the monoclinic phase is accompanied by a highly faulted intermediary phase. The subsequent growth behavior of monoclinic crystallites is consistent with a three -dimensional intrface-controlled, diffusion-limited growth process wit h a growth exponent between 3 and 4. Nanoindentation measurements of n anolaminates with 5-nm thick zirconia lavers give a hardness of simila r to 8 GPa for the upper strata where the morphology of the tetragonal zirconia layers contains an intrinsic roughness. The hardness increas es to similar to 10 GPa closer to the substrate where the laminar morp hology is more pronounced, Young's modulus is between 156 and 195 GPa for these same nanolaminates. (C) 1998 Elsevier Science S.A. All right s reserved.