The microstructure and stability of CVD aluminia coatings
Doctoral thesis, 1994
This thesis has examined topics of relevance for increasing the understanding of alumina coatings produced by Chemical Vapour Deposition. The work consists of three parts: (i) The microstructure of Al2O3 coatings; (ii) the .kappa.-Al2O3 -> .alpha.-Al2O3 phase transformation; and (iii) structure and diffraction data for .kappa.-Al2O3. Analytical transmission and scanning electron microscopy in combination with x-ray diffractometry have been the main experimental techniques. The detailed microstructures, such as crystal structures, surface morphologies, twinning, dislocation density, porosity, orientation relationships, grain size and chemical compositions of both single- and multilayer coatings of .alpha.-Al2O3 and .kappa.-Al2O3 were examined and provided the basis for a model describing the preferential nucleation of a particular alumina polymorph (.alpha./.kappa.) on a particular bonding or modification layer. In addition, in multilayer coatings epitaxy was frequently found and the orientation relationships could be determined using electron diffraction in TEM. The reason for and the general form of these relationships could be described using crystallographic information from the TEM investigations. The .kappa. -> .alpha. transformation in different single- and multilayer coatings of .kappa.-Al2O3 was examined. Features of the transformation, such as transformation rate and adhesion were related to the microstructure of the coatings and a model for the transformation growth was presented. A mechanism for intergranular cracking due to the volume change associated with the transformation was reported. X-ray powder diffraction data for .kappa.-Al2O3 are reported. The peak intensities down to 3% of the maximum intensity in the 2.OMEGA. range 3 - 150 °C; are given. The lattice parameters were found to be a=4.834(1) Å, b=8.310(1) Å, c=8.937(1) Å and the theoretical density was calculated to be 3.773 g/cm3. A model for the structure of .kappa.-Al2O3 is presented which is based on comparisons of computer simulations of XRD patterns with experimental data, in combination with previous knowledge of .kappa.-Al2O3, such as thermal stability and density.