Microstructure of Alumina (Al2O3) Grown by Oxidation of FeCrAl Alloys and by Chemical Vapour Deposition

Doctoral thesis, 2009

This thesis consists of two parts: the detailed microstructure of (i) CVD alumina multilayer coatings, and (ii) alumina scales formed on three different FeCrAl alloys. The coatings and oxide scales were characterized by using analytical electron microscopy and different surface analysis techniques. The first part of this work investigates the interfacial structure in hot-wall CVD TiC/Al2O3 and TiN/Al2O3 multilayer coatings deposited both on cemented carbide substrates (which are commercially used) and on α-Al2O3 single crystals (model systems) with different surfaces. In TiN/κ Al2O3 multilayers epitaxial columns were frequent and the same orientation relationships were found when deposited on both types of substrates. One difference between the two cases (cemented carbide substrates and model substrates) was that γ-Al2O3 could grow (for a short distance) on the TiN layers in the former case, while no γ-Al2O3 was found in the latter case. In addition, when TiN/κ-Al2O3 multilayers were deposited on the model substrates the κ→α phase transformation occurred. The microstructure of the transformed α-Al2O3 layers was different compared to as-deposited α-Al2O3, e.g. several voids and dislocations formed within the transformed α-Al2O3 layers. Also TiC/α-Al2O3 multilayers have been deposited on different surfaces of single crystals of α-Al2O3. In this case, the TiC layers were oxidized in-situ prior to the alumina deposition. The general microstructure of the alumina and TiC layers was very different when deposited onto different surfaces of α-Al2O3 single crystal substrates. The second part of this work documents detailed microstructural investigations of three different alumina-forming FeCrAl alloys exposed to high temperature oxidation under various conditions. The oxidation behaviour of the investigated alloys follows a similar trend no matter if they are cast or manufactured by powder metallurgy. Before oxidation, on the surface of the FeCrAl alloys, a thin native oxide containing Fe, Cr and Al, with relatively high Cr content was formed. After oxidation at relatively low temperatures, 500‒600°C, an oxide containing a mixture of Fe, Cr and Al with a thickness of less than 100 nm formed on the Cr-rich native oxide. At higher temperatures (700°C or above) two-layered alumina scales formed, separated by a Cr-rich band that is believed to be the remnant of the pre-existing native oxide and hence represent the original alloy/gas interface. Accordingly the inner alumina layer is formed by inward oxygen diffusion, while the outer part forms by outward cation diffusion. The inner alumina layer consists of α-Al2O3 in both dry and wet O2. The outer alumina layer initially forms rapidly growing metastable phases, which transform to α-Al2O3 with time. The phase transformation starts at the Cr-rich band and proceeds outwards. The presence of water vapour inhibits the phase transformation, which is believed to be due to the stabilization of γ-Al2O3 by water vapour. Hence, a higher oxidation rate was observed in presence of water vapour.