Annealing of aluminium oxide/iridium coatings in oxidizing atmosphere

Abstract

The deleterious effects of oxidation on the life and performance of structural carbon materials during high temperature heating are well established [1]. The principal properties required of coatings on carbonaceous material for oxidation protection at high temperature include thermal stability, good adhesion and minimum or no chemical reactions. Since it is unlikely that any single coating material will meet these requirements, the most effective way to cope with this challenge is to use bilayer or multilayer coatings. Studies related to high-temperature annealing of A1203/Ir coatings on isotropic graphite (IG) substrate in inert gas atmosphere showed that asdeposited amorphous or y-A1203 coatings were converted into oL-A1203 [2]. However , the results also showed that a very thin layer of AlIr interracial compound was formed. Since it was difficult to coat the iridium and A1203 coatings on all sides of the IG substrate by present coating system, annealing of A1203/Ir coatings on IG substrate in oxidizing atmosphere was impossible because oxygen burned off the IG. For this purpose A1203/Ir coatings were deposited on o;-A120 3 substrate and subsequently annealed in an oxidizing atmosphere. In this work the effect of annealing in an oxidizing atmosphere on the structural morphology and chemical reaction between A1203 and iridium coatings were investigated. RF magnetron sputtering was used for aluminium oxide and iridium coating deposition on 01-A1203 substrates. A detailed description of the preparation of the aluminium oxide and iridium coatings are reported elsewhere [3, 4]. Fig. 1 shows a schematic illustration of Al203/Ir coating deposition on oc-A120 3 substrate. First, an iridium coating of about 4 mm x 8 mm was deposited on the ol-A120 3 substrate and secondly, in order to cover the iridium coating completely, an A120 3 coating 8 m m x substrate