AlNi coatings on the internal surfaces of tubes

The oxidation resistance of nickel based superalloys used for high temperature components in gas turbine can be increased by the formation of high temperature intermetallic NiAl compounds. In this study the internal surfaces of Inconel tubes, with an internal diameter of 1 mm and a total length of 280 mm were aluminised by means of a CVD process using AlCl 3 and At as precursors. Subsequently, the aluminised samples were given a heat treatment under vacuum. Afterwards, the thickness and the composition homogeneity of the resulting NiAl layers before and after heat treatment were examined by electro-optical techniques that included optical and scanning electron microscopes and EDS X-ray microanalysis. The homogeneity of the resulted coatings was correlated to the deposition temperature and deposition time.