Reactive sputter deposition of alumina films on superalloys and their high-temperature corrosion resistance

Abstract

The deposition of stoichiometric alumina films by means of d.c. reactive sputtering was investigated. The oxidation and hot corrosion performance of the Al 2O 3 films on Ni-base superalloys were evaluated. The oxygen partial pressure ( P O 2 ) could dramatically affect the chemical compositions and microstructures of the deposited films, and two P O 2 thresholds were revealed, showing close relations with the increase or decrease of P O 2 during sputtering. When P O 2 varied through the P O 2 thresholds, the total gas pressure, the sputtering voltage and the depositing rate exhibited abrupt changes. If P O 2 was lower than the P O 2 thresholds, the sputtered films were mainly composed of metallic aluminum with little alumina. When P O 2 was higher than the P O 2 thresholds the films were composed only of amorphous alumina. The Al 2O 3 films deposited on superalloys can reduce their oxidation rates by 3–8 times. It was observed that Cr 2O 3 grew both in the cracks within the Al 2O 3 films and at the film-substrate interface at 1273 K or below. At 1373 K, the Al 2O 3 film was gradually displaced by a mixed layer of Al 2O 3 and Cr 2O 3. It is proposed that the oxidation of the specimens coated with Al 2O 3 is controlled predominantly by inward diffusion of anions through the Al 2O 3 films at 1273 K or below, and by outward diffusion of cations through the (Al, Cr) 2O 3 layers formed during the degradiation of the Al 2O 3 film at the higher temperature, 1373 K. The Al 2O 3 films showed beneficial effects on the hot-corrosion resistance of superalloys GH30 and K38G. The Al 2O 3 films on specimens of GH30 eliminated internal sulfidation and oxidation, while those on K38G alleviated internal sulfidation and oxidation.