REACTIVE SPUTTER-DEPOSITION OF ALUMINA FILMS ON SUPERALLOYS AND THEIRHIGH-TEMPERATURE CORROSION-RESISTANCE

The deposition of stoichiometric alumina films by means of d.c. reacti ve sputtering was investigated. The oxidation and hot corrosion perfor mance of the Al2O3 films on Ni-base superalloys were evaluated. The ox ygen partial pressure (P-O2) could dramatically affect the chemical co mpositions and microstructures of the deposited films, and two P-O2 th resholds were revealed, showing dose relations with the increase or de crease of P-O2 during sputtering. When P-O2 varied through the P-O2 th resholds, the total gas pressure, the sputtering voltage and the depos iting rate exhibited abrupt changes. If P-O2 was lower than the P-O2 t hresholds, the sputtered films were mainly composed of metallic alumin um with little alumina. When P-O2 was higher than the P-O2 thresholds the films were composed only of amorphous alumina. The Al2O3 films dep osited on superalloys can reduce their oxidation rates by 3-8 times. I t was observed that Cr2O3 grew both in the cracks within the Al2O3 fil ms and at the film-substrate interface at 1273 K or below. At 1373 K, the Al2O3 film was gradually displaced by a mixed layer of Al2O3 and C r2O3. It is proposed that the oxidation of the specimens coated with A l2O3 is controlled predominantly by inward diffusion of anions through the Al2O3 films at 1273 K or below, and by outward diffusion of catio ns through the (Al, Cr)(2)O-3 layers formed during the degradiation of the Al2O3 film at the higher temperature, 1373 K. The Al2O3 films sho wed beneficial effects on the hot-corrosion resistance of superalloys GH30 and K38G. The Al2O3 films on specimens of GH30 eliminated interna l sulfidation and oxidation, while those on K38G alleviated internal s ulfidation and oxidation.