Hanyi Lou

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 Al2O3 films on Ni-base superalloys were evaluated. The oxygen partial pressure (PO2) could dramatically affect the chemical compositions and microstructures of the deposited films, and two PO2 thresholds were revealed, showing close relations with the increase or decrease of PO2 during sputtering. When PO2 varied through the PO2 thresholds, the total gas pressure, the sputtering voltage and the depositing rate exhibited abrupt changes. If PO2 was lower than the PO2 thresholds, the sputtered films were mainly composed of metallic aluminum with little alumina. When PO2 was higher than the PO2 thresholds the films were composed only of amorphous alumina. The Al2O3 films deposited on superalloys can reduce their oxidation rates by 3–8 times. It was observed that Cr2O3 grew both in the cracks within the Al2O3 films 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 Cr2O3. It is proposed that the oxidation of the specimens coated with Al2O3 is controlled predominantly by inward diffusion of anions through the Al2O3 films at 1273 K or below, and by outward diffusion of cations through the (Al, Cr)2O3 layers formed during the degradiation of the Al2O3 film at the higher temperature, 1373 K. The Al2O3 films showed beneficial effects on the hot-corrosion resistance of superalloys GH30 and K38G. The Al2O3 films on specimens of GH30 eliminated internal sulfidation and oxidation, while those on K38G alleviated internal sulfidation and oxidation.

The mechanism of scale adhesion on sputtered microcrystallized CoCrAl films

The oxidation mechanisms of sputtered microcrystalline Co−30Cr−5Al coatings were investigated by an acoustic emission technique, scatch test, transmission electron microscope (TEM), which was compared with CoCrAlY alloy. The results indicated that the beneficial effects of microcrystallization on the scale adhesion of Co−30Cr−5Al alloy are as follows: (1) The sputtered CoCrAl coating possesses a columnar structure, and oxidation along the columnar grains may form many micropegs which can anchor the scale to the metallic substrate, enhancing bonding of the scale. (2) The grain size of the sputtered coating is several orders of magnitude smaller than that of the cast alloy, and the grain size of oxide scales formed on the former is finer than that of the latter. The finer oxide scale may relieve the growth stresses during isothermal oxidation and partial thermal stresses during cooling by plastic deformation through grain sliding. The microcrystalline coating is more plastic than the cast alloy, which may relieve a certain amount of thermal stresses of the oxide scales. On the basis of oxide adhesion and plasticity, microcrystallization is more beneficial than the addition of reactive elements.

Rehealing ability of oxide scales formed on microcrystalline K38G coatings

The rehealing ability of the protective oxide scales formed on a microcrystalline coating of the Ni-base superalloy K38G has been studied. The results indicated that the oxide scales on the coating may be rehealed when the original scales are destroyed, and the coating still has excellent resistance against high-temperature oxidation. When the original Al2O3 oxide scales spall, mixed-oxide scales of both (Al, Cr)2O3 and TiO2 can form on the surface. The mixed scales are very protective and adherent.

The oxidation resistance of a sputtered, microcrystalline TiAl-intermetallic-compound film

The oxidation behavior of a cast TiAl intermetallic compound and its sputtered microcrystalline film was investigated at 700–900°C in static air. At 700°C, both the cast alloy and its sputtered microcrystalline film exhibited excellent oxidation resistance. No scale spallation was observed. However, at 800–900°C, the oxidation kinetics for the cast TiAl alloy followed approximately a linear rate law, which indicates that it has poor oxidation resistance over this temperature range. The poor oxidation resistance of TiAl was due to the formation of an Al2O3+TiO2 scale which spalled extensively during cooling. Nevertheless, the sputtered, TiAl-microcrystalline film exhibited very good oxidation resistance. The oxidation kinetics followed approximately the parabolic rate law at all temperatures. Although the composition of the scales was the same as that of scales formed on the cast alloy, the scales formed on the sputtered microcrystalline-TiAl film are adherent strongly to the substrate. No scale spallation was found at 700–850°C, while a small amount of spallation was observed only at 900°C. This indicates that microcrystallization can improve the oxidation resistance of the TiAl alloy.

Effect of nanocrystallization on the oxidation behavior of a Ni-8Cr-3.5Al alloy

Magnetron-sputter deposition was used to produce a Ni–8Cr–3.5Al(wt.%) nanocrystalline coating on substrates of the same alloy. Theoxidation behavior of the cast Ni–8Cr–3.5Al alloy and itssputtered coating were investigated at 1000°C in air. Complex,layered-oxide scales composed of Cr2O3 outer layer,mixed spinel NiAl2O4 and NiCr2O4middle layer, and α-Al2O3 inner layer were formedon the Ni–8Cr–3.5Al nanocrystalline coating during 200-hroxidation, whereas Cr2O3, with some NiCr2O4external layer with internal Al2O3, formed on the castalloy. Because of the formation of this α-Al2O3inner layer on the coating, the sputtered Ni–8Cr–3.5Al coatingshowed better oxidation resistance than the cast alloy. The effect ofnanocrystallization on oxide formation is discussed. It was indicated thatthe formation of this α-Al2O3 inner layer was closely related to therapid diffusion of Al through grain boundaries in the nanocrystallinecoating and the relatively high Cr content in Ni–8Cr–3.5Al.

Cyclic oxidation resistance of CoCrAl alloys and their sputtered microcrystalline coatings with and without yttrium

Abstract The cyclic oxidation of the normal-grained CoCrAl alloys and their sputtered microcrystalline coatings with and without yttrium was investigated at 1000°C in air. The results indicated that the normal-grained Co30Cr5Al alloy exhibited very poor resistance against cyclic oxidation. After only one or two cycles, the initially formed Al 2 O 3 scale severely spalled off, and then Cr 2 O 3 and CoCr 2 O 4 spinels were formed as a surface layer, while Al 2 O 3 only existed in the form of internal oxides. In contrast, the normal-grained CoCrAl alloy with yttrium and the two kinds of sputtered microcrystalline coatings with and without yttrium showed excellent cyclic oxidation resistance at 1000°C. Neither scale spallation nor obvious internal oxidation was observed. The scales formed on the latter three kinds of samples were mainly composed of Al 2 O 3 . The reasons for the beneficial effects of reactive elements on the oxidation resistance have been well documented in the literature. In this paper, the mechanisms of the beneficial effect of microcrystallization on the oxidation resistance have been proposed.

Effective of Ta, Ti and TiN barriers on diffusion and oxidation kinetics of sputtered CoCrAlY coatings

Abstract The effect of Ta, Ti and TiN barriers on the diffusion of aluminium from sputtered CoCrAlY coatings to Ni-base superalloy substrates and on the oxidation kinetics of the coatings at 1000°C has been studied. The results show that the depth ( h ) of the β+γ growth zone between the coating and the substrate is proportional to the root of the annealing time ( t ), from which a method to evaluate the inhibition degree of aluminium diffusion has been proposed. It was found that all the three barriers could inhibit the diffusion of aluminium from the coatings to the substrates. The inhibition order is as follows: Ta > TiN > Ti. However, the oxidation kinetics was not improved by the additions of the barriers. The TiN barrier has a little influence on the oxidation kinetics of the CoCrAlY coating, while Ta and Ti barriers have a determental effect.

Oxidation Behavior of CoCrAlY Coated Intermetallic Ni3Al-Base Alloy IC6

The oxidation of CoCrAlY coated Ni3Al-base intermetallic alloy IC6 in air at the temperature range 1000–1100°C was investigated. The results show that after 300-hour oxidation at 1000° the Al2O3 scale formed on CoCrAlY coating is in good condition with no spallation, but at 1050 and 1100 ° after about 60 hr oxidation the scale began to spall, the internal oxidation was observed in the diffusion zone beneath CoCrAlY coating, which may result from the serious degradation of CoCrAlY coating. Key Words: Ni3Al, intermetallics, CoCrAlY Coating, Oxidation,

Hot Corrosion Resistance of Reactive Sputtered Al2O3 Coating

Ni-base superalloys GH30 and K38G were coated with Al2O3 coatings by means of reactive sputtering. Hot corrosion was performed at 1123 K either by immersing the specimens half in the sulphate melt or by applying sulphate deposit on the specimens. The Al2O3 coatings had beneficial effects on the hot corrosion resistance of GH30 and K38G. The Al2O3 coatings on specimens of GH30 eliminated internal sulphidation and oxidation while those on K38G alleviated internal sulphidation and oxidation.