High-temperature oxidation of sprayed NiAl–Re coatings subjected to magnetic treatment

Abstract

The effect of structural changes that occur in heat-resistant detonation-sprayed NiAl-Re coatings treated in the magnetic field on their oxidation is examined. It is shown that the surface of coatings subjected to magnetic treatment is rich in nickel and layers of nickel oxides form more intensively and to a greater depth (as compared with those without magnetic treatment) until a continuous layer of alumina forms in the scale. The removal of the nickel-rich layer uncovers aluminum-rich surface where a continuous protective layer of Al 2 O 3 scale can be formed in high-temperature oxidation. INTRODUCTIONWe previously established that the microstructure and chemical composition of NiAl-Re alloy could vary under the magnetic field [1-2]. This is a promising effect for improving the properties of heat-resistant β-NiAl coatings, whose development is a very important area in aviation materials science. It is also reported that NiAl coatings are advantageous as a binding layer in thermal-barrier coatings along with widely used alloys such as MCrAlY (CoCrAlY and NiCrAlY) [3]. Numerous research efforts in this area are intended to ensure the fracture toughness of materials over a wide temperature range suitable for commercial applications and the high-temperature oxidation resistance that is possible when a continuous protective Al 2 O 3 layer forms on the coating. The problem is that higher aluminum content needed to form alumina over the coating makes the intermetallide base brittle.Detonation-sprayed β-NiAl + γ-Re coatings have higher fracture toughness than NiAl coatings [4]. This is because rhenium inclusions serve as obstacles and traps for cracks. There are differences in the oxidation of the detonation-sprayed NiAl and NiAl-Re coatings at temperatures between 800 and 1100°C in air [5]. For example, scale consisting of alumina with inclusions of spinel NiAl 2 O 4 forms on the NiAl coating at 1000°C. The scale on the NiAl-Re coating is layered: the outer layer is spinel and the inner Al 2 O 3 . This scale structure is typical of nickelrich Ni-Al alloys [6,7]. The presence of a spinel layer, whose mechanical properties are weaker than those of alumina, is undesirable [8]. Alloys with the composition corresponding to intermetallide NiAl contain substitution defects: some nickel atoms are located in the aluminum sublattice, i.e., nickel atoms that occupy vacancies in the aluminum sublattice are antistructural [9,10]. To form an oxide layer with desirable composition, the optimal