| 其他摘要 | The thesis is focusing on corrosion behaviors of high-temperature protective MCrAlY coatings. Due to its superior advantage in forming dense and adherent protective oxide scales (i.e. Al2O3/Cr2O3), MCrAlY overlay coating series have been widely adopted for protections on hot components serving in aircraft engines and gas turbines. During services, no protective MCrAlY coating is able to be absolutely immune to harms induced by corrosions, of which the corrosion forms can be generally classified as room-temperature ambient corrosion, high-temperature oxidation and salt-induced hot corrosions. Thus, it is important and meaningful to investigate the corrosion behavior and mechanism for MCrAlYs, because the serving life of them depends greatly on the corrosion attacks.
A NiCoCrAlYSiB coating was deposited onto a directionally solidified nickel-base superalloy DZ125 using arc ion plating (AIP) method. The analyzing approaches involve mass gain measuring, microstructure observing, phase identification, chemical composition detecting, which they were performed with the aid of high-precision optical balance, scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), electron probe micro- analyzer (EPMA), X-ray photoelectron spectrum (XPS) and X-ray diffractometer (XRD).
According to the investigation purpose the NiCoCrAlYSiB coating specimens experienced different heat treatments, resulting in four types of samples of as-deposited, vacuum-annealed, pre-oxidized and short-term oxidized. Then they underwent identical salt spraying test for 200 h in a neutral NaCl solution mist, followed by an additional isothermal oxidation at 1000 ℃. The result shows a discriminated corrosion consequence to the four kinds of coating specimens, of which the as-deposited and as-annealed NiCoCrAlYSiB coatings suffered observable ambient corrosion and high-temperature oxidation while satisfactorily enhanced results were achieved for the pre-oxidized and the shortly oxidized. The good behavior of the pre-oxidized and oxidized coatings in both salt spray corrosion and subsequent high-temperature oxidation undoubtedly owes much to the dense and continuous alumina scale formed during previous heat treatments. It is confirmed that the pre-oxidized coating behaved even better than the oxidized because of its effectiveness in forming protective alumina scale.
With the aim to fabricate MCrAlY coating accommodating more Al reservoir, an AIP co-deposition and a low-pressure chemical vapor deposition (LP-CVD) technique were employed. The CVD method proved the advantage over the co-deposition because the gas-phase aluminized coating was more homogeneous with true concentration grads. The optimized gas-phase aluminization craft is to maintain samples in a CVD chamber (below 100 Pa) at 1000 ℃ for 4-6 h with the reacting agents of 96-98 wt.% FeAl powder and 2-4 wt.% NH4Cl. The obtained Al-gradient coating has a multilayer structure: the Al-rich outer layer, the Cr-rich internal layer and the normal NiCoCrAlYSiB bottom layer with fine dispersions of β phase. There exists a grain size evolution from the top layer to the bottom. In the medium internal layer a Y3Al intermetallic and an α-Cr stacking fault were observed and characterized using high resolution electron microscopy.
The gradient coating surpassed the normal NiCoCrAlYSiB in 1100 ℃ isothermal oxidation, 1000 ℃ cyclic oxidation and 900 ℃ hot corrosions in both pure Na2SO4 salt and the Na2SO4/NaCl (75:25, w/w) mixture. Since it accommodates much more Al reservoir, the gradient NiCoCrAlYSiB coating can easily sustain the fixing and repairing process for the α-Al2O3 scale. However, the normal NiCoCrAlYSiB suffered unsatisfactory consequence in the mentioned oxidations because it lacks helpful Al after long-time exposures. Similar results happened to the hot corrosions in the two kinds of salts, which the gradient coating maintained a continuous α-Al2O3 scale at last in contrast to the disastrous result for the normal NiCoCrAlYSiB. Especially, the structural grad of the gradient NiCoCrAlYSiB coating was reserved after 200 h hot corrosion in pure Na2SO4 because of the less corrosive feature of pure Na2SO4 and that the hot corrosion temperature is lower than aluminization at 1000 ℃. By this way, the gradient NiCoCrAlYSiB takes the advantages in forming and repairing α-Al2O3 scale in all types of high temperature behaviors, resulting in an extended long-term service life for itself.
A sacrificed fluxing mechanism is able to describe the hot corrosion behaviors of MCrAlY coatings. The MCrAlYs utilize their beneficial element Al to resist harms induced by hot corrosions at some costs. The basic fluxing of Al2O3 brings two sides of impact: the destruction of the alumina scale and the reduction of alkalinity. Then if the Al content at the surface layer is high enough, the selective oxidation of Al2O3 can take place near the fluxing area. That is to say, by sacrificing some Al2O3 (or Al) a MCrAlY coating can obtain extended service life in return as long as it contains high levels of Al reservoir. The gradient coating is one of them. |
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