| 其他摘要 | The thesis is focusing on the fabrication and high-temperature corrosion behaviors of MCrAlY coatings. Due to theirs superior advantage in forming dense and adherent protective oxide scales, MCrAlY overlay coatings have been widely adopted for protections on hot components serving in aircraft engines and gas turbines. During services, MCrAlY coatings degradated with the decrease of Al, due to the unceasing growth and reformation of protective alumina and interdiffusion between coating and substrate. Once the Al content decreased to the limit of form protective alumina, damage would happen to MCrAlY coatings. So including as much as possible Al within the region of not deteriorates their mechanical properties greatly is one development direction of MCrAlY coatings. To find a kind of MCrAlY coating with high Al content and slow interdiffusion speed is meaningful.
Two kinds of conventional NiCoCrAlSiY coatings (M14 and M19 coatings) and another gradient NiCoCrAlSiY coating were prepared by ion arc plating (AIP) on single crystal DD32. Microstructure of the three coatings was characterized, isothermal oxidation behaviors under 1000 °C and 1100 °C, cyclic oxidation behaviors under 1000 °C and hot corrosion behaviors in Na2SO4/K2SO4 (75:25，w/w) and Na2SO4/NaCl (75:25，w/w) mixed salt under 900 °C of the coatings together with the DD32 substrate were investigated.
The M14 and M19 coatings were composed of darker β-NiAl, brighter γ’/γ and some α-Cr, but more darker β phase for the M19 coating for its relative higher Al content. A much higher Al content gradient NiCoCrAlSiY coating, consisting of Al enriched outer zone and Cr enriched internal zone, was prepared using arc ion plating method and subsequent annealing treatment.
All those coatings exhibited good at 1000 °C isothermal oxidation test and only compact and continuous alumina scales could be observed on the surface. Isothermal oxidation at 1100 °C indicated that plenty of cracking and spallation had occured to the oxide films of M14 and M19 coatings after 200 h, besides, Kirkendall holes was observed in the M14 coating. While for the gradient coating, the oxide film adhered well to substrate, besides, owing to the interdiffusion of Cr and Re/W, a continuous layer of Cr(Re, W) precipitate formed at the IDZ and acted as an in-situ diffusion barrier to inhibit the element interdiffusion effectively, which make the gradient the slowest degradation speed.
The gradient coating exhibited best among the coatings at 1000 °C cyclic oxidation test, and β phase was the main phase after 200 cycles in contrast to the disappearance of β for the M14 and M19 coating after 62 cycles and 200 cycles, respectively. Massgain fluctuated at 170 cycles and inner oxidation appeared after 200 cycles for the M14 coating.
All the coatings behaved well at 900 °C hot corrosion in Na2SO4/K2SO4 (75:25, w/w) mixed salt. The massgains and degradation were slow, and the oxide films were compact and continuous after corrosion for 100 h. While serious corrosion occurred to all coatings in Na2SO4/NaCl (75:25, w/w) mixed salt. Corrosion cavity and sulfuration were obvious in the upper part of all the coatings and beneficial element was consumed greatly.
High Al content coatings can reduce the alkalinity of melting salt at the sacrifice of some surface Al2O3. As long as the Al2O3 film could be repaired in time, accelerated corrosion could be suppressed. The gradient coating is one of them, which reserved a high Al content after corrosion for 100 h in Na2SO4 + NaCl mixed salt. Cl2, acting as an activator through the repetition of chlorination and oxidation, can accelerate the corrosion of metal components, which was responsible for the quicker corrosion rate in the mixed salt contained NaCl. |
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