Research On Preparation And High-temperature Protectiveness Of NiAl Cladded Coatings Reinforced By Ceria Nano-particles

As big thrust-weight radio is becoming development trend in advanced gas turbine engines, the operating temperature of turbine blades continues to rise. The metal bonding layer in thermal barrier coating is bound to take on a higher requirement in the field of high-temperature protectiveness. The current MCrAlY coating, which is commonly used in high-temperature field,depending on chromium oxide as the main protective oxide film, however, was highly volatile in the high temperature oxidation and hot corrosion environments. So, the research in developing new anti-oxidation and anti-corrosion, which is superior to traditional metal bonding layer in thermal barrier coatings, is becoming imperative. The conventional aluminide coating, depending on the excellent thermal stability of NiAl intermetallic compound, has played an important role in high-temperature field. Based on the current research, a new NiAl coating, which consisted of β-NiAl phase, was prepared on the surface of GH4033 by laser cladding in this paper. And different percent nano-CeO₂ was introduced into NiAl coatings. Then, microstructure and micro-hardness between NiAl coatings modified by different percent nano-CeO₂ and the original NiAl coating were analyzed, and the high temperature protective properties of NiAl coatings reinforced by active effect and nanostructure effect were also discussed in different aspects such as grain refining, oxyphilic element diffusion, β-NiAl phase transition and oxide film formation mechanism.The major work and the results obtained are as follows:（1） In the analysis, dilution ratio was regarded as the main evaluating indicator of NiAl coatings, integrating surface flatness and cross-sectional geometry. During the laser cladding process on the surface of GH4033, the effect of laser power, scanning speed and tablet thickness on the laser cladding quality was studied. And the best cladding parameters under a large spot diameter were obtained by orthogonal experiments. The parameters optimized in the experiment were 950W（laser power）, 300mm/min（scanning speed） and 0.7mm（tablet thickness）.（2） By analyzing microstructure between NiAl coatings modified by 1.5wt.%, 3 wt.% and 4.5 wt.% nano-CeO₂ and original NiAl coating, including the crystalline form, grain size, phase composition, microstructure hardness and so on, microstructure evolution of NiAl coatings modified by different percent nano-CeO₂ was discussed. It showed that: Comparing with the original NiAl coating, geometry of NiAl coatings modified by different percent nano-CeO₂ turned from typical dual arc to flat. Pores, segregation and other defects of the coatings were disappeared. And the grains were refined under the effect of nano-CeO₂. The main component of the coatings modified by nano-CeO₂ was β-NiAl phase. Among NiAl coatings modified by different percent nano-CeO₂, coatings modified by 3wt.% nano-CeO₂ were better.（3） High temperature oxidation behavior between NiAl coatings modified by 1.5wt.%, 3 wt.% and 4.5 wt.% nano-CeO₂ and original NiAl coating were studied at 1200℃. The results show that: Comparing with the original NiAl coatings, nano-CeO₂ improved the oxidation resistance of NiAl coating significantly. The diffusion rate of the oxyphilic element was much less than that of the original NiAl coating. After isothermal oxidation for 100 h, there was no loose on the surface of NiAl coatings modified by different percent nano-CeO₂. Meanwhile, the main component of oxide film was composed of Al2O3. The oxidation kinetics also suggest that NiAl coatings modified by 3wt.% nano-CeO₂ owned a better oxidation resistance in the experiment.（4） Hot corrosion behavior between NiAl coatings modified by 1.5wt.%, 3 wt.% and 4.5 wt.% nano-CeO₂ and original NiAl coating were studied at 1200℃. The results show that: the loose phenomenon happened on the surface of original NiAl coatings after 80 h hot corrosion. Meanwhile, the oxide film on the surface of the original NiAl coating has been basically off and serious internal corrosion happened in some parts of the coating. However, NiAl coatings modified by different percent nano-CeO₂ formed α-Al2O3-Cr2O3 oxide system on the surface of the coatings, and the internal corrosion depth decreased. In addition, there was no significant loss and bulge on the surface of the coatings. Among NiAl coatings modified by different percent nano-CeO₂, coatings modified by 3wt.% nano-CeO₂ owned a better hot corrosion resistance in the experiment.（5） The thermal shock behavior between NiAl coatings modified by 1.5wt.%, 3 wt.% and 4.5 wt.% nano-CeO₂ and original NiAl coating were studied at 1000℃. The results show that: Some cracks appeared on the surface of original NiAl coating after 30 thermal shock cycles. Meanwhile, the cracks were alse found at the interface between coating and substrate. But there is no obvious cracks at the interface of the coatings modified by 1.5wt.%, 3 wt.% and 4.5 wt.% nano-Ce O₂. Comparing with the original NiAl coating, the number of cracks on the surface of the coatings modified by nano-CeO₂ is rare. And it was difficult to see the cracks between coating and substrate. Coating organization improved by nano-CeO₂ could greatly inhibit the formation of cracks during the thermal shock. Among NiAl coatings modified by different percent nano-CeO₂, coatings modified by 3wt.% nano-CeO₂ owned a better thermal shock resistance in the experiment.