High Temperature Oxidation Behavior of Nano-Al2O3 Modified β-NiAl Coating
To improve the high temperature oxidation behavior of the aluminide coating,nano-Al2O3 particles were added and uniformly dispersed in the coating.Al2O3-modified and Al2O3-free δ-Ni2Al3 coating systems were prepared by aluminizing electroplated Ni-Al2O3 and Ni films on Ni substrates at 620 ℃.Accordingly,the two δ-Ni2Al3 coating systems were vacuum annealed at 1 000 ℃ for 10 min to obtain Al2O3-modified and Al2O3-free β-NiAl coating systems.Then,these two β-NiAl coating systems were oxidized at 1 000 ℃ for 20 h.The structures and morphologies of these aluminide coatings before and after oxidation were observed through X-ray diffraction(XRD)and scanning electron microscopy(SEM)/energy dispersive spectroscopy(EDS)analysis.After 10 minutes of vacuum annealing at 1 000 ℃,the Al2O3-modified δ-Ni2Al3 coating system degraded to Al rich β-NiAl phases at the depth of XRD detection,while the Al2O3-free coating degraded to Ni rich β-NiAl phases.The cross-sectional morphologies of these two coatings were consistent with the XRD results.The oxidation kinetics curves of the modified and unmodified β-NiAl coating systems at 1 000 ℃ showed that the overall oxidation weight gain of the coating decreased after the addition of nano-Al2O3 particles.According to the difference in oxidation rates,the oxidation process of these two coatings could be divided into two stages:stage Ⅰ with a fast oxidation rate and stage Ⅱ with a slow oxidation rate.There was not much difference in the oxidation rate between these two coatings after entering stage Ⅱ.It was found that the oxidation rate of stage Ⅰ for the Al2O3-modified coating was relatively high compared to the Al2O3-free coating.However,the transition time from stage Ⅰ to stage Ⅱ occurred earlier for the Al2O3-modified coating.Once the oxidation process entered stage Ⅱ,the oxidation rate significantly decreased.After 20 h of oxidation,the oxide scales on surfaces of both coatings were α-Al2O3.The Al2O3-modified coating degraded to Ni rich β-NiAl and γ'-Ni3Al phases,while the unmodified coating completely degraded to γ'-Ni3Al phases within the range detected by XRD.The results showed that the degradation of the coating was reduced after the addition of nano-Al2O3 particles.From the cross-sectional morphology of the Al2O3-modified coating,it was observed that the oxide scale of the modified coating was dense and well-bonded to the substrate after oxidation.Meanwhile,some small-sized cavities were found at the oxide scale/coating interface and inside the coating.However,the surface of the unmodified coating exhibited significant spallation after oxidation and large-sized cavities were found in the spallation area.Cavities exceeding 5 μm also presented at the oxide scale/coating interface where the oxide scale had not peeled off.These observations indicated that the adhesion of the oxide scale was improved after modification with nano-Al2O3 particles.The results above show that the addition of the nano Al2O3 particles can reduce the oxidation rate and increase the oxide adhesion of the aluminide coating,resulting in a lighter degradation of the Al2O3-modified coating under the same oxidation conditions.
万方数据
