MCrAlY coatings, as a group of high temperature coatings, have extensively been considered for protecting gas turbine blades against oxidation and hot corrosion. At temperatures of more than 1000uC, interdiffusion between superalloy substrate and coating is one of the most important factors for controlling the lifetime of these coatings. In this paper, thin layers of a-Al 2 O 3 were used as a diffusion barrier between superalloy substrate and coating using electron beam physical vapour deposition method. These barrier layers successfully blocked the diffusion of substrate alloy elements such as tungsten, molybdenum and, in particular, titanium into the coating. Results showed that using these diffusion barrier layers between superalloy substrate and coating can considerably improve the oxidation resistance of NiCrAlY coatings so that the parabolic rate constant in the samples with micrometric diffusion barrier layer is about one-third of that in the samples without it.
Thermal barrier coatings have been widely considered owing to increasing efficiency of gas turbine engines. These coatings are often available in ceramic, thermally grown oxide and metallic bond layers. The function of metallic bond layer in these coatings is adhesion improvement and substrate protection. This layer can also be applied for electron beam-physical vapour deposition (EB-PVD). Yet, EB-PVD method cannot control evaporation process to be able to induce a deposit with uniform chemical composition along the coating thickness. The present study investigates composition variation during evaporation process of Ni-20Cr-11Al-0?3Y alloy using computational models and experimental results along with transition time prediction by computational model. To this end, EB-PVD by an electron gun with power of 3 kW in 4-6610 25 mbar vacuums was used. The results indicated that applying an initial molten pool with a composition similar to that of equilibrium pool (Ni-2Cr-15Al-0?5Y) decreases transition time by 70%.
In the present study, MoO 3 nanopowder was produced by evaporation of molybdenum oxide powder at high temperature (around 1,000 C) and its subsequent cooling in inert gas. The effect of different parameters such as pressure, temperature, and type of inert gas on the morphology and particle size were examined. The samples were characterized in terms of structure, morphology, particle size, and surface area using complementary techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and the Brunauer-EmmettTeller (BET) method. Particles of the synthesized nanopowder had spherical shape with specific surface area of 18.64 m 2 g À1 and the average diameter of about 31 nm in optimum conditions.
NiCrAlY coating was deposited on the IN-738LC Ni-based superalloy by cathodic arc deposition at the substrate temperatures of 400°C and 700°C. The effects of the substrate temperature on the microstructure and the isothermal oxidation behaviour of the coatings were then investigated. SEM image and XRD analysis were used to characterize the deposited coatings. It was found that when the substrate temperature was raised from 400°C to 700°C, the degree of coating crystallinity was increased from 23% to 58%. After the 20-hr oxidation at 1000°C in air, the γ ′ -depletion layer was formed in the substrate. The growth rate constants of the γ ′ -depletion layer after the 100-hr oxidation were 1.3×10 −4 μm (s b ) −1 and 6.4×10 −5 μm (s b ) −1 for the deposited coatings at 400°C and 700°C, respectively. The results, therefore, indicated that with increasing the substrate temperature from 400°C to 700°C, the oxidation resistance was improved.
In this paper aluminum nano-powder was produced by evaporation and condensation in a low pressure argon gas atmosphere. The process was modified according to the argon pressure to decrease the aluminum nano-particle size. Therefore, the argon gas pressure was changed from 0.1 to 1 mbar by change of injection flow rate, while other parameters were maintained constant. To evaluate the morphology of produced nano-powders at each argon pressure, the nano-powders were analyzed by Brunauer- Emmett- Teller (BET) method and transmission electron microscopy (TEM). It was found that the final particle diameter decreased with argon pressure. This decrease of particle size reaches a minimum of 85 nm, which corresponds to the pressure of 1 mbar. This powder had a specific surface area of 26.22 m2g-1.
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