| In order to improve the lifetime and reliability of engine turbine blades, advanced protective coatings and techniques are needed urgently for the turbine blade technology to block the harmful effects of temperature and environmental media on the superalloy matrix as a physical barrier. In the present work, NiCrAlYSi coatings have been deposited on the surfaces of nickel-base single crystal alloy and simulated turbine blade using a vacuum arc plating method, to obtain a better understanding on the optimization of the deposition process and high-temperature properties of NiCrAlYSi coatings.Vacuum arc plating method is used to deposit NiCrAlYSi coating on the surfaces of nickel-base single crystal alloy to evaluate the influence of the operating current on the microstructure and thickness of coatings. When the arc current is between 620±10A, low arc current may cause the instability and blow-out phenomenon of arc, which makes the coating structure uneven or porous. However, when the arc current is between 720±10A, the target materials may be slightly damaged. Only when the arc current is located between 670±10A, the coating process is relatively steady to obtain a dense and uniform microstructure. When the deposition time is less than 150 min, the thickness of NiCrAlYSi coating is proportional to the deposition time. In this case, the coating thickness is uniform, and the variations in coating thickness at different positions of turbine blades are generally less than 3μm.The coating thickness of vacuum arc plated NiCrAlYSi coating on the surface of Ni-based single crystal superalloy is about 25μm after a deposition duration of 90 min. After a vacuum diffusion treatment at 870 ± 10 oC for 32 h, the thickness of diffusion layer is about 5μm. In this case, the coating has an excellent bonding with the substrate. After high-temperature tensile tests at 760 oC, the Ni-based single crystal superalloy with a NiCrAlYSi coating exhibits the tensile strength of 1115 MPa, the yield strength of 922 MPa, the elongation of 14.5%, and the fracture surface shrinkage of 21.5%, respectively. Furthermore, after high-temperature tensile tests at 980 oC, the Ni-based single crystal superalloy with a NiCrAlYSi coating has a tensile strength of 885 MPa, a yield strength of 822.5MPa, an elongation of 31.5%, and a fracture surface shrinkage of 37.3%, respectively. As contrasted with the uncoated Ni-based single crystal superalloy, NiCrAlYSi coating increases its high temperature endurance performance by 15.9% and 63.5%, respectively, from 239 h to 276 h at 980 oC and from 167 h to 273 h at 1100 oC. However, the NiCrAlYSi coating slightly reduces the fatigue limit by 7.4% from 470 MPa to 436 MPa at 700 oC, as contrasted with the uncoated Ni-based single crystal superalloy.High temperature oxidation-resistance and hot corrosion resistance of NiCrAlYSi coating vacuum arc plated on Ni-based single crystal superalloy were evaluated by hgh-temperature isostatic or cyclic oxidation tests and hot corrosion tests in a molten salt mixture of 75% Na2SO4 + 25% Na Cl, respectively. As contrasted with the uncoated Ni-based single crystal superalloy, the NiCrAlYSi coating significantly enhances both the isostatic oxidation resistance at 1100 oC and the cyclic oxidation resistance at 1000 oC. After isothermal oxidation tests at 1100 oC for 100 h in air, the average oxidation rate of coating sample is only 0.087g/m2 h, which is less than 0.1 g/m2 h and conforms to a completely antioxidant level. After isothermal oxidation test at 1100 oC for 100 h or cyclic oxidation tests at 1000 oC for 250 times, the surface oxidized products on Ni-based single crystal superalloy without NiCrAlYSi coating consists mainly of Ni O, Ni Al2O4 and Ni Ta2O6, and these oxide films peel off seriously. Part of these oxidized regions forms some tumor-like oxides, and an internal oxidation phenomenon is observed. However, a relatively dense α-Al2O3 oxide film is formed on the surface of NiCrAlYSi coating on Ni-based single crystal alloy after isothermal oxidation test at 1100 oC for 100 h or cyclic oxidation tests at 1000 oC for 250 times.As contrasted with the uncoated Ni-based single crystal superalloy, the NiCrAlYSi coating improved the hot corrosion resistance when subjected to a molten salt environment of 75 wt.% Na2SO4 + 25 wt.% Na Cl at 900 oC. A relatively continuous Al2O3 oxide film is formed on the surface of NiCrAlYSi coating, however, some YAl O3 particles are generated at localized regions, which results in a decrease in Al content and the degradation of NiCrAlYSi coating. Both internal oxidation and sulphurization phenomena are observed in the matrix of Ni-based single crystal superalloy close to the interface, such as the formation of Al2O3. |
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