Nikel esaslı süperalaşım üzerinde bağ kaplamasının oluşturulması ve karakterizasyonu

The gas turbine produced energy is directly related the operating temperature therefore higher operating temperatures is required for energy production. The requirements need higher temperature capability materials and associated technologies such as improved oxidation and environmental coatings. Therefore, for elevated temperature application, Nickel-base superalloys are the best choice and they have become the standard for industrial gas turbines hot gas path components such as buckets, nozzles, and shrouds. Even though the base material provide great features for this kind of application the component has surface damage caused by oxidation and hot corrosions shortened the service life. Therefore, the blade protection is essential, and the solutions lead the protective coating layer of the turbine blades. For protection of the hot gas path component and better working condition of turbine, the coating must meet the engine operating condition requirements. It must withstand hot corrosion, oxidation, and erosion and static and alternate stresses applied to the blade surface; to this end the coating must have the requisite combination of strength and ductility. Besides, it must show good stability and not degrade the blade material mechanical properties. In the light of previous information, the thesis focus on the production of aluminide coating on the nickel based superalloys, Inconel 718 (IN718) by cold gas dynamic spray coating method. The method is promising technique because its working temperature relatively low when compares with the thermal barrier coatings methods. The main action of the study is examining the diffusion behavior of cold sprayed aluminum, nickel and other constituent elements of superalloy and understanding the formation of diffusion zones. Accordingly, some part of the specimens is coated only with unalloyed aluminum and remaining parts are coated with unalloyed aluminum and nickel separate steps (First Al is coated on substrate than Ni is coted on Al surface). In both sample groups, Al activity is higher than Ni (Al diffusion rate higher than Ni-DAl>DNi), because of being in elemental state. Especially, only Al coated and annealed at higher temperature (700°C, 850°C, 900°C for 24h- 950°C, and 1000°C, 1050°C for 1 h) samples clearly show three layered coatings and roughly name of these layers are, from surface to substrate, refractory element precipitates, NiAl, and Carbide layers. The multilayered coating is also called as "inward diffusion coatings" because of the high diffusion rate of Al than Ni (DAl>DNi). The samples that are coated only Al and annealed at 500°C, 600°C for 24h, represent the basic formation of Al3Ni and Al3Ni2 intermetallic that are the production of first interaction of Al and Ni below the 640°C (Melting temperature of Al). Moreover,thisis a result of predominant inward diffusion of aluminum from the aluminum layer, which causes the coating layer to grow into the substrate starting from its surface. The standard three layered (multilayered) structure is formed at specimens that are annealed at 700°C, 850°C, 900°C, 950°C, 1000°C,1050°C. The first zone of the multilayer is outer zone that includes refractory elements precipitates (α-Cr, Fe) in the AlNi matrix and that is formed by dominant diffusion of Al (inward diffusion) throughout to the substrate. In addition, AlNi phase is hyper stoichiometric (NiAl containing greater than 50 Al %at.). Second zone (middle zone) is formed by both inward and outward diffusion of Al and Ni that creates uniform AlNi phases (hypo stoichiometric AlNi- Al <50 % at. ). Finally, third zone, created by only outward diffusion of Ni and the reduction of Ni in this region causes the precipitation of the various elements originally present in solid solution in the superalloy in the form of several complex precipitates. These are some carbides (MC, M23C6) and refractor elements' phases (σ-Cr, Fe). Consequently, higher activity of Al leads to the complex structure of multilayered coating and at each layer, different diffusion. Aluminum has very high activity and content in the first zone. It causes the lack of sufficient solubility of certain substrate elements like Cr, Ti, Mo in the zone. In the scope of the thesis, outwardly diffused microstructre can not be produced due to the high activity of aluminum. On the other hand, the second and third zone could be given as an example for that kind of structure. The different annealing time and temperature (700°C, 850°C, 900°C, 950°C, 1000°C,1050°C) do not make any change in the microstructure except distinct thickness for the zones The first and third zones are mechanically weak and brittle due to the formation of refractory precipitates and carbides; they should be eliminated from the coating by additional processes or changing the composition of coating powders. The both aluminum and nickel coated samples are annealed at 850°C, 900°C for 24h, 950°C, 1000°C, and 1050°C for 1h. The similar three-layered structure is observed between aluminum-substrate, and aluminum–nickel. However, in the middle zone, porous and cracked type area is produced. The structure is formed due to high activity of aluminum, because aluminum is completely consumed before creating a diffusional bond between substrate-aluminum and aluminum-nickel. Therefore,nickel coating on the aluminum does not meet the requirement of diffusional coating. The diffusion activities of elements and created phases are supported by characterization test results of X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS) The Al coated samples are subjected to hardness test to measure the hardness of each zone. According to results, the first and third zone value is highest because of refractory precipitates and carbides.