Uçak motorlarında kullanılan ınconel 718 süper alaşımının atmosferik plazma yöntemiyle kaplanarak sürünme davranışının incelenmesi

Abstract

In engineering applications, the material; tensile, compression, creep, wear, buckling, corrosion and fatigue strengths are the mechanical properties that directly affect the working life of the material. There are engineering parameters that trigger these features in the background. These; effects such as applied stress, shape and magnitude of stress, ambient temperature, ambient conditions (corrosive environment), design directly affect the durability of the material during usage. While choosing the material in engineering, choosing the proper material according to the environment in which the material will be used is important in terms of material strength and cost. For this reason, it is not enough to choose the right material in terms of engineering, it is also necessary to choose the suitable material in terms of cost. While choosing the material in engineering design, the material must be made in the production process in thermo-mechanical processes (heat treatment, rolling, surface treatments, forging, coating, etc.) that increase the service life. The selection of engineering materials to be used in aviation is very important. The selected engineering material should be long-life. For this reasons; superalloys are used extensively in aviation, especially in high temperature environments in certain areas, the use of superalloys alone is not sufficient. Additional processes should be carried out according to the usage of area for the materials. In recent years, plasma spray coating is the most common thermal barrier coating (TBC) in industrial applications, plasma coating with inert gas, nitrogen or a secondary gas, hydrogen or helium under direct current with water-cooled tungsten electrode or annular water, it works by generating high voltage between the cooled copper anode. The gas is heated, decomposed and plasma flame is formed in ionized form, the flame reaches about 16650 ºC to spray any inorganic material with sufficient temperature, the inorganic material melts without decomposition. The inert gas applies a force towards the plasma flow of the coating powder, where the coating powder melts and plasticizes and precipitates on the base material at speeds of approximately 80 m/sec to 300 m/sec. With the developer cooling techniques, the surface temperature of the base material is kept low so that there is no heat treatment effect. The purpose of thermal barrier coatings; it is to increase the creep life of the material by reducing the temperatures coming to the main material used at high temperatures. Thermal barrier coatings (TBC) are usually a combination of multiple layer coatings, the top layer provides thermal insulation and the ceramic layer has the lowest thermal conductivity and is known as ZrO2 zirconia. The ceramic insulating layer is deposited on the base material alloy by interacting with the so-called oxidation resistant bond layer. The bond layer is the diffusion of aluminum, for example the bond coating of platinum aluminum or NiCrAlY. A thermally growing Al2O3 oxide layer is formed between the ceramic coating and the bond coating. The approximate thickness of each layer, respectively; ceramic layer changes from 125μm to 1000μm, the thickness of the bond coating layer changes from 50μm to 125μm, and the thermally growing oxide layer from 0.5μm to 10μm. Inconel 718 superalloy, which is widely used a material in aviation, is an alloy with high yield strength, tensile strength, fracture toughness, corrosion resistance and as well as sufficient creep strength at high temperatures, but it cannot be used for a long time at high temperatures only by aging heat treatment, additional protective processes are required for its use. One of the suitable processes to enhance for creep strength of this material is a thermal barrier coating of the material surface. In this study, aged inconel 718 specimens were coated with 100 µm bond coating (NiCrAlY ) and 250 µm (ZrO2) yttrium stabilized thermal barrier coating (TBC) by atmospheric plasma coating method. Creep rupture-tests were performed at temperatures of 750 °C, 800 °C and stresses between 250-350 MPa for TBC coated and uncoated Inconel 718 specimens. What makes this study different from similar studies is that creep-rupture tests were performed on TBC coated and uncoated Inconel 718 samples and the effect of TBC coating on creep properties of Inconel 718 material were investigated. Thus, it was determined that the creep-rupture life decreased with the increase in temperature and stress for all samples which is an expected result, at the same time, it was also observed that the creep-rupture life of the coated samples were longer than that of the uncoated samples at the same stress and temperatures. On the other hand, using Comsol finite element software, it has been determined that the temperature on the core of the base material varies depending on the rupture time. When the rupture time is short, the temperature difference between the surface and center of the coated samples is higher compared to uncoated samples, but when the rupture time is long, the temperature difference between surface and center of all samples were close each other for coated and uncoated samples. This result showed that TBC coating is more effective at the creep conditions when creep lifes are short.