Abdullah Cahit Karaoglanli

Thermal Shock and Cycling Behavior of Thermal Barrier Coatings (TBCs) Used in Gas Turbines

Gas turbine engines work as a power generating facility and are used in aviation industry to provide thrust by converting combustion products into kinetic energy [1-3]. Basic concerns regarding the improvements in modern gas turbine engines are higher efficiency and per‐ formance. Increase in power and efficiency of gas turbine engines can be achieved through increase in turbine inlet temperatures [1,4]. For this purpose, the materials used should have perfect mechanical strength and corrosion resistance and thus be able to work under aggressive environments and high temperatures [2]. The temperatures that turbine blades are exposed to can be close to the melting point of the superalloys. For this reason, internal cooling by cooling channels and insulation by thermal barrier coatings (TBCs) is used in order to lower the temperature of turbine blades and prevent the failure of superalloy substrates [1-4]. By utilizing TBCs in gas turbines, higher turbine inlet temperatures are allowed and as a result an increase in turbine efficiency is obtained [5]. TBCs are employed in a variety of areas such as power plants, advanced turbo engine combustion chambers, turbine blades, vanes and are often used under high thermal loads [6-11]. Various thermal shock tests are conducted by aerospace and land gas turbine manufacturers in order to develop TBCs and investigate the quality control characteristics. Despite that fact, a standardized method is still lacking. The reason lies behind the difficulty of finding a testing method that can simulate all the service and loading conditions. Present testing systems developed by the engine manufacturers for simulation of real thermal conditions in engines consist of; burner rig thermal shock testing units, jet engine thermal shock testing units and furnace cycle tests [16-20]. In this study, thermal cycle and thermal shock behavior of TBC systems under service conditions are examined, and a collection of testing methods used in evaluation of performance and endur‐

Effect of sintering on mechanical properties of cold sprayed thermal barrier coatings

In this study, thermal barrier coatings were deposited on nickel-based super-alloy with CoNiCrAlY bond and yttria partially stabilised zirconia top coats, using cold gas dynamic spraying and atmospheric plasma spraying techniques, respectively. High temperature oxidation behaviour and sintering effect on the mechanical properties of the coatings were studied. The main objective of this work is to explain the alteration in coating structure depending on temperature, and the mechanisms that lead to alterations in mechanical properties such as Young’s modulus and hardness. Structural examination after oxidation revealed a decrease in porosity content and crack healing on ceramic top coatings resulting from sintering effect.

Comparison of Oxidation and Thermal Shock Performance of Thermal Barrier Coatings

Operating temperatures in gas turbine engines have reached to 1200°C with the latest developments in coating technology. Thermal shock and furnace oxidation tests are widely used to determine thermal barrier coating (TBC) performance and its durability in aircraft applications. This paper presents the results of thermal shock and furnace oxidation tests, carried out with regard to the microstructure and TGO (thermally grown oxide) growth behavior of TBC systems. Isothermal oxidation behavior of TBCs was evaluated through examination of microstructure, formation, and growth behavior of TGO layers at 1200°C for different time periods in furnace oxidation tests. On the other hand, thermal shock behavior of TBC was evaluated through examination of its durability at 1200°C with thermal shock test, which was carried out until the coating failure became visible. The relationship between the TGO growth and oxidation behavior was discussed using furnace oxidation test results.

A Comparative Study of the Microabrasion Wear Behavior of CoNiCrAlY Coatings Fabricated by APS, HVOF, and CGDS Techniques

The current study focuses on the microabrasion wear and microstructural properties of CoNiCrAlY coatings fabricated on nickel-based superalloy substrates using atmospheric plasma spraying (APS), high-velocity oxygen fuel (HVOF), and cold gas dynamic spraying (CGDS) methods. The microabrasion tests were performed on the samples for different durations in order to understand the wear mechanism of thermally sprayed coatings and influence of the coating microstructure on the wear mechanism. The microstructures of as-sprayed coatings and wear mechanisms on the worn coatings were investigated. Initial surface topography was examined using a surface profilometer. Coating hardness measurements were performed with a microhardness tester. The lateral fracture was observed as the wear mechanism on the samples. The wear resistance of the coatings has changed with the surface features of the samples depending on the coating production process.

State of the Art Thermal Barrier Coating (TBC) Materials and TBC Failure Mechanisms

Thermal barrier coatings (TBCs) are widely used in the aviation industry to improve the service life of components being exposed to high temperatures. Providing a higher resistance compared to conventional coatings, TBCs also improve the performance and lifetime of materials through their thermal insulating characteristic. Resistance of gas turbine components, particularly turbine blades, vanes and combustion chambers, is required to be improved against failures such as corrosion, oxidation and thermal shock as well, since turbine inlet temperatures should be increased to improve the performance of gas turbine engines. Accordingly, it is aimed to obtain a better resistance and durability against the failure mechanisms through enhancement of the methods and materials used in thermal barrier coatings. In this study, thermal barrier coatings used in gas turbines as well as their structure, also the relevant failure mechanisms and the new material groups used in TBCs are discussed.

Wear Behavior of Severe Shot Peened and Thermally Oxidized Commercially Pure Titanium

In this study, severe shot peening (SSP) has been performed to commercially pure titanium (Grade 2) specimens in order to expose severe plastic deformations to the material surface. S230 shot media with higher air pressure conditions have been selected for SSP. A nanocrystalline layer has been created by means of severe plastic deformation. The nanograined structure has been investigated via XRD, FESEM, and microhardness methods. SSP specimens have been subjected to thermal oxidation treatment in air at 500 and 700 °C for 2 and 16 h. Severe shot peened commercially pure titanium has a thicker oxide layer when compared to as received ones. The results show that both SSP and thermal oxidation improve the wear characteristics.

Effect of Boron Nitride Coating on Wear Behavior of Carbide Cutting Tools in Milling of Inconel 718

Boron nitride based tribological coatings promise hope in tribological applications thanks to their excellent lubrication and heat resistance properties. However, the applicability of these coatings on cutting tools in machining applications is not well known and it needs to be revealed. Therefore, in this study, a boron nitride (BN) coating was deposited on carbide milling tools. Inconel 718 was used as workpiece material in face milling tests to determine the wear behavior of the BN coated carbide tools. Surface roughness and tool wear was recorded in relation with cutting length. Wear mechanisms on the coated carbide tools were determined using scanning electron microscopy in combination with energy dispersive spectroscopy. Abrasive and adhesive wear was found as main failure mechanisms on the worn tools. Approximately two times longer tool life was obtained with the BN coated carbide tools.

Evaluation of Hot Corrosion Behavior of APS and HVOF Sprayed Thermal Barrier Coatings (TBCs) Exposed to Molten Na 2 SO 4 + V 2 O 5 Salt at 1000 °C

Hot corrosion is a very destructive failure mechanism for thermal barrier coatings (TBCs) during service conditions. In the present study, a CoNiCrAlY is coated using atmospheric plasma spray (APS) and high velocity oxy-fuel (HVOF) techniques which produces TBCs that were exposed to 50% Na2SO4 and 50% V2O5 molten salts at 1000 °C. The top surface was visually inspected at the end of each four-hour cycle to determine the lifetime of TBCs. The failure criteria required for the termination of the hot corrosion cycles was assumed to be 25% cracking and spallation of top coats. X-Ray Diffraction (XRD) analysis was performed before and after hot corrosion tests in order to observe new phases which may occur in the top coating. After scanning electron microscopy (SEM) images of cross-section samples of hot corrosion tests are completed, samples were taken. As a result, it has been observed that the lifetimes of HVOF bond-coated TBCs are longer.

Solid particle erosion behavior of thermal barrier coatings produced by atmospheric plasma spray technique

ABSTRACT Thermal barrier coatings (TBCs) are commonly applied specifically for aerospace applications in which they are subjected to air-borne particles. Therefore, solid particle erosion behavior of all coating layer has been an important phenomenon and erosion behavior of various TBCs has been widely investigated in literature. In the present study, CoNiCrAlY and yttria stabilized zirconia (ZrO2 + 8% Y2O3) powders were deposited on Inconel 718 nickel based super alloy substrate. Atmospheric plasma spraying technique was applied for the deposition of the metallic bond coat and the ceramic top coats. Erosion tests were carried out under various particle impingement angles with an air jet erosion tester. Afterwards, eroded surfaces of the specimens were investigated with a three-dimensional (3D) optical surface profilometer (noncontact) and scanning electron microscope. The erosion rates, the areal surface roughness values, the 3D surface topographies, and the surface morphology of the specimens were evaluated based on the particle impingement angle to understand the solid particle erosion behavior of the produced coatings. The maximum erosion rates occurred at 60° impingement angle which is an indication of semi-ductile/semi-brittle erosion behavior. Furthermore, the surface roughness values and surface topographies also dramatically varied depending on the impingement angle. Deeper and wider erosion craters formed at 60° impact angle and the erosion craters were visualized by profilometer analysis.

Oxidation Behavior of NiCr/YSZ Thermal Barrier Coatings (TBCs)

Abstract Nichrome (NiCr) coatings are widely used to provide resistance against oxidation and corrosion in many machine components. TBCs must include bond coatings that are resistant to oxidation resulting fromhigh-temperature operating conditions. In the present study, NiCr powders were sprayed on nickel-based superalloy Inconel 718 substrates using atmospheric plasma spray (APS) technique. Bond-coated substrates were coated with yittria stabilized zirconia (YSZ). As such, the TBC samples were kept at 1000°C for 8 h, 24 h and 50 h in high temperature furnace and their isothermal oxidation behavior was investigated. Microstructure and phase change properties of TBCs before and after isothermal oxidation were then studied and analyzed.