Papers by Keyword: Thermally Grown Oxide

Abstract: TBCs (Thermal Barrier Coatings) is deposited on gas turbine blades to protect the substrate from a combustion gas flow. One of the serious problems occurred in gas turbine is TBC delamination which is caused by startup, steady and stop operation in service. TBC delamination results from subjecting to both cyclic thermal stress and evolution of internal stress due to thermally grown oxide (TGO). In this study, the finite element code which can simulate thermal and internal stress fields generated in TBC was developed. The developed code involves the follows: inelastic constitutive equation for ceramic coating, bilinear-type constitutive equation for bond coating and Chaboche-type inelastic constitutive equation for the substrate, and mass transfer equation in consideration of oxygen diffusion and chemical reaction with aluminum. Thermal cycling simulation was conducted using the developed code. It was confirmed that maximum stress and its location in the ceramic coating/bond coating interface were matched with the associated experimental results.

Abstract: Thermally grown oxide (TGO) plays important roles in thermal barrier coating system (TBC) for high temperature application such as in aircraft gas turbine engine blades (GTE). The TGO formed between the bond coat and topcoat interface can increase oxidation resistance to creep of GTE blades by minimizing oxygen diffusion into the metal substrate. In this research a NiCoCrAlYTa metallic bond coat was deposited on Inconel 625 substrate using two methods of deposition namely; high velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS). After coating process, both types of samples underwent pre-oxidation in argon furnace for 12-24 hours at 1000 °C. Results showed that the TGO formation for samples in which the bond coat deposited via HVOF method produced much thinner and continuous TGO formation compared to APS deposition. This TGO characteristic is very useful to lengthen the lifetime of the metals substrate.

Abstract: Formation of thin and continuous layer of thermally grown oxide (TGO) in thermal barrier coating (TBC) are essential in order to avoid coating failure for high temperature applications. As-sprayed high velocity oxy-fuel (HVOF) bond coat can provide more uniform TGO layer in TBC system and much less oxide compare to air plasma spray (APS). In this paper, both APS and HVOF method are used to deposit NiCoCrAlYTa bond coat on Inconel 625 substrate followed by topcoat, YSZ deposition. Pre-oxidation process was done in normal oxygen furnace at 1000°C for 12 to 24 hours to study the characteristic of TGO formation via these two different methods. From the result obtained, it shows that HVOF method provide better TGO formation as compared to APS.

Abstract: TBC (Thermal Barrier Coating) with YSZ-Al₂O₃/YSZ as top coat (TC) and MCrAlY alloy as bond coat (BC) marked as YSZ-Al₂O₃/YSZ double layer TBC was used for thermal barrier material on rocket nozzle wall. Several test have been conducted (thermal test and SEM-EDX). Thermal test was divided into two part, Thermal Torch and Non-isothermal Oxidation Test which is used for measure the resistance of TBC structure against oxyacethylene flame penetration as a miniature of thermal static test for rocket nozzle until the specimen was failure and used to measure the resistance against non-isothermal oxidation as well as investigate the growth of TGO (Thermally Grown Oxide) layer on interface between TC/BC for thermal torch and non-isothermal oxidation test respectively. This test was performed at 1100^o C with 10°C/min heating rate. The results showed that the improvement of feed rate on topcoat powder gave the effect on the surface morphology of topcoat. Lower feed rate bring in a coarse surface morphology and tend to be porous. Al₂O₃/YSZ composite layer affect on the growth of TGO layer after thermal test, which indicates the lower growth of TGO especially for 15%Al₂O₃/8YSZ. TGA test showed that the sample began to oxidize at temperature range c.a 1000 – 1030°C, however 1^st derivative analysis indicated that 15% Al₂O₃/8YSZ, 14 g/min was the most stable sampel. Moreover this material has a good resistance from torch destruction.

Abstract: Thermal barrier coating. Thermal fatigue. Exposure time. Thermal fatigue test is one of the most widely used method to evaluate the durability of thermal barrier coating (TBC). However, thermal fatigue test can be concluded in totally different results according to the test variations. Especially, Exposure time of thermal fatigue test can affect the delamination life cycle of TBC. In this study, using the same test equipment which Kim et al. used, thermal fatigue tests were performed with different holding time at high temperature, and the test results by Kim et al. and those by this study were compared. In addition, delamination map was come to perfection from the test results to define more accurate thermal fatigue life.

Abstract: The 8 wt. % yttria stabilized zirconia top coat (TC) and the CoNiCrAlY bond coat (BC) were sprayed onto the surface of newly developed fine-grained cast polycrystalline nickel-based superalloy Inconel 713LC by means of atmospheric plasma spraying (APS). As-prepared samples were isothermally exposed at the temperature of 1050 °C for 200 hours in an ambient atmosphere. Structural changes in the thermal barrier coatings (TBC) system after thermal exposure were studied by means of scanning electron microscope equipped with an energy dispersive microanalyzer. Critical weak points were identified on both the substrate-bond coat and bond coat-top coat interfaces.

Abstract: The microstructural evolution related to the thickness of thermal barrier coating (TBC) and their thermal stabilities have been investigated with a specific attention to defect species as well as to its morphology with the thermal exposure time. The TBCs with different thicknesses of 600 and 2,000 µm were prepared by air plasma spray (APS) process and the thermal exposure tests were performed at 950^C in a furnace with a dwell time of 100 hrs till 500 hrs. The thickness of thermally grown oxide (TGO) layer in the TBC with 2,000 µm is thinner than that with 600 µm. Also, the TBC with 2,000 µm is more efficient in improving the oxidation resistance of bond coat than that with 600 µm. Vickers indentation methods are used to evaluate the interfacial stabilities. Indentation impression and crack formation of the TBC of 600 µm is easily occurred in comparison with that of 2,000 µm, showing relatively longer cracks, independent of thermal exposure. However, the crack formation and propagation through the interface does not observed in the TBC with 2,000 µm, showing crack propagation through the top coat near the interface. These results imply that the interfacial stability of TBC can be also improved with increasing the coating thickness.

Abstract: In this work, the displacement instability of thermally grown oxide (TGO) occurring near the surface groove in Fecralloy substrate subjected to multiple purely thermal cycling, has been simulated coupled with crack propagation in TGO layer using finite element method. Due to insufficiency in the traditional criterion of crack opening displacement (COD), a modified COD criterion is employed to model crack propagation in TGO layer as thermal cycles in which the required input parameters are continually modified as thermal cycling based on the calculation result. The comparison between FEA and experimental results shows a good agreement indicating the validity and accuracy of the simulation, which may provide a solution for future works on more complicated case such as in thermal-mechanical cycling.

Abstract: In this paper, we explored the relationship between thermal aging and the residual stresses developed within a thermal barrier coating (TBC) and the underlying thermally grown oxide (TGO). Superalloy model specimens (CMSX4) with a curved geometry designed to simulate key features of turbine blades coated with air plasma sprayed (APS) 7 wt.% Y₂O₃-stabilised ZrO₂ applied to an Amdry 995 bond coat were oxidised in the temperature range 900°C to 975°C for various lengths of time. Stress measurements on both the TBC and the underlying TGO were made at positions of known curvature using Raman and photo-stimulated piezo-luminescence spectroscopy (PLPS). The residual stress values reflect the interaction between TGO and TBC, which has the potential to be used as an indicator of service lifetime. The results are discussed with respect to the origin of the stress changes and the long term integrity of the coating.

Abstract: Thermal barrier coating (TBC), which consisted of a NiCoCrAlY bond coat (BC) and a ZrO₂-8 wt.%Y₂O₃ topcoat (TC), was fabricated on the nickel-base superalloy by air plasma spray (APS). The BC and TBC was treated by vacuum heat treatment (VHT). The oxidation of coating with and without VHT has been performed at 1050°C. Oxidation behavior of coatings and thermally grown oxide(TGO) scale were studied by SEM with EDS. As shown in the results, after treating by VHT, a continuous Al₂O₃ layer formed more rapidly on the VHT coating than that formed on the APS coating, which can act as a diffusion barrier to suppress the formation of other detrimental oxides. The pre-oxidation treatments reduced the growth rate and extend the steady-state growth stage. The TGO in VHT TBC was still a single layer Al₂O₃ oxide after 120h. However, after same oxidation time, the TGO in APS TBC produced a dual-layer oxide consisting of an inner Al₂O₃ layer and outer spinel layer. Therefore the VHT improves the oxidation resistance of APS coating.