Papers by Keyword: Thermal Barrier Coating (TBC)

Abstract: Thermal barrier coatings (TBC) have been used to improve the efficiency of turbine engine by providing the capability to sustain significant temperature gradient across the coating. TBC failure occurs easily at the interface between the metallic bondcoat and topcoat. Alumina was proposed as a potential candidate as an interlayer to improve the oxidation resistance of thermal barrier coating due to its low oxygen diffusivity against the harsh environment. The mechanical properties, thermal behaviour and high temperature oxidation resistance of the coatings formed by gas tunnel type plasma spraying were investigated in this study. The results showed that this system exhibits the improvement of mechanical properties of the coating and oxidation resistance. This interlayer is preferred in order to minimize the detrimental effect of phase transformation of γ- Al2O3 to α-Al2O3.

Abstract: Thermal barrier coatings (TBCs), that reduce the temperature in the underlying substrate material, are an essential requirement for the hot section components of industrial gas turbines. Recently, in order to take full advantage of the potential of the TBC systems, experimental and analytical investigations in TBC systems have been performed. However there is a little information on the deformation behavior of the top coating. In addition, the effects of the thermal exposure and the process parameters on the mechanical properties of the top coating have never been clarified. From these backgrounds, the effects of the process variables in APS and the thermal exposure on the mechanical properties were investigated in order to optimize the APS process of top coatings. The experimental results indicated that the mechanical properties of the APS-TBC, i.e. the tensile strength and the elastic modulus, were significantly changed by the process variables and the long term thermal exposure. The microstructural investigation was also carried out and the relationship between the mechanical properties and the porosity was discussed.

Abstract: The subcommittee on superalloys and coatings, The Society for Materials and Science, Japan (JSMS) was established in 1997. As second stage of the committee, we investigated a fundamental study on thermal plasma sprayed thermal barrier coatings (TBCs). Especially, relationships between spray conditions and coating properties are discussed. It was used 4 spraying conditions such as 1) standard condition, 2) larger particle used and lower velocity, 3) normal particle used and lower velocity, and 4) standard condition and heating up the substrate to 473K. As a round robin test, porosity ratio, residual stress, and bonding strength were evaluated by several techniques for the specimens, which were sprayed by above 4 conditions. As a result, Young’s modulus, and tensile strength increased with decreasing porosity ratio. In case of tests for residual stress evaluation, as-sprayed specimens have small tensile residual stresses. After thermal exposure, residual stresses shifted to compressive.

Abstract: Elastic modulus of air plasma sprayed (APSed) YSZ (; ZrO2 stabilized by 8 wt. pct. Y2O3) top coat specimen, which is frequently used for thermal barrier coating (TBC) system for advanced gas turbines, was measured by employing the macro-, micro-, and nano-indentation methods. The elastic modulus was measured, following the Oliver-Pharr method. It was shown that the elastic modulus of the YSZ, as well as the microstructure, was significantly influenced by the spraying conditions employed. Especially the size of ceramic powders used was found to have the most pronounced effect. It was also shown that the elastic modulus revealed significant size effect: that is, there were significant differences in elastic modulus measured by the instruments on the macro-, micro-, and nano-levels. This size effect was discussed, correlating with some relating phenomena: crackings, sink-in, pile-up and spalling; as well as with the characteristic microstructures of the sprayed top coat.

Abstract: Thermal barrier coating (TBC) was carried out on Ni based super-alloy specimens, and edge indent tests were carried out at 1073K under different displacement rate. In addition, the specimens were heated and held at 1273K for long time, and the edge-indent tests of the specimens were performed both at 1273K and at room temperature. The results showed that the delamination load Pd and the delamination energy Ed measured at 1073K after short time holding were larger than those measured at room temperature, and the Pd and Ed slightly decreased with increasing displacement rate at 1073K while they slightly increased at 293K. The Pd and Ed at 1273K increased with increasing holding time and reached a maximum at 1400ks. With further increase in holding time, the Pd and Ed largely decreased. The Pd and Ed measured at 1273K were smaller than those at room temperature.

Abstract: With the knowledge of heterogeneous characteristics of thermal barrier coating materials at mesoscopic level, a coupled thermo-mechanical-damage (TMD Model) model was introduced and used to numerically quantify the thermal stresses and crack development of in thermal barrier coatings (TBCs) composite subjected to decreased temperatures. The effect of different surface precrack morphologies, such as precrack length and precrack density, on an interface crack subjected to thermal loading caused by a temperature change is presented. It provides us with a more sensible physical intuition and a more accurate mathematical for optimizing the design and the processing of ceramic coatings subjected to the coupled thermal-mechanical loading.

Abstract: This paper investigated the thermo-mechanical bending failure characteristic of air plasma-sprayed (APS) thermal barrier coating (TBC) system at 1000 oC by three-point bending test. A through-width surface pre-crack on the centre surface of the top ceramic coating (TC) in TBC sample was firstly designed and made by air plasma spraying technology. The fracture characteristic and spallation phenomena of the TBC were investigated under bending test in air at 1000 oC. The effects of constant displacement rate, span width of bending equipment and thermal aging time on the thermo-mechanical bending failure of the APS TBC system were slight at 1000 oC. Using SEM observations and EDX analysis, the interface cracks initiation, propagation and coalescence were discussed in detail. Furthermore, the fracture surface located within the TBC close to the TBC/TGO interface for all APS TBC samples under bending at 1000 oC.

Abstract: Thermal barrier coating (TBC) of a gas turbine blade suffers from high temperature oxidation. It is known that thermally growth oxidation (TGO) grows at the interface between ZrO2- 8%Y2O3 and CoNiCrAlY, and the TGO degrades the adhesive strength. The purpose of this study is to clarify the TGO growth process. Thermal aging tests of ZrO2-8%Y2O3 / CoNiCrAlY TBC systems under various temperature conditions were carried out. TGO growth process was observed by an electron probe micro analyzer (EPMA). Both TGO of Al and TGO of other elements were examined, and the thickness of the TGOs were examined. Results are summarized as follows. (1) The delamination occurred at a ZrO2-8%Y2O3 layer beside the interface. (2) The growth rate of complicated TGO which consisted of Co, Ni and Cr increased with an increase of temperature. However, the complicated TGO wasn’t related with the delamination life. (3) The delamination could occur if the thickness of Al TGO increased more than about 3 μm.

Abstract: An original instrumented microindenter capable of testing materials up to 1000°C in an inert atmosphere has been developed. The method of neural networks is used to solve the inverse problem, in order to determine the constitutive equation of the materials tested. To obtain a data basis for the training and validation of the neural network, finite element simulations were carried out for various sets of material parameters. To reduce the number of simulations a representative sampling of the loading-strain responses is performed using an unsupervised network, so-called self-organizing map.

Abstract: New gadolinium-yttrium zirconate thermal barrier coating(TBC) material is deposited by electron beam PVD method, as an alternative to YSZ TBC layer for gas turbine blade applications. XRD analysis reveals that the new TBC material consists of thermally stable pyrochlore structure. Hertzian and nanoindentation evaluations reveal that gadolinium zirconate materials show superior properties as a TBC candidate material with high mechanical properties. The Y2O3 doping improved hardness and elastic modulus of TBC layers. The indentation stress-strain curves by Hertzian indentation and the load-penetration depth curves by nanoindentation indicate that the new TBC layer has higher damage resistance combined with superior thermal insulation properties rather than commercial YSZ coatings.