Stability Of Thermal Barrier Coatings Research Articles

A comparative study of calcium–magnesium–aluminum–silicon oxide mitigation in selected self-healing thermal barrier coating ceramics

Abstract

A Simulation Study on the Crack Propagation Behavior of Nanostructured Thermal Barrier Coatings with Tailored Microstructure

The initiation and propagation of cracks are crucial to the reliability and stability of thermal barrier coatings (TBCs). It is important and necessary to develop an effective method for the prediction of the crack propagation behavior of TBCs. In this study, an extended finite element model (XFEM) based on the real microstructure of nanostructured TBCs was built and employed to elucidate the correlation between the microstructure and crack propagation behavior. Results showed that the unmelted nano-particles (UNPs) that were distributed in the nanostructured coating had an obvious “capture effect” on the cracks, which means that many cracks easily accumulated in the tensile stress zone of the adjacent UNPs and a complex microcrack network formed at their periphery. Arbitrarily oriented cracks mainly propagated parallel to the x-axis at the final stage of thermal cycles and the tensile stress was the main driving force for the spallation failure of TBCs. Correspondingly, I and I–II mixed types of cracks are the major cracking patterns.

Effects of microstructure design and feedstock species in the bond coat on thermal stability of thermal barrier coatings

The effects of the microstructure design and feedstock species in a bond coat on the thermal stability of thermal barrier coatings (TBCs) were investigated through thermal shock (TS) tests. Three kinds of feedstock (AMDRY 9951 : cobalt (Co)-based feedstock, AMDRY 997 : nickel (Ni)-based feedstock, and AMDRY 9624 : Ni-based feedstock) were employed for the single layer bond coat and the first layer in the double layer bond coat, which were prepared by high velocity oxy-fuel process. The second layer in the bond coat and the top coat was formed by air plasma spray process using Ni-based metallic powder (AMDRY 962) and 8YSZ (METCO 204 C-NS), respectively. The TS tests were performed till more than 50% of the region spalled in the TBC. The TBC system with the Co-based bond coat showed a better thermal durability than those with the Ni-based bond coats. The TBC systems with the double layer in the bond coat showed a better lifetime performance than those with the single layer, independent of the feedstock species.

Research on the Stability of Thermal Barrier Coatings under Thermal Cyclic Loading

Thermal barrier coating systems (TBCs) are widely used in turbines. However, premature failures have impaired the use of TBCs and cut down their lifetime. The thermally grown oxide layer (TGO) thickening and the material thermal expansion misfit under thermal cyclic loading significantly affect the interfacial stress field and stability of TBCs. In this study, the stability evaluation method of TBCs under thermal cyclic loading based on energy is established using the visco-elastoplastic and shakedown theorem. The semicircular shape interface is used to simplify the complicated interfacial undulations in FEA model. And actual TGO thickness obtained from experiment is used to simulate the bond coat oxidation. Then the effect of TGO thickening on the stability and stress field of TBCs under thermal cyclic loading is analyzed through the numerical simulation. It is concluded that estimating from the stress-strain evolution behavior, the local stability of the TBCs decreases with the TGO thickening, and assessing from energy, TBCs shows instable with TGO thickening.

Microstructure Evolution and Interface Stability of Thermal Barrier Coatings with Vertical Type Cracks in Cyclic Thermal Exposure

In this study, the effects of intrinsic feature of microstructure in thermal barrier coatings (TBCs) with and without vertical cracks on the microstructure and mechanical properties were investigated in cyclic thermal exposure. The hardness values of TBCs with vertical cracks were higher than those without vertical cracks, showing a good agreement with microstructure. The TBC prepared without vertical cracks using the 204-NS was delaminated after 250 cycles in the cyclic thermal exposure test. The TBCs with and without vertical cracks prepared with 204 C-NS and the TBC with vertical cracks prepared with 204 NS showed a sound condition without any cracking at the interface or spalling of top coat. After the thermal exposure of 381 cycles, the hardness values were increased in the survived TBC specimens, and the thicknesses of TGO layer for the TBCs with 204 C-NS and 204 NS were measured as in the ranges of 5-9 and 3-7 μm, respectively. In the thermal shock test, the advantage of vertical cracks for thermal durability of TBC could be well investigated, showing relatively longer sustained cycles in the TBCs with vertical cracks. The TBCs with vertical cracks are more efficient in improving thermal durability than those without vertical cracks in cyclic thermal exposure.

Thermal and Mechanical Characteristics of Thermal Barrier Coatings in Cyclic Thermal Fatigue Systems

In this study, the relationship between microstructural evolution and mechanical properties of thermal barrier coatings (TBCs) has been investigated through different thermal fatigue systems, electric thermal fatigue (ETF) and flame thermal fatigue (FTF), including the thermal stability through the interface between the bond and top coats. The TBC system with the thicknesses of 300 µm in both the top and bond coats was prepared with METCO 204 NS and AMDRY 962, respectively, with the air plasma spray (APS) system using 9MB gun. To observe the oxidation resistance and thermal stability of TBC, the thermal exposure tests were performed with both thermal fatigue tests at a surface temperature of 850 °C with a temperature difference of 200 °C between the surface and bottom of sample, for 12,000 EOH in designed apparatuses. The hardness values are slightly increased due to the densification of top coat with increasing the thermal exposure time in both thermal fatigue tests. The influence of thermal fatigue condition on the microstructural evolution and interfacial stability of TBC is discussed.

The effects of spray parameters on the microstructure and thermal stability of thermal barrier coatings formed by solution precursor flame spray (spfs)

Solution precursor spraying is a new method of applying thermal barrier coatings (TBCs) on metallic substrates. In this work, the injection of a solution precursor has been made axially into an oxy-acetylene flame to deposit 7wt.% yttria stabilized zirconia (7wt.% YSZ) coatings. For this purpose a home-made atomizer has been inserted into the nozzle of a flame spray torch. The precursor used was an aqueous solution containing zirconium and yttrium salts to make a solid solution of 93wt.% ZrO2 and 7wt.% Y2O3 (7YSZ) in the coating. A martensitic stainless steel plate has been used as substrate on which a bond coat of NiCrAlY alloy was applied by atmospheric plasma spray process. The effects of spray distance and atomizing gas (oxygen and hydrogen) on the microstructure, phase composition, thermal shock and thermal exposure resistance of the coatings have been examined. The results show that the use of H2 as the precursor atomizing gas instead of oxygen increases the enthalpy of the flame the result of which is coatings with the least non decomposed precursor and highest crystalline YSZ. This effect causes the severe cracking at thermal shock and thermal exposure tests in coatings made by oxygen atomizing due to higher amount of non-decomposed precursor. By contrast, coatings made by hydrogen atomizing showed very little cracking because of increased enthalpy of the flame and a small quantity of non-decomposed precursor.

Effect of dopants on the phase stability of zirconia-based plasma sprayed thermal barrier coatings

The influence of stabilizer type on the phase stability of thermal barrier coatings (TBCs) produced by air plasma spraying was explored. Together with the widely used zirconia-stabilized with yttria, other novel compositions, such as dysprosia-stabilized zirconia, yttria–lanthana-stabilized zirconia and ceria-stabilized zirconia were also investigated. The effect of isothermal heat treatment on the phase stability was explored. Results suggest that decomposition of the “non-transformable” tetragonal phase occurs to a greater or lesser extent for all dopants at these temperatures. The effect of Al 2O 3 and SiO 2 content was also explored. The rate of decomposition depends on the dopant kind, amount and on the presence of Al 2O 3 and SiO 2 impurities.

Residual Stress and Stability of Thermal Barrier Coatings during Cyclic Thermal Loading

The residual stress accumulation of thermal barrier coatings (TBC) during thermal cycling has been simulated by thermal elastic-plastic finite element method. Stability process of thermal barrier coatings during thermal cycling is studied by Melan's static shakedown theorem considering thermal gradients and thermal expansion misfit. The influences of the interfacial morphology of top coat / bond coat (TC/BC) on residual stress and stability process are analyzed. Thus, the residual stress and stability process are affected remarkably by interfacial topography between TC and BC. Accidented interface cause the sudden change of the interfacial residual stress which is harmful to interface bonding intensity and stabilization. It is useful information for studying of the life and final spallation of TBC system.

Adsorption of Al, O, Hf, Y, Pt, and S Atoms on α-Al2O3(0001)

We use first-principles density functional theory to investigate adsorption of Al, O, Hf, Y, Pt, and S-atoms on the α-Al2O3(0001) surface. We identify stable adsorption sites and predict binding energies and structures. We find that Al, Hf, and Y preferentially adsorb on threefold-hollow sites, transfer electrons to the surface, and form ionic bonds to the three oxygen atoms. In contrast, the most stable adsorption site for Pt and S is ontop an oxygen atom, and we do not observe significant charge transfer. We find a binding order of S < Pt < O < Al ≪ Y < Hf, which reflects both the ease with which the early transition metals Hf and Y ionize, as well as the (nearly) closed-shell repulsions influencing the adsorption of O, Pt, and S. We use these results to rationalize some observations regarding the stability of thermal barrier coatings.

Phase and microstructural stability of solution precursor plasma sprayed thermal barrier coatings

The phase and microstructural stability of thermal barrier coatings (TBCs) deposited using the solution precursor plasma spray (SPPS) process is studied as a function of thermal cycles at 1121 °C. In the SPPS process, an aqueous chemical precursor feedstock, that results in ZrO 2 –7 wt.% Y 2 O 3 (7YSZ) ceramic coating, is injected into the plasma jet and the coating is deposited on a metal substrate. The resulting coating has the desired non-transformable tetragonal phase structure and this phase is stable throughout the thermal cycling test. SPPS TBCs consist of ultra-fine splats and unmelted particles that include some non-pyrolyzed precursor. The non-pyrolyzed precursor in the coating was observed to decompose and crystallize during initial thermal cycling, which results in an increase of coating hardness. No sign of sintering of the ultra-fine splats in the coating was observed after 1090 cycles. The spacing of through-coating-thickness cracks (160–190 μm) does not change with thermal cycling, while the opening of cracks increases from around 0.4 μm at as-sprayed state to 1.7 μm at 40 cycles, and to 3.1 μm at 800 cycles.