Modelling of catastrophic stress development due to mixed oxide growth in thermal barrier coatings

In thermal barrier coatings (TBCs) of heavy-duty gas turbines, thermally grown oxide (TGO) develops in two stages, i.e. firstly, a thin layer of dense protective α-Al₂O₃ forms slowly, and then, a layer of porous detrimental mixed oxide (MO) between top coat (TC) and α-Al₂O₃ appears. During long-term isothermal oxidation at high temperature, the failure of TBCs usually occurs when a critical thickness of MO is reached, but the exact failure mechanism is still largely unclear, let alone the related stress development. In this paper, we analyze the stress evolution and the resultant failure modes due to the whole-layer growth of uniform MO. The results show that it is MO, rather than α-Al₂O₃, that is mainly responsible for the micro-cracking and/or delamination in TBCs. The fast growth of expansive MO induces catastrophic stresses, which leads to micro-cracking in the α-Al₂O₃ layer. The cracking of α-Al₂O₃ layer reduces the oxidation resistance and further accelerates the MO growth. Our theoretical analysis provides a reasonable explanation of the experimental results.     

Stage-sensitive microstructural evolution of nanostructured TBCs during thermal exposure

Nanostructured thermal barrier coatings (TBCs) often exhibit bimodal structure comprised of both nanozones and lamellar zones, and therefore, their sintering behaviour can be different from that of conventional coatings. In this study, changes in the microstructure and properties of nanostructured TBCs were investigated. The results show that their microstructural evolution is highly time-sensitive during long thermal exposure at 1150?°C. In stage I (0–20?h), changes in mechanical properties were significant. The dominant microstructural change was faster healing of flat pores, whereas the macroscopic structure seemed less affected. In stage II (20–500?h), the changes in properties were much slighter and some large macroscopic voids appeared. In brief, the microscopic healing of pores in lamellar zones leads to a significant change in mechanical properties in stage I, whereas sintering of the nanozones leads to macroscopic voids in stage II.     

CMAS环境下电子束物理气相沉积热障涂层的热循环行为及失效机制

航空发动机涡轮叶片工作时表面经常产生CaO-MgO-Al₂O₃-SiO₂(简称CMAS)等沉积物。本文中研究了电子束物理气相沉积(EB-PVD)制备ZrO₂热障涂层(TBCs)在CMAS环境下的热循环行为及失效机制。结果表明, 在1200℃热冲击条件下, 表面涂覆CMAS的热障涂层的热循环寿命低于100次, 而未涂覆CMAS的涂层寿命达到500次以上, CMAS 的存在加速了热障涂层的剥落失效。在1200℃经过210次循环后, ZrO₂陶瓷层与CMAS之间形成了约8 μm厚的互反应区, 其形成主要与CMAS中Ca²⁺内扩散有关。CMAS环境下热障涂层陶瓷层产生大量横向裂纹, 涂层的失效主要以陶瓷层片状剥落为主。     

A novel CMAS-resistant material based on thermodynamic equilibrium design: Apatite-type Gd10(SiO4)6O3

Calcium-magnesium-alumino-silicates (CMAS) melt attack has been a critical issue for the thermal barrier coatings (TBCs) with ever-increasing engine operating temperature. In this study, a novel CMAS-resistant material apatite-type Gd₁₀(SiO₄)₆O₃ is developed for TBCs application based on thermodynamic equilibrium design. The chemical reaction of Gd₁₀(SiO₄)₆O₃ bulk and CMAS melt is investigated at 1300°C. The CMAS corrosion resistance of Gd₁₀(SiO₄)₆O₃ bulk is evaluated and compared with the well-studied CMAS-resistant material Gd₂Zr₂O₇ (GZO). It is found that Gd₁₀(SiO₄)₆O₃ shows a significantly enhanced CMAS resistance, including lower intrinsic CMAS infiltration rate (~1.09 μm/h^1/2) and smaller infiltration upper limit (50-62 μm) for a 20 mg/cm² CMAS deposition. More importantly, for Gd₁₀(SiO₄)₆O₃, the CMAS infiltration only alters the composition but does not change the crystal structure or destroy microstructural integrity. The reaction mechanism is elucidated as following two stages: (a) surface Gd₁₀(SiO₄)₆O₃ quickly transforms into Ca₂Gd₈(SiO₄)₆O₂ in suit by interdiffusion with CMAS melt and then is thermodynamically stable with CMAS melt, thereby effectively inhibiting the further CMAS infiltration and (b) with the ongoing interdiffusion of Gd/Ca, the CMAS-infiltrated layer slowly thickens and follows a parabolic law. Meanwhile, the CMAS melt gradually precipitates Ca₂Gd₈(SiO₄)₆O₂ and CaAl₂Si₂O₈ (anorthite) until the melt is exhausted.     

Thermochemistry of calcium rare-earth silicate oxyapatites

The calcium rare-earth (RE) silicate oxyapatite, Ca₂RE₈(SiO₄)₆O₂ (RE = Yb, Er, Y, Dy, Nd, Gd, and Sm), powders were synthesized by the solid-state reaction method and characterized by X-ray diffraction (XRD), Raman spectroscopy, and elemental composition analysis. The thermodynamic properties of the oxyapatites have been investigated using high-temperature oxide melt calorimetry in molten 2PbO–B₂O₃ solvent at 805°C. The energetics of the oxyapatites related to ionic substitution on two crystallographic sites, M(1) and M(2), are discussed. The enthalpy of formation from the oxides becomes more exothermic as the ionic potential decreases or the ionic radius of the REs increases, which indicates increasing energetic stability in this order.     

Atmospheric plasma sprayed 7%-YSZ thick thermal barrier coatings with controlled segmentation crack densities and its thermal cycling behavior

Yttria-stabilized zirconia (YSZ)-coatings are deposited on Ni-based superalloy IN738 by atmospheric plasma spraying (APS). For the first time, controlled segmentation crack densities are manually developed in the coatings, even after the APS deposition. This method allows to user to control segmentation densities as well as cracks depth, which could be designed as per coating thickness and required application. Thermal cycling test shows promising strain tolerance behavior for the segmented coatings, whereas coating without segmentation could not sustain even for its first thermal cycle period. Further, microstructural studies reveal that a very thin layer of TGO was formed and obvious no coating failure or spallation was observed after thermal cycling test at 1150 °C for 500 cycles.     

CMAS (CaO–MgO–Al2O3–SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers

Calcium–magnesium–alumina–silicate (CMAS) corrosion significantly affects the durability of thermal barrier coatings (TBCs). In this study, Y₂O₃ partially stabilized ZrO₂ (YSZ) TBCs are produced by electron beam-physical vapor deposition, followed by deposition of a Pt layer on the coating surfaces to improve the CMAS resistance. After exposure to 1250 °C for 2 h, the YSZ TBCs were severely attacked by molten CMAS, whereas the Pt-covered coatings exhibited improved CMAS resistance. However, the Pt layers seemed to be easily destroyed by the molten CMAS. With increased heat duration, the Pt layers became thinner. After CMAS attack at 1250 °C for 8 h, only a small amount of Pt remained on the coating surfaces, leading to accelerated degradation of the coatings. To fully exploit the protectiveness of the Pt layers against CMAS attack, it is necessary to improve the thermal compatibility between the Pt layers and molten CMAS.     

The effects of heat treatment and gas atmosphere on the thermal conductivity of APS and EB-PVD PYSZ thermal barrier coatings

The effects of heat treatment and gas atmosphere on thermal conductivity of atmospheric plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) partially Y₂O₃ stabilized ZrO₂ (PYSZ) thermal barrier coatings (TBCs) were investigated. Two-layer samples that had an EB-PVD coating deposited on bond coated nickel-base superalloy IN625 substrates, free-standing APS and EB-PVD coatings as well as a quasi-free-standing EB-PVD PYSZ coating (coating on semitransparent sapphire) were included in the study. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C in vacuum and under argon gas using the laser flash technique. To investigate the effect of heat treatment on thermal conductivity, coatings were annealed at 1100 °C in air. For both the APS and EB-PVD PYSZ coatings the first 100 h heat treatment caused a significant increase in thermal conductivity that can be attributed to microstructural changes caused by sintering processes. Compared to the measurements in vacuum, the thermal conductivity of APS coatings increased by about 10% under argon gas at atmospheric pressure, whereas for the EB-PVD coatings, the influence of gas on thermal conductivity was relatively small. The effect of gas on the thermal conductivity of APS and EB-PVD PYSZ coatings can be attributed to amount, shape, and spatial arrangement of pores in the coating material.     

Effect of morphology on thermal conductivity of EB-PVD PYSZ TBCs

Partially yttria stabilized zirconia (PYSZ) based thermal barrier coatings (TBC) manufactured by electron beam-physical vapour deposition (EB-PVD) protect turbine blades, working under severe service conditions in aero engines and stationary turbines. These coatings show a high strain tolerance relying on their unique morphology which is comprised of weakly bonded, preferred-oriented columns, voids between feather-like sub-columns and, finally, of intra-columnar closed pores.The results obtained in this work demonstrate that variation of the EB-PVD process parameters alters the resulting columnar morphology and porosity of the coatings. The physical properties and, most importantly, thermal conductivity, are greatly affected by these morphological alterations. This study investigates three morphologically different EB-PVD PYSZ TBC top coats in terms of the spatial and geometrical characteristics of their porosity and correlates those with the thermal conductivity values measured in as-coated state and after heat treatment at 1100 °C for 1 h and 100 h. Changes in the open and closed porosity caused by heat-treatment are characterized by small-angle neutron scattering (SANS), Brunauer-Emmett-Teller Method (BET) and scanning electron microscope (SEM). Correlation of shape and surface-area changes in all porosity types of the analysed coatings revealed that the thermal conductivity of these coatings is influenced primarily by size and shape distribution of the pores and secondarily by the pore surface-area available at the cross section perpendicular to the heat flux.     

The spalling modes and degradation mechanism of ZrO2-8 wt.% Y2O3/CVD-Al2O3/Ni-22Cr-10Al-1Y thermal-barrier coatings

State-of-the-art conventional thermal-barrier coatings consist of a thermalinsulating, partially-stabilized ZrO₂ top coat and a bond coat. In this study, a continuous alumina-diffusion-barrier layer was deposited and interposed between the top coat and bond coat by chemical-vapor deposition (CVD). Both the conventional and the experimental TBC systems were cyclically tested at 1000°C, 1050°C, 1100°C, and 1150°C to evaluate and compare oxidation, performance, and fracture behavior. Introduction of the intermediate CVD-Al₂O₃ layer effectively suppressed the oxidation rate of the bond coat and sufficiently altered its oxidation behavior. The thermal-cyclic life of TBCs was improved by the new system. The failure of the ZrO₂-8 wt.% Y₂O₃/CVD-Al₂O₃/Ni-22Cr-10Al-1Y TBC specimens was observed to propagate mainly along the lamellar splats of the top coat, and secondarily along the top coat/CVD-Al₂O₃ interface.