ICMCTF2011 Session A2-2: Thermal and Environmental Barrier Coatings

Silicon-based ceramics are promising candidates for structural components in gas turbine engines. However, the passivating oxide (SiO₂) that forms on their surface reacts with water vapor present in the combustion atmosphere, causing coating recession and necessitating the use of environmental barrier coatings (EBCs). EBC lifetime depends upon the phase stability of the coating, its chemical compatibility with and coefficient of thermal expansion relative to the Si-based substrate, and its mechanical and chemical robustness in the combustion environment. We report the use of synchrotron-derived high-energy X-rays to examine residual stresses, phase transformations, and reactivity with calcium-magnesium-aluminosilicate glass deposits in two EBC systems. In situ and ex situ X-ray studies of barium-strontium-aluminosilicate (BSAS) and ytterbium silicate coatings, both on SiC/SiC substrates, will be described.

Thermal barrier coatings used in modern gas turbines typically consist of a single-phase compound, such as tetragonal or cubic yttria-stabilized zirconia or various rare earth zirconates. In addition, the state-of-the-art coatings applied by EB-PVD have specific columnar structure in which each column is essentially a single crystal grown normally to the substrate surface. In this work, a different type of TBC architecture is explored. Using a proprietary multi-source PVD method, a set of multi-phase coatings based on the ternary yttria-zirconia-tantala system has been deposited. The new coatings also develop columnar morphology, which provides thermal strain compliance, but unlike the conventional TBC, the multi-phase columns have distinct nano-crystalline structure. This paper presents microstructural characterization and thermal conductivity measurements of such multi-phase nano-crystalline coatings. For comparison, several bulk ceramic specimens with similar chemistries, prepared from liquid precursors, have been evaluated. The issues of microstructural stability during high temperature aging, as well as the role of phase and grain boundaries are discussed. It is argued that the concept of multi-phase TBC provides a wide choice of compounds and various combinations of oxide phases that may be used for new and advanced thermal barrier coatings.

The deposition of Yttria partially Stabilized Zirconia (YpSZ) for Thermal Barrier Coating application (TBC) is a current topic of interest. The TBC must exhibit high thickness (100-300 μm), vertical cracks in order to be a strain tolerant layer, and high porosity to decrease the thermal conductivity. Deposition technique such as Electron Beam Physical Vapor Deposition (EBPVD) or Atmospheric Plasma Spray (APS) are usual techniques to obtain 10-20 % porous YpSZ layers. New plasma processes using a suspension of YpSZ as raw material are used to deposit layers containing nano-pores. In this paper, a solution of nitrate salt is introduced into a low pressure plasma discharge (600 Pa, 120 W) to obtain YpSZ layers. Recent works have shown that the plasma process permits to synthesize YpSZ layer exhibiting a very low thermal diffusivity (0.85×10^{- 7} m² . s^-1 / 1100°C ) compared to coatings obtained by APS (3×10^{- 7} m² . s^-1 / 1100°C ). Observations of the layers by SEM showed that YpSZ was highly porous : a high number of pores with a micro and a nano-metric size was revealed.

Several analyses were used to study the characteristics and the stability of the YpSZ layers obtained in the low pressure plasma reactor. Optical emission spectroscopy proved that the oxidant chemistry in the plasma is responsible for the formation of the oxide and the elimination of the nitrates at low temperature (T< 300°C onto the layer). SEM, water porosimetry and XRD analyses were performed on the deposit to study the effect of the parameters (composition, power, post-treatment, concentration of the solution, heat treatment) on the structure, the morphology and the stability of YpSZ coatings. For example, it was observed that YpSZ is 50 % porous and that the nanostructures of the coating resist at high temperature conditions ( 1100°C / 50 h).

A range of presented investigations will concern characteristics of the TBC layers type RE₂Zr₂O₇ (RE = Gd, La, Sm, Nd). The layers were placed on a nickel superalloys type AMS5599 with an bond-coat type NiCrAlY, obtained by the VPS (vacuum plasma spraying) method. Thickness of the bond-coat is approx. 125µm in all cases. A ceramic layer was obtained in a result of plasma spraying by the APS method with powders of a general formula RE₂Zr₂O_7, obtained by a spray drying method. Thickness of obtained layers was comprised within a range 250-300µm. Rectangular samples were obtained of dimensions 40x20x2 mm.

A range of investigations, presented in the paper will comprise:

· evaluation of microstructure of a ceramic layer from a point of view of thickness, quality of a layer and quantity characteristics of cracks and pores architecture;

· evaluation of thermal diffusivity of a ceramic layer within a range of temperature 25-1100°C;

· determination of thermal conductivity of the TBC layers type RE₂Zr₂O₇;

· evaluation of electric properties by an impedance spectroscopy method – dependence of impedance and electric permittivity as a function of frequency of electric field within a range 10¹-10⁶Hz and temperature within a range 20-1000°C;

· obtained results in investigations on electric properties of these materials will be correlated with investigations on thermal diffusivity and investigations of electron scanning microscopy and optical spectroscopy within a range of waves from 0.2-1.1 µm;

· basing on obtained impedance spectra for these materials and knowledge of dimensions and shapes of ceramic grains, which form these materials, a complex character of their electric conductivity was defined;

· analysis of obtained results of effective conductivity of these materials, basing on the Maxwell‑ Wagner multi-phase model.

Results, presented in the paper, are effects of long-running investigations, carried out by the Department of Materials Science in the Silesian University of Technology, and these investigations concerned heat- resisting coatings and layers type TBC.

Financial support of Structural Funds in the Operational Programme –Innovative Economy (IE OP) financed from the European Regional Development Fund -Project No POIG.01.01.02-00-015/09 is gratefully acknowledged.

To analyse experimental set-up, surface strain field measurement is achieved using Digital Image Correlation (DIC) technique. This study shows that very high level of strain localisation is obtained due to crystal plasticity along {111} plane of the FCC single crystal used for the substrate. A Finite Element Analysis (FEA) is performed taking into account crystal plasticity to simulate the substrate single crystal behaviour. Thus, we clarify the physics of the onset of spallation events: both strain fields, experimental and numerical, allow to infer that substrate strain localisation triggers TC breakaway at low temperatures. Moreover, this study allows to estimate the dispersion level in TBC lifetime due to industrial tolerance range for single crystal orientation.

This study was supported by turbine manufacturer SNECMA (SAFRAN).

[1] L. Rémy, A. Alam, and A. Bickard. Thermo-mechanical creep-fatigue of coated systems. ASTM, STP 1428:98–111, 2003.

[2] M. Harvey, C. Courcier, V. Maurel, and L. Rémy. Oxide and TBC spallation in beta-nial coated systems under mechanical loading. Surface and Coatings Technology, 203(5-7):432–436, 12/25 2008.

[3] C. Courcier, V. Maurel, L. Rémy, and A. Phelippeau. Damage based life prediction model for ebpvd thermal barrier coatings under thermal fatigue. LCF6 - Sixth International Conference on Low Cycle Fatigue, Berlin, Germany September 8 - 12, 2008.

[4] C. Courcier, V. Maurel, L. Rémy, and S. Quilici, Interfacial damage based life model for EB-PVD thermal barrier coating, Surface and Coatings Technology, in submission.

Various defects, such as pores, cracks, and splat boundaries, are unavoidably formed in thermal barrier coatings (TBCs) during a plasma spray coating, which exert critical impact on the thermomechanical properties of TBCs, such as elastic modulus, thermal conductivity, and coefficient of thermal expansion [1]. TBC samples were prepared by TriplexPro^TM-200 system using different commercialized powders and the microstructure of TBC was controlled by the reheating the surface of TBC. The relatively porous TBC was prepared with METECO 204 C-NS and the relatively dense TBC with METECO 204 NS. The microstructure of the top coat in TBCs was just controlled, and the bond coat with about 300 mm thickness in the both top coats was prepared with AMDRY 962. The top coats were coated onto the bond coat, and then the surface of each TBC was reheated by plasma without powder feeding in same equipment. The rapid cooling process of the reheated top coat created vertical type cracks on the top coat. The microstructural characterizations of the relatively porous and dense TBCs with the vertical cracks were analyzed through mathematical approaches and compared to each other. A couple of governing partial differential equations was derived based on the thermoelastic theory [2], and a finite volume model was developed to evaluate the thermoelastic characteristics like temperature distribution profiles, displacement, and stresses, induced by a thermal fatigue for the governing equations. The radial stress of the vertical-cracked TBCs displayed the reverse undulation in the dense model and larger extension was developed in the porous model compared with the dense model.