Thermal and Environmental Barrier Coatings

Wednesday, May 1, 2013 8:00 AM in Room San Diego

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8:00 AM A2-3-1 Environmental Barrier Coatings for Turbine Engines: Current Status and Future Directions
Dongming Zhu (NASA Glenn Research Center, US)
Ceramic environmental barrier coatings (EBC) and SiC/SiC ceramic matrix composites (CMCs) will play a crucial role in future turbine engine systems because of their ability to significantly increase engine operating temperatures, reduce engine weight and cooling requirements. Advanced EBC systems for low emission CMC combustors and turbine airfoils are currently being developed under the NASA Fundamental Aeronautics and Intergraded System Research Programs to meet engine emission and performance goals. This paper emphasizes the NASA’s EBC system development paths for SiC/SiC ceramic matrix composites, focusing on the evolution of advanced EBC systems and material stability challenges that have been overcome in the last decade. Advanced EBC-CMC component testing and demonstrations under the NASA development programs will be discussed, and degradation modes and temperature capabilities of the state-of-the-art environmental barrier coating systems on SiC/SiC CMCs will be reviewed. The next generation turbine EBC systems still needs to be validated when using new compositions such as multicomponent hafnia, rare earth - transition metal alloyed silicates and high stability EBC bond coat systems. The EBC system development aiming at prime-reliant, enabling turbine engine CMC turbine airfoil temperature capability and long-term durability will largely benefit from the advances in the coating vapor processing techniques and sophisticated simulated engine high pressure and high velocity combustion environmental durability testing, combined with creep and fatigue loading conditions.
8:40 AM A2-3-3 Y2SiO5 Coatings Fabricated by RF Magnetron Sputtering
Peter Mechnich (German Aerospace Center (DLR), Germany)
Owing to its high stability, Yttrium monosilicate (Y2SiO5) is considered a very promising candidate material for protective coatings of ceramics and ceramic matrix composites. In particular, protection against high-temperature oxidation and recession in water-vapor rich environments is required. In the preset work, Y2SiO5 coatings were deposited on CVD-SiC pre-coated C/SiC, and Al2O3-based ceramic substrates by RF-magnetron sputtering, respectively. Despite a polycrystalline Y2SiO5 target was employed, XRD-analysis reveals that Y2SiO5 coatings are non-crystalline in the as deposited state. Microstructural analysis shows a dense, glaze-like coating morphology, exhibiting a good conformity. As deposited, glassy Y2SiO5 is stoichiometrically homogenous, and virtually free of cracks and macropores. Annealing performed at 1200°C induces crystallization of the low-temperature polymorph X1-Y2SiO5. Moreover, formation of closed macroporosity is observed, which is due to the coalescence of former nanopores. Annealing of the CVD-SiC coated substrates in reducing and oxidizing atmospheres produces multiple, serious effects. Crack formation is explained by the glaze-like properties along with thermal expansion mismatch. In reducing atmosphere a significant decomposition of Y2SiO5 to Y2O3 is observed at the coating surface. On the other hand, in oxidizing atmosphere the SiC substrate forms SiO2 which reacts with Y2SiO5 to highly porous Y2Si2O7 at the coating/substrate interface. In case of oxide substrates, no undesired interface reactions occur below the eutectic temperature, therefore sputtered Y2SiO5 coatings may be suitable protective overlays for hot flowing water-vapor rich atmospheres.
9:00 AM A2-3-4 Optimum Design of High Temperature Thermal Radiation Energy Reflection Coatings for SiC/SiC Components
Yutaka Kagawa (National Institute for Materials Science, Japan)

Thermal radiation energy reflection coatings using all oxide ceramic materials have been designed. All oxide ceramics, such as Al2O3, ZrO2 etc., base multi-scale laminate structure is modified to obtain maximum efficiency of reflectance of thermal radiation energy from high temperature radiation source. Systematic analysis has been done for optimization of the reflectance using interaction behavior between electromagnetic wave and the multilayer laminate structure. Effects of individual layer shape/dimension and interface morphology between oxide ceramics on the reflectance are also incorporated in the analysis. Thermal radiation energy transfer model, which includes high temperature heat source Ts and coating temperature Tc, (Ts>Tc) is used for optimization of the microstructure. Discussions are made for future application of the coatings for SiC/SiC components; especially contribution for simple cooling system and reduction of cooling energy loss.

9:40 AM A2-3-6 Tridimensional Analysis of Interfacial Defects Consequences on Delamination of Thermal Barrier Coatings
Romain Soulignac (Mines-ParisTech, France)

Thermal barrier coatings (TBC) used for aerospace turbine blades endure severe thermal and mechanical loadings. Modeling their lifetime is a major challenge for aeronautical industry. The considered material is the Ni-base superalloy AM1 coated with (Ni, Pt) Al and Y2O3 yttria-stabilized ZrO2 zirconia. A previous lifetime model [1] was established using mechanical compression tests on thermally damaged specimens: the maximum strain before spallation was defined as a failure criterion, which evolves with damage.

This study aims to model the delamination propagation of the ceramic layer. Mechanical compression tests were conducted in order to measure the delamination area during the propagation. The delaminated area was measured using image analysis and evaluated as a function of the applied mechanical loading. The delamination rate was derived from this measurement performed for different ageing at high temperature. Infrared thermography is used as non-destructive tests (NDT) for damaged areas estimation. An image correlation software also allows us to obtain local strains during the test.

A macroscopic cohesive zone model (CZM) was finally identified to model the propagation of the delamination. Based on original 3D measurements [2], interfacial roughness and measured porosity are meshed. A finite element analysis (FEA) including CZM and microstructure details was performed to assess the criticality of the interfacial defects.

REFERENCES

[1] C. Courcier and V. Maurel and L. Rémy and S. Quilici and I. Rouzou and A. Phelippeau, Interfacial damage based life model for EB-PVD thermal barrier coating, Surface and Coatings Technology 205 (2011) 3763-3773.

[2] V. Maurel and R. Soulignac and L. Helfen and F. N’Guyen and T.F. Morgeneyer and A. Köster and L. Rémy, 3d damage evolution measurement in tbc using synchrotron laminography. Oxidation of Metals, Accepted for publication, 2012.

10:00 AM A2-3-7 Adsorption of Various REs Atoms on NiAl and Al2O3 Surface: An Implication for Grain Boundary Diffusion in Thermal Barrier Coatings
Tian Zhang, Hongbo Guo (Beihang University, China)
We use first-principles density functional theory to investigate adsorption of five reactive elements (REs), including Hf, Zr, Y, Dy and La atoms, on the β-NiAl(110) surface and α-Al2O3(0001) surface. We find that Hf, Zr, Y, Dy and La atoms all preferentially adsorb on Ni-Ni bridge sites on β-NiAl(110) surface while on α-Al2O3(0001) surface, the energetically favorable sites for all five REs are threefold-hollow sites. On the β-NiAl(110) surface, a binding order of Y, Dy < La < Hf, Zr was obtained while on the α-Al2O3(0001) surface, the binding order of Dy, Y < La < Zr < Hf was achieved. As known, in first-principles theory, a grain boundary can be viewed as composed of two surfaces, thus the surface adsorption of REs can partially reflect their properties in grain boundaries. Thereby, the adsorption energies in the present studies may provide some implications about the inhibition ability of REs on the diffusion of Al and O atoms in grain boundaries of β-NiAl based bond coat alloy and alumina layer in thermal barrier coatings.
10:20 AM A2-3-8 Microstructure and Thermal Oxidation Behavior of Yttria-Stabilized Hafnia Coatings
Ernesto Rubio, Mohammed Noor-A-Alam, Steve Stafford, Chintalapalle Ramana (University of Texas at El Paso, US)
High-temperature coatings are critical technologies for future power-generation systems and industries. Thermal barrier coatings (TBCs), which are usually the ceramic materials applied as thin coatings, protect engine components and allow further increase in engine temperatures for higher efficiency. Thus, the durability and reliability of the coating systems have to be more robust in compared to current natural gas based engines. A near and mid-term target is to develop TBC architecture with a 1300 °C surface temperature tolerance. Understanding the structural evolution thermal behavior of the TBC-bond coat interface, specifically the thermally grown oxide (TGO), is fundamentally important. In the present work, the attention is directed towards y ttria-stabilized hafnia (YSH) coatings on alumina (α-Al2O3) to simulate the TBC-TGO interface and understand the phase evolution, microstructure and thermal oxidation of the coatings. YSH coatings were grown on α-Al2O3 substrates by sputter deposition by varying coating thickness in a wide range of ~30-1000 nm. The effect of coating thickness variation on the structure, morphology and the residual stress was investigated using X-ray diffraction (XRD) and high resolution scanning electron microscopy (SEM) . Thermal oxidation behavior of the coatings is evaluated using the isothermal oxidation measurements under static conditions. X-ray diffraction analyses revealed the existence of monoclinic hafnia phase for relatively thin coatings indicating that the interfacial phenomena are dominant in phase stabilization. The evolution towards pure stabilized cubic phase of hafnia with the increment of coating thickness is observed. The SEM results show the morphology of the sample with different thicknesses, and the average grain size increase in the range ~15-500 nm. Residual stress was calculated employing XRD techniques by changing the magnitudes of ψ-angle .Relation between residual stress and structural change is also studied. The results obtained on the thermal oxidation behavior indicate that the YSH coatings show initial mass gain in the first 6 hours and sustained structure for extended hours of thermal treatment.
10:40 AM A2-3-9 Tribocorrosion Mechanisms in Laser Deposited Titanium-based Smart Tribological Composite Smart Coating
Peter Olubambi, Masego Lepule, Babatunde Obadele (Tshwane University of Technology, South Africa); Joseph Olatunde Borode (Federal University of Technology, Nigeria)

Although titanium based alloys and composites have been widely utilized as smart materials for varying engineering applications, their relatively poor tribological properties have limit their effective applications as smart tribological composite coatings in the emerging surface engineering technology. In this study, the tribocorrosion mechanisms of titanium-based smart composite coatings deposited using laser materials deposition on austenitic stainless steel is investigated. Titanium and nickel powders were blended with zirconium in a turbular mixer, and the blended feedstock powders were laser deposited onto 316 austenitic stainless steel at varying laser processing parameters. Tribocorrosion behaviour of the composite coatings in sulphuric acid under varying loads was studied under sliding wear-corrosion set up using a CETR UMT-2 tribometer. Results revealed that the TiNi and TiNi-ZrO2 in a pseudoplastic state exhibited lower coefficient of friction as compared with 316. Wear resistance of TiNi alloy was mainly dependent on the recoverable strain limit, and also, on the effects of the reinforcing zirconium particles. Nevertheless, the coatings displayed a comparatively lower corrosion sesistance than the based stainless steel.