ICMCTF2015 Session A2-1: Thermal and Environmental Barrier Coatings

Thermal barrier coatings (TBCs) are essential in advanced gas turbines that require higher operating temperatures for increased efficiency and performance. As operating temperatures rise TBCs are vulnerable to degradation by ingested siliceous contaminants; the deposits generally referred to as CMAS (calcium-magnesium-alumino-silicates) melt during engine operation and infiltrate the porous ceramic top coat, compromising its strain tolerance. The ensuing stiffening can lead to delaminations through the top coat and at the TBC/TGO interface. A recently identified mechanism that involves bond coat cavitation can also lead to spallation of the TBC with the delamination occurring entirely within the bond coat. While bond coat void formation is not a new phenomenon, the correlation between CMAS infiltration of the TBC and cavitation has not been studied previously. Examination of specimens displaying signs of this damage suggests that creep deformation of the bond coat material motivated by stresses associated with the stiffening of the TBC can lead to void nucleation, growth, and coalescence. Numerical modeling, complemented by laser gradient experiments, is used to understand the mechanism behind the degradation and the evolution of the stresses during thermal cycling and high temperature dwells.

Because of their high Cr- and Al-contents and the presence of the reactive element Y, MCrAlY coatings exhibit a great resistance to high temperature corrosion and oxidation. However, they are generally not perfect alumino-formers. This constitutes a key issue for thermal barrier coating system applications for which Pt-modified β-NiAl or Pt-rich γ-Ni+γ’-Ni₃Al bond-coatings are often preferred. In the aim of overperforming these laters, NiCoCrAlYTa coatings were modified by a Pt overlayer and the effect of such Pt addition was investigated. This included the study of the influence of Pt on the coating microstructure, the impact on the oxidation resistance of the bond-coating as well as on the lifetime of thermal barrier coating systems. In addition, the influence of the manufacturing (NiCoCrAlYTa and Pt deposition processes, surface preparations) was analyzed after thermal cycling tests and degradation mechanisms were proposed. Due to an extensive Al diffusion toward the external Pt-rich zone during the bond-coating heat treatment, enrichment in Al occurred in the sub-surface and martensite formed. This favored Al selective oxidation, increased the oxide scale adherence, lead to a decrease in the oxidation rate and finally resulted in longer thermal barrier coating system lifetimes. Thermal cycling tests highlighted the relevance of manufacturing dense, well interdiffused and oxide-free NiCoCrAlYTa coatings, as well as suitably smoothing NiCoCrAlYTa surfaces before and after the modification by Pt.

Infiltration of CaO-MgO-Al₂O₃-SiO₂ (CMAS) in TBCs is an issue of concern for the aeronautical industry. From the existing research it is known that as soon as CMAS melts it infiltrates the TBC structure, chemically attacks the TBC, changes its phase composition and ultimately leads to the spallation of the TBC. Few investigations have shown the presence of anhydrite (CaSO₄) within CMAS deposits. These investigations could not provide a complete understanding of the presence and the effect of CaSO₄ on CMAS induced TBCs damage in this study. A systematic approach in understanding the effect of CaSO₄ in CMAS has been taken. Five different CMAS compositions with and without CaSO₄ were synthesized in the laboratory and their infiltration behaviours were investigated by depositing them on EB-PVD 7YSZ samples and subsequent heating at 1225 and1250°C. In addition, mass spectroscopy was applied on CaSO₄ containing CMAS and the vaporization behaviour of sulphur was studied at high temperature. XRD was applied on the molten CMAS compositions and differences in phase formation due to the presence of CaSO₄ were analysed. Based on this information, it is proved that sulphur which was present in the form of anhydrite evaporates as SO₃ during the high temperature heating and extra CaO adds to the CMAS composition. This CaO-enriched CMAS is found to be more destructive than the initial CMAS. Hence it can be emphasised that CaSO₄ presence in CMAS has no direct effect on the infiltration, but rather an indirect effect in changing the CMAS composition.

This study concerns the effect of volcanic ash content on the microstructure, stiffness and spallation lifetime of plasma sprayed YSZ coatings. These were produced using two different YSZ powders, containing 8 mol% and 38 mol% of Y₂O₃, with the latter formulation being claimed to produce coatings more resistant to CMAS-induced degradation. Following previous work on the ingestion [1] of Volcanic Ash (VA) into aeroengines, and on their tendency to promote [2, 3] spallation of Thermal Barrier Coatings (TBCs), two types of Icelandic VA, from Laki and Hekla volcanoes, were deposited onto plasma sprayed YSZ, at different dosage levels. Microstructural interactions between the YSZ and the penetrating VA were studied using XRD and EDX element mapping, after heat treatment at 1500˚C for periods up to 60 h. Free-standing coatings were used to measure the Young modulus and coatings attached to alumina substrates were employed to measure spallation lifetimes (in a periodic quenching furnace). Penetration of the VA into YSZ samples accelerates the sintering and associated stiffening of these coatings, and hence promotes spallation by raising the strain energy release rate during quenching. It is clear from microstructural examination that this is the main mechanism responsible for the reduced spallation lifetime, rather than the VA reaching the interface and impairing its toughness. It was found that the yttria level did have an effect on the microstructural changes, and on the associated sintering effects, and the details of this are explained.

[1] Shinozaki, M. Roberts, K.A., van de Goor, B and T.W. Clyne, Deposition of ingested volcanic ash on surfaces in the turbine of a small jet engine. Adv. Eng. Mats., 2013. 15: p. 986-994.

[2] Shinozaki, M. and T.W. Clyne, A methodology, based on sintering-induced stiffening, for prediction of the spallation lifetime of plasma sprayed coatings. Acta Mater., 2013. 61: p.579-588.

[3] Shinozaki, M. and T.W. Clyne, The effect of vermiculite on the degradation and spallation of plasma sprayed thermal barrier coatings. Surf. & Coating Techn., 2013. 216: p.172-177.

Although silicon-based ceramics (i.e., SiC, Si₃N₄) have been identified as some of the most promising materials systems for high-temperature structural applications in engine environments, a passivating SiO₂ surface layer, which forms on SiC and Si₃N₄ in oxygen-containing environments, becomes unstable in combustion environments, resulting in the volatilization of the silica layer and component recession. Environmental barrier coatings (EBCs) have been identified as protection from these harsh turbine environments. In this study, the interaction of two candidate EBC topcoats, Yb₂Si₂O₇ and Yb₂SiO₅, with CMAS was investigated in detail using quantitative laboratory-based X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction. Of particular interest was the formation of a new phase, an ytterbium-silicate oxyapatite (Ca₂Yb₈(SiO₄)₆O₂) in both ytterbium silicates.

Ceramic coatings processed by thermal spraying are involved in many industrial applications such as thermal barrier coatings (TBCs), electrical insulating and anti-corrosive systems. Due to the multilayered structure of TBCs, the mismatch of coefficients of thermal expansion (CTE) between each layer generates an equi-biaxial compressive stress state during thermal cycling. Usually, service loading conditions result in both thermal and mechanical loadings which conducts to the ceramic layer failure. The equi-biaxial stress state due to thermal cycling could be modified by mechanical loading varying the biaxiality stress ratio underwent by the TBC. Hence, to clarify the mechanisms of failure, the influence of bi-axial loading on crack growth is investigated. To that end, pre-cracked ceramic coatings were prepared by applying a laser shock debonding process: LASAT technique (Laser Shock Adhesion Test). The advantage of this contactless method is to easily control the size of a crack at the metal/ceramic interface. For this study, a polycristalline cobalt base alloy substrate and an alumina coating deposited by Air Plasma Spraying (APS) are considered. A coplanar biaxial fatigue machine is employed to prescribe various biaxiality stress ratios to a cruciform specimen. Top surface observation enables crack growth measurement under mechanical loading. In order to discuss the influence of biaxial loading, both uniaxial and biaxial tensile/compression tests results are compared. Finally, analysis of observed interfacial cracking enables to discuss the relevance of existing FE models.

Keywords: ceramic coating, biaxial loading, crack growth, interface

Bi-Layer thermal barrier systems made of a bottom layer of standard yttria stabilized zirconia (YSZ) and a top layer of gadolinium zirconate (GZO) offer the potential to increase the operating temperature of gas turbines or aero engines beyond the temperature limit of single layer YSZ systems. This is due to the exceeding phase stability of gadolinium zirconate at temperatures above 1200°C compared with yttria stabilized zirconia.

In this work, the mechanical stability of such novel thermal barrier coating systems prepared by atmospheric plasma spraying (APS) was investigated after isothermal oxidation at 1050°C. 4-point bend testing with in-situ acoustic emission measurement served for determining the critical strain to failure of the coating, while the use of the acoustic emission technique enabled distinction of individual failure modes in the bi-layer system, such as, shear failure of the top GZO-layer, delamination along the GZO/YSZ interface, or shear failure of the bottom YSZ-layer.

The measured critical strain values in combination with careful metallographic inspection were used to establish a fracture mechanics based lifetime model in form of mechanical stability diagrams. With the use of these diagrams, the failure of each individual layer can be assessed with respect to the externally applied load. The developed lifetime model enables a lifetime assessment by comparing the critical strain to failure with component loading cycles.

Key Words: Bi-layer thermal barrier coatings, gadolinium zirconate, 4-point bend testing, acoustic emission.