ICMCTF2011 Session A2-1: Thermal and Environmental Barrier Coatings

Attention will be focussed on plasma sprayed coatings, although most of the points made apply equally to PVD coatings. A model of the (solid state) sintering process [1] will be briefly outlined, based on the variational principle. Comparisons will be presented between theory and experiment, for shrinkage (dilatometry), surface area (BET) and thermal conductivity. The roles of surface (non-densifying) and grain boundary (densifying) diffusion will be highlighted.

Work will then be described in which specimens have been subjected to extended periods at (isothermal) high temperature (1200-1500˚C) and periodically quenched to room temperature using gas jets, with automatic monitoring of spallation events via a webcam. These specimens comprised partially stabilised zirconia (PSZ) coatings plasma sprayed onto relatively thick alumina substrates, with and without the subsequent surface addition of particulate designed to represent CMAS incorporation. Prior roughening of the alumina surface, via laser processing, was employed to ensure adequate interfacial toughness. The thermal misfit strain induced during cooling of such specimens has a magnitude (~15-20 millistrain) similar to that for PSZ on a superalloy substrate, although it is of opposite sign. Since little or no chemical reaction is expected between substrate and coating at these temperatures for a zirconia-alumina combination, any observed spallation is likely to have been promoted by changes occurring within the coating, such as sintering-induced stiffness enhancement. Experimental data will be presented concerning such observed spallation events, and conclusions drawn about the significance of sintering effects for TBC stability, with and without CMAS incorporation.

[1] A Cipitria, IO Golosnoy & TW Clyne, A Sintering Model for Plasma Sprayed Zirconia TBCs, Acta Mater., vol.57 (2009) 980-1003.

Yttria partially stabilised zirconia (YSZ) is widely used for thermal barrier coatings (TBC) in power generation, deposited both by APS as well as by EB-PVD. Degradation by molten deposits, mostly calcium-magnesium-alumino-silicates, which enter the turbine from the environment and are known as CMAS, has been identified as a serious cause of failure. It infiltrates the pores and cracks, reacts with the YSZ and leads to its destabilisation. To understand better after which time does CMAS penetrate into which depth and react to which extent and to investigate potential protection against CMAS attack is the purpose of the current research work.

A model CMAS, composed in mol% from 38CaO, 6MgO, 5Al₂O₃, 50SiO₂ and 1Fe₂O₃ ultra-milled, molten two times for 4 h at 1400°C and milled again, was deposited on the surface of an APS thermal barrier coating from commercial YSZ. The samples were exposed to 1100°C and 1150°C for 50h, 100h and 1000h in air and analysed by X-ray diffraction with micro-focus (µ-XRD) and by field emission SEM. Scanning the sample surface by µ-XRD from the unaffected area to the attacked area in steps of 200 µm reveals already after 50h considerable portions of the monoclinic phase in the CMAS attacked area. In micrographs however, this structural degradation is not visible while the infiltration by CMAS is observed.

Vermiculite powder (~100 µm diameter) was selected as having a composition (42%O-36%Si-8%Mg-5%K-3%Al-3% Na-2.5%Fe-0.5%Ti) representative of volcanic ash. Its melting temperature is about 1330˚C. Selected loadings of this powder were introduced onto the surface of plasma sprayed YSZ coatings (detached from their substrates), either on one surface or on both surfaces. The mass of these additions, relative to that of the YSZ, was in the range 0.1 - 5.0%. A short heat treatment was initially given, designed to promote adhesion of the vermiculite particles, after which the exact loading was established via high precision weighing. The following measurements were then made, on samples that had been exposed for periods (up to 50 hours) at elevated temperature (up to 1500˚C):

• Composition profile measurements by EDX on transverse sections, to monitor the penetration (down grain boundaries) of species from the vermiculite

• XRD spectra from free surfaces, after serial sectioning, to monitor the vitreous phase (ie grain boundary phase) content as a function of depth, and also the possible formation of new crystalline phases

• Measurement of the average (in-plane) Young's modulus, by 4- point bend testing of samples "injected" with vermiculite on both sides, to monitor the progression of sintering

• Measurement of the curvature of samples "injected" with vermiculite on one side, to monitor differential sintering (provided it is occurring via mechanisms that generate a volume contraction)

These measurements were used to provide information about the effect of typical volcanic ash particulate on the acceleration of sintering. It is shown that there is potentially a highly significant effect, with associated impairment of thermal protection and thermo- mechanical stability. Finally, preliminary experiments have been carried out to measure the deposition efficiency of the vermiculite powder on (uncoated) blades within a small jet engine, under typical operating conditions. These studies included the effect of blade surface roughness, particle size and engine speed. While these efficiencies are often quite low, it is shown that, for relatively low ash cloud particulate burdens, the deposition rates could in some cases be in a range such that substantially accelerated sintering would be promoted.

Turbines for aircraft propulsion and power generation need Thermal Barrier Coatings (TBCs) which allow higher gas temperature and improve lifetime. The zirconia layer reduces surface temperature and oxidation damage is limited by the thermally grown oxide. However, during thermal cycling the coating can delaminate by crack nucleation, propagation, and coalescence phenomena.

In this study, the LAser Shock Adhesion Test (LASAT, Fig.1) was applied to EB-PVD TBCs with different aging periods of one-hour cycles at 1100°C. A high energy pulsed laser is focused on the superalloy substrate surface opposite to the zirconia coat. The compressive shock wave propagates through the TBC layers and is reflected in a tensile shock wave at the zirconia free surface. Increasing the laser energy deposited on the superalloy surface, the zirconia-(Ni,Pt)Al bond coat interface remains intact for low levels of laser energy and fails for high levels with possible coating spallation for highest levels. Two laser spallation methods have been used to discriminate specimens depending on their thermal cycling. For the conventional method, by determining the minimum laser energy (“LASAT threshold”) necessary to crack interface, results proved that oxidation reduces the LASAT threshold and the corresponding measured interfacial strength. Features of the interfacial cracks were observed by SEM cross-sectional and top-surface views of laser shocked regions. For both specimens, the failure path (Fig.2) was located along the zirconia-alumina interface or within zirconia in instability sites as “pinched-off” regions. This failure morphology activated by laser shock was very similar to damage induced by crack nucleation and propagation under thermal cycling for this TBC system.

Due to large cracks at the alumina-zirconia interface and light transmitting properties of ceramic layers, optical response of TBCs changes and the interfacial damage generated by LASAT is directly detected by visual inspection (Fig.3). The change in TBC whiteness induced by failure allows the direct detection and measurement of the delamination crack front without cross-sectional observations. By plotting interfacial failure diameter versus deposited laser energy (graph 1), the decrease in crack resistance at the interface is demonstrated by the continuous gap of damage diameter between as-deposited and cycled specimens. A new LASAT method is established.

Further numerical FEM calculations of shock wave mechanics have been developed for TBCs to determine the interface stress level and simulate damaging phenomena. This work showed that LASAT is a powerful testing tool to investigate fracture resistance of TBCs.

Characterization and observations of ferroelastic switching have become increasingly important in the study of thermal barrier materials because of the important role they play in the high temperature toughness of tetragonal zirconia materials. In this paper we discuss the observations of ferroelastic domains in ceria-stabilized zirconia by confocal Raman spectroscopic mapping. Evidence of ferroelastic switching at the crack tip has been observed using this technique as well as measurements of the crystal orientation of individual grains. We will also discuss the use of this technique to complete measurements of coercive stress using a high temperature diamond anvil cell.

Air plasma sprayed thermal barrier coatings (TBCs) with MCrAlY (M = Ni, Co) bondcoats on Ni-based superalloys were tested in laboratory air at 1000 and 1100°C under various temperature cyclic conditions. Metallographic cross-sections of the exposed specimens were studied by optical metallography and scanning electron microscopy (SEM). The analytical studies indicated that in all studied testing conditions TBC-failure was initiated in the convex regions at the alumina scale / bondcoat interface. The crack propagation through the TBC was found to be the major lifetime limiting step.

The lifetime of a given TBC-system was strongly affected by the testing parameters. The parameters of importance are hot and cold dwell times, exact cold dwell temperature and cooling rate, which all have an effect on the crack propagation rate in the TBC. The lifetimes are longer with longer hot dwells and slower cooling rates, whereas the extension can be as large as a factor of two to three, as compared to shorter dwells and higher cooling rates. Furthermore, specimen geometry and preparation procedure for the test were also found to have an effect on the TBC lifetime.

The presented results indicate that careful selection and exact definition of the specimen preparation and testing procedures are absolutely necessary for a reliable lifetime determination in one testing laboratory, comparison of data between different laboratories and extrapolation of laboratory results to lower coating application temperatures.

To account for extremly high heating rates in reality, a laser-cycling set-up was developed to qualify different coating systems. This set-up consists of a high-power diode laser (3kW) and realizes temperatures up to 1500°C. Furthermore, it is possible to realize high thermal gradients inside the specimen with an additional cooling.

Failure mechanisms of the Thermal Barrier Coating systems are investigated with this set-up. In preliminary experiments the interface between bond coat and substrate fails which is also theoretically confirmed by FEM-simulation.

thermally grown oxides. Processing such TBC by the so-called dip-coating route results in either thin or thick, fairly adherent coatings showing non-oriented microstructures with randomly structured pore network. This specific microstructure confers to the barrier an optimum compromise between a satisfactory low thermal conductivity to properly insulate the system and a good lateral compliance to resist without fracture the thermo-mechanical stresses generated in service. After soft chemical processing has been completed, sintering of the TBC is ensured by a controlled heat treatment that generates the formation of a fairly regular surface crack network as a result of the bi-axial loading due to the ceramic shrinkage. Upon cyclic oxidation, the behaviour of the TBC strongly depends on the characteristics of the initial crack network that can further enhance under the detrimental effects of the thermomechanical stress generated by the cumulative cycles. Ultimately, the development of cracks that can perfectly connect to each other can provoke the detachment of individual yttria particles resulting in the onset for spallation and catastrophic failure of the TBC.