ICMCTF2014 Session A2-2: Thermal and Environmental Barrier Coatings

Yttria Stabilized Zirconia (YSZ) thermal barrier coatings (TBCs) have been extensively used for over 40 years to insulate hot section gas turbine components because of their favorable combination of properties. One of the key properties of YSZ is a very high thermal expansion coefficient, which reduces the thermal expansion mismatch between the TBC and the underlying metal. Many oxide ceramics with lower thermal expansion coefficients than YSZ have been evaluated as potential second generation TBCs and have been rejected because of poor thermal cyclic durability.

The degradation during service of the current standard Yttria Stabilized Zirconia (YSZ) Thermal Barrier Coatings (TBCs) protecting critical components in gas turbine engines is mainly caused by the initiation and the propagation of microcracks at the interface with the bond coat, which makes its early detection difficult. The development of reliable predictive models for TBCs spallation is hindered by the difficulty of accessing to the effective interface temperature through conventional means without compromising the integrity of the coating. This results in strongly conservative margins being imposed to allow safe operation.

In this context there have been a growing interest in the application of phosphor thermometry methods for the diagnostic of TBCs. The partial transparency of YSZ in the visible range of the spectrum allows to collect local information conveyed by the phosphorescence emissions from optically excited luminescent layers integrated within the depth of the TBC. This functionalisation can be obtained by the introduction of optically active components such as trivalent lanthanide ions directly into the crystal structure of YSZ, thus without detrimental alterations of the coating properties. Reported here is the fabrication feasibility study of such multilayer TBC structures by a sol-gel process alternative to standard electron beam physical vapour deposition and plasma spraying methods used for YSZ coatings for future applications in through thickness measure of temperature and early spallation monitoring.

9.7at%-YSZ phosphors have been synthesised via a sol-gel route by small additions of luminescent centers including Eu³⁺, Dy³⁺, Er³⁺, Sm³⁺ and Tm³⁺ ions. The microstructure as well as the room temperature spectral and temporal responses of these materials were investigated to optimise both their luminescence and microstructural properties.. Different multilayer prototypes integrating functionalised layers were successfully deposited by dip-coating on an industrial grade single crystal nickel superalloy. The optical performance of these designs were evaluated for future applications in punctual and 2D temperature measurements throughout the depth of the TBCs. In addition TBC samples containing pre-calibrated delaminated areas were produced and characterised as a first attempt to study the effects of interface decohesion on the luminescent emissions from doped YSZ sublayers. Preliminary results are promising for the use of fully integrated YSZ-type phosphor layers for monitoring temperature profiles and sensing damage evolution in TBCs systems exposed to cyclic oxidation conditions.

Two commercially available zirconia stabilized powders, namely Metco 204NS, ZrO₂+ 8 wt. % Y₂O₃ and Metco, 205NS, ZrO2 + 24 wt.% CeO₂ + 2.5 wt.% Y₂O₃ were attritor milled to obtain the particles sizes smaller than ten micrometers. The fine solids were formulated in a suspension composed of 20 wt.% powder, 40 wt.% water, and 40 wt.% ethanol. The suspensions were used for plasma spraying using two torches and SG-100 of Praxair and Triplex of Sulzer Metco. The suspension was injected into plasma jet through a continuous stream injector installed inside (SG-100) or outside of plasma torch (Triplex). The spray processes were carried out by varying the spray distance, the torch scan velocity, the electric power input and, finally, the stainless steel substrates roughness. The latter was achieved by the sand blasting using alumina grit. The temperature of the coatings during deposition was monitored using a pyrometer. The coatings were prepared metallographically and their microstructure was characterized using field emission scanning electron microscope and transmission electron microscope. The microstructure was correlated to the operational spray parameters. Finally, thermal diffusivity of obtained coatings was tested using two different set ups in low (up to 300°C) and high temperatures (up to 800°C). Moreover, the some mechanical properties of the coatings were tested using scratch and indentation tests.

Spherical Al particles in the range of 1 to 20 µm are deposited as slurry on the surface of a Ni- or Fe-based alloy according to the PARTICOAT concept (www.particoat.eu). During the heat treatment they oxidize to a topcoat from sintered hollow alumina spheres while forming an aluminized diffusion zone in the substrate. The topcoat effectuates as a thermal barrier by gas phase insulation and the diffusion zone forms a protective alumina layer.

Sheets of austenitic steels such as Alloy 321 were coated by spraying or tape casting using slurries with Al particles in a size range of 1 to 20 µm. Boron was added to the slurry to achieve a higher sintering degree and better adhesion to the substrate. To investigate the thermal barrier effect and the behaviour under exposure to temperature with the thermal gradient between the exposed side and the cooled backside, an experimental set-up was designed, which allows to heat the sample from one side while being cooled by airflow on the backside. The temperature is measured by thermocouples on both sides as a function of the time. An electric radiation heater or a Bunsen burner is used as heat source.

The coating provides a notable thermal barrier effect due to gas phase insulation by the hollow alumina sphere structure. A topcoat with a thickness of 300 µm for example, reduces the temperature at the backside by 350°C without and by 550°C with backside cooling when exposing the surface to 800°C. The temperature reduction by gas phase insulation remains stable over the investigated times of up to 100 h. The metastable alumina phases γ-Al₂O₃ and θ-Al₂O₃ are observed at the interface to the aluminium-rich diffusion zone confirming the reduction of temperature at the metal surface.

Recently, the plasma electrolytic oxidation (PEO), or the so-called micro-arc oxidation (MAO) process has been widely studied and applied in industries due to its ability to create functional oxide layers on light metals. In this work, the hybrid method of PEO and hot-dipped aluminizing (HDA) was employed to deposit composite ceramic coatings on the surface of carbon steel plate. The HDA of carbon steel plate was executed at 700^oC for 5 mins. The duty cycle and frequency of PEO were adjusted to fabricate six different HDA-PEO coatings. The chemical composition and microstructure of coatings were determined by a field emission electron probe microanalyzer (FE-EPMA), X-ray diffractometer (XRD) and scanning electron microscopy (SEM), respectively. The hardness and adhesion of coatings were determined by the micro hardness tester and scratch tester. The corrosion resistance of coatings was evaluated by the potentiodynamic polarization test in 3.5 wt.% NaCl aqueous solution. Effects of duty cycle and frequency on the microstructure, mechanical property and corrosion resistance of HDA-PEO coatings were discussed in this work.

Keywords: plasma electrolytic oxidation, hot-dipped aluminizing, duty cycle, frequency, scratch tester, corrosion test

As the operating temperature of turbine engines has increased, so has the prevalence of molten calcium magnesium alumino-silicate (CMAS) deposits infiltrating thermal barrier coatings (TBCs). The molten CMAS fills the pores in the TBC, which stiffens the coating, magnifying the stresses generated from the thermal strains, which increases the tendency for cracking. The aim of this presentation is to examine the effect that temperature gradients have on the interaction between silicate deposits and TBC systems. A thermal gradient test, in which a CO₂ laser is employed to impose a controllable thermal gradient, is used to investigate the interaction between CMAS and 7YSZ and GZO EB-PVD TBCs. The experimental results are then used to guide the development of expressions that describe the nature of silicate infiltration into the TBC, the evolution of coating elastic modulus, and the generation and release of stresses.

Innovative turbine concepts to assist in carbon capture are being considered that may result in higher H₂O and/or CO₂ concentrations than typical experience with natural gas fired land based turbines. Furnace cyclic testing has been used to assess thermal barrier coating (TBC) lifetime with superalloy 1483 and X4 substrates and high velocity oxygen fuel (HVOF)-NiCoCrAlYHfSi bond coatings at 1100°C. Average air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coating lifetimes were 5-6 times longer and interdiffusion was higher when 100h cycles were used to simulate base-load operation, rather than the 1h standard of aeroengines. With 100h cycles in air with 10%H₂O the average lifetime with X4 substrates increased 60% compared to 1483 substrates, while the difference was 40% in 1h cycles. Additions of 10-50%H₂O or 90%CO₂ did not strongly affect TBC lifetime on the 1483 substrates. Photo-stimulated luminescence spectroscopy (PSLS) and 3D microscopy were used to measure residual stress in the alumina scale and surface roughness, respectively, on specimens without a YSZ top coating. The average compressive stress in the scale was lowest in the 10%H₂O/airenvironment, however this difference did not correlate to changes in the TBC lifetime.

Research sponsored by the U. S. Department of Energy, Office of Fossil Energy, Coal and Power R&D.

Failure characteristics of EB-PVD YSZ TBCs with Pt-modified NiAl bond coat were examined with furnace cycling at 1100°C with dwell time of 1 hour using photo-stimulated luminescence spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. To examine the influence of EB-PVD topcoat during thermal cycling, the other side of the button specimen was only coated with (Ni,Pt)Al. Rumpling occurred on both sides but the amplitude of interface roughness increased more rapidly when the ceramic topcoat was absent. However, the TGO grew faster for the YSZ-coated (Ni,Pt)Al bondcoat. The compressive residual stress of the TGO scale initially increased, then gradually decreased on both sides. While the magnitude of peak compressive stress was similar (3 to 4 GPa), the decrease in the magnitude as a function of thermal cycling was faster for the bare (Ni,Pt)Al side, especially for the TGO developed on ridges. Interfacial and strain energy of the TGO scale were also estimated. Evolutions in phase constituents and microstructure examined by electron microscopy and relevant selected electron area diffraction analyses. Results from luminescence, microscopy and ensuing analyses were correlated with the failure characteristics of the TBCs to elucidate failure mechanisms.

Instrumented nanoindentation enables examination of materials’ surface mechanical properties with greater resolution and accuracy than ever before. Many applications not only require testing materials at small scale but also at high temperatures before component reliability in the working conditions can be confirmed. Nanomechanical characterization at high temperatures has been limited due to a number of challenges related to the instrumentation.

We have recently developed a unique method that allows for complex characterization of a material’s surface at elevated temperatures. A newly-designed, radically-different, low-drift heating stage for precise control of temperature up to 600 ^oC, combined with an improved nanoscale dynamic mechanical testing capability and in-situ scanning probe microscopy-based imaging, resulted in drift-free nanomechanical properties measurements over extended time.

Such experiments are particularly beneficial for investigation of materials such as superalloys and bond coats which were designed and optimized to serve in extreme atmospheric conditions. Based on the example of an intermetallic PtNiAl bond coat, we will demonstrate how our recently developed methodology for improved dynamic mechanical testing at elevated temperatures has been utilized for dynamic nanoscale creep characterization. Mechanical property mapping, for directly imaging mechanical response and properties with submicron spatial resolution, can further advance the understanding of material behavior.