YSZ Thermal Barrier Coatings Research Articles

Microstructure and surface residual stress of plasma sprayed nanostructured and conventional ZrO2‐8wt%Y2O3 thermal barrier coatings

AbstractThe nanostructured agglomerate feedstock which can be used for plasma spraying was obtained successfully by the reconstituting nanoparticle technique. Nanostructured and conventional ZrO2‐8wt%Y2O3 (8YSZ) thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying (APS) on NiCoCrAlY coated 45# steel substrates. The microstructure and phase composition of feedstock and corresponding coating were characterized by SEM and XRD. The elastic modulus was tested by the nanoindenter instrument. The surface residual stresses were examined by XRD stress test equipment. The numerical value probability statistics of surface residual stress by the Weibull distribution theory was implemented under different thermal spray parameters. The value of residual stresses increased with increasing the thickness of the ceramic coating. The surface residual stress of nanostructured 8YSZ coating is lower than that of the conventional 8YSZ coating. A physical mechanical model is put forward to explain the causation of the variation of residual stresses. At the same time, the variation characteristic of surface residual stress is discussed by introducing the closed form solution of residual stress of laminar layer plate. Copyright © 2010 John Wiley & Sons, Ltd.

Preparation and characteristics of three-layer YSZ–(YSZ/Al 2O 3)–YSZ TBCs

Novel three-layer YSZ–(YSZ/Al 2O 3)–YSZ (6 wt.% Y 2O 3 partially stabilized ZrO 2) thermal barrier coatings (TBCs) were successfully prepared on Ni-based superalloy substrate using composite sol–gel and pressure filtration microwave sintering (PFMS) techniques. The coatings were evaluated for the cyclic oxidation resistance, thermal barrier effect and the presences of phases and microstructures. FE-SEM results indicate that the coatings were dense and crack-free. The coatings maintained their structural integrity when they were exposed at 1100 for 100 h. They exhibited superior oxidation resistance, spallation resistance and thermal insulation property compared with single-layer YSZ coatings. Moreover, the detailed mechanisms were discussed in order to understand the improved performance of the three-layer TBCs.

Preparation and Characterization of 8YSZ Thermal Barrier Coatings on Rare Earth-Magnesium Alloy

Thermal barrier coatings (TBCs) of zirconia stabilized by 8 wt.% yttria (8YSZ) on top of rare earth-magnesium alloy were fabricated by atmospheric plasma spraying. Three coatings were prepared by controlling the substrate temperature with different cooling media including cold water, compressed air, and natural cooling during deposition. The phase compositions, mechanical properties, and thermal shock resistance of these coatings were investigated. These coatings are very helpful to the improvement of corrosion and wear resistances and thermal protection of magnesium alloy. During plasma spraying, the substrate temperature affects the microstructure of substrate and the bonding strength between the coating and the substrate. The coating whose substrate was cooled by compressed air during plasma spraying has a good thermal shock resistance. The coating failure is mainly induced by the substrate oxidation and thermal expansion mismatch between the coating and the substrate.

High temperature oxidation interfacial growth kinetics in YSZ thermal barrier coatings with bond coatings of NiCoCrAlY with 0.25% Hf

The results of an experimental study of the high-temperature isothermal oxidation behavior and microstructural evolution in two variations of air plasma sprayed ceramic thermal barrier coatings (TBCs) are discussed in the paper. Two types of TBC specimens were produced for testing. These include a standard and vertically cracked APS. High temperature oxidation was carried out at 900, 1000, 1100 and 1200°C. The experiments were performed in air under isothermal conditions. At each temperature, the specimens were exposed for 25, 50, 75 and 100h. The corresponding microstructures and microchemistries of the TBC layers were examined using scanning electron microscopy and energy dispersive X-ray spectroscopy. Changes in the dimensions of the thermally grown oxide layer were determined as functions of time and temperature. The evolution of bond coat microstructures/interdiffusion zones and thermally grown oxide layers were compared in the TBC specimens with standard and vertically cracked microstructures.

Mechanical Properties of LaTi2Al9O19 and Thermal Cycling Behaviors of Plasma-Sprayed LaTi2Al9O19/YSZ Thermal Barrier Coatings

Recently, extensive efforts have been made to develop new thermal barrier coating (TBC) materials which can operate at temperatures above 1523 K over a long term. In this article, LaTi2Al9O19 (LTA) was synthesized by solid-state reaction at 1773 K, and the mechanical properties of the LTA bulk were evaluated. The microhardness is about 14 GPa, comparable to that of YSZ bulk, whereas the Young’s modulus is about 44 GPa, lower than the value of YSZ. However, the fracture toughness of 0.8-1 MPa m1/2 is much lower than that of bulk YSZ. A double-ceramic-layer LTA/YSZ TBC structure was proposed and the TBC sprayed by plasma spraying. Thermal cycling tests of the TBC specimens were performed at 1373 K with a dwell time of 10 min. The LTA remained good stability with ZrO2 and Al2O3. However, the single layer LTA TBC was cracked at the LTA/bond coat interface after about 300 cycles, due to its poor thermal shock resistance, while the YSZ TBC yielded a lifetime of about 1000 cycles. The LTA/YSZ TBC remained intact even after 3000 cycles, exhibiting a promising potential as new TBC materials.

Effect of Young's modulus evolution on residual stress measurement of thermal barrier coatings by X-ray diffraction

Subjected to thermal cycling, the apparent Young's modulus of air plasma-sprayed (APS) 8 wt.% Y 2O 3-stabilized ZrO 2 (8YSZ) thermal barrier coatings (TBCs) was measured by nanoindentation. Owing to the effects of sintering and porous microstructure, the apparent Young's modulus follows a Weibull distribution and changes from 50 to 93 GPa with an increase of thermal cycling. The evolution of residual stresses in the top coating of an 8YSZ TBC system was determined by X-ray diffraction (XRD). The residual stresses derived from the XRD data are well consistent with that obtained by the Vickers indention. It is shown that the evolution of Young's modulus plays an important role in improving the measurement precision of residual stresses in TBCs by XRD.

Effects of piezo-spectroscopic coefficients of 8 wt.% Y2O3 stabilized ZrO2 on residual stress measurement of thermal barrier coatings by Raman spectroscopy

Abstract By using specially designed freestanding 8 wt.% Y 2 O 3 stabilized ZrO 2 (8YSZ) specimens, a linear relationship is found between the Raman peak shift, Δ ω , and applied uniaxial compressive stress, σ u , i.e., Δ ω = Π u σ u with Π u being the piezo-spectroscopic coefficient. The linear relationship is used to determine the in-plane residual stress in air plasma-sprayed 8YSZ thermal barrier coatings (TBCs). It is shown that the Π u of the 8YSZ is an exponential function of the thermal cycle N for a given thermal cycling process: Π u = 37.6exp(− N /25.9) + 13. Based on these two relationships, the actual residual stress of 8YSZ TBCs determined by the Raman spectroscopy method is quantitatively consistent with that obtained by the X-ray diffraction.

Hot-corrosion behaviors of overlay-clad yttria-stabilized zirconia coatings in contact with vanadate–sulfate salts

A dense clad overlay with chemical inertness was achieved on top of the plasma-sprayed YSZ thermal barrier coatings by laser in order to protect them from hot-corrosion attack. The Al 2O 3-clad YSZ coating exhibited good hot-corrosion behavior in contact with salt mixture of vanadium pentoxide (V 2O 5) and sodium sulfate (Na 2SO 4) for a longtime of 100 h at 1173 K. The LaPO 4-clad YSZ coating showed corrosion resistance inferior to the Al 2O 3-clad one. Yttria was leached from YSZ by reaction between Y 2O 3 and V 2O 5, which caused progressive destabilization transformation of YSZ from tetragonal ( t) to monoclinic ( m) phase. The chemical inertness of the clad layers and the restrained infiltration of the molten corrosive salts by the dense clad layers were primary contributions to improvement of the hot-corrosion resistances.

Determination of Scattering and Absorption Coefficients for Plasma‐Sprayed Yttria‐Stabilized Zirconia Thermal Barrier Coatings at Elevated Temperatures

The temperature dependence of the scattering and absorption coefficients for a set of freestanding plasma‐sprayed 8 wt% yttria‐stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) was determined at temperatures up to 1360°C in a wavelength range from 1.2 μm up to the 8YSZ absorption edge. The scattering and absorption coefficients were determined by fitting the directional‐hemispherical reflectance and transmittance values calculated by a four‐flux Kubelka–Munk method to the experimentally measured hemispherical‐directional reflectance and transmittance values obtained for five 8YSZ thicknesses. The scattering coefficient exhibited a continuous decrease with increasing wavelength and showed no significant temperature dependence. The scattering is primarily attributed to the relatively temperature‐insensitive refractive index mismatch between the 8YSZ and its internal voids. The absorption coefficient was very low (<1 cm−1) at wavelengths between 2 μm and the absorption edge and showed a definite temperature dependence that consisted of a shift of the absorption edge to shorter wavelengths and an increase in the weak absorption below the absorption edge with increasing temperature. The shift in the absorption edge with temperature is attributed to strongly temperature‐dependent multiphonon absorption. While TBC hemispherical transmittance beyond the absorption edge can be predicted by a simple exponential decrease with thickness, below the absorption edge, typical TBC thicknesses are well below the thickness range where a simple exponential decrease in hemispherical transmittance with TBC thickness is expected. [Correction added after online publication August 11, 2009: “edge to a shorter wavelengths” has been updated as “edge to shorter wavelengths.”]

Prediction of Thermal Conductivity of Composite Materials

In this work, we developed a Web system for designing constitution and structure of a composite and predict its thermal conductivity. Two simulation methods, analytical method and finite element method, are available fitting for different requirements on computational efficiency and accuracy. This system has been used to predict the thermal conductivity of composites such as Mo/Al2O3, SiC/Al alloy, YSZ thermal barrier coatings, etc.. The results are in good agreement with the experimental data, so the reliability and effectiveness of the system has been proved.

Effect of Microstructure on Elastic Modulus of YSZ Thermal Barrier Coatings

Effect of Microstructure on Elastic Modulus of YSZ Thermal Barrier Coatings

Effect of solution concentration on splat formation and coating microstructure using the solution precursor plasma spray process

The Solution Precursor Plasma Spray (SPPS) process had been successfully used to deposit 7YSZ thermal barrier coatings. In this research, the effects of solution precursor concentration on 7YSZ splat formation and coating microstructure are studied. With increasing solution concentration, solution viscosity increases and surface tension decreases. Solution concentration has no effect on precursor pyrolysis and crystallization temperatures. The average atomized droplets size of ∼ 35 μm is the same for low and high concentration solutions. By contrast, splat formation is greatly dependent on precursor concentration. Low concentration precursors experience surface precipitation and lead to shell formation. The deposits consist of semi-pyrolyzed material and result in a soft, porous coating. When the substrate temperature is raised to 450 °C, spongy deposits are formed. High concentration solutions are beneficial for volume precipitation within droplets. Solid particles are formed, melted and form splats on contact with the substrate. The build-up of dense splats provides a dense coating.

Nanostructured YSZ thermal barrier coatings engineered to counteract sintering effects

Thermal spray zirconia–8 wt% yttria (YSZ) deposits have been employed as thermal barrier coatings (TBCs) in the hot sections of gas turbines. The use of nanostructured YSZ represents an alternative for improving the performance of these coatings. Despite some initial positive research results, there are still fundamental questions to be answered on the applicability of nanostructured YSZ coatings as TBCs. These questions are related to sintering effects, which could significantly increase the thermal diffusivity/conductivity and elastic modulus values of these types of coatings in high temperature environments. In this study, nanostructured and conventional YSZ coatings were heat-treated at 1400 °C for 1, 5 and 20 h. It was observed that the nanostructured coatings counteract sintering effects, due to the presence of a bimodal microstructure exhibiting regions with different sintering rates: (i) matrix (low rate) and (ii) nanozones (high rate). Important sintering-affected properties, like thermal diffusivity and elastic modulus were studied. The thermal diffusivity and elastic modulus values of the nanostructured YSZ coatings were significantly lower than those of conventional YSZ coatings, even after an exposure to a temperature of 1400 °C for 20 h. This study demonstrates that nanostructured YSZ coatings can be engineered to counteract sintering effects and exhibit significantly lower increases in thermal diffusivity and elastic modulus values in high temperature environments when compared to those of conventional YSZ coatings.

Hertzian and Nanoindentations on EBPVD-Coated Gadolinium Zirconate Thermal Barrier Materials

New gadolinium-yttrium zirconate thermal barrier coating(TBC) material is deposited by electron beam PVD method, as an alternative to YSZ TBC layer for gas turbine blade applications. XRD analysis reveals that the new TBC material consists of thermally stable pyrochlore structure. Hertzian and nanoindentation evaluations reveal that gadolinium zirconate materials show superior properties as a TBC candidate material with high mechanical properties. The Y2O3 doping improved hardness and elastic modulus of TBC layers. The indentation stress-strain curves by Hertzian indentation and the load-penetration depth curves by nanoindentation indicate that the new TBC layer has higher damage resistance combined with superior thermal insulation properties rather than commercial YSZ coatings.

Microstructure Evolution of Nd<sub>2</sub>O<sub>3</sub> and Yb<sub>2</sub>O<sub>3</sub> Co-Doped YSZ Thermal Barrier Coatings during High Temperature Exposure

10mol% Nd2O3 and Yb2O3 co-doped YSZ thermal barrier coatings were produced by electron beam physical vapor deposition (EB-PVD). Compared to the traditional YSZ coating, the deposited coating has shown tree-like microstructure in each column. Due to this, the co-doped coating is more porous than the YSZ coating. The microstructure evolution of the coating during high temperature exposure at 1373 K was studied. The tree-like microstructure disappeared due to joining of sub-grains during sintering. Thermal growth oxide (TGO) grew quickly at the first few hours and then the growth of TGO became slow in the subsequent high temperature exposure. Cracks generated and propagated in the ceramic top coat and along the interface of the top coat and TGO layer. Finally, the coalescence of such cracks resulted in failure of the TBCs.

Low thermal conductivity thermal barrier coating deposited by the solution plasma spray process

This work seeks to develop an innovative thermal barrier coating (TBC) that will exhibit low thermal conductivity and high durability compared with that of current TBCs. To achieve this objective, a multiple component co-doped zirconia chemistry was selected for the topcoat of the TBC system, and a new process – Solution Plasma Spray (SPS or SPPS) – was employed to produce desirable microstructural features: the co-doped zirconia TBC contains ultrafine splats, high volume porosity and vertical cracks, for lower thermal conductivity and better durability, respectively. Test results verified a low thermal conductivity of 0.55–0.66 W/K·m over a temperature range of R.T. to 1300 °C, 1/3X that of EB-PVD 8YSZ TBCs, and its thermal cycling durability was 2.5X that of APS 8YSZ TBCs and 1.5X that of EB-PVD 8YSZ TBCs.

Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings

The use of the luminescence from europium doped yttria-stabilized zirconia thermal barrier coatings for non-contact thermometry has been previously studied up to 850 °C [Choy, K.L. et al., Surface Engineering, 16 [6] (2000) 496] and 1150 °C [M.M. Gentleman and D.R. Clarke, Surface and Coatings Technology, 188–189 (2004) 93]. In this contribution we examine the effect of long-term, high temperature aging and martensitic phase transformation on both the details of the luminescence spectrum and the temperature-dependent lifetime of the luminescence. It is found that even after prolonged aging, the wavelength of the 5D 0 → 7F 2 emission peak shifts only very slightly with increasing percentage of transformation to the monoclinic phase. Preliminary data suggests that the effect on the decay lifetime up to 1150 °C is very slight, expressed in a slight shift of the onset of thermal quenching to lower temperatures; the slope of the characteristic decay time-constant versus temperature appears relatively unaffected.

Environmental Effect of Hot Corrosion on Creep and Fatigue Failure of Thermal Barrier Coating Systems

In order to clarify the failure behavior of plasma sprayed thermal barrier coating (TBC) systems under the complicated modes of thermal-mechanical-chemical loadings, the stress rupture property evaluation and failure analysis were conducted for Y2O3-ZrO2 (YSZ) and CaO-SiO2-ZrO2 (C2S-CZ) TBC systems in air and two kinds of high-temperature corrosive environments. Static creep loading was found to bring about the typical creep failure for TBC systems even in the aggressive environment so called hot corrosion almost in similar manner to the case in air. On the contrary, it was revealed that the dynamic fatigue loading tends to cause a significant failure life reduction of TBC systems both in air and in corrosive environments. For YSZ TBC system, the penetration crack preexisting through the top-coat layer tends to provide a nucleation site for the fatigue crack even in air, and more significantly a short circuit path for the corrosive species in hot corrosive environment. For C2S-CZ system, on the contrary, the top-coat / bond-coat interface tends to provide easily the nucleation site for a main crack to propagate thereafter toward both the alloy interior and outer surface. Under lower stress level at 950°C, however, the oxide-induced crack closure together with crack tip blunting attributed mainly to the high reactivity of Ca compounds as a major constituent of the TC is effective to suppress substantially the crack propagation, so as to cause the prolonged failure life as compared to YSZ system even in aggressive gaseous environment.

Tailored microstructure of EB-PVD 8YSZ thermal barrier coatings with low thermal conductivity and high thermal reflectivity for turbine applications

This paper discusses microstructural changes produced by two novel approaches using electron beam-physical vapor deposition (EB-PVD) in which periodic strain fields/microporosity was incorporated within the large columnar grains of ZrO 2–8 wt.% Y 2O 3 (8YSZ). The traditional columnar microstructure of partially stabilized zirconia has been slightly modified to produce a lower thermal conductive thermal barrier coating (TBC) by periodically interrupting the condensing vapor resulting in microstructural modifications with diffuse or sharp interfaces and morphological changes on the submicron scale without changing the composition of the TBC. These microstructural modifications resulted in a 20–30% reduction in the thermal conductivity, 28–56% increase in hemispherical reflectance, improved oxidation cyclic life (over 100%), and better strain tolerance as compared to standard ZrO 2–8 wt.% Y 2O 3 deposited on platinum–nickel–aluminide and CoNiCrAlY bond-coated MAR-M-247 test samples. The TBC with tailored microstructures were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), steady-state laser heat flux technique, hemispherical reflectance and thermal cyclic oxidation tests.

Microscopic observation of degradation behavior in yttria and ceria stabilized zirconia thermal barrier coatings under hot corrosion

In this study, the effects of yttria stabilizer replacement with ceria on hot corrosion properties of thermal barrier coatings (TBCs) have been investigated in microscopic respects. Yttria stabilized zirconia (YSZ; ZrO 2–8 wt.%Y 2O 3) and ceria stabilized zirconia (CSZ; ZrO 2–25 wt.%CeO 2–2.5 wt.%Y 2O 3) thermal barrier coatings (TBCs) were fabricated to have similar microstructures, followed by hot corrosion tests with a NaVO 3 salt at 900 °C. Very dense YSZ TBCs with porosities below 4% were also fabricated in order to investigate the effects of coating defects on hot corrosion behavior. Results revealed that CSZ TBCs were better resistant to hot corrosion environment than YSZ TBCs in terms of phase stability and overall damage by the salt. From studies on microscopic failure behavior, it was found that YSZ and CSZ TBCs showed a different degradation behavior and that coating defects, such as pores, microcracks and splat boundaries, play important roles as effective paths for the salt in hot corrosion. From the present results, a possible microscopic degradation mechanism and effects of coating microstructures for zirconia-based TBCs under hot corrosion are discussed.