Grain-Boundary Grooving of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma-sprayed 7 wt% Y₂O₃–ZrO₂ thermal barrier coatings with annealing from 800° to 1400°C. Mullins's thermal grooving theories have been applied to plasma-sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain-boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000°C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100°C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200°C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat-treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.     

Transformation of Electron-Beam Physical Vapor-Deposited 8 wt% Yttria-Stabilized Zirconia Thermal Barrier Coatings

Yttria-stabilized (8.6 mol% YO_1.5) zirconia thermal barrier coatings evolve at high temperatures from the "non-transformable," metastable tetragonal-prime phase in their as-deposited condition to a mixture of the tetragonal and cubic phases. The kinetics of the transformation at 1200° and 1425°C are reported based on X-ray diffraction measurements. Complementary Raman spectroscopy measurements indicate a sharpening of the tetragonal bands at 263 and 465 cm⁻¹ that is attributed to a systematic decrease in disorder of the Y³⁺ and oxygen vacancies with annealing. No transformation to the monoclinic form of zirconia is observed immediately after high-temperature treatment. However, partial transformation to monoclinic occurs after a prolonged time (months) at room temperature in those samples treated at 1425°C, indicating the development of isothermal martensite.     

Determination of Scattering and Absorption Coefficients for Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

Prediction of radiative transport through translucent thermal barrier coatings (TBCs) can only be performed if the scattering and absorption coefficients and index of refraction of the TBC are known. To date, very limited information on these coefficients, which depend on both the coating composition and the microstructure, has been available for the very commonly utilized plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs. In this work, the scattering and absorption coefficients of freestanding plasma-sprayed 8YSZ coatings were determined from room-temperature normal-incidence directional-hemispherical reflectance and transmittance spectra over the wavelength range from 0.8 to 7.5 μm. Spectra were collected over a wide range of coating thickness from 60 to almost 900 μm. From the reflectance and transmittance spectra, the scattering and absorption coefficients as a function of wavelength were obtained by fitting the reflectance and transmittance values predicted by a four flux model to the experimentally measured values at all measured 8YSZ thicknesses. While the combined effects of absorption and scattering were shown in general to exhibit a nonexponential dependence of transmittance on specimen thickness, it was shown that for sufficiently high absorption and optical thickness, an exponential dependence becomes a good approximation. In addition, the implications of the wavelength dependence of the plasma-sprayed 8YSZ scattering and absorption coefficients on (1) obtaining accurate surface-temperature pyrometer measurements and on (2) applying mid-infrared reflectance to monitor TBC delamination are discussed.     

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."]     

The Measurement of Residual Strains Within Thermal Barrier Coatings Using High-Energy X-Ray Diffraction

The residual strains through the entire thickness of the zirconia layer of pristine and heat-treated thermal barrier coatings (TBCs) were mapped to help elucidate the failure mechanisms of TBCs. The strains were measured using 80.72 keV synchrotron radiation and a transmission geometry. The heat-treated TBC showed that a compressive strain formed in the zirconia layer of the TBC on cooling but this strain was diluted and reversed by the oxidation-driven expansion of the underlying metals. It also showed large (0.0024) out-of-plane tensile strains in the zirconia layer just above its interface with a thick underlying oxide layer.     

Measurement of Phase Abundance in Magnesia-Partially-Stabilized Zirconia by Rietveld Analysis of X-ray Diffraction Data

The relative abundance of the cubic ( c ), tetragonal ( t ), monoclinic ( m ), and orthorhombic ( o ) polymorphs of ZrO₂, and the δ phase, Mg₂Zr₅O₁₂, present in samples of 3.4-wt%-magnesia-partially-stabilized zirconia have been determined by Rietveld analysis of X-ray powder diffraction data. The samples studied correspond to the as-fired (AF), and subetectoid-aged maximum-strength (MS) and thermalshock (TS) states, with their surfaces in the ground or polished condition. The polymorph abundances of the bulk and near-surface regions are discussed in relation to the type of surface treatment. Grinding produces significant quantities of both m - and o -ZrO₂ in the near-surface regions of all samples. The m content increases from about 5 wt% in the bulk, to 10, 24, and 33 wt% in AF, MS, and TS material, respectively, while the o content increases from trace amounts to about 11 wt% in all samples. The m and o phases both increase at the expense of t -ZrO₂, and the transformation is accompanied by significant lattice distortion and/or crystal size reduction. Thus, measurement of only the 'ground-surface-monoclinic' content does not give an accurate indication of the total amount of transformable t -ZrO₂ in ceramics of this kind. Polishing removes some of the ground-surface m -ZrO₂ in MS and TS, and all of the m -ZrO₂ in AF material. The o -ZrO₂ produced by grinding also declines substantially in AF and MS, but is not removed by polishing of TS. As a result, the bulk composition cannot be guaranteed, in the general case, to be accessible by X-ray analysis of polished surfaces.     

Phase Formation and Stability in Reactively Sputter Deposited Yttria-Stabilized Zirconia Coatings

Yttria-stabilized zirconia (YSZ) coatings were produced by reactively cosputtering metallic zirconium and yttrium targets in an argon and oxygen plasma using a system with multiple magnetron sputtering sources. Coating crystal structure and phase stability, as functions of Y₂O₃ content, substrate bias, and annealing temperature, were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results demonstrated that highly (111)-oriented tetragonal and cubic zirconia structures were formed in 2 and 4.5 mol% Y₂O₃ coatings, respectively, when the coatings were grown with an applied substrate bias. Conversely, coatings deposited with no substrate bias had random tetragonal and cubic structures. XRD analysis of annealed coatings showed that the cubic zirconia in 4.5 mol% Y₂O₃ coatings exhibited structural stability at temperatures up to 1200°C. Transformation of the tetragonal to monoclinic phase occurred in 2 mol% Y₂O₃ coating during high-temperature annealing, with the fraction of transformation dependent on bias potential and annealing temperature.     

Phase Transformation in EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings during Annealing

Electron-beam physical-vapor-deposited thermal barrier coatings consisting of ZrO₂ stabilized by 7 wt% Y₂O₃ were investigated in regard to phase transformation after annealing. Free-standing ceramic layers were heat-treated in air, for up to 200 h, in the temperature range 1200°—1400°C and then analyzed by X-ray diffractometry. Based on information obtained from the {111} and {400} peaks, the phase composition and the Y₂O₃ content in the phases were calculated. At the start of transformation, small grains of a low-Y₂O₃ t phase and a c phase formed. After >30 h at 1300°C and at 1400°C, a mixture of a t phase deficient in Y₂O₃, an m phase, and a c phase formed after cooling, with the Y₂O₃ contents in the phases roughly predicted by the phase diagrams. The results of the present study are discussed here in detail and compared with data for plasma-sprayed coatings.     

Structural Evolution of Thermal-Sprayed Yttria-Stabilized ZrO2 Thermal Barrier Coatings with Annealing—A Neutron Diffraction Study

We studied the influence of the annealing temperature on the atomic structure of yttria-stabilized tetragonal zirconia (YSZ) which was deposited as a 300 μm thick thermal barrier coating (TBC) on nickel superalloy substrates by plasma spraying. To obtain neutron powder diffraction patterns of the barrier coatings we used an experimental technique where the sample is randomly rotated in the neutron beam. The time-averaged neutron diffraction pattern was then analyzed using the Rietveld refinement technique without any need for corrections. This allowed the comparison of the average crystals structures from bulk tetragonal samples obtained via common ceramic routes and those of micrometer thick films deposited on substrates using plasma spray or other nonequilibrium techniques.     

Creep Behavior of Plasma-Sprayed Zirconia Thermal Barrier Coatings

Thermally sprayed ceramic coatings deposited from nanostructured feedstock powder have often demonstrated improved properties relative to coatings produced from conventional powders. This type of coating has been reported to exhibit better wear resistance and higher adhesion strength compared with conventional deposits. Powder consisting of hollow spherical particles has been reported to produce coating with lower unmelted particles and lower thermal conductivity. In this study, the thermo-mechanical properties of plasma-sprayed yttria-stabilized zirconia coatings deposited using each of these types of powder were investigated. Creep strain and creep rate were measured using free-standing thick coatings loaded in a four-point bend configuration at temperatures ranging from 800° to 1200°C in air under a range of loads. The creep exponent and activation energy were determined.     

Chemical and Structural Changes in Manganese-Doped Yttria-Stabilized Zirconia Studied by Electron Energy Loss Spectroscopy Combined with Electron Diffraction

Solid solution of manganese in yttria-stabilized zirconia (YSZ) may occur in the electrolyte of solid oxide fuel cells. Possible changes in valence, coordination, and site occupancy of Mn in YSZ are of interest. Also, subsequent structural modification of the cubic YSZ, as well as the possible ordering of vacancies, has important consequences for the ionic conductivity. Electron energy loss spectroscopy was used to measure the O K and the Mn L edge of Mn in solid solution in a zirconia host lattice. The ratio Mn L ₃/ L ₂was determined for some manganese oxides and for Mn in solid solution. It is shown that the L ₃/ L ₂ratio does not simply reflect the oxidation state of the Mn ions in solid solution. Selected area diffraction experiments were also made in the TEM. This showed ordering of the cations and the anion vacancies at high doping levels. It is concluded that the number of O ions surrounding each Mn ion may be very important in interpreting the obtained L ₃/ L ₂ratio.     

Mechanism of Thermal Transport in Zirconia and Yttria-Stabilized Zirconia by Molecular-Dynamics Simulation

We present results of molecular-dynamics simulations of the thermal conductivity, κ, of ZrO₂ and Y₂O₃-stabilized ZrO₂ (YSZ). For both pure ZrO₂ and YSZ with low concentrations of Y₂O₃, we find that the high-temperature κ is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y₂O₃ concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also find that the few highest frequency modes are localized and do not contribute to κ.     

Damage Evolution and Stress Analysis in Zirconia Thermal Barrier Coatings during Cyclic and Isothermal Oxidation

The failure mechanisms of air-plasma-sprayed ZrO₂ thermal barrier coatings with various microstructures were studied by microscopic techniques after thermal cycling. The elastic modulus ( E ) and hardness ( H ) of the coatings were measured as functions of the number of thermal cycles. Initially, both E and H increased by ∼60% with thermal cycling because of sintering effects. However, after ∼80 cycles (0.5 h at 980°C), the accumulated damage in the coatings led to a significant decrease of ∼20% of the maximum value in both E and H . These results were correlated with stresses measured by a spectroscopic technique to understand specific damage mechanisms. Stress measurement and analysis revealed that the stress distribution in the scale was a complex function of local interface geometry and damage in the top coat. Localized variations in geometry could lead to variations in measured hydrostatic stresses from −0.25 to −2.0 GPa in the oxide scale. Protrusions of the top ZrO₂ coat into the bond coat were localized areas of high stress concentration and acted as damage-nucleation sites during thermal and mechanical cycling. The net compressive hydrostatic stress in the oxide scale increased significantly as the scale spalled during thermal cycling.     

Topographical and Microstructural Property Evolution of Air Plasma‐Sprayed Zirconia Thermal Barrier Coatings

The effects of process parameters on thermal barrier coating (TBC) formation and microstructural properties have been studied. Further understanding of the evolution of properties such as porosity and hardness is an important aspect in the design of efficient TBCs. Plasma‐sprayed yttria‐stabilized zirconia was coated onto mild steel substrates. The torch was held perpendicular to the substrate to form cone‐shaped deposits. Standoff distance (SOD) (80, 90, and 120 mm) and time (15, 30, and 60 s) were altered to investigate the microstructural property relationships of the coatings. Shape characteristics of the coatings were measured via a coordinate measuring machine, and surface roughness measurements were acquired using a 3D optical profiler. The deposition efficiency and coating roughness were affected by SOD and the evolving contour of the underlying surface. Hardness and porosity profiles were mapped to display the effect of process parameters. Dynamic parameters such as particle trajectory, evolving impact angle and dwell time affected changes in porosity, hardness, and density for each coating profile.     

Thermal Conductivity of Plasma-Sprayed Monolithic and Multilayer Coatings of Alumina and Yttria-Stabilized Zirconia

The temperature dependence of the thermal conductivity of plasma-spray-deposited monolithic coatings, as well as multilayer coatings that consisted of Al₂O₃ and ZrO₂ that was stabilized by 8% Y₂O₃ (YSZ), was investigated. The coatings exhibited a large reduction in thermal conductivity at all temperatures, when compared to the bulk monolithic Al₂O₃ and YSZ. This reduction was due to porosity as well as thermal resistance that was caused by interfaces in the coatings. The largest decrease in the thermal conductivity of the coatings, relative to monolithic fully dense materials, was due to splat interfaces within each layer, as well as the coating/substrate interface. On the other hand, the multilayer coatings showed little variation in the thermal conductivity, relative to the number of layers, which suggests that the influence of interlayer interfaces on heat transfer is relatively small. A one-dimensional analysis of steady-state heat transfer has been presented to illustrate the significance of porosity, splat interfaces, and interlayer interfaces, with respect to the overall thermal conductivity of multilayer coatings.     

Influence of Tetragonality on Tetragonal-to-Monoclinic Phase Transformation during Hydrothermal Aging in Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The influence of tetragonality, which is defined as the lattice parameter ratio c / a , on the tetragonal-to-monoclinic phase transformation during hydrothermal aging was investigated in yttria-stabilized zirconia coatings. The yttria content was adjusted in the range of 4–8 mass% (denoted as x YZ, where x = 4–8 and YZ represents the yttria-stabilized zirconia). The tetragonality of the zirconia in the as-sprayed coatings was less than that in the powders. To change the tetragonality for each yttria content, the coatings were annealed at 1273 K before aging. Without annealing, the phase transformation was prevented only in 8YZ. When annealing was applied, an increase of the tetragonality (i.e., recovery of the tetragonality) was observed, and transformation during hydrothermal aging was also suppressed in 6YZ. It was concluded that the increase in tetragonality that occurred without a change in the yttria content was suggested to be caused by the lattice relaxation of the tetragonal phase, and this relaxation is believed to cause a reduction of the critical yttria concentration, thus preventing the phase transformation.     

Effect of Isothermal Heat Treatment on Plasma-Sprayed Yttria-Stabilized Zirconia Studied by Impedance Spectroscopy

Impedance spectroscopy (IS) was used to measure electrical responses of various plasma-sprayed yttria-stabilized zirconia (YSZ) materials. Coupled with X-ray diffraction, scanning electron microscopy, and microindentation, the effects of heat treatment on microstructure, phase composition, and mechanical properties were investigated. Results showed electrical responses for YSZ grains and grain boundaries varied with size and position of silver electrodes used. YSZ heat treated at 950°C showed micro-crack closure: at 1150°C, microstructural and phase changes, and at 1450°C, formation of a resistive monoclinic phase. The aim was to develop that IS as a non-destructive tool for thermal barrier coatings; results suggest IS can monitor microstructural and phase changes in YSZ under certain conditions.     

Long-Term Behavior and Application Limits of Plasma-Sprayed Zirconia Thermal Barrier Coatings

Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO₂-based plasma-sprayed thermal barrier coatings (TBCs) with 8 mol% CeO₂, 19.5 mol% CeO₂/1.5 mol% Y₂O₃, 35 mol% CeO₂, and 4.5 mol% Y₂O₃ after long-term heat treatments at typical operation temperatures (1000°–1400°C) are presented. Experimental studies include X-ray diffractometry, mechanical testing, and scanning electron microscopy. Thermal cycling experiments also have been performed. TBCs with 8 mol% CeO₂ contain mainly the tetragonal equilibrium phase and, therefore, show rapid failure because of the high amount of tetragonal → monoclinic phase transformation, even after relatively short heat treatments (1250°C/1 h). In the case of the other systems that consist mainly of the tetragonal or cubic nonequilibrium phases, TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ or 35 mol% CeO₂ reveal a smaller amount of monoclinic phase after long-term heat treatments (1250°C/1000 h) compared with TBCs containing 4.5 mol% Y₂O₃. TBCs containing 35 mol% CeO₂ show a higher degree of sintering than the TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ and, therefore, a greater increase of the elastic modulus. Among the systems investigated, TBCs containing 4.5 mol% Y₂O₃ exhibit the highest resistance to failure in thermal-cycling experiments.