Thermomechanical behavior and phase relationships of plasma-sprayed zirconia coatings (original) (raw)
Acta Materialia, 2008
The effect of porosity on the thermal diffusivity and elastic modulus has been studied on artificially aged, free-standing thermal barrier coatings (TBCs) produced by air plasma spray (APS). The activation energy of the sintering phenomenon was estimated from the variation in diffusivity with time and temperature. X-ray diffraction was used to evaluate the phase stability of 7 wt.% yttria partially stabilized zirconia (YPSZ) coatings. The thermal diffusivity and elastic modulus as measured by photothermal techniques and three-point bending, respectively, are reported as a function of the ageing time. Correlations between the thermal and mechanical parameters are investigated by suitable models based on the microstructural features revealed by electron microscopy. The reliability of porosity information provided by image analysis and used as input for the modelling is critically discussed.
Phase transformations in air plasma-sprayed yttria-stabilized zirconia thermal barrier coatings
DYNA, 2014
Phase transformations in air plasma-sprayed thermal barrier coatings composed of ZrO2-8 wt.% Y2O3 (zirconia-8 wt.% yttria) are studied using X-Ray diffraction and Rietveld refinement measurements. Samples of TBC deposited onto Inconel 625 substrate were fabricated and heat treated at two different conditions: exposition to 1100ºC up to 1000 hours and exposition to temperatures between 700ºC and 1100ºC during 50 hours. According to Rietveld refinement measurements, the content of the cubic phase in the top coat increases with time and temperature; it starts at 7.3 wt.% and reaches 15.7 wt.% after 1000 hours at 1100ºC. The presence of a cubic phase in high amounts is undesirable due its lower mechanical properties compared with the tetragonal phase. After 800 hours of exposure to high temperature, the amount of Y 2 O 3 in the tetragonal phase reduces to 6.6 wt.% and a fraction of this phase transforms to a monoclinic structure during cooling. The monoclinic phase reached 18.0 wt.% after 1000 hours. This phase is also undesirable, not only due to its higher thermal conductivity, but also because the tetragonal-to-monoclinic transformation implies a volume change of circa 5%, which favors crack formation and propagation and compromises the coating integrity.
Strain, 2010
Thermal barrier coatings (TBCs) are widely adopted to protect mechanical components in gas turbine engines operating at high temperature. Basically, the surface temperature of these components must be low enough to retain material properties within acceptable bounds and to extend component life. From this standpoint, air plasma-sprayed (APS) ceria and yttria co-stabilized zirconia (CYSZ) is particularly promising because it provides enhanced thermal insulation capabilities and resistance to hot corrosion. However, essential mechanical properties, such as hardness and Young's modulus, have been less thoroughly investigated. Knowledge of Young's modulus is of concern because it has a significant effect on strain tolerance and stress level and, hence, on durability. The focus of the present study was to determine the mechanical properties of APS CYSZ coatings. In particular, X-ray diffraction (XRD) is adopted for phase analysis of powders and as-sprayed coatings. In addition, scanning electron microscopy (SEM) and image analysis (IA) are employed to explore coating microstructure and porosity. Finally, the Young's modulus of the coating is determined using nanoindentation and a resonant method. The results obtained are then discussed and a cross-check on their consistency is carried out by resorting to a micromechanical model.
Ceramics International, 2010
Thick plasma sprayed thermal barrier coatings are suitable for thermal and hot corrosion protection of metal components in land-based turbine and diesel engines. In this work, ceria-yttria co-stabilized zirconia coatings were deposited by atmospheric plasma spraying in a mixture of nontransformable tetragonal t 0 and cubic c zirconia phases. Free-standing coatings were isothermally annealed at 1315 8C for different times and their crystal structure was studied by XRD. No phase decomposition occurred. Columnar grains grew in the molten splats with increasing annealing time according to a preferential direction and, after 50 h of heat treatment, they were partially replaced by equiaxed grains. Both in-plane and out-ofplane thermal expansion coefficients (CTEs) were measured from coating expansion during heating. The CTE was slightly sensitive to thermal exposure in out-of-plane direction, whereas it kept almost constant in plane direction. The specific heat capacity Cp of annealed coatings, measured by differential scanning calorimetry (DSC), decreased in comparison with as-sprayed coating, due to high-temperature sintering.
Effect of dopants on the phase stability of zirconia-based plasma sprayed thermal barrier coatings
Journal of the European Ceramic Society, 2010
The influence of stabilizer type on the phase stability of thermal barrier coatings (TBCs) produced by air plasma spraying was explored. Together with the widely used zirconia-stabilized with yttria, other novel compositions, such as dysprosia-stabilized zirconia, yttria-lanthana-stabilized zirconia and ceria-stabilized zirconia were also investigated. The effect of isothermal heat treatment on the phase stability was explored. Results suggest that decomposition of the "non-transformable" tetragonal phase occurs to a greater or lesser extent for all dopants at these temperatures. The effect of Al 2 O 3 and SiO 2 content was also explored. The rate of decomposition depends on the dopant kind, amount and on the presence of Al 2 O 3 and SiO 2 impurities.
Phase Evolution upon Aging of Air Plasma Sprayed t′-Zirconia Coatings: II-Microstructure Evolution
Journal of the American Ceramic Society, 2013
The correlation between microstructural and phase evolution in aged, yttria-partially-stabilized zirconia, air plasma-sprayed coatings is discussed. Freestanding coatings with the dense, vertically cracked structure were isothermally aged at 1482°C (2700°F) in air. Characterization of the resulting microstructures was conducted using transmission electron microscopy, then compared with a parallel analysis of the phase evolution via synchrotron X-ray diffraction (XRD) described in Part I. Additional context was provided by related studies on vapor-deposited coatings. Several salient points can be extracted from these assessments. XRD was further validated as a practical method for studying phase stability after clarification of how the possible phases are defined, including the following: (i) the nature of the t′ phase observed in XRD after phase decomposition has begun and (ii) the relationship between the Y-rich tetragonal (t″) and Y-rich cubic (c) phases reported to coexist via XRD. A strong relationship between the initial microstructure and the subsequent phase destabilization is also reported. As a result, phase evolution is proposed to proceed via two competing routes. The interplay between these mechanisms dictates the incubation time for monoclinic formation within a given coating. †
Journal of Thermal Spray Technology, 2013
Plasma generated by the SG-100 torch was applied to spray suspension formulated with the use of ZrO 2 + 8 wt.% Y 2 O 3 (8YSZ) and ZrO 2 + 24 wt.% CeO 2 + 2.5 wt.% Y 2 O 3 (24CeYSZ) as solid phases. The suspensions were formulated with the use of 20 wt.% solid phase, 40 wt.% water, and 40 wt.% ethanol. The plasma spray parameters were optimized by keeping constant: (a) the electric power of 40 kW and (b) the working gas compositions of 45 slpm for Ar and 5 slpm for H 2 . On the other hand, the spray distance was varied from 40 to 60 mm and the torch linear speed was varied from 300 to 500 mm/s. The coatings were sprayed onto stainless steel substrates, and their thicknesses were in the range from 70 to 110 lm. The coating microstructures were analyzed with a scanning electron microscope. Mechanical properties were tested with the different methods including the indentation and scratch tests. The indentation test, carried out with various loads ranging from 100 to 10,000 mN, enabled to determine elastic modulus and Martens microhardness. YoungÕs modulus of the coatings was in the range of 71-107 GPa for 8YSZ and 68-130 GPa for 24CeYSZ coatings. The scratch test enabled the authors to find the scratch macrohardness.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2000
Yttria (8 wt.%) stabilized zirconia (YSZ) with a NiCrAlY bond coat was atmospherically plasma sprayed on mild steel substrates using various processing parameters including YSZ coating thickness, bond coat thickness, stand off distance, and substrate temperature. The cracking behavior of these coatings under four point bending load was examined using an acoustic emission (AE) recorder. The numbers of AE events exhibited during the elastic and plastic deformation of coatings were analyzed. Using multi-linear regression analysis, the number of AE events was correlated to the spray parameters. This analysis revealed that coatings with thicker YSZ top coat and NiCrYAl bond coat sprayed on a heated substrate at shorter stand off distance exhibited more AE activity and released higher AE energy under the bending. The greater emission activity and higher AE energy were evidence of severe cracking. 0 2000 Elsevier Science S.A. All rights reserved.
Phase Transformations of Plasma-Sprayed Zirconia-Ceria Thermal Barrier Coatings
Journal of the American Ceramic Society, 2002
Phase constituents and transformations of plasma-sprayed thermal barrier coatings (TBCs) with CeO 2 -stabilized ZrO 2 (CSZ; 16 -26 wt% CeO 2 ) have been investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The as-coated CSZ coatings with 16 and 18 wt% CeO 2 consisted only of the nonequilibrium tetragonal (t) phase. A mixture of the t and the nonequilibrium cubic (c) phases was observed for the as-coated CSZ coatings containing 20 -26 wt% CeO 2 . During 65 min cyclic oxidation at 1135°C (45 min hold time) in air, the t or the mixture of the t and the c phases decomposed to the equilibrium tetragonal (t) and the equilibrium cubic (c) phases. Some of the t phase transformed to the monoclinic (m) phase on cooling. More m phase was observed to develop in the CSZ coating containing 16 wt% CeO 2 than in the other coatings. More m phase was observed on the top surface than on the bottom surface of the CSZ coating. Spalling of the plasmasprayed CSZ coating during thermal cycling occurred after 230 cycles for the CSZ coating containing 16 wt% CeO 2 , whereas the lifetime of the CSZ coatings with 18 -26 wt% CeO 2 ranged between 320 and 340 cycles.
Strain gradients in plasma-sprayed zirconia thermal barrier coatings
Surface & Coatings Technology, 1998
Neutron diffraction was used to measure the residual strain field in plasma-sprayed zirconia thermal barrier coatings (TBCs). Data were collected at the British neutron spallation source of ISIS (Didcot), on ENGIN, a recently installed TOF (time-of-flight) instrument designed for residual strain depth profiling. Its particular geometry permitted a direct measurement of the interplanar distances of crystallographic planes lying parallel to the component surface as well as the measurement of zero-strain reference samples. The latter were annealed samples of the three present phases: zirconia (top coat), NiCoCrAlY (bondcoat), and copper (substrate). In this way ϵ33, the strain component perpendicular to the sample surface, was determined at several positions inside the component, for all the present phases. The results of this analysis, consisting of a strain profile throughout the entire cross-section of the coated component, were integrated by those obtained by a destructive testing, performed after TOF data collection, consisting in the measurement of curvature change of the ceramic after substrate removal by chemical attack.
Ceramics International, 2011
Nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by Atmospheric Plasma Spraying (APS). X-ray diffraction (XRD) was used to investigate their phase composition, while scanning electron microscopy (SEM) was employed to examine their microstructure. The coatings showed a unique and complex microstructure composed of well-melted splats with columnar crystal structure, partially melted areas, which resembled the morphology of the powder feedstock, and equiaxed grains. Vickers microhardness of nanostructured zirconia coatings was similar to that of the conventional ones and strongly depended on the indentation load. Otherwise, a higher thermal shock resistance was found. This effect was addressed to the retention of nanostructured areas in coating microstructure and to the corresponding high porosity. #
Acta Materialia, 2009
The sintering model described in Part I, which relates to free-standing plasma-sprayed thermal barrier coatings, is extended here to the case of a coating attached to a rigid substrate. Through-thickness shrinkage measurements have been carried out for coatings attached to zirconia substrates, and these experimental data are compared with model predictions. The model is then used to explore the influence of the substrate material (zirconia vs. a nickel superalloy), and of the in-plane coating stiffness. Both differential thermal expansion stresses and tensile stresses arising from the constraint imposed on in-plane shrinkage can be relaxed via two diffusional mechanisms: Coble creep and microcrack opening. This relaxation allows progression towards densification, although the process is somewhat inhibited, compared with the case of a free-standing coating. Comparison of the stored elastic strain energy with the critical strain energy release rate for interfacial cracking allows estimates to be made of whether debonding is energetically favoured. 1
Acta Materialia, 2003
Quantitative microstructure characterization to better understand processing-microstructure-property correlations is of considerable interest in plasma sprayed coating research. This paper quantifies, by means of small-angle neutron scattering (SANS) data, microstructure (porosity, opening dimensions, orientation and morphologies) in plasma sprayed partially-stabilized zirconia (PSZ) coatings, primarily used as thermal barrier coatings. We report on the investigation of the influence of feedstock characteristics on microstructure and establish its influence on the resultant thermal and mechanical properties. The microstructural parameters determined by SANS studies are then assembled into a preliminary model to develop a predictive capability for estimating the properties of these coatings. Thermal conductivity and elastic modulus were predicted using finite element analysis and ultimately compared to experimental values.
High-temperature mechanical behavior of plasma sprayed lanthanum zirconate coatings
Ceramics International, 2014
Rare-earth zirconates are potential materials for thermal barrier coatings. Their properties are not still well known, due to any lacks in processing and characterization techniques. To this purpose lanthanum zirconate coatings were herein manufactured by plasma spraying. The coatings exhibited high porosity, due to the presence of pores, splat boundaries and microcracks. The high-temperature evolution of mechanical properties was investigated by arranging specific bending tests up to 1500 1C using SiC testing assembly which allows to reconstruct and measure the sample curvature and the deformation. At higher temperatures the coatings showed an inelastic behavior, related to their unique microstructure, as well as a toughening effect.
JOURNAL OF FACULTY OF ENGINEERING & TECHNOLOGY, 2015
Wear and friction behaviour of yttria stabilised zirconia coatings are very sensitive to the structure of the material and test parameters such as temperature, applied load, sliding speed, and environment. The present study describes the friction, and sliding wear behaviours of plasma sprayed yttria stabilized zirconia coating (ZrO 2 -8wt.%Y 2 O 3 (YSZ) deposited on a stainless steel substrate with NiAl bond coat. Tribological properties of the coating were assessed under lubrication condition at loads of 4N and 8N. The frictional behaviour of coating was assessed at a constant temperature of 50°C, while wear characteristics of the coating were investigated at 50°C and 100°C. The experimental results of this study showed a slight decrease in frictional coefficient with increasing load. However, the coating wear rate was slightly increased with increasing load and temperature. The coating wear mainly involved materials transferred from the counter body and pulling-out from the coatin...
Journal of the European Ceramic Society, 2007
Widely used in turbines for propulsion and power generation, thermal barrier coatings (TBCs) increase the efficiency of turbine engines by allowing them to work at higher temperatures, due to their thermal insulating properties. Typically TBC systems consist of a metallic bondcoat (BC) and a ceramic topcoat (TC). Previous research has revealed that ceramic TCs possess an amplitude-dependent mechanical behaviour and that they can be used as damping treatments, due to their good damping properties. The microstructure and the properties of ceramic TCs vary significantly depending on the employed deposition technique. This work investigates the differences in the mechanical behaviour of yttria-stabilised zirconia (YSZ with 8 wt% yttria) TC deposited by atmospheric plasma spraying (APS) and electron beam-physical vapour deposition (EB-PVD), by means of tests run with the amplitude dependent damping (ADD) test rig and of scanning electron microscopy (SEM) analysis.
Materials Research, 2004
The increase of the petroleum cost in the last decades revitalized the interest for lighter and more economic vehicles. Simultaneously, the demand for safe and unpolluted transports grows. The application of thermal barriers coatings (TBC) on combustion chamber and on flat surface of pistons reduces the thermal losses of the engines, resulting in higher temperatures in the combustion chamber. This fact contributes to the improvement of the thermal efficiency (performance) and for the reduction of incomplete combustion. Supported on these initial ideas, thermal barriers coatings constituted by CaO partially stabilized zirconia were produced and their microstructure examined. This coating still presents some drawbacks associated with thermal stresses and permeability to oxidizing gases, which will, eventually, lead to failure of the TBC by spallation. The failure may, in general, be associated to one of three factors: oxide growth at the ceramic-metal interface, formed during thermal cycling; stress build-up due to thermal cycling; and metal-oxide interface segregation, mainly of S. However, it is also relevant to understand the behavior of TBC's under isothermal oxidation. Therefore, this paper investigates the effect of oxidation on the adherence of thermal sprayed coatings. The adherence was measured by linear scratching tests, widely used for thin coatings. Plasma sprayed calcia partially stabilized zirconia was used as TBC and Ni-5%Al as bond coat, with Al substrates. Coated samples were submitted to heat treatments at 500 °C, for 50 h. The microstructures were examined by optical light microscopy, X-ray diffraction, profilometry and SEM.
Scientific Reports, 2020
The phase stability and microstructure of ZrO2–5CaO and ZrO2–24MgO mixed coating (wt%) by air plasma spraying on 304 stainless steel substrates were investigated. A Ni–5Al (wt%) metallic bond coating was firstly sprayed between the substrate and the ceramic top layer. The results were compared with the individual coatings of ZrO2–5CaO and ZrO2–24MgO for a better understanding of the correlation between their microstructures and mechanical properties. Mixed zirconia coating was found to have a mixture of cubic and tetragonal phases that stabilized under different plasma spray conditions. Microscopic observations and elemental composition analysis of as-sprayed mixed coating showed that modified ceramic-matrix grains had been formed. Microsized ZrO2–5CaO particles were embedded in the matrix grain creating an intragranular microstructure. Results indicated that ceramic-matrix grains provided a diffusion barrier for the growth of oxides induced stress near and onto the bond layer that ...
Chemical aspects of plasma spraying of zirconia-based thermal barrier coatings
Acta Materialia, 2008
Zirconia-based thermal barrier coatings (TBCs) of nominal chemical composition 8 wt.% Y 2 O 3 -ZrO 2 and 25.5 wt.% CeO 2 -2.5 Y 2 O 3 -ZrO 2 were prepared by atmospheric plasma spray and low-pressure plasma spray by selecting different deposition parameters. The surface chemical composition has been investigated by X-ray photoelectron spectroscopy in order to study the variation of surface chemical composition induced by the plasma-spraying process as a function of deposition parameters. The results reveal the occurrence of chemical-physical reactions such as stabilizing oxide depletion and enrichment, reduction to lower valence states, impurity segregation phenomena and the formation of new species. The chemical information was confirmed by differential thermal analysis measurements, which indicates that chemical aspects in plasma spraying are relevant and should be considered in designing reliable TBCs for maximum performance in aerospace applications.