Thermomechanical behavior and phase relationships of plasma-sprayed zirconia coatings (original) (raw)
Related papers
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.
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.