Plasma-sprayed Yttria-stabilized Zirconia Coatings Research Articles

Characterization of air plasma-sprayed yttria-stabilized zirconia coatings deposited with nitrogen

Air plasma-sprayed (APS) yttria-stabilized zirconia (YSZ) conventional and nanostructured coatings were deposited with nitrogen as the primary plasma gas. Nanostructured coatings were deposited as a function of critical plasma spray parameter (CPSP). Microstructural characteristics, e.g., phase distribution, porosity, microhardness, percentage of partially melted (PM) regions, surface roughness, and wear performance, were evaluated for deposited coatings. Effect of CPSP on wear performance of coatings was most prominent at highest normal load considered. Conventional and nanostructured coatings deposited with nitrogen showed different trends for specific wear rate as a function of normal load. While specific wear rate for conventional coatings was nearly constant, it increased with increasing load for nanostructured coatings. Thus, even though nanostructured coatings provided better wear resistance at lower normal loads, conventional YSZ coatings become more wear resistant at higher loads. For example, at highest normal load considered, specific wear rate of conventional coatings was around 12 % smaller than that for corresponding nanostructured coatings.

Propagation feature of cracks in plasma-sprayed YSZ coatings under gradient thermal cycling

The cracking behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) during thermal gradient cycling test was investigated. The relationship between the size of the bright spot observed on YSZ coating surface and the size of the crack in thermal barrier coatings (TBCs) was examined by introducing pre-cracks in TBCs assisted with finite element analysis (FEA). The cracking behavior during thermal cycling, and the relationship between the residual lifetime of TBCs and the crack size were examined. Results show that the bright spot observed on YSZ surface corresponds to the crack in the coating and thus the size of bright spot increases with crack size. It was found that the crack size increases linearly with thermal cycle number when the relative thermal cycle number is less than 85% of thermal cyclic lifetime. The crack propagation acceleration at the later stage of lifetime is attributed to coalescence of numerous cracks. The obvious coalescence of cracks in TBCs begins to take place when the residual lifetime becomes approximately 15%, which leads to a rapid decrease of the residual lifetime of YSZ coating with the increase of thermal cycle number.

Effect of Dimensions of Non-bonded Lamellar Interfaces on the Stress Distribution in APS-TBCs

Cracking behavior has been considered for the top coating in the thermal barrier coating system. The effect of the dimension and position of non-bonded lamellar interfaces on the stress distribution is investigated by the simulation method. Plasma-sprayed yttria-stabilized zirconia coatings are deposited at different spraying distances. The crack propagation behavior is studied by statistical analysis after the coatings have experienced different thermal cycles. The simulation results reveal that the normal stresses at the tips of non-bonded lamellar interfaces increase with the increment of the length of interfaces and with the decrease of the distance between them. The stress level almost stays the same with the changing of lamellar thickness. Experimental results indicate the correctness of the simulation. It is found that the length of non-bonded lamellar interfaces in the as-deposited coatings shows an increasing tendency with the spraying distance. The total crack length of coatings increases with the number of thermal cycles and the spraying distance after thermal cycling tests.

Abrasive Wear Analysis of Plasma-Sprayed LaCeYSZ Nanocomposite Coatings Using Experimental Design and ANN

The abrasive wear characteristics of plasma-sprayed nanostructured yttria-stabilized zirconia (YSZ) coatings on Inconel 718 substrates was evaluated using AFS 50/70-grade silica sand as abrasives. This article depicts the dependence of abrasive wear characteristics of plasma-sprayed nanocomposite LaCeYSZ coatings on abrading distance, keeping the applied load constant. The influence of four operating parameters—that is, load, wheel speed, time, and temperature with four different levels each—on the performance output (i.e., abrasion wear rate) is studied using Taguchi's L16 orthogonal array design and analysis of variance (ANOVA). Out of the four parameters, load has been found to be most significant factor followed, by speed of the abrasive wheel and temperature influencing abrasion. The morphology of the worn-out surface also showed microcutting and small crater formation in the binder matrix caused by the repetitive impacts of abrasive particles. It was observed that coating with nano-LaCeYSZ grains exhibited higher wear resistance compared to conventional YSZ coating and the reason may be attributed to embedded crack-arresting nanozones, which toughen the coating. An artificial neural network (ANN) approach is then implemented taking into account training and test procedures to predict the triboperformance under different operating conditions. This technique helps in saving time and resources for a large number of experimental trials and successfully predicts the wear rate of the coatings both within and beyond the experimental domain.

Comparison of different models for the determination of the absorption and scattering coefficients of thermal barrier coatings

The thermal radiative properties of thermal barrier coatings (TBCs) are becoming more important as the inlet temperatures of advanced gas-turbine engines are continuously being pushed higher in order to improve efficiency. To determine the absorption and scattering coefficients of TBCs, four-flux, two-flux and Kubelka–Munk models were introduced and used to characterize the thermal radiative properties of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings. The results show that the absorption coefficient of YSZ is extremely low for wavelengths <6μm and the scattering coefficient is high and decreases with increasing wavelength. The obvious deviation of absorption and scattering coefficients obtained by the Kubelka–Munk model from those values calculated by four-flux and two-flux models indicates that surface reflection is an important parameter which cannot be neglected when characterizing the radiative property of the coating. The excellent agreement of predicted reflectance and transmittance spectra by the two-flux and four-flux models for coating thicknesses >200μm suggests that when the coating thickness is larger than around twice the average scattering distance, the collimated flux can be simply treated as a diffuse flux inside the coating, and thus the two-flux model can be used to determine the absorption and scattering coefficients as a simplification of the four-flux model.

Developing empirical relationships to estimate porosity and microhardness of plasma-sprayed YSZ coatings

Plasma-sprayed yttria-stabilized zirconia (YSZ) coating has been considered to be a good protective coating material for high-temperature applications on account of its superior properties and life cycle costs. However, thermal barrier coatings (TBCs) have engineering reliability problems in tailoring the microstructure and mechanical properties towards achieving both prime reliance and manufacturing reproducibility. In this work, empirical relationships were developed to estimate TBCs performance characteristics (porosity and microhardness) by incorporating independently controllable atmospheric plasma spray operational parameters (input power, standoff distance and powder feed rate) using the response surface methodology (RSM). A central composite rotatable design with three factors and five levels was chosen to minimize the number of experimental conditions. Within the scope of the design space, the input power and the standoff distance appeared to be the most significant two parameters affecting the coating quality characteristics among the three investigated process parameters. Further, correlating the spray parameters with coating properties enables the identification of characteristics regime to achieve desired quality of YSZ coatings.

Relationship Between the Microstructure and Thermal Conductivity of Plasma-Sprayed ZrO2 Coatings

Plasma-sprayed yttria-stabilized zirconia coatings have a complex microstructure consisting of a variety of pores and cracks. These microstructure features which are determined by the spray process are known to influence the thermal conductivity of coatings. In this article, the microstructure features such as total porosity, large pores, and small pores were quantified by means of scanning electron microscopy (SEM) and image analysis, and for each spray process, the particle velocity and particle temperature were measured prior to impact onto the substrate using the online monitoring system (Spray Watch 2i). Multiple linear regression was used to find the relationship between the particle state and the spray gun parameters. The linear regression models were also investigated between the particle state and the microstructure features, in addition, between the microstructure features and the thermal conductivity. The comprehensive correlation of spray process-microstructure-thermal conductivity was established for plasma-sprayed ZrO2 coatings.

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium–magnesium–alumina–silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm 2/s to 0.7 mm 2/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.

The adhesion strength and indentation toughness of plasma-sprayed yttria stabilized zirconia coatings

In this work, yttria stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spray (APS) technique under various powder feed rates and spray distances. The microstructure and phase analysis were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Tensile adhesion test (TAT) and interfacial indentation test (IIT) methods were used to evaluate adhesion of coatings. The effect of spray parameters on the coatings adhesion as well as its toughness was investigated. The obtained results revealed that with increasing powder feed rate and spray distance, adhesion of the coatings reaches to a maximum and then reduces. Interfacial toughness values change in the same manner. Adhesion/toughness behaviors of the YSZ coatings were related to microstructural characteristics including the volume fraction and size of pores and unmelted particles.

Characterization of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings by Cathodoluminescence

The occurrence of monoclinic zirconia phase has an important impact on the performance of thermal barrier coatings (TBC) of yttria-stabilized zirconia (YSZ). Therefore, a reliable method is needed to detect its contents and to investigate also its spatial distribution within the parent microstructure. This was the motivation to apply cathodoluminescence (CL) spectroscopy. YSZ coatings with different porosities were manufactured by atmospheric plasma spraying. CL analysis yielded monoclinic phase contents of 5.2 ± 1.6% for the high-porous sample and 3.4 ± 0.5% for the low-porous sample. The results were qualitatively confirmed by x-ray diffraction (XRD). However, due to its lower detection sensitivity the XRD results are quantitatively on lower level. Owing to its synthesis method, the applied powder feedstock showed a considerable content of monoclinic phase. The lower the particle temperatures were the larger fraction of monoclinic phase remained untransformed. This has to be considered when spraying high-porous TBCs.

Process-Controlled Plasma-Sprayed Yttria-Stabilized Zirconia Coatings: New Insights from Ultrasmall-Angle X-ray Scattering

A multicomponent microstructure model is applied in ultrasmall-angle X-ray scattering studies of two groups of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs). One group was sprayed from a single powder feedstock using controlled processing conditions. The other group included three different feedstock morphologies (obtained from different manufacturing methods), each with a similar particle size distribution and sprayed under the same average controlled processing conditions. The microstructure is quantitatively related to the feedstock morphology and processing conditions. Relationships are explored among these microstructures and the coating properties (e.g., thermal conductivity, elastic modulus). The degree of microstructural anisotropy is demonstrated to be pore-size dependent, being more pronounced for larger pores, and more sensitive to feedstock morphology (powder processing) than to spray processing. The microstructure analysis indicates two broad distributions of interlamellar pores, which combined, account for 70%–80% of the pore volume. The total porosity is found to increase with decreasing particle temperature or velocity. For all coatings, a negative linear relationship exists between thermal conductivity and total porosity. Comparison of the new analysis is made with earlier small-angle neutron scattering results, and implications are considered for a more general application of this metrology in TBC microstructure design.

Plasma-sprayed YSZ coatings: Microstructural features and resistance to molten metals

Yttria Stabilized Zirconia (YSZ) coatings deposited by plasma spraying are widely used as thermal barriers onto metallic substrates, due to their attractive thermal properties and chemical inertia. In the present study, YSZ coatings were deposited onto steel substrates and their microstructure was investigated with X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The micro- and nano-scopic structural features were correlated to the performance of the coatings against corrosion by molten metals, which could simulate the industrial conditions of numerous applications. More precisely, in-scale porosity of the coatings and crystal parameters modifications were systematically examined and correlated to the corrosion behaviour in molten zinc, tin and aluminium baths for a 6 days testing period. The structural stability of the ceramic coating at high temperature, as well as the reactivity of the molten metal on the ceramic surface were preliminarily evaluated by thermogravimetric analysis and differential scanning calorimetry.

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V 2O 5 reaction and melt infiltration in plasma-sprayed 7 wt% Y 2O 3–ZrO 2 (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V 2O 5 with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV 2O 7 (for reactions at 750 °C and below), m-ZrO 2 and YVO 4. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V 2O 5 was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V 2O 5 at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V 2O 5.

Investigation of reactions between vanadium oxide and plasma-sprayed yttria-stabilized zirconia coatings

The phase evolution occurring during the reaction between corrosive V 2O 5 ( T m = 690 °C) and a plasma-sprayed 7 wt.% Y 2O 3–ZrO 2 (YSZ) coating from 700 to 900 °C has been investigated in situ by X-ray diffraction. The temperature and time of interaction between the V 2O 5 and YSZ coating determines the phases observed. Between 700 and 750 °C, reaction products of ZrV 2O 7 and YVO 4 were observed within minutes of reaching the test temperature. m-ZrO 2 was observed after 220 and 60 min at 700 and 750 °C, respectively. The simultaneous formation of both ZrV 2O 7 and YVO 4 at the beginning of the reaction along with the delay of the m-ZrO 2 formation suggests similar reactivity between both Zr and Y with V 2O 5. The weight percent of the ZrV 2O 7 phase began to diminish after 150 and 60 min at 700 and 750 °C, respectively. For reaction temperatures of 800 and 900 °C, there is a rapid decrease in the amount of t′-ZrO 2 and a rapid increase in the amount of m-ZrO 2 with reaction time. YVO 4 was also observed at these reaction temperatures. SEM and TEM microstructural observations confirmed the phases detected from the in situ XRD experiments. Reactions between YSZ and V 2O 5 suggest that the formation of a liquid phase due to the high solubility of both zirconia and yttria in vanadia is the dominate mechanism that damages the coating. The thermal conductivity of a plasma-sprayed YSZ coating reacted with up to 1 wt.% V 2O 5 did not significantly change due to the small volume affected.

Study on gas permeation behaviour through atmospheric plasma-sprayed yttria stabilized zirconia coating

Gas permeation behaviour through atmospheric plasma-sprayed 8 mol% yttria stabilized zirconia (YSZ) electrolyte coating was studied experimentally. YSZ coatings were fabricated using different powder feedstock. The temperature and velocity of in-flight particles during spraying were measured with a diagnostic system. The results showed that particle temperature and velocity were significantly influenced by the size of powders. The gas permeability of these coatings was estimated by a specific instrument with pure O 2, N 2 and H 2. It was found that the gas permeability was reduced by decreasing the size of powder. Gas permeation behaviour through plasma-sprayed YSZ coating was studied. Transition flow was compatible to gas permeation behaviour for all three plasma-sprayed YSZ coatings. The relationship between gas permeation behaviour and coating microstructure is discussed in this article.

Microwave sintering of plasma-sprayed yttria stabilized zirconia electrolyte coating

In this work, microwave (MW) was used to sinter atmospheric plasma-sprayed 8 mol% yttria stabilized zirconia (YSZ) electrolytes coatings. Three types of coatings were studied: pure YSZ coating, 5 wt% BaTiO 3 filled YSZ coating and 10 wt% BaTiO 3 filled YSZ coating. Microstructures and ionic conductivities of as-sprayed and MW-sintered coatings were characterized. The results indicated that that the pure YSZ coating can hardly be sintered by the MW sintering and it can only be heated up to 900 °C. For two composite coatings, BaTiO 3 particles distributed in the composite coating act as a good MW susceptor and thereby, a good heat source for sintering YSZ matrix. After a MW sintering at 1450 °C for about 30 min, an obvious structural modification was noticed: the lamellar and columnar crystal features of as-sprayed plasma-sprayed YSZ coatings were modified to equiax crystal in the sintered coatings. Ionic conductivity of sintered 5 wt% BaTiO 3 filled YSZ composite coatings were elevated to about twice of as-sprayed coating. A further increase of BaTiO 3 content to 10 wt% in the composite coating enhanced the sintering efficiency, however, the ionic conductivity of composite coatings was decreased.

Plasma-sprayed yttria-stabilized zirconia coatings on type 316L stainless steel for pyrochemical reprocessing plant

Type 316L stainless steel (SS) is one of the candidate materials proposed for application in pyrochemical reprocessing plants. In the present work, yttria-stabilized zirconia coatings of 300 μm were applied over type 316L SS with a metallic bond coating of 50 μm by an optimized plasma spray process, and were assessed for the corrosion behaviour in molten LiCl–KCl medium at 873 K for periods of 5 h, 100 h, 250 h and 500 h. The as-coated and tested samples were examined by optical microscopy and SEM for homogeneity, penetration of molten salt through coating and corrosion of type 316L SS substrate. The results indicated that the yttria-stabilized zirconia coatings performed well without significant degradation and corrosion attack. Laser melting of the coated samples using CO 2 laser was attempted to consolidate the coatings. The development of large grains with segmented cracks was noticed after laser melting, though the coating defects have been eliminated.

Improving Plasma-Sprayed Yttria-Stabilized Zirconia Coatings for Solid Oxide Fuel Cell Electrolytes

Using a D-optimal design of experiments, the influences of feedstock powder and plasma gases on deposition efficiency, gas tightness, and the electrochemical behavior of vacuum plasma-sprayed yttria-stabilized zirconia for solid oxide fuel cell electrolytes were examined. In-flight particle temperature and velocity, measured by online particle diagnostics, were correlated with plasma and deposit properties. Electrochemical testing of cells was performed to determine the influence of gas tightness and microstructure of electrolyte deposit on cell behavior.

Laser surface treatment of plasma-sprayed yttria-stabilized zirconia coatings

Un equipo laser continuo de CO 2 , con potencia nominal de 1 kW, se utilizo para sellar la superficie revestida con ZrO 2 8 % Y 2 O 3 por proyeccion por plasma sobre un substrato de acero AISI 1045. Se investigaron los efectos del tratamiento de fusion con laser sobre la microestrutura y la resistencia a la corrosion del recubrimiento. La resistencia a la corrosion se analizo por medidas electroquimicas en una solucion de NaCl al 3 %. Las micrografias mostraron que la superficie del revestimiento presento varias grietas pequenas, sin embargo, no fueron observados poros. La microestructura de la capa sellada presento una estructura en forma de columnas con crecimiento perpendicular a la superficie libre. El sellado a laser mejora la resistencia a la corrosion de los recubrimientos y aumenta la microdureza.

Influence of Hydrogen on the Microstructure of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The influence of secondary hydrogen and current on the deposition efficiency (DE) and microstructure of yttria-stabilized zirconia (YSZ) coatings was evaluated. To better understand the influence of the spray process on coating consistency, a YSZ powder, −125 +44 µm, was sprayed with nitrogen/hydrogen parameters and a 9 MB plasma gun from Sulzer Metco. DE and coating porosity, which were produced using two different spray gun conditions yielding the same input power, were compared. Amperage was allowed to vary between 500 and 560 A, and hydrogen was adjusted to maintain constant power, while nitrogen flow was kept at a fixed level. Several power conditions, ranging from 32 to 39 kW, were tested. Different injection geometries (i.e., radial with and without a backward component) were also compared. The latter was found to produce higher in-flight temperatures due to a longer residence time of the powder particles in the hotter portion of the plasma. Porosity was based on cross-sectional micrographs. In-flight particle temperature and velocity measurements were also carried out with a special sensor for each condition. Test results showed that DE and coating density could vary significantly when a different hydrogen flow rate was used to maintain constant input power. On the other hand, DE was found to correlate very well with the temperature of the in-flight particles. Therefore, to obtain more consistent and reproducible DE and microstructures, it is preferable to maintain the in-flight particle temperature around a constant value instead of keeping a constant input power by adjusting the secondary hydrogen flow rate.