Y2O3 Stabilized ZrO2 Coatings Research Articles

Development of ZrO2 and YSZ coatings deposited by PE-CVD below 800 °C for the protection of Ni alloys

One possibility to increase the safety of structural Ni alloys is the use of yttria stabilized zirconia coatings in order to avoid deleterious thermal effects on their mechanical properties and corrosion resistance above 800 °C. However, the deposition of YSZ with a fully cubic structure below 800 °C cannot be achieved by traditional processing routes. In this work, we show the deposition of ZrO2 and Y2O3-stabilized ZrO2 coatings by Plasma Enhanced Chemical Vapor Deposition (PE-CVD). To this purpose, the effect of deposition temperature (500–800 °C), plasma power (50–250 W) and precursor concentration (0.5–2 g) on the microstructure, composition and crystalline phase of ZrO2 and YSZ coatings was studied. Contrary to previous reports, we show that it is feasible to use zirconium (IV) acetylacetonate [Zr (acac)4] to produce nanostructured zirconia coatings with low carbon content (1.8 at.%) and metastable tetragonal phase. A fully cubic YSZ structure was achieved with the use of Y-acetylacetonate with concentrations above 12 mol% at 700 °C and obtain a columnar microstructure.

Comparison of CMAS corrosion and sintering induced microstructural characteristics of APS thermal barrier coatings

The microstructural features of high-temperature sintered and CaO-MgO-Al2O3-SiO2 (CMAS) corroded air plasma sprayed Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) under the thermal gradient condition were comparatively studied. As-sprayed YSZ has a lamellar structure and the lamellae are composed of closely aligned columnar crystals. The sintered and the CMAS corroded YSZ coatings maintain the t’-ZrO2 phase as the as-sprayed YSZ coating. The sintered YSZ remains the lamellar structure with reduced interlamellar gaps and grains coarsening. After the CMAS corrosion, the top layer of the YSZ coating keeps its lamellar structure consisting of some columnar grains with the CMAS infiltration into the intergrain gaps and the formation of striped Zr2Y2O7. The typical lamellar structure transforms into more equiaxed grains in the middle and bottom layers of the ceramic coating along with significant infiltration of amorphous CMAS and anorthite formation in the bottom layer owing to the high contents of Ca and Al.

Investigation of three steps of hot corrosion process in Y2O3 stabilized ZrO2 coatings including nano zones

Phase transformation of tetragonal ZrO2 to monoclinic phase and also increment of bond coat oxidation kinetic (TGO thickening) can substantially restrict the life time of thermal barrier coating systems (TBCs). So, nanostructured and conventional Y2O3 stabilized ZrO2 coatings were evaluated in fused V2O5-Na2SO4 salts during thermal exposure in air. Microstructural characterization showed lower hot corrosion products (monoclinic zirconia, YVO4 crystals) formation and reduction of TGO thickness in thermal barrier coating system consisting of nanostructured Y2O3 stabilized ZrO2 (YSZ) top coat. It was found that inhomogeneities, pores and micro-cracks played a principal role in the molten salts infiltration into the YSZ coating during three steps of hot corrosion process. In the nanostructured YSZ coating with tri-model structure, nano zones which surrounded by fully molten parts could fill the aforementioned defects and could act as barrier for the oxygen and corrosive molten salts penetration into the TBC.

Surface Properties of Different Travel Speeds and Powder-Feeder Rates for Plasma-Sprayed Coatings on Aluminum Substrates

The surface properties of plasma-sprayed partial Y2O3 stabilized ZrO2 coatings with different travel speed and powder-feeder rate of sprayed gun on aluminum substrates have been reported. Several coatings analytical parameters were studied and these factors affecting coating properties were carried out in the plasma-sprayed experiments. Experimental results have shown that the structure of the sprayed coatings that makes relatively diverse in texture for most of the coatings was examined by a SEM. The hardened strength with a low level of a slant of powder-feeder rate became significantly weaker as the travel speed was decreased. Interestingly, the structure of the specimens for the travel speed of 30mm/s is more homogenous than that of the others, indicating a coating with good homogenous structures, while the interfaced structure of the specimens for the powder-feeder rate of 25 mm/s is more homogenous than that of the others, indicating a coating with good adhesive structures. The tendency of the effect of travel speed of sprayed gun on the hardened surface of coatings at powder feeder rate of sprayed gun varied to 30g/min from 20g/min seems to be apparent.

High‐Temperature Corrosion of EB‐PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay

High‐temperature interaction of sol–gel‐derived artificial volcanic ash (AVA) matching the bulk composition of the April 15, 2010 Eyjafjallajokull (Iceland) volcanic eruption with a standard 4 mol% (7 wt%) Y2O3‐stabilized ZrO2 (YSZ) electron‐beam physical vapor deposition (EB‐PVD) thermal barrier coating and a corresponding YSZ powder is investigated in order to access possible implications of similar volcanic ashes on the performance of coated turbine engine airfoils. Up to 900°C, AVA deposits and EB‐PVD YSZ do not show significant interaction. Viscous flow above the glass transition of AVA (Tg∼930°C) yields proceeding wetting of EB‐PVD YSZ coatings. At 1100°C, the YSZ surface is covered by a dense glaze‐like AVA overlay. At 1200°C, AVA is mostly infiltrating the coating, leaving a crystalline plagioclase‐ and hematite‐type residue at the interface. Moreover, some ZrSiO4 is formed at the expense of YSZ. The overall thermochemical effects on short‐term exposure of the EB‐PVD YSZ coating to a small AVA load were moderate, in particular before complete infiltration. On the other hand, AVA acts as a solvent for the stabilizing Y2O3 beyond 1000°C and a progressive depletion of Y2O3 in the YSZ is observed at the AVA/YSZ interface. Detrimental effects on YSZ phase stability and hence coating lifetime cannot be ruled out for long‐term exposure and higher AVA loads.

Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

Abstract The effects of La2O3 addition on thermal conductivity, phase stability and thermal cycle life of Y2O3 stabilized ZrO2 plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La2O3 addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.

An experimental technique to evaluate the effective thermal conductivity of Y2O3 stabilized ZrO2 coatings

An experimental device was set up to determine thermal resistance and conductivity of 8% yttria-stabilized zirconia deposited by plasma spray method on cylindrical specimen. In this experimental setup, coated surface of the sample was exposed to a high temperature environment and inner metal surface was cooled by flowing air, simulating actual gas turbine applications. Overall heat resistance at the outside surface of thermal barrier coating was adopted to assess thermal advantage due to the thermal barrier coating deposited on air-cooled cylindrical specimen. 28% less heat was extracted at 1000°C by applying 1.2 mm thick thermal barrier coating. Temperatures of the outside surface of the coated samples increased with increasing coating thickness with respect to the same furnace temperature since the sample with thicker coating was less thermally conductive and retarded heat transfer. The overall heat resistances of samples between the outside surface of sample and the flowing air inside the sample assembly were estimated. Then, the thermal conductivity of coating could be determined from the difference of overall thermal resistances of two selected samples with varying coating thickness.

Effect of Heat Treatment on the Thermal Conductivity of Plasma-Sprayed Thermal Barrier Coatings

The effect of heat treatment on the thermal conductivity of plasma-sprayed Y2O3 stabilized ZrO2 (YSZ) and Al2O3 coatings was investigated. A heat treatment of 1300 °C in flowing argon for 50 h was found to significantly increase the thermal conductivity of the coatings when compared to measurements in the assprayed condition. Transmission electron microscopy (TEM) examination of the microstructures of the coatings in the as-sprayed and heat-treated conditions revealed that sintering of microcracks at the splat interfaces was the main cause for the increase in thermal conductivity. In the YSZ coatings, complete closure of microcracks was frequently observed. In contrast, microcrack closure in the Al2O3 coatings was characterized by the isolated necking of particles across a microcrack rather than complete closure. A model for thermal conductivity in a solid containing oriented penny-shaped cracks was used to explain the observed increase in thermal conductivity after heat treatment.