Standard Yttria-stabilized Zirconia Research Articles

A comparative study of calcium–magnesium–aluminum–silicon oxide mitigation in selected self-healing thermal barrier coating ceramics

Abstract

Development of Advanced Nickelate-Based Oxygen Electrodes for Solid Oxide Cells

The present work deals with the development of lanthanide nickelate based oxygen electrodes (Ln2NiO4+ δ, with Ln = Nd, La or Pr) as the oxygen electrode for both solid oxide fuel cell (SOFC) and solid oxide electrolysis (SOEC) applications. The have been widely investigated as they present high oxygen diffusion parameters, making them attractive as SOFC oxygen electrodes. However, the main drawback of these nickelate phases is their reactivity with standard YSZ (yttria stabilized zirconia) or CGO (cerium gadolinium oxide) electrolytes, as typical sintering temperatures are usually higher than 1000 ºC in order to get strong adhesion at the electrode/electrolyte interface. Two strategies will be employed to avoid the undesired reactivity: the direct formation of the nickelate phase into YSZ scaffolds by infiltration and the development of active barrier layer.

Laser Cladding of Embedded Sensors for Thermal Barrier Coating Applications

The accurate real-time monitoring of surface or internal temperatures of thermal barrier coatings (TBCs) in hostile environments presents significant benefits to the efficient and safe operation of gas turbines. A new method for fabricating high-temperature K-type thermocouple sensors on gas turbine engines using coaxial laser cladding technology has been developed. The deposition of the thermocouple sensors was optimized to provide minimal intrusive features to the TBC, which is beneficial for the operational reliability of the protective coatings. Notably, this avoids a melt pool on the TBC surface. Sensors were deposited onto standard yttria-stabilized zirconia (7–8 wt % YSZ) coated substrates; subsequently, they were embedded with second YSZ layers by the Atmospheric Plasma Spray (APS) process. Morphology of cladded thermocouples before and after embedding was optimized in terms of topography and internal homogeneity, respectively. The dimensions of the cladded thermocouple were in the order of 200 microns in thickness and width. The thermal and electrical response of the cladded thermocouple was tested before and after embedding in temperatures ranging from ambient to approximately 450 °C in a furnace. Seebeck coefficients of bared and embedded thermocouples were also calculated correspondingly, and the results were compared to that of a commercial standard K-type thermocouple, which demonstrates that laser cladding is a prospective technology for manufacturing microsensors on the surface of or even embedded into functional coatings.

Hot Corrosion Behavior of YSZ and CaZrO3/YSZ Composite Thermal Barrier Coatings in Contact with 50V2O5 + 50Na2SO4 Salts

A novel thermal barrier coating system was formulated to resist hot corrosion environments. In this coating system, 5% CaZrO3 was added to conventional yttria-stabilized zirconia (YSZ). The above composite coating system was compared with the standard YSZ system in the presence of a mixture of 50% Na2SO4 and 50% V2O5 at 950 °C. The results demonstrated that the lifetime of the CaZrO3-added composite system in this highly hostile environment was longer compared with the standard YSZ system. This is due to the fact that the preferential reaction of NaVO3 shifted from yttria to calcia, forming CaV2O4 instead of YVO4. The preferential reactions are discussed in terms of free energy changes and acid–base theory of molten salts with ceramic oxides. Furthermore, calculations of lattice distortion also proved that the CaZrO3-added composite system demonstrated less distortion, thus increasing the overall lifetime of the coating system.

High Temperature Thermal Properties of Columnar Yttria Stabilized Zirconia Thermal Barrier Coating Performed by Suspension Plasma Spraying

Performance enhancement of gas turbines is a main issue for the aircraft industry. Over many years, a large part of the effort has been focused on the development of more insulating Thermal Barrier Coatings (TBCs). In this study, Yttria Stabilized Zirconia (YSZ) columnar structures are processed by Suspension Plasma Spraying (SPS). These structures have already demonstrated abilities to get improved thermal lifetime, similarly to standard YSZ TBCs performed by EB-PVD. Thermal diffusivity measurements coupled with differential scanning calorimetry analysis are performed from room temperature up to 1100 °C, first, on HastelloyX substrates and then, on bilayers including a SPS YSZ coating. Results show an effective thermal conductivity for YSZ performed by SPS lower than 1 W.m-1K-1 whereas EB- PVD YSZ coatings exhibit a value of 1.5 W.m-1K-1.

Functionally Graded Thermal Barrier Coatings with Improved Reflectivity and High-Temperature Capability

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat isolative topcoat. In modern TBCs the ceramic topcoat is further divided into layers with different functions. One example is the double layer system in which conventional yttria stabilized zirconia (YSZ) is used as bottom and new materials as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. Examples of such systems will be shown. In modern gas turbines the increased temperatures and gas pressures lead to an increased fraction of radiative heat flow. Coatings with increased reflectivity can be used to avoid the direct heating of the metallic substrates by this radiation. An effective method to produce such coatings is suspension plasma spraying. These reflective coatings are deposited on top of the TBC system and will lead to a further grading and improved performance of the coating.

Recent Developments in the Field of Thermal Barrier Coatings

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat-isolative topcoat. Several recent research activities are concentrating on developing improved bondcoat or topcoat materials; for the topcoat especially, those with reduced thermal conductivity are investigated. Using advanced topcoat materials, the ceramic coating can be further divided into layers with different functions. One example is the double-layer system in which conventional yttria-stabilized zirconia (YSZ) is used as bottom and new materials such as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. In addition, new functions are introduced within the TBCs. These can be sensorial properties that can be used for an improved temperature control or even for monitoring remaining lifetime. Further increased application temperatures will also lead to efforts for a further improvement of the reflectivity of the coatings to reduce the radiative heat transfer through the TBC.

Optical Nondestructive Condition Monitoring of Thermal Barrier Coatings

This paper describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and online condition monitoring in gas turbines. Increases in turbine inlet temperature in the pursuit of higher efficiency will make it necessary to improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation, it will also be necessary to devise techniques for online condition monitoring and NDE. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving NDE for TBCs. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia (YSZ)) with a rare-earth activator to provide the coating with luminescence when excited with UV light. This paper describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented along with the results of comparative thermal cycle testing of TBSCs, produced using a production standard air plasma spray system. The latter show the durability of TBSCs to be similar to that of standard YSZ TBCs and indicate that the addition of the rare-earth dopant is not detrimental to the coating. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer, which in turn destabilizes the coating system as it becomes thicker. Experimental data are presented, indicating that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers covered by as much as 500 μm of standard TBC material. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made, and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.56 mm thickness.