Zirconates with high melting points were investigated for
application as materials for thermal barrier coatings at operating temperatures >1300°C. SrZrO3, BaZrO3, and La2Zr2O7
powders were synthesized and sintered to compacts with
various levels of porosity. The sintering results indicated a promising low-sintering activity of the investigated materials. Thermal properties of these dense materials were determined. Thermal expansion coefficients were slightly lower than those of Y2O3-stabilized ZrO2 (YSZ); thermal conductivities of SrZrO3 and BaZrO3 were comparable or slightly higher than those of YSZ. La2Zr2O7 had a lower thermal conductivity. SrZrO3 was not suitable for application as a thermal barrier coating because of a phase transition at temperatures between 700° and 800°C. Mechanical properties (hardness, fracture toughness, and Young’s modulus) of dense BaZrO3 and La2Zr2O7 samples were determined by indentation techniques and showed lower hardness and Young’s modulus compared to YSZ. BaZrO3 and La2Zr2O7 powders were optimized for application as powders for plasma spraying. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner at 1200°C showed a massive attack of the BaZrO3 coating, with loss of BaO. On the other hand, the La2Zr2O7 coating showed excellent thermal stability and thermalshock behavior.

Zirconates as New Materials for Thermal Barrier Coatings

To enhance the insulating properties of a thermal barrier coating, one has to focus on new materials with lower intrinsic thermal conductivity than established yttria-stabilized zirconia. Substances with pyrochlore structure were investigated. Starting from lanthanum zirconate, substitutions of the lanthanum by other trivalent rare-earth elements were made, and the thermal conductivity and the thermal expansion coefficient of the manufactured materials were measured. A complete substitution of the lanthanum led to increased thermal expansion coefficients, whereas the partial substitution did not show an appreciable effect. The thermal conductivities of the modified materials were lower than that of the pure lanthanum zirconate for temperatures <1000°C for all amounts and elements of substitution. A comparison of the observed values with calculated values of the thermal conductivities showed a relatively good agreement.

Thermal Conductivity and Thermal Expansion Coefficients of the Lanthanum Rare-Earth-Element Zirconate System

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.

Recent Developments in the Field of Thermal Barrier Coatings

Pyrochlore materials La2Zr2O7 and Gd2Zr2O7 have been used to produce thermal barrier coating systems by atmospheric plasma spraying. The materials have been applied as single-layer coatings with only a topcoat made of pyrochlore material. In addition, double-layer systems with a first layer of yttria-stabilized zirconia (YSZ) and a top layer made of pyrochlore material were produced. These systems have been tested in thermal cycling test rigs at surface temperatures between 1200-1450°C and the results were compared to the behavior of YSZ coatings. Single-layer coatings had a rather poor thermal cycling performance. On the other hand, double-layer systems showed similar results to YSZ coatings at temperatures below about 1300°C. At higher temperatures the double-layer coatings produced from our own powders revealed excellent thermal cycling behavior. At the highest test conditions, lifetime was thereby orders of magnitude better than that of YSZ coatings. Results indicate that an increase of the maximum surface temperature in gas turbines by at least 100 K becomes possible with the new coatings. Coatings produced from commercial powders showed a somewhat reduced performance.

New Thermal Barrier Coatings Based on Pyrochlore/YSZ Double Layer Systems

https://www.ncnr.nist.gov/programs/sans/pdf/publications/0081.pdf

Comprehensive microstructural characterization and predictive property modeling of plasma-sprayed zirconia coatings

This paper summarizes prior and on-going machine evaluations of thermal barrier coatings (TBC) for power generation, that is large industrial gas turbine applications. Rainbow testing of TBCs on turbine nozzles, shrouds, and buckets are described along with a test of combustor liners. General Electric Power Generation has conducted more than IS machine tests on TBC turbine nozzles with various coatings. TBC performance has been quite good, and additional testing, including TBCs on shrouds and buckets, is continuing. Included is a brief comparison of TBC requirements for power generation and aircraft turbines.

/pdf/tbc-experience-in-land-based-gas-turbines-37e30luj15.pdf

TBC experience in land based gas turbines

A method of determining blade tip clearance in a gas turbine includes (a) measuring the distance A between an ultrasonic sensor (16) and a stator shroud surface (14), (b) measuring the distance between B a radio frequency sensor (20) and rotating blade tips (13) of the gas turbine; and (c) subtracting the distance measured by the ultrasonic sensor (16) from the distance measured by the radio frequency sensor (20). The apparatus includes an ultrasonic sensor for measuring the distance between a stator shroud surface (14) and the ultrasonic sensor (16), a radio frequency sensor (20) for measuring the distance between rotating blade tips (13) of the gas turbine and the radio frequency sensor; and a computer system (24) for receiving and subtracting the ultrasonic sensor measurements from the radio frequency sensor measurements.

Method and apparatus for measuring turbine blade tip clearance

GE is continuing work on the development of Melt-Infiltrated Ceramic Matrix Composites (MI-CMC) for use in industrial gas turbine engine components. Long-term environmental degradation of test samples under realistic engine conditions is being determined using a unique high-pressure combustion rig apparatus. Rig testing is also being used to evaluate an F-class 1st stage shroud system incorporating an MI-CMC inner shroud component. While large, advanced engines, such as the F and H classes, offer the greatest benefits for using MI-CMC components, initial engine tests have been done using a GE-2 (2MW) machine to reduce costs and risk. Long term (1000 hours) engine testing results for single piece GE-2 shrouds are also described.Copyright © 2001 by ASME

Rig and Gas Turbine Engine Testing of MI-CMC Combustor and Shroud Components

A sensor for measuring Wobbe index of a fuel is provided. The sensor includes a substrate and a diaphragm layer. The diaphragm layer includes a first layer having at least one heating element configured to sense energy content in a fuel, wherein the heating element includes a doped poly-silicon carbide that is disposed on the substrate. The diaphragm layer also includes a second layer including an undoped poly-silicon carbide layer configured to prevent oxidation of the first layer. The sensor further includes a sensing layer having a catalyst suspended in a support structure. The sensor also includes a cavity formed under the diaphragm layer and is configured to provide thermal isolation of the heating element.

Wobbe index sensor system

A method for measuring a lower heating value of a gaseous fuel. The method includes mixing a gaseous fuel with air to provide a combustible air-fuel mixture. The air-fuel mixture is directed to flow across a flow surface (52) of a first micro-hotplate (50) maintained at a constant temperature. A change in power required to maintain a constant temperature of the first micro-hotplate flow surface due to a convective and conductive heat transfer from the first micro-hotplate flow surface to the air-fuel mixture is measured. The air-fuel mixture is directed to flow across a flow surface (62) of a second micro-hotplate (60) maintained at a constant temperature. The air-fuel mixture is combusted as the air-fuel mixture flows across the second micro-hotplate flow surface. A change in power required to maintain a constant temperature of the second micro-hotplate flow surface due to the combustion of the air-fuel mixture is measured.

Robert Michael Orenstein

Papers

TBC experience in land based gas turbines

Method and apparatus for measuring turbine blade tip clearance

Rig and Gas Turbine Engine Testing of MI-CMC Combustor and Shroud Components

Wobbe index sensor system

Method, sensor and system for measuring a lower heating value and a wobbe index of a gaseous fuel