It is known that the relative performance of thermal barrier coatings is largely dependent upon the oxidation properties of the bond coat utilized in the system. Also, the oxidation properties of diffusion-type bond coats (aluminides and their modifications) are functions of the superalloy substrate used in blade applications. Therefore, the performance of a given coating system utilizing a diffusion-type bond coat can significantly vary from one superalloy to another. Toward the objective of developing coating systems with more universal applicability, it is essential to understand the mechanisms by which the superalloy substrate can influence the coating performance. In this study, we examined the relative performance of yttria-stabilized zirconia/platinum aluminide coating system on alloys CMSX-4 and MAR M 002DS representing single-crystal and directionally-solidified alloy systems respectively using thermal exposure tests at 1150 °C with a 24-h cycling period to room temperature. Changes in coating microstructure were characterized by various electron-optical techniques. Experiment showed that the coating system on alloy MAR M 002DS had outperformed that on alloy CMSX-4, which could be related to the high thermal stability of the bond coat on alloy MAR M 002DS. From a detailed microstructural characterization, this difference in behavior could be explained at least partially in terms of variation in chemical composition of the two alloys, which was also reflected on the exact failure mechanism of the coating system.

Titanium is a key element in nickel-base superalloys needed with aluminum to achieve the desired volume fraction of the strengthening γ ′ -phase. However, depending upon its concentration, titanium can degrade the adherence of aluminum oxide by forming TiO 2 particles near the oxide-metal interface. This effect is extended to thermal barrier coating systems where in this case, the bond coat replaces the superalloy as the underlying substrate. Noting that the onset of failure of thermal barrier coating systems coincides with the first spall of the thermally grown oxide, titanium level in the superalloy can have an important effect on the useful life of the coating. Therefore, this study was undertaken to examine the effect of titanium on the performance of a thermal barrier coating system. Included in the study were two Ni-base superalloys with similar chemical composition except for the Ti content and a Pt-containing bond coat consisting of γ ′ + γ -phases all top coated with zirconia stabilized by 7 wt % yttria. Coating performance was evaluated from thermal exposure tests at 1150 ° C with a 24 h cycling period to room temperature. Various electron-optical techniques were used to characterize the microstructure. The coating system on the low-Ti alloy was found to outperform that on the high-Ti alloy. However, for both alloys, failure was observed to occur by loss of adhesion between the thermally grown oxide and underlying bond coat.

Current technology of thermal barrier coating systems used in gas turbine blade applications relies on the use of a metallic bond coat, which has a twofold function: (i) it develops a thin layer of aluminum oxide enhancing the adhesion of the ceramic top coat and (ii) it provides an additional resistance to oxidation. It was the objective of this study to develop an understanding of the role of platinum in bond coats of the diffusion-type deposited on a nickel-based superalloy. Two Pt-containing bond coats were included in the study: (i) a platinum-aluminide and (ii) a bond coat formed by interdiffusion between an electroplated layer of platinum and the superalloy substrate. In both cases, the top ceramic coat was yttria-stabilized zirconia. For reference purposes, a simple aluminide bond coat free of Pt was also included in the study. Thermal exposure tests at 1150 ° C with a 24 h cycling period at room temperature were used to compare the coating performance. Microstructural features were characterized by various electron-optical techniques. Experimental results indicated that Pt acts as a “cleanser” of the oxide-bond coat interface by decelerating the kinetics of interdiffusion between the bond coat and superalloy substrate. This was found to promote selective oxidation of Al resulting in a purer Al 2 O 3 scale of a slower growth rate increasing its effectiveness as “glue” holding the ceramic top coat to the underlying metallic substrate. However, the exact effect of Pt was found to be a function of the state of its presence within the outermost coating layer. Of the two bond coats studied, a surface layer of Pt-rich gamma prime phase ( L 1 2 superlattice) was found to provide longer coating life in comparison with a mixture of PtAl 2 and beta phase. This could be related to the effectiveness of gamma prime phase as a sink for titanium minimizing its detrimental effect on the adherence of aluminum oxide.

Plugging material in some of the film cooling channels of a failed aero-gas turbine engine first stage turbine blade is analyzed using the energy dispersive X-ray spectroscopy in an environmental scanning electron microscope. The objective of the analysis was to identify the nature and source of the plugging material that appears to have caused overheating and eventual failure of some of the blades. The results of the analysis indicate that the plugging material, which occurs as a dense aggregate of 0.1 m diameter fibers, is mainly composed of Zr, Y, and O. In addition, the material shows presence of micron size particles dispersed between the fibers. The analysis of the particles indicates they are fluoride-rich compounds, possibly of yttrium or calcium. Small or trace amounts of Ca, Na, and Mg are also observed in the plugging material. The analysis of the areas surrounding the plugged cooling channels shows presence of Cr–Co–Ni–aluminide bond coat and a discontinuous platinum coat over the bond coat. In contrast, the areas surrounding the fractured surface and melted edge show significant presence of calcium fluoride and Mg–Al–silicate. The analysis of melted edge shows presence of all the elements representing various coating layers as well as the impurities; however, Zr and Y were not detected in the melted areas.

Although low-temperature premixed compression ignition (PCI) combustion in a light-duty diesel engine offers dramatic and simultaneous reductions in nitric oxides ( N O x ) and soot, associated increases in unburned hydrocarbons (HC) and carbon monoxide (CO) become unacceptable. Production diesel oxidation catalysts (DOCs) are effective in oxidizing the increased levels of HC and CO under lean combustion conditions. However, the low-temperature∕high CO combination under rich PCI conditions, designed as a lean N O x trap (LNT) regeneration mode, generally renders the DOC ineffective. The objectives of this study are to characterize the oxidizing efficiency of a production DOC under lean and rich PCI conditions, and attempt to identify probable causes for the observed ineffectiveness under rich PCI. The study uses several tests to characterize the behavior of the DOC under lean PCI and rich PCI combustion conditions, including (1) steady-state feed gas characterization, (2) transient feed gas characterization, (3) air injection (4) insulated air-fuel sweep, and (5) combustion mode switching. The DOC never becomes effective under rich PCI for any of the tests, suggesting that the platinum-based catalyst may be incorrect for use with rich PCI. Furthermore, combustion mode switching between lean PCI and rich PCI (mimicking LNT loading and regeneration) demonstrates diminishing effectiveness of the DOC during and after continuous mode transitioning.

As part of an ongoing effort to develop a microscale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, modeling, and experimental assessment of a catalytic combustion system. Previous work has indicated that homogenous gas-phase microcombustors are severely limited by chemical reaction timescales. Storable hydrocarbon fuels, such as propane, have been shown to blow out well below the desired mass flow rate per unit volume. Heterogeneous catalytic combustion has been identified as a possible improvement. Surface catalysis can increase hydrocarbon-air reaction rates, improve ignition characteristics, and broaden stability limits. Several radial inflow combustors were micromachined from silicon wafers using deep reactive ion etching and aligned fusion wafer bonding. The 191 mm 3 combustion chambers were filled with platinum-coated foam materials of various porosity and surface area. For near stoichiometric propane-air mixtures, exit gas temperatures of 1100 K were achieved at mass flow rates in excess of 0.35 g ∕ s . This corresponds to a power density of ∼ 1200 MW ∕ m 3 ; an 8.5-fold increase over the maximum power density achieved for gas-phase propane-air combustion in a similar geometry. Low-order models, including time-scale analyses and a one-dimensional steady-state plug-flow reactor model, were developed to elucidate the underlying physics and to identify important design parameters. High power density catalytic microcombustors were found to be limited by the diffusion of fuel species to the active surface, while substrate porosity and surface area-to-volume ratio were the dominant design variables.

Thermal barrier coatings (TBCs) provide an alloy surface temperature reduction when applied to turbine component surfaces. Thermal barrier coatings can be used as a tool for the designer to augment the power and/or enhance the efficiency of gas turbine engines. TBCs have been used successfully in the aerospace industry for many years, with only limited use for industrial gas turbine applications. Industrial gas turbines operate for substantially longer cycles and time between overhauls, and thus endurance becomes a critical factor. There are many factors that affect the life of a TBC including the composition and microstructure of the base alloy and bond coating. Alloys such as Mar-M 247, CMSX-4, and CMSX-10 are materials used for high temperature turbine environments, and usually require protective and/or thermal barrier coatings for increased performance. Elements such as hafnium, rhenium, and yttrium have shown considerable improvements in the strength of these alloys. However, these elements may result in varying effects on the coatability and environmental performance of these alloys. This paper discusses the effects of these elements on the performance of thermal barrier coatings. [S0742-4795(00)02603-X]

Cyclic oxidation behavior of aluminide, platinum modified aluminide, and MCrAlY coatings has been investigated at three temperatures. Aluminide and platinum modified coatings were deposited on GTD 111 material using an outward diffusion process. CoCrAlY coating was applied on GTD-111 by Electron Beam Physical Vapor Deposition (EB-PVD). The oxidation behavior of these coatings is characterized by weight change measurements and by the variation of β phase present in the coating. The platinum modified aluminide coating exhibited the highest resistance to oxide scale spallation (weight loss) during cyclic oxidation testing. Metallographic techniques were used to determine the amount of β phase and the aluminum content in a coating as a function of cycles. Cyclic oxidation life of these coatings is discussed in terms of the residual β and aluminum content present in the coating after exposure. These results have been used to calibrate and validate a coating life model (COATLIFE) developed at the Material Center for Combustion Turbines (MCCT). [S0742-4795(00)00801-2]

A fast-response probe for the measurement of total temperature in unsteady or fluctuating compressible flows is discussed. The operation of the device is based on the measurement of transient heat flux at different probe operating temperatures. The heat flux gauges used in the present investigation were thin film platinum resistance thermometers painted onto the stagnation region of hemispherical fused quartz probes. The quartz probes had a diameter of approximately 3 mm. Uncertainty estimates indicate that temperature measurements with an accuracy of better than ±3 K are possible. As a demonstration of the accuracy and utility of the device, total temperature and convective heat transfer coefficient measurements were obtained in a number of supersonic nozzle tests and in a compressible turbulent jet experiment.

The hot gas path section components of land based turbines require materials with superior mechanical properties and good hot corrosion and oxidation resistance. These components are generally coated with either a diffusion coating (aluminide or platinum aluminide) or with an overlay coating (MCrAlY) to provide additional hot corrosion and/or oxidation protection. These coatings degrade due to inward and outward diffusion of elements during service. Outward diffusion of aluminum results in formation of a protective oxide layer on the surface. When the protective oxide spalls, Aluminum in the coating diffuses out to reform the oxide layer. Accelerated oxidation and failure of coating occur when the Al content in the coating is insufficient to reform a continuous alumina film. This paper describes development of a coating life predictions model that accounts for both oxidation and oxide spallation under thermal mechanical loading as well as diffusion of elements that dictate the end of useful life. Cyclic oxidation data for aluminide and platinum aluminide coatings were generated to determine model constants. Applications of this model for predicting cyclic oxidation life of coated materials are demonstrated. Work is underway to develop additional material data and to qualify the model for determining actual blade and vane coating refurbishment intervals.

Quartz crystal microbalance (QCM) and pressure measurements are used for determination of jet fuel thermal stability in a batch reactor. The QCM is able to monitor extremely small amounts of deposition in situ, while the pressure measurements provide qualitative data on the oxidation process. The dependence of the deposition amount was monitored as a function of the oxygen availability for two fuels. Also, the effect of QCM electrode materials was investigated. Deposition and oxidation were compared for the following electrode materials: gold, aluminum, silver, and platinum. We also studied the effect of dilution on oxidation and deposition. Jet fuel was diluted with increasing amounts of hydrocarbon solvent. It was observed that this dilution procedure can help characterize a fuel’s effective antioxidant concentration. Fuel dilution is also shown to be a good technique for improving thermal stability characteristics of poor fuels. Additionally we have studied the temperature effect on deposition for two fuels over the range 140 to 180°C.

Results from an analysis of cracked first-stage blades (or buckets) from two General Electric MS7001E industrial/electric utility gas turbines are presented. Numerous cracks were observed along the leading-edge and midchord regions of the pressure and suction surfaces. In one unit cracks were found after 874 start-stop cycles, which included 218 trips from load and 11,000 service hours. Buckets in the sister engine were examined after 1800 cycles, which included 218 trips from load and 24,000 service hours. In both cases, cracks initiated in the platinum aluminide coating and propagated into the IN-738LC base metal. For the 11,000-hour bucket, 20-mil (0.5-mm) deep cracks were observed, and for the 24,000-hour bucket, the leading edge cracks had grown to the leading edge cooling hole, a distance of 0.2 in. (5 mm). The number of cycles to crack initiation was in good agreement with thermal mechanical fatigue (TMF) predictions from the REMLIF computer program, which is part of the Electric Power Research Institute’s (EPRI) Life Management System. The cracking was greatly accelerated by the large number of trips experienced. The extensive crack propagation that occurred is thought to have been strongly assisted by oxygen and sulfur penetration along the grain boundaries. The coating on the leading edge degraded from the original platinum-aluminide plus beta phases to a gamma prime phase after 24,000 hours of service, but it was still protective except where it was cracked. Where the coating was cracked, environmental attack of the interdiffusion zone and base metal occurred, resulting in spallation of the coating and preferential grain boundary attack. Operating and maintenance considerations for optimizing bucket life in demanding cyclic duty environments are also discussed.

The performance of yttria-stabilized zirconia (YSZ), ceria-stabilized zirconia (CSZ), and magnesia-stabilized zirconia (MSZ) coatings was evaluated using an atmospheric burner rig; test environment contained compounds of vanadium, sodium, and sulfur. The coatings were deposited by plasma spraying and electron beam physical vapor deposition (EB-PVD); sputtered sealant layers of hafnia, alumina, and platinum were deposited on the YSZ coating. The tests were performed for up to 500 hours at 1650°F and 1300°F. The tests were designed to simulate the deposit chemistry and sulfur trioxide partial pressures expected in a marine gas turbine engine operating on contaminated fuel. YSZ, CSZ, and MSZ coatings all underwent reaction in the burner rig environment; the reaction products and their effects on spallation were varied. MSZ was by far the most reactive, readily forming MgSO 4 in both 1650°F and 1300°F tests. The observed reaction products provided a measure of “protection” for the bond coat by preventing molten salt infiltration for the duration of the test. The mechanism of ceramic spallation is discussed. Sputtered overlayers of platinum, hafnia, and alumina did not prevent salt infiltration and reaction with the underlying ceramic, although no reaction product between the overlayer and the salt was observed.

Nickel and cobalt-base superalloy blades and vanes in the hot sections of all gas turbines are coated to enhance resistance to hot corrosion. Pack cementation aluminizing, invented in 1911, is the most widely used coating process. Corrosion resistance of aluminide coatings can be increased by modification with chromium, platinum, or silicon. Chromium diffusion coatings can be used at lower temperatures. Formation and degradation mechanisms are reasonably well understood and large-scale manufacturing processes for these coatings are gradually being automated. Pack cementation and related diffusion coatings serve well for most aircraft engine applications. The trend for industrial and marine engines is more toward the use of overlay coatings because of the greater ease of designing these to meet a wide variety of corrosion conditions.

The hot corrosion resistance of several protective coatings that had been applied to MAR-M-509 nozzle guide vanes and exposed in a utility gas turbine has been evaluated. The coatings included basic aluminide, rhodium-aluminide, platinum-rhodium-aluminide, and palladium-aluminide diffusion coatings, and cobalt-chromium-aluminum-yttrium (CoCrAlY) and ceramic overlay coatings. A combination of metallographic examination of vane cross sections and energy dispersive X-ray analysis (EDS) was employed in the evaluation. The results showed that none of the coatings was totally resistant to corrosive attack. The CoCrAlY and platinum-rhodium-aluminide coatings exhibited the greatest resistance to hot corrosion. The CoCrAlY coated vanes were, however, susceptible to thermal fatigue cracking. Except for the poor performance of the palladium-aluminide coating, the precious metal aluminides offered the best protection against corrosion. Hot isostatically pressing coatings was not found to be beneficial, and in one case appeared detrimental.

This paper describes an experimental heat transfer investigation around the leading edge of a high-pressure film-cooled gas turbine rotor blade. The measurements were performed in the VKI isentropic compression tube facility using platinum thin film gauges painted on a blade made of machinable glass ceramic. Free-stream to wall temperature ratio, Reynolds, and Mach numbers were selected from actual aeroengines conditions. Heat transfer data obtained without and with film cooling in a stationary frame are presented. The effects of coolant to free-stream mass weight ratio and temperature ratio were successively investigated. Heat transfer modifications due to incidence angle variations were interpreted with the aid of inviscid flow calculation methods.

In the continuing quest for increased turbine efficiency, the part played by blade profile shape remains crucial. Three turbine vanes with successively increased aerodynamic loading were tested in the High Speed Cascade Wind Tunnel at DFVLR Braunschweig. In addition to wake traverses, measurements of the boundary layer behavior were made. These consisted of (1) use of a constant temperature anemometer to measure the fluctuating heat transfer rate on an array of thin film platinum thermometers deposited on the vanes and (2) a flattened, traversing pitot probe held against the vane surface. Transition measurement by these techniques is described.