Dennis S. Fox

SiC Recession Caused by SiO2 Scale Volatility under Combustion Conditions: II, Thermodynamics and Gaseous-Diffusion Model

In combustion environments, volatilization of SiO2 to Si-O-H(g) species is a critical issue. Available thermochemical data for Si-O-H(g) species were used in the present study to calculate boundary-layer-controlled fluxes from SiO2. Calculated fluxes were compared to volatilization rates of SiO2 scales grown on SiC, which were measured in a high-pressure burner rig, as reported in Part I of this paper. Calculated volatilization rates also were compared to those measured in synthetic combustion gas furnace tests. Probable vapor species were identified in both fuel-lean and fuel-rich combustion environments, based on the observed pressure, temperature, and velocity dependencies, as well as on the magnitude of the volatility rate. Water vapor was responsible for the degradation of SiO2 in the fuel-lean environment. SiO2 volatility in fuel-lean combustion environments was attributed primarily to the formation of Si(OH)4(g), with a small contribution of SiO(OH)2(g). Reducing gases such as H2 and/or CO, in combination with water vapor, contributed to the degradation of SiO2 in the fuel-rich environment. The model to describe SiO2 volatility in a fuel-rich combustion environment gave a less satisfactory fit to the observed results. Nevertheless, it was concluded-given the known thermochemical data-that SiO2 volatility in a fuel-rich combustion environment is best described by the formation of SiO(g) at 1 atm total pressure and the formation of Si(OH)4(g), SiO(OH)2(g), and SiO(OH)(g) at higher pressures. Other Si-O-H(g) species, such as Si2(OH)6, may contribute to the volatility of SiO2 under fuel-rich conditions; however, complete thermochemical data are unavailable at this time.

SiC and Si3N4 Recession Due to SiO2 Scale Volatility Under Combustor Conditions

SiC and Si3N4 materials were tested under various turbine engine combustion environments, chosen to represent either conventional fuel-lean or fuel-rich mixtures proposed for high speed aircraft. Representative CVD, sintered, and composite materials were evaluated in both furnace and high pressure burner rig exposure. While protective SiO2 scales form in all cases, evidence is presented to support paralinear growth kinetics, i.e. parabolic growth moderated simultaneously by linear volatilization. The volatility rate is dependent on temperature, moisture content, system pressure, and gas velocity. The burner tests were used to map SiO2 volatility (and SiC recession) over a range of temperature, pressure, and velocity. The functional dependency of material recession (volatility) that emerged followed the form: exp(-Q/RT) * Px * vy. These empirical relations were compared to rates predicted from the thermodynamics of volatile SiO and SiOxHv reaction products and a kinetic model of diffusion through a moving ...

Thermal and Environmental Barrier Coating Development for Advanced Propulsion Engine Systems

Ceramic thermal and environmental barrier coatings (TEBCs) are used in gas turbine engines to protect engine hot-section components in the harsh combustion environments, and extend component lifetimes. Advanced TEBCs that have significantly lower thermal conductivity, better thermal stability and higher toughness than current coatings will be beneficial for future low emission and high performance propulsion engine systems. In this paper, ceramic coating design and testing considerations will be described for turbine engine high temperature and high-heat-flux applications. Thermal barrier coatings for metallic turbine airfoils and thermal/environmental barrier coatings for SiC/SiC ceramic matrix composite (CMC) components for future supersonic aircraft propulsion engines will be emphasized. Further coating capability and durability improvements for the engine hot-section component applications can be expected by utilizing advanced modeling and design tools.

Thermographic characterization of impact damage in SiC/SiC composite materials

SiC/SiC composite materials targeted as turbine components for next generation aero-engines are being investigated at NASA Glenn Research Center. In order to examine damage mechanisms in these materials, SiC/SiC coupons were impacted with 1.59 mm diameter steel spheres at increasing velocities from 115 m/s to 400 m/s. Pulsed thermography, a nondestructive evaluation technique that monitors the thermal response of a sample over time, was utilized to characterize the impact damage. A thermal standard of similar material was fabricated to aid in the interpretation of the thermographic data and to provide information regarding thermography system detection capabilities in 2.4 mm thick SiC/SiC composite materials. Flat bottom holes at various depths with aspect ratios greater than 2.5 were detectable in the thermal images. In addition, the edges of holes at depths of 1.93 mm into the sample were not as resolvable as flat bottom holes closer to the surface. Finally, cooling behavior was characterized in SiC/SiC materials and used to determine impact damage depth within an 8.5% error of a known depth.

Direct Mass Spectrometric Identification of Silicon Oxychloride Compounds

Silicon oxychloride compounds formed during the high temperature reaction of Si and SiC with Cl2/O2 mixtures at 930 and 950 C have been directly observed with an atmospheric pressure mass spectrometer sampling system. Molecules with the formulas Si2OCl6 and Si3OCl8 have been identified. These compounds are very likely formed by a gas phase reaction between the oxygen gas in the system and silicon tetrachloride which is produced by chlorination of the silicon-based material.

The Application of Thermal Diffusivity Imaging to Sic-Fiber-Reinforced Silicon Nitride

Strong, tough, high temperature ceramic matrix composites are currently being developed for application in advanced heat engines. These new materials require new nondestructive inspection and material characterization techniques to insure the final integrity, as well as reduce the time require for development. One of the most promising of these new materials is SiC fiber-reinforced silicon nitride ceramic matrix composite (SiCf/ Si3N4). The high temperature thermal and mechanical performance of ceramic matrix composites is strongly dependent on the thermal diffusivity and interfacial bond strength of the material. Previous work has shown a interaction between the thermal diffusivity and the fracture toughness of SiCf/Si3N4. A thermal imaging technique has been developed to provide rapid large area measurements of the thermal diffusivity perpendicular to the fiber direction in these composites. Results are presented for a series of SiCf/Si3N4 (reaction bonded silicon nitride) composite samples heat-treated under various conditions. These results are compared with previous experimental and analytical work in this field.

Numerical and Experimental Studies of a Film Cooled Pulsed Detonation Tube

The Constant Volume Combustion Cycle Engine (CVCCE) program at the NASA Glenn Research Center is assessing the feasibility of creating a hybrid gas turbine engine. In the hybrid engines under study, the constant pressure combustor is replaced with a pulsed detonative combustor to achieve near constant volume burning and increased thermodynamic efficiency. The design of a practical, long-life pulse detonation combustor requires the design of components that are capable of enduring the severe thermal environment created by repetitive detonations. In the current study, a film cooled superalloy combustor liner test article was designed and manufactured. The temperature and stress distributions along the liner were calculated as a function of the hot gas heat flux using the finite element method for both film cooled and un-cooled cases. Unsteady CFD simulations of the dynamics of cooling flows subjected to a forced-detonation in a hydrogen-air mixture were performed. The design of the practical cooled liner closely followed the results from previous 2-D film cooling CFD analysis and 3-D film cooling CFD analysis described herein. Film cooling effectiveness was demonstrated experimentally on a film cooled combustor section in a repetitive detonative environment on the CVCCE testbed.

Oxidation Behavior of Prospective Silicon Nitride Materials for Advanced Microturbine Applications

Various laboratory tests have shown that high-pressure water vapor environments combined with elevated temperatures and intermediate gas velocities (current facilities limited to about 50 m/s) can cause grain boundary degradation and material recession in silica formers. Recent tests include burner rig testing conducted by NASA [1], Honeywell Engines & Systems [2], Siemens Power Generation [3], CRIEPI in Japan [4, 5], “Keiser rig” testing at Oak Ridge National Laboratory (ORNL) [6], and engine testing in the Allison 501K industrial gas turbine [7]. This paper presents a summary of oxidation test data of candidate silicon nitride materials for advanced microturbine applications. These data are of interest to microturbine component designers in order to determine the limits of safe unprotected component operation with respect to the given turbine environment, as well as to understand the behavior of ceramic microturbine components once local spallation of the protective environmental barrier coating has occurred.This paper intends to give materials and engine development engineers some guidance with respect to the different test facility capabilities and the prevailing oxidation/recession mechanisms to better understand/interprete the oxidation test results when developing new ceramic material compositions and environmental barrier coating systems.Copyright

Thermal Diffusivity Imaging of Ceramic Composites

Strong, tough, high temperature ceramic matrix composites are currently being developed for application in advanced heat engines. One of the most promising of these new materials is a SiC fiber-reinforced silicon nitride ceramic matrix composite (SiCf/Si3N4). The interfacial shear strength in such composites is dependant on the integrity of the fiber’s carbon coating at the fiber-matrix interface. The integrity of the carbon rich interface can be significantly reduced if the carbon is oxidized. Since the thermal diffusivity of the fiber is greater than that of the matrix material, the removal of carbon increases the contact resistance at the interface reducing the thermal diffusivity of the composite. Therefore thermal diffusivity images can be used to characterize the progression of carbon depletion and degradation of the composite. A new thermal imaging technique has been developed to provide rapid large area measurements of the thermal diffusivity perpendicular to the fiber direction in these composites. Results of diffusivity measurements will be presented for a series of SiCf/Si3N4 (reaction bonded silicon nitride) composite samples heat-treated under various conditions. Additionally, the ability of this technique to characterize damage in both ceramic and other high temperature composites will be shown.Copyright