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Opila, Elizabeth J.

Publications and source records attributed to Opila, Elizabeth J..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Evaluation of Ultra-High Temperature Ceramics for Propulsion Application

Ultra-high temperature ceramic (UHTC) materials consisting of zirconium diboride or hafnium diboride plus silicon carbide offer a combination of properties that make them candidates for airframe leading edges on sharp-bodied reentry vehicles. These UHTC perform well in the environment for such applications, i.e. low pressure air. The purpose of this study was to examine three of these materials under conditions more representative of a propulsion environment, i.e. higher oxygen and total pressure. Results of stren$h and fracture toughness measurements, furnace oxidation, and thermal shock exposures are presented. The poor oxidation resistance of UHTCs is the predominant factor limiting their applicability to propulsion.

Levine, Stanley R.↗
NASA NTRS2001

Thermodynamics of Volatile Silicon Hydroxides Studied

Silicon-based ceramics are promising candidate structural materials for heat engines. The long-term stability of these materials to environmental degradation is dependent on the formation and retention of a protective SiO2 layer. It is well known that SiO2 forms stable volatile hydroxides in the presence of water vapor at elevated temperatures. Combustion conditions, which characteristically are at high velocities, contain significant water vapor pressures, and high temperatures tend to promote continuous formation of these hydroxides with resulting material degradation. For the degradation of silicon-based ceramics to be predicted, accurate thermodynamic data on the formation of silicon hydroxides are needed.

Copland, Evan H.↗
NASA NTRS2001

Oxidation of ZrB2-SiC

In this paper the oxidation behavior of ZrB2-20 vol% SiC is examined. Samples were exposed in stagnant air in a zirconia furnace (Deltech, Inc.) at temperatures of 1327, 1627, and 1927 C for ten ten-minute cycles. Samples were removed from the furnace after one, five, and ten cycles. Oxidized material was characterized by mass change when possible, x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Oxidation kinetics, oxide scale development, and matrix recession were monitored as a function of time and temperature. Oxidation and recession rates of ZrB2 - 20 vol% SiC were adequately modeled by parabolic kinetics. Oxidation rates of this material are rapid, allowing only very short-term application in air or other high oxygen partial pressure environments.

Opila, Elizabeth J.↗
NASA NTRS2001

Evaluation of Ultra-High Temperature Ceramics for Aeropropulsion Use

Among the ultra-high temperature ceramics (UHTC) are a group of materials consisting of zirconium diboride or hafnium diboride plus silicon carbide, and in some instances, carbon. These materials offer a good combination of properties that make them candidates for airframe leading edges on sharp-bodied reentry vehicles. These UHTC perform well in the environment for such applications, i.e., air at low pressure. The purpose of this study was to examine three of these materials under conditions more representative of a propulsion environment, i.e., higher oxygen partial pressure and total pressure. Results of strength and fracture toughness measurements, furnace oxidation and high velocity thermal shock exposures are presented for ZrB2 plus 20 volume % SiC, ZrB2 plus 14 volume % SiC plus 30 volume % C, and SCS-9a SiC fiber reinforced ZrB2 plus 20 volume % SiC. The poor oxidation resistance of UHTCs is the predominant factor limiting their applicability to propulsion applications.

Levine, Stanley R.↗
NASA NTRS2001

Thermodynamics of Silicon-Hydroxide Formation in H2O Containing Atmospheres

The formation of volatile silicon-hydroxide species from SiO2 in water containing atmospheres has been identified as a potentially important mode of degradation of Si-based ceramics. Availability of thermodynamic data for these species is a major problem. This study is part of an ongoing effort to obtain reliable, experimentally determined thermodynamic data for these species. The transpiration method was used to measure the pressure of Si-containing vapor in equilibrium with SiO2 (cristobalite) and Ar + H2O(g) with various mole fractions of water vapor, X(sub H2O), at temperatures ranging from 1000 to 1780 K. Enthalpies and entropies for the reaction, SiO2(s) + 2H2O(g) = Si(OH)4(g), were obtained, at X(sub H2O) = 0.15 and 0.37, from the variation of lnK with 1/T according to the 'second law method'. The following data were obtained: delta(H)deg = 52.9 +/- 3.7 kJ/mole and delta(S)deg = -68.6 +/- 2.5 J/mole K at an average temperature of 1550 K, and delta(H)deg = 52.5+/-2.0 kJ/mole and delta(S)deg= -69.7 +/- 1.5 J/moleK at an average temperature of 1384 K, for X(sub H2O)= 0.15 and 0.37, respectively. These data agree with results from the literature obtained at an average temperature of 1600 K, and strongly suggest Si(OH)4(g) is the dominant vapor species. Contradictory results were obtained with the determination of the dependence of Si-containing vapor pressure on the partial pressure of water vapor at 1187 and 1722 K. These results suggested the Si-containing vapor could be a mixture of Si(OH)4 + SiO(OH)2. Further pressure dependent studies are in progress to resolve these issues.

Copland, Evan↗
NASA NTRS2000

Ceramic Matrix Composites (CMC) Life Prediction Method Development

Advanced launch systems (e.g., Reusable Launch Vehicle and other Shuttle Class concepts, Rocket-Based Combine Cycle, etc.), and interplanetary vehicles will very likely incorporate fiber reinforced ceramic matrix composites (CMC) in critical propulsion components. The use of CMC is highly desirable to save weight, to improve reuse capability, and to increase performance. CMC candidate applications are mission and cycle dependent and may include turbopump rotors, housings, combustors, nozzle injectors, exit cones or ramps, and throats. For reusable and single mission uses, accurate prediction of life is critical to mission success. The tools to accomplish life prediction are very immature and not oriented toward the behavior of carbon fiber reinforced silicon carbide (C/SiC), the primary system of interest for a variety of space propulsion applications. This paper describes an approach to satisfy the need to develop an integrated life prediction system for CMC that addresses mechanical durability due to cyclic and steady thermomechanical loads, and takes into account the impact of environmental degradation.

Levine, Stanley R.↗
NASA NTRS2000

The Paralinear Oxidation of SiC in Combustion Environments

SiC is proposed for structural applications in high pressure, high temperature. high gas velocity environments of turbine and rocket engines. These environments are typically composed of complex gas mixtures containing carbon dioxide, oxygen, water vapor, and nitrogen. It is known that the primary oxidant for SiC in these environments is water vapor.

Opila, Elizabeth J.↗
NASA NTRS2000

Paralinear Oxidation of CVD SiC in Simulated Fuel-Rich Combustion

The oxidation kinetics of CVD SiC were measured by thermogravimetric analysis (TGA) in a 4H2 (central dot) 12H2O (central dot) 10CO (central dot) 7CO2 (central dot) 67N2 gas mixture flowing at 0.44 cm/s at temperatures between 1300 and 1450 C in fused quartz furnace tubes at I atm total pressure. The SiC was oxidized to form solid SiO2. At less than or = 1350 C, the SiO2 was in turn volatilized. Volatilization kinetics were consistent with the thermodynamic predictions based on SiO formation. These two simultaneous reactions resulted in overall paralinear kinetics. A curve fitting technique was used to determine the linear and parabolic rate constants from the paralinear kinetic data. Volatilization of the protective SiO2 scale resulted in accelerated consumption of SiC. Recession rates under conditions more representative of actual combustors were estimated from the furnace data.

Fox, Dennis S.↗
NASA NTRS2000

Corrosion Issues for Ceramics in Gas Turbines

The requirements for hot-gas-path materials in gas turbine engines are demanding. These materials must maintain high strength and creep resistance in a particularly aggressive environment. A typical gas turbine environment involves high temperatures, rapid gas flow rates, high pressures, and a complex mixture of aggressive gases. Figure 26.1 illustrates the requirements for components of an aircraft engine and critical issues [1]. Currently, heat engines are constructed of metal alloys, which meet these requirements within strict temperature limits. In order to extend these temperature limits, ceramic materials have been considered as potential engine materials, due to their high melting points and stability at high temperatures. These materials include oxides, carbides, borides, and nitrides. Interest in using these materials in engines appears to have begun in the 1940s with BeO-based porcelains [2]. During the 1950s, the efforts shifted to cermets. These were carbide-based materials intended to exploit the best properties of metals and ceramics. During the 1960s and 1970s, the silicon-based ceramics silicon carbide (SiC) and silicon nitride (Si3N4) were extensively developed. Although the desirable high-temperature properties of SiC and Si3N4 had long been known, consolidation of powders into component-sized bodies required the development of a series of specialized processing routes [3]. For SiC, the major consolidation routes are reaction bonding, hot-pressing, and sintering. The use of boron and carbon as additives which enable sintering was a particularly noteworthy advance [4]. For Si3N4 the major consolidation routes are reaction bonding and hot pressing [5]. Reaction-bonding involves nitridation of silicon powder. Hot pressing involves addition of various refractory oxides, such as magnesia (MgO), alumina (Al2O3), and yttria (y2O3). Variations on these processes include a number of routes including Hot Isostatic Pressing (HIP), gas-pressure sintering, sinter-HIPing, and Encapsulation-HIPing. It is important to note that each process involves the addition of secondary elements, which later were shown to dramatically influence oxidation and corrosion behavior. As dense bodies of silicon-based ceramics became more readily available, their desirable high temperature properties were confirmed. These materials retained strength to very high temperatures (i.e. 1300-1400 C). Further, they were lightweight and made from abundant materials. SiC and Si3N4 therefore emerged as leading ceramic candidates for components in heat engines, designed to operate at higher temperatures for better performance and fuel efficiency. The first US programs for ceramics in heat engines have been reviewed [6]. Selected programs on ceramic engine parts are summarized here in regard to their contributions to understanding the corrosion behavior of a heat engine environment.

Jacobson, Nathan S.↗
NASA NTRS2000

Oxidation and Corrosion of Ceramics and Ceramic Matrix Composites

Ceramics and ceramic matrix composites are candidates for numerous applications in high temperature environments with aggressive gases and possible corrosive deposits. There is a growing realization that high temperature oxidation and corrosion issues must be considered. There are many facets to these studies, which have been extensively covered in some recent reviews. The focus of this paper is on current research, over the past two years. In the authors' view, the most important oxidation and corrosion studies have focused on four major areas during this time frame. These are; (I) Oxidation of precursor-based ceramics; (II) Studies of the interphase material in ceramic matrix composites; (III) Water vapor interactions with ceramics, particularly in combustion environments; and (IV) Development of refractory oxide coatings for silicon-based ceramics. In this paper, we shall explore the most current work in each of these areas.

Jacobson, Nathan S.↗
NASA NTRS1999

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

Silicon carbide (SiC) and Si3N4 materials were tested in various turbine engine combustion environments chosen to represent either conventional fuel-lean or fuel-rich mixtures proposed for high-speed aircraft. Representative chemical vapor-deposited (CVD), sintered, and composite materials were evaluated by furnace and high-pressure burner rig exposures. Although protective SiO2 scales formed in all cases, the evidence presented supports a model based on 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 thus used to map SiO2 volatility (and SiC recession) over a range of temperatures, pressures, and velocities. The functional dependency of material recession (volatility) that emerged followed the form A[exp(-Q / RT)](P(sup x)v(sup y). These empirical relations were compared with rates predicted from the thermodynamics of volatile SiO and SiOxHy reaction products and a kinetic model of diffusion through a moving boundary layer. For typical combustion conditions, recession of 0.2 to 2 micrometers/hr is predicted at 1200 to 1400 C, far in excess of acceptable long-term limits.

Smialek, James L.↗
NASA NTRS1999

The Oxidation Rate of SiC in High Pressure Water Vapor Environments

CVD SiC and sintered alpha-SiC samples were exposed at 1316 C in a high pressure burner rig at total pressures of 5.7, 15, and 25 atm for times up to 100h. Variations in sample emittance for the first nine hours of exposure were used to determine the thickness of the silica scale as a function of time. After accounting for volatility of silica in water vapor, the parabolic rate constants for Sic in water vapor pressures of 0.7, 1.8 and 3.1 atm were determined. The dependence of the parabolic rate constant on the water vapor pressure yielded a power law exponent of one. Silica growth on Sic is therefore limited by transport of molecular water vapor through the silica scale.

Opila, Elizabeth J.↗
NASA NTRS1999

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(-QIRT) * P(exp x) * v(exp y). These empirical relations were compared to rates predicted from the thermodynamics of volatile SiO and SiO(sub x)H(sub Y) reaction products and a kinetic model of diffusion through a moving, boundary layer. For typical combustion conditions, recession of 0.2 to 2 micron/h is predicted at 1200- 1400C, far in excess of acceptable long term limits.

Smialek, James L.↗
NASA NTRS1999

Corrosion of Ceramic Materials

Non-oxide ceramics are promising materials for a range of high temperature applications. Selected current and future applications are listed. In all such applications, the ceramics are exposed to high temperature gases. Therefore it is critical to understand the response of these materials to their environment. The variables to be considered here include both the type of ceramic and the environment to which it is exposed. Non-oxide ceramics include borides, nitrides, and carbides. Most high temperature corrosion environments contain oxygen and hence the emphasis of this chapter will be on oxidation processes.

Opila, Elizabeth J.↗