Characterizing the Chemical Stability of High Temperature Materials for Application in Extreme Environments

The chemical stability of high temperature materials must be known for use in the extreme environments of combustion applications. The characterization techniques available at NASA Glenn Research Center vary from fundamental thermodynamic property determination to material durability testing in actual engine environments. In this paper some of the unique techniques and facilities available at NASA Glenn will be reviewed. Multiple cell Knudsen effusion mass spectrometry is used to determine thermodynamic data by sampling gas species formed by reaction or equilibration in a Knudsen cell held in a vacuum. The transpiration technique can also be used to determine thermodynamic data of volatile species but at atmospheric pressures. Thermodynamic data in the Si-O-H(g) system were determined with this technique. Free Jet Sampling Mass Spectrometry can be used to study gas-solid interactions at a pressure of one atmosphere. Volatile Si(OH)4(g) was identified by this mass spectrometry technique. A High Pressure Burner Rig is used to expose high temperature materials in hydrocarbon-fueled combustion environments. Silicon carbide (SiC) volatility rates were measured in the burner rig as a function of total pressure, gas velocity and temperature. Finally, the Research Combustion Lab Rocket Test Cell is used to expose high temperature materials in hydrogen/oxygen rocket engine environments to assess material durability. SiC recession due to rocket engine exposures was measured as a function of oxidant/fuel ratio, temperature, and total pressure. The emphasis of the discussion for all techniques will be placed on experimental factors that must be controlled for accurate acquisition of results and reliable prediction of high temperature material chemical stability

Influence of Alumina Reaction Tube Impurities on the Oxidation of Chemically-Vapor-Deposited Silicon Carbide

Pure coupons of chemically vapor deposited (CVD) SiC were oxidized for 100 h in dry flowing oxygen at 1300 C. The oxidation kinetics were monitored using thermogravimetry (TGA). The experiments were first performed using high-purity alumina reaction tubes. The experiments were then repeated using fused quartz reaction tubes. Differences in oxidation kinetics, scale composition, and scale morphology were observed. These differences were attributed to impurities in the alumina tubes. Investigators interested in high-temperature oxidation of silica formers should be aware that high-purity alumina can have significant effects on experiment results

Oxidation of Carbon Fibers in Water Vapor Studied

T-300 carbon fibers (BP Amoco Chemicals, Greenville, SC) are a common reinforcement for silicon carbide composite materials, and carbon-fiber-reinforced silicon carbide composites (C/SiC) are proposed for use in space propulsion applications. It has been shown that the time to failure for C/SiC in stressed oxidation tests is directly correlated with the fiber oxidation rate (ref. 1). To date, most of the testing of these fibers and composites has been conducted in oxygen or air environments; however, many components for space propulsion, such as turbopumps, combustors, and thrusters, are expected to operate in hydrogen and water vapor (H2/H2O) environments with very low oxygen contents. The oxidation rate of carbon fibers in conditions representative of space propulsion environments is, therefore, critical for predicting component lifetimes for real applications. This report describes experimental results that demonstrate that, under some conditions, lower oxidation rates of carbon fibers are observed in water vapor and H2/H2O environments than are found in oxygen or air. At the NASA Glenn Research Center, the weight loss of the fibers was studied as a function of water pressure, temperature, and gas velocity. The rate of carbon fiber oxidation was determined, and the reaction mechanism was identified

Oxidation of high entropy ultra-high temperature ceramics

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Synergistic degradation mechanisms of SiC/BN/SiC in oxidizing environments at intermediate temperatures under load

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Volatile Reaction Products From Silicon-Based Ceramics in Combustion Environments Identified

Silicon-based ceramics and composites are prime candidates for use as components in the hot sections of advanced aircraft engines. These materials must have long-term durability in the combustion environment. Because water vapor is always present as a major product of combustion in the engine environment, its effect on the durability of silicon-based ceramics must be understood. In combustion environments, silicon-based ceramics react with water vapor to form a surface silica (SiO2) scale. This SiO2 scale, in turn, has been found to react with water vapor to form volatile hydroxides. Studies to date have focused on how water vapor reacts with high-purity silicon carbide (SiC) and SiO2 in model combustion environments. Because the combustion environment in advanced aircraft engines is expected to contain about 10-percent water vapor at 10-atm total pressure, the durability of SiC and SiO2 in gas mixtures containing 0.1- to 1-atm water vapor is of interest. The reactions of SiC and SiO2 with water vapor were monitored by measuring weight changes of sample coupons in a 0.5-atm water vapor/0.5-atm oxygen gas mixture with thermogravimetric analysis

Stability of oxides/environmental barrier coating candidate materials in hightemperature, high-velocity steam

Stability of oxides/Environmental Barrier Coating (EBC) candidate materials in high-temperature, high-velocity steam has been characterized using a steam-jet furnace modeled after Lucato et al [1]. The objective of this work is to quantify stability of oxides for use as coatings on SiC-based composites in turbine engine environments with the long term goal of developing thermochemical life prediction models for EBCs. SiO2, TiO2, Y2O3, and rare earth silicates were exposed in one atmosphere steam flowing at approximately 170 m/s at temperatures between 1200 and 1400°C for times up to 375 h. Oxide recession, attributed to formation of volatile metal hydroxides, was measured for SiO2, TiO2 and Y2O3. The SiO2 recession rates were consistent with values predicted assuming loss of material was limited by transport of Si(OH)4(g) through a laminar gas boundary layer. TiO2 single crystal recession was slightly less than SiO2 but too rapid for use in a turbine environment. Y2O3 recession was not measureable within the sensitivity of techniques used here. Y2Si2O7 exposed in the steam-jet furnace was selectively depleted of SiO2 by the reaction: Y2Si2O7 + 2H2O(g) = Y2SiO5 + Si(OH)4(g) (1) A porous surface layer of Y2SiO5 formed after exposure of Y2Si2O7 and was confirmed by X-ray Diffraction Analysis (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS). Key microstructural features observed in addition to the porosity include grain refinement, faceting, and grain fall out. The growth rate of the porous layer decreased with time at 1300°C, although the depletion depth varied significantly across the surface, possibly due to preferred crystallographic orientations for the depletion reaction. The silica depletion depth decreased with increasing temperature. The depletion depths were uniform at 1200°C as shown in Figure 1. At 1400°C the porous surface layers sintered rapidly, closing off paths for water vapor ingress into the material and thus minimizing SiO2 depletion by Reaction (1). Y2SiO5 was significantly more stable than Y2Si2O7. Significant SiO2 depletion of the monosilicate was not observed within the sensitivity of the techniques used here

Oxidation of SiC/BN/SiC Composites in Reduced Oxygen Partial Pressures

SiC fiber-reinforced SiC composites with a BN interphase are proposed for use as leading edge structures of hypersonic vehicles. The durability of these materials under hypersonic flight conditions is therefore of interest. Thermogravimetric analysis was used to characterize the oxidation kinetics of both the constituent fibers and composite coupons at four temperatures: 816, 1149, 1343, and 1538 C (1500, 2100, 2450, and 2800 F) and in oxygen partial pressures between 5% and 0.1% (balance argon) at 1 atm total pressure. One edge of the coupons was ground off so the effects of oxygen ingress into the composite could be monitored by post-test SEM and EDS. Additional characterization of the oxidation products was conducted by XPS and TOF-SIMS. Under most conditions, the BN oxidized rapidly, leading to the formation of borosilicate glass. Rapid initial oxidation followed by volatilization of boria lead to protective oxide formation and further oxidation was slow. At 1538C in 5% oxygen, both the fibers and coupons exhibited borosilicate glass formation and bubbling. At 1538C in 0.1% oxygen, active oxidation of both the fibers and the composites was observed leading to rapid SiC degradation. BN oxidation at 1538C in 0.1% oxygen was not significant

Micro-scale observation of cracking in SiC/BN/SiC ceramic matrix composites

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Borosilicate wetting on ceramic matrix composites and Si-based substrates

The oxidation of Ceramic Matrix Composites (CMCs) is a complex process due to the combined oxidation of ceramic fibers, matrix, and an interphase. In previous work, the oxidation of CVD SiC coated SiC/BN/SiC CMCs with a single uncoated face exposing the fibers, matrix, and interphase was studied at temperatures of 800, 1200, and 1300°C. The thermal oxidation of the exposed face was characterized to understand crack-sealing in CMCs during use. During thermal oxidation, the exposed face of the CMC was sealed by borosilicate glass formation. Oxide droplets were observed to form at BN/SiC interfaces. In this work, stand-alone borosilicate glass cylinders situated on SiC, Si, or SiO2 substrates were heated in air and visualized in situ using a heating microscope. Changes in oxide morphology, volatility, and wetting were characterized as a function of borosilicate and substrate composition. Results from stand-alone glasses were compared with the observations from exposed CMC faces to elucidate mechanisms of composite sealing during thermal oxidation