Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview

The processes of passive oxidation, deposit-induced corrosion, active oxidation, scale/substance interactions, and scale volatility are presently studied in the case of high-purity SiC and Si3N4 in pure oxygen, giving attention to such secondary elements in the ceramics as water and CO2 oxidants, combustion environment impurities, and thermal cycling. Deposit-induced corrosion is discussed for the cases of NaSO4 as well as vanadate and oxide-slag deposits; issues associated with the active-to-passive oxidation transition are noted.

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Corrosion of Silicon-Based Ceramics in Combustion Environments

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 UHTCperform 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 vol.% SiC, ZrB2 plus 14 vol.% SiC plus 30 vol.% C, and SCS-9a SiC fiber reinforced ZrB2 plus 20 vol.% SiC. The poor oxidation resistance of UHTCs is the predominant factor limiting their applicability to propulsion applications. # 2002 Elsevier Science Ltd. All rights reserved.

Evaluation of ultra-high temperature ceramics foraeropropulsion use

Rare earth silicates have been investigated to evaluate their potential as an advanced environmental barrier coating (EBC) having higher temperature capability than current EBCs. Volatility data in high steam environments indicate that rare earth monosilicates (RE 2 SiO 5 ; RE: rare earth element) have lower volatility than the current barium strontium aluminum silicate (BSAS) EBC top coat in combustion environments. Rare earth silicates also exhibit superior chemical compatibility compared to BSAS. The superior chemical compatibility and low volatility are key attributes to achieve higher temperature capability. The chemical compatibility is especially critical for EBCs on Si 3 N 4 because of the high chemical reactivity of some of the oxide additives in Si 3 N 4 . In simulated combustion environments, EBCs with a rare earth monosilicate top coat exhibit superior temperature capability and durability compared to the current state-of-the art EBCs with a BSAS top coat.

Rare earth silicate environmental barrier coatings for SiC/SiC composites and Si3N4 ceramics

Accident tolerant fuel cladding development: Promise, status, and challenges

Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3-2SiO2) or mullite + BSAS (1-xBaO-xSrO-Al2O3-2SiO2) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, silica volatility, phase stability, and thermal conductivity. Variants of this family of EBCs were applied to monolithic SiC and melt infiltrated SiC/SiC composites. Reaction between BSAS and silica results in low melting (approx. 1300 C) glasses at T > 1400 C, which can cause the spallation of the EBC. At temperatures greater than 1400 C, the BSAS top coat also degrades by formation of a porous structure, and it suffers significant recession via silica volatilization in water vapor-containing atmospheres. All of these degradation mechanisms can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. There was evidence of rapid sintering at temperatures as low as 1300 C, as inferred from the sharp increase in thermal conductivity.

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Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

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 Recession Due to Sio2 Scale Volatility Under Combustion Conditions: Part 2; Thermodynamics and Gaseous Diffusion Model

A high-pressure sampling mass spectrometer was used to detect the volatile species formed from SiO2 at temperatures between 1200C and 1400C in a flowing water vapor/oxygen gas mixture at 1 bar total pressure. The primary vapor species identified was Si(OH)4. The fragment ion Si(OH)3+,' was observed in quantities 3 to 5 times larger than the parent ion Si(OH)4+. The Si(OH)3+ intensity was found to have a small temperature dependence and to increase with the water vapor partial pressure as expected. In addition, SiO(OH)+ believed to be a fragment of SiO(OH)2, was observed. These mass spectral results were compared to the behavior of silicon halides.

Mass Spectrometric Identification of Si–O–H(g) Species from the Reaction of Silica with Water Vapor at Atmospheric Pressure

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 ...

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Dennis S. Fox

Papers

Rare earth silicate environmental barrier coatings for SiC/SiC composites and Si3N4 ceramics

Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

Sic Recession Due to Sio2 Scale Volatility Under Combustion Conditions: Part 2; Thermodynamics and Gaseous Diffusion Model

Mass Spectrometric Identification of Si–O–H(g) Species from the Reaction of Silica with Water Vapor at Atmospheric Pressure

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