Silica Formers Research Articles

Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y1.2/Yb0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.

High-temperature oxidation behavior of polymer-derived SiHfBCN ceramic nanocomposites

Within this study, the oxidation behavior of SiHfBCN ceramic powders and monoliths was studied at temperatures from 1200 to 1400°C. Both powder and monolithic samples exhibited parabolic oxidation behavior characterized by very low rates (10−9–10−8mg2cm−4h−1). The activation energy of 112.9kJmol−1, which was determined for the SiHfBCN powder, is comparable to that of other silica formers such as silicon or SiC and relates to the diffusion of molecular oxygen through silica scale. Whereas, the values determined for the SiHfBCN ceramic monoliths (174 and 140kJmol−1, depending on the Hf content) indicate the complex nature of their oxidation process, leading at temperatures below 1300°C to a continuous oxide scale consisting of borosilicate, silica, m-and t-HfO2. At higher temperatures, the oxide scale consists of silica, HfSiO4 as well as m-and t-HfO2 and becomes discontinuous, probably due to the evaporation of boria.

Surface Scale Formation on Solid Oxide Fuel Cell Proximal Balance of Plant Components

Abstract Chromium containing alloys used as solid oxide fuel cell (SOFC) interconnects can generate volatile chrome species that deposit as Cr2O3(s) at the SOFC cathode/electrolyte interface under modest current densities (∼0.5 A/cm2). Deposition of chromic oxide at this interface increases overpotential losses, thereby degrading fuel cell performance and efficiency. Balance of plant components have not received attention as a chromium source but can produce volatile Cr species through direct thermal contact with the hot cell stack. In this work, materials representative of BoP component alloys were exposed to dry air at temperatures between 600°C and 800°C for 72 h. The material classes tested include austenitic steel, ferritic steel, alumina formers, silica formers, and a specialty ferritic with elevated alumina and silica content. The surface scales formed on each alloy were identified using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Thin surface scales were formed that included Cr-, Fe-, Al-, and Si-oxides as well as Mn–Cr spinel. The surface composition estimated from the analytical data is used to thermodynamically calculate the abundance of volatile chromium species over the alloys. Using the calculated vapor composition and assumed rate efficiencies, it is possible to calculate the mass of Cr2O3 that will deposit on the SOFC surface. Up to 4.3 cm2 of SOFC active area can be deactivated in 2000 h of operation assuming a 1% efficiency of volatilization of chrome species and a 1% efficiency of deposition of Cr2O3. This is estimated to be approximately 5% of the starting active area of an ∼20 W SOFC. Degradation of SOFC performance is expected to scale almost linearly with stack size.

Oxidation and Volatilization of Silica Formers in Water Vapor

At high temperatures, SiC and Si3N4 react with water vapor to form a SiO2 scale. SiO2 scales also react with water vapor to form a volatile Si(OH)4 species. These simultaneous reactions, one forming SiO2 and the other removing SiO2, are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved. After steady state is achieved, the oxide found on the surface is a constant thickness, and recession of the underlying material occurs at a linear rate. The steady‐state oxide thickness, the time to achieve steady state, and the steady‐state recession rate can be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate also can be determined from parameters that describe a water‐vapor‐containing environment. Accordingly, maps have been developed to show these steady‐state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of SiO2 formers in water‐vapor‐containing environments, such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si3N4.

Accelerated oxidation of SiC CMC's by water vapor and protection via environmental barrier coating approach

Abstract Silicon carbide fiber reinforced silicon carbide matrix composites (SiC/SiC CMC's) are attractive materials for use in gas turbine hot sections due to the potential for high temperature mechanical properties and overall lower density than metals. Potential SiC/SiC CMC gas turbine components include combustion liners, and turbine shrouds, vanes, and blades. Engine design with SiC/SiC CMC's will allow optimization for performance, efficiency, and/or emissions. However, SiC/SiC CMC's are silica formers under oxidizing conditions and have been shown experimentally to undergo accelerated oxidation due to exposure to steam in high temperature combustion environments such as found in the gas turbine hot section. Oxidation by steam in a flowing gas stream has been shown to exhibit paralinear behavior and result in unacceptable recession of the surface. Thus, prior to the successful introduction of SiC/SiC CMC's for long life use in gas turbines, the problem of accelerated oxidation needs to be addressed and resolved. To this end, one approach has been the development of the environmental barrier coating (EBC) to prevent accelerated oxidation by limiting oxidant access to the surface of the silica former. This paper will review the accelerated oxidation of silica formers such as silicon carbide, the experimental testing confirming the problem, and EBC approaches resolving the problem.

Materials for temperatures above 1500°C in oxidizing atmospheres. Part III: Contact reaction tests

The topic Materials for Temperatures above 1500 °C in Oxidizing Atmospheres is discussed in a 3 part publication. In the first part [1] a literature survey had led to two potential material systems consisting either of particle reinforced MoSi 2 or of a MoSi 2 based oxidation resistant coating on creep resistant RSiC. Both systems were verified experimentally in a second publication [2] including the development of optimum oxidation resistant MoSi 2 composite materials and of an optimum coating system on RSiC. The present part deals with the contact corrosion behavior of these silica formers if other refractory materials are used in combination with these material systems as it is often the necessity in practical applications. Therefore, the contact reactions of MoSi 2 with 10 other high temperature oxide based structural materials were investigated at 1600 °C in air. The results are mainly documented by microprobe investigations with respect to the contact reaction products formed. It was found that only two material combinations are conceivable for long-term use in contact with MoSi 2 . The results are in agreement with the considerations discussed in part one of this publication [1].

Materials for temperatures above 1500°C in oxidizing atmospheres. Part I: Basic considerations on materials selection

In the first part of this paper the results of a literature review are presented. Here we provide an overview of the materials that are suitable for long term service in oxidizing atmospheres at temperatures of 1500°C and higher. Criteria are a sufficient stability against the surrounding environment, low vapor pressure of the material or the reaction product formed and sufficient creep resistance. A further aspect is that reactions between contacting materials of different compositions should not lead to the formation of low melting eutectics at the contact areas. The second part reports about results from thermogravimetric measurements on the oxidation resistance of several potential materials. The third part deals with the experimental investigations concerning the “contact-corrosion” of selected high-temperature resistant materials (Al2O3, ZrO2, CeO2, La2O3, Y2O3, HfO2, and the Spinel (MgO · Al2O3)) in direct contact with a SiO2-scale forming material (MoSi2) at 1600°C in air, since most of the potential materials for such high temperatures are silica formers. Parts II and III will appear in the next 2 issues of this journal. Werkstoffe für oxidierende Atmosphären und Temperaturen oberhalb von 1500°C. Teil I: Grundlegende Überlegungen zur Werkstoffauswahl Im ersten Teil des Beitrags werden die Ergebnisse einer Literaturrecherche vorgestellt. In einem Überblick werden Werkstoffe bzw. Phasen diskutiert, die für einen Langzeiteinsatz unter oxidierenden Bedingungen bei Temperaturen von 1500°C und höher geeignet erscheinen. Kriterien sind eine ausreichende Stabilität gegenüber der Umgebung, niedriger Dampfdruck der Werkstoffe oder der Reaktionsprodukte, die mit der Umgebung gebildet werden, und eine ausreichende Kriechfestigkeit. Ein weiterer Aspekt ergibt sich aus der Reaktion beim Kontakt zwischen Werkstoffen unterschiedlicher Zusammensetzung, bei dem sich keine niedrig schmelzenden Eutektika an den Kontaktflächen bilden sollten. Der zweite Teil des Beitrags berichtet über Ergebnisse von thermogravimetrischen Messungen bezüglich der Oxidationsbeständigkeit verschiedener potentieller Werkstoffe. Der dritte Teil behandelt experimentelle Untersuchungen im Hinblick auf „Kontaktkorrosion”︁ ausgewählter Hochtemperaturwerkstoffe (Al2O3, ZrO2, CeO2, La2O3, Y2O3, HfO2 und der Spinell MgO · Al2O3) im direkten Kontakt mit einem SiO2-Deckschichtbildner (MoSi2) bei 1600°C in Luft. Hintergrund dieser Versuche zur „Kontaktkorrosion”︁ ist, daß die meisten potentiellen Werkstoffe für die Anwendung bei solch hohen Temperaturen SiO2-Bildner sind. Die Teile II und III erscheinen in den nächsten beiden Heften.

Subparabolic Oxidation Behavior of Silicon Carbide at 1300°C

The phenomenon of subparabolic oxide growth rate, more the rule than the exception in the oxidation of silica formers, is examined in light of data sets acquired in SiC oxidation experiments at 1300°C. Negative deviation from parabolic kinetics is shown to correlate with oxide crystallinity. The data sets were analyzed to ascertain the predominant and contributing growth laws, and tested with models that describe the overall kinetics as combinations of the familiar growth laws.

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 thermogra‐vimetry (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 experimental results.