Electronic materials
`Edited by C.N.R. Rao and V. Narayanamurti
`
`Ceramics, composites and intergrowths
`Edited by Frank Zok and Alan Atkinson
`
`Polymers
`Edited by J.E. Mark and B.D. Viers
`
`1
`
`UTC 2021
`General Electric v. United Technologies
`IPR2016-01289
`
`

`

`Current Opinion in Solid State & Materials Science
`Vol 5 No 4 August 2001
`Anthony K Cheetham usA, Sir John Meurig Thomas uK, Editors
`
`Editorial Board
`
`Alexis T Bell USA
`Alexander M Bradshaw Germany
`C Jeffrey Brinker USA
`Robert W Cahn UK
`C Richard A Catlow UK
`David E Cox USA
`EA Davis UK
`Mark E Davis USA
`Harry G Drickamer USA
`Hellmut Eckert Germany
`Stephen R Elliott uK
`Gerhard Ertl Germany
`Anthony G Evans usA
`Gerard Ferey France
`Richard H Friend UK
`Pratibha L Gai
`WA Gambling UK
`G Neville Greaves uK
`Jerzy Haber Poland
`Masaki Hasegawa Japan
`Larry L Hench USA
`Archie Howie UK
`Colin J Humphreys UK
`Hiroo lnokuchi Japan
`Hiizu lwamura Japan
`Bruce A Joyce UK
`Makoto Kikuchi Japan
`Jacek Klinowski UK
`Helmut Knozinger Germany
`Haruo Kuroda Japan
`Stephen Mann uK
`
`Yusei Maruyama Japan
`Ian E Maxwell Netherlands
`Yoshimasa Murata Japan
`Venkatesh Narayanamurti USA
`Robert E Newnham USA
`John M Newsam USA
`Bruce M Novak USA
`Stephen Payne USA
`JB Pendry UK
`Alexander Pines usA
`CNR Rao India
`Bernard Raveau France
`M Wyn Roberts UK
`Arndt Simon Germany
`Raymond E Smallman UK
`Naohiro Soga Japan
`Hans W Spiess Germany
`Brian CH Steele uK
`J Fraser Stoddart UK
`A Marshall Stoneham UK
`Galen D Stucky USA
`Ulrich w Suter Switzerland
`David A Tirrell USA
`Rutgar A van Santen Netherlands
`Ian M Ward UK
`John W White Australia
`George M Whitesides usA
`Fred Wudl USA
`Adriano Zecchina Italy
`Jeffrey I Zink USA
`
`2001 Contents
`
`The subject of solid state and materials science is
`divided into thirteen major sections, each of which
`is reviewed once a year. Each issue contains two
`or three of the major sections, and the amount of
`space devoted to each section is related to its
`importance.
`
`Amorphous Materials
`Edited by S Elliot and Craig Taylor
`
`Biomaterials
`Edited by John Hunt and Molly Shoichet
`
`Ceramics, Composites and lntergrowths
`Edited by Frank Zok and Alan Atkinson
`
`Characterization
`Edited by Manfred Ruehle and Hellmut Eckert
`
`Electronic Materials
`Edited by Ram Rao and Venky Narayanamurti
`
`Metals and Alloys
`Edited by Robert Cahn and Masaharu Yamaguchi
`
`Modelling
`Edited by Richard Catlow and Prof. Alistair Cormack
`
`Molecular Crystals
`Edited by Guatam Desiraju and Juerg Hulliger
`
`Optical and Magnetic Materials
`Edited by Dan Hewak
`
`Polymers
`Edited by James Mark and Brent Viers
`
`Solid Catalysts and Porous Solids
`Edited by Pratibha L Gai and Mike Anderson
`
`Surface Science
`Edited by Wyn Roberts and Sir John Meurig Thomas
`
`Synthesis and Reactivity
`Edited by Gerard Ferey and Allan Jacobson
`
`Pergamon
`
`2
`
`

`

`Current Opinion in Solid State & Materials Science
`
`Publication information: Current Opinion in
`Solid State & Materials Science (ISSN 1359-
`0286). For 2001, volume 5 is scheduled for
`publication. Subscription prices are available
`upon request from the Publisher or from the
`Regional Sales Office nearest you or from this
`journal's website (http:/ / www.elsevier.com/
`locate/cossms). Further information is available
`on this journal and other Elsevier Science
`products through Elsevier's website: (http:/
`/ www.elsevier. com). Subscriptions are accepted
`on a prepaid basis only and are entered on a
`calendar year basis. Issues are sent by standard
`mail (surface within Europe, air delivery outside
`Europe). Priority rates are available upon
`request. Claims for missing issues should be
`made within six months of the date of dispatch.
`
`© 2001 Elsevier Science Ltd. All rights
`reserved .
`This journal and the individual contributions
`contained in it are protected under copyright by
`Elsevier Science Ltd. and the following terms
`and conditions apply to their use:
`
`Photocopying
`Single photocopies of single articles may be
`made for personal use as allowed by national
`copyright laws. Permission of the Publisher and
`payment of a fee is required for all other
`photocopying, including multiple or systematic
`copying, copying for advertising or promotional
`purposes, resale, and all forms of document
`delivery. Special rates are available for
`educational institutions that wish to make
`photocopies for non-profit educational classroom
`use.
`Permissions may be sought directly from Global
`Rights Dept, Elsevier Science, P.O. Box 800,
`Oxford OX5 1DX, UK; phone: (+ 44) 1865
`843830, fax: ( + 44) 1865 853333, e-mail:
`permissions@elsevier.co.uk. You may also
`contact Global Rights directly through Elsevier's
`home page (http://www.elsevier.com), by
`selecting 'Obtaining Permissions'.
`In the USA, users may clear permissions and
`
`make payments through the Copyright
`Clearance Center, Inc., 222 Rosewood Drive,
`Danvers, MA 01923, USA; phone: ( + 1) (978)
`7508400, fax: (+1) (978) 7504744, and in the
`UK through the Copyright Licensing Agency
`Rapid Clearance Service (CLARCS) , 90
`Tottenham Court Road, London W1 P OLP, UK;
`phone: ( + 44) 20 7631 5555;
`fax: (+ 44) 20 7631 5500. Other countries may
`have a local reprographic rights agency for
`payments.
`
`Derivative Works
`
`Subscribers may reproduce tables of contents or
`prepare lists of articles including abstracts for
`internal circulation within their institutions.
`Permission of the Publisher is required for
`resale or distribution outside the institution.
`
`Permission of the Publisher is required for all
`other derivative works, including compilations
`and translations.
`
`Electronic Storage or Usage
`
`Permission of the Publisher is required to store
`or use electronically any material contained in
`this journal, including any article or part of an
`article.
`
`Except as outlined above, no part of this
`publication may be reproduced, stored in a
`retrieval system or transmitted in any form or by
`any means, electronic, mechanical,
`photocopying, recording or otherwise, without
`prior written permission of the Publisher.
`
`Address permissions requests to: Elsevier
`Science Rights & Permissions Department, at
`the mail, fax and e-mail addresses noted above.
`
`Notice
`
`No responsibility is assumed by the Publisher
`for any injury and/or damage to persons or
`property as a matter of products liability,
`
`negligence or otherwise, or from any use or
`operation of any methods, products, instructions
`or ideas contained in the material herein.
`Because of rapid advances in the medical
`sciences, in particular, independent verification
`of diagnoses and drug dosages should be
`made.
`
`Although all advertising material is expected to
`conform to ethical (medical) standards, inclusion
`in this publication does not constitute a
`guarantee or endorsement of the quality or
`value of such product or of the claims made of
`it by its manufacturer.
`Author enquiries
`
`For enquiries relating to the submission of
`articles (including electronic submission), the
`status of accepted articles through our Online
`Article Status Information System (OASIS),
`author Frequently Asked Questions and any
`other enquiries relating to Elsevier Science,
`please consult http://www.elsevier.com/locate/
`authors /
`For specific enquiries on the preparation of
`electronic artwork, consult http:/
`/www .elsevier.com I locate/ authorartwork/
`Contact details for questions arising after
`acceptance of an article, especially those
`relating to proofs, are provided when an article
`is accepted for publication.
`
`Current Opinion in Solid State & Materials
`Science is indexed and/or abstracted by
`Advanced Polymers Abstracts, Cambridge
`Scientific Abstracts (Materials Information)
`Metadex, Ceramics Technology Digest,
`Chemical Abstracts Services, Chemistry Citation
`Index, Composites Industry Abstracts, Current
`Contents (Physical, Chemical & Earth Sciences,
`Engineering, Computing & Technology),
`Engineered Materials Abstracts, Fluid Abstracts
`(Process Engineering, Fluid Abstracts, Civil
`Engineering and FLUIDEX), lnspec Information
`Services, Materials Science Citation Index, Metal
`Abstracts, SciSearch.
`
`Publishing Office: Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington, Oxford OX5 1 GB, UK.
`Advertising information: Advertising orders and enquiries can be sent to: USA, Canada and South America: Mr Tino de Carlo, The Advertising Department, Elsevier Science Inc.,
`655 Avenue of the Americas, New York, NY 10010-5107, USA; phone: (+1) (212) 633 3815; fax: (+1) (212) 633 3820; e-mail: t.decarlo@elsevier.com. Japan: The Advertising
`Department, Elsevier Science K.K., 9-15 Higashi-Azabu 1-chome, Minato-ku, Tokyo 106-0044, Japan; phone: (+81) (3) 5561 5033; tax: (+81) (3) 5561 5047. Europe and ROW:
`Rachel Leveson-Gower, The Advertising Department, Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK; phone: (+44) (1865) 843565; fax: (+44)
`(1865) 843976; e-mail: r.leveson-gower@elsevier.co.uk
`Orders, claims, and product enquiries: please contact the Customer Support Department at the Regional Sales Office nearest you:
`New York: Elsevier Science, PO Box 945, New York, NY t 0159-0945, USA; phone: ( + 1) (212) 633 3730 [toll tree number for North American customers: 1-888-4E5-INFO (437-4636)];
`fax: (+1) (212) 633 3680; e-mail: usinfo-f@elsevier.com
`Amsterdam: Elsevier Science, PO Box 211 , 1000 AE Amsterdam, The Netherlands; phone: (+31) 20 4853757; fax: (+31) 20 4853432; e-mail: nlinfo-f@elsevier.nl
`Tokyo: Elsevier Science, 9-15 Higashi-Azabu 1-chome, Minato-ku, Tokyo 106-0044, Japan; phone: (+81) (3) 5561 5033; fax: (+81) (3) 5561 5047; e-mail: info@eJsevier.co.jp
`Singapore: Elsevier Science, No. 1 Temasek Avenue, # 17-01 Millenia Tower, Singapore 039192; phone: (+65) 434 3727; fax: (+ 65) 337 2230; e-mail: asiainfo@elsevier.com.sg
`Rio de Janeiro: Elsevier Science, Rua Sele de Setembro 111 / 16 Andar, 20050-002 Centro, Rio de Janeiro-RJ, Brazil; phone: (+55) (21) 509 5340; fax: (+55) (21) 5071 991; e-mail:
`elsevier@campus.com.br [Note (Latin America): for orders, claims and help desk information, please contact the Regional Sales Office in New York as listed above]
`
`Author enquiries
`
`For enquiries relating to the submission of articles (including electronic submission), the status of accepted articles through our Online Article Status Information System (OASIS), author
`Frequently Asked Questions and any other enquiries relating to Elsevier Science, please consult http://www.elsevier.com/Jocate/authors/
`
`For specific enquiries on the preparation of electronic artwork, consult http://www.eJsevier.com /locate/authorartwork/
`
`Contact details for questions arising after acceptance of an article, especially those relating to proofs, are provided when an article is accepted for publication.
`Periodicals postage paid at Rahway, NJ. Current Opinion in Solid State & Materials Science (ISSN 1359-0286) is published bimonthly by Elsevier Science Ltd, The Boulevard, Langford
`Lane, K1dhngton, Oxford OX5 1 GB, UK. The annual subscription in the USA is $1247.00.
`Current Opinion in Solid State & Materials Science is circulated by Mercury International Ltd 365 Blair Road Avenel NJ 07001 USA. POSTMASTER: please send address corrections
`to Current Opinion in Solid State & Materials Science, c/o Customer Services, Elsevier S~ience Inc., 655 Avenue '01 the Am~ricas, New York, NY 10010, USA.
`@) The paper used in this publication meets the requirements of ANSI/NISO 239.48-1992 (Permanence of paper)
`
`3
`
`

`

`PERGAMON
`
`Current Opinion in Solid State and Materials Science 5 (2001) 301-309
`
`Current Opinion in
`Solid State &
`Materials Science
`
`Oxidation and corrosion of cerarmcs and ceramic matrix composites
`Nathan S. Jacobson 3 ·*, Elizabeth J. Opila\ Kang N. Leeb
`"NASA Glenn Research Center, Cleveland, OH 44135, USA
`bCleveland State University/NASA Glenn Research Center, Cleveland, OH 44135, USA
`
`Abstract
`
`Ceramics and ceramic matrix composites are candidates for numerous applications in high temperature environments with corrosive
`gases and deposits. High temperature ox.idation and corrosion issues must be considered for these materials. The most important oxidation
`and corrosion studies have recently focused on four major areas. These are: (I) oxidation of precursor-based ceramics, (II) studies of the
`interpbase material in ceramic matrix composites, (ill) water vapor interactions with ceran1ics, particularly in combustion environments,
`(IV) development of refractory oxide coatings for silicon-based ceramics. This paper explores the most current work in each of these
`areas. © 2001 Elsevier Science Ltd. All rights reserved.
`
`1. Introduction
`
`Ceramics and ceramic matrix composites are candidates
`for numerous applications in high temperature environ(cid:173)
`ments with aggressive gases and possible corrosive de(cid:173)
`posits. 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 exten(cid:173)
`sively covered in some recent reviews [**1,**2].
`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, (ill) water vapor
`interactions with ceramics, particularly in combustion
`environments, (IV) development of refractory oxide coat(cid:173)
`ings for silicon-based ceramics. In this paper, we shall
`explore the most cun-ent work in each of these areas.
`
`2. Oxidation of precursor derived ceramics
`
`Precursor-derived ceramics in the system Si-B-C-N
`are promising new candidates for high temperature appli-
`
`'''Corresponding author. Tel. : + l-216-433-5498; fax : + 1-216-433-
`5544.
`E-mail addresses: nathan.s.jacobson@grc.nasa.gov (N.S. Jacobson),
`opila@grc.nasa.gov (E.J. Opila), kang.n.lee@grc.nasa.gov (K.N. Lee).
`
`cations. Some of the outstanding properties attributed to
`these materials include thermal stability of the amorphous
`phase to temperatures as high as 1900°C [*3], a stage III
`creep rate below detectible measurement levels [4], and the
`lowest reported oxidation rates of any non-oxide material
`known to date [*3].
`Much of the work on these materials is focused on
`processing routes from ceramic-precursor materials [5].
`Descriptions of oxidation behavior to date are based on a
`limited number of high temperature exposures in air. It is
`proposed [*3, *6] that a thin amorphous dual layer oxide
`which forms on SiBN 3C when exposed in air at high
`temperatures is responsible for the excellent oxidation
`resistance of this class of materials. Secondary neutral
`mass spectroscopy (SNMS) [*3] as well as energy disper(cid:173)
`sive spectroscopy (EDS) [*6] show that the outer layer is
`composed of silica containing some boron and carbon,
`while the inner layer is a B-N phase containing some
`silicon and oxygen. Thermochemical calculations show
`that BN can exist in equilibrium with Si0 2 at low oxygen
`partial pressures [**7] which is consistent with the SNMS
`and EDS observations. Oxidation rate constants deter(cid:173)
`mined from oxide thickness measurements for SiBN 3 C
`powder oxidized in air at temperatures between 1000 and
`1600°C are reported to be slightly lower those found for
`CVD Si 3N 4in oxygen [*6]. Care must be taken in compar(cid:173)
`ing oxidation rates obtained at different oxygen partial
`pressures. For example, the rate of oxide formation on
`SiBN 3C (kP=3.4Xl0 - 2 (µm/!h based on a 1.3-µm film
`grown in 50 h at 1500°C in air [*3]), is not slower than
`SiC and Si 3N4 as claimed. Pure CVD forms of SiC and
`Si 3N4 yield oxidation rates of 9.5Xl0 - 2 and 1.lXl0- 1
`
`1359-0286/01 /$ - see front matter © 2001 Elsevier Science Ltd. AU rights reserved.
`PII: S 1359-0286(01 )00009-2
`
`4
`
`

`

`302
`
`N.S. Jacobson et al. I Currell/ Opi11io11 in Solid State and Materials Science 5 (2001) 301-309
`
`(µ,m) 2 /h, respectively, at 1500°C in oxygen [8]. Oxidation
`rates of SiBN 3 C would be expected to be about 1.6 X 10- 1
`(µ,m)2 /h at an oxygen partial pressure of 1 atm. At lower
`temperatures, SiBN 3 C oxidation rates are comparable to
`CVD Si 3N.i but lower than CVD SiC. Clearly these
`oxidation rates for SiBN 3C are very low, but entirely
`comparable to pure Si 3N.i in this temperature range.
`The work of Nickel and coworkers [**7,9,10] is the first
`systematic work to assess the oxidation behavior of these
`materials. In an excellent summary [**7] Nickel raises
`some of the key
`issues for the Si-C-B-N system.
`Thermal stability of the nitrogen in the base mateiial is an
`issue. In addition, because these materials are silica
`formers the following problems of silica stability are
`expected to be equally detrimental for the Si-B-C-N
`materials: active oxidation, impurity dominated passive
`oxidation, water-vapor interactions, and hot corrosion. In
`addition, boria is very volatile at high temperatures espe(cid:173)
`cially in water vapor. These issues must be examined with
`careful experimental studies.
`In some preliminary experimental work, Nickel [9] finds
`thin scales in some cases and thick, bubbled scales in other
`cases, sometimes within the same sample. Nickel's work
`demonstrates some of the difficulties in characterizing the
`oxidation behavior of the Si-B-C-N materials. First,
`weight changes due to oxidation are small relative to those
`for SiC and Si 3N4 for the same amount of material
`oxidized. For example when Si 3N4 is oxidized 13 g weight
`gain will be measured for every mole (60 g) of Si0 2
`formed due to concurrent nitrogen loss as a gas. For
`SiBCN 3 , however, only 2 g weight gain will be observed
`for 95 g of Si0 2 + f B 20 3 formed due to concurrent
`nitrogen and carbon losses as gases. In addition, volatiliza(cid:173)
`tion of boria can lead to very low mass gains even for
`thick oxide scale formation. These relatively low weight
`gains suggest much better lower oxidation rates than
`reality. Thus comparisons of oxidation rates on a weight
`basis alone are not valid. Careful microstructural analysis
`of oxide thickness and material recession must be con(cid:173)
`ducted to compare oxidation rates to other silica-formers.
`Second, porosity in the as-pyrolyzed materials leads to
`internal oxidation, again making interpretation of weight
`change results difficult. Statements about relative oxidation
`rates of porous materials are meaningless unless
`the
`porosity is characterized (percent porosity, pore size and
`distribution).
`Third, complete removal of hydrogen dming pyrolysis is
`needed or samples can be destroyed by release of residual
`gases on oxidation. Baldus et al. [* ll] propose that Si-H
`bonds are more prone to oxidation and must be eliminated
`for good oxidation resistance. This can be accomplished by
`pyrolysis at temperatures of at least 1450°C.
`Finally, inhomogeneous materials are a problem, since
`oxidation properties can vary from region to region of
`material. As processing methods are optimized, homoge(cid:173)
`neous materials are expected to become more available,
`enabling systematic oxidation studies.
`
`3. The problem of easily oxidizable phases in
`composites
`
`Fiber-reinforced SiC and Si 3N 4 have numerous desir(cid:173)
`able properties such as high fracture toughness, strength
`retention at high temperatures, good thermal conductivity,
`and light weight. However, nearly all proposed types of
`composites based SiC and Si 3N4 contain some type of
`easily oxidizable second phase such as carbon or boron
`nitride (BN). To achieve the full potential of these
`materials, open cracks are permissible. Yet these very
`cracks provide hot-gas-paths to the readily oxidizable
`phases. Thus recent studies have focused on understanding
`the mechanism of the oxidation of this second phase in
`hopes of eventually mitigating the damage.
`Carbon-fiber reinforced SiC is one leading candidate
`composite [** 12]. Carbon fibers are relatively inexpensive
`and stable in inert atmospheres to very high temperatures.
`However the oxidation of these fibers is a critical issue.
`Initial studies of oxidation of carbon fiber reinforced SiC
`have been done by Lamouroux et al.
`[13]. At low
`temperatures the chemical reaction of oxygen with carbon
`is rate controlling, at intennediate temperatures diffusion
`through the crack is rate controlling, and at the highest
`temperatures the matrix is sealed with silica.
`Recently, Halbig et al. [14] have examined the effect of
`air oxidation on stressed C-fiber/SiC-matrix composites.
`These readily oxidize leading to extensive oxidation in a
`stressed state. Specimens were stressed at 69 and 172 MPa
`from 350 to 1500°C. Only the specimens tested at 550°C
`and below showed time to failure of 25 h or more. The
`specimens
`tested at
`temperatures greater than 550°C
`showed time to failure of 17-142 min. Analytical and
`finite element models are developed to model oxygen
`concentrations through a crack bridged by carbon fibers
`[**15]. Qualitatively these predict a small change in
`oxygen concentration for the diffusion controlled cases and
`a large change in oxygen concentration for a reaction
`controlled case. Fig. 1 illustrates these effects.
`Given the severity of the oxidation problem with carbon
`fiber
`reinforcements,
`some major composite design
`changes are needed. Lamoureux et al. [*16] have sug(cid:173)
`gested a matrix composed of a sequence of B 4 C 1 __ JB 4 C/
`B 4 C 1 __ J SiC layers to improve oxidation resistance via the
`fol1owing:
`
`(a) Crack deflection by these layers generally creates a
`natTow, complex path for entering oxygen
`(b) Sealing of the crack occurs more readily due to
`borosilicate glass formation
`
`Their results do indicate improved performance over SiC
`matrices. However, point (b) must be discussed in more
`detail. Relative to pure silica, the lower viscosity borosili(cid:173)
`cate glass flows more easily and hence 'plugs' any porosity
`or cracks. Such an approach may be useful over the short
`term, but for long-term operation borosilicates tend to lose
`
`5
`
`

`

`Reaction
`
`Starting Matrix
`• 6x6 Fiber Tow
`Array
`
`• • • • • •
`
`750°c
`25 ksi
`
`12so0c
`25 ksi
`
`•
`
`• • • • • • • • • • • •
`Controlled •••••• • • • • • •
`•••••• • • • • • •
`••••••
`•
`•
`•
`•
`• • •
`•
`•
`••••••
`•
`•
`•
`•
`•
`•••• •••
`•
`•
`•
`•
`•
`••••••
`••••••
`••••••
`••••••
`••••••
`••••••
`••••••
`••••••
`••••••
`••••••
`
`Diffusion
`Controlled
`
`• • • • • •
`
`• • • •
`
`• •••
`••••
`
`• • • •
`
`Fig. 1. Cross section illustrations of the oxidation patterns at two stages of oxidation of carbon fiber tows for reaction controlled and diffusion controlled kinetics. The
`microstructures are from 0/90 degree C-fiber/SiC matrix composites under stressed oxidation until failure. The dark gray areas are where a fiber was consumed from oxidation
`and the black areas are voids. (From Ref. [15) reprinted with permission of the American Ceramic Society).
`
`<:
`0
`
`;;;-t
`
`~
`!?..
`
`Q
`::!
`n;
`~
`~
`~:
`;;;·
`~ :g:
`~
`~
`ti 5..
`~
`~
`~-
`;;;-
`g,
`§.
`" "' ...,,
`w
`~
`""" c:,
`......
`L
`%
`
`w
`0 w
`
`6
`
`

`

`304
`
`N.S. Jacobson et al. I Current Opinion in Solid State and Materials Science 5 (2001) 301-309
`
`the boria component due to vaporization of boria. This
`leaves behind pure silica. If a substantial number of cracks
`a.re filled with silica, then the composite is weakened.
`Other composite architectures involve SiC fibers in SiC
`or Si 3N 4 ma.trices. Common SiC fibers become unstable at
`temperatures greater than a.bout 1200°C and hence are not
`desirable. However stoichiometric SiC fibers have been
`developed and as their price decreases, composites of these
`fibers in a SiC or Si 3N 4 matrix are then feasible. In order
`for proper load transfer to the fibers, the fibers cannot bond
`to the matrix. Thus coatings which are non-reactive with
`the matrix are required. These fiber coatings, termed
`'interphases' , are of these three types [**12):
`
`l. Thin layer ( < 1 µ,m) of anisotropic pyrocarbon
`2. Layered refractory oxide -
`magnetoplumbite-type
`oxides and mica-type oxides
`3. Hexagonal boron nitride (BN)
`
`There are a number of critical environmental issues to be
`addressed with ea.ch of these schemes. As discussed,
`carbon oxidizes and is completely removed as a gas.
`Models have been developed for this [17].
`The concept of a layered refractory oxide seems attrac(cid:173)
`tive, but there are only a very limited number of these
`compounds available. Further, it has been pointed out that
`although these oxides may not oxidize further, they are
`poor protective coatings for SiC fibers [18]. Silica has the
`lowest known permeability to oxygen; anything else would
`show faster oxygen transport.
`The third concept is hexagonal boron nitride. BN has a
`number of problems in an oxidizing environment [19,*20].
`Oxidation to B 20/l) starts at about 850°C. This B 20 3(1)
`reacts readily with water vapor to form various HxByOz(g)
`species. Thus in a moisture containing environment, BN
`effectively volatilizes. Less crystalline forms of BN and
`high oxygen content BN appear to be more reactive with
`water vapor. These structural/impurity effects on reactivity
`are important and need to be explored further.
`In a composite with BN fiber coatings, degradation can
`occur via two mechanisms, as illustrated in Fig. 2. BN
`oxidizes to form B 20 3(1), which reacts with any available
`Si0 2 to form borosilicate glass, as shown in Fig. 2a. After
`longer times, the B 20 3 is leached out of the borosilicate by
`reaction with water vapor. Alternatively, BN oxidizes to
`form B 20 3(1), which in turn is volatilized by reaction with
`water, as shown in Fig. 2b. More et al. [21] have observed
`similar effects. They also find that oxygen-containing BN
`leads to SiC attack deeper within the composite.
`The attack of BN interphases has also been modeled by
`several investigators [*20,22; Luthra and Meschter, private
`communication]. As discussed, BN volatilizes and leaves
`behind the anular region shown in Fig. 2b. Simultaneously
`the SiC fiber and matrix walls oxidize and this oxidation is
`enhanced by the presence of boron.
`In summary, easily oxidiza.ble second phases are a
`
`Fig. 2. Micrographs of composites with one edge ground off and exposed
`to oxidizing gases. The top photo (courtesy of Q. Nguyen) shows
`borosilicate glass formation [I 00 h in oxygen at 816°C] and the bottom
`photo shows volatilization due to H-B-O(g) specie formation (100 h in
`humid air at 500°C) (From Ref. (*20] reprinted with permission of the
`American Ceramic Society).
`
`central issue with composites. By the very architecture of
`these materials, the second phases would be exposed to
`high oxygen potentials. In order for these composites to
`reach application, it is essential to address this problem.
`
`4. Water vapor interactions with silicon-based
`ceramics
`
`Silicon-based ceramics and composites are proposed for
`many applications in combustion environments. These
`environments contain substantial amounts of water vapor
`as a product of the combustion of hydrocarbon fuels. It has
`been shown that water vapor plays several roles in the
`degradation of silicon-based ceramics. The first effect is to
`increase the intrinsic oxidation rate of silicon-based materi(cid:173)
`als relative to rates observed in dry oxygen or air. This
`effect is due to the higher solubility of water in the silica
`scale relative to oxygen. The increase in oxidation rates as
`
`7
`
`

`

`N.S. Jacobson et al. I Current Opinion in Solid State and Materials Science 5 (2001) 301-309
`
`305
`
`a function of water vapor partial pressure has been
`quantitatively examined in several recent papers [*23,24].
`It is generally agreed that the oxidation rate is proportional
`to P(H 20) with a power law exponent of one. Results
`obtained experimentally with oxygen as a carrier gas
`should be corrected for the contribution of oxygen to the
`total oxidation process [Meschter, private communication].
`The second effect of water vapor on silicon-based
`ceramics, which is of wide interest in the last few years, is
`the effect of volatility of silica on mate1ial recession rates.
`Silica volatilizes by the following reaction:
`
`(1)
`
`High pressure burner rig studies showed linear recession
`rates of SiC and Si 3N4 due to this effect [25,**26]. A
`paralinear kinetic model was developed to explain these
`results [**26, **27]. It is shown that the linear volatility
`rate of silica and the linear recession rate of silica formers
`(k 1) have the following dependence:
`
`The low velocity, high pressure rig is a useful tool,
`however, for other purposes. This type of rig is useful for
`simulating environmental damage internal to composites
`where gas velocity effects are not important. Another use
`for the high pressure low velocity rig is to screen materials
`which are needed as diffusion barriers to water vapor. Such
`mate1ials may be needed as a barrier layer between silicon(cid:173)
`based mate1ials and environmental barrier coatings, which
`will be discussed in the next section.
`At very high gas velocities experienced by
`turbine
`blades the model shown in Eq. (2) above fails to account
`for recession occurring at the leading edge [*32]. In
`addition, little oxide is found on the material surface
`[Opila, unpublished work]. The lack of surface oxide raises
`questions about the validity of applying Eq. (2) under
`these conditions. A recent paper [*37] maps out the
`pressure/velocity conditions where the paralinear model is
`applicable. Microstrnctural and recession data obtained
`under different pressure and velocity conditions are needed
`to complete this map.
`
`(2)
`
`5. Coatings
`
`This has been substantiated in a number of other burner
`rigs
`[28,29]
`and
`land-based
`turbine
`environments
`[30,**31,*32]. It is very important when trying to simulate
`surface recession in turbine environments that a high gas
`velocity is used, in addition to a high partial pressure of
`water vapor. The volatility effect is barely discernible for a
`gas velocity of 4X 10 - 2 mis (50% water vapor/1 atm total
`pressure/ 1200-1400°C).
`Recent work in a low velocity (5 X 10 - 4 m/ s) high
`pressure (10 atm totaJ pressure, 1.5 atm water vapor) rig
`has shown rapid degradation of silicon-based ceramics but
`by a different mechanism [* *31 ,33,*34,35,*36]. In these
`low velocity conditions the volatility of silica is minimal.
`Because the water vapor partial pressure is high, oxidation
`is enhanced, first, due to the high solubility of water vapor
`in silica as mentioned above, and second, these high
`oxidation rates generate large amounts of gaseous products
`[*23]. The oxide scales become 1iddled with pores as the
`product gases accumulate. The oxidation rate is no longer
`controlled by solid state diffusion of oxidant through the
`silica scale. Instead, many short circuit paths through the
`pores are available for gaseous transport of the oxidant.
`Very thick oxide layers grow, however, they are non(cid:173)
`protective and linear oxidation and recession rates are
`observed [33,35]. In the literature there are cases where the
`degradation of silicon-based materials in low velocity
`conditions are compared to degradation occurring in high
`velocity conditions [**3 1, *34, *36]. It is important to
`recognize that the linear recession in low velocity and high
`velocity environments occurs by two entirely different
`mechanisms and simulations of turbine environments
`require high gas velocities for accurate prediction of
`surface recession.
`
`The use of silicon-based ceramic components in ad(cid:173)
`vanced gas turbine engines depends on the successful
`development of methods to prevent the volatilization of
`silica in water vapor. Application of external environmen(cid:173)
`tal barrier coatings (EBCs) is a promising approach to
`providing such a protection [38]. In addition such coatings
`provide protection against other forms of corrosion, such
`as deposit-induced corrosion.
`Key issues that must be considered in selecting coating
`materials are as follows [*39]: (1) the coating possesses
`environmental durability in water vapor. (2) The coating
`possesses a coefficient of thermal expansion (CTE) close
`to that of the substrate to minimize CTE mismatch stress.
`(3) The coating maintains a stable phase under thermal
`exposure. A volumettic change typically accompanies a
`phase transfo1mation, disrnpting the integrity of the coat(cid:173)
`ing. ( 4) The coating is chem