OURNAL OF THE
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`MAR 11 1999
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`GE-1025.001
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`GE-1025.002
`
`

`
`j(XJRNAL OF THE EUROPEAN CERAMIC SOCIETY
`
`llllll II lllllllllll 111111111111111111111111111111
`0955 -2219(1999)18:16; 1- E
`
`(Abstracted/indexed in: Applied Mechanics Reviews; ASM International/Materials Information;
`Ceramic Abstracts; Current Contents/Engineering, Computing and Technology; Current Contents/
`Physical, Chemical and Earth Sciences; Fluid Engineering Abstracts (BHRA ); FLU/DEX;
`IN SPEC; Ma terials Science Citation Index; Science Citation Index)
`
`CONTENTS
`
`Volume 18 Number 16 1998
`Corrosion of Ceramics, 5th Conference of the European Ceramic Society,
`Versailles, France, 22-26 June 1997
`iii Preface
`2307 Modelling of the oxidation kinetics of a yttria-doped hot-pressed silicon nitride
`F. A. COSTA OLIVEIRA, D. J. BAXTER & J. UNGEHEUER (The Netherlands)
`2313 High oxidation resistance of hot pressed silicon nitride containing yttria and lanthania
`F. MONTEVERDE & A . BELLOSI (Italy)
`2323 Corrosion of a dense, low-additive Si3N4 in high temperature combustion gases
`D. J. BAXTER (The Netherlands), T. GRAZIANI (Italy) , H.-M. WANG &
`R. A. MCCAULEY (USA)
`2331 Hot-corrosion of silicon carbide in combustion gases at temperatures above the dew
`point of salts
`M. CARRUTH , D. BAXTER , F. OLIVEIRA (The Netherlands) & K. COLEY (UK)
`2339 Features of corrosion resistance of AIN- SiC ceramics in air up to 1600°C
`V. A . LAVRENKO (Ukraine) , M. DESMAISON-BRUT (France}, A . D. PANASYUK (Ukraine) &
`J. DESMAISON (France)
`2345 Oxidation protection coatings for C/SiC based on yttrium silicate
`J. D. WEBSTER , M. E. WESTWOOD, F. H. HAYES, R. J. DAY, R. TAYLOR (UK), A . DURAN ,
`M. APARICIO (Spain) , K. REBSTOCK & W. D. VOGEL (Germany)
`2351 Mullite based oxidation protection for SiC- C/C composites in air at temperatures up to
`1900 K
`H. FRITZE , J. JOJIC, T. WITKE , C. RUSCHER (Germany), S. WEBER , S. SCHERRER (France),
`R. WEIB , B. SCHULTRICH & G. BORCHARDT (Germany)
`2365 Hot-corrosion behaviour of silica and silica-formers : external versus internal control
`C. BERTHOLD & K . G. NICKEL (Germany)
`2373 Corrosion of zirconia ceramics in acidic solutions at high pressures and temperatures
`M . SCHACHT, N. BOUKIS, E. DINJUS, K. EBERT, R. JANSSEN , F. MESCHKE &
`N. CLAUSSEN (Germany)
`2377 Corrosion effects of glass on YSZ electrolytes
`C. M. S. RODRIGUES , J. A . LABRINCHA & F. M . B. MARQUES (Portugal)
`2383 Corrosion of ceramics in halogen-containing atmospheres
`D. W. READEY (USA)
`2389 Selection of materials for use at temperatures above 1500°C in oxidizing atmospheres
`K. BUNDSCHUH , M . SCHUZE , C. MULLER , P. GREIL & W. HEIDER (Germany)
`
`This journal is part of ContentsDirect, the free alerting service which sends tables of contents by e-mail
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`www .elsevier.co.jp
`
`ELSEVIER
`
`02036
`
`GE-1025.003
`
`

`
`P I I : S 0 9 5 5 - 2 2 I 9 ( 9 8 ) 0 0 2 4 1 - 6
`
`Journal of the European Ceramic Society 18 (1998) 2345--2350
`© 1998 Elsevier Science Limited
`Printed in Great Britain. All rights reserved
`0955-22 19/99/$--see front matter
`
`Oxidation Protection Coatings for C/SiC Based
`on Yttrium Silicate
`J. D. Webster, a M. E. Westwood,a F. H. Hayes, a* R. J. Day,a R. Taylor,a
`A. Duran,b M. Aparicio,b K. Rebstockc and W. D. Vogelc
`
`"Manchester Materials Science Centre, University of Manchester & UMIST, Grosvenor Street, Manchester Ml 7HS, UK
`hCSIC, Instituto de Ceramica y Vidrio, 28500 Arganda de! Rey, Madrid, Spain
`cDaimler-Benz Aerospace, Dornier Research, Friedrichshafen, Germany
`
`Abstract
`
`The factor which currently precludes the use of car(cid:173)
`bon fibre reinforced silicon carbide (CJ SiC) in high
`temperature structural applications such as gas tur(cid:173)
`bine engines is the oxidation of carbon fibres at
`temperatures greater than 400°C. It is, therefore,
`necessary to develop coatings capable of protecting
`Cf SiC components from oxidation for extended per(cid:173)
`iods at J600°C. Conventional coatings consist of
`multi/ayers of different materials designed to seal
`cracks by forming glassy phases on exposure to
`oxygen . The objective of this work was to develop a
`coating which was inherently crack resistant and
`would, therefore, not require expensive sealing lay(cid:173)
`ers. Yttrium silicate has been shown to possess the
`required properties for use in oxidation protection
`coatings. These requirements can be summarised as
`being low Young 's modulus, low thermal expansion
`coefficient, good erosion resistance, and low oxygen
`permeability. The development of protective coatings
`based on a SiC bonding layer combined with an
`) outer y ttrium silicate erosion resistant layer and
`oxygen barrier is described. Thermodynamic com(cid:173)
`puter calculations and finite element analy sis have
`been used to design the coating. CJ SiC samples have
`been coated using a combination of chemical vapour
`deposition and slip casting. The behaviour against
`oxidation of the coating has been evaluated. © 1998
`Elsevier S cience Limited. All rights reserved
`
`1 Introduction
`
`The last 30 years have seen a steady development
`in the range of ceramic materials with potential for
`high temperature engineering applications. One
`such application is the use of ceramic matrix com-
`
`*To whom correspondence should be addressed.
`
`2345
`
`posites (CMCs) as structural hot section compo(cid:173)
`nents of propulsion systems for the next generation
`of commercial supersonic aircraft. The inclusion of
`these materials in gas turbine engines would allow
`significantly higher operating temperatures than is
`possible with conventional nickel based super(cid:173)
`alloy materials. Such utilisation of CMCs offers
`improvements in thermal efficiency which can be
`translated into decreased specific fuel consumption,
`decreased weight and consequently lower stresses
`in rotating components and higher thrust to weight
`ratios. Raising the operating temperature would
`increase the thermodynamic efficiency of the engine
`and reduce the level of pollutant emission. These
`effects are achieved by moving closer to stoichio(cid:173)
`metric combustion of the fuel and reducing the
`amount of air which must be diverted through the
`turbine components for cooling purposes.
`The combination of high strength and damage
`tolerant fracture behaviour of carbon fibre rein(cid:173)
`forced ceramics makes them candidates for this
`application. In non-oxidising environments, the
`mechanical properties of carbon fibre reinforced
`ceramics are retained to temperatures in excess of
`2000°C.1 The factor which currently inhibits the
`application of such materials in gas turbine engines
`is the oxidation of carbon fibres at temperatures
`above 400°C. To prevent oxidation requires elim(cid:173)
`inating access of oxygen to the carbon fibres.
`Hence there is a need for external coatings capable
`of preventing oxidation of carbon fibre reinforced
`ceramics for extended periods at temperatures in
`the region of 1600°C. Such coatings have been
`reviewed by Westwood et al. 2 These coatings are
`extremely prone to cracking, particularly when
`used under conditions of thermal cycling on a
`CMC substrate with a substantially lower thermal
`expansion coefficient. Thus, conventional coat(cid:173)
`ings consist of multilayers of different materials
`designed to seal cracks by forming glassy phases on
`exposure to oxygen.
`
`GE-1025.004
`
`

`
`2346
`
`J. D. Webster et al.
`
`0.40 ~
`
`I
`
`I
`
`0.35
`
`0.30
`
`Carbon fibre reinforced silicon carbide compo(cid:173)
`site manufactured by Daimler-Benz Aerospace
`(Dornier Research) was used as the substrate for
`the coating layers. Yttrium silicate, Y 2Si05, has
`been suggested as a suitable outer coating layer
`and promising oxidation protection results have
`been reported.3 Figure 1 shows the architecture of
`the coating.
`Previous work in this area has typically been
`based on a 'trial and error' approach with minimal
`theoretical input to the design of coatings. The use
`of thermodynamic modelling and finite element
`analysis at the design stage was intended to act as a
`filtering process for potential coatings, allowing
`experimental work to be concentrated on the most
`promising coatings. Oxidation studies were carried
`out on the coated composite samples using con(cid:173)
`tinuous mass measurement during
`isothermal
`exposure to air at 1600°C.
`
`2 Thermodynamic Calculations
`
`The software which has been used is the MTDA TA
`package4
`supplied by
`the National Physical
`Laboratory, UK. A methodology has been
`developed to apply thermodynamic modelling to
`oxidation protection coatings. 5 The chemical com(cid:173)
`patibility between coating components and reac(cid:173)
`tions of the coating with an oxidising environment
`have been investigated.
`The chemical compatibility between Y 2Si05 and
`SiC has been assessed by performing calculations
`on the Y - Si- 0 - C system. The equilibrium phases
`for the composition 2Y- 2Si- 50-1 C (corresponding
`to Y 2Si05- SiC) have been calculated as a function
`of temperature (Fig. 2). The calculated amounts of
`Y 2Si05 and SiC do not change between 0 and
`1750°C, indicating that the two compounds will
`not react with each other over that temperature
`range.
`The exposure of the coating to oxygen has been
`simulated by calculating the equilibrium species at
`1600°C as a function of increasing oxygen con(cid:173)
`centration for the composition 2Y- 2Si- (5 + x)0- 1 C
`
`Yttrium silicate oxygen barrier I erosion
`
`resistant layer
`
`SiC Bonding layer
`
`C/SiC Substrate
`
`Y2Si0s
`
`~
`
`0.25
`
`0
`
`0.20
`
`O> "" '-
`<D </)
`:§,
`</) § 0.15
`
`0.10
`
`0.05
`
`0.00
`
`,...
`
`0
`
`SiC
`
`I
`500
`
`-
`
`-
`
`I
`1000
`TIC
`
`I
`1500
`
`Fig. 2. Calculated equilibrium phases up to l 700°C for the
`composition 2Y-2Si-50-IC.
`
`(Y 2Si05 + SiC + x02). The results of these calcula(cid:173)
`tions are shown in Fig. 3. The reactions involved in
`the oxidation of a Y2Si05 + SiC coating are
`thought to be:
`
`Y2SiOs(s) + SIC(s) + ~02(g) - t Y2SI20 1(s) + CO(g)
`(1)
`
`Y 2SiOs(s) + SiC(s) + 202(g) - t Y 2Sii01(s) + C02(g)
`(2)
`
`0.35
`
`0.30
`
`0.25
`
`Y2Si0s
`
`O> "" 0.20
`'-
`U1 Q)
`;::;
`Q)
`Cl.
`
`</)
`<fl
`
`~ 0.15
`~
`
`0.10
`
`0.05
`
`0.00
`
`5
`
`6
`
`7
`
`8
`
`g
`
`n(O)/mol
`
`Fig. 1. Yttrium silicate-based coating architecture.
`
`Fig. 3. Calculated equilibrium species at l 600°C as a function of
`oxygen concentration for the composition 2Y-2Si-(5 + x)0-1 C.
`
`GE-1025.005
`
`

`
`Oxidation protection coatings for C/ SiC based on yttrium silicate
`
`2347
`
`3 Finite Element Analysis
`
`A finite element analysis methodology6 has been
`developed using the LUSAS software to determine
`the stresses in the coating which develop on ther(cid:173)
`mal cycling due to the thermal expansion coeffi(cid:173)
`cient mismatch with the substrate. Hence it is
`possible to estimate the degree of cracking likely to
`occur. Figure 4 shows the stresses which are gen(cid:173)
`erated in the coated composite on cooling from
`1200°C to room temperature.
`Figure 4 shows that the 50 µm SiC bond layer is
`under a tensile stress of approximately 1 ·5 GPa and
`so will definitely crack. However, the outer Y 2Si05
`layer is only under a tensile stress of approximately
`100 MPa and so is not expected to crack. This is
`because Y 2Si05 has an extremely low coefficient of
`thermal expansion ( 4.g x 1o- 6 0c- 1) 3 and it also has
`an exceptionally low Young's Modulus (approxi(cid:173)
`mately 20 GPa). 3 It is these two mechanical prop(cid:173)
`erty parameters which have to be minimised in
`order to reduce the stresses generated in a coating
`layer and thus minimise the degree of cracking.
`
`to 4 min). An alkali-free strong base, tetramethyl(cid:173)
`ammonium hydroxide (TMAH), was selected as a
`dispersant to achieve slip stability based on the
`results of isoelectric point measurements for Y 20 3
`and Si02. Viscosities as low as 8-lOmNsm- 2 were
`obtained for these slips. Two slip compositions
`were selected to give coatings with different com(cid:173)
`positions. One coating was composed of princi(cid:173)
`pally Y 2Si05 while the other was a mixture of
`Y 2SiOs and Y 2Si207.
`Yttrium silicate coatings were obtained on C/SiC
`substrates with SiC bond layers by slurry dipping.
`Four layers of yttrium silicate were required to
`achieve a thickness of 100 µm on CVD SiC bond
`layers.
`The sintering process is complex since the simul(cid:173)
`taneous presence of oxide and non-oxide materials
`seriously limits the compatibility conditions. The
`selected sintering parameters were 3 hat l600°C in
`flowing argon (P 02 = 10- 3 bar). The possible reac(cid:173)
`tions between Si02 and Y 20 3 during sintering are:
`SI02(s) + Y203(s) ---> Y2SiOs(s)
`
`(3)
`
`4 Coating Manufacture
`
`C/SiC substrate material with a CVD (20 µm) SiC
`bond layer was supplied by Daimler-Benz Aerospace
`(Dornier Research), Germany. The yttrium silicate
`layer was then deposited by slip casting and sintering.
`Aqueous Y 20 3/Si02 slips were prepared from
`Y20 3 powder of mean diameter 3·5 µm, and
`microcrystalline Si02 powder of mean diameter
`2·5 µm. Two homogenising procedures were used:
`ball milling with A 120 3 balls and jar for 4 h, and
`high speed shear mixing limited to short times (up
`
`The XRD pattern shown in Fig. 5 for the mixed
`Y 2Si05/Y 2Si20 7 coating confirms that these reac(cid:173)
`tions have occurred, indicating the presence of
`Y 2Si05 as the main phase, along with a consider(cid:173)
`able amount of Y 2Si20 7.
`
`5 Oxidation Studies
`
`Oxidation behaviour has been investigated by con(cid:173)
`tinuous isothermal mass change measurements in
`
`SiC Bond Layer
`
`-
`-
`- ...- -
`
`1600
`
`1400
`
`c;s 1200
`D..
`
`:E - 1000
`en en e 800
`.....
`rn
`
`600
`
`400
`
`200
`
`-0 -
`
`-200
`
`---
`
`Yttrium Silicate Outer Layer
`""-
`
`C0"1'0Site
`---
`·--
`-
`-
`Distance through coated composite (microns)
`
`---
`
`~
`~ -
`-- ~
`-~
`
`.
`
`1200
`
`Fig. 4. Calculated stresses in SiC/Y 2Si05 coated C/SiC on cooling from 1200°C to room temperature.
`
`GE-1025.006
`
`

`
`2348
`
`J. D. Webster et al.
`
`0 Y2Si05 (ASTM 36-1476)
`0 Y2Sip7 (ASTM 40-0034)
`
`0
`
`SiY coating/Sic, l .600°C,
`3h, Ar flow, SiC bed
`
`0
`
`0
`
`00
`
`SiY bulk, l.600°C, 3h, air
`
`IO
`
`15
`
`20
`
`25
`
`30
`
`35
`20 (degree)
`
`40
`
`45
`
`50
`
`55
`
`60
`
`Fig. 5. Mixed XRD pattern for the mixed Y 2Si05/Y 2Si 20 7 coating after 3 hours at l 600°C.
`
`air at 1600°C. Details of the experimental setup
`can be found elsewhere. 7 The results of oxidation
`tests on the two types of coating are shown in Fig. 6.
`Work at Daimler-Benz Aerospace (Dornier
`Research) has shown that the critical mass loss at
`which the mechanical properties of the C/SiC
`composite used in this work are degraded suffi(cid:173)
`ciently to cause failure is 3%. It can be seen from
`the results in Fig. 6 that the SiC/Y 2Si0 5 coating
`provides negligible protection from oxidation. The
`mixed SiC/Y2Si05 + Y2Si20 7 coating, however,
`protects the composite for 53 h. The reason for this
`difference becomes apparent in examination of
`samples by scanning electron microscopy. Figure 7
`shows the surface of the Y 2Si05 coating. It is clear
`
`that this layer is extremely porous, explaining the
`rapid oxidation of the underlying composite. The
`surface of the mixed Y 2Si0 5 + Y 2Sii0 7 coating is
`shown in Fig. 8. It is clear that the degree of por(cid:173)
`osity in this coating is much lower, thus explaining
`the enhanced protective effect. The effect of the
`additions Y 2Si20 7 is to lower the temperature at
`which effective sintering can be achieved. The sud(cid:173)
`den mass loss of the Y 2Si05 + Y 2Si20 7 coated
`sample shown in Fig. 6 after approximately 50 h
`has been attributed to the oxidation of the SiC
`bond layer causing spallation of the outer coating.
`This is shown in Fig. 9. The relatively crack free
`appearance of the coatings and the absence of any
`evidence of phases other than SiC, Si02, Y 2Si05,
`
`5
`
`0
`
`20
`
`40
`
`60
`
`BO
`
`100
`
`120
`
`140
`
`Tin. (hours)
`
`Fig. 6. Isothermal oxidation behaviour of C/SiC with coatings based on yttrium silicate at 1600°C.
`
`GE-1025.007
`
`

`
`Oxidation protection coatings for C/ SiC based on y ttrium silicate
`
`2349
`
`and Y 2Si20 7 verifies the results of finite element
`analysis and thermodynamic calculations.
`
`6 Conclusions
`
`It has been shown that thermodynamic calcula(cid:173)
`tions and finite element analysis can be successfully
`used to predict the performance of oxidation pro(cid:173)
`tection coatings for C/SiC. A method has been
`developed to deposit Y 2Si05 based coatings on SiC
`coated CjSiC by slip casting and sintering. The
`process variables have been investigated and the
`optimum parameters determined. It has been
`found that the coating must be of a high quality in
`order to provide significant protection against oxi(cid:173)
`dation. This can be achieved by using a mixed
`Y 2Si05 + Y 2Si20 7 outer layer. These coatings pro(cid:173)
`vide protection against oxidation at 1600° C as
`evidenced by isothermal mass change measure(cid:173)
`ments and scanning electron microscopy. The fail(cid:173)
`ure mechanism of such coatings has been found to
`be spallation caused by the oxidation of the SiC
`bond layer.
`
`Acknowledgements
`
`This work was undertaken as part of a Brite
`Euram Project involving Daimler-Benz Aerospace
`(Germany), CSIC (Spain), Sintec CVD (UK),
`Ceramic Holland (The Netherlands) and Uni(cid:173)
`versity of Manchester & UMIST (UK). The coop(cid:173)
`in
`the Brite Euram
`eration of .the partners
`consortium and the CEC is acknowledged. The
`authors would also like to thank Dr H. J. Seifert
`and co-workers at the Max Planck Institute, Stutt(cid:173)
`gart for providing assessed thermodynamic data
`for the Y 20rSi02 system prior to publication.
`
`References
`
`l. Strife, J. R. and Sheehan, J. E., Ceramic coatings for car(cid:173)
`bon-carbon composites. Amer. Ceram. Soc. Bull., 1988,
`67, 369- 374.
`2. Westwood, M. E., Webster, J. D ., Day, R. J. , Hayes, F.
`H . and Taylor, R., Oxidation protection of ceramic matrix
`composites with carbon fibre reinforcement. J. Mater.
`Sci., 1996, 31, 1389- 1397.
`3. Ogura, Y. , Kondo, M. and Morimoto, T. Y2Si05 as
`oxidation resistant coating for C/C composites, Proceed(cid:173)
`ings of the Tenth International Conference on Composite
`Materials, Whistler, British Columbia, Canada, 14-18
`August 1995, Edited by A. Poursartip and K. Street,
`Woodhead Publishing Limited, volume IV, 1995, pp. 767-
`774.
`4. Davies, R. H ., Dinsdale, A. T., Hodson, S. M., Grisby, J.
`A. , Pugh, N . J., Barry, T. I. and Chart, T. G ., MTDATA
`- The NPL Databank for metallugy thermochemistry,
`User Aspects of Phase Diagrams, ed. F . H. Hayes. The
`Institute of Metals, London, 1991 , pp. 140-152.
`
`Fig. 7. SEM micrograph of the surface of the Y 2Si05 coating.
`
`Fig. 8. SEM micrograph of the surface of the Y 2Si05/Y 2Sii07
`coating.
`
`Fig. 9. Spallation of Y 2Si05 + Y 2Si20 7 coating layer.
`
`GE-1025.008
`
`

`
`2350
`
`J. D . Webster et al.
`
`5. Webster, J. D., Hayes, F. H., Taylor, R., Rebstock, K.
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