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`vii
`
`ANNOUNCEMENT
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`Important announcement for all authors submitting papers for publication in the Journal of Materials Science.
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`From February 1995 all authors are requested to submit articles to a new address as follows:
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`Professor W Bonfield
`Journal of Materials Science
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`2
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`
`JOURNAL OF MATERIALS SCIENCE 31(1996) 138971397
`
`Review
`
`Oxidation protection for carbon fibre
`
`composites
`
`M. E. WESTWOOD*, J. D. WEBSTER, R. J. DAY, F. H. HAYES, R. TAYLOR
`Manchester Materials Science Centre, University of Manchester and UMIS T,
`Grosvenor Street, Manchester M1 7HS, UK
`
`Carbon fibre-reinforced ceramic matrix composites are promising candidate materials for
`high-temperature structural applications such as gas turbine blades. In oxidizing
`environments at temperatures above 400°C, however, carbon fibres are rapidly oxidized.
`There is, therefore, a need to coat the composite in order to protect it against oxidation.
`This review identifies the requirements of an effective oxidation protection system for
`carbon fibre—reinforced ceramics and summarizes the work which has been carried out
`
`towards this goal over the last 50 years. The most promising coatings are those composed
`of several ceramic layers designed to protect against erosion, spallation and corrosion, in
`addition to possessing a self—healing capability by the formation of glassy phases on
`exposure to oxygen.
`
`1. Introduction
`Ceramic matrix composite materials reinforced with
`carbon fibres have received an increasing amount of
`attention in recent years. Much of the attention has
`been focused on developing these materials for use as
`high-temperature structural components in advanced
`gas-turbine engines. The driving force behind this re-
`search is the desire to increase the efficiency of these
`engines which dictates that
`they must operate at
`higher temperatures. Thus new materials must be
`found that can withstand this hostile, reactive environ-
`ment.
`
`‘
`
`The high strength and exceptional fracture tough-
`ness of these composites. combined with their re-
`fractory properties and their resistance to erosion,
`corrosion and wear, make them ideal candidates for
`this application [1]. In an inert atmosphere or in
`vacuum, carbon fibres retain their strength, modulus
`and other mechanical properties to temperatures in
`excess of 2000 “C which is much higher than those
`tolerated by other materials. A major problem arises,
`however, when composites containing carbon fibres
`are exposed to an oxidizing environment. Once the
`temperature is above 400 0C, the carbon fibres react
`with oxygen and are rapidly burnt away. Regardless of
`exposure time, successful operation in an oxidizing
`environment therefore requires the composite to have
`a protective coating of an appropriate combination of
`refractory materials in order to prevent oxygen at-
`tacking the substrate.
`
`The development of an effective oxidation protec—
`tion system for carbon has been in progress for more
`than 50 years and many different materials have been
`tried. The most promising systems include function-
`ally active layers which are designed to react with any
`oxygen which penetrates through cracks in the outer
`layer to form a glass which flows and seals these
`cracks. This enables the oxygen to be consumed before
`it can reach the substrate and the system is intended to
`be self-healing.
`This paper is a review of the literature which dis-
`cusses the fundamentals of oxidation protection and
`the various protection systems for carbon fibre-rein-
`forced ceramic matrix composites (CMCs) and sum-
`marizes developments which have been made in recent
`years.
`
`2. Requirements of integrated oxidation
`protection systems and their
`components
`The factors which must be considered when designing
`an oxidation protection system are well established.
`They are summarized in Fig. 1.
`The primary function of an oxidation protection
`system is to isolate the substrate material from the
`oxidizing environment. To achieve this,
`the system
`must perform a number of functions.
`At least one component of the system must form
`an effective barrier to the ingress of oxygen. This
`
`‘To whom all correspondence should be addressed.
`
`0022—246]
`
`LC 1996 Chapman & Hall
`
`1389
`
`3
`
`

`

`Ox
`
`en in
`
`yg
`
`Volatility
`
`Adhesiong'
`
`Carbon out
`
`Compatibility —
`mechanical and
`chemical
`
`coating
`
`SUbstrate
`
`Figure 1 Considerations when designing an integrated oxidation
`protection system [2].
`
`should be matched as closely as possible to avoid )5
`generation of large stresses during heating and H}
`ing. It is also important that the components of
`system do not react with each other to form und ?
`able phases, dissociate at high temperatures, or “11'
`.
`go phase transformations involving large vol
`changes during heating or cooling in the temperat 1
`range of interest.
`To avoid non—uniform attack of the protection 8
`
`tion
`
`(— Sealant Required—b (—lntrinsic
`
`Prnt
`
` illll
`
`l-l‘1m“
`Range—>
`
`Figure 2 General oxidation behaviour of coated CMCs with carbon
`fibre reinforcement [4].
`
`component of the system must therefore have a low
`oxygen permeability. Ideally, a material can be used
`which forms an adherent in situ oxide. In addition, it is
`equally important to minimize the diffusion of carbon
`outwards from the substrate. This is specially vital if
`some layers in the system contain oxides because these
`may be reduced by carbon.
`In service, it is inevitable that cracks will form in the
`outer layer of a coating because the coefficient of
`thermal expansion (CTE) of the substrate is so low
`compared to the coating materials. This mismatch
`causes microcracks to form on cooling down from the
`coating deposition temperature. For the system to
`protect against oxidation for long periods of time, it
`must possess a self—healing capability [2]. At least one
`of the internal layers must be either a glass or a glass-
`forming compound which can flow into and seal any
`cracks which develop. In order to create an oxidation
`protection system capable of providing protection
`over a large range of temperatures, it is necessary to
`seal
`the cracks which develop in the temperature
`range from the oxidation threshold of the composite
`(400 °C for C/SiC) to the microcracking temperature
`of the protective coating [3]. The microcracking tem-
`perature is the temperature at which the coating was
`originally fiaw free, i.e. the deposition temperature for
`Chemical vapour deposited (CVD) coatings or the
`sintering temperature for slurry coatings. This general
`oxidation behaviour of coated CMCs with carbon
`
`is represented schematically in
`
`fibre reinforcement
`Fig. 2.
`It is necessary to establish a good adherence be—
`tween the substrate and the coating, and between the
`different layers of the system. This requires good wet—
`ting properties and use of the correct manufacturing
`route. A suitable bond layer may provide the neces-
`sary resistance to the outward diffusion of carbon as
`well as forming a strong bond at the surface of the
`substrate. The bond layer should not penetrate excess-
`ively into the substrate.
`Mechanical and chemical compatibility must be
`ensured between the coating system and between the
`various layers of the system. The coefficients of ther-
`mal expansion of layers in contact with each other
`
`1390
`
`.
`
`*j
`3.1. Erosion protection layers/primary
`’f
`oxygen barriers
`If coated composites are to be used in aerospace 3fl A.
`turbines they will be exposed to high-velocity h '
`particles and reduced ambient pressures where VaP".
`ization becomes a threat to lifetime. It is apparem’
`therefore, that hard ceramic outer coatings with-(11:2?
`vapour pressures are required for these applicatl
`
`_
`'
`
`,7
`
`
`
`requirement for a primary oxygen barrier.
`The mechanical properties of the coating must a
`be neglected. The coating must be able to withst 4
`any stresses generated at the surface of the componen '
`A low modulus is desirable in order to accommodatg
`strains generated by thermal expansion coefficien‘fi‘ '
`mismatches during thermal cycling.
`‘
`‘
`This combination of properties cannot be met bf
`any single material. It is necessary, therefore, to develofi -
`multi—layer coatings where each layer performs a spa:- '
`cific function, allowing the coating as a whole to '
`provide the required oxidation protection.
`
`V
`
`‘p
`
`i
`
`'
`
`3. Fundamentals of multi-layer oxidation 7.
`protection systems
`a
`It is generally accepted that in order to protect a 06*
`ramic matrix composite from oxidation over a rangfi
`or high temperatures, a multi-layer system is required
`The protection mechanisms of single layers of multiii ;
`layers of the same material do not work over
`I.
`ranges of temperature and may not protect against
`various oxidation mechanisms which exist over such .
`temperature ranges. The main advantage in the dove? '_
`opment of a multi-layer coating is the possibility g
`associating complementary specific properties and 6f
`gaining the advantages of each layer while limitinfi
`their drawbacks [1].
`ll
`In a multi-layer system, the individual layers Shelli?
`be strategically stacked with respect to the substratgi
`In this way, protection can be provided over the Whig?"
`temperature range of interest. Various layers are
`tivated at different temperatures or under certain 00 I
`ditions such as the opening of a crack. The stacking!
`order of the layers must also take into account the -
`mal expansion coefficients and chemical compatibilityik‘;
`
`_
`
`.-
`g
`
`4
`
`

`

`
`
`3.1,]; I Temperatures for oxide vapour pressures of 10‘3 mm [8, 9]
`
`v103
`
`2250
`
`rho2
`
`2239
`
`ZrO 2
`
`2239
`
`ECU
`
`2027
`
`A1203
`
`1905
`
`CaO
`
`1875
`
`Tao2
`
`1780
`
`Sio2
`
`1770
`
`MgO
`
`1695
`
`2475
`
`.J (00
`
` mo2
`
`
`CTE between the coating and substrate for an oxide
`coating than a non—oxide coating. This mismatch can
`lead to coating spallation. Hence it is advantageous to
`coat the substrate with a non-oxide material which
`
`forms an appropriate oxide scale on exposure.
`A disadvantage of non-oxide outer layers which
`form oxide scales is the erosion of the scale by vaporiz-
`ation and excessive flow in high-velocity gas streams.
`These problems may be overcome by the use of over-
`lay coatings. Such overlayers are discussed later. A
`more serious problem with oxide scales is the buildup
`of gaseous oxidation products between the scale and
`the non-oxide ceramic. Equlibrium gas presssures at
`the scale/ceramic interface increase with increasing
`temperature. When a temperature is reached where
`the gas pressure at the interface is greater than the
`ambient pressure, the scale becomes frothy and por-
`ous. This allows very rapid attack of the underlying
`ceramic.
`
`Because it is desirable to combine the requirement
`for an erosion protection layer with that for a primary
`oxygen barrier, the oxygen permeability of candidate
`materials must be considered. HfOz, ZrOZ, and ThOZ
`can be ruled out on the grounds that their oxygen
`permeabilities are too high [11713]. These materials
`may, however, be useful as erosion protection layers
`when the primary oxygen barrier requirement is fulfil-
`led by another layer. The permeabilites of A1203 and
`Y203 are lower, but are still higher than that of
`vitreous silica, suggesting that SiC and Si3N4 may
`well possess the best combination of properties for
`use as an erosion protection layer/primary oxygen
`barrier.
`
`In the past, iridium has been tried as an outer
`erosion layer for protecting CMCs from oxidation at
`very high temperatures. Iridium is attractive because
`of its 2440 DC melting point and its extremely low
`oxygen permeability at
`temperatures as high as
`2200 °C [7]. Although excellent protection has been
`demonstrated for short
`times in the 2000—2100°C
`
`range, difficulties in fabricating high-quality coatings
`and the cost and availability of iridium were deter-
`rents to further development. Other major drawbacks
`for the use of iridium are the high thermal expansion
`mismatch between the metal and the coating and
`erosion problems due to the formation of volatile ers
`[14]. The thermal expansion mismatch is probably
`more significant because Ir03 formation can be con-
`trolled by the application ofa thin oxide overlayer [7].
`To summarize, it is apparent that SiC and Si3N4 are
`the most attractive materials in terms of the combina-
`
`tion of properties required. due to their formation of
`a glassy silica scale. Outer coatings of these materials
`are primary barriers to oxygen, provide a hard cro-
`sion-resistant outer surface, and act to resist erosion
`
`1391
`
`
`
`the most common primary oxygen barriers are
`n carbide, SiC and silicon nitride, Si3N4. They
`in refractory and oxidation-resistant due to the
`ation of a slow-growing silica, SiOZ, scale upon
`. tion. Silica exhibits a relatively low vapour pres-
`‘to temperatures as high as 1650 “C as well as low
`n diffusivity [2]. The silica-based ceramics also
`i‘i- the most attractive thermal expansion com-
`ility with the substrate.
`,_e oxidation of Si3N4 differs from that of SiC in
`its oxidation rate is two to three orders of magni-
`x lower in the well-studied temperature range of
`V. t 1400 °C. Also, while SiC yields SiO2 as the only
`not of oxidation, Si3N4 oxidation produces an
`layer of silicon oxynitride, SiZNZO beneath the
`silica scale. It has been proposed that this inner
`, 20 layer constitutes a better barrier than Si02 to
`1 en diffusion by virtue of a tighter network struc-
`, and hence is responsible for the slower oxidation
`~ of Si3N4 [5]. Silicon oxynitride is considered to
`it of the most stable nitrides in oxidizing atmo-
`res at high temperature and could therefore be an
`'llent candidate material for a primary oxygen
`‘er in a protection system [6].
`any hard oxide ceramics have also been con-
`d as potential outer layers. Sheehan [7] has
`;, ulated that a value of 10'3 mm is an appropriate
`i 'rnum vapour pressure for a material to be used as
`‘ osion protection layer. Table I shows the temper—
`'e at which various oxides have a vapour pressure
`70‘3 mm [8, 9].
`,, can be seen from Table I that all of these mater-
`? appear to be suitable for use as an erosion protec-
`.»._- layer, having vapour pressures below the critical
`i e 0f10‘ 3 mm at temperatures up to a minimum of
`5°C. However. most of these materials have fea-
`“- which are undesirable for coating use. CaO and
`I are both sensitive to moisture. The high toxicity
`60 is a further disadvantage, as is the radioactivity
`I
`' h02. Titanium oxide is very sensitive to the oxy-
`- concentration of the environment and may suffer
`'5 phase instability and the formation of suboxides
`ch are less refractory than TiOl. In order for HfOz
`,1 ZrO2 to be useful as erosion protection layers,
`must be stabilized against undesirable phase
`nges which occur during heating and cooling. It
`been shown that an outer HfOZ layer has less
`.x'dency to crack than Y203 [10]. Vitreous SiOz is
`00d barrier to oxygen but lacks the hardness re—
`5 ed of an erosion protection layer. The usefulness
`V ilica is, therefore, limited to that ofa scale formed
`4 a silicon-containing non-oxide ceramic. The ther-
`1 expansion coefficients of oxide ceramics are gen-
`'= 11y higher
`than the corresponding borides or
`'l'bides, meaning that there is a greater mismatch in
`
`5
`
`

`

`
`
`the primary oxygen barrier from reaching the A;
`strate, and to be capable of sealing cracks which
`inevitably form during service. Both these functi
`can be performed by one or more functionally a *7
`layers. The use of glassy materials for such purpo-
`has been considered for many years. In 1934, a pa ,J
`i
`was issued to the National Carbon Company _
`a coating method to render carbon articles oxidati
`resistant at high temperatures [17]. The coating .71,
`scribed in the patent consisted ofa SiC inner layer
`an outer glaze based on 8203. Glazes on P205
`,
`SiOz are also mentioned.
`The use of borate glasses has been extensiv-
`studied. Borate glasses have many properties .i-i
`make them useful materials in oxidation protecti
`applications. The viscosity of 3203 in the temperat i
`range 600—11003C is such that it will form thin p I _
`tective films. The tendency of 3203 to wet SiC a
`Si3N4 and flow into coating cracks makes it an ml
`lent sealant material for extended-life applicatio;
`In systems where the outer coating is a non-oxi ,-
`ceramic which forms a silica scale, B203 to n1 ~
`an excellent
`sealant
`in the
`temperature ra 1..
`600—1100 °C where the viscosity of silica is too high tq:
`allow crack-sealing behaviour. The usefulness of .2.
`ate glasses is limited, however, by vaporization z.
`temperatures above 1000 °C [18] and moisture semi-i.
`tivity [19]. Hydrolysis of B203 when exposed to a
`bient moisture causes the glass to swell and crumblm
`This can cause coating spallation at room temperature»
`due to glass swelling or spallation during heating duq' ’
`to moisture release [4]. Hydrated borates are highly:
`volatile and so there may be glass depletion at relat-
`tively low temperatures in moist environments [234—-
`These properties,
`together with the comparatively.
`high vapour pressure and oxygen diffusivity, mead.-
`that 8203 is generally used as a secondary oxidatiuq—t
`barrier beneath the outer coating which acts to
`'
`imize the glass depletion.
`temperatures:
`In order to provide protection at
`above 1100 0C, it is necessary to use a sealant
`'
`‘
`superior high-temperature properties to those *
`8203. Silica is stable up to approximately 1800°Cg
`but at
`lower temperatures is too viscous to v“.
`cracks effectively. Glasses must, therefore, be modified”
`in order to protect the composite over the full tern ‘9 '1 i
`ature range.
`.W' "
`Volatilization can be reduced by increasing the “UP
`cosity of borate glasses. This may be achieved it
`adding up to 25 mol % of a refractory oxide to
`8203 sealant. Suitable oxides include TiOz, Z ' ~.
`HfOz, Al203, Y203, Sc203, Lazo3, $02 and C60 ,
`The choice of oxide is determined by the temperat“! 1
`range over which the effect is required, For exampl‘!“
`alumina is a suitable addition in the 550'9000q
`range,
`titania for
`intermediate temperatures.
`‘
`either zirconia or hafnia for a high-temperatut’e mi
`gime in the range 1200—16000C [20]. The mots
`sensitivity of borate glasses can be reduced by the
`of Li20 additions, but unfortunately lithia has
`,
`disadvantage of reducing viscosity and so incre
`volatilization. This may be corrected, however, by "e
`addition of a third oxide such as silica.
`'
`
`-.v"
`
`D
`
`and vaporization of glasses used in other layers for
`crack sealing. Oxides such as Al203 and Y203 are
`worthy of consideration as oxygen barriers, whilst
`other oxides such as HfOz. ZrOz, and Th0; may be
`of use as erosion-protection layers. Iridium may offer
`useful oxygen barrier properties but exhibits a large
`thermal expansion mismatch with the substrate,
`potentialy causing spallation problems.
`Unfortunately, silicon-based ceramics have poor
`environmental durability in atmospheres containing
`molten salts, water vapour, or hydrogen, Molten salts
`such as Na2804 or Na2C03 are formed from impu-
`rities in an engine‘s fuel and air intake and dissolve the
`protective silica scale that exists on the surface of SiC
`and Si 3N4. Water vapour reacts with the silica scale to
`form gaseous Si(OH)4, and hydrogen reduces the sil-
`ica scale to gaseous SiO [15]. All of these reactions
`can lead to an accelerated or catastrophic degrada-
`tion, and thus limit
`the lifetime of the protection
`system. SiC and Si3N4 erosion layers therefore require
`an overcoat
`to ensure environmental protection if
`their full potential is to be realized.
`It has been proposed that a tantalum pentoxide
`overcoating could protect silicon—based outer erosion
`layers from the various forms of corrosion [16]. An
`additional outer layer of a material such as TaZOS,
`SiZNZO or ZrOZ may also be used to minimize ero-
`sion and excessive flow of vitreous silica. Mullite
`
`(3A12032Si02) appears most promising as an over-
`coating because of its chemical compatibility with
`silicon-based ceramics, low CTE, and relatively high
`chemical stability. Mullite coatings applied by con—
`ventional plasma spraying are amorphous but then
`tend to crack on thermal cycling due to a volumetric
`contraction associated with crystallization. Lee et a1.
`[15] have now developed a fully crystalline, plasma-
`sprayed mullite coating which has dramatically im-
`proved adherence and thermal shock resistance. The
`superior coating adherence is obtained by first
`roughening the SiC surface prior to deposition of the
`mullite overlayer. The precipitation of amorphous
`mullite is prevented by decreasing the cooling rate.
`This is achieved by heating the SiC layer above the
`crystallization temperature. This new mullite coating
`has been applied to an SiC substrate and subjected
`to cyclic oxidation between room temperature and
`120071400°C in air. For a 20h cycle oxidation test,
`the weight gain was half that of an uncoated substrate.
`In contrast, conventionally sprayed mullite coatings
`did not render any oxidation protection to the sub-
`strate because of severe cracking of the coating. Pre-
`liminary tests have shown that these novel mullite
`coatings are promising for protection against molten
`salt corrosion, and mullite—yttria—stabilized zirconia
`(YSZ) dual layer coatings are promising for protection
`in oxidizing/reducing gases and water vapour [15].
`
`3.2. Functionally active layers
`In order to provide effective oxidation protection over
`long periods of time and under conditions of thermal
`cycling,
`it
`is necessary for a coating system to be
`capable of preventing any oxygen which penetrates
`
`1392
`
`6
`
`

`

`Functional layer partially
`transformed into sealing glass
`
` Cracked outer layer
`
`Functional layer
`
`S u bstrate
`
`‘
`
`i 3 Conversion of functional layer material to glass sealant on exposure to oxygen.
`
`
`
`’
`
`fe crack-sealing properties of silica-based glasses
`‘ be improved by the addition of alkali oxides such
`iZO or B203. However.
`the downside to this
`each is that both these additions compromise the
`ntages pure silica presents over borate glasses
`use both the moisture sensitivity and oxygen per—
`ility of the glass are increased. This could result
`mage due to moisture uptake, chemical incom-
`ility with outer coatings, or accelerated oxygen
`ort to the substrate. Both LiZO and 3203 have
`cantly higher vapour pressures than SiOz and
`. may present volatility problems over
`long
`’ds at high temperatures [2].
`9 summarize, silica-based glasses offer a temper-
`2 advantage over borates, but do not perform as
`at the lower end of the temperature range. Low—
`:- rature performance can be improved by fiuxing
`* oxide additions but this is at the expense of high-
`p rature performance. The upper
`temperature
`for the use of borate or silicate glasses in contact
`__ carbon is 1500 °C [2]. so if a ceramic matrix
`posite reinforced with carbon fibres is to be used
`gher temperatures, a refractory carbide interlayer
`quired to prevent a carbothermic reduction reac-
`. occurring.
`order to overcome the deficiencies of binary
`‘
`ms, a sealant of composition lOTiOZ—ZOSiOz—
`202 has been developed [21]. The SiOz and
`f 2 additions have the effect of increasing the moist-
`" resistance of the glass,
`reducing volatility at
`“i temperatures, increasing the viscosity of the sea-
`, and preventing corrosion of any SiC layers in
`tact with the sealant. Increasing the TiOz/BZO3
`‘0 reduces the high-temperature viscosity of the
`1% nt, whereas increasing the Si02/B203 ratio
`. es the low-temperature viscosity to increase rap—
`] which is detrimental to oxidation performance in
`7'500—800 0C temperature range.
`system
`the
`to
`glassy
`sealant
`belonging
`.‘s—SiOZ—AIZO3 has been developed to protect
`} Cs reinforced with carbon fibres [22]. The coating
`firmed by either spraying with a gun or by brush
`'lication of a liquid suspension. After drying, the
`ture is heat treated at a temperature sufficient to
`'sform the layer into an insoluble, self-healing
`,a glass.
`' more recent approach to the use of glass sealants
`functionally active layers is to use glass—forming
`V‘ pounds which, when oxidized. form glasses. This
`hod has the advantage that oxygen which pen-
`tes the outer layers is actively absorbed rather
`'
`simply slowed down. Fig. 3 shows schematically
`~ way in which such layers work.
`
`Several boron- and silicon-containing compounds
`have been suggested for use in functionally active
`layers. Boron carbide, B4C, is of interest for such an
`application because it oxidizes much more rapidly
`than silicon-based materials, thus giving faster sealing.
`On exposure to oxygen, B4C undergoes the reaction
`
`B4C ‘i‘ 402 —' 2B203 +
`This occurs between 700 and 900 0C and is accom-
`
`panied by a 250% volume increase which ensures
`excellent crack-sealing properties [7]. The main disad-
`vantage of 34C is the formation ofa gaseous oxidation
`product. Gas buildup underneath a coating can cause
`spallation or reduce the effectiveness of the 3203
`sealant by causing it to become frothy and porous.
`Several borides have been considered for functional
`
`layer use. The most common are titanium and silicon
`borides. TiB2 exhibits exceptional hardness and chem-
`ical stability and therefore offers considerable promise
`for use in highly erosive and corrosive environments
`such as those found within gas turbine engines. The
`oxidation of TiB2 produces a two-phase mixture of
`B203 glass and TiOZ:
`
`Courtois er al. [3] have deposited titanium diboridc
`by CVD as a sealant layer underneath an SiC outer
`layer to protect C/SiC composites and found it to be
`a promising material for crack scaling in the temper-
`ature range 700—11000C. The silicon borides have an
`advantage over other potential functional layer mater-
`ials because they oxidize to form a borosilicate glass
`with no other products. Also, the composition of the
`resulting glass can be determined by altering the com-
`position of the boride. This means that sealants can be
`produced for specific temperature ranges.
`Another candidate material
`for use as a crack
`
`sealant is molybdenum disilicide. MoSiz. The initial
`oxidation of MoSi2 in air is strongly dependent on
`0; partial pressure and temperature. At high temper-
`ature and high oxygen partial pressure, for example,
`1420°C and > 104 Pa, MoSi2 oxidizes to form vol-
`atile M003 and vitreous SiOz. As the silica layer
`coarsens it becomes more difficult for oxygen to dif-
`fuse through the SiOz scale and the specimen becomes
`resistant to further oxidation. This process is referred
`to as passive oxidation. At low partial pressures, gas-
`eous SiOZ forms and no protective layer exists. this is
`active oxidation. Below 600 °C, M003 is not volatile.
`which, in turn, prevents the formation of a continuous
`amorphous SiO2 layer. This results in a spalling of
`layers from the surface. referred to as MoSiz pest [23].
`It should also be noted that MoSiz undergoes a
`
`1393
`
`7
`
`

`

`
`
`5. Systems described in the current
`literature
`
`The simplest systems consist of either a single layer
`or several layers of the same material. These system -
`provide only limited protection because they are "
`prone to failure by spallation and oxygen penetration, .
`and have no internal sealing capability. An example is
`_
`a single layer of SiC deposited by slurry dipping [28};
`;
`Such coatings are not worthy of further consideratiol, _
`1
`as viable oxidation protection systems except in low:
`temperature, short-term applications.
`i '
`The protective properties of refractory coating ‘
`such as SiC can be dramatically improved by coverinfi-
`with a glassy layer. The glassy layer usually consists 91
`a borate—based glass, applied either as a slurry [29] 9’ :-
`by dipping in molten 8203 [30]. These systems pro'
`vide some self-healing capability up to around 1000 0‘3
`but are limited by borate volatility at higher temper?!
`atures, the major volatile being HBO; [31]. Hoffmafm _
`at al. [32] have protected C/C composites with a SIG
`coating applied by CVD and an outer layer of silica;
`Problems associated with this system are the lack 0‘
`low-temperature crack sealing due to the high visc_0&‘!t
`ity of silica and. the poor erosion resistance at hlsll .
`temperatures where the lower viscosity of silica allow ‘
`excessive flow and some volatilization.
`.
`"'
`One system which shows some promise for pOSSIbh V
`incorporation into a