Attachment 2a: Copy of Document 2 from the University of Illinois at
`Urbana-Champaign Library
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`Attachment 2b: University of Illinois at Urbana-Champaign Library catalog record for
`Document 2
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`Attachment 2c: British National Bibliography record for Document 2
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`Attachment 2d: Statewide Illinois Library Catalog record for Document 2
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`Attachment 2e: British Library catalog record for Document 2
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`Attachment 2f: RERO catalog record for Document 2
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`Attachment 2g: Google Scholar list of publications citing Document 2
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`Attachment 2h: Copy of a publication citing Document 2
`984
`Chem. Mater.1996, 8,984-1003
`
`Reviews
`Materials Chemistry Issues Related to Advanced
`Materials Applications in the Automotive Industry
`Chaitanya K. Narula,*,† John E. Allison,‡ David R. Bauer,§ and
`Haren S. Gandhi⊥
`ChemistryDepartment,Material Sciences Department, Polymer Science Department, and
`Chemical Engineering Department,
`FordMotor Co., P.O. Box 2053, MD 3083,
`Dearborn, Michigan 48121
`Received December 8, 1995. Revised Manuscript Received March 13, 1996
`The driving force behind the research on advanced materials has largely been aerospace
`and defense applications.
`In such applications the customer-induced limitations and
`infrastructural economic factors are secondary to the application-oriented parameters.
`Automobiles, on the other hand, represent the primary consumers of advanced materials in
`civilian applications. Automobiles are high-technology, low-cost machines which need to
`be robust for various climatic conditions and driver behavior. Advanced materials play an
`important role in the design and fabrication of various components of automobiles, and the
`use of new materials continues to increase. Recent interest in developing highly fuel efficient
`vehicles with low emissions has focused efforts toward materials, designs, and devices and
`is spurring research into advanced materials for weight reduction. The goal is to achieve
`fuel efficiency by weight reduction and a more efficient powertrain. In this review article,
`we summarize the efforts of the 1970s and the 1980s on the development of ceramic gas
`turbine engines with emphasis on materials processing and properties. This is followed by
`a discussion of metal-matrix composites and reinforced plastics, the structural materials
`of current interest. The materials issues related to automotive exhaust reduction catalysts
`are discussed since materials continue to play an important role in designing catalysts to
`meet the new EPA regulations. The understanding of materials chemistry is expected to
`play an important role in designing new materials and developing new processes that can
`be used economically to mass produce vehicle components. We also summarize reports based
`on ceramic precursor technology and sol-gel processes that show promise in the fabrication
`of automotive components.
`
`X
`
`Introduction
`nomics, and (iv) government regulations.1 Recent inter-
`est in developing highly fuel efficient vehicles with low
`Automobiles are probably the most common high-
`emissions has focused efforts toward materials, designs,
`technology machines that an average person operates
`and devices. The two commonly used acronyms ULEV
`on a daily basis in developed countries. Since the early
`and PNGV refer to “ultralow emission vehicles” and
`days of automobile development, the basic vehicle design
`“partnership for a new generation vehicle (a cooperative
`has consisted of an engine, wheels, a steering wheel,
`program between GM, Ford, Chrysler, and the Depart-
`brakes, and driver and passenger compartments. The
`ment of Energy)” and represent the goals for emissions
`first noticeable change occurred in the driver and
`and fuel economy. The ULEV need to meet non-
`passenger compartments from stagecoach-type designs
`methane organic gases/CO/NOx emission standards of
`to closed compartments over a century ago. Improve-
`0.04/1.7/0.2 g/mile for 50 000 miles and of 0.055/2.1/0.3
`ments over the past several decades have brought major
`g/mile for 100 000 miles. The PNGVs need to attain fuel
`changes in automobiles. Compared to early automo-
`efficiency of 82.5 miles/gal. New materials and tech-
`biles, today’s vehicles are aerodynamic, have a large
`nologies will be needed to meet these goals.
`It is
`number of safety features, employ a variety of advanced
`important to mention that the adoption of a technology
`materials and composites in various parts, and contain
`is not just an addition to a vehicle; it requires major
`several electronic components. The driving forces for
`changes in the overall design of the vehicle itself.2
`research are (i) improvement in quality in a highly
`The traditional materials of interest to the automobile
`competitive market, (ii) environmental issues, (iii) eco-
`industry have been (i) metals which form most of the
`body and engine components, (ii) organic polymers
`* Author for correspondence.
`which are plastics and rubbers for internal trim, dash-
`† Chemistry Department.
`boards, tires, wipers, bumpers, and some body compo-
`‡ Material Sciences Department.
`§ Polymer Science Department.
`nents, (iii) oils, and (iv) paints.3 Over the years, the
`⊥Chemical Engineering Department.
`use of electronics in cars has increased and is expected
`X Abstract published in Advance ACS Abstracts,April 15, 1996.
`
`S0897-4756(95)00588-6 CCC: $12.00 © 1996 American Chemical Society
`
`104
`
`

`

`Attachment 2h: Copy of a publication citing Document 2
`Reviews
`Chem. Mater.,Vol.8, No. 5, 1996985
`to grow further. It was recognized quite early that the
`Table 1. Specific Modulus of Some Important Materialsa
`reduction in the weight of an automobile for both
`specific modulus
`gasoline and diesel engines, and burning diesel at
`material
`(104 MN m-2)
`MP/dec P (°C)
`elevated temperatures (1315 °C) will result in improved
`SiC
`17.2
`2600
`fuel efficiency and reduced tail-pipe emissions. At
`AlN
`10.3
`2450
`Si3N4
`11.7
`1900
`present, materials limitations prevent the operation of
`Al2O3
`9.0
`2050
`diesel vehicles at high temperatures. The fuel efficiency
`12.4
`2530
`BeO
`of the modern gasoline vehicle is achieved by a reduction
`BN
`4.8
`2700
`in the weight of the automobile and the treatment of
`C
`42.0
`3500
`emissions with a three-way catalyst. Further improve-
`SiO2(vitreous)
`3.1
`1710
`3.8
`1500
`steel
`ments of modern gasoline vehicles can be achieved with
`Al
`3.0
`660
`the use of advanced materials to reduce the weight of
`aFrom: Dobson, M. M. Silicon Carbide Alloys; Evans, P. E.,
`vehicles without compromising safety.
`Ed.; The Parthenon Press: Lancashire, England, 1986; p 2.
`The purpose of this review is to summarize the
`research on the automotive applications of advanced
`ity, low thermal expansion and good thermal shock
`materials. In the 1970s, ceramics were proposed as the
`resistance of materials also constitute selection criteria.
`materials of choice4 because they provide the ideal
`Thus silicon carbide and silicon nitride are the only
`combination of being lightweight and able to function
`acceptable materials because aluminum nitride is un-
`at high temperatures. This led to the fabrication of gas
`stable in moisture, alumina does not have good thermal
`turbine and adiabatic diesel engines. However, those
`shock resistance, beryllium oxide is toxic, carbon has a
`efforts were largely abandoned due to the limited
`low oxidation stability, and boron nitride is difficult to
`reliability of ceramics, their catastrophic failure, and the
`fabricate.
`exorbitant cost of machining them. There is still some
`Although a ceramics based gas turbine engine and
`interest in these devices for PNGV, although nonadia-
`parts of the adiabatic engine were fabricated in the
`batic diesel engine based technologies offer better fuel
`1970s, they are not in use because ceramics are quite
`economy and will become suitable for mass production
`brittle and have intrinsic cracks which grow under
`provided advances in catalysis can bring NOx and
`stress.9 Here we provide a brief description of the gas
`particulate emissions to acceptable levels.
`turbine and adiabatic engine with emphasis on the
`In the following sections, we will summarize earlier
`materials needs. Structural materials for these engines
`efforts on the ceramics in gas turbines and adiabatic
`were identified to be partially stabilized zirconia (PSZ),
`engines. This will be followed by the description of the
`silicon carbide (SiC), silicon nitride (Si3N4), lithium
`materials of current interest. Space requirements force
`aluminum silicate (LAS), and magnesium aluminum
`us to limit our discussion to metal matrix composites,
`silicate (MAS) with operating temperatures ranging
`fiber-reinforced plastics, and automotive catalysts. Other
`from 800 °C for PSZ to 1500 °C for SiC. We will limit
`important applications of ceramics are in optical coat-
`our discussion to the fabrication processes, properties,
`ings,5 energy-storage materials and energy-conversion
`and limitations of silicon carbide and silicon nitride
`materials,6 and chemical sensor devices7 which will not
`materials which were employed with some success. At
`be discussed here. Since this review emphasizes ap-
`present, a Si3N4 turbocharger rotor is in production and
`plications, we will first describe devices prior to materi-
`is used in several Japanese automobiles.10
`als related issues. This will be followed by the summary
`We will limit our discussion of ceramic precursors for
`of earlier efforts, ongoing research that could have an
`silicon carbide or silicon nitride to those which have
`impact in automotive industry, and future directions.
`been employed either in the fabrication of complex
`Since the number of publications and patents on auto-
`shapes or as binders. There are only a few examples of
`motive applications of materials is very large, we limit
`the applications of ceramic precursors in the fabrication
`our discussion to the review of the relevant work rather
`of complex shapes. These examples do not show the
`than attempt to create a comprehensive summary.
`advantages of ceramic precursor pyrolysis over classical
`Some recent reports based on ceramic precursor
`methods to obtain SiC or Si3N4 components. A green
`technology and sol-gel processes show promise in
`body made of preceramic polymer undergoes significant
`designing vehicle components and are described along
`density changes (factor of 2-3) and weight loss in the
`with the discussion on the processing of materials. An
`form of gaseous byproducts. The compacting of the
`obvious omission is CVD-based developments. For
`green body causes shrinkage and gas evolution which
`automotive applications, the CVD-based processes are
`introduces porosity. The maximum volume11 change
`limited to electronics, optical, and chemical sensor
`during polymer-ceramic conversion is given as
`applications due to economic reasons.8
`ψ ) Râ - 1
`Structural Ceramics in the Automotive
`where R is the mass of ceramic/mass of polymer and â
`Industry
`is the density of polymer/density of ceramic.
`Structural ceramics need to tolerate high stress at low
`Maximum porosity (π) during a volume-invariant
`strain, implying that they should have a high elastic
`conversion is given as
`modulus. The commonly used parameter for the com-
`parison of various materials is the specific modulus
`π ) 1 - Râ
`(elastic modulus/specific gravity) which expresses usable
`In most polymer-ceramic conversions, significant vol-
`strength per unit weight. Table 1 shows the specific
`ume change and porosity formation are observed. In
`modulus and melting point/decomposition point of some
`the case for all organosilicon polymers, Râ is smaller
`materials. High-temperature strength, chemical stabil-
`
`105
`
`

`

`Attachment 2h: Copy of a publication citing Document 2
`986 Chem. Mater., Vol. 8, No. 5, 1996
`
`Reviews
`
`Figure 1. Gas flow path of a vehicular gas turbine made of
`ceramics.
`than unity, suggesting that the fabrication of fully dense
`bodies is probably impossible without shrinkage.12,13
`Active filler-controlled pyrolysis of preceramic poly-
`mers overcomes the problems of shrinkage and massive
`porosity associated with fabrication of ceramic compo-
`nents from preceramic polymers.13 In this approach,
`active fillers react with the pyrolysis byproducts of
`preceramic polymers. Thus, this approach combines the
`classical method approach with ceramic precursor tech-
`nology since the reaction of active fillers with gaseous
`byproducts proceeds under the reaction conditions of
`classical methods. This method seems to be more
`suitable for the preparation of ceramic composites and
`fabrication of composite components. Unfortunately,
`the physical properties of materials prepared by this
`method have not been determined.
`Gas Turbine Engine. Turbines have been in use
`in power plants, aircraft turboprops, helicopter engines,
`aircraft jet engines, marine engines, and hydrofoil craft
`engines and are considered a proven technology. These
`turbines are usually fabricated from costly superalloys,
`which makes them too expensive to be useful in a high-
`volume,
`low-cost industry such as automobiles. A
`ceramic-based turbine, on the other hand, could be
`prepared in large volumes at an acceptable cost. A
`vehicular gas turbine made of ceramics was developed
`in the 1970s and 1980s, and its entire gas flow path is
`shown in Figure 1.4 The objective of this research was
`to exclude cooling and demonstrate that the components
`can survive a 175 h duty cycle at 1055 °C and 25 h at
`1372 °C involving several starts and shutdowns. A
`design of this type was expected to offer several advan-
`tages such as low exhaust emission, multi-fuel capabil-
`ity, a simple low-maintenance machine, and low oil
`consumption.
`Initially, the ceramics of choice for the gas turbine
`engine were silicon nitride and silicon carbide. Later,
`silicon nitride (both reaction-bonded and hot-pressed)
`was found to be more suitable in the fabrication of
`various components.
`Adiabatic Engine. Figure 2 shows a simplified
`schematic adiabatic engine descried by Bryzik and
`Kamo and developed under tank-automotive command
`and the Cummins Engine Co. program.14 The engine
`is adiabatic in a thermodynamic sense, but there is still
`some heat loss. The diesel combustion chamber is
`insulated to allow operation at near adiabatic condi-
`tions.
`In addition, the thermal energy lost to the
`exhaust system is converted to useful power by turbo-
`machinery and there is no conventional forced cooling.
`
`Figure 2. Adiabatic engine. Reprinted with permission from
`SAE Paper Number 830314, copyright 1983 Society of Auto-
`motive Engineers, Inc.
`The insulated components are the piston, cylinder head
`valves, cylinder liner, exhaust valves, and exhaust ports.
`Thus the reduction in the lost energy and elimination
`of a conventional cooling system results in improved fuel
`efficiency and economy. Feasibility studies have been
`concentrated in the 250-500 HP range for military
`trucks. The materials for the application in adiabatic
`engines need to have the following characteristics:
`insulation capability, high expansion coefficient, high
`temperature capability, high strength, fracture tough-
`ness, high thermal shock resistance, low-friction and
`wear characteristics, and low cost. The material prop-
`erties are as follows:
`temperature limit (°C) >982,
`fracture toughness (KIC in MPa) >8.0, flexural strength
`(MPa) >800, thermal conductivity (K) <0.01, thermal
`shock resistance (∆T, °C) >500, and coefficient of
`expansion (10-6/°C) >10.14 Partially stabilized zirconia
`(PSZ) was considered to be a suitable material due to
`its good thermal shock resistance and flexural strength.15
`Ceramic coatings were found to be acceptable for
`insulation or corrosion resistance in high-temperature
`applications.16
`Another material of interest in developing low-fric-
`tion, unlubricated diesel engines was silicon carbide.17
`The interest in silicon carbide stems from properties
`such as wear resistance, lightweight, high-temperature
`stability, low coefficient of friction, potential low-cost
`fabrication, and corrosion resistance. A solid silicon
`carbide cylinder and pistons were tested for 47.5 h and
`performed reasonably well. Ultimately, failure occurred
`due to the failure of the piston/piston pin connection.
`Lithium aluminosilicate, LiAlSi4O12 (â-spodumene),
`was also investigated for manufacturing components of
`the ceramic diesel engine.17 Pistons and liners fabri-
`cated from lithium aluminosilicate were tested by the
`light-load firing test. After 3 min, the piston disinte-
`grated and the liner cracked. The failure mechanism
`is believed to involve the metal-to-ceramic interface.
`Materials for Gas Turbine and Adiabatic
`Engine Components
`The preceding discussion provides information on the
`selection criteria for the materials for the gas turbine
`
`106
`
`

`

`Attachment 2h: Copy of a publication citing Document 2
`Reviews
`Chem. Mater.,Vol.8, No. 5, 1996987
`and adiabatic engine components. In this section, we
`matrix and the reinforcing phase should be already
`describe structural details, classical method of prepara-
`debonded or should separate as the crack tip ap-
`proaches. Analysis of crack propagation in whisker-
`tion, and properties of silicon nitride and silicon carbide.
`reinforced silicon nitride has been carried out by Hutch-
`A large number of ceramic precursor routes for silicon
`inson.28 Finally, machining also affects the toughness
`carbide and silicon nitride are known and are sum-
`of silicon nitride bodies. For example, Tajima and
`marized in a recent book.18 Some recent developments
`in the use of preceramics as binders show promise and
`Urashima report that the coarse diamond abrasion of
`will be discussed. At least one report claims reproduc-
`fine-grain silicon nitride reduces its strength from
`ible shrinkage when preceramics are used as binders;
`∼1500 to ∼800 MPa.29
`this might reduce machining and the cost of fabricating
`The fabrication of components from silicon nitride
`components. If fabrication does not require machining
`powder is generally carried out by slip casting, injection
`and results in achieving properties comparable to those
`molding, or compression molding. Each method pro-
`achieved with the injection molding process, ceramic
`vides components of different characteristics. For ex-
`precursors as binders may be competitive.
`ample, slip cast Si3N4 has high strength, injection
`Silicon Nitride. Excellent strength, thermal shock
`molded Si3N4 material shows good thermal shock re-
`resistance, and oxidation behavior make silicon nitride
`sistance, and compression molded Si3N4 materials ex-
`a material of choice for gas turbine engines.19 Silicon
`hibit high hardness, good wear resistance, and low
`nitride was first prepared by Wo¨hler in 1857. Subse-
`thermal expansion. After fabrication, the components
`quently, its preparation from the carbothermal nitri-
`are machined, which substantially increases their cost.
`dation of silicon oxide, direct nitridation of silicon, and
`
`Reaction-Sintered Silicon Nitride. In reaction sinter-
`the decomposition of silicon amide [Si(NH)2] was re-
`ing, silicon powder is isostatically pressed, injection
`ported. Silicon nitride exists in R- and â-forms consist-
`molded, or slip-cast after demping with an organic
`ing of 8- and 12-membered rings of silicon and nitrogen
`binder. The nitridation of the compact is carried out
`that form interleaved sheets. Each silicon atom is
`at 1300-1450 °C. The process is unique in that no
`surrounded by four nitrogen atoms and occupies a
`dimensional changes occur during nitridation. In gen-
`tetrahedral site. The R-form, in addition, contains 0.04
`eral, reaction bonded silicon nitride components have
`oxygen/silicon. Of these two phases, â-Si3N4 is a long-
`high porosity. The properties of the reaction bonded
`grained material with high fracture toughness, and
`silicon nitride components also depend on the quality
`R-Si3N4 is equiaxed grained with low fracture tough-
`and particle size of the silicon powder, and temperature
`ness.20 Thus a high volume fraction of R-Si3N4 is
`of nitridation. For example, the modulus of rupture of
`necessary for the fabrication of high-strength parts.21
`a sample prepared at 1400 °C from -400 mesh silicon
`The R f â phase transformation is related to the
`powder is 23 400 ( 1380 psi.30 The modulus of rupture
`formation of elongated grains with high aspect ratio,
`of a sample prepared from 10 µm silicon nitridation at
`which is believed to increase fracture toughness to about
`1450 °C is 32 000 ( 4350 psi.31 Flame spraying also
`6 MPa m1/2.22 Grain growth is a function of sintering
`furnishes samples with a high modulus of rupture
`additives, impurities, and Si3N4.23 Highly elongated
`(33 900 ( 2200 psi,32 above 40 000 psi33 ). Purity (98-
`grains develop by gas-pressure sintering with a fracture
`99% vs 99-99.99%) does not seem to affect bend
`toughness of 9-11 MPa m1/2.24 Toughness increase is
`strength.30 The materials for fabrication of a gas
`also accomplished by whisker reinforcing.25 The tough-
`turbine engine should be capable of operating at 1260
`involving frictional
`ening contribution of whiskers,
`°C with a stress of 10 000 psi for 200 h while undergoing
`motion along the interface, is described by the following
`less than 0.5% creep strain.
`equation:26
`The primary advantage of injection molding is that
`complex shapes can be fabricated with minimum finish
`machining. However, the strength of injection molded
`silicon nitride components is far less than the stress in
`the disk of the wheel of a turbine rotor.
`Injection
`molding of silicon powder containing 0.02 wt % calcium
`gave a material of desired specification upon nitridation
`at 1282 °C with 1.8% H2 containing N2.34 The improved
`creep resistance is attributed to a fine-grained micro-
`structure and increased refractories at the grain bound-
`ary phase.
`In slip-casting, a stable suspension of silicon powder
`is poured into an absorbent mold. The mold absorbs
`most of the liquid, and the cast thus formed is subse-
`quently dried and sintered. The highest density cast
`is obtained from the lowest viscosity suspension. The
`nitridation results in high-density silicon nitride ware.
`The density and modulus of rupture of samples nitrided
`at 1093 °C for 18 h, 1260 °C for 24 h, and 1460 °C for
`18 h were 2.73 g/cm3 and 40 000 psi.35
`
`Hot-Pressed Silicon Nitride. Hot-pressed silicon ni-
`tride is sufficiently strong for the fabrication of rotor
`disks. Hot-pressed silicon nitride powders can be fully
`densified in the presence of small amounts of hot-
`
`DKpo ≈ {(σr)3dwfγrEc/12Erτi}1/2
`where τi is the shear resistance of the interface.
`Thus, toughness increases with increasing whisker
`contribution and whisker diameter. High whisker
`strength is needed to sustain frictional forces resisting
`whisker pullout. Since silicon nitride grains can also
`act like reinforcing whiskers, there is substantial re-
`search in progress to optimize grain size.27
`The structure of the interface between reinforcing
`grains and the matrix is very important for silicon
`nitride sintered components. The interface between the
`
`DKfb ≈ {(σr)2dwfγrEc/24Erγi}1/2
`where γr/γi ) µσr/τdb. The terms σr, dw, fγr, Er, γi and
`τdb, Ec, and µ represent tensile strength, diameter,
`volume fraction, fracture energy, and Young’s modulus
`of the reinforcing phase, interfacial debonding energy,
`Young’s modulus of the composite, and the friction
`coefficient along the interface, respectively. The whis-
`ker pullout toughening contribution is described as
`follows:
`
`107
`
`

`

`4.3
`3.2
`
`84-39
`28-12
`
`Reviews
`Table 2. Some Properties of Silicon Carbide
`coeff of
`Young’s
`thermal
`thermal
`modulus
`conductivity
`expansion R
`(104 MN m-2)
`K(wm-1 °C-1)
`(10-6 °C-1)
`40.7
`4.8
`100-150
`34.5
`4.4
`100-50
`34.0
`70
`41.4
`41.3
`27.6
`
`Attachment 2h: Copy of a publication citing Document 2
`988 Chem.Mater., Vol. 8, No. 5, 1996
`pressing aids.36 Terwilliger and Lange suggest that at
`high temperatures a liquid phase is formed which
`promotes densification by a solution-reprecipitation
`mechanism.37 The microstructure of the silicon nitride
`type
`powder influences the strength and fracture energy of
`sintered (R)
`hot-pressed silicon nitride.21
`reaction bonded
`The grain boundary governs the mechanical proper-
`(20% v/v free Si)
`ties of the components at high temperatures. A liquid
`SiC fiber-Si
`phase, proposed to be magnesium silicate, forms on the
`composite
`grain boundaries when hot pressing is done in the
`CVD
`REFEL SiC
`presence of MgO.38 The formation of magnesium cal-
`sintered Si3N4
`cium silicate has been shown by the Auger analysis of
`(alumina additive)
`the fracture surface. A depth profile of the samples also
`suggests incorporation of oxide impurities in the bound-
`cally pressed to form green compacts which are then
`ary phase.39 Thus, hot pressing aids and oxide impuri-
`pyrolyzed under argon or ammonia to obtain SiCN or
`ties can be expected to influence the high-temperature
`Si3N4 parts, respectively. These parts are X-ray amor-
`strength of hot-pressed silicon nitride. For example, a
`phous and crack-free and attain 93% density. The
`sample of R-Si3N4 from Westinghouse containing about
`mechanical strength is 375 MPa and Vickers hardness
`100 ppm calcium was hot pressed with 5 wt % MgO and
`is 9.5 GPa for SiCN-based parts.
`was found to have a flexural strength of 60 Ksi at 1400
`Silicon Carbide. The low fracture toughness of
`°C.40
`silicon carbide has limited its applications in the
`Ceramic Precursors in Fabrication of Silicon Nitride
`automotive industry, but it has found some use in
`Bodies. The past 10 years have seen an explosion of
`automobile exhaust-valve systems and turbocharger
`research on ceramic precursors for silicon nitride ma-
`rotors.46 The possible number of polytypes for silicon
`terials.18 A large number of papers and patents claim
`carbide have been calculated to be 93 813 567,47 and
`the suitability of ceramic precursors in the fabrication
`approximately 200 polytypes have been identified.48 The
`of components of various shapes and forms. Unfortu-
`common polytypes are 2H, 6H, 15R, and 3C. The
`nately, information on actual fabrications of components
`chemistry of sintering aids and metallic impurities
`and their properties is nonexistent. Ceramic precursor
`influence â f R phase transformations.49 The tough-
`routes would be ideal if the precursors (i) are soluble in
`ness of silicon carbide can be improved by sintering with
`common organic solvents and can be molded into the
`alumina, and the high toughness has also been cor-
`shape of the components and (ii) form silicon nitride in
`related with the aspect ratio of grains.50 Aluminum-
`high yields. However, shrinkage and high porosity can
`doped â-SiC powders also show similar grain structure
`be expected due to densification and gas evolution,
`in sintered bodies.51 The proposed sintering mechanism
`respectively. A more practical application of a ceramic
`suggests the formation of aluminum silicate which
`precursor is as a binder which was first employed by
`promotes liquid-phase sintering.52
`Yajima for silicon carbides.41
`There are several known methods for the fabrication
`In a patent, Lukacs claims that a mixture of silicon
`of SiC components. The sintering of pure SiC powders
`nitride powder, poly[(methylvinyl)silazane],
`[(CH3-
`shows no shrinkage. Pressureless sintering of SiC is
`SiHNH)0.8(CH3SiCHdCH2NH)0.2]n, and sintering aids
`considered to be an important development.53 This is
`can be injection molded and cured at 150 °C.42 After
`done with B and C addition, but the fracture toughness
`curing, densified and sintered silicon nitride bodies are
`of these materials is low. Control of composite micro-
`obtained which retain the shape of the article. Unfor-
`structure by liquid-phase sintering has also been at-
`tunately, there is no information on the properties of
`tempted.54 Injection molding is considered to be suit-
`the silicon nitride components. Schwark et al. report
`able for mass-scale production and is described in detail.
`the application of isocyanate modified polysilazane43
`Some properties of silicon carbide, prepared by various
`prepared from the reaction of phenylisocyanate with
`methods, are summarized in Table 2.
`[(MeSiHNH)0.8(CH2dCHSiMeNH)0.2]x as a binder. In
`
`Injection Molding of Silicon Carbide Components.
`this process, the polymer is mixed with silicon nitride
`Silicon carbide components need to have a Weibull
`and sintering aids and injected at 3500 MPa into a
`strength of 80 Ksi and a Weibull modulus of 16 to be
`heated (>150 °C) mold.44 The resulting silicon nitride
`considered suitable. The fabrication process needs to
`green bodies with 8 wt % binder exhibit a four-point
`be simple and suitable for mass production. Consider-
`strength above 8 MPa. On firing, a shrinkage of less
`ing these two parameters, injection molding was rec-
`than 10% is observed. The flexural strength and
`ognized to be method of choice.
`In this process, a
`fracture toughness of the sintered silicon nitride bodies
`powder of silicon carbide and sintering aids is suspended
`are claimed to be comparable to traditional injection
`in an organic binder and the resulting slurry is injection
`molded bodies, although no data are presented.
`molded into complex shapes. After removal of the
`Riedel et al.45 prepared ceramic parts from polysila-
`binder, the green body is sintered to obtain the final
`zane NCP 200 which are first thermally treated to an
`compact. The method employed by Whalen et al. to
`infusible, trisilylated nitrogen structure:
`fabricate turbochargers is described here.55
`Commercial â-silicon carbide (Ibiden UF lot 0166),
`350 °C
`[-RSiH-NH-]n98
`amorphous boron (Starck lots 3506), and lampblack
`[-RSiH-NH-]n-m[-RSiN-]m + mH2
`carbon (Monsanto TL 246) were first analyzed to
`determine purity. The silicon carbide contained 1.6 wt
`The infusible material is then powdered and isostati-
`% oxygen which is typical of ultrafine powders (due to
`
`108
`
`

`

`Attachment 2h: Copy of a publication citing Document 2
`Reviews
`Chem. Mater.,Vol.8, No. 5, 1996989
`the SiO2 coating) and 0.7 wt % free carbon. The carbon
`components. There are some important results on the
`and boron also showed some oxygen due to adsorbed
`fabrication of ceramic components using ceramic pre-
`cursors as binders which will be summarized here.
`moisture. The mean particle sizes for SiC, B, and C
`were 1.18, 2.70, and 5.62 µm, respectively. Polytype
`The application of ceramic precursors as binder is not
`analysis of SiC was carried out by X-ray diffraction. The
`a new concept. Yajima showed that R-SiC (particle size
`SiC was primarily 3C (85%) and the remaining material
`3 µm, purity 99.99%) mixed with polycarbosilane (aver-
`was a disordered phase. The SEM micrographs showed
`age mol wt 800)57 can be pressed into green bodies (10
`that the particles were equiaxed and rounded with a
`× 30 × 4 mm). These green bodies can then be sintered
`few grains showing well-defined cleavage faces.
`at 700-1400 °C without shrinkage or expansion.58
`A 55.5 vol % powder (97% SiC powder, 2% carbon
`black, and 1% amorphous boron) was mixed with a 44.5
`vol % thermoplastic binder. The mixture was warmed
`and homogenized in a high-shear Haake mill. The
`homogeneous mixture was injection molded into test
`bars. The dewaxing of the bars was carried out under
`vacuum conditions. The resulting bars showed a mean
`warpage of 0.001 94 in. and mean linear shrinkage of
`3.89%. The bars were sintered at 2100 °C in vacuum
`for 10 min. The average density of the bars was 94%
`of theoretical density (3.17 g/cm3) as sintered and
`increased to 95.5% after machining.
`The bars showed reduced oxygen (0.006%) and excess
`carbon (2%). The higher carbon content is due to the
`addition of carbon black as a sintering aid. The micro-
`structure of the bars was comprised of uniform particles
`of 6 µm with evenly distributed small pores and no
`evidence of needle or feather morphology which is
`typical of oversintering. There was a random distribu-
`tion of low density inclusions of sizes up to 80 microme-
`ters. The X-ray diffraction analysis showed 52.9% 3C
`SiC with a balance of disordered SiC. At room temper-
`ature, the mean modulus of rupture, Weibull charac-
`teristic, and Weibull modulus were 43.3 Ksi, 45.8 Ksi,
`and 8.0, respectively. The flexural stress rupture
`measurements at 1400 °C and applied stress of 25 and
`30 Ksi showed behavior typical of heterogenous materi-
`als with a failure time range of 5-163 h.
`SEM analysis shows four types of defects in the
`sintered bars. The first type of defect is caused by
`poorly sintered SiC particles. This defect has nearly
`spherical 25-30 µm shapes and is probably formed from
`agglomerates which do not contain sintering aids. The
`second type of defect is caused by grain growth which
`is also spherical in shape. The third p