Handbook of Plastics, Elastomers and Composites Part 9 potx

Although CMCs are not as widely used at this time, there are notable applications that are indicative of their great potential.The main types of reinforcements used in composite material

320 Chapter Four

30 Bergland, Lars in S.T Peters, ed Handbook of Composites, 2nd ed., p 116, Chapman

& Hall, London, 1998

31 Loos, Alfred C and Springer, George, S.J., Comp Mater., March 17, 1983, pp.

34 Ko, Frank K., in Engineered Materials Handbook, Vol 1, Composites, Theodore

Re-inhart, Tech Chairman, ASM International, 1987, p 519

35 Hancock, P and Cuthbertson, R.C., J Mat Sci., 5, 762–768, 1970.

36 Kohkonen, K.E and Potdar, N., in S.T Peters, ed Handbook of Composites, 2nd ed.,

Chapman and Hall, p 598, London, 1998

37 Abrate, S., in P.K Mallick, ed., Composites Engineering Handbook, Marcel Dekker,

New York, NY, 1997, p 783

38 Freeman, W.T and Stein, B A., Aerospace America, Oct 1985, pp 44–49.

39 Heil, C., Dittman, D., and Ishai, O., Composites (24) no 5, 1993, pp 447–450.

40 Chan, W.S., in P.K Mallick, ed., Composites Engineering Handbook, Marcel Dekker,

New York, NY, 1997, pp 357–364

41 Baker, A., P.K Mallick, ed., Composites Engineering Handbook, Marcel Dekker, p.

tion, Long Beach, CA, Jan 1979

45 Nelson, W.D., Bunin, B.L., and Hart-Smith, L.J., Critical Joints in Large Composite

Aircraft Structure, in Proc 6th Conf Fibrous Composites in Structural Design, Army

Materials and Mechanics Research Center Manuscript Report AMMRC MS 83-2(1983), pp U-2 through II-38

46 Baker, A., P.K Mallick, ed., Composites Engineering Handbook, Marcel Dekker, p.

674, New York, NY, 1997

47 Peters, S.T., Humphrey, W D., and Foral, R., Filament Winding, Composite Structure Fabrication, 2nd ed., SAMPE Publishers, 1999, Covina, CA, pp 9–13.

48 Kranbuehl, David E., in International Encyclopedia of Composites, Vol 1, pp.

531–543, Stuart M Lee, ed., VCH Publishers, New York

49 Whitney, J.M., Daniel, I.M., and Pipes, R.B., Experimental Mechanics of Fiber forced Composite Materials, SESA Monograph No 4, The Society for Experimental

Rein-Stress Analysis, Brookfield Center, Connnecticut, 1982

50 Carlsson, L.A and Pipes, R.B., Experimental Characterization of Advanced ite Materials, Prentice-Hall, Englewood Cliffs, N.J., 1987.

Compos-51 Munjal, A., SAMPE Quarterly, Jan 1986.

52 Safe Handling of Advanced Composite Materials Components, Health Association,Arlington, VA, April 1989

5 Metal Matrix Composites, Ceramic Matrix Composites, Carbon Matrix Composites, and Thermally Conductive Polymer

The development of composite materials and the related design and manufacturingtechnologies is one of the most important advances in the history of materials Compositesare multifunctional materials having unprecedented mechanical and physical propertiesthat can be tailored to meet the requirements of a particular application Some compositesalso exhibit great resistance to high-temperature corrosion, oxidation, and wear Theseunique characteristics provide the engineer with design opportunities not possible withconventional monolithic (unreinforced) materials Composites technology also makes pos-sible the use of an entire class of solid materials, ceramics, in applications for whichmonolithic versions are unsuited because of their great strength scatter and poor resistance

to mechanical and thermal shock Furthermore, many manufacturing processes for posites are well adapted to the fabrication of large, complex structures This allows consol-idation of parts, which can reduce manufacturing costs

com-In recent years, carbon fibers with thermal conductivities much greater than that of per have been developed These reinforcements are being used in polymer, metal, and car-bon matrices to create composites with high thermal conductivities that are being used inapplications for which thermal management is important Discontinuous versions of thesefibers are also being incorporated in thermoplastic injection molding compounds, improv-ing their thermal conductivity by as much as two orders of magnitude or more Thisgreatly expands the range of products for which injection molded polymers can be used

■ Thermal control and electronic packaging

■ Automobile, train, and aircraft structures

■ Mechanical components, such as brakes, drive shafts, and flywheels

■ Tanks and pressure vessels

■ Dimensionally stable components

■ Process industries equipment requiring resistance to high-temperature corrosion, tion, and wear

oxida-■ Offshore and onshore oil exploration and production

■ Marine structures

■ Sports and leisure equipment

■ Biomedical devices

■ Civil engineering structures

The resulting increases in production volumes have helped to reduce material prices, creasing their attractiveness in cost-sensitive applications

in-It should be noted that biological structural materials occurring in nature are typicallycomposites Common examples are wood, bamboo, bone, teeth, and shell Furthermore,use of artificial composite materials is not new Bricks made from straw-reinforced mudwere employed in biblical times This material also has been widely used in the American

Southwest for centuries, where it is known as adobe In current terminology, it would be called an organic fiber-reinforced ceramic matrix composite.

To put things in perspective, it is important to consider that modern composites ogy is only several decades old This is an extremely short period of time compared withother materials, such as metals, which go back millennia In the future, improved and newmaterials and processes can be expected It is also likely that new concepts will emerge,such as greater functionality, including integration of electronics, sensors, and actuators There is no universally accepted definition of a composite material A good description

technol-of a composite is a material consisting technol-of two or more distinct materials bonded together.1This differentiates composites from materials such as alloys

Solid materials can be divided into four categories (polymers, metals, ceramics, andcarbon) We consider carbon as a separate class because of its unique characteristics Wefind both reinforcements and matrix materials in all four categories This results in the po-tential for a limitless number of new material systems having unique properties that cannot

be obtained with any single monolithic material Table 5.1 shows the types of materialcombinations that are now in use

Composites are usually classified by the type of material used for the matrix The fourprimary categories of composites are polymer matrix composites (PMCs), metal matrixcomposites (MMCs), ceramic matrix composites (CMCs), and carbon matrix composites(CAMCs) The last category, CAMCs, includes carbon/carbon composites (CCCs), whichconsist of carbon matrices reinforced with carbon fibers For decades, CCCs were the onlysignificant type of CAMC However, there are now other types of composites utilizing acarbon matrix Notable among these is silicon carbide fiber-reinforced carbon, which isbeing used in military aircraft gas turbine engine components

The characteristics of the four classes of matrix materials used in composites differ ically Table 5.2 presents properties of selected matrix materials from the four classes

rad-Note that the densities, moduli, strengths, and failure strains differ greatly These and otherdifferences result in composite materials that have very dissimilar characteristics.

Composites are now important commercial and aerospace materials.2 At this time,PMCs are the most widely used composites MMCs are employed in a significant and in-creasing number of commercial and aerospace applications, such as automobile engines,electronic packaging, cutting tools, circuit breaker contact pads, high-speed and precisionmachinery, and aircraft structures CCCs are used in high-temperature, lightly loaded ap-plications, such as aircraft brakes, rocket nozzles, glass processing equipment, and heattreatment furnace support fixtures and insulation Although CMCs are not as widely used

at this time, there are notable applications that are indicative of their great potential.The main types of reinforcements used in composite materials include aligned continu-ous fibers, discontinuous fibers, whiskers (elongated single crystals), particles, and numer-

Reinforcement Matrix Polymer Metal Ceramic Carbon

Modulus GPa (Msi)

Tensile strength MPa (Ksi)

Tensile failure strain (%)

Thermal conductivity W/mK (Btu/hr·ft·°F)

Coefficient

of thermal expansion ppm/K (ppm/°F) Epoxy Polymer 1.8 (0.065) 3.5 (0.5) 70 (10) 3 0.1 (0.06) 60 (33) Aluminum

re-In addition, carbon fibers are combined with glass or aramid to improve impact resistance.Laminates combining composites and metals, such as “Glare,” which consists of layers ofaluminum and glass fiber-reinforced epoxy, are being used in aircraft structures to improvefatigue resistance.

Composites are strongly heterogeneous materials That is, the properties of a compositevary considerably from point to point in the material, depending on the material phase inwhich the point is located Monolithic ceramics, metallic alloys, and intermetallic com-pounds are usually considered to be homogeneous materials, as a first approximation.Many artificial composites, especially those reinforced with fibers, are anisotropic,which means their properties vary with direction (the properties of isotropic materialsare the same in every direction) This is a characteristic they share with a widely usednatural fibrous composite, wood As for wood, when structures made from artificial fi-brous composites are required to carry load in more than one direction, they are typicallyused in laminated form It is worth noting that the strength properties of some metalsalso vary with direction This is typically related to the manufacturing process, such asrolling

With the exception of MMCs, composites do not display plastic behavior as monolithicmetals do, which makes composites more sensitive to stress concentrations However, theabsence of plastic deformation does not mean that composites should be considered brittlematerials like monolithic ceramics The heterogeneous nature of composites results incomplex failure mechanisms that impart toughness Fiber-reinforced materials have beenfound to produce durable, reliable structural components in countless applications.2 Forexample, PMCs have been used in production boats, electrical equipment, and solid rocketmotors since the 1950s, and in aircraft since the early 1970s The technology has pro-gressed to the point where the entire empennage (tail section) of the Boeing 777 is made

■ Tailorable mechanical and physical properties

■ High strength

■ High modulus

■ Low density

■ Excellent resistance to fatigue, creep, creep rupture, corrosion, and wear

Composites are available with tailorable thermal and electrical conductivities that rangefrom very low to very high Composites are available with tailorable coefficients of ther-mal expansion (CTEs) ranging from –2 to + 60 ppm/K (1 to 30 ppm/°F)

As for monolithic materials, each of the four classes of composites has its own lar attributes For example, CMCs tend to have particularly good resistance to corrosion,oxidation, and wear, along with high-temperature capability

particu-rials, including carbon, aramid, glasses, oxides, boron, and so on Carbon, glass, andaramid fibers are probably the most important at this time

There are dozens of different types of commercial carbon fibers The stiffest versionshave moduli of 965 GPa (140 Msi) Strengths top out at 7 GPa (1,000,000 lb/in2) Carbonfibers are made from several types of precursor materials, polyacrylonitrile (PAN), petro-leum pitch, coal tar pitch, and rayon Except for a few applications initially developedmany years ago, rayon-based carbon fibers are no longer of great importance Characteris-tics of the two types of pitch-based fibers tend to be similar but very different from thosemade from PAN The key types of carbon fibers are standard modulus (SM) PAN, interme-diate modulus (IM) PAN, ultrahigh modulus (UHM) PAN, and ultrahigh modulus (UHM)pitch The strongest UHS carbon fibers are forms of intermediate modulus (IM) fibers.Carbon fiber cost varies greatly The least expensive industrial versions are now availablefor about USD 10/kg (USD 5/lb)

The outstanding mechanical properties of composite materials has been a key reasonfor their extensive use in structures However, composites also have important physicalproperties, especially low, tailorable coefficient of thermal expansion (CTE) and high ther-mal conductivity, which are key reasons for their selection in an increasing number of ap-plications Key examples are electronic packaging and thermal management.19–21

Many composites, such as PMCs reinforced with carbon and aramid fibers, and siliconcarbide particle-reinforced aluminum, have low CTEs, which are advantageous in applica-tions requiring dimensional stability Examples include spacecraft structures, instrumentstructures, and optical benches.22 By appropriate selection of reinforcements and matrixmaterials, it is possible to produce composites with near-zero CTEs

Coefficient of thermal expansion tailorability provides a way to minimize thermalstresses and distortions that often arise when dissimilar materials are joined For exam-ple, the CTE of silicon carbide particle-reinforced aluminum depends on particle con-tent By varying the amount of reinforcement, it is possible to match the CTEs of avariety of key engineering materials, such as steel, titanium, and alumina (aluminum ox-ide).23

The ability to tailor CTE is important in many applications For example, titanium tings are often used with carbon/epoxy (C/Ep) structures instead of aluminum, because thelatter has a much larger CTE that can cause high thermal stresses under thermal cycling.Another application for which CTE is important is electronic packaging, because thermalstresses can cause failure of ceramic substrates, semiconductors, and solder joints

fit-A unique and increasingly important property of some composites is exceptionally highthermal conductivity This is leading to increasing use of composites in applications forwhich heat dissipation is a key design consideration In addition, the low densities of com-posites make them particularly advantageous in thermal control applications for whichweight is important An important recent breakthrough is the development of injectionmolded PMCs with thermal conductivities as high as 100 W/m·K (58 Btu/hr·ft·°F) This isdiscussed in Sec 5.5

There are a large and increasing number of thermally conductive PMCs, MMCs, andCAMCs One of the most important types of reinforcements for these materials is pitch fi-bers.11 PAN-based fibers have relatively low thermal conductivities.10 However, pitch-based fibers with thermal conductivities more than twice that of copper are commerciallyavailable These ultrahigh thermal conductivity (UHK) reinforcements also have very highstiffnesses and low densities Fibers made by chemical vapor deposition (CVD), also

called vapor-grown fibers, have reported thermal conductivities as high as 2000 W/m·K

(1160 Btu/hr·ft·°F), about five times that of copper.24 Fibers made from another form ofcarbon, diamond, also have the potential for thermal conductivities in this range PMCsand CCCs reinforced with UHK carbon fibers are being used in a wide range of applica-

Composites are complex, heterogeneous, and often anisotropic material systems Their

properties are affected by many variables, including in situ constituent properties;

rein-forcement form, volume fraction, and geometry; properties of the interphase, the region

where the reinforcement and matrix are joined (also called the interface); and void

con-tent The process by which the composite is made affects many of these variables.26 posites containing the same matrix material and reinforcements, when combined bydifferent processes, may have very different properties

Com-Several other important things must be kept in mind when considering composite erties For one, most composites are proprietary material systems made by proprietaryprocesses There are few industry or government specifications for composites as there arefor many structural metals However, this is also the case for many monolithic ceramicsand polymers, which are widely used engineering materials Despite their inherently pro-prietary nature, there are some widely used composite materials made by a number ofmanufacturers that have similar properties Notable examples are standard modulus (SM)and intermediate modulus (IM) carbon fiber-reinforced epoxy

prop-Another critical issue is that properties are sensitive to the test methods by which theyare measured, and there are many different test methods used throughout the industry.27,28Furthermore, test results are very sensitive to the skill of the technician performing them.Because of these factors, it is very common to find significant differences in reportedproperties of what is nominally the same composite material

There is often a great deal of confusion among those unfamiliar with composites aboutthe effect of reinforcement form The properties of composites are very sensitive to rein-forcement form, volume fraction, and internal reinforcement geometry

It is important to keep in mind that one of the key problems with using discontinuousfiber reinforcement is that it is often difficult to control fiber orientation For example,material flow can significantly align fibers in some regions This affects all mechanicaland physical properties, including modulus, strength, CTE, thermal conductivity, etc Forexample, if there is significant flow in a region, strength properties perpendicular to thefiber direction in this area may be low This has been a frequent source of componentfailures

Traditional fabric reinforcements have fibers oriented at 0 and 90° For the sake of pleteness, we note that triaxial fabrics, which have fibers at 0°, +60°, and –60°, are nowcommercially available Composites using a single layer of this type of reinforcement areapproximately quasi-isotropic, which means that they have the same in-plane elastic (butnot strength) properties in every direction Their thermal conductivity and CTE are alsoapproximately isotropic in the plane of the fabric

com-Composites also offer a number of significant manufacturing advantages over monolithicmetals and ceramics For example, fiber-reinforced polymers and ceramics can be fabri-cated in large, complex shapes that would be difficult or impossible to make with othermaterials.26,29–31 The ability to fabricate complex shapes allows consolidation of parts,which reduces machining and assembly costs Some processes allow fabrication of parts

in their final shape (net shape) or close to their final shape (near-net shape), which alsoproduces manufacturing cost savings The relative ease with which smooth shapes can bemade is a significant factor in the use of composites in boats, aircraft, and other applica-tions for which aerodynamic considerations are important Manufacturing considerationsfor each of the four classes of composites are discussed in the following sections, alongwith their properties

5.5 Polymer Matrix Composites

Chapter 4 presents a thorough discussion of polymer matrix composites focused primarily

on structural applications This chapter emphasizes physical properties, but mechanicalproperties are presented for completeness

The high thermal conductivities of some PMCs has led to their increasing use in cations like spacecraft structures and electronic packaging components, e.g., printed circuitboard heat sinks, heat spreaders, and heat sinks used to cool microprocessors The addition

appli-of thermally conductive carbon fibers and ceramic particles to thermoplastics has openedthe door to use of injection molded parts for which plastics previously could not be usedbecause of their low thermal conductivities We consider some examples in this section.There are important issues that must be discussed before presenting composite proper-ties The traditional structural materials are primarily metal alloys for most of which thereare industry and government standards The situation is very different for composites.Most reinforcements and matrices are proprietary materials for which there are no stan-dards In addition, many processes are proprietary This is similar to the current situationfor most polymers and ceramics The matter is further complicated by the fact that thereare many test methods in use to measure mechanical and physical properties.27,28 As a re-sult, there are often conflicting material property data in the usual sources used by engi-neers, published papers, and manufacturers’ literature The data presented in this chapterrepresent a carefully evaluated distillation of information from many sources However, inview of the uncertainties discussed, the properties presented in this chapter should be con-sidered approximate values

Polymers are relatively weak, low-stiffness materials with low thermal conductivitiesand high coefficients of thermal expansion To obtain materials with mechanical propertiesthat are acceptable for structural applications, it is necessary to reinforce them with contin-uous or discontinuous fibers The addition of ceramic or metallic particles to polymers re-sults in materials that have increased modulus As a rule, strength typically does notincrease significantly and may actually decrease However, there are many particle-rein-forced polymers used in electronic packaging, primarily because of their physical proper-ties For these applications, ceramic particles such as alumina, aluminum nitride, boronnitride, and even diamond are added to obtain an electrically insulating material withhigher thermal conductivity and lower CTE than the monolithic base polymer Metallicparticles such as silver and aluminum are added to create materials that are both electri-cally and thermally conductive These materials have replaced lead-based solders in someapplications There are also magnetic composites made by incorporating ferrous or perma-nent magnet particles in various polymers A common example is magnetic tape used torecord audio and video

328 Chapter Five

Polymer matrices generally are relatively weak, low-stiffness, viscoelastic materials.They also have very low thermal and electrical conductivities The strength and stiffness

of PMCs come primarily from the fiber phase

In a vacuum, resins outgas water and organic and inorganic chemicals, which can dense on surfaces with which they come in contact This can be a problem in optical sys-tems and electronic packaging Outgassing can result in corrosion and affect surfaceproperties critical for thermal control, such as absorptivity and emissivity Outgassing can

con-be controlled by resin selection and baking out the component, followed by storage in adry environment

For a wide range of applications, composites reinforced with continuous fibers are themost efficient structural materials at low to moderate temperatures Consequently, we fo-cus on them Table 5.3 presents room-temperature mechanical properties of unidirectionalpolymer matrix composites reinforced with key fibers: E-glass, aramid, boron, standardmodulus (SM) PAN (polyacrylonitrile) carbon, intermediate modulus (IM) PAN carbon,ultrahigh modulus (UHM) PAN carbon, ultrahigh modulus (UHM) pitch carbon, and ul-trahigh thermal conductivity (UHK) pitch carbon The fiber volume fraction is 60 percent,

a typical value

The properties presented in Table 5.3 are representative of what can be obtained atroom temperature with a well made PMC employing an epoxy matrix Epoxies are widelyused, provide good mechanical properties, and can be considered a reference matrix mate-rial Properties of composites using other resins may differ from these This has to be ex-amined on a case-by-case basis

The properties of PMCs, especially strengths, depend strongly on temperature Thetemperature dependence of polymer properties differs considerably This is also true fordifferent epoxy formulations, which have different cure and glass transition temperatures The properties shown in Table 5.3 are axial, transverse and shear moduli, Poisson’s ra-tio, tensile and compressive strengths in the axial and transverse directions, and in-plane

shear strength The Poisson’s ratio presented is called the major Poisson’s ratio It is

de-fined as the ratio of the magnitude of transverse strain divided by the magnitude of axialstrain when the composite is loaded in the axial direction Note that transverse moduli andstrengths are much lower than corresponding axial values

Carbon fibers display nonlinear stress-strain behavior Their moduli increase under creasing tensile stress and decrease under increasing compressive stress This makes themethod of calculating modulus critical Various tangent and secant definitions are usedthroughout the industry, contributing to the confusion in reported properties For example,

in-on in-one program, it was found that the fiber supplier, prepreg supplier, and end user wereall using different definitions of modulus, resulting in significantly different values The moduli presented in Table 5.3 are based on tangents to the stress-strain curves atthe origin Using this definition, tensile and compressive moduli are usually very similar.However, this is not the case for moduli computed using various secants These typicallyproduce compression moduli that are significantly lower than tensile moduli, because thestress-strain curves are nonlinear

As a result of the low transverse strengths of unidirectional laminates, they are rarelyused in structural applications The design engineer selects laminates with layers in severaldirections to meet requirements for strength, stiffness, buckling, etc There are an infinitenumber of laminate geometries that can be selected For comparative purposes, it is useful

to consider quasi-isotropic laminates, which have the same elastic properties in all tions in the plane

direc-Laminates have quasi-isotropic elastic properties when they have the same percentage

of layers every 180/n degrees, where n≥ 3 The most common quasi-isotropic laminateshave layers that repeat every 60°, 45°, or 30° We note, however, that strength properties inthe plane are not isotropic for these laminates, although they tend to become more uniform

330 Chapter Five

as the angle of repetition becomes smaller Laminates have quasi-isotropic CTE and

coef-ficient of expansion when they have the same percentage of layers in every 180/m degrees, where m≥ 2 For example, laminates with equal numbers of layers at 0° and 90° havequasi-isotropic thermal properties

Table 5.4 presents the mechanical properties of quasi-isotropic laminates having equalnumbers of layers at 0°, +45°, -45°, and 90° The elastic moduli of all quasi-isotropic lam-inates are the same for a given material Note that the moduli and strengths are much lowerthan the axial properties of unidirectional laminates made of the same material In manyapplications, laminate geometry is such that the maximum axial modulus and tensile andcompressive strengths fall somewhere between axial unidirectional and quasi-isotropicvalues

Table 5.5 presents physical properties of selected unidirectional composite materialshaving a typical fiber volume fraction of 60 percent The densities of all of the materialsare considerably lower than that of aluminum, and some are lower than that of magne-sium This reflects the low densities of both fibers and matrix materials The low densities

of most polymers give PMCs a significant advantage over most MMCs and CMCs, allother things being equal

As Table 5.5 shows, all of the composites have relatively low axial CTEs This resultsfrom the combination of low fiber axial CTE, high fiber stiffness, and low matrix stiffness.The CTE of most polymers is very high Note that the axial CTEs of PMCs reinforcedwith aramid fibers and some carbon fibers are negative This means that, contrary to thegeneral behavior of most monolithic materials, they contract when heated The transverseCTEs of the composites are all positive, and their magnitudes are much larger than themagnitudes of the corresponding axial CTEs This results from the high CTE of the matrixand a Poisson effect caused by constraint of the matrix in the axial direction and lack ofconstraint in the transverse direction The transverse CTE of aramid composites is particu-larly high, in part because the fibers have a relatively high positive radial CTE

The axial thermal conductivities of composites reinforced with glass, aramid, boron,and a number of the carbon fibers are relatively low In fact, E-glass and aramid PMCs areoften used as thermal insulators As Table 5.5 shows, most PMCs have low thermal con-ductivities in the transverse direction, as a result of the low thermal conductivities of thematrix and the fibers in the radial direction Through-thickness conductivities of laminatestend to be similar to the transverse thermal conductivities of unidirectional composites Table 5.6 shows the in-plane thermal conductivities and CTEs of quasi-isotropic lami-nates made from the same materials as in Table 5.5 Here again, a fiber volume fraction of

60 percent is assumed

Note that the CTEs of the quasi-isotropic composites are higher than the axial values ofcorresponding unidirectional composites However, the CTEs of quasi-isotropic compositesreinforced with aramid and carbon fibers are still very small By appropriate selection of fi-ber, matrix, and fiber volume fraction, it is possible to obtain quasi-isotropic materials withCTEs very close to zero The through-thickness CTEs of these laminates are typically posi-tive and relatively large However, this is not a significant issue for most applications Oneexception is optical mirrors, for which through-thickness CTE can be an important issue.The in-plane thermal conductivity of quasi-isotropic laminates reinforced with UHMpitch carbon fibers is similar to that of aluminum alloys, while UHK pitch carbon fibersprovide laminates with a conductivity over 50 percent higher Both materials have densi-ties about 35 percent lower than aluminum

As mentioned earlier, through-thickness thermal conductivities of laminates tend to besimilar to the transverse thermal conductivities of unidirectional composites, which arerelatively low If laminate thickness is small, this may not be a severe limitation However,low through-thickness thermal conductivity can be a significant issue for thick laminatesand for very high thermal loads This issue needs to be addressed on a case-by-case basis

T

T

T

334 Chapter Five

A significant recent advance in PMC technology is the development of able materials with much higher thermal conductivities than those available in the past.Unreinforced polymers have thermal conductivities in the range of 0.2 W/m·K (0.1 Btu/hr·ft·°F) A number of commercially available PMCs consisting of thermoplastic matricesreinforced with discontinuous carbon fibers have reported thermal conductivities rangingfrom 2 W/m·K (1.2 Btu/hr·ft·°F) to as high as 100 W/m·K (58 Btu/hr·ft·°F).33 Matrices in-clude PPS, nylon 6, polycarbonate and liquid crystal polymers These materials are electri-cally conductive Electrically insulating PMCs reinforced with thermally conductivediscontinuous ceramic reinforcements have reported thermal conductivities of up to 15 W/m·K (8.8 Btu/hr·ft·°F)

injection-mold-Thermally conductive injection molding compounds are now being used in a significantand increasing number of applications for which unreinforced polymers and traditional in-jection molding compounds, because of their low thermal conductivities, are not accept-able As discussed earlier, the mechanical and physical properties of fiber-reinforcedinjection molded PMCs are affected by fiber orientation induced by material flow In addi-tion to the ability to dissipate heat, another advantage of these PMCs is that their use re-sults in lower temperatures and thermal gradients, which tends to reduce distortion On thenegative side, as reinforcement volume fraction increases, fracture toughness decreases.Nevertheless, these materials present the design engineer with a greater range of optionsfor injection molded parts than in the past

Figure 5.1 shows a stepper motor having an injection molded carbon fiber-reinforcedPPS combination enclosure-heat sink.34 In this application, the PMC replaces die-cast alu-minum Although the composite thermal conductivity of 20 W/m·K (12 Btu/hr·ft·°F), islower than that of die cast aluminum, it meets the required maximum operating tempera-ture of 60°C, which is acceptable for this application The reason is that the thermal design

is convection limited The lower composite thermal conductivity only increases operatingtemperature by 2 to 5°C Injection molded thermally conductive thermoplastic PMCs arealso being used in electronic packaging Figure 5.2 shows a microprocessor heat spreader

5.6 Metal Matrix Composites

MMCs consist of metals reinforced with a variety of ceramic and carbon fibers, whiskers,and particles.35 There are wide ranges of materials that fall in this category An importantexample is a material consisting of tungsten carbide particles embedded in a cobalt matrix,which is used extensively in cutting tools and dies This composite, often referred to as a

Figure 5.1 Thermally conductive carbon reinforced PPS injection molded stepper motor

fiber-enclosure—heat sink (Courtesy of Cool mers, Inc.)

Poly-cermet, cemented carbide, or simply (but incorrectly) tungsten carbide, has much better

fracture toughness than monolithic tungsten carbide, which is a brittle ceramic material.Another interesting MMC, tungsten carbide particle-reinforced silver, is a key circuitbreaker contact pad material Here, the composite provides good electrical conductivityand much greater hardness and wear resistance than monolithic silver, which is too soft to

be used in this application Ferrous alloys reinforced with titanium carbide particles havebeen used for many years in numerous aerospace and commercial production applications,including dies, engine valves, and aircraft fuel pumps Compared to the monolithic basemetals, they offer better wear resistance, higher stiffness, and lower density

Another notable commercial MMC application is the Honda Prelude engine block,which has cylinder walls reinforced with a combination of aluminum oxide (alumina) andcarbon fibers, enabling elimination of cast iron cylinder liners.36 MMCs are also beingused in high-speed electronics manufacturing equipment and in photolithography tablesand other equipment used for production of microprocessor chips Other applications in-clude aircraft structures, aircraft engine fan exit guide vanes, and automobile and trainbrake rotors.37

One of the most important uses for MMCs is in electronic packaging and thermal agement.24,36,38 For example, silicon carbide particle-reinforced aluminum, often called

man-Al/SiC in the electronics industry, is being used in high-volume production parts such as

microprocessor lids and power modules for hybrid electric vehicles such as the ToyotaPrius Other MMCs used in packaging are carbon fiber reinforced-aluminum and copper,beryllium oxide (beryllia) particle-reinforced beryllium and silicon/aluminum Here, theadvantages are high stiffness, high thermal conductivity, and low density and CTE Twotraditional packaging materials, copper/tungsten and copper/molybdenum, can also beconsidered MMCs The CTEs of these composites can be tailored by varying the ratio ofthe two constituent metals A major drawback is that both have high densities

Monolithic metallic alloys are among the most widely used structural materials By inforcing them with continuous fibers, discontinuous fibers, whiskers, and particles, newmaterials are created with enhanced or modified properties, such as higher strength and

re-Figure 5.2 Injection-molded carbon fiber-reinforced thermoplastic microprocessor heat

spreader (Courtesy of Cool Polymers, Inc.)

336 Chapter Five

stiffness, better wear resistance, lower CTE, etc In some cases, the improvements are matic

dra-The greatest increases in strength and modulus are achieved with continuous fibers, at

least in the direction parallel to the fibers, called the axial or longitudinal direction As for

PMCs, transverse properties are dominated by the properties of the matrix and interface

The latter is more properly referred to as the interphase region However, because the

ma-trices are in themselves structural materials, transverse strength properties are frequentlygreat enough to permit use of unidirectional MMCs in some structural applications, which

is usually not possible for PMCs One example is the boron fiber-reinforced aluminumstruts used on the Space Shuttle Orbiter.39

One of the major advantages of MMCs reinforced with continuous fibers over PMCs isthat many, if not most, unidirectional MMCs have much greater transverse strengths thatallow them to be used in a unidirectional configuration In general, the axial moduli andstrengths of the composites are much greater than those of the monolithic base metals usedfor the matrices However, MMC transverse strengths are often somewhat lower thanthose of the parent matrix materials

The key particle-reinforced MMCs, include titanium carbide-reinforced steel, num reinforced with silicon carbide and with alumina particles, titanium carbide particle-reinforced titanium, and titanium boride-reinforced titanium

alumi-Aluminum reinforced with silicon carbide particles, the focus of this section, is arguablythe most important of the newer types of MMCs The low cost of the aluminum matrix andsilicon carbide particle makes these composites of particular interest There are wideranges of materials falling in this category They are made by a variety of processes, whichare discussed later in this section Properties depend on the type of particle, particle volumefraction, matrix alloy, and the process used to make them Table 5.7 presents representativecomposite properties for three particle volume fractions, 25, 55, and 70 percent In general,

as particle volume fraction increases, modulus and yield strength increase, and fracturetoughness, tensile ultimate strain, and CTE decrease Particle reinforcement also improveselevated temperature strength properties and fatigue resistance The ability to tailor CTE byvarying particle volume fraction is a key attribute of these materials

There are a variety of processes to make silicon carbide particle-reinforced aluminum,including powder metallurgy, stir casting, and pressure and pressureless infiltration Thelast two, as well as remelt casting, can make net shape or near-net shape parts

5.7 Carbon Matrix Composites

Carbon matrix composites (CAMCs) consist of a carbon matrix reinforced with any bination of fibers, whiskers, or particles.19–21 For many years, the only significant CAMCswere carbon/carbon composites (CCCs), in which the reinforcements are discontinuous orcontinuous carbon fibers In the last few years, a new proprietary carbon matrix materialsystem was developed that has a silicon carbide fiber reinforcement This material is nowbeing used for engine flaps on a military aircraft engine One of the key reported advan-tages of this new material is that it has a higher CTE than CCCs, reducing the tendency ofprotective ceramic coatings to crack The focus of this section is on CCCs

com-CCCs are used in a variety of applications, including electronic packaging, spacecraftradiator panels, rocket nozzles, reentry vehicle nose tips, the Space Shuttle Orbiter leadingedges and nose cap, aircraft brakes, heat treating furnaces, and glass-making equipment

As for PMCs, there are many different CCC materials having widely different ical and physical properties The primary advantages of CCCs are

mechan-■ High strength compared with competing materials at very high temperatures

■ High stiffness

In addition, CCCs are less brittle than monolithic carbon.

The primary disadvantages are

■ Susceptibility to oxidation at temperatures above about 370 to 500°C (700 to 930°F)

■ Low interlaminar (through-thickness) tensile and shear strengths for materials with 2Dreinforcement

■ Microcracking at low stresses in some directions for 3D composites

■ High cost of many systems

The variables affecting properties include type of fiber, reinforcement form, geometry,and volume fraction and matrix characteristics Because of the low interlaminar strengthproperties of CCCs, many applications, particularly those with thick walls, often usethree-dimensional reinforcement

As mentioned earlier, one of the most significant limitations of CCCs is oxidation dition of oxidation inhibitors to the matrix and protective coatings raises the threshold sub-stantially In inert atmospheres, CCCs retain their properties to temperatures as high as2400°C (4300°F)

Ad-Carbon matrices are typically weak, brittle, low-stiffness materials As a result, transverseand through-thickness elastic moduli and strength properties of unidirectional CCCs are low.Because of this, three-dimensional reinforcement forms are used in some applications

As for all composites, properties of CCCs depend on those of the reinforcement, trix, fiber-matrix interphase, and the process by which they are made Table 5.8 presentsmechanical and physical properties of CCCs reinforced with unbalanced fabrics having awarp-to-fill ratio of 4:1 The reinforcements are two types of carbon fibers that have veryhigh thermal conductivities, P120, and K1100 For comparison, copper has a thermal con-ductivity of about 400 W/m·K (230 Btu/hr·ft·°F) The combination of high thermal con-ductivity and low density makes CCCs attractive candidates for thermal management andelectronic packaging In addition, CCCs have very low CTEs, leading to their use as ther-mal doublers with carbon fiber-reinforced PMC structures The unique combination ofproperties possessed by CCCs, combined with a lack of outgassing, also makes them at-tractive for optical subsystems There are a variety of fibrous carbonaceous materials atvarious stages of development that have even higher thermal conductivities than the CCCs

ma-reported here For example, highly oriented pyrolytic graphite, also called thermal lytic graphite, has a reported in-plane thermal conductivity of as high as 1700 W/m·K

pyro-(980 Btu/hr·ft·°F)

There are two basic types of processes used to make CAMCs.19–21 The first is chemicalvapor infiltration (CVI) CVI is a process in which gaseous chemicals are reacted or de-composed, depositing a solid material on a fibrous preform In the case of CAMCs, hydro-carbon gases like methane and propane are broken down, and the material deposited is thecarbon matrix The second class of processes involves infiltration of a preform with poly-mers or pitches, which are then converted to carbon by pyrolysis (heating in an inert atmo-sphere).19–21 After pyrolysis, the composite is heated to high temperatures to graphitizethe matrix To minimize porosity, the process is repeated until a satisfactory density is

achieved This is called densification Common matrix precursors are phenolic and furan

resins, and pitches derived from coal tar and petroleum

9.0–26 (1.3–3.8)349–412 (202–238)24–47 (14–27)

303–344 (44–50) 338–420 (49–61)

124–145 (18–21) 9.6–16 (1.4–2.3)400–483 (231–279)56–72 (32–42)