Composite Material Structure and Processing 1.1 Introduction Composite materials are multiphase materials obtained through the artificial com- bination of different materials in order to attain properties that the individual com- ponents by themselves cannot attain.They are not multiphase materials in which the different phases are formed naturally by reactions,phase transformations, or other phenomena.An example is carbon fiber reinforced polymer.Composite materials should be distinguished from alloys,which can comprise two more com- ponents but are formed naturally through processes such as casting.Composite materials can be tailored for various properties by appropriately choosing their components,their proportions,their distributions,their morphologies,their de- grees of crystallinity,their crystallographic textures,as well as the structure and composition of the interface between components.Due to this strong tailorability, composite materials can be designed to satisfy the needs of technologies relat- ing to the aerospace,automobile,electronics,construction,energy,biomedical and other industries.As a result,composite materials constitute most commercial engineering materials. An example of a composite material is a lightweight structural composite that is obtained by embedding continuous carbon fibers in one or more orientations in a polymer matrix.The fibers provide the strength and stiffness,while the polymer serves as the binder.In particular,carbon fiber polymer-matrix composites have the following attractive properties: Low density(lower than aluminum) High strength (as strong as high-strength steels) High stiffness(stiffer than titanium,yet much lower in density) Good fatigue resistance Good creep resistance Low friction coefficient and good wear resistance Toughness and damage tolerance(as enabled by using appropriate fiber orien- tations) Chemical resistance(chemical resistance controlled by the polymer matrix) 1
1 Composite Material Structure and Processing 1.1 Introduction Composite materials are multiphase materials obtained through the artificial combination of different materials in order to attain properties that the individual components by themselves cannot attain. They are not multiphase materials in which the different phases are formed naturally by reactions, phase transformations, or other phenomena. An example is carbon fiber reinforced polymer. Composite materials should be distinguished from alloys, which can comprise two more components but are formed naturally through processes such as casting. Composite materials can be tailored for various properties by appropriately choosing their components, their proportions, their distributions, their morphologies, their degrees of crystallinity, their crystallographic textures, as well as the structure and composition of the interface between components. Due to this strong tailorability, composite materials can be designed to satisfy the needs of technologies relating to the aerospace, automobile, electronics, construction, energy, biomedical and other industries. As a result, composite materials constitute most commercial engineering materials. An example of a composite material is a lightweight structural composite that is obtained by embedding continuous carbon fibers in one or more orientations in a polymer matrix. The fibers provide the strength and stiffness, while the polymer serves as the binder. In particular, carbon fiber polymer-matrix composites have the following attractive properties: Low density (lower than aluminum) High strength (as strong as high-strength steels) High stiffness (stiffer than titanium, yet much lower in density) Good fatigue resistance Good creep resistance Low friction coefficient and good wear resistance Toughness and damage tolerance (as enabled by using appropriate fiber orientations) Chemical resistance (chemical resistance controlled by the polymer matrix) 1
2 1 Composite Material Structure and Processing Corrosion resistance Dimensional stability (can be designed for zero CTE) Vibration damping ability Low electrical resistivity High electromagnetic interference(EMI)shielding effectiveness High thermal conductivity. Another example of a composite is concrete,which is a structural composite obtained by combining (through mixing)cement (the matrix,ie.,the binder, obtained by a reaction-known as hydration-between cement and water),sand (fine aggregate),gravel(coarse aggregate),and optionally other ingredients that are known as admixtures.Short fibers and silica fume(a fine SiO2 particulate)are examples of admixtures.In general,composites are classified according to their matrix material.The main classes of composites are polymer-matrix,cement- matrix,metal-matrix,carbon-matrix and ceramic-matrix composites. Polymer-matrix and cement-matrix composites are the most common,due to the low cost of fabrication.Polymer-matrix composites are used for lightweight structures(aircraft,sporting goods,wheel chairs,etc.),in addition to vibration damping,electronic enclosures,asphalt(composite with pitch,a polymer,as the matrix),solder replacement,etc.Cement-matrix composites in the form of con- crete(with fine and coarse aggregates),steel-reinforced concrete,mortar(with fine aggregate,but no coarse aggregate)or cement paste(without any aggregate)are used for civil structures,prefabricated housing,architectural precasts,masonry, landfill cover,thermal insulation,sound absorption,etc. Carbon-matrix composites are important for lightweight structures (e.g.,Space Shuttles)and components(e.g.,aircraft brakes)that need to withstand high tem- peratures,but they are relatively expensive due to the high cost of fabrication. Carbon-matrix composites suffer from their tendency to be oxidized(2C +O2 2CO),thereby becoming vapor. Carbon fiber carbon-matrix composites,also called carbon-carbon composites, are the most advanced form of carbon,as the carbon fiber reinforcement makes them stronger,tougher,and more resistant to thermal shock than conventional graphite.With the low density of carbon,the specific strength(strength/density), specific modulus(modulus/density)and specific thermal conductivity(thermal conductivity/density)of carbon-carbon composites are the highest among com- posites.Furthermore,the coefficient of thermal expansion is near zero. Ceramic-matrix composites are superior to carbon-matrix composites in terms of oxidation resistance,but they are not as well developed as carbon-matrix composites.Metal-matrix composites with aluminum as the matrix are used for lightweight structures and low-thermal-expansion electronic enclosures,but their applications are limited by the high cost of fabrication and by galvanic corrosion. Metal-matrix composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion (CTE)and increase the strength and modulus.If a relatively graphitic kind of carbon fiber is used,the thermal conductivity can also be enhanced.The combination of low CTE and high
2 1 Composite Material Structure and Processing Corrosion resistance Dimensional stability (can be designed for zero CTE) Vibration damping ability Low electrical resistivity High electromagnetic interference (EMI) shielding effectiveness High thermal conductivity. Another example of a composite is concrete, which is a structural composite obtained by combining (through mixing) cement (the matrix, i.e., the binder, obtained by a reaction – known as hydration – between cement and water), sand (fine aggregate), gravel (coarse aggregate), and optionally other ingredients that are known as admixtures. Short fibers and silica fume (a fine SiO2 particulate) are examples of admixtures. In general, composites are classified according to their matrix material. The main classes of composites are polymer-matrix, cementmatrix, metal-matrix, carbon-matrix and ceramic-matrix composites. Polymer-matrix and cement-matrix composites are the most common, due to the low cost of fabrication. Polymer-matrix composites are used for lightweight structures (aircraft, sporting goods, wheel chairs, etc.), in addition to vibration damping, electronic enclosures, asphalt (composite with pitch, a polymer, as the matrix), solder replacement, etc. Cement-matrix composites in the form of concrete (with fine and coarse aggregates), steel-reinforced concrete, mortar (with fine aggregate, but no coarse aggregate) or cement paste (without any aggregate) are used for civil structures, prefabricated housing, architectural precasts, masonry, landfill cover, thermal insulation, sound absorption, etc. Carbon-matrix composites are important for lightweight structures (e.g., Space Shuttles) and components (e.g., aircraft brakes) that need to withstand high temperatures, but they are relatively expensive due to the high cost of fabrication. Carbon-matrix composites suffer from their tendency to be oxidized (2C + O2 → 2CO), thereby becoming vapor. Carbon fiber carbon-matrix composites, also called carbon-carbon composites, are the most advanced form of carbon, as the carbon fiber reinforcement makes them stronger, tougher, and more resistant to thermal shock than conventional graphite. With the low density of carbon, the specific strength (strength/density), specific modulus (modulus/density) and specific thermal conductivity (thermal conductivity/density) of carbon-carbon composites are the highest among composites. Furthermore, the coefficient of thermal expansion is near zero. Ceramic-matrix composites are superior to carbon-matrix composites in terms of oxidation resistance, but they are not as well developed as carbon-matrix composites. Metal-matrix composites with aluminum as the matrix are used for lightweight structures and low-thermal-expansion electronic enclosures, but their applications are limited by the high cost of fabrication and by galvanic corrosion. Metal-matrix composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion (CTE) and increase the strength and modulus. If a relatively graphitic kind of carbon fiber is used, the thermal conductivity can also be enhanced. The combination of low CTE and high
1.1 Introduction 3 thermal conductivity makes them very attractive for electronic packaging (e.g., heat sinks).Besides good thermal properties,their low density makes them par- ticularly desirable for aerospace electronics and orbiting space structures;orbiters are thermally cycled by moving through the Earth's shadow. Compared to the metal itself,a carbon fiber metal-matrix composite is char- acterized by a higher strength-to-density ratio (i.e.,specific strength),a higher modulus-to-density ratio (i.e.,specific modulus),better fatigue resistance,bet- ter high-temperature mechanical properties(a higher strength and a lower creep rate),a lower CTE,and better wear resistance. Compared to carbon fiber polymer-matrix composites,a carbon fiber metal- matrix composite is characterized by higher temperature resistance,higher fire resistance,higher transverse strength and modulus,a lack of moisture absorp- tion,a higher thermal conductivity,a lower electrical resistivity,better radiation resistance,and absence of outgassing. On the other hand,a metal-matrix composite has the following disadvantages compared to the metal itself and the corresponding polymer-matrix composite: higher fabrication cost and limited service experience. Fibers used for load-bearing metal-matrix composites are mostly in the form of continuous fibers,but short fibers are also used.The matrices used include aluminum,magnesium,copper,nickel,tin alloys,silver-copper,and lead alloys. Aluminum is by far the most widely used matrix metal because of its low density, low melting temperature(which makes composite fabrication and joining relatively convenient),low cost,and good machinability.Magnesium is comparably low in melting temperature,but its density is even lower than aluminum.Applications include structures(aluminum,magnesium),electronic heat sinks and substrates (aluminum,copper),soldering and bearings(tin alloys),brazing (silver-copper), and high-temperature applications(nickel). Although cement is a ceramic material,ceramic-matrix composites usually refer to those with silicon carbide,silicon nitride,alumina,mullite,glasses and other ceramic matrices that are not cement. Ceramic-matrix fiber composites are gaining increasing attention because the good oxidation resistance of the ceramic matrix(compared to a carbon matrix) makes the composites attractive for high-temperature applications (e.g.,aerospace and engine components).The fibers serve mainly to increase the toughness and strength(tensile and flexural)of the composite due to their tendency to be partially pulled out during the deformation.This pullout absorbs energy,thereby tough- ening the composite.Although the fiber pullout is advantageous,the bonding between the fibers and the matrix must still be sufficiently strong for the fibers to strengthen the composite effectively.Therefore,control over the bonding between the fibers and the matrix is important for the development of these composites. When the reinforcement is provided by carbon fibers,the reinforcement has a second function,which is to increase the thermal conductivity of the composite, as the ceramic is mostly thermally insulating whereas carbon fibers are thermally conductive.In electronic,aerospace,and engine components,the enhanced ther- mal conductivity is attractive for heat dissipation
1.1 Introduction 3 thermal conductivity makes them very attractive for electronic packaging (e.g., heat sinks). Besides good thermal properties, their low density makes them particularly desirable for aerospace electronics and orbiting space structures; orbiters are thermally cycled by moving through the Earth’s shadow. Compared to the metal itself, a carbon fiber metal-matrix composite is characterized by a higher strength-to-density ratio (i.e., specific strength), a higher modulus-to-density ratio (i.e., specific modulus), better fatigue resistance, better high-temperature mechanical properties (a higher strength and a lower creep rate), a lower CTE, and better wear resistance. Compared to carbon fiber polymer-matrix composites, a carbon fiber metalmatrix composite is characterized by higher temperature resistance, higher fire resistance, higher transverse strength and modulus, a lack of moisture absorption, a higher thermal conductivity, a lower electrical resistivity, better radiation resistance, and absence of outgassing. On the other hand, a metal-matrix composite has the following disadvantages compared to the metal itself and the corresponding polymer-matrix composite: higher fabrication cost and limited service experience. Fibers used for load-bearing metal-matrix composites are mostly in the form of continuous fibers, but short fibers are also used. The matrices used include aluminum, magnesium, copper, nickel, tin alloys, silver-copper, and lead alloys. Aluminum is by far the most widely used matrix metal because of its low density, lowmeltingtemperature(whichmakescompositefabricationandjoiningrelatively convenient), low cost, and good machinability. Magnesium is comparably low in melting temperature, but its density is even lower than aluminum. Applications include structures (aluminum, magnesium), electronic heat sinks and substrates (aluminum, copper), soldering and bearings (tin alloys), brazing (silver-copper), and high-temperature applications (nickel). Although cement is a ceramic material, ceramic-matrix composites usually refer to those with silicon carbide, silicon nitride, alumina, mullite, glasses and other ceramic matrices that are not cement. Ceramic-matrix fiber composites are gaining increasing attention because the good oxidation resistance of the ceramic matrix (compared to a carbon matrix) makes the composites attractive for high-temperature applications (e.g., aerospace and engine components). The fibers serve mainly to increase the toughness and strength (tensile and flexural) of the composite due to their tendency to be partially pulled out during the deformation. This pullout absorbs energy, thereby toughening the composite. Although the fiber pullout is advantageous, the bonding between the fibers and the matrix must still be sufficiently strong for the fibers to strengthen the composite effectively. Therefore, control over the bonding between the fibers and the matrix is important for the development of these composites. When the reinforcement is provided by carbon fibers, the reinforcement has a second function, which is to increase the thermal conductivity of the composite, as the ceramic is mostly thermally insulating whereas carbon fibers are thermally conductive. In electronic, aerospace, and engine components, the enhanced thermal conductivity is attractive for heat dissipation
4 1 Composite Material Structure and Processing A third function of the reinforcement is to decrease the drying shrinkage in the case of ceramic matrices prepared using slurries or slips.In general,the drying shrinkage decreases with increasing solid content in the slurry.Fibers are more ef- fective than particles at decreasing the drying shrinkage.This function is attractive for the dimensional control of parts made from the composites. Fiber-reinforced glasses are useful for space structural applications,such as mirror back structures and supports,booms and antenna structures.In low Earth orbit,these structures experience a temperature range from -100 to 80C,so they need an improved thermal conductivity and a reduced coefficient of thermal expansion.In addition,increased toughness,strength and modulus are desirable. Due to the environmental degradation resistance of carbon fiber reinforced glasses, they are also potentially useful for gas turbine engine components.Additional attractions are low friction,high wear resistance,and low density. The glass matrices used for fiber-reinforced glasses include borosilicate glasses (e.g.,Pyrex),aluminosilicate glasses,soda lime glasses and fused quartz.Moreover, a lithia aluminosilicate glass-ceramic and a CaO-MgO-Al2O3-SiO2 glass-ceramic have been used. 1.2 Composite Material Structure The structure of a composite is commonly such that one of the components is the matrix while the other components are fillers bound by the matrix,which is often called the binder.For example,in carbon fiber reinforced polymer,which is important for lightweight structures,the polymer is the matrix,while the carbon fiber is the filler.In case of a structural composite,the filler usually serves as a reinforcement.For example,carbon fiber is a reinforcement in the polymer- matrix composite. Composites can be classified according to the matrix material,which can be a polymer,a metal,a carbon,a ceramic or a cement (e.g.,Portland cement).They can also be classified according to the shape of the filler.A composite that has particles as the filler is said to be a particulate composite.For example,concrete is a particulate composite in which cement is the matrix and sand and stones are the two types of particles that are present together.A composite with fibers used as the filler is said to be a fibrous composite. The components in a composite can also take the form of layers.An example is laminate flooring that consists of layers of polymer,paper and fiberboard that are joined together during fabrication. A composite material can be in bulk or film form.The film form can be such that the composite is a standalone film or a film that is attached to a substrate.Less commonly,a composite material takes the form of particles or fibers;i.e.,a single particle or fiber consisting of more than one component
4 1 Composite Material Structure and Processing A third function of the reinforcement is to decrease the drying shrinkage in the case of ceramic matrices prepared using slurries or slips. In general, the drying shrinkage decreases with increasing solid content in the slurry. Fibers are more effective than particles at decreasing the drying shrinkage. This function is attractive for the dimensional control of parts made from the composites. Fiber-reinforced glasses are useful for space structural applications, such as mirror back structures and supports, booms and antenna structures. In low Earth orbit, these structures experience a temperature range from –100 to 80°C, so they need an improved thermal conductivity and a reduced coefficient of thermal expansion. In addition, increased toughness, strength and modulus are desirable. Due to the environmental degradation resistance of carbon fiber reinforced glasses, they are also potentially useful for gas turbine engine components. Additional attractions are low friction, high wear resistance, and low density. The glass matrices used for fiber-reinforced glasses include borosilicate glasses (e.g., Pyrex), aluminosilicate glasses, soda lime glasses and fused quartz. Moreover, a lithia aluminosilicate glass-ceramic and a CaO-MgO-Al2O3-SiO2 glass-ceramic have been used. 1.2 Composite Material Structure The structure of a composite is commonly such that one of the components is the matrix while the other components are fillers bound by the matrix, which is often called the binder. For example, in carbon fiber reinforced polymer, which is important for lightweight structures, the polymer is the matrix, while the carbon fiber is the filler. In case of a structural composite, the filler usually serves as a reinforcement. For example, carbon fiber is a reinforcement in the polymermatrix composite. Composites can be classified according to the matrix material, which can be a polymer, a metal, a carbon, a ceramic or a cement (e.g., Portland cement). They can also be classified according to the shape of the filler. A composite that has particles as the filler is said to be a particulate composite. For example, concrete is a particulate composite in which cement is the matrix and sand and stones are the two types of particles that are present together. A composite with fibers used as the filler is said to be a fibrous composite. The components in a composite can also take the form of layers. An example is laminate flooring that consists of layers of polymer, paper and fiberboard that are joined together during fabrication. A composite material can be in bulk or film form. The film form can be such that the composite is a standalone film or a film that is attached to a substrate. Less commonly, a composite material takes the form of particles or fibers; i.e., a single particle or fiber consisting of more than one component
1.2 Composite Material Structure 5 1.2.1 Continuous Fiber Composites A fibrous composite involving continuous fibers is particularly attractive as a struc- tural material due to the high strength and modulus of the fibers,which bear most of the load.A continuous carbon fiber polymer-matrix composite is an example. The use of steel reinforcing bars(called "rebars")to reinforce concrete gives steel reinforced concrete,which is another example(even though steel rebars are not re- ferred to as fibers).A fibrous composite is also attractive in that it can be tailored by choosing the orientation of the fibers.A common configuration involves the fibers being in the form of plies.A ply,also known as a lamina,is a sheet that has fibers with the same orientation in the plane of the sheet.Each lamina has thousands of fibers along its thickness because each fiber tow consists of thousands of fibers. The composite is made up of a number of laminae such that the fiber orientations can be different among the laminae.For example,the fibers in the consecutive laminae can be oriented at 0,90,+45 and-45,resulting in a two-dimensionally "quasi-isotropic"configuration.A conventional system of notation for describing the lay-up configuration of the laminae is illustrated below. [O]s means an eight-lamina composite with all laminae having fibers in the same direction (0).[0/90]2s (where the subscript "s"means "symmetric")means an eight-lamina composite with the stacking order 0,90,0,90,90,0,90,0,where the first four laminae and the remaining four laminae are mirror images and the mirror plane is the center plane of the composite.[0/45/90/-45]s means an eight- lamina composite with the stacking order 0,45,90,-45,-45,90,45,0,where the first four laminae and the remaining four laminae are mirror images.[0/45/90/- 45]2s means a 16-lamina composite with the stacking order 0,45,90,-45,0,45,90, -45,-45,90,45,0,-45,90,45,0,where the first eight laminae and the remaining eight laminae are mirror images.[0/45/90/-45]2s means a 16-lamina composite with the stacking order0,45,90,-45,0,45,90,-45,-45,90,45,0,-45,90,45,0°， where the first eight laminae and the remaining eight laminae are mirror images. [0/45/90/-45]3s means a 24-lamina composite with the stacking order 0,45,90, -45,0,45,90,-45,0,45,90,-45,-45,90,45,0,-45,90,45,0,-45,90,45,0°，where the first 12 laminae and the remaining 12 laminae are mirror images. An optical micrograph of the interlaminar interface between two laminae that are at 90 to one another (i.e.,a crossply configuration).The interlaminar interface is the region between the two parallel lines that are separated by 8.7 um.The fibers above the interface are in the plane of the paper,whereas those below the interface are perpendicular to the paper The direction perpendicular to the laminae is known as the through-thickness direction.The interface between two adjacentlaminae is known as the interlaminar interface,which is the mechanically weak link in the laminate.Figure 1.1 is an optical micrograph of the interlaminar interface between two laminae that are 90 relative to one another (i.e.,a crossply configuration).This means that the through-thickness direction is relatively weak mechanically,and delamination (local separation of the laminae from one another)is a common form of damage in these composites.When the fibers are carbon fibers,which are much more conductive electrically than the polymer matrix,the through-thickness direction
1.2 Composite Material Structure 5 1.2.1 Continuous Fiber Composites A fibrous composite involving continuous fibers is particularly attractive as a structural material due to the high strength and modulus of the fibers, which bear most of the load. A continuous carbon fiber polymer-matrix composite is an example. The use of steel reinforcing bars (called “rebars”) to reinforce concrete gives steel reinforced concrete, which is another example (even though steel rebars are not referred to as fibers). A fibrous composite is also attractive in that it can be tailored by choosing the orientation of the fibers. A common configuration involves the fibers being in the form of plies. A ply, also known as a lamina, is a sheet that has fibers with the same orientation in the plane of the sheet. Each lamina has thousands of fibers along its thickness because each fiber tow consists of thousands of fibers. The composite is made up of a number of laminae such that the fiber orientations can be different among the laminae. For example, the fibers in the consecutive laminae can be oriented at 0, 90, +45 and –45°, resulting in a two-dimensionally “quasi-isotropic” configuration. A conventional system of notation for describing the lay-up configuration of the laminae is illustrated below. [0]8 means an eight-lamina composite with all laminae having fibers in the same direction (0°). [0/90]2s (where the subscript “s” means “symmetric”) means an eight-lamina composite with the stacking order 0, 90, 0, 90, 90, 0, 90, 0°, where the first four laminae and the remaining four laminae are mirror images and the mirror plane is the center plane of the composite. [0/45/90/–45]s means an eightlamina composite with the stacking order 0, 45, 90, –45, –45, 90, 45, 0°, where the first four laminae and the remaining four laminae are mirror images. [0/45/90/– 45]2s means a 16-lamina composite with the stacking order 0, 45, 90, –45, 0, 45, 90, –45, –45, 90, 45, 0, –45, 90, 45, 0°, where the first eight laminae and the remaining eight laminae are mirror images. [0/45/90/–45]2s means a 16-lamina composite with the stacking order 0, 45, 90, –45, 0, 45, 90, –45, –45, 90, 45, 0, –45, 90, 45, 0°, where the first eight laminae and the remaining eight laminae are mirror images. [0/45/90/–45]3s means a 24-lamina composite with the stacking order 0, 45, 90, –45, 0, 45, 90, –45, 0, 45, 90, –45, –45, 90, 45, 0, –45, 90, 45, 0, –45, 90, 45, 0°, where the first 12 laminae and the remaining 12 laminae are mirror images. An optical micrograph of the interlaminar interface between two laminae that are at 90° to one another (i.e., a crossply configuration). The interlaminar interface is the region between the two parallel lines that are separated by 8.7μm. The fibers above the interface are in the plane of the paper, whereas those below the interface are perpendicular to the paper The direction perpendicular to the laminae is known as the through-thickness direction. The interface between two adjacent laminae is known as the interlaminar interface, which is the mechanically weak link in the laminate. Figure 1.1 is an optical micrograph of the interlaminar interface between two laminae that are 90° relative to one another (i.e., a crossply configuration). This means that the through-thickness direction is relatively weak mechanically, and delamination (local separation of the laminae from one another) is a common form of damage in these composites. When the fibers are carbon fibers, which are much more conductive electrically than the polymer matrix, the through-thickness direction