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Lecture-Polymeric and Composite materials.ppt

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Lecture-Polymeric and Composite materials.ppt

  1. 1. Polymeric and Composite Materials
  2. 2. POLYMERS AND COMPOSITE MATERIALS 1. Fundamentals of Polymer Technology 2. Thermoplastic Polymers 3. Thermosetting Polymers 4. Elastomers 5. Composites--Technology and Classification 6. Composite Materials 7. Guide to the Processing of Polymers and Composite Materials
  3. 3. Polymer A compound consisting of long-chain molecules, each molecule made up of repeating units connected together  There may be thousands, even millions of units in a single polymer molecule  The word polymer is derived from the Greek words poly, meaning many, and meros (reduced to mer), meaning part  Most polymers are based on carbon and are therefore considered organic chemicals
  4. 4. Types of Polymers  Polymers can be separated into plastics and rubbers  As engineering materials, it is appropriate to divide them into the following three categories: 1. Thermoplastic polymers 2. Thermosetting polymers 3. Elastomers where (1) and (2) are plastics and (3) are rubbers
  5. 5. Thermoplastic Polymers - Thermoplastics Solid materials at room temperature but viscous liquids when heated to temperatures of only a few hundred degrees  This characteristic allows them to be easily and economically shaped into products  They can be subjected to heating and cooling cycles repeatedly without significant degradation  Symbolized by TP
  6. 6. Thermosetting Polymers - Thermosets  Cannot tolerate repeated heating cycles as thermoplastics can  When initially heated, they soften and flow for molding  Elevated temperatures also produce a chemical reaction that hardens the material into an infusible solid  If reheated, thermosets degrade and char rather than soften  Symbolized by TS
  7. 7. Elastomers (Rubbers) Polymers that exhibit extreme elastic extensibility when subjected to relatively low mechanical stress  Some elastomers can be stretched by a factor of 10 and yet completely recover to their original shape  Although their properties are quite different from thermosets, they share a similar molecular structure that is different from the thermoplastics
  8. 8. Market Shares  Thermoplastics are commercially the most important of the three types  About 70% of the tonnage of all synthetic polymers produced  Thermosets and elastomers share the remaining 30%  On a volumetric basis, the current annual usage of polymers exceeds that of metals
  9. 9. Examples of Polymers  Thermoplastics:  Polyethylene, polyvinylchloride, polypropylene, polystyrene, and nylon  Thermosets:  Phenolics, epoxies, and certain polyesters  Elastomers:  Natural rubber (vulcanized)  Synthetic rubbers, which exceed the tonnage of natural rubber
  10. 10. Reasons Why Polymers are Important  Plastics can be molded into intricate part shapes, usually with no further processing  Very compatible with net shape processing  On a volumetric basis, polymers:  Are cost competitive with metals  Generally, require less energy to produce than metals  Certain plastics are transparent, which makes them competitive with glass in some applications
  11. 11. General Properties of Polymers  Low density relative to metals and ceramics  Good strength-to-weight ratios for certain (but not all) polymers  High corrosion resistance  Low electrical and thermal conductivity
  12. 12. Limitations of Polymers  Low strength relative to metals and ceramics  Low modulus of elasticity (stiffness)  Service temperatures are limited to only a few hundred degrees  Viscoelastic properties, which can be a distinct limitation in load-bearing applications  Some polymers degrade when subjected to sunlight and other forms of radiation
  13. 13. Synthesis of Polymers  Nearly all polymers used in engineering are synthetic  They are made by chemical processing  Polymers are synthesized by joining many small molecules together into very large molecules, called macromolecules, that possess a chain-like structure  The small units, called monomers, are generally simple unsaturated organic molecules such as ethylene C2H4
  14. 14. Polyethylene  Synthesis of polyethylene from ethylene monomers: (1) n ethylene monomers, (2a) polyethylene of chain length n; (2b) concise notation for depicting polymer structure of chain length n
  15. 15. Polymerization  As a chemical process, the synthesis of polymers can occur by either of two methods: 1. Addition polymerization 2. Step polymerization  Production of a given polymer is generally associated with one method or the other
  16. 16. Addition Polymerization  In this process, the double bonds between carbon atoms in the ethylene monomers are induced to open up so they can join with other monomer molecules  The connections occur on both ends of the expanding macromolecule, developing long chains of repeating mers  It is initiated using a chemical catalyst to open the carbon double bond in some of the monomers
  17. 17. Addition Polymerization  Model of addition (chain) polymerization: (1) initiation, (2) rapid addition of monomers, and (3) resulting long chain polymer molecule with n mers at termination of reaction
  18. 18. Step Polymerization  In this form of polymerization, two reacting monomers are brought together to form a new molecule of the desired compound  As reaction continues, more reactant molecules combine with the molecules first synthesized to form polymers of length n = 2, then length n = 3, and so on  In addition, polymers of length n1 and n2 also combine to form molecules of length n = n1 + n2, so that two types of reactions are proceeding simultaneously
  19. 19. Step Polymerization  Model of step polymerization showing the two types of reactions occurring: (left) n-mer attaching a single monomer to form a (n+1)-mer; and (right) n1-mer combining with n2-mer to form a (n1+n2)-mer.
  20. 20. Some Examples  Polymers produced by addition polymerization:  Polyethylene, polypropylene, polyvinylchloride, polyisoprene  Polymers produced by step polymerization:  Nylon, polycarbonate, phenol formaldehyde
  21. 21. Degree of Polymerization  Since molecules in a given batch of polymerized material vary in length, n for the batch is an average  The mean value of n is called the degree of polymerization (DP) for the batch  DP affects the properties of the polymer  Higher DP increases mechanical strength but also increases viscosity in the fluid state, which makes processing more difficult
  22. 22. Molecular Weight  The sum of the molecular weights of the monomers in the molecule  MW = n times the molecular weight of each repeating unit  Since n varies for different molecules in a batch, the molecular weight must be interpreted as an average
  23. 23. Typical Values of DP and MW Polymer Polyethylene Polyvinylchloride Nylon Polycarbonate DP(n) 10,000 1,500 120 200 MW 300,000 100,000 15,000 40,000
  24. 24. Polymer Molecular Structures  Linear structure – chain-like structure  Characteristic of thermoplastic polymers  Branched structure – chain-like but with side branches  Also found in thermoplastic polymers  Cross-linked structure  Loosely cross-linked, characteristic of elastomers  Tightly cross-linked, characteristic of thermosets
  25. 25. Polymer Molecular Structures Linear Branched Loosely cross-linked Tightly cross-linked
  26. 26. Effect of Branching on Properties  Thermoplastic polymers always possess linear or branched structures or a mixture of the two  Branches increases entanglement among the molecules, which makes the polymer  Stronger in the solid state  More viscous at a given temperature in the plastic or liquid state
  27. 27. Effect of Cross-Linking on Properties  Thermosets possess a high degree of cross-linking; elastomers possess a low degree of cross-linking  Thermosets are hard and brittle, while elastomers are elastic and resilient  Cross-linking causes the polymer to become chemically set  The reaction cannot be reversed  The polymer structure is permanently changed; if heated, it degrades or burns rather than melt
  28. 28. Crystallinity in Polymers  Both amorphous and crystalline structures are possible, although the tendency to crystallize is much less than for metals or non-glass ceramics  Not all polymers can form crystals  For those that can, the degree of crystallinity (the proportion of crystallized material in the mass) is always less than 100%
  29. 29. Crystalline Polymer Structure  Crystallized regions in a polymer: (a) long molecules forming crystals randomly mixed in with the amorphous material; and (b) folded chain lamella, the typical form of a crystallized region
  30. 30. Crystallinity and Properties  As crystallinity is increased in a polymer  Density increases  Stiffness, strength, and toughness increases  Heat resistance increases  If the polymer is transparent in the amorphous state, it becomes opaque when partially crystallized
  31. 31. Low Density & High-Density Polyethylene Polyethylene type Low density High density Degree of crystallinity 55% 92% Specific gravity 0.92 0.96 Modulus of elasticity 140 MPa (20,000 lb/in2) 700 MPa (100,000 lb/in2) Melting temperature 115C (239F) 135C (275F)
  32. 32. Some Observations About Crystallization  Linear polymers consist of long molecules with thousands of repeated mers  Crystallization involves folding back and forth of the long chains upon themselves  The crystallized regions are called crystallites  Crystallites take the form of lamellae randomly mixed in with amorphous material  A crystallized polymer is a two-phase system  Crystallites interspersed in an amorphous matrix
  33. 33. Factors for Crystallization  Slower cooling promotes crystal formation and growth  Mechanical deformation, as in the stretching of a heated thermoplastic, tends to align the structure and increase crystallization  Plasticizers (chemicals added to a polymer to soften it) reduce crystallinity
  34. 34. Thermal Behavior of Polymers  Specific volume (density)-1 as a function of temperature
  35. 35. Additives  Properties of a polymer can often be beneficially changed by combining it with additives  Additives either alter the molecular structure or  Add a second phase, in effect transforming the polymer into a composite material
  36. 36. Types of Additives by Function  Fillers – strengthen polymer or reduce cost  Plasticizers – soften polymer and improve flow  Colorants – pigments or dyes  Lubricants – reduce friction and improve flow  Flame retardents – reduce flammability of polymer  Cross-linking agents – for thermosets and elastomers  Ultraviolet light absorbers – reduce degradation from sunlight  Antioxidants – reduce oxidation damage
  37. 37. Thermoplastic Polymers (TP)  Thermoplastic polymers can be heated from solid state to viscous liquid and then cooled back down to solid  Heating and cooling can be repeated many times without degrading the polymer  Reason: TP polymers consist of linear and/or branched macromolecules that do not cross-link upon heating  Thermosets and elastomers change chemically when heated, which cross-links their molecules and permanently sets these polymers
  38. 38. Mechanical Properties of Thermoplastics  Low modulus of elasticity (stiffness)  E is much lower than metals and ceramics  Low tensile strength  TS is about 10% of metal  Much lower hardness than metals or ceramics  Greater ductility on average  Tremendous range of values, from 1% elongation for polystyrene to 500% or more for polypropylene
  39. 39. Strength vs. Temperature  Deformation resistance (strength) of polymers as a function of temperature
  40. 40. Physical Properties of Thermoplastics  Lower densities than metals or ceramics  Typical specific gravity for polymers are 1.2 (compared to ceramics (~ 2.5) and metals (~ 7)  Much higher coefficient of thermal expansion  Roughly five times the value for metals and 10 times the value for ceramics  Much lower melting temperatures  Insulating electrical properties
  41. 41. Commercial Thermoplastic Products and Raw Materials  Thermoplastic products include  Molded and extruded items  Fibers and filaments  Films and sheets  Packaging materials  Paints and varnishes  Starting plastic materials are normally supplied to the fabricator in the form of powders or pellets in bags, drums, or larger loads by truck or rail car
  42. 42. Thermosetting Polymers (TS)  TS polymers are distinguished by their highly cross-linked three-dimensional, covalently-bonded structure  Chemical reactions associated with cross-linking are called curing or setting  In effect, formed part (e.g., pot handle, electrical switch cover, etc.) becomes a large macromolecule  Always amorphous and exhibits no glass transition temperature
  43. 43. General Properties of Thermosets  Rigid - modulus of elasticity is two to three times greater than thermoplastics  Brittle, virtually no ductility  Less soluble in common solvents than thermoplastics  Capable of higher service temperatures than thermoplastics  Cannot be remelted - instead they degrade or burn
  44. 44. Cross-Linking in TS Polymers  Three categories: 1. Temperature-activated systems 2. Catalyst-activated systems 3. Mixing-activated systems  Curing is accomplished at the fabrication plants that make the parts rather than the chemical plants that supply the starting materials to the fabricator
  45. 45. Temperature-Activated Systems Curing caused by heat supplied during part shaping operation (e.g., molding)  Starting material is a linear polymer in granular form supplied by the chemical plant  As heat is added, material softens for molding, but continued heating causes cross-linking  Most common TS systems  The term “thermoset" applies best to these polymers
  46. 46. Catalyst-Activated Systems Cross-linking occurs when small amounts of a catalyst are added to the polymer, which is in liquid form  Without the catalyst, the polymer remains stable and liquid  Once combined with the catalyst it cures and changes into solid form
  47. 47. Mixing-Activated Systems Mixing of two chemicals results in a reaction that forms a cross-linked solid polymer  Elevated temperatures are sometimes used to accelerate the reactions  Most epoxies are examples of these systems
  48. 48. TS vs. TP Polymers  TS plastics are not as widely used as the TP  One reason is the added processing costs and complications involved in curing  Largest market share of TS = phenolic resins with  6% of the total plastics market  Compare polyethylene with  35% market share  TS Products: countertops, plywood adhesives, paints, molded parts, printed circuit boards and other fiber reinforced plastics
  49. 49. Elastomers Polymers capable of large elastic deformation when subjected to relatively low stresses  Some can be extended 500% or more and still return to their original shape  Two categories: 1. Natural rubber - derived from biological plants 2. Synthetic polymers - produced by polymerization processes like those used for thermoplastic and thermosetting polymers
  50. 50. Characteristics of Elastomers  Elastomers consist of long-chain molecules that are cross-linked (like thermosetting polymers)  They owe their impressive elastic properties to two features: 1. Molecules are tightly kinked when unstretched 2. Degree of cross-linking is substantially less than thermosets
  51. 51. Elastomer Molecules  Model of long elastomer molecules, with low degree of cross-linking: (left) unstretched, and (right) under tensile stress
  52. 52. Elastic Behavior of Elastomer Molecule  When stretched, the molecules are forced to uncoil and straighten  Natural resistance to uncoiling provides the initial elastic modulus of the aggregate material  Under further strain, the covalent bonds of the cross-linked molecules begin to play an increasing role in the modulus, and stiffness increases  With greater cross-linking, the elastomer becomes stiffer, and its modulus of elasticity is more linear
  53. 53. Stiffness of Rubber  Increase in stiffness as a function of strain for three grades of rubber: natural rubber, vulcanized rubber, and hard rubber
  54. 54. Vulcanization Curing to cross-link most elastomers  Vulcanization = the term for curing in the context of natural rubber (and certain synthetic rubbers)  Typical cross-linking in rubber is one to ten links per hundred carbon atoms in the linear polymer chain, depending on degree of stiffness desired  Considerably less than cross-linking in thermosets
  55. 55. Natural Rubber (NR)  NR = polyisoprene, a high molecular-weight polymer of isoprene (C5H8)  It is derived from latex, a milky substance produced by various plants, most important of which is the rubber tree that grows in tropical climates  Latex is a water emulsion of polyisoprene (about 1/3 by weight), plus various other ingredients  Rubber is extracted from latex by various methods that remove the water
  56. 56. Vulcanized Natural Rubber  Properties: High tensile strength, tear strength, resilience (capacity to recover shape), and resistance to wear and fatigue  Weaknesses: degrades when subjected to heat, sunlight, oxygen, ozone, and oil  Some of these limitations can be reduced by additives  Market share of NR  22% of total rubber volume (natural plus synthetic)
  57. 57. Natural Rubber Products  Largest single market for NR is automotive tires  Other products: shoe soles, bushings, seals, and shock absorbing components  In tires, carbon black is an important additive  It reinforces the rubber, serving to increase tensile strength and resistance to tear and abrasion  Other additives: clay, kaolin, silica, talc, and calcium carbonate, as well as chemicals that accelerate and promote vulcanization
  58. 58. Synthetic Rubbers  Development of synthetic rubbers was motivated largely by world wars when NR was difficult to obtain  Tonnage of synthetic rubbers is now more than three times that of NR  The most important synthetic rubber is styrene-butadiene rubber (SBR), a copolymer of butadiene (C4H6) and styrene (C8H8)  As with most other polymers, the main raw material for synthetic rubbers is petroleum
  59. 59. Thermoplastic Elastomers (TPE) A thermoplastic that behaves like an elastomer  Elastomeric properties not from chemical cross-links, but from physical connections between soft and hard phases in the material  Cannot match conventional elastomers in elevated temperature, strength and creep resistance  Products: footwear; rubber bands; extruded tubing, wire coating; molded automotive parts, but no tires
  60. 60. COMPOSITE MATERIALS 1. Technology and Classification of Composite Materials 2. Metal Matrix Composites 3. Ceramic Matrix Composites 4. Polymer Matrix Composites 5. Guide to Processing Composite Materials
  61. 61. Composite Material Defined A materials system composed of two or more distinct phases whose combination produces aggregate properties different from those of its constituents  Examples:  Cemented carbides  Plastic molding compounds with fillers  Rubber mixed with carbon black  Wood (a natural composite as distinguished from a synthesized composite)
  62. 62. Why Composites are Important  Composites can be very strong and stiff, yet very light in weight  Strength-to-weight and stiffness-to-weight ratios are several times greater than steel or aluminum  Fatigue properties are generally better than for common engineering metals  Toughness is often greater  Possible to achieve combinations of properties not attainable with metals, ceramics, or polymers alone
  63. 63. Disadvantages and Limitations  Properties of many important composites are anisotropic  May be an advantage or a disadvantage  Many polymer-based composites are subject to attack by chemicals or solvents  Just as the polymers themselves are susceptible  Composite materials are generally expensive  Manufacturing methods for shaping composite materials are often slow and costly
  64. 64. Possible Classification of Composites 1. Traditional composites – composite materials that occur in nature or have been produced by civilizations for many years  Examples: wood, concrete, asphalt 2. Synthetic composites - modern material systems normally associated with the manufacturing industries  Components are first produced separately and then combined to achieve the desired structure, properties, and part geometry
  65. 65. Components in a Composite Material Most composite materials consist of two phases: 1. Primary phase - forms the matrix within which the secondary phase is imbedded 2. Secondary phase - imbedded phase sometimes referred to as a reinforcing agent, because it usually strengthens the composite material  The reinforcing phase may be in the form of fibers, particles, or various other geometries
  66. 66. Classification of Composite Materials 1. Metal Matrix Composites (MMCs) - mixtures of ceramics and metals, such as cemented carbides 2. Ceramic Matrix Composites (CMCs) - Al2O3 and SiC imbedded with fibers to improve properties 3. Polymer Matrix Composites (PMCs) - polymer resins imbedded with filler or reinforcing agent  Examples: epoxy and polyester with fiber reinforcement, and phenolic with powders
  67. 67. Functions of the Matrix Material  Primary phase provides the bulk form of the part or product made of the composite material  Holds the imbedded phase in place, usually enclosing and often concealing it  When a load is applied, the matrix shares the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent
  68. 68. Reinforcing Phase  Function is to reinforce the primary phase  Reinforcing phase (imbedded in the matrix) is most commonly one of the following shapes: fibers, particles, or flakes
  69. 69. Physical Shapes of Imbedded Phase Possible physical shapes of imbedded phases in composite materials: (a) fiber, (b) particle, and (c) flake
  70. 70. Fibers Filaments of reinforcing material, usually circular in cross section  Diameters from ~ 0.0025 mm to about 0.13 mm  Filaments provide greatest opportunity for strength enhancement of composites  Filament form of most materials is significantly stronger than the bulk form  As diameter is reduced, the material becomes oriented in the fiber axis direction and probability of defects in the structure decreases significantly
  71. 71. Continuous Fibers vs. Discontinuous Fibers  Continuous fibers - very long; in theory, they offer a continuous path by which a load can be carried by the composite part  Discontinuous fibers (chopped sections of continuous fibers) - short lengths (L/D = roughly 100)  Whiskers = discontinuous fibers of hair-like single crystals with diameters down to about 0.001 mm (0.00004 in) and very high strength
  72. 72. Fiber Orientation – Three Cases  One-dimensional reinforcement, in which maximum strength and stiffness are obtained in the direction of the fiber  Planar reinforcement, in some cases in the form of a two-dimensional woven fabric  Random or three-dimensional in which the composite material tends to possess isotropic properties
  73. 73. Fiber Orientation Fiber orientation in composite materials: (a) one-dimensional, continuous fibers; (b) planar, continuous fibers in the form of a woven fabric; and (c) random, discontinuous fibers
  74. 74. Materials for Fibers  Fiber materials in fiber-reinforced composites  Glass – most widely used filament  Carbon – high elastic modulus  Boron – very high elastic modulus  Polymers - Kevlar  Ceramics – SiC and Al2O3  Metals - steel  Most important commercial use of fibers is in polymer composites
  75. 75. Particles and Flakes  A second common shape of imbedded phase is particulate, ranging in size from microscopic to macroscopic  Flakes are basically two-dimensional particles - small flat platelets  Distribution of particles in the matrix is random  Strength and other properties of the composite material are usually isotropic
  76. 76. Interface between Constituent Phases in Composite Material  For the composite to function, the phases must bond where they join at the interface  Direct bonding between primary and secondary phases
  77. 77. Interphase  In some cases, a third ingredient must be added to bond primary and secondary phases  Called an interphase, it is like an adhesive
  78. 78. Alternative Interphase Form Formation of an interphase consisting of a solution of primary and secondary phases at their boundary
  79. 79. Properties of Composite Materials  In selecting a composite material, an optimum combination of properties is often sought, rather than one particular property  Example: fuselage and wings of an aircraft must be lightweight, strong, stiff, and tough  Several fiber-reinforced polymers possess these properties  Example: natural rubber alone is relatively weak  Adding carbon black increases its strength
  80. 80. Three Factors that Determine Properties 1. Materials used as component phases in the composite 2. Geometric shapes of the constituents and resulting structure of the composite system 3. How the phases interact with one another
  81. 81. Example: Fiber Reinforced Polymer  Model of fiber-reinforced composite material showing direction in which elastic modulus is being estimated by the rule of mixtures
  82. 82. Example: Fiber Reinforced Polymer (continued)  Stress-strain relationships for the composite material and its constituents  The fiber is stiff but brittle, while the matrix (commonly a polymer) is soft but ductile
  83. 83. Variations in Strength and Stiffness Variation in elastic modulus and tensile strength as function of direction relative to longitudinal axis of carbon fiber-reinforced epoxy composite
  84. 84. Importance of Geometric Shape: Fibers  Most materials have tensile strengths several times greater as fibers than as bulk materials  By imbedding the fibers in a polymer matrix, a composite material is obtained that avoids the problems of fibers but utilizes their strengths  Matrix provides the bulk shape to protect the fiber surfaces and resist buckling  When a load is applied, the low-strength matrix deforms and distributes the stress to the high-strength fibers
  85. 85. Other Composite Structures  Laminar composite structure – conventional  Sandwich structure  Honeycomb sandwich structure
  86. 86. Laminar Composite Structure  Conventional laminar structure - two or more layers bonded together in an integral piece  Example: plywood, in which layers are the same wood, but grains oriented differently to increase overall strength
  87. 87. Sandwich Structure: Foam Core  Relatively thick core of low-density foam bonded on both faces to thin sheets of a different material
  88. 88. Sandwich Structure: Honeycomb Core  Alternative to foam core  Foam or honeycomb achieve high ratios of strength-to-weight and stiffness-to-weight
  89. 89. Other Laminar Composite Structures  FRPs - multi-layered, fiber-reinforced plastic panels for aircraft, boat hulls, other products  Printed circuit boards - layers of reinforced copper and plastic for electrical conductivity and insulation, respectively  Snow skis - layers of metals, particle board, and phenolic plastic  Windshield glass - two layers of glass on either side of a sheet of tough plastic
  90. 90. Metal Matrix Composites (MMCs) Metal matrix reinforced by a second phase  Reinforcing phases: 1. Particles of ceramic  These MMCs are commonly called cermets 2. Fibers of various materials  Other metals, ceramics, carbon, and boron
  91. 91. Cermets MMC with ceramic contained in a metallic matrix  The ceramic often dominates the mixture, sometimes up to 96% by volume  Bonding can be enhanced by slight solubility between phases at elevated temperatures used in processing  Cermets can be subdivided into 1. Cemented carbides – most common 2. Oxide-based cermets – less common
  92. 92. Cemented Carbides One or more carbide compounds bonded in a metallic matrix  Common cemented carbides are based on tungsten carbide (WC), titanium carbide (TiC), and chromium carbide (Cr3C2)  Tantalum carbide (TaC) and others are less common  Metallic binders: usually cobalt (Co) or nickel (Ni)
  93. 93.  Photomicrograph (about 1500X) of cemented carbide with 85% WC and 15% Co (photo courtesty of Kennametal Inc.) Cemented Carbide
  94. 94.  Typical plot of hardness and transverse rupture strength as a function of cobalt content Cemented Carbide Properties
  95. 95. Applications of Cemented Carbides  Tungsten carbide cermets (Co binder)  Cutting tools, wire drawing dies, rock drilling bits, powder metal dies, indenters for hardness testers  Titanium carbide cermets (Ni binder)  Cutting tools; high temperature applications such as gas-turbine nozzle vanes  Chromium carbide cermets (Ni binder)  Gage blocks, valve liners, spray nozzles
  96. 96. Ceramic Matrix Composites (CMCs) Ceramic primary phase imbedded with a secondary phase, usually consisting of fibers  Attractive properties of ceramics: high stiffness, hardness, hot hardness, and compressive strength; and relatively low density  Weaknesses of ceramics: low toughness and bulk tensile strength, susceptibility to thermal cracking  CMCs represent an attempt to retain the desirable properties of ceramics while compensating for their weaknesses
  97. 97. Ceramic Matrix Composite  Photomicrograph (about 3000X) of fracture surface of SiC whisker reinforced Al2O3 (photo courtesy of Greenleaf Corp.)
  98. 98. Polymer Matrix Composites (PMCs) Polymer primary phase in which a secondary phase is imbedded as fibers, particles, or flakes  Commercially, PMCs are more important than MMCs or CMCs  Examples: most plastic molding compounds, rubber reinforced with carbon black, and fiber-reinforced polymers (FRPs)
  99. 99. Fiber-Reinforced Polymers (FRPs) PMC consisting of a polymer matrix imbedded with high-strength fibers  Polymer matrix materials:  Usually, a thermosetting plastic such as unsaturated polyester or epoxy  Can also be thermoplastic, such as nylons (polyamides), polycarbonate, polystyrene, and polyvinylchloride  Fiber reinforcement is widely used in rubber products such as tires and conveyor belts
  100. 100. Fibers in PMCs  Various forms: discontinuous (chopped), continuous, or woven as a fabric  Principal fiber materials in FRPs are glass, carbon, and Kevlar 49  Less common fibers include boron, SiC, and Al2O3, and steel  Glass (in particular E-glass) is the most common fiber material in today's FRPs  Its use to reinforce plastics dates from around 1920
  101. 101. Common FRP Structures  Most widely used form of FRP is a laminar structure  Made by stacking and bonding thin layers of fiber and polymer until desired thickness is obtained  By varying fiber orientation among layers, a specified level of anisotropy in properties can be achieved in the laminate  Applications: boat hulls, aircraft wing and fuselage sections, automobile and truck body panels
  102. 102. FRP Properties  High strength-to-weight and modulus-to-weight ratios  A typical FRP weighs only about 1/5 as much as steel  Yet strength and modulus are comparable in fiber direction  Good fatigue strength  Good corrosion resistance, although polymers are soluble in various chemicals  Low thermal expansion for many FRPs
  103. 103. FRP Applications  Aerospace – much of the structural weight of today’s airplanes and helicopters consist of advanced FRPs  Example: Boeing 787  Automotive – some body panels for cars and truck cabs  Low-carbon sheet steel still widely used due to its low cost and ease of processing  Sports and recreation  FRPs used for boat hulls since 1940s  Fishing rods, tennis rackets, golf club shafts, helmets, skis, bows and arrows
  104. 104. Other Polymer Matrix Composites  Other PMCs contain particles, flakes, and short fibers  Called fillers when used in molding compounds  Two categories: 1. Reinforcing fillers – used to strengthen or otherwise improve mechanical properties 2. Extenders – used to increase bulk strength and reduce cost per unit weight, with little or no effect on mechanical properties
  105. 105. Guide to Processing Composite Materials  The two phases are typically produced separately before being combined into the composite part  Processing techniques to fabricate MMC and CMC components are similar to those used for powdered metals and ceramics  Molding processes are commonly used for PMCs with particles and chopped fibers  Specialized processes have been developed for FRPs
  106. 106. Guide to the Processing of Polymers  Polymers are nearly always shaped in a heated, highly plastic state  Common operations are extrusion and molding  Molding of thermosets is more complicated because of cross-linking  Thermoplastics are easier to mold, and a greater variety of molding operations are available  Rubber processing has a longer history than plastics, and rubber industries are traditionally separated from plastics industry, even though processing is similar
  107. 107. Thanks