Ch09

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Ch09

  1. 1. COMPOSITE MATERIALS <ul><li>Technology and Classification of Composite Materials </li></ul><ul><li>Metal Matrix Composites </li></ul><ul><li>Ceramic Matrix Composites </li></ul><ul><li>Polymer Matrix Composites </li></ul><ul><li>Guide to Processing Composite Materials </li></ul>
  2. 2. Composite Material Defined <ul><li>A materials system composed of two or more distinct phases whose combination produces aggregate properties different from those of its constituents </li></ul><ul><li>Examples: </li></ul><ul><ul><li>Cemented carbides (WC with Co binder) </li></ul></ul><ul><ul><li>Plastic molding compounds with fillers </li></ul></ul><ul><ul><li>Rubber mixed with carbon black </li></ul></ul><ul><ul><li>Wood (a natural composite as distinguished from a synthesized composite) </li></ul></ul>
  3. 3. Why Composites are Important <ul><li>Composites can be very strong and stiff, yet very light in weight </li></ul><ul><ul><li>Strength‑to‑weight and stiffness‑to‑weight ratios are several times greater than steel or aluminum </li></ul></ul><ul><li>Fatigue properties are generally better than for common engineering metals </li></ul><ul><li>Toughness is often greater </li></ul><ul><li>Possible to achieve combinations of properties not attainable with metals, ceramics, or polymers alone </li></ul>
  4. 4. Disadvantages and Limitations <ul><li>Properties of many important composites are anisotropic </li></ul><ul><ul><li>May be an advantage or a disadvantage </li></ul></ul><ul><li>Many polymer‑based composites are subject to attack by chemicals or solvents </li></ul><ul><ul><li>Just as the polymers themselves are susceptible </li></ul></ul><ul><li>Composite materials are generally expensive </li></ul><ul><li>Manufacturing methods for shaping composite materials are often slow and costly </li></ul>
  5. 5. Possible Classification of Composites <ul><li>Traditional composites – composite materials that occur in nature or have been produced by civilizations for many years </li></ul><ul><ul><li>Examples: wood, concrete, asphalt </li></ul></ul><ul><li>Synthetic composites - modern material systems normally associated with the manufacturing industries </li></ul><ul><ul><li>Components are first produced separately and then combined to achieve the desired structure, properties, and part geometry </li></ul></ul>
  6. 6. Components in a Composite Material <ul><li>Most composite materials consist of two phases: </li></ul><ul><li>Primary phase - forms the matrix within which the secondary phase is imbedded </li></ul><ul><li>Secondary phase - imbedded phase sometimes referred to as a reinforcing agent , because it usually strengthens the composite material </li></ul><ul><ul><li>The reinforcing phase may be in the form of fibers, particles, or various other geometries </li></ul></ul>
  7. 7. Our Classification of Composite Materials <ul><li>Metal Matrix Composites (MMCs) ‑ mixtures of ceramics and metals, such as cemented carbides and other cermets </li></ul><ul><li>Ceramic Matrix Composites (CMCs) ‑ Al 2 O 3 and SiC imbedded with fibers to improve properties </li></ul><ul><li>Polymer Matrix Composites (PMCs) ‑ polymer resins imbedded with filler or reinforcing agent </li></ul><ul><ul><li>Examples: epoxy and polyester with fiber reinforcement, and phenolic with powders </li></ul></ul>
  8. 8. Functions of the Matrix Material <ul><li>Primary phase provides the bulk form of the part or product made of the composite material </li></ul><ul><li>Holds the imbedded phase in place, usually enclosing and often concealing it </li></ul><ul><li>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 </li></ul>
  9. 9. Reinforcing Phase <ul><li>Function is to reinforce the primary phase </li></ul><ul><li>Reinforcing phase (imbedded in the matrix) is most commonly one of the following shapes: fibers, particles, or flakes </li></ul><ul><li>Also, secondary phase can take the form of an infiltrated phase in a skeletal or porous matrix </li></ul><ul><ul><li>Example: a powder metallurgy part infiltrated with polymer </li></ul></ul>
  10. 10. Physical Shapes of Imbedded Phase <ul><li>Possible physical shapes of imbedded phases in composite materials: (a) fiber, (b) particle, and (c) flake </li></ul>
  11. 11. Fibers <ul><li>Filaments of reinforcing material, usually circular in cross section </li></ul><ul><li>Diameters from ~ 0.0025 mm to about 0.13 mm </li></ul><ul><li>Filaments provide greatest opportunity for strength enhancement of composites </li></ul><ul><ul><li>Filament form of most materials is significantly stronger than the bulk form </li></ul></ul><ul><ul><li>As diameter is reduced, the material becomes oriented in the fiber axis direction and probability of defects in the structure decreases significantly </li></ul></ul>
  12. 12. Continuous Fibers vs. Discontinuous Fibers <ul><li>Continuous fibers - very long; in theory, they offer a continuous path by which a load can be carried by the composite part </li></ul><ul><li>Discontinuous fibers (chopped sections of continuous fibers) - short lengths (L/D = roughly 100) </li></ul><ul><ul><li>Whiskers = discontinuous fibers of hair-like single crystals with diameters down to about 0.001 mm (0.00004 in) and very high strength </li></ul></ul>
  13. 13. Fiber Orientation – Three Cases <ul><li>One‑dimensional reinforcement, in which maximum strength and stiffness are obtained in the direction of the fiber </li></ul><ul><li>Planar reinforcement, in some cases in the form of a two‑dimensional woven fabric </li></ul><ul><li>Random or three‑dimensional in which the composite material tends to possess isotropic properties </li></ul>
  14. 14. Fiber Orientation <ul><li>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 </li></ul>
  15. 15. Materials for Fibers <ul><li>Fiber materials in fiber‑reinforced composites </li></ul><ul><ul><li>Glass – most widely used filament </li></ul></ul><ul><ul><li>Carbon – high elastic modulus </li></ul></ul><ul><ul><li>Boron – very high elastic modulus </li></ul></ul><ul><ul><li>Polymers - Kevlar </li></ul></ul><ul><ul><li>Ceramics – SiC and Al 2 O 3 </li></ul></ul><ul><ul><li>Metals - steel </li></ul></ul><ul><li>Most important commercial use of fibers is in polymer composites </li></ul>
  16. 16. Particles and Flakes <ul><li>A second common shape of imbedded phase is particulate , ranging in size from microscopic to macroscopic </li></ul><ul><ul><li>Flakes are basically two‑dimensional particles ‑ small flat platelets </li></ul></ul><ul><li>Distribution of particles in the matrix is random </li></ul><ul><ul><li>Strength and other properties of the composite material are usually isotropic </li></ul></ul>
  17. 17. Interface between Constituent Phases in Composite Material <ul><li>For the composite to function, the phases must bond where they join at the interface </li></ul><ul><li>Direct bonding between primary and secondary phases </li></ul>
  18. 18. Interphase <ul><li>In some cases, a third ingredient must be added to bond primary and secondary phases </li></ul><ul><li>Called an interphase , it is like an adhesive </li></ul>
  19. 19. Alternative Interphase Form <ul><li>Formation of an interphase consisting of a solution of primary and secondary phases at their boundary </li></ul>
  20. 20. Properties of Composite Materials <ul><li>In selecting a composite material, an optimum combination of properties is often sought, rather than one particular property </li></ul><ul><ul><li>Example: fuselage and wings of an aircraft must be lightweight, strong, stiff, and tough </li></ul></ul><ul><ul><ul><li>Several fiber‑reinforced polymers possess these properties </li></ul></ul></ul><ul><ul><li>Example: natural rubber alone is relatively weak </li></ul></ul><ul><ul><ul><li>Adding carbon black increases its strength </li></ul></ul></ul>
  21. 21. Three Factors that Determine Properties <ul><li>Materials used as component phases in the composite </li></ul><ul><li>Geometric shapes of the constituents and resulting structure of the composite system </li></ul><ul><li>How the phases interact with one another </li></ul>
  22. 22. Example: Fiber Reinforced Polymer <ul><li>Model of fiber‑reinforced composite material showing direction in which elastic modulus is being estimated by the rule of mixtures </li></ul>
  23. 23. Example: Fiber Reinforced Polymer (continued) <ul><li>Stress‑strain relationships for the composite material and its constituents </li></ul><ul><li>The fiber is stiff but brittle, while the matrix (commonly a polymer) is soft but ductile </li></ul>
  24. 24. Variations in Strength and Stiffness <ul><li>Variation in elastic modulus and tensile strength as a function of direction relative to longitudinal axis of carbon fiber‑reinforced epoxy composite </li></ul>
  25. 25. Importance of Geometric Shape: Fibers <ul><li>Most materials have tensile strengths several times greater as fibers than as bulk materials </li></ul><ul><li>By imbedding the fibers in a polymer matrix, a composite material is obtained that avoids the problems of fibers but utilizes their strengths </li></ul><ul><ul><li>Matrix provides the bulk shape to protect the fiber surfaces and resist buckling </li></ul></ul><ul><ul><li>When a load is applied, the low‑strength matrix deforms and distributes the stress to the high‑strength fibers </li></ul></ul>
  26. 26. Other Composite Structures <ul><li>Laminar composite structure – conventional </li></ul><ul><li>Sandwich structure </li></ul><ul><li>Honeycomb sandwich structure </li></ul>
  27. 27. Laminar Composite Structure <ul><li>Conventional laminar structure - two or more layers bonded together in an integral piece </li></ul><ul><li>Example: plywood, in which layers are the same wood, but grains oriented differently to increase overall strength </li></ul>
  28. 28. Sandwich Structure: Foam Core <ul><li>Relatively thick core of low density foam bonded on both faces to thin sheets of a different material </li></ul>
  29. 29. Sandwich Structure: Honeycomb Core <ul><li>Alternative to foam core </li></ul><ul><li>Foam or honeycomb achieve high ratios of strength‑to‑weight and stiffness‑to‑weight </li></ul>
  30. 30. Other Laminar Composite Structures <ul><li>FRPs - multi‑layered, fiber‑reinforced plastic panels for aircraft, boat hulls, other products </li></ul><ul><li>Printed circuit boards - layers of reinforced copper and plastic for electrical conductivity and insulation, respectively </li></ul><ul><li>Snow skis - layers of metals, particle board, and phenolic plastic </li></ul><ul><li>Windshield glass - two layers of glass on either side of a sheet of tough plastic </li></ul>
  31. 31. Metal Matrix Composites (MMCs) <ul><li>Metal matrix reinforced by a second phase </li></ul><ul><li>Reinforcing phases: </li></ul><ul><ul><li>Particles of ceramic </li></ul></ul><ul><ul><ul><li>These MMCs are commonly called cermets </li></ul></ul></ul><ul><ul><li>Fibers of various materials </li></ul></ul><ul><ul><ul><li>Other metals, ceramics, carbon, and boron </li></ul></ul></ul>
  32. 32. Cermets <ul><li>MMC with ceramic contained in a metallic matrix </li></ul><ul><li>The ceramic often dominates the mixture, sometimes up to 96% by volume </li></ul><ul><li>Bonding can be enhanced by slight solubility between phases at elevated temperatures used in processing </li></ul><ul><li>Cermets can be subdivided into </li></ul><ul><ul><li>Cemented carbides – most common </li></ul></ul><ul><ul><li>Oxide‑based cermets – less common </li></ul></ul>
  33. 33. Cemented Carbides <ul><li>One or more carbide compounds bonded in a metallic matrix </li></ul><ul><li>Common cemented carbides are based on tungsten carbide (WC), titanium carbide (TiC), and chromium carbide (Cr 3 C 2 ) </li></ul><ul><ul><li>Tantalum carbide (TaC) and others are less common </li></ul></ul><ul><li>Metallic binders: usually cobalt (Co) or nickel (Ni) </li></ul>
  34. 34. <ul><li>Photomicrograph (about 1500X) of cemented carbide with 85% WC and 15% Co (photo courtesty of Kennametal Inc.) </li></ul>Cemented Carbide
  35. 35. <ul><li>Typical plot of hardness and transverse rupture strength as a function of cobalt content </li></ul>Cemented Carbide Properties
  36. 36. Applications of Cemented Carbides <ul><li>Tungsten carbide cermets (Co binder) </li></ul><ul><ul><li>Cutting tools, wire drawing dies, rock drilling bits, powder metal dies, indenters for hardness testers </li></ul></ul><ul><li>Titanium carbide cermets (Ni binder) </li></ul><ul><ul><li>Cutting tools; high temperature applications such as gas‑turbine nozzle vanes </li></ul></ul><ul><li>Chromium carbide cermets (Ni binder) </li></ul><ul><ul><li>Gage blocks, valve liners, spray nozzles </li></ul></ul>
  37. 37. Ceramic Matrix Composites (CMCs) <ul><li>Ceramic primary phase imbedded with a secondary phase, usually consisting of fibers </li></ul><ul><li>Attractive properties of ceramics: high stiffness, hardness, hot hardness, and compressive strength; and relatively low density </li></ul><ul><li>Weaknesses of ceramics: low toughness and bulk tensile strength, susceptibility to thermal cracking </li></ul><ul><li>CMCs represent an attempt to retain the desirable properties of ceramics while compensating for their weaknesses </li></ul>
  38. 38. Ceramic Matrix Composite <ul><li>Photomicrograph (about 3000X) of fracture surface of SiC whisker reinforced Al 2 O 3 (photo courtesy of Greenleaf Corp.) </li></ul>
  39. 39. Polymer Matrix Composites (PMCs) <ul><li>Polymer primary phase in which a secondary phase is imbedded as fibers, particles, or flakes </li></ul><ul><li>Commercially, PMCs are more important than MMCs or CMCs </li></ul><ul><ul><li>Examples: most plastic molding compounds, rubber reinforced with carbon black, and fiber‑reinforced polymers (FRPs) </li></ul></ul>
  40. 40. Fiber‑Reinforced Polymers (FRPs) <ul><li>PMC consisting of a polymer matrix imbedded with high‑strength fibers </li></ul><ul><li>Polymer matrix materials: </li></ul><ul><ul><li>Usually a thermosetting plastic such as unsaturated polyester or epoxy </li></ul></ul><ul><ul><li>Can also be thermoplastic, such as nylons (polyamides), polycarbonate, polystyrene, and polyvinylchloride </li></ul></ul><ul><ul><li>Fiber reinforcement is widely used in rubber products such as tires and conveyor belts </li></ul></ul>
  41. 41. Fibers in PMCs <ul><li>Various forms: discontinuous (chopped), continuous, or woven as a fabric </li></ul><ul><li>Principal fiber materials in FRPs are glass, carbon, and Kevlar 49 </li></ul><ul><ul><li>Less common fibers include boron, SiC, and Al 2 O 3 , and steel </li></ul></ul><ul><li>Glass (in particular E‑glass) is the most common fiber material in today's FRPs </li></ul><ul><ul><li>Its use to reinforce plastics dates from around 1920 </li></ul></ul>
  42. 42. Common FRP Structures <ul><li>Most widely used form of FRP is a laminar structure </li></ul><ul><ul><li>Made by stacking and bonding thin layers of fiber and polymer until desired thickness is obtained </li></ul></ul><ul><ul><li>By varying fiber orientation among layers, a specified level of anisotropy in properties can be achieved in the laminate </li></ul></ul><ul><li>Applications: boat hulls, aircraft wing and fuselage sections, automobile and truck body panels </li></ul>
  43. 43. FRP Properties <ul><li>High strength‑to‑weight and modulus‑to‑weight ratios </li></ul><ul><ul><li>A typical FRP weighs only about 1/5 as much as steel </li></ul></ul><ul><ul><li>Yet strength and modulus are comparable in fiber direction </li></ul></ul><ul><li>Good fatigue strength </li></ul><ul><li>Good corrosion resistance, although polymers are soluble in various chemicals </li></ul><ul><li>Low thermal expansion for many FRPs </li></ul>
  44. 44. FRP Applications <ul><li>Aerospace – much of the structural weight of today’s airplanes and helicopters consist of advanced FRPs </li></ul><ul><ul><li>Example: Boeing 787 </li></ul></ul><ul><li>Automotive – some body panels for cars and truck cabs </li></ul><ul><ul><li>Low-carbon sheet steel still widely used due to its low cost and ease of processing </li></ul></ul><ul><li>Sports and recreation </li></ul><ul><ul><li>FRPs used for boat hulls since 1940s </li></ul></ul><ul><ul><li>Fishing rods, tennis rackets, golf club shafts, helmets, skis, bows and arrows </li></ul></ul>
  45. 45. Other Polymer Matrix Composites <ul><li>Other PMCs contain particles, flakes, and short fibers </li></ul><ul><li>Called fillers when used in molding compounds </li></ul><ul><li>Two categories: </li></ul><ul><ul><li>Reinforcing fillers – used to strengthen or otherwise improve mechanical properties </li></ul></ul><ul><ul><li>Extenders – used to increase bulk and reduce cost per unit weight, with little or no effect on mechanical properties </li></ul></ul>
  46. 46. Guide to Processing Composite Materials <ul><li>The two phases are typically produced separately before being combined into the composite part </li></ul><ul><ul><li>Processing techniques to fabricate MMC and CMC components are similar to those used for powdered metals and ceramics </li></ul></ul><ul><ul><li>Molding processes are commonly used for PMCs with particles and chopped fibers </li></ul></ul><ul><ul><li>Specialized processes have been developed for FRPs </li></ul></ul>

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