Engineering Materials-istanbul .technical University

C.Ergun Mak214E

Composites

Mechanical & Physical Properties •

•

•

•

•

• •

High Strength: Metals Ceramics High Ductility Metals Polymers High Toughness Metals Polymers Good Wear Resistance Ceramics Metals Good Corrosion Resistance Polymers Ceramics Good Electrical Conductivity Metals Low Density Polymers

• None of the single materials can satisfy all demanded properties at the same time. • But combination for the desired combination of the properties. • Composite materials: The combination of two or more materials to obtain the desired properties that can not be obtained in the original materials. • Improved properties: 9Stiffness, 9Strength, 9Hardness, 9Weight, 9Corrosion Resistance, 9Conductivity

1

C.Ergun Mak214E

The specific strength vs. temperature for several composites and metals. 2

1

C.Ergun Mak214E

Types of Composites

1.

Particulate Composites a. Dispersion-Strengthened Composites b. True Particulate Composites i. Cemented carbides ii. Abrasives iii. Electrical Contacts iv. Polymers v. Cast Metal Particulate Comp. 2. Fiber-Reinforced Composites 3. Laminar Composites 4. Sandwich Structures

(a) plywood is a laminar composite (b) a fiber-reinforced polymer matrix composites (c) concrete; a particulate composite; sand or gravel in a cement matrix 3

C.Ergun Mak214E

1a. Dispersion-Strengthened Composites 1. Contains very hard oxide particles 10 to 250 nm in diameter called “dispersoids” in a metal matrix. 2. Prevents dislocation motions and provides strengthening. 3. No coherency between the dispersoids and matrix. 4. No overaging; Good high temperatures porperties. 5. No dissolution in dispersoids or no reaction between matrix and dispersoids. Examples: ¾ SAP: Sintered Al powder up to 14% Al2O3 content. Fabricated with powder metallurgy. ¾ TD-Ni (Thoria dispersed Nickel): First thorium and Nickel powder mixed and pressed to compact the green. Then sintered in oxygen allowing internal oxidation of thorium forming thoria.

4

2

C.Ergun Mak214E

TD-Ni. The dispersed ThO2 particles with a diameter of 300 nm or less

SAP can be used upto about 300°C. 5

C.Ergun Mak214E

1b. True Particulate Composites

i) Cemented carbides (Cermets)– Hard ceramic particles bonded with a soft metallic matrix. • WC very hard and too brittle to be a cutting tool. Co matrix provides good toughness. • Produced with powder metallurgy: Co and WC powders sintered together. • Other hard ceramics; TiC, TaC, etc.

ii) Abrasives: Grinding and cutting wheels. • Hard particles bonded by glass or polymeric matrix. • Hard particles; Al2O3, SiC or BN.

Microstructure of WC / 20% cobaltcemented carbide

6

3

C.Ergun Mak214E

1b. True Particulate Composites

iii) Electrical Contacts: • Used for electrical contacts in switches and relays • A good combination of wear resistance and high electrical conductivity.

The steps in producing a silver-tungsten electrical composite: (a) Press the W powders, (b) a low-density porous compact green state, (c) Sintering, 7 (d) Infiltration of liquid silver into the pores.

C.Ergun Mak214E

1b. True Particulate Composites

iv) Polymers: Polymers containing fillers and extenders are particulate composites. a) Fillers: Carbon black in vulcanized rubber to improve strength, stiffness, hardness, wear and heat resistance. b) Extenders: CaCO3, Silica, talc, etc. For reducing the cost. c) Elastomers: to increase toughness. d) Lead (Pb): to use in nuclear application to absorb radiation.

The effect of clay on the properties of polyethylene.

8

4

C.Ergun Mak214E

1b. True Particulate Composites

v) Cast Metal Particulate Composites: • Al casting containing SiC composites; for vehicle applications • Thioxotropic behavior: In solid + liquid region, metal slurry behaves like a solid if no stress, but can easily flows under pressure therefore can be injected into dies: Al-Mg alloys used in auto rims, engine blocks, etc. • Compocasting: Molten alloy cooled to about 40% solid + 60% liquid region and vigorous stirring to break the dendritic structure (similar to Thioxocasting, but used in composite making), then add the hard particles.

SiC reinforced Al casting alloy, segregated to interdendritic 9 regions of the casting

C.Ergun Mak214E

2. Fiber Reinforced Composites Incorporating fibers in matrix: 9Fibers: Strong, stiff but brittle 9Matrix: Softer and ductile Combined properties: • Matrix transmits the force to the fibers • Fibers carry the most of the applied load Improvement in: ¾Fatigue resistance, ¾Strength ¾Stiffness ¾Strength / weight ratio ¾Both at room T and high T

10

5

C.Ergun Mak214E

11

C.Ergun Mak214E

2. Fiber Reinforced Composites

Factors affecting the properties of fiber reinforced composites: (a) Length and Diameter of Fibers, (b) Amount of Fibers, (c) Orientation of Fibers, (d) Fiber Properties, (e) Matrix Properties, (f) Bonding between Matrix and fibers (a) Fiber length and diameter: Fibers; short fibers, long fibers, continuous fibers. Diameters generally vary in the range of 10 - 150µm. Length should be higher than a specific value called critical fiber length.

l cr =

σ f ⋅d 2 ⋅τ i

l
Very small reinforcing effect Only finite reinforcing effect

l > 15 lc

Full Strengthening (like continious fibers)

Increasing the length of chopped fibers in a matrix increases the strength. 12

6

C.Ergun Mak214E

2. Fiber Reinforced Comp. (b) Amount of Fibers: • As the volume fraction of fibers increases, the strength of composites increases. • But generally less than 80% due to the wetting problems. (c) Orientation of Fibers: • Short and randomly distributed fiber: Relatively isotropic properties. • Long unidirectional fibers: anisotropic properties. Best strength and modulus when the fibers are parallel to the loading direction (longitudinal). Worst results when the fibers perpendicular (transverse) • Some arrangements possible to meet the strength requirements: 0/90o, 0/45/90o)

Increase in amount of reinforement, increase in elasticity and strength

Contribution is maximum when 13 stress and fiber direction is paralle.

C.Ergun Mak214E

A 3D weave for fiberreinforced composites.

(a) Tapes containing aligned fibers can be joined to produce a multi-layered different orientations to produce a quasi-isotropic composite. (b) 0°/+45°/90° composite 14

7

C.Ergun Mak214E

2. Fiber Reinforced Composites

(d) Fibers: • Polymer fibers (Kevlar, Nylon, PE) • Metals (Be, Boron, W) • Ceramics (E, S glass, Carbon) • Whiskers (Alumina, Grapite, SiC, etc) (e) Matrix Properties: Supports the fibers, transmits the stress to the fibers, protects the fibers from the external effects, also controls the electrical, chemical, and thermal properties. 9Polymer Matrix Composites: 9Metal Matrix Composites: 9Ceramic Matrix Composites (e) Bonding and failure: If bonding between fiber and matrix is low, strength and toughness is low. Sometimes coating layer on fibers to increase the bonding between fiber and matrix such as “Sizing”: silane coating on glass fibers

15

C.Ergun Mak214E

Polymer Matrix Fiber Composites

¾Common in many applications such as aerospace, racing, etc. ¾High strength metal fibers metals, polymers or ceramics, • Carbon fibers for high stiffness: For sporting goods. • Kevlar fibers for high strength: Bulletproof clotting for lightweight ballistic protections. • PE fibers relatively high strength combined with toughness and damage resistance: Sails for racing yachts. • Hybrid composites with a mixture of two or more different fibers: Kevlar + Carbon for toughness and stiffness, Kevlar + Glass for improved stiffness.

For better fracture toughness: 9Long fibers 9Amorphous matrix 9Thermoplastic elastomer matrix 9Interpenetrating network polymer

16

8

C.Ergun Mak214E

Metal Matrix Fiber Composites

Reinforce with metal or ceramic fibers for improved properties. 9Al reinforced with borsic fibers for aerospace applications: space shuttle struts 9Cu reinforced with SiC; High strength propellers for ships. 9Al reinforced with Al2O3 fibers; Pistons for diesel engines 9Al reinforced with SiC fibers or whiskers; Aerospace applications stiffeners missle fins. 9Al reinforced with Carbon fibers; Antenal mast in Hubble telescope 9Superalloys reinforced with metal (W) or ceramic fibers (SiC or B4C) high strength at high T; Jet engine parts. 9Ti or Ti3Al reinforced with SiC fibers; Turbine blades and disks. 9Nb3Sn-Cu composites; super-conducting applications. 17

C.Ergun Mak214E

Ceramic Matrix Fiber Composites

Carbon-Carbon composites, 9Stronger at high T upto 3000oC than lower T. 9Applications: Nose cone and leading edges of high performance aerospace vehicles(planes), brake discs for racing cars, commercial jet aircrafts and biomedical applications.

Pay special attention to carbo-carbon composite: No lost even increase in mechanical properties upon heating.

18

9

C.Ergun Mak214E

Ceramic Matrix Fiber Composites

Ceramic fiber / ceramic matrix composites: “POOR BONDING” at fiber / matrix interface; improved fracture toughness. (a) Propagation of crack around the fiber and “bridging” some fiber in the cracked surface; (b) pulling out the fiber.

Two failure modes: (a) Extensive pull-out of SiC fibers in a glass matrix provides good composite toughness. (b) Bridging of some fibers across a crack enhances the toughness of a ceramic-matrix 19 composite

C.Ergun Mak214E

3. Laminar Composites Why laminar composites? To improve corrosion resistance, low cost, high strength, light weight, superior wear or abrasion resistance, for better appearance, and unusual thermal expansion coef. Types: 1. Thin coatings, 2. Thicker protective surfaces, 3. Claddings, 4. Bimetallics, 5. Laminates, 6. Fiber reinforced composites. 20

10

C.Ergun Mak214E

Laminar composites: large variety of laminar composites of a variety of different applications. 1) Laminates: Layers joined by adhesives 9Arall 9Glare 2) Clad metals: 9Silver coinage 9Alclad 3) Bimetallics

21

C.Ergun Mak214E

Types of Laminar Composites

1) Micro laminates: Layers joined by adhesives Applications: Safety glasses, insulations in motors, for gears, for printed circuits, decorative items such as Formica and furniture. Schematic diagram of an aramidaluminum laminate, Arall, which has potential for aerospace applications.

Microlaminates: 1. Arall (Al / aramid laminates), 2. Glare (glass / Al laminates) For a combination of strength, stiffness, corrosion resistance, light weight, good fatigue resistance (blocking crack growth), good resistance to lightning strike, good formability and machinability.

22

11

C.Ergun Mak214E

Types of Laminar Composites

2) Clad metals: metal / metal composites, corrosion resistance with high strength. • Silver coinage: A silver colored Cu 80% Ni alloy bonded to a Cu 20% Ni alloy for low cost. • Alclad: commercially pure Al bonded to high strength Al alloy for the combination of corrosion resistance, strength and light weight. Pure Al protects high strength alloy against corrosion. For aircraft, heat exchangers, buildings, storage tanks. 3) Bimetallics : Two metals in a laminar composites. •Very different therermal expansion coef. •Reversible and repeatable expansion characteristics for reliability, •A high modulus of elasticity to do a work. For temperature indicators and controllers. Temperature measurements.

C.Ergun Mak214E

Clading is used to produce bimetallics!! 23

4. Sandwich and Honeycomb Structures

Sandwich materials: Lightweight and stiff structures 9Thin layer of facing materials, 9Light weight filler materials, Neither the facing material nor the filler material is rigid but the combination is very stiff. Examples: ¾Corrugated paper used for packaging. ¾Honeycomb materials for aircrafts Honeycomb: 9 A lightweight and stiff structures 9 Materials may be used such as Al, fiber glass, paper, aramid, etc. 9 May be filled by foam or fiberglass for sound and vibration absorption. 24

12

C.Ergun Mak214E Some

important definition: Refer to the glossaries at the end of each chapter for more definitions

Cemented carbides (Cermets) - Particulate composites containing hard ceramic particles bonded with a soft metallic matrix. Electrical Contacts - Materials used for electrical contacts in switches and relays must have a good combination of wear resistance and electrical conductivity. Polymers based particulate composites - Many engineering polymers that contain fillers and extenders are particulate composites. Rule of Mixtures - Some properties of the laminar composite materials parallel to the lamellae are estimated from the rule of mixtures. Producing Laminar Composites - (a) roll bonding, (b) explosive bonding, (c) coextrusion, and (d) brazing. Laminates - Laminates are layers of materials joined by an organic adhesive. Cladding - A laminar composite produced when a corrosion-resistant or high-hardness layer of a laminar composite formed onto a less expensive or higher-strength backing. Bimetallic - A laminar composite material produced by joining two strips of metal with different thermal expansion coefficients, making the material sensitive to temperature changes. Aspect ratio - The length of a fiber divided by its diameter. Delamination - Separation of individual plies of a fiber-reinforced composite.

25

C.Ergun Mak214E

Processing the Composites Steps in manufacturing of composites: 1. Producing the fibers 2. Arranging the fibers 3. Manufacturing the composites Producing the fibers ¾ Metallic fibers (Be, Boron, W) ¾ Glass fibers (E and S glass, Carbon) ¾ Polymer fibers (Kevlar, Nylon, PE) ¾ Whiskers (Al2O3, Graphite, SiC, etc) Fibers: 1.Boron fiber 2.Carbon fiber 3.Whisker

26

13

C.Ergun Mak214E

Comparison of the specific strength and specific modulus of fibers versus metals and polymers. 27

C.Ergun Mak214E

1. Producing the Fibers

1.Boron fiber: Very reactive 9 CVD (chemical vapor deposition) process; Vaporization of BCl3 and deposition on very fine W filaments substrates (carrier). 9Similar technique used to produce SiC deposition on C fiber substrates with also CVD method.

28

14

C.Ergun Mak214E

1. Producing the Fibers

2. Carbon fiber: Produced by carburizing or Graphitizing. Graphitizing 9Starting material is a polymer such as PAN (polyacrylonitrile) or pitch (aromatic compound) 9Carburizing at high T (1000-2000oC) just C backbone remains. Carburizing for high strength. 9Graphitizing at high T (2500-3000oC) for high elastic modulus 9Drawing is important to align to get the desired orientation. 3.

Whiskers: 9 Single crystal with aspect ratios 20 to 1000. 9 No mobile dislocations so no slip; very high strength. 29

C.Ergun Mak214E

2. Arranging the Fibers Divided into 3 groups: ¾Bundled filaments ¾Short fibers ¾Continuous fibers

1.Bundled filaments: Filaments bundled together. 9 Yarns: A twisted bundle of more than 10000 continuous fibers 9 Tow: An untwisted bundle of more than 10000 continuous fibers 9 Rovings: An untwisted bundle of less than 10000 continuous fibers

30

15

C.Ergun Mak214E

2. Arranging the Fibers-Cont.

2.Short fiber --Staples: Chopped short fiber with a length of 1 cm or less. 9 Easy to mix into polymer matrix composites. 9 Good for sheet or bulk molding. 9 Random orientation in matrix providing isotropic properties. 3.Long and continuous fibers: 9 Mats: Non-woven randomly oriented fibers loosely held together by a polymer resin. 9 Fabrics: Woven, braided or knitted in 2D or 3D and impregnated in a polymer resin. 9 Tapes: Single filament thick strips of prepregs with the filaments either in unidirectional or woven fiber. Several layers of tapes are joined for structures. Upper and lower faces may cover with metal foils and joined with diffusion bonds Prepreg: Layers of fibers, mats or fabrics in un-polymerized resins. Stacked to shape a desired part and polymerized to join the layers 31

C.Ergun Mak214E

3.Manufacturing the Composites

1.Short fiber reinforced composites: a. Mixing: Mixing the fibers with polymer with liquid or polymer matrix. b. Manufacturing the composites: Some conventional methods 9 Injection molding for polymer matrix comp. 9 Casting for production of MMCs 9 Spray up method: spraying the mixture against a form and curing. 2.Continuous fiber (short or long) reinforced composites: ¾ ¾ ¾ ¾ ¾ ¾

Hand lay out technique: Pressure bag molding: Match die molding: Filament winding: Pultrusion: Metal Matrix Composites (MMCs):

32

16

C.Ergun Mak214E

Manufacturing the Composites, Cont.

2. Continous fiber (short or long) reinforced composites: Unidicrectionally alligned fibers, mats and fabrics. ¾Hand lay out technique: 9Place the tapes, mats or fabrics against a form. 9Saturate with a polymer resin. 9Roll to assure good contact between the layers and to free the pores. 9Cure the polymer. 9Good for fiber glass vehicle bodies etc. But slow and labor intensive. 33

C.Ergun Mak214E

Manufacturing the Composites, Cont.

¾Pressure bag molding: 9Similar to hand lay out technique but pressure provides good bonding. 9Large polymer matrix composties such as skins of military aircrafts. ¾ Match die molding: short fibers of mats into a two part die. When closed the composite shaped.

(b) pressure bag molding, (c) matched die molding.

Close die deformation of composites 34 composed of several tapes

17

C.Ergun Mak214E

Manufacturing the Composites, Cont.

¾Filament winding: 9 Wrapping the fiber around a form or a mandrel: Gradual increase in thickness up to even several feet with build up the material. 9 Dipping the filament into the polymer matrix resin before, during or after winding. 9 Curing completes the production. 9 Good for pressure tanks, rocket motor casings, etc.

Producing composite shapes by filament winding. 35

C.Ergun Mak214E

Manufacturing the Composites, Cont.

¾ Pultrusion: 9Fiber drawn from the spools 9Passed the fiber from a polymer resin bath. 9Gathered together in a die to produce particular shape. 9Curing the resin immediately in an oven in the continuous production line. 9Good for continuous, simple shaped products with a cross section of round, rectangle, pipe, plate, or sheet. 9Subsequent processes for complicated shapes such as sporting goods, ski poles, fishing poles, golf club shafts.

36

18

C.Ergun Mak214E

Manufacturing the Composites, Cont.

¾ Metal Matrix Composites (MMCs): Casting methods base on forcing liquid metal around fibers: 1. Capillary rise 2. Pressure casting 3. Vacuum infiltration 4. Continuous casting

37

C.Ergun Mak214E

Manufacturing the Composites, Cont.

Manufacturing methods: • Deformation boding good for claddings and bimetallics braking the surface oxide films and providing atom to atom contact. • Adhesive bonding for individual plies. (A) Roll Bonding, (B) Explosive Bonding, (C) Coextrusion, (D) Brazing.

Production of fiber tapes by encasing fibers between metal cover sheets by diffusion bonding. 38

19

C.Ergun Mak214E

Manufacturing the Composites, Cont. Corrugation method for producing a honeycomb core, Material (such as aluminum) is corrugated between two rolls. The corrugated sheets are joined together with adhesive, Cut to the desired thickness. Honeycomb sandwich structure • A hexagonal cell honeycomb core, • Joined to two face sheets by means of adhesive sheets, • Exceptionally lightweight yet stiff, and strong. 39

20