Ch09

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

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


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 (WC with Co binder) Plastic molding compounds with fillers Rubber mixed with carbon black Wood (a natural composite as distinguished from

a synthesized composite)


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


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


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


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


Our Classification of Composite Materials

1. Metal Matrix Composites (MMCs) ‑ mixtures of ceramics and metals, such as cemented carbides and other cermets

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


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


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

Also, secondary phase can take the form of an infiltrated phase in a skeletal or porous matrix Example: a powder metallurgy part infiltrated with

polymer


Physical Shapes of Imbedded Phase

Possible physical shapes of imbedded phases in composite materials: (a) fiber, (b) particle, and (c) flake


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


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


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


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


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


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


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


Interphase

In some cases, a third ingredient must be added to bond primary and secondary phases

Called an interphase, it is like an adhesive


Alternative Interphase Form

Formation of an interphase consisting of a solution of primary and secondary phases at their boundary


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


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


Example: Fiber Reinforced Polymer

Model of fiber‑reinforced composite material showing direction in which elastic modulus is being estimated by the rule of mixtures


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


Variations in Strength and Stiffness

Variation in elastic modulus and tensile strength as a function of direction relative to longitudinal axis of carbon fiber‑reinforced epoxy composite


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


Other Composite Structures

Laminar composite structure – conventional Sandwich structure Honeycomb sandwich structure


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


Sandwich Structure: Foam Core

Relatively thick core of low density foam bonded on both faces to thin sheets of a different material


Sandwich Structure:Honeycomb Core

Alternative to foam core

Foam or honeycomb achieve high ratios of strength‑to‑weight and stiffness‑to‑weight


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


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


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


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)


Photomicrograph (about 1500X) of cemented carbide with 85% WC and 15% Co (photo courtesty of Kennametal Inc.)

Cemented Carbide


Typical plot of hardness and transverse rupture strength as a function of cobalt content

Cemented Carbide Properties


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


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


Ceramic Matrix Composite

Photomicrograph (about 3000X) of fracture surface of SiC whisker reinforced Al2O3 (photo courtesy of Greenleaf Corp.)


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)


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


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


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


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


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


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 and reduce cost per unit weight, with little or no effect on mechanical properties


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

Ch09

Business

fibers fiber materials

form of fibers

processing composite

composite materials

polymer composites

important composites

matrix material primary

natural composite