engineering materials 1. metals and their alloys 2. ceramics 3. polymers 4. composites ©2012 john...

ENGINEERING MATERIALS

1. Metals and their alloys

2. Ceramics

3. Polymers

4. Composites

©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes

Metals and Their Alloys

A metal is a category of materials generally characterized by properties of ductility, malleability, luster, and high electrical and thermal conductivity Includes both metallic elements and their alloys

An alloy is a metal composed of two or more elements, at least one of which is metallic

Generally classified into two groups:

1. Ferrous – steels and cast irons

2. Nonferrous – aluminum, copper, etc.©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes

Iron-Carbon Phase Diagram

Binary phase diagram for iron‑carbon system, up to about 6% carbon


Steels

Iron-carbon alloy containing from 0.02% to 2.11% carbon (most steels are 0.05% and 1.1% C)

Steels often contain other alloying ingredients, such as manganese, chromium, nickel, and molybdenum

Categories of steels include:

1. Plain carbon steels

2. Low alloy steels

3. Stainless steels

4. Tool steels


Plain Carbon Steels

Carbon is the principal alloying element, with only small amounts of other elements (about 0.5% manganese is normal)

Strength of plain carbon steels increases with carbon content, but ductility is reduced

High carbon steels can be heat treated to form martensite, making the steel very hard and strong


Tensile strength and hardness as a function of carbon content in plain carbon steel (hot rolled)


Properties of Steel

AISI-SAE Designation Scheme

Specified by a 4‑digit number system: 10XX, where 10 indicates plain carbon steel, and XX indicates carbon % in hundredths of percentage points For example, 1020 steel contains 0.20% C Developed by American Iron and Steel Institute

(AISI) and Society of Automotive Engineers (SAE), so designation often expressed as AISI 1020 or SAE 1020


Plain Carbon Steels

1. Low carbon steels - less than 0.20% C Applications: automobile sheetmetal parts, plate

steel for fabrication, railroad rails2. Medium carbon steels - between 0.20% and 0.50% C

Applications: machinery components and engine parts such as crankshafts and connecting rods

3. High carbon steels - greater than 0.50% C Applications: springs, cutting tools and blades,

wear-resistant parts


Low Alloy Steels

Iron‑carbon alloys containing additional alloying elements in amounts totaling less than 5% by weight

Mechanical properties superior to plain carbon steels for given applications

Higher strength, hardness, hot hardness, wear resistance, and toughness Heat treatment is often required to achieve these

improved properties


High strength Steel applications in new Mercedes Benz structure

Around 72 percent of all the bodyshell panels for the Mercedes-Benz E-Class e.g. are made from ultra-high-strength steel – a new record in passenger-car development. Three to four times the tensile strength of conventional high-strength steel grades. They are used at points where the material can be exposed to exceptionally high stresses during an accident – as a material for the B-pillars and the side roof frames to provide side impact protection,

AISI-SAE Designation Scheme

AISI‑SAE designation uses a 4‑digit number system: YYXX, where YY indicates alloying elements, and XX indicates carbon % in hundredths of % points

Examples:13XX - Manganese steel

20XX - Nickel steel

31XX - Nickel‑chrome steel

40XX - Molybdenum steel

41XX - Chrome‑molybdenum steel


Stainless Steel (SS)

Highly alloyed steels designed for corrosion resistance Principal alloying element is chromium, usually

greater than 15% Cr forms a thin impervious oxide film that

protects surface from corrosion Nickel (Ni) is another alloying ingredient in certain SS

to increase corrosion protection Carbon is used to strengthen and harden SS, but

high C content reduces corrosion protection since chromium carbide forms to reduce available free Cr


Properties of Stainless Steels

In addition to corrosion resistance, stainless steels are noted for their combination of strength and ductility These properties generally make stainless steel

difficult to work in manufacturing• But not impossible! (Jim comment)

Significantly more expensive than plain C or low alloy steels


Types of Stainless Steel

Classified according to the predominant phase present at ambient temperature:

1. Austenitic stainless ‑ typical composition 18% Cr and 8% Ni

2. Ferritic stainless ‑ about 15% to 20% Cr, low C, and no Ni

3. Martensitic stainless ‑ as much as 18% Cr but no Ni, higher C content than ferritic stainless


Designation Scheme for Stainless Steels

We start over again (3 digits) !!

First digit indicates general type, and last two digits give specific grade within type Type 302 – Austenitic SS

18% Cr, 8% Ni, 2% Mn, 0.15% C Type 430 – Ferritic SS

17% Cr, 0% Ni, 1% Mn, 0.12% C Type 440 – Martensitic SS

17% Cr, 0% Ni, 1% Mn, 0.65% C


Cast Irons

Iron alloys containing from 2.1% to about 4% carbon and from 1% to 3% silicon

Highly suitable as casting metals Tonnage of cast iron castings is several times that of

all other cast metal parts combined, (excluding cast ingots in steel-making that are subsequently rolled into bars, plates, and similar stock)

Overall tonnage of cast iron is second only to steel among metals


Types of Cast Irons

Most important is gray cast iron Other types include ductile iron, white cast iron,

malleable iron, and various alloy cast irons Ductile and malleable irons possess chemistries

similar to the gray and white cast irons, respectively, but result from special processing treatments


Nonferrous Metals 'Not Iron' – how clever!

Metal elements and alloys not based on iron Most important - aluminum, copper, magnesium,

nickel, titanium, and zinc, and their alloys Although not as strong as steels, certain nonferrous

alloys have strength‑to‑weight ratios that make them competitive with steels in some applications

Many nonferrous metals have properties other than mechanical that make them ideal for applications in which steel would not be suitable


Aluminum and Magnesium

Aluminum (Al) and magnesium (Mg) are light metals They are often specified in

engineering applications for this feature

Both elements are abundant on earth, aluminum on land and magnesium in the sea Neither is easily extracted

from their natural states©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes

Aluminum and Its Alloys

High electrical and thermal conductivity Excellent corrosion resistance due to formation of a

hard thin oxide surface film Very ductile metal, noted for its formability Pure aluminum is relatively low in strength, but it can

be alloyed and heat treated to compete with some steels, especially when weight is taken into consideration– Where do you see a lot of aluminum today? What

other products are using it? ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes

Auto Industry Embracing Aluminum as lightweight replacement for steel

2015 Ford F150 Pickup truck – saves 750 lbs, likely 30MPG highway

Aluminum and Its Alloys

High electrical and thermal conductivity Excellent corrosion resistance due to formation of a

hard thin oxide surface film Very ductile metal, noted for its formability Pure aluminum is relatively low in strength, but it can

be alloyed and heat treated to compete with some steels, especially when weight is taken into consideration


Magnesium and Its Alloys

Lightest of the structural metals Available in both wrought and cast forms Relatively easy to machine In all processing of magnesium, small particles of the

metal (such as small metal cutting chips) oxidize rapidly Care must be exercised to avoid fire hazards Stuff burns REALLY well


Copper

One of the oldest metals known to mankind Low electrical resistivity ‑ commercially pure copper is

widely used as an electrical conductor Also an excellent thermal conductor One of the noble metals (gold and silver are also

noble metals), so it is corrosion resistant


Nickel and Its Alloys

Similar to iron in some respects: Magnetic Modulus of elasticity E for iron and steel

Differences with iron: Much more corrosion resistant - widely used as

(1) an alloying element in steel, e.g., stainless steel, and (2) as a plating metal on metals such as plain carbon steel

High temperature properties of Ni alloys are superior

(American nickel is 25% nickel)©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes

Nickel Alloys

Alloys of nickel are commercially important and are noted for corrosion resistance and high temperature performance

In addition, a number of superalloys are based on nickel

Applications: stainless steel alloying ingredient, plating metal for steel, applications requiring high temperature and corrosion resistance


Titanium and Its Alloys

Abundant in nature, constituting 1% of earth's crust (aluminum is 8%)

Density of Ti is between aluminum and iron Importance has grown in recent decades due to its

aerospace applications where its light weight and good strength‑to‑weight ratio are exploited– Boeing 787 – 15% titanium by weight (39,000

lb/ac for 787-8)


Applications of Titanium

In the commercially pure state, Ti is used for corrosion resistant components, such as marine components and prosthetic implants

Titanium alloys are used as high strength components at temperatures ranging up to above 550C (1000F), especially where its excellent strength‑to‑weight ratio is exploited

Alloying elements used with titanium include aluminum, manganese, tin, and vanadium


Zinc and Its Alloys

Low melting point makes it attractive as a casting metal, especially die casting

Also provides corrosion protection when coated onto steel or iron The term galvanized steel refers to steel coated

with zinc Widely used as alloy with copper (brass)


My first car - 1970 Camaro– 90,000 miles At 6 years old, rusted wheelwells, surface rust all over.

Before Auto Industry started to use zinc plated steel

Superalloys

High‑performance alloys designed to meet demanding requirements for strength and resistance to surface degradation at high service temperatures

Many superalloys contain substantial amounts of three or more metals, rather than consisting of one base metal plus alloying elements

Commercially important because they are very expensive

Technologically important because of their unique properties


Why Superalloys are Important

Room temperature strength properties are good but not outstanding

High temperature performance is excellent - tensile strength, hot hardness, creep resistance, and corrosion resistance at very elevated temperatures

Operating temperatures often ~ 1100C (2000F) Applications: gas turbines ‑ jet and rocket engines,

steam turbines, and nuclear power plants (systems that operate more efficiently at high temperatures)


Three Groups of Superalloys

1. Iron‑based alloys ‑ in some cases iron is less than 50% of total composition Alloyed with Ni, Cr, Co

2. Nickel‑based alloys ‑ better high temperature strength than alloy steels Alloyed with Cr, Co, Fe, Mo, Ti

3. Cobalt‑based alloys ‑ 40% Co and 20% chromium Alloyed with Ni, Mo, and W

Virtually all superalloys strengthen by precipitation hardening (hold at high temp for longer periods of time to optimize grain structure)


Ceramic Defined

An inorganic compound consisting of a metal (or semi‑metal) and one or more nonmetals

Important examples: Silica - silicon dioxide (SiO2), the main ingredient in

most glass products Alumina - aluminum oxide (Al2O3), used in various

applications from abrasives to artificial bones More complex compounds such as hydrous

aluminum silicate (Al2Si2O5(OH)4), the main ingredient in most clay products


Properties of Ceramic Materials

High hardness, electrical and thermal insulating, chemical stability, and high melting temperatures

Brittle, virtually no ductility - can cause problems in both processing and performance of ceramic products

Some ceramics are translucent, window glass (based on silica) being the clearest example


Three Basic Categories of Ceramics

1. Traditional ceramics ‑ clay products such as pottery, bricks, common abrasives, and cement

2. New ceramics ‑ more recently developed ceramics based on oxides, carbides, etc., with better mechanical or physical properties than traditional ceramics

3. Glasses ‑ based primarily on silica and distinguished by their noncrystalline structure


Traditional Ceramics

Based on mineral silicates, silica, and mineral oxides found in nature

Primary products are fired clay (pottery, tableware, brick, and tile), cement, and natural abrasives such as alumina

Products and the processes to make them date back thousands of years

Glass is also a silicate ceramic material and is sometimes included among traditional ceramics


Raw Materials for Traditional Ceramics

Mineral silicates, such as clays and silica, are among the most abundant substances in nature and are the principal raw materials for traditional ceramics

Another important raw material for traditional ceramics is alumina


Traditional Ceramic Products

Pottery and Tableware – based on clay usually combined with other minerals such as silica and feldspar

Brick and tile – based on low-cost clays and silica Refractories – alumina often used as a refractory

ceramic Abrasives – most grinding wheels are based on

alumina or silicon carbide


New Ceramics

Ceramic materials developed synthetically over the last several decades

Also refers to improvements in processing techniques that provide greater control over structures and properties of ceramic materials

New ceramics are based on compounds other than variations of aluminum silicate

New ceramics are usually simpler chemically than traditional ceramics; for example, oxides, carbides, nitrides, and borides


Oxide Ceramics

Most important oxide ceramic is alumina Al2O3 Alumina is also produced synthetically from bauxite Control of particle size and impurities, refinements in

processing methods, and blending with small amounts of other ceramic ingredients, strength and toughness of alumina are improved substantially compared to its natural counterpart

Alumina also has good hot hardness, low thermal conductivity, and good corrosion resistance


Products of Oxide Ceramics

Abrasives (grinding wheel grit) Bioceramics (artificial bones and teeth) Electrical insulators and electronic components Refractory brick Cutting tool inserts – REALLY hard Spark plug barrels Engineering components


Alumina ceramic components (photo courtesy of Insaco Inc.)


Carbide Ceramics

Includes silicon carbide (SiC), tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), and chromium carbide (Cr3C2)

Production of SiC dates from a century ago, and it is generally included among traditional ceramics

WC, TiC, and TaC are hard and wear resistant and are used in applications such as cutting tools

WC, TiC, and TaC must be combined with a metallic binder such as cobalt or nickel in order to fabricate a useful solid product


Nitrides

Important nitride ceramics are silicon nitride (Si3N4), boron nitride (BN), and titanium nitride (TiN)

Properties: hard, brittle, high melting temperatures, usually electrically insulating, TiN being an exception

Applications: Silicon nitride: components for gas turbines, rocket

engines, and melting crucibles Boron nitride and titanium nitride: cutting tool

materials and coatings


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


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 polymers2. Thermosetting polymers3. Elastomerswhere (1) and (2) are plastics and (3) is Rubber


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


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


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


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% about evenly, with a slight edge for the former

On a volumetric basis, current annual usage of polymers exceeds that of metals


Examples of Polymers

Thermoplastics: Polyethylene, polyvinylchloride, polypropylene,

polystyrene, and nylon Thermosets:

Phenolics, epoxies, and certain polyesters Elastomers:

Natural rubber (vulcanized) Synthetic rubber, which exceed the tonnage of

natural rubber (think car tires)


Polymer Applications

Reasons Why Polymers are Important

Plastics can be molded into intricate part shapes, usually with no further processing Very compatible with net shape processing

WHY IS THIS IMPORTANT? 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©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes

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


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


Synthesis of Polymers

Nearly all polymers used in engineering are synthetic (what's that?)

•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


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


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


Polymer Molecular Structures

LinearBranched

Loosely cross-linked Tightly cross-linked


Effect of Branching on Properties

Thermoplastic polymers always possess linear or branched structures, or a mixture of the two

Branches increase 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


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


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


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


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


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


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


Cross-Linking in Thermosetting 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


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


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 unstretched2. Degree of cross‑linking is substantially less

than thermosets


Vulcanization

Curing to cross‑link most elastomers Vulcanization = curing in the context of natural

rubber and some synthetic rubbers (just add sulphur) 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 Considerable less than cross‑linking in

thermosets


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


Stiffness of Rubber

Increase in stiffness as a function of strain for three grades of rubber: natural rubber, vulcanized rubber, and hard rubber


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


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– With some modern exceptions!

Products: footwear; rubber bands; extruded tubing, wire coating; molded automotive parts, but no tires


Composite Materials

A materials system composed of two or more distinct phases whose combination produces aggregate properties different from those of its constituents

Composites can be very strong and stiff, yet very light in weight

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 of Composite Materials - maybe

Properties of many important composites are anisotropic (what's that mean?) May be an advantage or a disadvantage

So Waterman thinks it is a great advantage, especially when you can orient the fibers in the exact direction that you want strength.

Many polymer‑based composites are subject to attack by chemicals or solvents (that's why we have coatings)

•Composite materials are generally expensive Manufacturing methods for shaping composite materials are

often slow and costly – another opportunity for you in the workplace!


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


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 Flakes


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


Materials for Fibers

Fiber materials in fiber‑reinforced composites Glass – most widely used filament Carbon – high elastic modulus Boron – very high elastic modulus Kevlar (a polymer) Al2O3 SiC

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


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 common2. 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)


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


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 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


engineering materials 1. metals and their alloys 2. ceramics 3. polymers 4. composites ©2012 john...

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