Age hardening (Precipitation Hardening)

Precipitation Hardening Stainless Steels Alloys, Properties, Fabrication Processes.

Background

Precipitation hardening stainless steels are chromium and nickel containing steels that provide an

optimum combination of the properties of martensitic and austenitic grades. Like martensitic

grades, they are known for their ability to gain high strength through heat treatment and they also

have the corrosion resistance of austenitic stainless steel.

The high tensile strengths of precipitation hardening stainless steels come after a heat treatment

process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is

achieved through the addition of one or more of the elements Copper, Aluminium, Titanium,

Niobium, and Molybdenum. The most well known precipitation hardening steel is 17-4 PH.

The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper

and 0.3% Niobium. 17-4 PH is also known as stainless steel grade 630. The advantage of

precipitation hardening steels is that they can be supplied in a solution treated condition, which is readily machineable. After machining or another fabrication method, a single, low temperature

heat treatment can be applied to increase the strength of the steel. This is known as ageing or

age-hardening. As it is carried out at low temperature, the component undergoes no distortion.

Characterisation

Precipitation hardening steels are characterised into one of three groups based on their final

microstructures after heat treatment. The three types are: martensitic (e.g. 17-4 PH), semi-

austenitic (e.g. 17-7 PH) and austenitic (e.g. A-286).

Martensitic Alloys

Martensitic precipitation hardening stainless steels have a predominantly austenitic structure at

annealing temperatures of around 1040 to 1065C. Upon cooling to room temperature, they

undergo a transformation that changes the austenite to martensite.

Semi-austenitic Alloys

Unlike martensitic precipitation hardening steels, annealed semi-austenitic precipitation

hardening steels are soft enough to be cold worked. Semi-austenitc steels retain their austenitic

structure at room temperature but will form martensite at very low temperatures.

Austenitic Alloys

Austenitic precipitation hardening steels retain their austenitic structure after annealing and

hardening by ageing. At the annealing temperature of 1095 to 1120C the precipitation hardening

phase is soluble. It remains in solution during rapid cooling. When reheated to 650 to 760C,

precipitation occurs. This increases the hardness and strength of the material. Hardness remains

lower than that for martensitic or semi-austenitic precipitation hardening steels. Austenitic alloys

remain nonmagnetic.

Properties

Strength

Yield strengths for precipitation-hardening stainless steels are 515 to 1415 MPa. Tensile

strengths range from 860 to 1520 MPa. Elongations are 1 to 25%. Cold working before ageing

can be used to facilitate even higher strengths.

Heat Treatment

The key to the properties of precipitation hardening stainless steels lies in heat treatment. After

solution treatment or annealing of precipitation hardening stainless steels, a single low

temperature age hardening stage is employed to achieve the required properties. As this treatment is carried out at a low temperature, no distortion occurs and there is only superficial

discolouration. During the hardening process a slight decrease in size takes place.

This shrinking is approximately 0.05% for condition H900 and 0.10% for H1150. Typical

mechanical properties achieved for 17-4 PH after solution treating and age hardening are given

in the following table. Condition designations are given by the age hardening temperature in F.

Table 1. Mechanical property ranges after solution treating and age hardening

Cond. Hardening Temp and

time Hardness (Rockwell C) Tensile Strength (MPa)

A Annealed 36 1100

H900 482C, 1 hour 44 1310

H925 496C, 4 hours 42 1170-1320

H1025 552C, 4 hours 38 1070-1220

H1075 580C, 4 hours 36 1000-1150

H1100 593C, 4 hours 35 970-1120

H1150 621C, 4 hours 33 930-1080

Typical Chemical Composition

Table 2. Typical chemical composition for stainless steel alloy 17-4PH

17-4 PH

C 0.07%

Mn 1.00%

Si 1.00%

P 0.04%

S 0.03%

Cr 17.0%

Ni 4.0%

Cu 4.0%

Nb+Ta 0.30%

Typical Mechanical Properties

Table 3. Typical mechanical properties for stainless steel alloy 17-4PH

Grade 17-4PH Annealed Cond 900 Cond 1150

Tensile Strength (MPa) 1100 1310 930

Elongation A5 (%) 15 10 16

Proof Stress 0.2% (MPa) 1000 1170 724

Elongation A5 (%) 15 10 16

Typical Physical Properties

Table 4. Typical physical properties for stainless steel alloy 17-4PH

Property Value

Density 7.75 g/cm3

Melting Point C

Modulus of Elasticity 196 GPa

Electrical Resistivity 0.080x10-6 .m Thermal Conductivity 18.4 W/m.K at 100C

Thermal Expansion 10.8x10-6 /K at 100C

Alloy Designations

Stainless steel 17-4 PH also corresponds to a number of following standard designations and

specifications. Table 5. Alternate designations for stainless steel alloy 17-4PH

Euronorm UNS BS En Grade

1.4542 S17400 - - 630

Corrosion Resistance

Precipitation hardening stainless steels have moderate to good corrosion resistance in a range of

environments. They have a better combination of strength and corrosion resistance than when

compared with the heat treatable 400 series martensitic alloys. Corrosion resistance is similar to

that found in grade 304 stainless steel. In warm chloride environments, 17-4 PH is susceptible to

pitting and crevice corrosion.

When aged at 550C or higher, 17-4 PH is highly resistant to stress corrosion cracking. Better

stress corrosion cracking resistance comes with higher ageing temperatures. Corrosion resistance

is low in the solution treated (annealed) condition and it should not be used before heat

treatment.

Heat Resistance

17-4 PH has good oxidation resistance. In order to avoid reduction in mechanical properties, it

should not be used over its precipitation hardening temperature. Prolonged exposure to 370-

480C should be avoided if ambient temperature toughness is critical.

Fabrication

Fabrication of all stainless steels should be done only with tools dedicated to stainless steel

materials or tooling and work surfaces must be thoroughly cleaned before use. These precautions

are necessary to avoid cross contamination of stainless steel by easily corroded metals that may

discolour the surface of the fabricated product.

Cold Working

Cold forming such as rolling, bending and hydroforming can be performed on 17-4PH but only

in the fully annealed condition. After cold working, stress corrosion resistance is improved by re-

ageing at the precipitation hardening temperature.

Hot Working

Hot working of 17-4 PH should be performed at 950-1200C. After hot working, full heat

treatment is required. This involves annealing and cooling to room temperature or lower. Then

the component needs to be precipitation hardened to achieve the required mechanical properties.

Machinability

In the annealed condition, 17-4 PH has good machinability, similar to that of 304 stainless steel.

After hardening heat treatment, machining is difficult but possible. Carbide or high speed steel

tools are normally used with standard lubrication. When strict tolerance limits are required, the

dimensional changes due to heat treatment must be taken into account

Welding

Precipitation hardening steels can be readily welded using procedures similar to those used for

the 300 series of stainless steels. Grade 17-4 PH can be successfully welded without preheating.

Heat treating after welding can be used to give the weld metal the same properties as for the

parent metal. The recommended grade of filler rods for welding 17-4 PH is 17-7 PH.

Applications

Due to the high strength of precipitation hardening stainless steels, most applications are in

aerospace and other high-technology industries. Applications include: Gears Valves and other

engine components High strength shafts Turbine blades Moulding dies Nuclear waste casks

Supplied Forms

17-4 PH is typically supplied by Aalco in the following forms: Round bar Hexagonal bar

Billet

Source: Aalco For more information on this source please visit Aalco

ALLOYS -- NOTES

ALLOYS An alloy is a homogeneous mixture of two or more elements, at least one of which is a metal,

and where the resulting material has metallic properties. The resulting metallic substance usually

has different properties (sometimes substantially different) from those of its components.

Contents

1 Properties

2 Classification

3 Terminology

4 See also

Properties:

Alloys are usually prepared to improve on the properties of their components. For instance, steel

is stronger than iron, its primary component. The physical properties of an alloy, such as density,

reactivity and electrical and thermal conductivity may not differ greatly from the alloy's

elements, but engineering properties, such as tensile strength, shear strength and Young's

modulus, can be substantially different from those of the constituent materials. This is sometimes

due to the differing sizes of the atoms in the alloylarger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors. This helps the

alloy resist deformation, unlike a pure metal where the atoms move more freely. Unlike pure

metals, most alloys do not have a single melting point. Instead, they have a melting range in

which the material is a mixture of solid and liquid phases. The temperature at which melting

begins is called the solidus, and that at which melting is complete is called the liquidus.

However, for most pairs of elements, there is a particular ratio which has a single melting point;

this is called the eutectic mixture.

Classification

Alloys can be classified by the number of their constituents. An alloy with two components is

called a binary alloy; one with three is a ternary alloy, and so forth. Alloys can be further

classified as either substitution alloys or interstitial alloys, depending on their method of

formation. In substitution alloys, the atoms of the components are approximately the same size

and the various atoms are simply substituted for one another in the crystal structure. An example

of a (binary) substitution alloy is brass, made up of copper and zinc. Interstitial alloys occur

when the atoms of one component are substantially smaller than the other and the smaller atoms

fit into the spaces (interstices) between the larger atoms.

Terminology

In practice, some alloys are used so predominantly with respect to their base metals that the

name of the primary constituent is also used as the name of the alloy. For example, 14 karat gold

is an alloy of gold with other elements. Similarly, the silver used in jewelry and the aluminium

used as a structural building material are also alloys. The term "alloy" is sometime used in

everyday speech as a synonym for a particular alloy. For example, automobile wheels made of

"aluminium alloy" are commonly referred to as simply "alloy wheels". The usage is obviously

indefinite, since steels and most other metals in practical use are also alloys.

See also

Look up alloy in Wiktionary, the free dictionary.

List of alloys

Intermetallics

Heat treatment

Retrieved from "http://en.wikipedia.org/wiki/Alloy"

List of alloys This is a list of alloys for which an article exists in Wikipedia (or is proposed but not yet

written). They are grouped by base metal, in order of increasing atomic number. Within these

headings they are in no particular order. Some of the main alloying elements are optionally listed

after the alloy names.

Contents

1 Alloys of magnesium

2 Alloys of aluminium

3 Alloys of potassium

4 Alloys of iron

5 Alloys of cobalt

6 Alloys of nickel

7 Alloys of copper

8 Alloys of zinc

9 Alloys of gallium

10 Alloys of zirconium

11 Alloys of silver

12 Alloys of indium

13 Alloys of tin

14 Rare earth alloys

15 Alloys of gold

16 Alloys of mercury

17 Alloys of lead

18 Alloys of bismuth

19 Alloys of uranium

Alloys of magnesium

Magnox (aluminium)

T-Mg-Al-Zn (Bergman phase) is a complex metallic alloy

Elektron

Alloys of aluminium

Main article: Aluminium alloys

Al-Li (lithium)

Duralumin (copper)

Nambe (aluminium plus seven other undisclosed metals)

Silumin (silicon)

AA-8000: used for building wire in the U.S. per the National Electrical Code

Magnalium (5% magnesium)/used in airplane bodies, ladders,etc.

Aluminium also forms complex metallic alloys, like -Al-Mg, '-Al-Pd-Mn, T-Al3Mn Alnico - alloy of aluminum, nickel, and cobalt used in magnets

Alloys of potassium

NaK (sodium)

Alloys of iron

See also: Category:Ferrous alloys

Steel (carbon) (category:steels)

o Stainless steel (chromium, nickel)

AL-6XN

Alloy 20

Celestrium

Marine grade stainless

Martensitic stainless steel

Surgical stainless steel (chromium, molybdenum, nickel)

o Silicon steel (silicon)

o Tool steel (tungsten or manganese)

o Bulat steel

o Chromoly (chromium, molybdenum)

o Crucible steel

o Damascus steel

o HSLA steel

o High speed steel

o Maraging steel

o Reynolds 531

o Wootz steel

Iron

o Anthracite iron (carbon)

o Cast iron (carbon)

o Pig iron (carbon)

o Wrought iron (carbon)

Fernico (nickel, cobalt)

Elinvar (nickel, chromium)

Invar (nickel)

Kovar (cobalt)

Spiegeleisen (manganese, carbon, silicon)

Ferroalloys (category:Ferroalloys)

o Ferroboron

o Ferrochrome

o Ferromagnesium

o Ferromanganese

o Ferromolybdenum

o Ferronickel

o Ferrophosphorus

o Ferrotitanium

o Ferrovanadium

o Ferrosilicon

Alloys of cobalt

Megallium

Stellite (chromium, tungsten, carbon)

o Talonite

Alnico

Vitallium

Alloys of nickel

German silver (copper, zinc)

Chromel (chromium)

Hastelloy (molybdenum, chromium, sometimes tungsten)

Inconel (chromium, iron)

Monel metal (copper, nickel, iron, manganese)

Nichrome (chromium, iron, nickel)

Nicrosil (chromium, silicon, magnesium)

Nisil (silicon)

Nitinol (titanium, shape memory alloy)

Cupronickel (bronze, copper)

Soft magnetic alloys

o Mu-metal (iron)

Alloys of copper

Main article: Copper alloys

Beryllium copper (beryllium)

Billon (silver)

Brass (zinc)

o Calamine brass (zinc)

o Chinese silver (zinc)

o Dutch metal (zinc)

o Gilding metal (zinc)

o Muntz metal (zinc)

o Pinchbeck (zinc)

o Prince's metal (zinc)

o Tombac (zinc)

Bronze (tin, aluminium or any other element)

o Aluminium bronze (aluminium)

o Bell metal (tin)

o Florentine bronze (aluminium or tin)

o Guann

o Gunmetal (tin, zinc)

o Glucydur

o Phosphor bronze (tin and phosphorus)

o Ormolu (Gilt Bronze) (zinc)

o Speculum metal (tin)

Constantan (nickel)

Corinthian brass (gold, silver)

Cunife (nickel, iron)

Cupronickel (nickel)

Cymbal alloys (Bell metal) (tin)

Devarda's alloy (aluminium, zinc)

Hepatizon (gold, silver)

Heusler alloy (manganese, tin)

Manganin (manganese, nickel)

Molybdochalkos (lead)

Nickel silver (nickel)

Nordic gold (aluminium, zinc, tin)

Shakudo (gold)

Tumbaga (gold)

Alloys of zinc

Zamak (aluminium, magnesium, copper)

Alloys of gallium

Galinstan

Alloys of zirconium

Zircaloy

Alloys of silver

Sterling silver (copper)

Britannia silver (copper)

Goloid (copper, gold)

Alloys of indium

Field's metal (bismuth, tin)

Alloys of tin

Britannium (copper, antimony)[1]

Pewter (lead, copper)

Solder (lead, antimony)

Rare earth alloys

Mischmetal (various rare earths)

Alloys of gold

Corinthian brass (copper)

Electrum (silver, copper)

Tumbaga (copper)

Rose gold (copper)

White gold

Alloys of mercury

Amalgam

Alloys of lead

Molybdochalkos (copper)

Solder (tin)

Terne (tin)

Type metal (tin, antimony)

Alloys of bismuth

Wood's metal (lead, tin, cadmium)

Rose metal (lead, tin)

Alloys of uranium

Staballoy (depleted uranium with other metals, usually titanium or molybdenum)

Bauschinger effect

The Bauschinger effect refers to a property of materials where the material's stress-strain

characteristics change as a result of the microscopic stress distribution of the material. For

example, an increase in tensile yield strength at the expense of compressive yield strength.

The Bauschinger effect is named after the German engineer Johann Bauschinger (de:Johann

Bauschinger).

While more tensile cold working increases the tensile yield strength, the local initial compressive

yield strength after tensile cold working is actually reduced. The greater the tensile cold working,

the lower the compressive yield strength.

The Bauschinger effect is normally associated with conditions where the yield strength of a

metal decreases when the direction of strain is changed. It is a general phenomenon found in

most polycrystalline metals.

The basic mechanism for the Bauschinger effect is related to the dislocation structure in the cold-

worked metal. As deformation occurs, the dislocations will accumulate at barriers and produce

dislocation pileups and tangles.

Based on the cold work structure, two types of mechanisms are generally used to explain the

Bauschinger effect.

First, local back stresses may be present in the material, which assist the movement of

dislocations in the reverse direction. Thus, the dislocations can move easily in the reverse

direction and the yield strength of the metal is lower. The pile-up of dislocations at grain

boundaries and Orowan loops around strong precipitates are two main sources of these back

stresses.

Second, when the strain direction is reversed, dislocations of the opposite sign can be produced

from the same source that produced the slip-causing dislocations in the initial direction.

Dislocations with opposite signs can attract and annihilate each other. Since strain hardening is

related to an increased dislocation density, reducing the number of dislocations reduces strength.

The net result is that the yield strength for strain in the opposite direction is less than it would be

if the strain had continued in the initial direction.

BRITTLE FRACTURE |

Version 4 - view current page

what is brittle fracture?

Basically, brittle fracture is a rapid run of cracks through a stressed material. The cracks usually

travel so fast that you can't tell when the material is about to break. In other words, there is very

little plastic deformation before failure occurs. In most cases, this is the worst type of fracture

because you can't repair visible damage in a part or structure before it breaks. In brittle fracture,

the cracks run close to perpendicular to the applied stress.

This perpendicular fracture leaves a relatively flat surface at the break. Besides having a nearly

flat fracture surface, brittle materials usually contain a pattern on their fracture surfaces. Some

brittle materials have lines and ridges beginning at the origin of the crack and spreading out

across the crack surface.

Other materials, like some steels have back to back V-shaped markings pointing to the origin of

the crack. These V-shaped markings are called chevrons. Very hard or fine grained materials

have no special pattern on their fracture surface, and amorphous materials like ceramic glass

have shiny smooth fracture surfaces.

Chevron Fracture Surface (Callister p. 185)

Radiating Ridge Fracture Surface (Callister pg. 186, copyright by John Wiley & Sons, inc.)

Types of Brittle Fracture The first type of fracture is transgranular. In transgranular fracture, the fracture travels

through the grain of the material. The fracture changes direction from grain to grain due

to the different lattice orientation of atoms in each grain. In other words, when the crack

reaches a new grain, it may have to find a new path or plane of atoms to travel on because

it is easier to change direction for the crack than it is to rip through. Cracks choose the

path of least resistance. You can tell when a crack has changed in direction through the

material, because you get a slightly bumpy crack surface.

The second type of fracture is intergranular fracture. Intergranular fracture is the crack

traveling along the grain boundaries, and not through the actual grains. Intergranular

fracture usually occurs when the phase in the grain boundary is weak and brittle ( i.e.

Cementite in Iron's grain boundaries). Think of a metal as one big 3-D puzzle.

Transgranular fracture cuts through the puzzle pieces, and intergranular fracture travels

along the puzzle pieces pre-cut edges.

Ductile to Brittle Fracture Transition

In fracture, there are many shades of gray. Brittle fracture and ductile fracture are fairly general

terms describing the two opposite extremes of the fracture spectrum. I will explain the factors

that make a material lean toward one type of fracture as opposed to the other type of fracture.

The first and foremost factor is temperature. Basically, at higher temperatures the yield

strength is lowered and the fracture is more ductile in nature. On the opposite end, at

lower temperatures the yield strength is greater and the fracture is more brittle in nature.

This relationship with temperature has to do with atom vibrations.

As temperature increases, the atoms in the material vibrate with greater frequency and

amplitude. This increased vibration allows the atoms under stress to slip to new places in

the material ( i.e. break bonds and form new ones with other atoms in the material). This

slippage of atoms is seen on the outside of the material as plastic deformation, a common

feature of ductile

fracture.

When temperature decreases however, the exact opposite is true. Atom vibration

decreases, and the atoms do not want to slip to new locations in the material. So when the

stress on the material becomes high enough, the atoms just break their bonds and do not

form new ones. This decrease in slippage causes little plastic deformation before fracture.

Thus, we have a brittle type fracture.

At moderate temperatures (with respect to the material) the material exhibits

characteristics of both types of fracture. In conclusion, temperature determines the

amount of brittle or ductile fracture that can occur in a material.

Another factor that determines the amount of brittle or ductile fracture that occurs in a material is

dislocation density. The higher the dislocation density, the more brittle the fracture will be in the

material. The idea behind this theory is that plastic deformation comes from the movement of

dislocations.

As dislocations increase in a material due to stresses above the materials yield point, it becomes

increasingly difficult for the dislocations to move because they pile into each other. So a material

that already has a high dislocation density can only deform but so much before it fractures in a

brittle manner.

The last factor is grain size. As grains get smaller in a material, the fracture becomes more

brittle. This phenomena is do to the fact that in smaller grains, dislocations have less space to

move before they hit a grain boundary. When dislocations can not move very far before fracture,

then plastic deformation decreases. Thus, the material's fracture is more brittle. In ending, I

would like to say that these are just the basics of brittle fracture. There are whole books written

on just brittle fracture. So, if this section interested you at all, go look the books up at your local

university library. Good Luck with your studies MSE 2034/44 students. .

CERAMICS - NOTES |

Version 3 - view current page

CeramicS

This article is about ceramic materials. For the fine art, see ceramics (art). Fixed Partial

Denture, or "Bridge" The word ceramic is derived from the Greek word

(keramikos). The term covers inorganic non-metallic materials whose formation is due

to the action of heat. Up until the 1950s or so, the most important of these were the

traditional clays, made into pottery, bricks, tiles and are like, along with cements and

glass. Clay based ceramics are described in the article on pottery. A composite material

of ceramic and metal is known as cermet. The word ceramic can be an adjective, and

can also be used as a noun to refer to a ceramic material, or a product of ceramic

manufacture. Ceramics is a singular noun referring to the art of making things out of

ceramic materials. The technology of manufacturing and usage of ceramic materials is

part of the field of ceramic engineering. Many ceramic materials are hard, porous and

brittle. The study and development of ceramics includes methods to mitigate problems

associated with these characteristics, and to accentuate the strengths of the materials

as well as to investigate novel applications. The American Society for Testing and

Materials (ASTM) defines a ceramic article as an article having a glazed or unglazed

body of crystalline or partly crystalline structure, or of glass, which body is produced

from essentially inorganic, non-metallic substances and either is formed from a molten

mass which solidifies on cooling, or is formed and simultaneously or subsequently

matured by the action of the heat.[1]

Contents

1 Types of ceramic materials 2 Examples of structural ceramics 3 Examples of whiteware ceramics 4 Classification of technical ceramics

o 4.1 Examples of technical ceramics 5 Properties of ceramics

o 5.1 Mechanical properties o 5.2 Electrical properties

5.2.1 Semiconductors 5.2.2 Superconductivity 5.2.3 Ferroelectricity and supersets 5.2.4 Positive thermal coefficient

6 Classification of ceramics o 6.1 In situ manufacturing o 6.2 Sintering-based methods

7 Other applications of ceramics 8 References 9 See also

Types of ceramic materials

For convenience ceramic products are usually divided into four sectors, and these are

shown below with some examples:

Structural, including bricks, pipes, floor and roof tiles Refractories, such as kiln linings, gas fire radiants, steel and glass making

crucibles Whitewares, including tableware, wall tiles, decorative art objects and sanitary

ware Technical, is also known as Engineering, Advanced, Special, and in Japan, Fine

Ceramics. Such items include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones. Frequently the raw materials do not include clays.

Examples of structural ceramics

Construction bricks. Floor and roof tiles. Sewage pipes

Examples of whiteware ceramics

Bone china Earthenware, which is often made from clay, quartz and feldspar. Porcelain, which are often made from kaolin Stoneware

Classification of technical ceramics

Technical ceramics can also be classified into three distinct material categories:

Oxides: Alumina, zirconia Non-oxides: Carbides, borides, nitrides, silicides Composites: Particulate reinforced, combinations of oxides and non-oxides.

Each one of these classes can develop unique material properties

Examples of technical ceramics

Barium titanate (often mixed with strontium titanate) displays ferroelectricity, meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in electromechanical transducers, ceramic capacitors, and data storage elements. Grain boundary conditions can create PTC effects in heating elements.

Bismuth strontium calcium copper oxide, a high-temperature superconductor Boron carbide (B4C), which is used in ceramic plates in some personnel,

helicopter and tank armor. Boron nitride is structurally isoelectronic to carbon and takes on similar physical

forms: a graphite-like one used as a lubricant, and a diamond-like one used as an abrasive.

Ferrite (Fe3O4), which is ferrimagnetic and is used in the magnetic cores of electrical transformers and magnetic core memory.

Lead zirconate titanate is another ferroelectric material. Magnesium diboride (MgB2), which is an unconventional superconductor. Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a

commonly used abrasive, and as a refractory material. Silicon nitride (Si3N4), which is used as an abrasive powder. Steatite is used as an electrical insulator. Uranium oxide (UO2), used as fuel in nuclear reactors. Yttrium barium copper oxide (YBa2Cu3O7-x), another high temperature

superconductor. Zinc oxide (ZnO), which is a semiconductor, and used in the construction of

varistors. Zirconium dioxide (zirconia), which in pure form undergoes many phase changes

between room temperature and practical sintering temperatures, can be chemically "stabilized" in several different forms. Its high oxygen ion conductivity recommends it for use in fuel cells. In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material.

Properties of ceramics

Mechanical properties

Ceramic materials are usually ionic or covalently-bonded materials, and can be

crystalline or amorphous. A material held together by either type of bond will tend to

fracture before any plastic deformation takes place, which results in poor toughness in

these materials. Additionally, because these materials tend to be porous, the pores and

other microscopic imperfections act as stress concentrators, decreasing the toughness

further, and reducing the tensile strength. These combine to give catastrophic failures,

as opposed to the normally much more gentle failure modes of metals. These materials

do show plastic deformation. However, due to the rigid structure of the crystalline

materials, there are very few available slip systems for dislocations to move, and so

they deform very slowly. With the non-crystalline (glassy) materials, viscous flow is the

dominant source of plastic deformation, and is also very slow. It is therefore neglected

in many applications of ceramic materials.

Electrical properties

Semiconductors There are a number of ceramics that are semiconductors. Most of these are transition metal oxides that are II-VI semiconductors, such as zinc oxide.

While there is talk of making blue LEDs from zinc oxide, ceramicists are most interested

in the electrical properties that show grain boundary effects. One of the most widely

used of these is the varistor. These are devices that exhibit the property that resistance

drops sharply at a certain threshold voltage. Once the voltage across the device

reaches the threshold, there is a breakdown of the electrical structure in the vicinity of

the grain boundaries, which results in its electrical resistance dropping from several

megohms down to a few hundred ohms. The major advantage of these is that they can

dissipate a lot of energy, and they self reset after the voltage across the device drops

below the threshold, its resistance returns to being high. This makes them ideal for

surge-protection applications. As there is control over the threshold voltage and energy

tolerance, they find use in all sorts of applications. The best demonstration of their

ability can be found in electrical substations, where they are employed to protect the

infrastructure from lightning strikes. They have rapid response, are low maintenance,

and do not appreciably degrade from use, making them virtually ideal devices for this

application. Semiconducting ceramics are also employed as gas sensors. When various

gases are passed over a polycrystalline ceramic, its electrical resistance changes. With

tuning to the possible gas mixtures, very inexpensive devices can be produced.

Superconductivity Under some conditions, such as extremely low temperature, some ceramics exhibit superconductivity. The exact reason for this is not known, but

there are two major families of superconducting ceramics.

Ferro electricity and supersets Piezoelectricity, a link between electrical and mechanical response, is exhibited by a large number of ceramic materials,

including the quartz used to measure time in watches and other electronics. Such

devices use both properties of piezoelectrics, using electricity to produce a mechanical

motion (powering the device) and then using this mechanical motion to produce

electricity (generating a signal). The unit of time measured is the natural interval

required for electricity to be converted into mechanical energy and back again. The

piezoelectric effect is generally stronger in materials that also exhibit pyroelectricity, and

all pyroelectric materials are also piezoelectric. These materials can be used to inter

convert between thermal, mechanical, and/or electrical energy; for instance, after

synthesis in a furnace, a pyroelectric crystal allowed to cool under no applied stress

generally builds up a static charge of thousands of volts. Such materials are used in

motion sensors, where the tiny rise in temperature from a warm body entering the room

is enough to produce a measurable voltage in the crystal. In turn, pyroelectricity is seen

most strongly in materials which also display the ferroelectric effect, in which a stable

electric dipole can be oriented or reversed by applying an electrostatic field.

Pyroelectricity is also a necessary consequence of ferroelectricity. This can be used to

store information in ferroelectric capacitors, elements of ferroelectric RAM. The most

common such materials are lead zirconate titanate and barium titanate. Aside from the

uses mentioned above, their strong piezoelectric response is exploited in the design of

high-frequency loudspeakers, transducers for sonar, and actuators for atomic force and

scanning tunneling microscopes.

Positive thermal coefficient Increases in temperature can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials,

mostly mixtures of heavy metal titanates. The critical transition temperature can be

adjusted over a wide range by variations in chemistry. In such materials, current will

pass through the material until joule heating brings it to the transition temperature, at

which point the circuit will be broken and current flow will cease. Such ceramics are

used as self-controlled heating elements in, for example, the rear-window defrost

circuits of automobiles. At the transition temperature, the material's dielectric response

becomes theoretically infinite. While a lack of temperature control would rule out any

practical use of the material near its critical temperature, the dielectric effect remains

exceptionally strong even at much higher temperatures. Titanates with critical

temperatures far below room temperature have become synonymous with "ceramic" in

the context of ceramic capacitors for just this reason.

Classification of ceramics

Non-crystalline ceramics: Non-crystalline ceramics, being glasses, tend to be formed

from melts. The glass is shaped when either fully molten, by casting, or when in a state

of toffee-like viscosity, by methods such as blowing to a mold. If later heat-treatments

cause this class to become partly crystalline, the resulting material is known as a glass-

ceramic. Crystalline ceramics: Crystalline ceramic materials are not amenable to a

great range of processing. Methods for dealing with them tend to fall into one of two

categories - either make the ceramic in the desired shape, by reaction in situ, or by

"forming" powders into the desired shape, and then sintering to form a solid body.

Ceramic forming techniques include shaping by hand (sometimes including a rotation

process called "throwing"), slip casting, tape casting (used for making very thin ceramic

capacitors, etc.), injection molding, dry pressing, and other variations. (See also

Ceramic forming techniques. Details of these processes are described in the two books

listed below.) A few methods use a hybrid between the two approaches.

In situ manufacturing

The most common use of this method is in the production of cement and concrete.

Here, the dehydrated powders are mixed with water. This starts hydration reactions,

which result in long, interlocking crystals forming around the aggregates. Over time,

these result in a solid ceramic. The biggest problem with this method is that most

reactions are so fast that good mixing is not possible, which tends to prevent large-scale

construction. However, small-scale systems can be made by deposition techniques,

where the various materials are introduced above a substrate, and react and form the

ceramic on the substrate. This borrows techniques from the semiconductor industry,

such as chemical vapour deposition, and is very useful for coatings. These tend to

produce very dense ceramics, but do so slowly.

Sintering-based methods

The principles of sintering-based methods is simple. Once a roughly held together

object (called a "green body") is made, it is baked in a kiln, where diffusion processes

cause the green body to shrink. The pores in the object close up, resulting in a denser,

stronger product. The firing is done at a temperature below the melting point of the

ceramic. There is virtually always some porosity left, but the real advantage of this

method is that the green body can be produced in any way imaginable, and still be

sintered. This makes it a very versatile route. There are thousands of possible

refinements of this process. Some of the most common involve pressing the green body

to give the densification a head start and reduce the sintering time needed. Sometimes

organic binders such as polyvinyl alcohol are added to hold the green body together;

these burn out during the firing (at 200350C). Sometimes organic lubricants are

added during pressing to increase densification. It is not uncommon to combine these,

and add binders and lubricants to a powder, then press. (The formulation of these

organic chemical additives is an art in itself. This is particularly important in the

manufacture of high performance ceramics such as those used by the billions for

electronics, in capacitors, inductors, sensors, etc. The specialized formulations most

commonly used in electronics are detailed in the book "Tape Casting," by R.E. Mistler,

et al., Amer. Ceramic Soc. [Westerville, Ohio], 2000.) A comprehensive book on the

subject, for mechanical as well as electronics applications, is "Organic Additives and

Ceramic Processing," by D. J. Shanefield, Kluwer Publishers [Boston], 1996. A slurry

can be used in place of a powder, and then cast into a desired shape, dried and then

sintered. Indeed, traditional pottery is done with this type of method, using a plastic

mixture worked with the hands. If a mixture of different materials is used together in a

ceramic, the sintering temperature is sometimes above the melting point of one minor

component - a liquid phase sintering. This results in shorter sintering times compared to

solid state sintering.

Other applications of ceramics

Ceramics are used in the manufacture of knives. The blade of the ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.

Ceramics such as alumina and boron carbide have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small-arms protective inserts (SAPI). Similar material is used to protect cockpits of some military airplanes, because of the low weight of the material.

Ceramic balls can be used to replace steel in ball bearings. Their higher hardness means that they are much less susceptible to wear and can often more than triple lifetimes. They also deform less under load meaning they have less contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. The major drawback to using ceramics is a significantly higher cost. In many cases their electrically insulating properties may also be valuable in bearings.

In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 F (3300 C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, as shown by Carnot's theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially

dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is unfeasible with current technology.

Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.

Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Most hydroxy apatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong-fully dense nano crystalline hydroxapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic natural bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.

Ceramics

The word ceramic is derived from the Greek word (keramikos). The term

covers inorganic non-metallic materials whose formation is due to the action of heat. Up

until the 1950s or so, the most important of these were the traditional clays, made into

pottery, bricks, tiles and are like, along with cements and glass. Clay based ceramics

are described in the article on pottery.

A composite material of ceramic and metal is known as cermet. The word ceramic can

be an adjective, and can also be used as a noun to refer to a ceramic material, or a

product of ceramic manufacture. Ceramics is a singular noun referring to the art of

making things out of ceramic materials. The technology of manufacturing and usage of

ceramic materials is part of the field of ceramic engineering.

Many ceramic materials are hard, porous and brittle. The study and development of

ceramics includes methods to mitigate problems associated with these characteristics,

and to accentuate the strengths of the materials as well as to investigate novel

applications.

The American Society for Testing and Materials (ASTM) defines a ceramic article as an

article having a glazed or unglazed body of crystalline or partly crystalline structure, or

of glass, which body is produced from essentially inorganic, non-metallic substances

and either is formed from a molten mass which solidifies on cooling, or is formed and

simultaneously or subsequently matured by the action of the heat.[1]

Types of ceramic materials

For convenience ceramic products are usually divided into four sectors, and these are

shown below with some examples:

Structural, including bricks, pipes, floor and roof tiles Refractories, such as kiln linings, gas fire radiants, steel and glass making

crucibles Whitewares, including tableware, wall tiles, decorative art objects and sanitary

ware Technical, is also known as Engineering, Advanced, Special, and in Japan, Fine

Ceramics. Such items include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, bio-

medical implants, jet engine turbine blades, and missile nose cones. Frequently the raw materials do not include clays.

Examples of structural ceramics

Construction bricks. Floor and roof tiles. Sewage pipes

Examples of whiteware ceramics

Bone china Earthenware, which is often made from clay, quartz and feldspar. Porcelain, which are often made from kaolin Stoneware

Classification of technical ceramics

Technical ceramics can also be classified into three distinct material categories:

Oxides: Alumina, zirconia Non-oxides: Carbides, borides, nitrides, silicides Composites: Particulate reinforced, combinations of oxides and non-oxides.

Each one of these classes can develop unique material properties

Examples of technical ceramics

Barium titanate (often mixed with strontium titanate) displays ferroelectricity, meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in electromechanical transducers, ceramic capacitors, and data storage elements. Grain boundary conditions can create PTC effects in heating elements.

Bismuth strontium calcium copper oxide, a high-temperature superconductor Boron carbide (B4C), which is used in ceramic plates in some personnel,

helicopter and tank armor.

Boron nitride is structurally isoelectronic to carbon and takes on similar physical forms: a graphite-like one used as a lubricant, and a diamond-like one used as an abrasive.

Ferrite (Fe3O4), which is ferrimagnetic and is used in the magnetic cores of electrical transformers and magnetic core memory.

Lead zirconate titanate is another ferroelectric material. Magnesium diboride (MgB2), which is an unconventional superconductor. Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a

commonly used abrasive, and as a refractory material. Silicon nitride (Si3N4), which is used as an abrasive powder. Steatite is used as an electrical insulator. Uranium oxide (UO2), used as fuel in nuclear reactors. Yttrium barium copper oxide (YBa2Cu3O7-x), another high temperature

superconductor. Zinc oxide (ZnO), which is a semiconductor, and used in the construction of

varistors. Zirconium dioxide (zirconia), which in pure form undergoes many phase changes

between room temperature and practical sintering temperatures, can be chemically "stabilized" in several different forms. Its high oxygen ion conductivity recommends it for use in fuel cells. In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material.

Properties of ceramics

Mechanical properties

Ceramic materials are usually ionic or covalently-bonded materials, and can be

crystalline or amorphous. A material held together by either type of bond will tend to

fracture before any plastic deformation takes place, which results in poor toughness in

these materials. Additionally, because these materials tend to be porous, the pores and

other microscopic imperfections act as stress concentrators, decreasing the toughness

further, and reducing the tensile strength. These combine to give catastrophic failures,

as opposed to the normally much more gentle failure modes of metals.

These materials do show plastic deformation. However, due to the rigid structure of the

crystalline materials, there are very few available slip systems for dislocations to move,

and so they deform very slowly. With the non-crystalline (glassy) materials, viscous flow

is the dominant source of plastic deformation, and is also very slow. It is therefore

neglected in many applications of ceramic materials.

Electrical properties

Semiconductors There are a number of ceramics that are semiconductors. Most of

these are transition metal oxides that are II-VI semiconductors, such as zinc oxide.

While there is talk of making blue LEDs from zinc oxide, ceramicists are most interested

in the electrical properties that show grain boundary effects.

One of the most widely used of these is the varistor. These are devices that exhibit the

property that resistance drops sharply at a certain threshold voltage. Once the voltage

across the device reaches the threshold, there is a breakdown of the electrical structure

in the vicinity of the grain boundaries, which results in its electrical resistance dropping

from several megohms down to a few hundred ohms. The major advantage of these is

that they can dissipate a lot of energy, and they self reset after the voltage across the

device drops below the threshold, its resistance returns to being high.

This makes them ideal for surge-protection applications. As there is control over the

threshold voltage and energy tolerance, they find use in all sorts of applications. The

best demonstration of their ability can be found in electrical substations, where they are

employed to protect the infrastructure from lightning strikes. They have rapid response,

are low maintenance, and do not appreciably degrade from use, making them virtually

ideal devices for this application.

Semiconducting ceramics are also employed as gas sensors. When various gases are

passed over a polycrystalline ceramic, its electrical resistance changes. With tuning to

the possible gas mixtures, very inexpensive devices can be produced.

Superconductivity Under some conditions, such as extremely low temperature,

some ceramics exhibit superconductivity. The exact reason for this is not known, but

there are two major families of superconducting ceramics.

Ferroelectricity and supersets

Piezoelectricity, a link between electrical and mechanical response, is exhibited by a

large number of ceramic materials, including the quartz used to measure time in

watches and other electronics. Such devices use both properties of piezoelectrics, using

electricity to produce a mechanical motion (powering the device) and then using this

mechanical motion to produce electricity (generating a signal). The unit of time

measured is the natural interval required for electricity to be converted into mechanical

energy and back again.

The piezoelectric effect is generally stronger in materials that also exhibit pyroelectricity,

and all pyroelectric materials are also piezoelectric. These materials can be used to

inter convert between thermal, mechanical, and/or electrical energy; for instance, after

synthesis in a furnace, a pyroelectric crystal allowed to cool under no applied stress

generally builds up a static charge of thousands of volts. Such materials are used in

motion sensors, where the tiny rise in temperature from a warm body entering the room

is enough to produce a measurable voltage in the crystal.

In turn, pyroelectricity is seen most strongly in materials which also display the

ferroelectric effect, in which a stable electric dipole can be oriented or reversed by

applying an electrostatic field. Pyroelectricity is also a necessary consequence of

ferroelectricity. This can be used to store information in ferroelectric capacitors,

elements of ferroelectric RAM.

The most common such materials are lead zirconate titanate and barium titanate. Aside

from the uses mentioned above, their strong piezoelectric response is exploited in the

design of high-frequency loudspeakers, transducers for sonar, and actuators for atomic

force and scanning tunneling microscopes.

Positive thermal coefficient

Increases in temperature can cause grain boundaries to suddenly become insulating in

some semiconducting ceramic materials, mostly mixtures of heavy metal titanates. The

critical transition temperature can be adjusted over a wide range by variations in

chemistry. In such materials, current will pass through the material until joule heating

brings it to the transition temperature, at which point the circuit will be broken and

current flow will cease. Such ceramics are used as self-controlled heating elements in,

for example, the rear-window defrost circuits of automobiles.

At the transition temperature, the material's dielectric response becomes theoretically

infinite. While a lack of temperature control would rule out any practical use of the

material near its critical temperature, the dielectric effect remains exceptionally strong

even at much higher temperatures. Titanates with critical temperatures far below room

temperature have become synonymous with "ceramic" in the context of ceramic

capacitors for just this reason.

Classification of ceramics

Non-crystalline ceramics: Non-crystalline ceramics, being glasses, tend to be formed

from melts. The glass is shaped when either fully molten, by casting, or when in a state

of toffee-like viscosity, by methods such as blowing to a mold. If later heat-treatments

cause this class to become partly crystalline, the resulting material is known as a glass-

ceramic.

Crystalline ceramics: Crystalline ceramic materials are not amenable to a great range

of processing. Methods for dealing with them tend to fall into one of two categories -

either make the ceramic in the desired shape, by reaction in situ, or by "forming"

powders into the desired shape, and then sintering to form a solid body. Ceramic

forming techniques include shaping by hand (sometimes including a rotation process

called "throwing"), slip casting, tape casting (used for making very thin ceramic

capacitors, etc.), injection molding, dry pressing, and other variations. (See also

Ceramic forming techniques. Details of these processes are described in the two books

listed below.) A few methods use a hybrid between the two approaches.

In situ manufacturing

The most common use of this method is in the production of cement and concrete.

Here, the dehydrated powders are mixed with water. This starts hydration reactions,

which result in long, interlocking crystals forming around the aggregates. Over time,

these result in a solid ceramic.

The biggest problem with this method is that most reactions are so fast that good mixing

is not possible, which tends to prevent large-scale construction. However, small-scale

systems can be made by deposition techniques, where the various materials are

introduced above a substrate, and react and form the ceramic on the substrate. This

borrows techniques from the semiconductor industry, such as chemical vapour

deposition, and is very useful for coatings.

These tend to produce very dense ceramics, but do so slowly.

Sintering-based methods

The principles of sintering-based methods is simple. Once a roughly held together

object (called a "green body") is made, it is baked in a kiln, where diffusion processes

cause the green body to shrink. The pores in the object close up, resulting in a denser,

stronger product.

The firing is done at a temperature below the melting point of the ceramic. There is

virtually always some porosity left, but the real advantage of this method is that the

green body can be produced in any way imaginable, and still be sintered. This makes it

a very versatile route.

There are thousands of possible refinements of this process. Some of the most

common involve pressing the green body to give the densification a head start and

reduce the sintering time needed. Sometimes organic binders such as polyvinyl alcohol

are added to hold the green body together; these burn out during the firing (at 200

350C).

Sometimes organic lubricants are added during pressing to increase densification. It is

not uncommon to combine these, and add binders and lubricants to a powder, then

press. (The formulation of these organic chemical additives is an art in itself. This is

particularly important in the manufacture of high performance ceramics such as those

used by the billions for electronics, in capacitors, inductors, sensors, etc.

The specialized formulations most commonly used in electronics are detailed in the

book "Tape Casting," by R.E. Mistler, et al., Amer. Ceramic Soc. [Westerville, Ohio],

2000.) A comprehensive book on the subject, for mechanical as well as electronics

applications, is "Organic Additives and Ceramic Processing," by D. J. Shanefield,

Kluwer Publishers [Boston], 1996.

A slurry can be used in place of a powder, and then cast into a desired shape, dried and

then sintered. Indeed, traditional pottery is done with this type of method, using a plastic

mixture worked with the hands.

If a mixture of different materials is used together in a ceramic, the sintering

temperature is sometimes above the melting point of one minor component - a liquid

phase sintering. This results in shorter sintering times compared to solid state sintering.

Other applications of ceramics

Ceramics are used in the manufacture of knives. The blade of the ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.

Ceramics such as alumina and boron carbide have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small-arms protective inserts (SAPI). Similar material is used to protect cockpits of some military airplanes, because of the low weight of the material.

Ceramic balls can be used to replace steel in ball bearings. Their higher hardness means that they are much less susceptible to wear and can often more than triple lifetimes. They also deform less under load meaning they have less

contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. The major drawback to using ceramics is a significantly higher cost. In many cases their electrically insulating properties may also be valuable in bearings.

In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 F (3300 C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, as shown by Carnot's theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is unfeasible with current technology.

Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.

Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Most hydroxy apatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong-fully dense nano crystalline hydroxapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic natural bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.

Ceramics-2 |

Version 3 - view current page

Ceramics A ceramic has traditionally been defined as an inorganic, nonmetallic solid that is prepared from powdered materials, is fabricated into products through the application of heat, and displays

such characteristic properties as hardness, strength, low electrical conductivity, and brittleness."

The word ceramic comes the from Greek word "keramikos", which means "pottery." They are

typically crystalline in nature and are compounds formed between metallic and nonmetallic

elements such as aluminum and oxygen (alumina-Al2O3), calcium and oxygen (calcia - CaO),

and silicon and nitrogen (silicon nitride-Si3N4).

Depending on their method of formation, ceramics can be dense or lightweight. Typically, they

will demonstrate excellent strength and hardness properties; however, they are often brittle in

nature. Ceramics can also be formed to serve as electrically conductive materials or insulators.

Some ceramics, like superconductors, also display magnetic properties. They are also more

resistant to high temperatures and harsh environments than metals and polymers. Due to ceramic

materials wide range of properties, they are used for a multitude of applications.

The broad categories or segments that make up the ceramic industry can be classified as:

Structural clay products (brick, sewer pipe, roofing and wall tile, flue linings, etc.)

Whitewares (dinnerware, floor and wall tile, electrical porcelain, etc.)

Refractories (brick and monolithic products used in metal, glass, cements, ceramics,

energy conversion, petroleum, and chemicals industries)

Glasses (flat glass (windows), container glass (bottles), pressed and blown glass

(dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical

fibers))

Abrasives (natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused

alumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure

blasting of materials)

Cements (for roads, bridges, buildings, dams, and etc.)

Advanced ceramics

o Structural (wear parts, bioceramics, cutting tools, and engine components)

o Electrical (capacitors, insulators, substrates, integrated circuit packages,

piezoelectrics, magnets and superconductors)

o Coatings (engine components, cutting tools, and industrial wear parts)

o Chemical and environmental (filters, membranes, catalysts, and catalyst supports)

The atoms in ceramic materials are held together by a chemical bond which will be discussed a

bit later. Briefly though, the two most common chemical bonds for ceramic materials are

covalent and ionic. Covalent and ionic bonds are much stronger than in metallic bonds and,

generally speaking, this is why ceramics are brittle and metals are ductile.

Ceramic Structures As discussed in the introduction, ceramics and related materials cover a wide range of objects.

Ceramics are a little more complex than metallic structures, which is why metals were covered

first. A ceramic has traditionally been defined as an inorganic, nonmetallic solid that is prepared from powdered materials and is fabricated into products through the application of heat. Most

ceramics are made up of two or more elements. This is called a compound. For example, alumina

(Al2O3) is a compound made up of aluminum atoms and oxygen atoms. The two most common

chemical bonds for ceramic materials are covalent and ionic. The bonding of atoms together is

much stronger in covalent and ionic bonding than in metallic. This is why ceramics generally

have the following properties: high hardness, high compressive strength, and chemical inertness.

This strong bonding also accounts for the less attractive properties of ceramics, such as low

ductility and low tensile strength. The absence of free electrons is responsible for making most

ceramics poor conductors of electricity and heat. However, it should be noted that the crystal

structures of ceramics are many and varied and this results in a very wide range of properties.

For example, while ceramics are perceived as electrical and thermal insulators, ceramic oxide

(initially based on Y-Ba-Cu-O) is the basis for high temperature superconductivity. Diamond and

silicon carbide have a higher thermal conductivity than aluminum or copper. Control of the

microstructure can overcome inherent stiffness to allow the production of ceramic springs, and

ceramic composites which have been produced with a fracture toughness about half that of steel.

Also, the atomic structures are often of low symmetry that gives some ceramics interesting

electromechanical properties like piezoelectricity, which is used in sensors and transducers. The

structure of most ceramics varies from relatively simple to very complex. The microstructure can

be entirely glassy (glasses only); entirely crystalline; or a combination of crystalline and glassy.

In the latter case, the glassy phase usually surrounds small crystals, bonding them together. The

main compositional classes of engineering ceramics are the oxides, nitrides and carbides.

COLD WORK AND HOT WORK

Cold working refers to plastic deformation that occurs usually, but not necessarily, at

room temperature.

Hot working refers to plastic deformation carried out above the recrystallization

temperature.

Warm working: as the name implies, is carried out at intermediate temperatures. It is a

compromise between cold and hot working.

The temperature ranges for these 3 categories of plastic deformation are given in the

next table in term of a ratio, where T is the working temperature and Tm is the melting

point of the metal, both on the absolute scale. Although it is a dimensionless quantity,

this ratio is known as the homologous temperature.

Definition:

As stated before, cold working refers to plastic deformation that occurs usually, but not

necessarily, at room temperature.

For example: Deforming lead at room temperature is a hot working process because the

recrystallization temperature of lead is about room temperature.

Cold and hot are relative terms.

Plastic deformation is a deformation in which the material does not return to its original

shape; this is the opposite of an elastic deformation.

Effects of Cold Working:

The behavior and workability of the metals depend largely on whether deformation

takes place below or above the recrystallization temperature.

Deformation using cold working results in:

Higher stiffness, and strength, but

Reduced malleability and ductility of the metal.

Anisotropy

-----------------------------------------------------------------------------------------------------

Hot Working

Definition

Hot working is the deformation that is carried out above the recrystallization

temperature.

In these circumstances, annealing takes place while the metal is worked rather than

being a separate process. The metal can therefore be worked without it becoming work

hardened. Hot working is usually carried out with the metal at a temperature of about

0.6 of its melting point.

Effects of hot working

At high temperature, scaling and oxidation exist. Scaling and oxidation produce

undesirable surface finish. Most ferrous metals needs to be cold worked after hot

working in order to improve the surface finish.

The amount of force needed to perform hot working is less than that for cold work.

The mechanical properties of the material remain unchanged during hot working.

The metal usually experiences a decrease in yield strength when hot worked.

Therefore, it is possible to hot work the metal without causing any fracture.

Quenching is the sudden immersion of a heated metal into cold water or oil. It is used to

make the metal very hard. To reverse the effects of quenching, tempering is used

(reheated of the metal for a period of time)

To reverse the process of quenching, tempering is used, which is the reheat of the

metal.

Methods used for Cold, Hot working

ROLLING -- FORGING ------

The advantages of hot working are

Lower working forces to produce a given shape, which means the machines involved don't have to be as strong, which means they can be built more cheaply;

The possibility of producing a very dramatic shape change in a single working step, without causing large amounts of internal stress, cracks or cold working;

Sometimes hot working can be combined with a casting process so that metal is cast and then immediately hot worked. This saves money because we don't have to pay for the energy to reheat the metal.

Hot working tends to break up large crystals in the metal and can produce a favourable alignment of elongated crystals (see DeGarmo Fig. 17-4 below).

Hot working can remove some kinds of defects that occur in cast metals. It can close gas pockets (bubbles) or voids in a cast billet; and it may also break up non-metallic slag which can sometimes get caught in the melt (inclusions).

The main problems, however, are

If the recrystallisation temperature of the worked metal is high e.g. if we are talking about steel, specialised methods are needed to protect the machines that work the metal. The working processes are also dangerous to human operators and very unpleasant to work near (see picture below for some idea why).

The surface finish of hot worked steel tends to be pretty crude because (a) the dies or rollers wear quite rapidly; (b) there is a lot of dimensional change as the worked object cools; and (c) there is the constant annoying problem of scale formation on the surface of the hot steel.

Of course smart people have found ways to minimise the problems or work around

them - more below - and as a result hot working is a very common and useful process.

We just have to be aware of its limits and follow the hot working operations with other

types of manufacturing process that can fix the problems that occur.

Cold working

As explained above, when we work a metal below the recrystallisation temperature,

there is accumulation of a kind of material damage at the atomic level, through the pile-

up of dislocations. However this is not necessarily a bad thing. Many useful engineering

objects are deliberately cold-worked as part of the manufacturing process to achieve

improved properties. One common example is fencing wire. It is cold-drawn in the final

stages, before being galvanised (plated with zinc) and coiled ready for sale. The cold

working stages increase the yeild stress of the wire, meaning we can pull harder on the

wire before it deforms plastically (stretches). That's helpful when you are stringing a

fence. However the cold working does not increase the ultimate strength of the

material. So in a sense, cold working uses up some of the safety margin of the material.

If a very strongly cold worked material is overloaded, it could well just break like a brittle

material with no warning. So we try to design cold working as a compromise. A little bit

can be good: too much could be dangerous.

The advantages of cold working are

A better surface finish may be achieved; Dimensional accuracy can be excellent because the work is not hot so it doesn't

shrink on cooling; also the low temperatures mean the tools such as dies and rollers can last a long time without wearing out.

Usually there is no problem with oxidative effects such as scale formation. In fact, cold rolling (for example) can make such scale come off the surface of a previously hot-worked object.

Controlled amounts of cold work may be introduced. As with hot working, the grain structure of the material is made to follow the

deformation direction, which can be good for the strength of the final product. Strength and hardness are increased, although at the expense of ductility. OH & S problems related to working near hot metal are eliminated.

However

There is a limit to how much cold work can be done on a given piece of metal. See the discussion above about accumulation of damage in the form of piled up dislocations. There are ways to get around this problem, see below.

Higher forces are required to produce a given deformation, which means we need heavily built, strong forming machines (= $$$).

A neat trick: cold work then normalise

Cold working has many advantages and is very much the more common type of metal

forming. However if a large overall deformation is desired, how can we do it using only

cold working? The answer is: do some cold work, then put the object through a heat-

treatment cycle to relieve the atomic-scale damage caused by the cold work. This is

called annealing or normalising the metal. It is done by heating the metal object above

the recrystallisation temperature, waiting a few minutes, then allowing it to cool. Of

course we have to pay for the energy to do the heating.

This type of cold-work/anneal/cold-work/anneal sequence is used by plumbers who

shape copper tube on a building site. When a piece of tube has to bent sharply, it is

done in easy stages with a proper annealing between each stage (usually done using a

hand-held gas flame). This ensures that metal won't crack during the bending

operations.

Think about working with a sheet of lead on a nice warm day in the Australian sun. The

lead will likely be above its recrystallisation temperature, with no special heating

required. This can actually be very useful. It means you can shape your sheet of lead

for hours - bend it back and forth, hammer it out, whatever - and it will probably accept

all the deformation with cracking. This is one of the reasons lead sheet was so popular

in ancient times as a roofing/guttering material (for those who could afford it). Any

strange shape needed could be hammered out of a sheet or even a lump, right on site,

and with no special furnaces or other technology.

The advantages of hot working are

Lower working forces to produce a given shape, which means the machines involved don't have to be as strong, which means they can be built more cheaply;

The possibility of producing a very dramatic shape change in a single working step, without causing large amounts of internal stress, cracks or cold working;

Sometimes hot working can be combined with a casting process so that metal is cast and then immediately hot worked. This saves money because we don't have to pay for the energy to reheat the metal.

Hot working tends to break up large crystals in the metal and can produce a favourable alignment of elongated crystals (see DeGarmo Fig. 17-4 below).

Hot working can remove some kinds of defects that occur in cast metals. It can close gas pockets (bubbles) or voids in a cast billet; and it may also break up non-metallic slag which can sometimes get caught in the melt (inclusions).

The main problems, however, are

If the recrystallisation temperature of the worked metal is high e.g. if we are talking about steel, specialised methods are needed to protect the machines that work the metal. The working processes are also dangerous to human operators and very unpleasant to work near (see picture below for some idea why).

The surface finish of hot worked steel tends to be pretty crude because (a) the dies or rollers wear quite rapidly; (b) there is a lot of dimensional change as the

worked object cools; and (c) there is the constant annoying problem of scale formation on the surface of the hot steel.

Of course smart people have found ways to minimise the problems or work around

them - more below - and as a result hot working is a very common and useful process.

We just have to be aware of its limits and follow the hot working operations with other

types of manufacturing process that can fix the problems that occur.

Cold working

As explained above, when we work a metal below the recrystallisation temperature,

there is accumulation of a kind of material damage at the atomic level, through the pile-

up of dislocations. However this is not necessarily a bad thing. Many useful engineering

objects are deliberately cold-worked as part of the manufacturing process to achieve

improved properties. One common example is fencing wire. It is cold-drawn in the final

stages, before being galvanised (plated with zinc) and coiled ready for sale. The cold

working stages increase the yeild stress of the wire, meaning we can pull harder on the

wire before it deforms plastically (stretches). That's helpful when you are stringing a

fence. However the cold working does not increase the ultimate strength of the

material. So in a sense, cold working uses up some of the safety margin of the material.

If a very strongly cold worked material is overloaded, it could well just break like a brittle

material with no warning. So we try to design cold working as a compromise. A little bit

can be good: too much could be dangerous.

The advantages of cold working are

A better surface finish may be achieved; Dimensional accuracy can be excellent because the work is not hot so it doesn't

shrink on cooling; also the low temperatures mean the tools such as dies and rollers can last a long time without wearing out.

Usually there is no problem with oxidative effects such as scale formation. In fact, cold rolling (for example) can make such scale come off the surface of a previously hot-worked object.

Controlled amounts of cold work may be introduced. As with hot working, the grain structure of the material is made to follow the

deformation direction, which can be good for the strength of the final product. Strength and hardness are increased, although at the expense of ductility. OH & S problems related to working near hot metal are eliminated.

However

There is a limit to how much cold work can be done on a given piece of metal. See the discussion above about accumulation of damage in the form of piled up dislocations. There are ways to get around this problem, see below.

Higher forces are required to produce a given deformation, which means we need heavily built, strong forming machines (= $$$).

A neat trick: cold work then normalise

Cold working has many advantages and is very much the more common type of metal

forming. However if a large overall deformation is desired, how can we do it using only

cold working? The answer is: do some cold work, then put the object through a heat-

treatment cycle to relieve the atomic-scale damage caused by the cold work. This is

called annealing or normalising the metal. It is done by heating the metal object above

the recrystallisation temperature, waiting a few minutes, then allowing it to cool. Of

course we have to pay for the energy to do the heating.

This type of cold-work/anneal/cold-work/anneal sequence is used by plumbers who

shape copper tube on a building site. When a piece of tube has to bent sharply, it is

done in easy stages with a proper annealing between each stage (usually done using a

hand-held gas flame). This ensures that metal won't crack during the bending

operations.

Think about working with a sheet of lead on a nice warm day in the Australian sun. The

lead will likely be above its recrystallisation temperature, with no special heating

required. This can actually be very useful. It means you can shape your sheet of lead

for hours - bend it back and forth, hammer it out, whatever - and it will probably accept

all the deformation with cracking. This is one of the reasons lead sheet was so popular

in ancient times as a roofing/guttering material (for those who could afford it). Any

strange shape needed could be hammered out of a sheet or even a lump, right on site,

and with no special furnaces or other technology.

Composite material

Composite materials (or composites for short) are engineered materials made from

two or more constituent materials with significantly different physical or chemical

properties and which remain separate and distinct on a macroscopic level within the

finished structure.

Background

The most primitive composite materials comprised straw and mud in the form of bricks

for building construction; the Biblical book of Exodus speaks of the Israelites being

oppressed by Pharaoh, by being forced to make bricks without straw. The ancient brick-

making process can still be seen on Egyptian tomb paintings in the Metropolitan

Museum of Art[1]. The most advanced examples perform routinely on spacecraft in

demanding environments. The most visible applications pave our roadways in the form

of either steel and aggregate reinforced portland cement or asphalt concrete. Those

composites closest to our personal hygiene form our shower stalls and bath tubs made

of fiberglass. Solid surface, imitation granite and cultured marble sinks and counter tops

are widely used to enhance our living experiences.

There are two categories of constituent materials: matrix and reinforcement. At least

one portion of each type is required. The matrix material surrounds and supports the

reinforcement materials by maintaining their relative positions. The reinforcements

impart their special mechanical and physical properties to enhance the mat