ceramics and processing

DEPARTMENT OF METALLURGY & MATERIALSMEHRAN UNIVERSITY OF ENGINEERING & TECHNOLOGY

TOPIC: INTRODUCTION

MEHRAN UNIVERSITY OF ENGINEERING AND TECHNOLOGY, JAMSHORO, SINDH, PAKISTAN

Topics Description Advanced Ceramics

Processing, Applications

Composite Materials

Processing, Applications

Polymers Processing, Applications, characterization, structure and Mechanical Properties

Super Alloys Processing, Applications, Mechanical Properties

Nano Materials (Introduction)Shape Memory Alloys

(Introduction)

Bio-Materials (Introduction)05/02/23 2

Topics Description Testing of Materials

Introduction, tensile Properties, Stress-Strain Curve, Mechanism and preventation of Failure, Mechanical Failure

Fracture Mechanics

Mechanism and preventation of Failure, Mechanical Failure

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Materials science is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering.

This science investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It includes elements of applied physics and chemistry.

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Materials science” involves investigating the relationships that exist between the structures and properties of materials.

Materials engineering” is, on the basis of these structure–property correlations, designing or engineering the structure of a material to produce a predetermined set of properties.

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Materials Engineering is all about understanding what things around us of are made of and how we can produce things out of different materials so that they have different properties.

Carrying out tests to see how strong a material is in a laboratory. However, it can also be very practical, for example, working in a huge production plant manufacturing materials from raw products. Materials engineering also takes you from the very small, such as nano-technology, to the very big, such as producing thousands of tons of steel a week in a blast furnace

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•Materials scientists and engineers are specialists who are totally involved in the investigation and design of materials.•Significant reductions in mechanical strength may result from exposure to elevated temperatures or corrosive environments•What will the finished product cost?The more familiar an engineer or scientist is with the various characteristics and structure–property relationships, as well as processing techniques of materials, the more proficient and confident he or she will be to make judicious materials choices based on these criteria.

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“If the Automotive industry had advanced at the same pace as the Computer industry, we would be driving cars, which gave a thousand kilometres to the litre and cost $25”.

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The continuing evolution of the materials world and the associated materials technologies is accelerating rapidly with each new technological development supplying more data to the knowledge bank.

New Materials capable of operating under : temperatures, higher speeds, longer life factors and lower maintenance costs

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A wide-ranging group of materials whose ingredients are clays, sand and felspar.

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Contain some of the following:Silicon & Aluminium as silicatesPotassium compoundsMagnesium compoundsCalcium compounds

Sand contains Silica and Feldspar or Aluminium Potassium Silicate.

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WhitewaresRefractoriesGlassesAbrasivesCements

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CeramicsMetals

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Made from natural clays and mixtures of clays and added crystalline ceramics.

These include:Whitewares Structural Clay Products Refractory Ceramics

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CrockeryFloor and wall tilesSanitary-wareElectrical porcelainDecorative ceramics

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Sinter and Serve

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Firebricks for furnaces and ovens. Have high Silicon or Aluminium oxide content.Brick products are used in the manufacturing plant for iron and steel, non-ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemical industries.

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Used to provide thermal protection of other materials in very high temperature applications, such as steel making (Tm=1500°C), metal foundry operations, etc.

They are usually composed of alumina (Tm=2050°C) and silica along with other oxides: MgO (Tm=2850°C), Fe2O3, TiO2, etc., and have intrinsic porosity typically greater than 10% by volume.

Specialized refractories, (those already mentioned) and BeO, ZrO2, mullite, SiC, and graphite with low porosity are also used.

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Main ingredient is Silica (SiO2) If cooled very slowly will form crystalline

structure. If cooled more quickly will form amorphous

structure consisting of disordered and linked chains of Silicon and Oxygen atoms.

This accounts for its transparency as it is the crystal boundaries that scatter the light, causing reflection.

Glass can be tempered to increase its toughness and resistance to cracking.

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Three common types of glass:Soda-lime glass - 95% of all glass,

windows containers etc.Lead glass - contains lead oxide to

improve refractive indexBorosilicate - contains Boron oxide,

known as Pyrex.

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Flat glass (windows)Container glass (bottles)Pressed and blown glass

(dinnerware)Glass fibres (home insulation)Advanced/specialty glass (optical

fibres)

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SoftenedGob

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Softened glass

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The strength of glass can be enhanced by inducing compressive residual stresses at the surface.

The surface stays in compression - closing small scratches and cracks.

Small Scratches

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Tempering: Glass heated above Tg but below the softening point Cooled to room temp in air or oil Surface cools to below Tg before interior when interior cools and contracts it draws the

exterior into compression.

Chemical Hardening: Cations with large ionic radius are diffused into the

surface This strains the “lattice” inducing compressive

strains and stresses.

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Many have tried to gain access with golf clubs and baseball bats but obviously the glass remains intact ! From time to time a local TV station intends to show videos of those trying to get at the cash!!

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Good electrical insulators and refractories. Magnesium Oxide is used as insulation

material in heating elements and cables. Aluminium Oxide Beryllium Oxides Boron Carbide Tungsten Carbide. Used as abrasives and cutting tool tips.

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Natural (garnet, diamond, etc.)Synthetic abrasives (silicon carbide,

diamond, fused alumina, etc.) are used for grinding, cutting, polishing, lapping, or pressure blasting of materials

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Used to produce concrete roads, bridges, buildings, dams.

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Advanced ceramic materials have been developed over the past half century

Applied as thermal barrier coatings to protect metal structures, wearing surfaces, or as integral components by themselves.

Engine applications are very common for this class of material which includes silicon nitride (Si3N4), silicon carbide (SiC), Zirconia (ZrO2) and Alumina (Al2O3)

Heat resistance and other desirable properties have lead to the development of methods to toughen the material by reinforcement with fibers and whiskers opening up more applications for ceramics

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Structural: Wear parts, bioceramics, cutting tools, engine components, armour.

Electrical: Capacitors, insulators, integrated circuit packages, piezoelectrics, magnets and superconductors

Coatings: Engine components, cutting tools, and industrial wear parts

Chemical and environmental: Filters, membranes, catalysts, and catalyst supports

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Rotor (Alumina)

Gears (Alumina)05/02/23 45

Ceramic Rotor

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Automotive Components in Silicon Carbide

Chosen for its heat and wear resistance

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Ceramic armour systems are used to protect military personnel and equipment.

Advantage: low density of the material can lead to weight-efficient armour systems.

Typical ceramic materials used in armour systems include alumina, boron carbide, silicon carbide, and titanium diboride.

The ceramic material is discontinuous and is sandwiched between a more ductile outer and inner skin.

The outer skin must be hard enough to shatter the projectile.

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Bulletproof glass (also known as ballistic glass, transparent armor or bullet-resistant glass) is a type of strong but optically transparent material that is particularly resistant to being penetrated when struck by bullets, but like all known materials, is not completely impenetrable

Index of refraction05/02/23 51

Most of the impact energy is absorbed by the fracturing of the ceramic and any remaining kinetic energy is absorbed by the inner skin, that also serves to contain the fragments of the ceramic and the projectile preventing severe impact with the personnel/equipment being protected.

Alumina ceramic/Kevlar composite system in sheets about 20mm thick are used to protect key areas of Hercules aircraft (cockpit crew/instruments and loadmaster station).

This lightweight solution provided an efficient and removable/replaceable armour system. Similar systems used on Armoured Personnel Carrier’s.

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Projectile

Outer hard skin

Ceramic-Discontinuous

Innerductileskin

PersonnelandEquipment

Ceramic Armor System

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Body armour and other components chosen for their ballistic properties.

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Extracted From: Fundamental of Modern Manufacturing

Processing of Traditional CeramicsProcessing of New CeramicsProcessing of CermetsProduct Design Considerations

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Ceramic materials divide into three categories: 1. Traditional ceramics – particulate

processing2. New ceramics – particulate processing3. Glasses – solidification processing

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Traditional ceramics are made from minerals occurring in nature Products include pottery, porcelain, bricks,

and cement New ceramics are made from

synthetically produced raw materials Products include cutting tools, artificial

bones, nuclear fuels, and substrates for electronic circuits

The starting material for all of these items is powder

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For traditional ceramics, the powders are usually mixed with water to temporarily bind the particles together and achieve the proper consistency for shaping

For new ceramics, substances other than water are used as binders during shaping

After shaping, the green parts are fired (sintered), whose function is the same as in powder metallurgy: To effect a solid state reaction which bonds

the material into a hard solid mass

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Figure 17.1 ‑ Usual steps in traditional ceramics processing: (1) preparation of raw materials, (2) shaping, (3) drying, and (4) firing

Part (a) shows the workpart during the sequence, while (b) shows the condition of the powders

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Shaping processes for traditional ceramics require the starting material to be a plastic paste This paste is comprised of fine ceramic

powders mixed with water The raw ceramic material usually occurs

in nature as rocky lumps, and reduction to powder is the purpose of the preparation step in ceramics processing

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Reducing particle size in ceramics processing by use of mechanical energy in various forms such as impact, compression, and attrition

Comminution techniques are most effective on brittle materials such as cement, metallic ores, and brittle metals

Two general types of comminution operations: 1. Crushing 2. Grinding

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Reduction of large lumps from the mine to smaller sizes for subsequent further reduction

Several stages may be required (e.g., primary crushing, secondary crushing), the reduction ratio in each stage being in the range 3 to 6

Crushing of minerals is accomplished by compression against rigid surfaces or by impact against surfaces in a rigid constrained motion

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Jaw CrusherLarge jaw toggles back and forth to crush

lumps against a hard, rigid surface

Crushing operations: (a) jaw crusher

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Roll CrusherCeramic lumps are squeezed between rotating rolls

Crushing operations: (c) roll crusher

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In the context of comminution, grinding refers to the operation of reducing the small pieces after crushing to a fine powder

Accomplished by abrasion, impact, and compaction by hard media such as balls or rolls

Examples of grinding include: Ball mill Roller mill Impact grinding

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Ball MillHard spheres mixed with stock are rotated

inside a large cylindrical container; the mixture is carried up the container wall as it rotates, and then pulled back down by gravity for grinding action

Mechanical methods of producing ceramic powders: (a) ball mill

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Roller MillStock is compressed against a flat horizontal grinding table by rollers riding over the table surface

Mechanical methods of producing ceramic powders: (b) roller mill

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1. Clay (hydrous aluminum silicates) - usually the main ingredient because of ideal forming characteristics when mixed with water

2. Water – creates clay-water mixture with suitable plasticity for shaping

3. Non‑plastic raw materials, such as alumina and silica - reduce shrinkage in drying and firing but also reduce plasticity of the mixture during forming

4. Other ingredients, such as fluxes that melt (vitrify) during firing and promote sintering, and wetting agents to improve mixing of ingredients

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Slip casting The clay-water mixture is a slurry

Plastic forming methods The clay is plastic

Semi‑dry pressing The clay is moist but has low plasticity

Dry pressing The clay is basically dry (less than 5%

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Figure 17.4 ‑ Four categories of shaping processes used for traditional ceramics, compared to water content and

pressure required to form the clay

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A suspension of ceramic powders in water, called a slip, is poured into a porous plaster of paris mold so that water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface

The slip composition is 25% to 40% water Two principal variations:

Drain casting - the mold is inverted to drain excess slip after a semi‑solid layer has been formed, thus producing a hollow product

Solid casting - to produce solid products, adequate time is allowed for entire body to become firm

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Sequence of steps in drain casting, a form of slip casting: (1) slip is poured into mold cavity, (2) water is absorbed into plaster mold to form a firm layer, (3) excess slip is poured out, and (4) part is removed from mold and trimmed

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The starting mixture must have a plastic consistency, with 15% to 25% water

Variety of manual and mechanized methods Manual methods use clay with more water

because it is more easily formed▪ More water means greater shrinkage in drying

Mechanized methods generally use a mixture with less water so starting clay is stiffer

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Hand modeling (manual method) Jiggering (mechanized method)Plastic pressing (mechanized

method)Extrusion (mechanized method)

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Creation of the ceramic product by manipulating the mass of plastic clay into the desired geometry Hand molding - similar only a mold or form is used to define portions of the part geometry Hand throwing on a potter's wheel is another refinement of handcraft methods

Potter's wheel = a round table that rotates on a vertical spindle, powered either by motor or foot‑operated treadle Products of circular cross‑section can be formed by throwing and shaping the clay, sometimes using a mold to provide the internal shape

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Jiggering Similar to potter's wheel methods, but hand throwing is replaced by mechanized techniques

Sequence in jiggering: (1) wet clay slug is placed on a convex mold; (2) batting; and (3) a jigger tool imparts the final product shape

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Forming process in which a plastic clay slug is pressed between upper and lower molds contained in metal rings

Molds are made of porous material such as gypsum, so when a vacuum is drawn on the backs of the mold halves, moisture is removed from the clay

The mold sections are then opened, using positive air pressure to prevent sticking of the part in the mold

Advantages: higher production rate than jiggering and not limited to radially symmetric parts

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Compression of clay through a die orifice to produce long sections of uniform cross‑section, which are then cut to required piece length

Equipment utilizes a screw‑type action to assist in mixing the clay and pushing it through die opening

Products: hollow bricks, shaped tiles, drain pipes, tubes, and insulators

Also used to make the starting clay slugs for other ceramics processing methods such as jiggering and plastic pressing

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Semi-dry Pressing Uses high pressure to overcome the clay’s low plasticity and force it into a die cavity

Semi‑dry pressing: (1) depositing moist powder into die cavity, (2) pressing, and (3) opening the die sections and ejection

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Process sequence is similar to semi‑dry pressing - the main distinction is that the water content of the starting mix is typically below 5%

Dies must be made of hardened tool steel or cemented carbide to reduce wear since dry clay is very abrasive

No drying shrinkage occurs, so drying time is eliminated and good dimensional accuracy is achieved in the final product

Typical products: bathroom tile, electrical insulators, refractory brick, and other simple geometries

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Water plays an important role in most of the traditional ceramics shaping processes

Thereafter, it has no purpose and must be removed from the clay piece before firing

Shrinkage is a problem during drying because water contributes volume to the piece, and the volume is reduced when it is removed

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Volume of clay as a function of water content Relationship shown here is typical; it varies for different clay compositions

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The drying process occurs in two stages: Stage 1 - drying rate is rapid and

constant as water evaporates from the surface into the surrounding air and water from the interior migrates by capillary action to the surface to replace it This is when shrinkage occurs, with the risk

of warping and cracking Stage 2 - the moisture content has been

reduced to where the ceramic grains are in contact Little or no further shrinkage occurs

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Typical drying rate curve and associated volume reduction (drying shrinkage) for a ceramic body in drying

Drying rate in the second stage of drying is depicted here as a straight line; the function is sometimes concave or convex

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Heat treatment process that sinters the ceramic material

Performed in a furnace called a kiln Bonds are developed between the

ceramic grains, and this is accompanied by densification and reduction of porosity

Therefore, additional shrinkage occurs in the polycrystalline material in addition to that which has already occurred in drying

In the firing of traditional ceramics, a glassy phase forms among the crystals which acts as a binder

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Application of a ceramic surface coating to make the piece more impervious to water and enhance its appearance

The usual processing sequence with glazed ware is: 1. Fire the piece once before glazing to harden

the body of the piece2. Apply the glaze3. Fire the piece a second time to harden the

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The manufacturing sequence for the new ceramics can be summarized in the following steps: 1. Preparation of starting materials2. Shaping3. Sintering4. Finishing

While the sequence is nearly the same as for the traditional ceramics, the details are often quite different

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Strength requirements are usually much greater for new ceramics than for traditional ceramics

Therefore, the starting powders must be smaller and more uniform in size and composition, since the strength of the resulting ceramic product is inversely related to grain size

Greater control of the starting powders is required

Powder preparation includes mechanical and chemical methods

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Many of the shaping processes for new ceramics are borrowed from powder metallurgy (PM) and traditional ceramics PM press and sinter methods have been

adapted to the new ceramic materials And some of the traditional ceramics

forming techniques are used to shape the new ceramics, such as: slip casting, extrusion, and dry pressing

The processes described here are not normally associated with the forming of traditional ceramics, although several are associated with PM

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Similar to dry pressing except it is carried out at elevated temperatures so sintering of the product is accomplished simultaneously with pressing

This eliminates the need for a separate firing step

Higher densities and finer grain size are obtained, but die life is reduced by the hot abrasive particles against the die surfaces

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Uses hydrostatic pressure to compact the ceramic powders from all directions

Avoids the problem of nonuniform density in the final product that is often observed in conventional uniaxial pressing

Same process used in powder metallurgy

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Ceramic particles are mixed with a thermoplastic polymer, then heated and injected into a mold cavity The polymer acts as a carrier and provides flow characteristics for molding Upon cooling which hardens the polymer, the mold is opened and the part is removed Because temperatures needed to plasticize the carrier are much lower than those required for sintering the ceramic, the piece is green after molding The plastic binder is removed and the remaining ceramic part is sintered

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Since the plasticity needed to shape the new ceramics is not normally based on water, the drying step required for traditional green ceramics can be omitted for most new ceramic products

The sintering step is still very much required Functions of sintering are the same as

before: 1. Bond individual grains into a solid mass2. Increase density3. Reduce or eliminate porosity

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Parts made of new ceramics sometimes require finishing, which has one or more of the following purposes: 1. Increase dimensional accuracy 2. Improve surface finish3. Make minor changes in part geometry

Finishing usually involves abrasive processes Diamond abrasives must be used to cut the

hardened ceramic materials 05/02/23 95

A family of composite materials consisting of carbide ceramic particles imbedded in a metallic binder

Classified as metal matrix composites because the metallic binder is the matrix which holds the bulk material together

However, the carbide particles constitute the largest proportion of the composite material, normally between 80% and 95% by volume

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The carbide powders must be sintered with a metal binder to provide a strong and pore‑free part Cobalt works best with WC, while nickel is

better with TiC and Cr3C2 Usual proportion of binder metal is 4% up to 20% Powders of carbide and binder metal are thoroughly mixed wet in a ball mill to form a homogeneous sludge The sludge is then dried in a vacuum or controlled atmosphere to prevent oxidation in preparation for compaction

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Most common process is cold pressing, used for high production of cemented carbide parts such as cutting tool inserts Dies must be oversized to account for shrinkage

during sintering (shrinkage can be 20% or more) For high production, the dies are made with

WC‑Co liners to reduce wear For smaller quantities, large flat sections may be

pressed and then cut into smaller pieces Other methods: isostatic pressing and hot

pressing

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It is possible to sinter WC (and TiC) without a metal binder, but the resulting material is less than 100% of true density Using a binder yields a structure virtually free of porosity

Sintering of WC‑Co involves liquid phase sintering The usual sintering temperatures for WC‑Co are 1370‑1425C (2500‑2600F), which is below cobalt's melting point of 1495C (2716F) Thus, the pure binder metal does not melt at the sintering temperature

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Figure 17.11 ‑ WC‑Co phase diagram

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However, WC dissolves in Co in the solid state so WC is gradually dissolved during the heat treatment, and its melting point is reduced so melting occurs As the liquid phase forms, it flows and wets the

WC particles, further dissolving the solid Presence of molten metal also serves to remove

gases from the internal regions of the compact These mechanisms cause a rearrangement

of the remaining WC particles into a closer packing, which results in significant densification and shrinkage of the WC‑Co mass

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Subsequent processing is usually required after sintering to achieve adequate dimensional control of the cemented carbide parts

Grinding with a diamond or other very hard abrasive wheel is the most common secondary operation performed for this purpose

Other secondary operations include Electric discharge machining Ultrasonic machining

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1. Raw Materials Preparation and Melting

2. Shaping Processes in Glassworking3. Heat Treatment and Finishing4. Product Design Considerations

Glass is one of three basic types of ceramics The others are traditional ceramics and new

ceramics Glass is distinguished by its

noncrystalline (vitreous) structure The other ceramic materials have a crystalline

structure

Glass products are commercially produced in an almost unlimited variety of shapes

Most products made in very large quantities: Light bulbs, beverage bottles, jars, light bulbs Window glass Glass tubing (e.g., for fluorescent lighting) Glass fibers

Other products are made individually: Giant telescope lenses

Methods for shaping glass are quite different from those for traditional and new ceramics

In glassworking, the principal starting material is silica, usually combined with other oxide ceramics that form glasses

Process sequence in shaping glass: Starting material is heated to transform it from a

hard solid into a viscous liquid It is then shaped while in this fluid condition When cooled and hard, the material remains in the

glassy state rather than crystallizing

Figure 12.1 The typical process sequence in glassworking: (1) preparation of raw materials and melting, (2) shaping, and (3) heat treatment.

The principal component in nearly all glasses is silica (SiO2) Primary source is natural quartz in sand

Other components are added in proportions to achieve the desired composition: Soda ash (source of Na2O), limestone (source of

CaO), aluminum oxide (Al2O3), and potash (source of K2O),

Recycled glass is usually added to the mixture too

The batch of starting materials to be melted is called a charge, and loading it into the furnace is called charging the furnace Melting temperatures for glass are around 1500C to

1600C (2700F to 2900F) Viscosity of molten glass is inversely

related to temperature Since shaping immediately follows melting, the

temperature at which the glass is tapped depends on the viscosity required for the shaping process

Shaping processes to fabricate glass products can be grouped into three categories: 1. Discrete processes for piece ware (bottles,

jars, plates, light bulbs) 2. Continuous processes for making flat glass

(sheet and plate glass) and tubing (laboratory ware, fluorescent lights)

3. Fiber‑making processes to produce fibers (for insulation and fiber optics)

Ancient methods of hand-working glass included glass blowing

Handicraft methods are still used today for making glassware items of high value in small quantities

However, most modern glass shaping processes are highly mechanized technologies for producing discrete pieces such as jars, bottles, and light bulbs in high quantities

Spinning – similar to centrifugal casting of metals

Pressing – mass production of flat products such as dishes and TV tube faceplates

Press-and-blow –production of wide-mouth containers such as jars

Blow-and-blow - production of smaller-mouth containers such as beverage bottles and incandescent light bulbs

Casting – large items such as astronomical lenses that must cool slowly to avoid cracking

Figure 12.2 Spinning of funnel‑shaped glass parts such as back sections of cathode ray tubes for TVs and computer monitors: (1) gob of glass dropped into mold; and (2) rotation of mold to cause spreading of molten glass on mold surface.

Figure 12.3 Pressing of flat glass pieces: (1) glass gob is fed into mold from furnace; (2) pressing into shape by plunger; and (3) plunger is retracted and finished product is removed (symbols v and F indicate motion (velocity) and applied force).

Figure 12.4 Press‑and‑blow forming sequence: (1) molten gob is fed into mold cavity; (2) pressing to form a parison; (3) the partially formed parison, held in a neck ring, is transferred to the blow mold, and (4) blown into final shape.

Figure 12.5 Blow‑and‑blow forming sequence: (1) gob is fed into inverted mold cavity; (2) mold is covered; (3) first blowing step; (4) partially formed piece is reoriented and transferred to second blow mold, and (5) blown to final shape.

If molten glass is sufficiently fluid, it can be poured into a mold

Massive objects, such as astronomical lenses and mirrors, are made by this method

After cooling and solidifying, the piece must be finished by lapping and polishing

Casting of glass is not often used except for special jobs

Smaller lenses are usually made by pressing

Processes for producing flat glass such as sheet and plate glass: Rolling of flat plate Float process

Process for producing glass tubes Danner process

Starting glass from melting furnace is squeezed through opposing rolls whose gap determines sheet thickness, followed by grinding and polishing for parallelism and smoothness

Figure 12.6 Rolling of flat glass

Rolling of Flat Plate

Molten glass flows onto surface of a molten tin bath, where it spreads evenly across the surface, achieving a uniform thickness and smoothness - no grinding or polishing is needed

Figure 12.7 The float process for producing sheet glass

Float Process

Molten glass flows around a rotating hollow mandrel through which air is blown while glass is drawn

Figure 12.8 Drawing of glass tubes by the Danner process.

Danner Process

Glass fiber products fall into two categories, with different production methods for each:

1. Fibrous glass for thermal insulation, acoustical insulation, and air filtration, in which the fibers are in a random, wool‑like condition Produced by centrifugal spraying

2. Long continuous filaments suitable for fiber reinforced plastics, yarns, fabrics, and fiber optics Produced by drawing

In a typical process for making glass wool, molten glass flows into a rotating bowl with many small orifices around its periphery

Centrifugal force causes the glass to flow through the holes to become a fibrous mass suitable for thermal and acoustical insulation

Continuous glass fibers of small diameter (lower limit ~ 0.0025 mm) are produced by pulling strands of molten glass through small orifices in a heated plate made of a platinum alloy

Drawing of continuous glass fibers

Drawing of Glass

Heating to elevated temperature and holding to eliminate stresses and temperature gradients; then slow cooling to suppress stress formation, then more rapid cooling to room temperature

Annealing temperatures are ~ 500C (900F) Annealing has the same function in

glassworking as in metalworking – to relieve stresses

Annealing is performed in tunnel‑like furnaces, called lehrs, in which the products flow slowly through the hot chamber on conveyors

Heating to a temperature somewhat above annealing temperature into the plastic range, followed by quenching of surfaces, usually by air jets

When the surfaces cool, they contract and harden while interior is still plastic

As the internal glass cools, it contracts, putting the hard surfaces in compression

Tempered glass is more resistant to scratching and breaking due to compressive stresses on its surfaces

Products: windows for tall buildings, all‑glass doors, safety glasses

When tempered glass fails, it shatters into many small fragments

Automobile windshields are not made of tempered glass, due to the danger posed by this fragmentation

Instead, conventional glass is used; it is fabricated by sandwiching two pieces of glass on either side of a tough polymer sheet

Should this laminated glass fracture, the glass splinters are retained by the polymer sheet and the windshield remains relatively transparent

Operations include grinding, polishing, and cutting

Glass sheets often must be ground and polished to remove surface defects and scratch marks and to make opposite sides parallel

In pressing and blowing with split dies, polishing is often used to remove seam marks from the product

Cutting of continuous sections of tube and plate is done by first scoring the glass with a glass‑cutting wheel or cutting diamond and then breaking the section along the score line

Decorative and surface processes performed on certain glassware products include: Mechanical cutting and polishing operations;

and sandblasting Chemical etching (with hydrofluoric acid, often

in combination with other chemicals) Coating (e.g., coating of plate glass with

aluminum or silver to produce mirrors)

Glass is transparent and has optical properties that are unusual if not unique among engineering materials For applications requiring transparency,

light transmittance, magnification, and similar optical properties, glass is likely to be the material of choice

Certain polymers are transparent and may be competitive, depending on design requirements

Glass is much stronger in compression than tension Components should be designed to be

subjected to compressive stresses, not tensile stresses

Glass is brittle Glass parts should not be used in applications

that involve impact loading or high stresses that might cause fracture

Certain glass compositions have very low thermal expansion coefficients and can tolerate thermal shock These glasses should be selected for applications

where this characteristic is important Design outside edges and corners with

large radii and inside corners with large radii, to avoid points of stress concentration

Threads may be included in glass parts However, the threads should be coarse