Download - Chapter 8 Glass and Ceramics

8/3/2019 Chapter 8 Glass and Ceramics

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GLASS AND CERAMICS


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glass is an inorganicproduct of fusion which

has been cooled to a rigidcondition without

crystallizing


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There is two types of glass

Glass

Glass-ceramic


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Glass-ceramicmaterials share manyproperties with both glass and more

traditional crystalline ceramics

It is formed as a glass, and then made to

crystalize partly by heat treatment glass-ceramics have no pores between

crystals


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High mechanical strength Low coefficient of thermal expansion (avoid

thermal shock)

very low heat conduction coefficient

Some can be made nearly transparent (1520% loss in a typical cooktop) for radiationin the infrared wavelengths

High temperature capabilities Good biological compatibilty

Good dielectric properties (application inelectronic)


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A glass-ceramic stove uses radiant heatingcoils or infrared halogen lamps as the heatingelements. The surface of the glass-ceramiccooktop above the burner heats up, but the

adjacent surface remains cool because of thelow heat conduction coefficient of thematerial.


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Commercial glass or Soda-lime glass:- bottles and jars, flat glass for windows or for

drinking glasses is known as commercialglass or soda-lime glass, as soda ash is usedin its manufacture.

Lead glass:

- make a wide variety of decorative glass

objects.


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Borosilicate glass:

- used for domestic kitchens and chemistrylaboratories

Glass Fibre:

- used in the reinforced plastics to make

protective helmets, boats, piping, car chassis,ropes, car exhausts and many other items.


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Glass produced by heating the raw materialsto certain temperature which melting occur.

Raw material:Sand (SiO2)

Lime stone (CaCO3)

Soda ash (Na2CO3)


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To achieve high optical transparency theglass product must be homogenous andpore free.

Homogeneity is achieved by complete meltingand mixing of raw ingredients.

Porosity result from small bubbles.

To eliminate the bubbles, proper adjustmentof the viscosity of the molten material isrequired.


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Pressing Blowing

Drawing

Sheet Fiber


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Pressing To produce thick wall pieces of glass

Such as plate and dishes Form by pressure application in a graphite-coated

cast iron mold having the desired shape.


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Blowing Usually done by hand especially for art object Example:

Glass jar

Bottles

Light bulb

From a raw gob of glass, a parisonor temporaryshape is form by mechanical pressing in a mold.

The pieces is inserted into a finishing or blow moldand force to conform to the mold contours by thepressure created by a blast of air.


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Drawing Use to form long glass pieces

Example : Sheet glass

Rod glass

Tube glass

Fiber glass (using sophisticated drawing operation)


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The float process of producing sheet glass


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Heat-treated glass is a general term used in theglass fabrication industry to describe glassthat has been processed through a temperingoven to change its strength and breakagecharacteristics

(i.e., the size and/or shape of the glass piecesafterbreakage). There are two distinct heat-treated

Types of Heat Treating Glasses

1. Annealing2. Glass Tempering


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The process of heat-treated by heatingannealed glass to a temperature ofapproximately 600 oC then slowly cooling it

The process remove undesirerable stressesfrom the glass

Important in retain many of the thermalstresses and decrease the overall strength

of the glass


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Tempered glass is made from annealed glassvia thermal tempering process

Tempered glass is heated above theannealing point

The glass is then rapidly cooled with forceddraft of air while inner portion of the glassremains free for a short of time.


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Tempered glass is stronger than annealedglass

When broken, tempered glass fractures intosmall fragments that reduces serious injuriescompared to annealed glass.


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Color Color is created by adding to the glass a substance

such as a metallic oxide. Iron can produce green orbrown glass; copper, light blue or red; cobalt, adeeper blue; tin, white; gold or selenium, red; etc.Colors are added in the form of frit, or crushedglass, or as glass rods or glass powder.

cobalt oxide has been added toproduce a bluish color


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Specific volume (1/r) vs Temperature (T):

Glasses:

-- do not crystallize

-- change in slope in spec. vol. curve at

glass transition temperature, Tg-- transparent - no grain boundaries to

scatter light

Crystalline materials:

-- crystallize at melting temp, Tm-- have abrupt change in spec.

vol. at Tm

Adapted from Fig. 14.16,Callister & Rethwisch 3e.

T

Specific volume

Supercooled

Liquid

solid

Tm

Liquid(disordered)

Crystalline

(i.e., ordered)

Tg

Glass

(amorphous solid)


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Edit your company slogan


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www.themegallery.com

An inorganic, non metallic solid prepared frompowdered materials

Fabricated into products through the application of

heat. The word of ceramics come from Greek wordkeramikos means pottery.


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Structural

Bricks, pipes, floor and roof tiles Refractories

Kiln linings, gas fire radiants, steel and glassmaking crucibles

WhitewaresTableware, wall tiles, decorative art objectsand sanitary ware

Technical

Tiles used in the Space Shuttle program, gasburner nozzles, ballistic protection, nuclearfuel uranium oxide pellets, bio-medicalimplants, jet engine turbine blades, andmissile nose cones.


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MECHANICAL- are usually ionic or covalent bonded materials,and can be crystalline or amorphous.

- poor toughness

-porous- decreasing toughness,reducing tensile strength

ELECTRICAL

- such as extremely low temperature, some

ceramics exhibit high temperaturesuperconductivity


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Compound formed between metallic andnonmetallic elements

- aluminium and oxygen (Al2O3)- calcium and oxygen (CaO)

- silicon and nitrogen (Si2N4)

Silicates (compounds of silicon and oxygen)


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Ceramics are used inthe manufacture ofknives.

The blade of the

ceramic knife willstay sharp for muchlonger than that of asteel knife, althoughit is more brittle andcan be snapped bydropping it on ahard surface


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Ceramics such asalumina and boroncarbide have beenused in ballistic

armored vests to repellarge-caliber rifle fire.

Similar material isused to protectcockpits of somemilitary airplanes,because of the lowweight of the material.


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The two main categories of ceramics aretraditional and advanced.

Traditional ceramics include objects made ofclay and cements that have been hardened by

heating at high temperatures. Traditionalceramics are used in:

Glasses-dishes

Clay Products flowerpot, crockery

Cement

wall tiles,plaster of paris


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Advanced ceramics include carbides, such as

silicon carbide, SiC; oxides, such as aluminumoxide, Al2O3; nitrides, such as silicon nitride,Si3N4; and many other materials, including themixed oxide ceramics that can act assuperconductors.

Advanced ceramics require modern processingtechniques, and the development of thesetechniques has led to advances in medicine andengineering.

Types: Microelectromechanical System

Optical Fibers

Ceramic Ball Bearings


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Hydroplasticforming

FABRICATION

TECHNIQUE

Slip Casting


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Clay mineral mixed with water become Highly plastic

Pliable

May be molded without cracking

Extremely low yield strength Technique using extrusion

Extrusion- stiff plastic ceramic mass is forcedthrough a die orifice having the desired cross-

sectional geometry


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Slip suspension of clay and/or other nonplasticmaterials in water.

The nature of the slip High specific gravity Yet be very fluid Pourable The cast piece must be free of bubbles Low drying shrinkage Relatively high strength

The properties of the mold influence the quality ofthe casting


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Solid CastingPoured into a porous mold

Water absorbed into the

mold

Leave solid layer on the

Mold wall the thickness of

which depends on the time

Continued until the desire

mold cavity become solid

Drain CastingPoured into a porous mold

Water absorbed into the

mold

Terminated when the solid

shell wall reaches the

desired thickness by

inverting the mold andpouring out the excess slip


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solid component

Adapted from Fig.14.22, Callister &Rethwisch 3e. (Fig.

14.22 is from W.D.Kingery, Introductionto Ceramics, JohnWiley and Sons, Inc.,1960.)

hollow component

pour slip

into mold

drain

mold

green

ceramic

pour slip

into mold

absorb water

into moldgreen

ceramic


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Evaporation

controlled by:

temperature

humidity rate of

airflow

Shrinkage

Minimize shrinkage:

- increased particlesize

- added nonplasticmaterials to the clay

Factor influenceshrinkage:

- body thickness

- water content of theform body

- clay particle size

Advantages of Drying:

- high temperatureused in conventionalmethods are avoided(drying temperature cankept below 50oC)



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Vitrification - gradual formation of a liquid gasthat flows into and fills some of the pore volume

Degree of vitrification - depends on temperature,time and composition.

Upon cooling fused phase form glassy matrix --dense, strong body

Final microstructure consists:

- vitrified phase

- unreacted quartz particle- porosity


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Firing temperature increase, vitrificationincrease.

* strength, durability and density alsoincreases

Application: Building bricks ordinarily fired at 900 c and

relatively porous

Firing of highly vitrified porcelain borders on

being optically translucent (takes place athigher temperature)

Complete vitrification is avoided duringfiring.


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Powder Pressing

Sintering


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