Manufacturing Engineering Technology in SI Units, 6 Edition...

Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 18:

Ceramics, Glasses and Superconductors:

Processing and Equipment

Copyright © 2010 Pearson Education South Asia Pte Ltd

Chapter Outline

1. Introduction

2. Shaping Ceramics

3. Forming and Shaping of Glass

4. Techniques for Strengthening and Annealing Glass

5. Design Considerations for Ceramics and Glasses

6. Processing of Superconductors


Introduction

Ceramics and glasses have characteristics of high-

temperature strength and hardness, low electrical and

thermal conductivity, chemical inertness, and

resistance to wear and corrosion

Methods employed for ceramics consist of crushing the

raw materials; shaping, drying, firing and applying

finishing operations


Shaping Ceramics

Techniques depend on the type of ceramics and their

shapes


Shaping Ceramics

Procedure involves the following steps:

1. Crushing or grinding the raw materials into very fine

particles

2. Mixing them with additives to impart certain desirable

characteristics

3. Shaping, drying, and firing the material


Shaping Ceramics

Ground particles are mixed with additives such as:

1. Binder

2. Lubricant

3. Wetting agent

4. Plasticizer

5. Agents

6. Deflocculent


Shaping Ceramics: Casting

Common casting process is slip casting or drain

casting

A slip is a suspension of colloidal ceramic particles in

an immiscible liquid

A slip must have sufficient fluidity and low enough

viscosity to flow easily into the mold


Shaping Ceramics: Casting

Large and complex parts can be made by slip casting

Doctor-blade Process

Thin sheets of ceramics can be made


Shaping Ceramics: Plastic Forming

Plastic forming can be carried out by extrusion,

injection molding and jiggering

Also called soft, wet, or hydroplastic forming

In extrusion, the tooling costs are low and production

rates are high


Shaping Ceramics: Pressing

Dry Pressing

Dry pressing is used for relatively simple shapes, such

as whiteware, refractories for furnaces, and abrasive

products

Organic and inorganic binders are added to the mixture

and also act as lubricants

Has high production rates and close control of

dimensional tolerances



Wet Pressing

The part is formed in a mold while under high pressure

in a hydraulic or mechanical press

Production rates are high

But part size is limited, difficulty in dimensional control

and tooling costs is high

Isostatic Pressing

Used for ceramics to obtain a uniform density

distribution throughout



Jiggering

A motion where the clay bat is formed by templates or

rollers

Confined to axi-symmetric parts and has limited

dimensional accuracy

Injection Molding

Used for precision forming of ceramics in high-

technology applications

Can produce thin sections


Shaping Ceramics:

Drying and Firing

Drying and firing of the part is to give proper strength

and hardness

Due to the tendency that the part can warp or crack

from moisture content and thickness

Low moisture gradient prevents a large, uneven

gradient in shrinkage from the surface to the interior

during drying


Shaping Ceramics:

Drying and Firing

Firing or sintering involves heating the part in a

controlled environment; improving ceramic strength

and hardness

Improvement in properties results from:

1. Development of a strong bond between the complex

oxide particles in the ceramic

2. Reduced porosity

Nanophase ceramics can be sintered at lower

temperatures than those used for conventional

ceramics.


Shaping Ceramics:

Finishing Operations

Other finishing operations are:

1. Grinding

2. Lapping and honing

3. Ultrasonic machining

4. Drilling

5. Electrical-discharge machining

6. Laser-beam machining

7. Abrasive water-jet cutting

8. Tumbling


Shaping Ceramics:


EXAMPLE 18.1

Dimensional Changes During the Shaping of Ceramic Components

A solid, cylindrical ceramic part is to be made with a final

length, L, of 20 mm. For this material, it has been

established that linear shrinkages during drying and firing

are 7 and 6%, respectively, based on the dried dimension.

Calculate (a) the initial length of the part and (b) the dried

porosity if the porosity of the fired part is 3%.


Shaping Ceramics:


Solution

Dimensional Changes During the Shaping of Ceramic Components

a. We have the information for below:

b. Since the final porosity is 3%,

As there is a linear shrinkage during firing is 6%,

Hence,


mm 77.2228.2107.107.01

mm 28.2106.0106.0

do

dd

d

d

LL

LLLL

LL

ffa VVV 97.003.01

f

f

d VV

V 2.106.01

3

19% isPorosity %812.1

97.0

d

a

V

V

Forming and Shaping of Glass

Glass is processed by melting and shaping it in molds,

with tools or by blowing

Strength of glass is improved by thermal and chemical

treatments or by laminating it with a thin sheet of tough

plastic

Glass products are categorized by:

1. Flat sheets or plates

2. Rods and tubing

3. Discrete products

4. Glass fibers


Forming and Shaping of Glass:

Flat-sheet and Plate Glass

Flat-sheet glass can be made by:

1. Float method: molten glass from the furnace is fed

into a long bath

2. Drawing process: passing the molten glass through a

pair of rolls

3. Rolling process: molten glass is squeezed between

powered rollers and forming a sheet



Tubing and Rods

Air is blown through the mandrel to prevent the glass

tube from collapsing

Making the glass tubes for fluorescent bulbs

Alternative process involves extrusion of a strip of

glass



Discrete Glass Products

Several processes are used to make discrete glass

objects:

1. Blowing: Making of hollow and thin-walled glass items




2. Pressing: similar to closed die forging




3. Centrifugal Casting: centrifugal force pushes the

molten glass against the mold wall and solidifies

4. Sagging: Shallow dish-shaped or lightly embossed

glass parts can be made

5. Glass Ceramics Manufacture:

Combination methods

used for ceramics and glasses



Glass Fibers

Continuous glass fibers are drawn through multiple

orifices in heated platinum plates

Glass wool is made by a centrifugal spraying process

in which molten glass is ejected (spun) from a rotating

head


Techniques for Strengthening and

Annealing Glass

Glass can be strengthened by finishing operations and

impart desired properties and surface characteristics

Thermal Tempering

Surfaces of the hot glass are cooled by a blast of air



Annealing Glass

Thermal Tempering

Compressive surface stresses improve the strength of

the glass

Higher the glass coefficient of thermal expansion and

lower its thermal conductivity, the stronger the glass

becomes

Due to high amount of energy stored in residual

stresses, tempered glass shatters into a large number

of pieces when broken



Annealing Glass

Chemical Tempering

Larger atoms will replace the smaller atoms on the surface of the glass

Residual compressive stresses is develop on the surface

At low temperatures, part distortion is minimal

Laminated Glass

A product of strengthening method called laminate strengthening

Shattered pieces are held together because of the toughness of the wire



Annealing Glass

Bulletproof Glass

Consists of glass laminated with a polymer sheet

(polycarbonate)

Capacity of a bulletproof glass to stop a bullet depends

on:

1. Type and thickness of the glass

2. Size, shape, weight, and speed of the bullet

3. Properties and thickness of the polymer sheet


Techniques for Strengthening and Annealing Glass:


Residual stresses is developed in glass products when

not cooled at a sufficiently low rate

Annealing process for stress-relief

Sharp edges and corners can be smoothed by:

1. Grinding

2. Fire polishing


Design Considerations for Ceramics

and Glasses

Ceramic and glass products require careful selection of

composition, processing methods, finishing operations,

and methods of assembly with other components

Limitations have to be balanced against hardness,

scratch resistance, compressive strength and a diverse

physical properties

Dimensional changes, warping, cracking during

processing and service life are significant selection

factors


Processing of Superconductors

2 basic types of superconductor:

1. Low-temperature superconductors

2. High-temperature superconductors

They can be available in powder form

Difficulty in manufacturing due to

1. Inherent brittleness

2. Anisotropy


Processing of Superconductors

CASE STUDY 18.1

Production of High-temperature Superconducting Tapes

Schematic illustration of the powder-in-tube process


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