Chapter 13: Applications and Processing of Ceramics

ISSUES TO ADDRESS...

• How do we classify ceramics ?

• What are some applications of ceramics ?

• How is processing of ceramics different than for metals ?

Chapter 13 - 1

Classification of Ceramics

Ceramic Materials

Glasses Clay products Refractories Abrasives Cements Advanced ceramics -optical -composite reinforce -containers/ household -whiteware -structural -bricks for high T (furnaces) -sandpaper -cutting -polishing

Adapted from Fig. 13.1 and discussion in Section 13.2-8,

Callister & Rethwisch 8e.

-composites -structural -engine rotors valves bearings -sensors

Chapter 13 - 2

Ceramics Application: Die Blanks

• Die blanks: -- Need wear resistant properties!

• Die surface: -- 4 m m polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate.

-- polycrystalline diamond gives uniform hardness in all directions to reduce wear.

Ao

die die

Ad

tensile force Adapted from Fig. 11.8(d),

Callister & Rethwisch 8e.

Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.

Chapter 13 - 3

Ceramics Application: Cutting Tools

• Tools: -- for grinding glass, tungsten, carbide, ceramics -- for cutting Si wafers -- for oil drilling • Materials: -- manufactured single crystal or polycrystalline diamonds in a metal or resin matrix.

-- polycrystalline diamonds resharpen by microfracturing along cleavage planes.

oil drill bits blades Single crystal diamonds polycrystalline diamonds in a resin matrix.

Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.

Chapter 13 - 4

Ceramics Application: Sensors

• Example: • Principle: ZrO 2 as an oxygen sensor Ca2+ Increase diffusion rate of oxygen to produce rapid response of sensor signal to change in oxygen concentration • Approach: Add Ca impurity to ZrO 2 : -- increases O 2 vacancies A substituting Ca 2+ removes a Zr 4+ ion ion and an O 2 ion. -- increases O 2 • Operation: diffusion rate - voltage difference produced when O 2 ions diffuse from the external surface through the sensor to the gas with an unknown, higher oxygen content reference gas surface.

-- magnitude of voltage difference  partial pressure of oxygen at the external surface sensor O2 diffusion reference gas at fixed oxygen content + voltage difference produced!

Chapter 13 - 5

Refractories

• Materials to be used at high temperatures (e.g., in high temperature furnaces). • Consider the Silica (SiO 2 ) - Alumina (Al 2 O 3 ) system.

• Silica refractories - silica rich - small additions of alumina depress melting temperature (phase diagram): 2200 T(ºC) 2000 Liquid (

L

) 3Al 2 O 3 -2SiO 2 mullite alumina +

L

1800 crystobalite +

L

1600 1400 0 mullite +

L

alumina + mullite mullite + crystobalite 20 40 60 80 100 Composition (wt% alumina) Fig. 12.27,

Callister & Rethwisch 8e.

(Fig. 12.27 adapted from F.J. Klug and R.H. Doremus,

J. Am. Cer.

Soc. 70 (10), p. 758, 1987.) Chapter 13 - 6

Advanced Ceramics: Materials for Automobile Engines

• Advantages: – Operate at high temperatures – high efficiencies – Low frictional losses – Operate without a cooling system – Lower weights than current engines • Disadvantages: – Ceramic materials are brittle – Difficult to remove internal voids (that weaken structures) – Ceramic parts are difficult to form and machine • Potential candidate materials: Si 3 N 4 , SiC, & ZrO 2 • Possible engine parts: engine block & piston coatings Chapter 13 - 7

Advanced Ceramics: Materials for Ceramic Armor

Components:

-- Outer facing plates -- Backing sheet

Properties/Materials:

-- Facing plates -- hard and brittle — fracture high-velocity projectile — Al 2 O 3 , B 4 C, SiC, TiB 2 -- Backing sheets -- soft and ductile — deform and absorb remaining energy — aluminum, synthetic fiber laminates Chapter 13 - 8

Ceramic Fabrication Methods (i)

GLASS FORMING PARTICULATE FORMING • Blowing of Glass Bottles: CEMENTATION • Pressing: plates, cheap glasses Parison mold Gob Pressing operation -- glass formed by application of pressure -- mold is steel with graphite lining • Fiber drawing: Compressed air Suspended parison Finishing mold Adapted from Fig. 13.8,

Callister & Rethwisch 8e

. (Fig. 13.8 is adapted from C.J. Phillips,

Glass: The Miracle Maker

, Pittman Publishing Ltd., London.) wind up Chapter 13 - 9

Sheet Glass Forming

• Sheet forming – continuous casting – sheets are formed by floating the molten glass on a pool of molten tin Adapted from Fig. 13.9,

Callister & Rethwisch 8e

. Chapter 13 - 10

Glass Structure

• Basic Unit: 4 Si0 4 tetrahedron Si4+ O2 Glass is noncrystalline ( amorphous) • Fused silica is SiO 2 to which no impurities have been added • Other common glasses contain impurity ions such as Na + , Ca 2+ , Al 3+ , and B 3+ • Quartz is crystalline SiO

2

: Na+ Si4+ O2 (soda glass) Adapted from Fig. 12.11,

Callister & Rethwisch 8e

. Chapter 13 - 11

Glass Properties

• Specific volume (1 /r ) vs Temperature (

T

): Specific volume Supercooled Liquid Liquid (disordered) • Crystalline materials : -- crystallize at melting temp,

T m

-- have abrupt change in spec.

vol. at

T m

Glass (amorphous solid) Crystalline (i.e., ordered)

T g T m

Adapted from Fig. 13.6,

Callister & Rethwisch 8e

. solid

T

• Glasses : -- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature ,

T g

-- transparent - no grain boundaries to scatter light Chapter 13 - 12

Glass Properties: Viscosity

• Viscosity, h : -- relates shear stress (  ) and velocity gradient

(dv/dy):

 glass  d

v

d

y

d

v

d

y

velocity gradient h  

dv

/

dy

h has units of (Pa-s) Chapter 13 - 13

Log Glass Viscosity vs. Temperature

• Viscosity decreases with

T

10 14 10 10 • • • soda-lime glass: 70% SiO 2 balance Na 2 O (soda) & CaO (lime) borosilicate (Pyrex): 13% B 2 O 3 , 3.5% Na 2 O, 2.5% Al 2 O 3 • Vycor: 96% SiO 2 , 4% B 2 O 3 fused silica: > 99.5 wt% SiO 2 strain point annealing point 10 6 10 2 1 200 Working range: glass-forming carried out 600 1000 1400 1800

T

melt

T

(ºC) Adapted from Fig. 13.7,

Callister & Rethwisch 8e

. (Fig. 13.7 is from E.B. Shand,

Engineering Glass

, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.) Chapter 13 - 14

Heat Treating Glass

• Annealing : -- removes internal stresses caused by uneven cooling.

• Tempering : -- puts surface of glass part into compression -- suppresses growth of cracks from surface scratches.

-- sequence: before cooling hot initial cooling cooler hot cooler at room temp.

compression tension compression -- Result: surface crack growth is suppressed.

Chapter 13 - 15

• Ceramic Fabrication techniques: -- glass forming (pressing, blowing, fiber drawing).

-- particulate forming (hydroplastic forming, slip casting, powder pressing, tape casting) -- cementation

Chapter 13 - 16

Ceramic Fabrication Methods (iia)

GLASS FORMING PARTICULATE FORMING CEMENTATION Hydroplastic forming: • Mill (grind) and screen constituents: desired particle size • Extrude this mass (e.g., into a brick)

Ao

force container ram billet container die holder extrusion

Ad

die • Dry and fire the formed piece Adapted from Fig. 12.8(c),

Callister & Rethwisch 8e

. Chapter 13 - 17

Ceramic Fabrication Methods (iia)

GLASS FORMING PARTICULATE FORMING CEMENTATION Slip casting: • Mill (grind) and screen constituents: desired particle size • Mix with water and other constituents to form slip • Slip casting operation pour slip into mold absorb water into mold “green ceramic” solid component pour slip into mold drain mold hollow component “green ceramic” Adapted from Fig. 13.12,

Callister & Rethwisch 8e

. (Fig. 13.12 is from W.D. Kingery,

Introduction to Ceramics

, John Wiley and Sons, Inc., 1960.) • Dry and fire the cast piece Chapter 13 - 18

Typical Porcelain Composition

(50%) 1. Clay (25%) 2. Filler – e.g. quartz (finely ground) (25%) 3. Fluxing agent (Feldspar) -- aluminosilicates plus K + , Na + , Ca + -- upon firing - forms low-melting-temp. glass Chapter 13 - 19

Hydroplasticity of Clay

• Clay is inexpensive • When water is added to clay -- water molecules fit in between layered sheets -- reduces degree of van der Waals bonding -- when external forces applied – clay particles free to move past one another – becomes hydroplastic • Structure of Kaolinite Clay: Adapted from Fig. 12.14,

Callister & Rethwisch 8e

. (Fig. 12.14 is adapted from W.E. Hauth, "Crystal Chemistry of Ceramics",

American Ceramic Society Bulletin

, Vol. 30 (4), 1951, p. 140.) charge neutral Shear charge neutral weak van der Waals bonding 4+ Si 3+ Al OH O 2 Shear Chapter 13 - 20

Drying and Firing

• Drying : as water is removed - interparticle spacings decrease – shrinkage .

Adapted from Fig. 13.13,

Callister & Rethwisch 8e

. (Fig. 13.13 is from W.D. Kingery,

Introduction to Ceramics

, John Wiley and Sons, Inc., 1960.) wet body partially dry completely dry Drying too fast causes sample to warp or crack due to non-uniform shrinkage • Firing : -- heat treatment between 900-1400 ºC - vitrification : liquid glass forms from clay and flux – flows between SiO 2 particles. (Flux lowers melting temperature).

Si0 2 particle (quartz) glass formed around the particle 70 m m Adapted from Fig. 13.14,

Callister & Rethwisch 8e

. (Fig. 13.14 is courtesy H.G. Brinkies, Swinburne University of Technology, Hawthorn Campus, Hawthorn, Victoria, Australia.) Chapter 13 - 21

Ceramic Fabrication Methods (iib)

GLASS FORMING PARTICULATE FORMING CEMENTATION Powder Pressing : used for both clay and non-clay compositions.

• Powder (plus binder) compacted by pressure in a mold - Uniaxial compression - compacted in single direction - Isostatic (hydrostatic) compression - pressure applied by - fluid - powder in rubber envelope Hot pressing - pressure + heat ( Chapter 13 - 22

Sintering

Sintering occurs during firing of a piece that has been powder pressed -- powder particles coalesce and reduction of pore size Aluminum oxide powder: -- sintered at 1700 ºC for 6 minutes.

15

m

m

Adapted from Fig. 13.16,

Callister & Rethwisch 8e

. Adapted from Fig. 13.17,

Callister & Rethwisch 8e

. (Fig. 13.17 is from W.D. Kingery, H.K. Bowen, and D.R. Uhlmann,

Introduction to Ceramics

, 2nd ed., John Wiley and Sons, Inc., 1976, p. 483.) Chapter 13 - 23

Tape Casting

• Thin sheets of green ceramic cast as flexible tape • Used for integrated circuits and capacitors • Slip = suspended ceramic particles + organic liquid (contains binders, plasticizers) Fig. 13.18,

Callister & Rethwisch 8e

. Chapter 13 - 24

Ceramic Fabrication Methods (iii)

GLASS FORMING PARTICULATE FORMING CEMENTATION • Hardening of a paste – paste formed by mixing cement material with water • Formation of rigid structures having varied and complex shapes • Hardening process – hydration (complex chemical reactions involving water and cement particles) • Portland cement – production of: -- mix clay and lime-bearing minerals - calcine (heat to 1400ºC) -- grind into fine powder Chapter 13 - 25

Summary

• Categories of ceramics: -- glasses -- clay products -- refractories -- cements -- advanced ceramics • Ceramic Fabrication techniques: -- glass forming (pressing, blowing, fiber drawing).

-- particulate forming (hydroplastic forming, slip casting, powder pressing, tape casting) -- cementation • Heat treating procedures -- glasses —annealing, tempering -- particulate formed pieces —drying, firing (sintering) Chapter 13 - 26

Reading:

ANNOUNCEMENTS

Core Problems: Self-help Problems:

Chapter 13 - 27