IE 337: Materials & Manufacturing Processes

Chapters 7, 12, 16 & 17

Lecture 13:

Ceramics, Glass and Powder Processing

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This Time

Ceramics Glass Processing Powder Processing: Ceramics and Metals Homework #5 on Thursday (2/25/10)

Ceramics

General properties Hard High wear resistance Brittle High compressive strength High elastic modulus High temperature resistance Good creep resistance Low conductivity Low thermal expansion Good chemical inertness

Glasses Clay products

Refractories Abrasives Cements Advanced ceramics

-optical -composite reinforce -containers/ -household

-whiteware -bricks

-bricks for high T (furnaces)

-sandpaper -cutting -polishing

-composites -structural

engine -rotors -valves -bearings

-sensors

Ceramics: Classification

Al2O3-SiO2 Si3N4

ZrO2

SiC

BN

Al2O3

AlN

WC

Diamond

ZrO2

Al2O3

Common Ceramics

Oxides: Al2O3, ZrO2

Nitrides: AlN, Si3N4, BN, TiN Carbides: WC, SiC, TiC, TaC Glasses: SiO2 + others Carbon: Graphite, Diamond

Processed as powders

15m

sinter

Whiteware Ceramics

Clay Quartz Feldspar

Processing Water addition, mixing Air removal Shaping Drying Coating Firing

Products Brick Structural Tile Drain / sewer pipe Decorative applications Bath / kitchen structures

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Refractory Material

Retain properties at high temperature Mechanical Chemical

Products Fire brick Insulating fibers Refractory linings Coatings

Silica Alumina Magnesium Oxide

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Abrasives

High hardness

Examples Silicon carbide Aluminum oxide Cubic boron nitride

Roughing Applications Grinding Cutting

Water-jet Sawing

Coatings

Super-Finishing Honing Lapping

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Glasses

Amorphous solid Vitreous (noncrystalline)

structure Amorphous Cooled to semi-solid

condition without crystallization

Subject to creep Silica Glass

Optical properties Thermal stability

Products Window glass Fiber optics Chemical containers Lenses

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Glass Ceramics

Crystalline solid 0.1 to 1.0 micron grains Use of nucleating agents

Glass Ceramic Efficient processing in

glassy state Net shape process Good mechanical

properties versus glass Low porosity Low thermal expansion Higher resistance to

thermal shock

Products Cookware Heat exchangers Missile radomes

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Cermets

Combination of metals & ceramics “Cemented” carbides Bound with high

temperature metal

Properties High hardness High temperature

resistance Improved toughness Improved strength Improved shock resistance

Applications Crucibles Jet nozzles High temperature brakes

Production Press powder in metal

mold Sintering in controlled

atmosphere

WC-Co

GLASS

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Shaping Methods for Glass

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

Glassworking: principal starting material is silica Usually combined with other oxide ceramics that form

glasses

Heated to transform it from a hard solid into a viscous liquid; it is then shaped into the desired geometry while in this fluid condition

When cooled and hard, the material remains in the amorphous state rather than crystallizing

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The typical process sequence in glassworking:

(1) preparation of raw materials and melting,

(2) shaping, and

(3) heat treatment

Glassworking Processes

Piece Ware Flat and Tubular Glass Glass Fibers

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Piece Ware Shaping Processes

Spinning – similar to centrifugal casting Pressing – for mass production of flat products

such as dishes, bake ware, and TV faceplates Blow forming – for production of smaller-mouth

containers such as beverage bottles and incandescent light bulbs

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

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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 spread molten glass on mold surface

Spinning

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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)

Pressing

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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

Blow Forming

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Casting

A low viscosity glass can be poured into a mold Uses: massive objects, such as astronomical

lenses and mirrors 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

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

Rolling

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

Float Process

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Forming of Glass Fibers

Products can be divided into 2 categories:

1. Discontinuous fibrous glass for 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

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Continuous glass fibers of small diameter are produced by pulling strands of molten glass through small orifices in a heated plate made of a platinum alloy

Drawing

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Heat Treatment

Annealing to eliminate stresses from temperature gradients Annealing temperatures are around 500C

followed by slow cooling

Tempering to make the glass more resistant to scratching and breaking due to compressive stresses on its surfaces Heating to a temperature above annealing,

followed by quenching of surfaces by air jets

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Finishing Operations

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

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)

Figure 16.1 A collection of powder metallurgy parts (photo courtesy of Dorst America, Inc.).

Powder Processing Parts

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Powder Processing

1. The Characterization of Engineering Powders

2. Production of Metallic Powders

3. Conventional Pressing and Sintering

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Powder Metallurgy (PM)

Metal processing technology in which parts are produced from metallic powders

Usual PM production sequence:1. Pressing - powders are compressed into desired shape to

produce green compact Accomplished in press using punch-and-die tooling

designed for the part

2. Sintering – green compacts are heated to bond the particles into a hard, rigid mass Performed at temperatures below the melting point of the

metal

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Why Powder Metallurgy is Important

PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining

PM process wastes very little material - ~ 97% of starting powders are converted to product

PM parts can be made with a specified level of porosity, to produce porous metal parts Examples: filters, oil‑impregnated bearings and

gears

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More Reasons Why PM is Important

Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy Tungsten filaments for incandescent lamp bulbs are made by

PM

Certain alloy combinations and cermets made by PM cannot be produced in other ways Non-equilibrium microstructures possible

PM compares favorably to most casting processes in dimensional control

PM production methods can be automated for economical production

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Engineering Powders

A powder can be defined as a finely divided particulate solid

Engineering powders include metals and ceramics

Geometric features of engineering powders: Particle size and distribution Particle shape and internal structure Surface area

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Measuring Particle Size

Most common method uses screens of different mesh sizes

Mesh count - refers to the number of openings per linear inch of screen A mesh count of 200 means there are 200 openings

per linear inch Since the mesh is square, the count is equal in both

directions, and the total number of openings per square inch is 2002 = 40,000

Higher mesh count = smaller particle size

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Screen Mesh

Figure 16.2 Screen mesh for sorting particle sizes.

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Particle Shapes in PM

Figure 16.3 Several of the possible (ideal) particle shapes in powder metallurgy.

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Observations

Smaller particle sizes generally show greater friction and steeper angles

Spherical shapes have the lowest interpartical friction

As shape deviates from spherical, friction between particles tends to increase

Easier flow of particles correlates with lower interparticle friction

Lubricants are often added to powders to reduce interparticle friction and facilitate flow during pressing

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Particle Density Measures

True density - density of the true volume of the material The density of the material if the powders were

melted into a solid mass

Bulk density - density of the powders in the loose state after pouring Because of pores between particles, bulk density is

less than true density

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Packing Factor

Bulk density divided by true density Typical values for loose powders range between

0.5 and 0.7 If powders of various sizes are present, smaller

powders will fit into spaces between larger ones, thus higher packing factor

Packing can be increased by vibrating the powders, causing them to settle more tightly

Pressure applied during compaction greatly increases packing of powders through rearrangement and deformation of particles

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Porosity

Ratio of volume of the pores (empty spaces) in the powder to the bulk volume

In principlePorosity + Packing factor = 1.0

The issue is complicated by possible existence of closed pores in some of the particles

If internal pore volumes are included in above porosity, then equation is exact

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Chemistry and Surface Films

Metallic powders are classified as either Elemental - consisting of a pure metal Pre-alloyed - each particle is an alloy

Possible surface films include oxides, silica, adsorbed organic materials, and moisture As a general rule, these films must be removed prior

to shape processing

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Production of Metallic Powders

In general, producers of metallic powders are not the same companies as those that make PM parts

Any metal can be made into powder form Three principal methods by which metallic

powders are commercially produced1. Atomization2. Chemical3. Electrolytic

In addition, mechanical methods are occasionally used to reduce powder sizes

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Coventional PM Sequence

Figure 16.7 Conventional powder metallurgy production sequence: (1) blending, (2) compacting, and (3) sintering; (a) shows the condition of the particles while (b) shows the operation and/or workpart during the sequence.42

Blending and Mixing of Powders

For successful results in compaction and sintering, the starting powders must be homogenized

Blending - powders of same chemistry but possibly different particle sizes are intermingled Different particle sizes are often blended to reduce

porosity

Mixing - powders of different elements/alloys are combined

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Compaction

Application of high pressure to the powders to form them into the required shape

Conventional compaction method is pressing, in which opposing punches squeeze the powders contained in a die

The workpart after pressing is called a green compact, the word green meaning not yet fully processed

The green strength of the part when pressed is adequate for handling but far less than after sintering

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Conventional Pressing in PM

Figure 16.9 Pressing in PM: (1) filling die cavity with powder by automatic feeder; (2) initial and (3) final positions of upper and lower punches during pressing, (4) part ejection.

Press for Conventional Pressing in PM

Figure 16.11 A 450 kN (50‑ton) hydraulic press for compaction of PM parts (photo courtesy of Dorst America, Inc.).

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Sintering

Heat treatment to bond the metallic particles, thereby increasing strength and hardness

Usually carried out at between 70% and 90% of the metal's melting point (absolute scale)

Generally agreed among researchers that the primary driving force for sintering is reduction of surface energy

Part shrinkage occurs during sintering due to pore size reduction

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Sintering Sequence

Figure 16.12 Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions.

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Sintering Cycle and Furnace

Figure 16.13 (a) Typical heat treatment cycle in sintering; and (b) schematic cross section of a continuous sintering furnace.

Limitations and Disadvantages

High costs High tooling and equipment costs Metallic powders are expensive Typically requires a unique material or geometry to justify

Problems in storing and handling metal powders Degradation over time, fire hazards with certain metals

Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing This is true for traditional punch and die

Variations in density throughout part may lead to yield issues especially for complex geometries

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Interparticle Friction and Powder Flow

Friction between particles affects ability of a powder to flow readily and pack tightly

A common test of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel

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Angle of Repose

Figure 16.4 Interparticle friction as indicated by the angle of repose of a pile of powders poured from a narrow funnel. Larger angles indicate greater interparticle friction.

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Powder Injection Molding

53shape

flow

dry/ debind

sinter (firing)

powder final

CERAMICS

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

Ceramics Processing

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Slip Casting

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

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, mold not drained

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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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SLIP CASTING

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Tape Casting

Polyester FilmCarrier

Slip

Dried Tape

Doctor Blade

Polyester Film Roll

Fabrication process for thin ceramic sheets

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Miniaturization of Complex Circuits

High Temperature Co-Fired Ceramic (HTCC) Low Temperature Co-Fired Ceramic (LTCC) Thick film metal traces are printed on several tape layers of ceramic and are

co-fired Tape layers are electrically connected through vias Significant miniaturization of circuit form factor with this technology

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Extrusion

Compression of clay through a die orifice to produce long sections of uniform cross‑section

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

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Extruder Sectional View

Components and features of a (single‑screw) extruder for plastics and elastomers

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Ceramic Extrusion: Examples

cordieritecatalytic converter

50 cells/cm2

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Powder Injection Molding (PIM)

Ceramic particles are mixed with a thermoplastic polymer, then heated and injected into a mold cavity. Polymer provides flow characteristics for molding

Mold-Filling Interactions

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Air trapWeld-line

Short shot

Flashing

Filler-polymer separation

Jetting

Die Pressing

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Semi-Dry Pressing

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

Joining

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Next Time

Chapter 30 & 31