1. Basic Concept of Ceramic Materials

Ceramic materials are generally inorganic, non-metallic materials formed by ionic or covalent bonding between metallic and non-metallic elements.

Representative constituent elements include:

• Aluminum

• Silicon

• Zirconium

• Magnesium

• Calcium

• Titanium

• Boron

• Carbon

• Nitrogen

• Oxygen

Ceramic materials commonly exist as oxides, nitrides, carbides, borides, and silicates.

Representative compounds include:

• Al₂O₃: Alumina

• ZrO₂: Zirconia

• SiC: Silicon carbide

• Si₃N₄: Silicon nitride

• AlN: Aluminum nitride

• BN: Boron nitride

• TiC: Titanium carbide

• B₄C: Boron carbide

• MgO: Magnesium oxide

• SiO₂: Silicon dioxide

Compared with metals, ceramic materials generally exhibit:

• High hardness

• High compressive strength

• Excellent wear resistance

• Excellent heat resistance

• High chemical stability

• Low density

• High electrical insulation

• Low ductility

• Low fracture toughness

• Vulnerability to impact and tensile loading

• High sensitivity to surface defects

Therefore, ceramics are not simply harder alternatives to metals. They are specialized engineering materials used where high temperature, wear, corrosion resistance, electrical insulation, thermal conductivity, or specific electrical functionality is required.

2. Bonding Structure of Ceramic Materials

2.1 Ionic Bonding

Ionic bonding is formed by electrostatic attraction between positive and negative ions.

Representative materials:

• Al₂O₃

• MgO

• ZrO₂

• CaO

Characteristics of ionic bonding:

• High bond energy

• High melting point

• High hardness

• Low electrical conductivity

• High brittleness

When atomic planes attempt to slip in an ionic crystal, ions with the same charge approach one another and generate strong repulsive forces.

As a result, dislocations cannot move as easily as in metals, and plastic deformation is highly restricted.

2.2 Covalent Bonding

Covalent bonding is formed when atoms share electrons.

Representative materials:

• SiC

• Si₃N₄

• BN

• B₄C

• Diamond

Characteristics of covalent bonding:

• Very high bond strength

• High hardness

• High heat resistance

• Low thermal expansion

• Excellent chemical stability

• Low ductility

Because covalent bonds are strongly directional, atomic movement within the crystal is difficult, and cracking is more likely than plastic deformation.

2.3 Mixed Bonding

Most practical ceramics do not consist entirely of pure ionic or pure covalent bonding.

For example, alumina exhibits both ionic and covalent bonding characteristics.

The relative proportion of these bonding types influences:

• Hardness

• Thermal conductivity

• Electrical insulation

• Fracture toughness

• Chemical stability

• Sinterability

3. Basic Classification of Ceramic Materials

Ceramic materials can generally be classified as follows.

3.1 Traditional Ceramics

These materials are based mainly on natural minerals or silicates.

Representative materials:

• Clay

• Porcelain

• Brick

• Tile

• Refractory materials

• Glass

• Cement

3.2 Structural Fine Ceramics

These are high-purity ceramics used for components requiring mechanical strength, hardness, wear resistance, and heat resistance.

Representative materials:

• Alumina

• Zirconia

• Silicon carbide

• Silicon nitride

• Boron carbide

• Boron nitride

• Titanium carbide

3.3 Functional Ceramics

These ceramics are used for electrical, magnetic, optical, piezoelectric, dielectric, or thermal-conduction functions.

Representative materials:

• Barium titanate

• PZT

• Ferrite

• Aluminum nitride

• Zinc oxide

• Transparent ceramics

• Solid electrolytes

3.4 Ceramic Composite Materials

These materials combine two or more ceramics or combine ceramics with other materials.

Representative types:

• Zirconia-toughened alumina

• Alumina-toughened zirconia

• Silicon-carbide-fiber-reinforced ceramics

• Carbon-fiber-reinforced silicon carbide

• Metal-ceramic composites

• Cemented carbide

4. General Composition of Ceramic Materials

Ceramic products are not composed only of the principal ceramic phase.

They generally contain:

1. Primary ceramic powder

2. Sintering aids

3. Binders

4. Dispersants

5. Plasticizers

6. Lubricants

7. Pore-forming agents

8. Stabilizers

9. Reinforcing phases

10. Impurities

4.1 Representative Composition Ranges

Binders and plasticizers are mostly removed during debinding before final sintering.

5. Relationship Between Ceramic Microstructure and Properties

5.1 Grain Size

Ceramics are often composed of many crystal grains.

As grain size decreases, the following generally improve:

• Strength

• Hardness

• Resistance to crack propagation

• Surface finish

• Potential sintered density

However, if the grains are excessively fine:

• Abnormal grain growth may occur during sintering.

• Grain-boundary phase volume may increase.

• High-temperature creep may increase.

• Thermal conductivity may decrease.

5.2 Grain Boundaries

A grain boundary is the interface between adjacent crystal grains.

Grain boundaries may contain:

• Impurities

• Glassy phases

• Sintering additives

• Secondary phases

• Fine pores

If the grain boundaries are weak, cracks may propagate along them.

A large amount of grain-boundary glass phase may result in:

• Improved sinterability

• Reduced high-temperature strength

• Increased creep

• Reduced thermal conductivity

• Reduced chemical stability

5.3 Porosity

Porosity is one of the main defects that reduces ceramic strength.

As porosity increases:

• Strength decreases.

• Elastic modulus decreases.

• Thermal conductivity decreases.

• Density decreases.

• Impact resistance decreases.

• Fluid permeability increases.

However, high porosity is intentionally used in filters, thermal insulators, and catalyst supports.

5.4 Secondary Phases

Dispersing another phase inside the ceramic can modify its properties.

Examples:

• Zirconia dispersed in alumina

• Glass-forming sintering aids in silicon nitride

• Boron and carbon added to silicon carbide

• Metal particles dispersed in a ceramic matrix

Secondary phases may provide:

• Crack deflection

• Crack bridging

• Transformation toughening

• Grain-growth control

• Thermal-conductivity adjustment

• Electrical conductivity

6. Alumina — Al₂O₃

6.1 Basic Composition

Alumina is a representative oxide ceramic composed mainly of aluminum oxide.

Theoretical elemental mass ratio:

• Aluminum: Approximately 52.9%

• Oxygen: Approximately 47.1%

6.2 Typical Composition by Grade

85–92% Alumina

Characteristics:

• Relatively low cost

• Easy to sinter

• Used for insulation and wear-resistant parts

• Lower strength and chemical resistance than high-purity alumina

95–96% Alumina

Characteristics:

• Balanced mechanical performance and cost

• Electrical insulation parts

• Wear-resistant tiles

• General industrial ceramic components

99% or Higher Alumina

Characteristics:

• High corrosion resistance

• High electrical insulation

• High heat resistance

• Suitable for high-purity processes

• Higher sintering and machining cost

6.3 Properties

Advantages:

• High hardness

• High compressive strength

• Excellent wear resistance

• Excellent electrical insulation

• Relatively high thermal conductivity

• Excellent chemical stability

• Stable raw-material supply

• Relatively economical among fine ceramics

Limitations:

• Low fracture toughness

• Vulnerable to impact

• Low tensile strength

• Limited thermal-shock resistance

• Difficult machining

• Sensitive to edge chipping

6.4 Main Applications

• Electrical insulators

• Ceramic substrates

• Pump seals

• Valve seats

• Nozzles

• Wear liners

• Bearing components

• Semiconductor-equipment components

• Medical components

• Wear-resistant parts in food machinery

7. Zirconia — ZrO₂

7.1 Basic Composition

Zirconia is a high-toughness oxide ceramic based on zirconium oxide.

Pure zirconia changes crystal phase with temperature.

• Monoclinic phase

• Tetragonal phase

• Cubic phase

Because these phase changes involve significant volume changes, stabilizers are required.

7.2 Representative Stabilized Compositions

3Y-TZP

3Y-TZP is yttria-stabilized tetragonal zirconia polycrystal.

8YSZ

8YSZ stabilizes the cubic phase and is used as an oxygen-ion conductor.

Mg-PSZ

7.3 Transformation Toughening

Tetragonal zirconia grains may transform into the monoclinic phase under stress at a crack tip.

This transformation causes a volume increase and generates compressive stress around the crack tip.

As a result:

• Crack propagation is suppressed.

• Fracture toughness increases.

• Flexural strength increases.

• Impact resistance improves.

This phenomenon is known as transformation toughening.

7.4 Properties

Advantages:

• High fracture toughness among ceramics

• High flexural strength

• Excellent wear resistance

• Low thermal conductivity

• Can be polished to a smooth surface

• Biocompatibility

• Oxygen-ion-conducting grades available

Limitations:

• Lower hardness than alumina

• Phase-stability issues at high temperature

• Possible low-temperature degradation

• Moisture- and stress-assisted degradation

• Low thermal conductivity may cause localized heat accumulation

• Relatively high cost

7.5 Main Applications

• Ceramic bearings

• Precision valves

• Cutting blades

• Pump components

• Dental materials

• Oxygen sensors

• Fuel cells

• Precision guides

• Wear-resistant components

8. Zirconia-Toughened Alumina — ZTA

8.1 Basic Composition

ZTA is a ceramic composite in which zirconia particles are dispersed in an alumina matrix.

Representative composition:

8.2 Properties

• Higher fracture toughness than alumina

• Retains high hardness

• Excellent wear resistance

• Better dimensional stability than zirconia

• Toughened through crack deflection and phase transformation

• Higher cost

• Requires precise sintering control

8.3 Main Applications

• Medical components

• Wear liners

• Cutting components

• Valves

• Pump seals

• High-load ceramic components

9. Silicon Carbide — SiC

9.1 Basic Composition

Silicon carbide is a non-oxide ceramic in which silicon and carbon are strongly covalently bonded.

9.2 Representative Grades

Reaction-Bonded Silicon Carbide

Representative composition:

• SiC: Approximately 80–95 wt.%

• Residual Si: Approximately 5–20 wt.%

Characteristics:

• Low dimensional change during processing

• Complex shapes can be produced

• Relatively low manufacturing cost

• Residual silicon may limit high-temperature or corrosive-environment performance

Pressureless-Sintered Silicon Carbide

Representative composition:

• SiC: Approximately 98–99.5 wt.% or higher

• B and C sintering aids: Approximately 0.1–2 wt.%

Characteristics:

• High corrosion resistance

• High heat resistance

• High hardness

• Difficult to sinter

Recrystallized Silicon Carbide

Characteristics:

• High porosity

• Excellent thermal-shock resistance

• Used for high-temperature structural parts and kiln furniture

9.3 Properties

Advantages:

• Very high hardness

• High thermal conductivity

• Low thermal expansion

• Excellent thermal-shock resistance

• High wear resistance

• High corrosion resistance

• Maintains strength at elevated temperature

• Low density

Limitations:

• Low fracture toughness

• Extremely difficult machining

• Vulnerable to impact

• Difficult sintering

• High cost for complex geometry

• Surface oxidation may occur in oxidizing atmospheres

9.4 Main Applications

• Mechanical seals

• Pump bearings

• Heat exchangers

• Semiconductor-equipment components

• Heater components

• High-temperature nozzles

• Wear liners

• Ballistic armor

• Abrasives

• Kiln furniture

10. Silicon Nitride — Si₃N₄

10.1 Basic Composition

Silicon nitride is a non-oxide ceramic composed of silicon and nitrogen.

10.2 Representative Composition

Because silicon nitride has poor intrinsic sinterability, sintering aids are commonly used.

Sintering aids form a grain-boundary glass phase and promote densification.

10.3 Microstructure

During sintering, the α phase may transform into elongated β-phase grains.

These elongated β-Si₃N₄ grains contribute to:

• Crack deflection

• Crack bridging

• Grain pullout

• Increased fracture toughness

10.4 Properties

Advantages:

• High fracture toughness

• High flexural strength

• Excellent thermal-shock resistance

• Low thermal expansion

• Low density

• Excellent fatigue resistance

• High elevated-temperature strength

• Good wear resistance

Limitations:

• High manufacturing cost

• Grain-boundary glass may increase high-temperature creep

• Difficult machining

• Possible long-term oxidation degradation

• Complex sintering process

10.5 Main Applications

• Ceramic bearing balls

• Turbocharger rotors

• Engine components

• Cutting tools

• High-temperature rollers

• Welding jigs

• Semiconductor components

• High-speed rotating parts

11. Aluminum Nitride — AlN

11.1 Basic Composition

Aluminum nitride combines high thermal conductivity with high electrical insulation.

11.2 Representative Composition

Oxygen impurities can significantly reduce thermal conductivity.

11.3 Properties

Advantages:

• Very high thermal conductivity

• Excellent electrical insulation

• Thermal expansion similar to silicon

• High heat resistance

• Low dielectric loss

• Suitable for semiconductor substrates

Limitations:

• Can react with moisture

• Hydrolysis must be considered

• High raw-material cost

• Complex sintering process

• Difficult machining

• Surface oxidation may occur

11.4 Main Applications

• Semiconductor heat-dissipation substrates

• Power-module substrates

• LED thermal-management substrates

• Electrically insulating heat sinks

• High-frequency electronic components

• Vacuum-equipment components

12. Boron Nitride — BN

12.1 Basic Composition

Boron nitride is a ceramic composed of boron and nitrogen.

Representative structures:

• Hexagonal boron nitride

• Cubic boron nitride

12.2 Hexagonal Boron Nitride

Its layered structure is similar to graphite.

Properties:

• Low coefficient of friction

• Excellent high-temperature lubricity

• High electrical insulation

• Good thermal conductivity

• Poor wettability by molten metals

• Machinable

• Low strength

• Limited high-temperature oxidation resistance

Main applications:

• High-temperature lubricants

• Molten-metal nozzles

• Electrical insulation parts

• High-temperature jigs

• Mold-release coatings

• Semiconductor components

12.3 Cubic Boron Nitride

Properties:

• Hardness second only to diamond

• Suitable for machining ferrous metals

• High thermal stability

• High wear resistance

Main applications:

• Cutting tools

• Abrasives

• Machining of hardened materials

• Ultra-precision machining

13. Boron Carbide — B₄C

13.1 Basic Composition

Boron carbide is an ultra-hard ceramic composed of boron and carbon.

13.2 Properties

• Extremely high hardness

• Very low density

• High wear resistance

• Excellent neutron absorption

• High chemical stability

• Very low fracture toughness

• Vulnerable to impact

• Extremely difficult machining

• Difficult sintering

13.3 Main Applications

• Ballistic armor

• Nozzles

• Blasting components

• Abrasives

• Nuclear-reactor control materials

• Wear liners

14. Barium Titanate — BaTiO₃

14.1 Basic Composition

Barium titanate is a representative dielectric and ferroelectric ceramic.

14.2 Properties

• High dielectric constant

• Ferroelectric behavior

• Piezoelectric behavior

• Temperature-dependent phase transformation

• Dielectric properties can be adjusted by composition

• Vulnerable to mechanical impact

14.3 Main Applications

• Multilayer ceramic capacitors

• Dielectric components

• Piezoelectric elements

• Sensors

• Actuators

• Temperature-compensation devices

15. PZT Piezoelectric Ceramics

15.1 Basic Composition

PZT is a piezoelectric ceramic composed of lead, zirconium, and titanium.

Representative formula:

• Pb(Zr,Ti)O₃

Representative composition:

The zirconium-to-titanium ratio controls piezoelectric and dielectric properties.

15.2 Properties

• High piezoelectric coefficient

• High electromechanical coupling coefficient

• High dielectric constant

• Fast response

• Brittle

• Limited temperature stability

• Environmental regulation required because of lead content

15.3 Main Applications

• Ultrasonic transducers

• Piezoelectric actuators

• Pressure sensors

• Accelerometers

• Precision positioning

• Ultrasonic cleaners

• Medical ultrasound elements

16. Ferrites

16.1 Basic Composition

Ferrites are magnetic ceramics composed of iron oxide and other metal oxides.

Representative types:

• Mn-Zn ferrite

• Ni-Zn ferrite

• Barium ferrite

• Strontium ferrite

16.2 Representative Composition

Mn-Zn Ferrite

• Fe₂O₃

• MnO

• ZnO

Typical composition:

Ni-Zn Ferrite

16.3 Properties

• High electrical resistivity

• Low eddy-current loss

• Magnetic functionality

• Good high-frequency performance

• Brittle

• Low tensile strength

• Sensitive to impact

16.4 Main Applications

• Transformer cores

• Inductors

• EMI filters

• Antenna cores

• Permanent magnets

• Sensors

• Motor magnets

17. Glass

17.1 Basic Composition

Glass is an amorphous inorganic material without long-range crystalline order.

Representative soda-lime glass composition:

Representative borosilicate-glass composition:

17.2 Properties

• Transparency

• Electrical insulation

• Corrosion resistance

• Smooth surface

• Low tensile strength

• Sensitivity to surface scratches

• Brittleness

• Thermal-shock performance depends on composition

17.3 Main Applications

• Protective windows

• Observation windows

• Optical components

• Laboratory equipment

• Sensor covers

• Observation windows in food-processing equipment

• Electrical insulation parts

18. Refractory Ceramics

Refractories are materials that retain shape and strength at high temperature.

Representative types:

• Alumina refractories

• Silica refractories

• Magnesia refractories

• Silicon-carbide refractories

• Zirconia refractories

18.1 Alumina Refractories

Representative composition:

• Al₂O₃: Approximately 45–99 wt.%

• SiO₂: Balance

• Other oxides: Small amount

Applications:

• Kilns

• Furnaces

• High-temperature piping

• Burner components

18.2 Magnesia Refractories

Representative composition:

• MgO: Approximately 80–98 wt.%

• CaO, SiO₂, Fe₂O₃: Small amount

Characteristics:

• Good resistance to basic slags

• High-temperature stability

• Limited thermal-shock resistance

19. Porous Ceramics

Porous ceramics are intentionally manufactured with internal pores.

Typical porosity:

• Approximately 10–90%

Methods of forming pores:

• Addition of pore-forming agents

• Foaming

• Freeze casting

• Partial sintering

• Template methods

Properties:

• Low density

• Low thermal conductivity

• High specific surface area

• Fluid permeability

• Low mechanical strength

Main applications:

• Filters

• Catalyst supports

• Thermal insulation

• Sound absorbers

• Vacuum chucks

• Gas diffusers

• Molten-metal filters

20. Ceramic Composite Materials

20.1 Ceramic-Matrix Composites

These materials contain fibers or particles dispersed in a ceramic matrix.

Representative systems:

• SiC fiber/SiC matrix

• Carbon fiber/SiC matrix

• Al₂O₃ fiber/Al₂O₃ matrix

Properties:

• Crack bridging

• Crack deflection

• Improved fracture toughness

• Improved thermal-shock resistance

• High-temperature strength

• High manufacturing cost

20.2 Cermets

Cermets combine ceramic and metallic phases.

Representative examples:

• TiC-Ni

• TiCN-Ni

• WC-Co

The ceramic phase provides hardness and wear resistance, while the metallic phase provides toughness and bonding strength.

21. Cemented Carbide — WC-Co

21.1 Basic Composition

Cemented carbide consists of tungsten-carbide particles bonded by a cobalt matrix.

Representative composition:

21.2 Relationship Between Composition and Properties

Increasing WC Content

• Increases hardness

• Increases wear resistance

• Increases compressive strength

• May reduce toughness

Increasing Co Content

• Increases fracture toughness

• Increases impact resistance

• Reduces hardness

• Reduces wear resistance

TiC, TaC, and NbC

• Improve hot hardness

• Suppress diffusion wear

• Adjust cutting performance

• Modify thermal-shock behavior

21.3 Main Applications

• Cutting tools

• Dies

• Drills

• Wear-resistant nozzles

• Punches

• Rolls

• Mining tools

• High-pressure components

22. Mechanical Properties of Ceramics

22.1 Hardness

Ceramics have high hardness because their atomic bonds are strong and dislocation movement is difficult.

As hardness increases:

• Wear resistance generally increases.

• Cutting-tool life increases.

• Abrasiveness increases.

• Machining cost increases.

• Risk of brittle fracture may increase.

22.2 Compressive Strength

Ceramics are very strong under compression.

Reasons:

• Cracks tend to close under compressive stress.

• Atomic bonds are strong.

• Large compressive loads can be supported without plastic deformation.

However, under tension and bending, cracks open and fracture occurs more easily.

22.3 Tensile and Flexural Strength

Because direct tensile testing of ceramics is difficult, flexural strength is often used instead.

Factors affecting flexural strength include:

• Surface roughness

• Surface cracks

• Internal porosity

• Grain size

• Specimen size

• Machining direction

• Test speed

• Temperature

22.4 Fracture Toughness

Fracture toughness represents resistance to crack growth.

General tendencies:

• Zirconia: Relatively high

• Silicon nitride: Medium to high

• Alumina: Medium to low

• Silicon carbide: Low

• Boron carbide: Very low

In ceramics, fracture toughness may be more important than strength when evaluating impact resistance and defect sensitivity.

22.5 Weibull Statistics

Ceramic strength varies statistically because it depends on the size and distribution of defects.

The Weibull modulus represents the degree of strength scatter.

• High Weibull modulus: Low strength variation

• Low Weibull modulus: High strength variation

Design should consider reliability and probability of failure rather than using only average strength.

22.6 Fatigue

Ceramics can also fail under cyclic loading.

Fatigue is accelerated especially under:

• Moisture

• High temperature

• Repeated tensile loading

• Surface defects

• Corrosive fluids

Moisture may weaken atomic bonds at the crack tip and cause slow crack growth.

23. Thermal Properties

23.1 Heat Resistance

Ceramics generally have higher heat resistance than metals and polymers.

However, actual service temperature may be limited by:

• Oxidation

• Phase transformation

• Grain-boundary glass

• Thermal shock

• High-temperature creep

• Chemical reaction

• Metal joints

23.2 Thermal Conductivity

Thermal conductivity varies greatly among ceramics.

High-thermal-conductivity materials:

• AlN

• SiC

• BeO

• High-purity Al₂O₃

Low-thermal-conductivity materials:

• ZrO₂

• Porous ceramics

• Glass

• Refractories

23.3 Thermal Expansion

Ceramics with low thermal-expansion coefficients show low thermal deformation.

However, when joined to metals, thermal-expansion mismatch can generate stress.

Possible problems:

• Joint cracking

• Delamination

• Warpage

• Fracture during cooling

• Thermal-cycle fatigue

23.4 Thermal Shock

Thermal shock is cracking caused by rapid temperature change.

Thermal-shock resistance improves with:

• Low thermal-expansion coefficient

• High thermal conductivity

• High fracture toughness

• Low elastic modulus

• Small section thickness

• Gradual geometric transitions

Materials with good thermal-shock resistance:

• SiC

• Si₃N₄

• Some BN grades

• Cordierite

Materials with relatively poor thermal-shock resistance:

• General alumina

• Some zirconia grades

• Ordinary glass

24. Electrical Properties

24.1 Electrical Insulation

Many ceramics contain few free electrons and therefore provide high electrical insulation.

Representative insulating materials:

• Al₂O₃

• AlN

• Si₃N₄

• Glass

• BN

24.2 Dielectric Properties

Dielectric ceramics can store electrical energy in an electric field.

Representative materials:

• BaTiO₃

• TiO₂

• Al₂O₃

• Glass-ceramics

Key evaluation items:

• Dielectric constant

• Dielectric loss

• Dielectric breakdown strength

• Temperature coefficient

• Frequency characteristics

24.3 Ionic Conductivity

In some ceramics, ions move more readily than electrons.

Representative materials:

• Stabilized zirconia

• Lithium-based solid electrolytes

• Beta-alumina

Main applications:

• Oxygen sensors

• Fuel cells

• Solid-state batteries

• Sodium-ion conductors

25. Ceramic Manufacturing Processes

25.1 Powder Production

Main methods:

• Milling

• Precipitation

• Sol-gel processing

• Pyrolysis

• Vapor-phase synthesis

• Combustion synthesis

Important powder characteristics:

• Particle size

• Particle-size distribution

• Particle shape

• Purity

• Surface area

• Agglomeration state

25.2 Mixing and Dispersion

The following may be added to ceramic powder:

• Binder

• Plasticizer

• Dispersant

• Sintering aid

• Lubricant

• Solvent

Poor dispersion may cause:

• Porosity

• Strength variation

• Warpage

• Density variation

• Cracking

• Abnormal grain growth

25.3 Forming

Representative forming methods:

• Uniaxial pressing

• Cold isostatic pressing

• Slip casting

• Tape casting

• Extrusion

• Injection molding

• Gel casting

• Additive manufacturing

25.4 Debinding

Debinding removes organic binders and plasticizers from the green body.

If debinding is too rapid, it may cause:

• Internal pressure buildup

• Cracking

• Swelling

• Delamination

• Porosity

25.5 Sintering

Sintering heats a powder compact below its melting point to promote particle bonding and densification.

During sintering:

• Particle necks grow.

• Pores shrink.

• Density increases.

• Grains grow.

• Shrinkage occurs.

• Phase transformation may occur.

Representative sintering methods:

• Pressureless sintering

• Reaction bonding

• Hot pressing

• Hot isostatic pressing

• Spark-plasma sintering

• Liquid-phase sintering

25.6 Post-Machining

Because sintered ceramics are extremely hard, diamond tools are commonly used.

Post-machining methods:

• Diamond grinding

• Lapping

• Polishing

• Laser machining

• Ultrasonic machining

• Electrical-discharge machining

• Waterjet cutting

26. Ceramic Machining and Design Principles

26.1 Avoid Sharp Corners

Ceramics are highly sensitive to stress concentration.

Countermeasures:

• Provide sufficient fillets.

• Use chamfers.

• Avoid abrupt section changes.

• Apply radii at groove ends.

26.2 Prefer Compression over Tension

Ceramic components should be designed so that compressive stress predominates wherever possible.

Structures to avoid:

• Thin cantilevers

• Eccentric tension

• Local bending

• Point contact

• Forced interference fits

26.3 Maintain Uniform Thickness

Abrupt thickness changes can cause cracking due to sintering shrinkage and thermal stress.

26.4 Fastening Design

Directly threading ceramic parts is generally limited.

Alternatives include:

• Metal inserts

• Adhesive bonding

• Clamping

• Compression fastening

• Brazing

• Soldering after metallization

• Flange structures

26.5 Contact Stress

Ceramics are vulnerable to high local contact stress caused by point contact.

Countermeasures:

• Increase contact area.

• Use an elastic intermediate layer.

• Reduce alignment error.

• Improve surface finish.

• Prevent edge contact.

27. Joining Ceramics to Metals

27.1 Mechanical Fastening

• Bolts

• Clamps

• Shrink fits

• Flanges

Considerations:

• Thermal-expansion mismatch

• Local stress

• Assembly tolerance

• Plastic deformation of metal components

27.2 Adhesive Bonding

Advantages:

• Stress distribution

• Dissimilar-material joining

• Suitability for complex geometry

Limitations:

• Temperature limits

• Chemical exposure

• Long-term creep

• Surface preparation requirements

27.3 Brazing

A metal layer may be formed on the ceramic surface, or an active-metal braze may be used.

Representative active metals:

• Ti

• Zr

Applications:

• Vacuum components

• Electronic packages

• Sensors

• Electrodes

27.4 Metallization

A metallic layer is formed on the ceramic surface.

Representative methods:

• Mo-Mn metallization

• Thin-film deposition

• Thick-film printing

• Electroless plating

• Sputtering

28. Friction and Wear

Ceramics often have excellent wear resistance because of their high hardness, but they are not automatically suitable for every tribological application.

Items to consider:

• Mating material

• Lubrication

• Surface roughness

• Contact pressure

• Speed

• Temperature

• Particle contamination

• Thermal shock

• Microcracking

Representative tribological ceramics:

• SiC

• Al₂O₃

• Si₃N₄

• ZrO₂

• BN

29. Corrosion and Chemical Stability

Ceramics are generally corrosion resistant, but no ceramic is stable in every chemical environment.

Examples:

• Alumina may be attacked by prolonged exposure to strong acids or alkalis.

• SiC may oxidize in some high-temperature oxidizing atmospheres.

• AlN is sensitive to moisture.

• Zirconia may undergo low-temperature degradation under moisture and stress.

• Glass is vulnerable to hydrofluoric acid.

• Silicon nitride may oxidize at high temperature.

Therefore, exact chemical concentration and temperature must be considered.

30. Ceramic Materials in Food Machinery

30.1 Applicable Materials

• Alumina

• Zirconia

• Silicon carbide

• Silicon nitride

• Glass

• Ceramic coatings

30.2 Main Components

• Mechanical seals

• Valve seats

• Cutting blades

• Pump components

• Wear-resistant nozzles

• Grinding components

• Precision guides

• Observation windows

• Wear liners

30.3 Advantages

• Corrosion resistance

• Low metal-ion release

• High wear resistance

• Resistance to cleaning chemicals

• Suitability for high-temperature cleaning

• Potential for non-stick surfaces

• Hygienic surface finishing

30.4 Considerations

• Sharp fragments may form if fracture occurs.

• Metal detection is difficult.

• Impact loading must be controlled.

• Microcrack inspection may be required.

• Food-contact compliance must be verified.

• Surface porosity must be controlled.

• Thermal shock during cleaning must be reviewed.

• Coating delamination may occur.

31. Ceramics in Semiconductor and Electrical Equipment

31.1 Representative Materials

• Al₂O₃

• AlN

• SiC

• Si₃N₄

• Quartz

• BN

31.2 Main Applications

• Insulating substrates

• Electrostatic chucks

• Heaters

• Wafer supports

• Insulating bushings

• Vacuum components

• Thermal-management components

• Sensor housings

31.3 Main Requirements

• High purity

• Low particle generation

• Low outgassing

• High electrical insulation

• High thermal conductivity

• Plasma resistance

• Dimensional stability

• Precision surfaces

32. Comparison of Major Ceramic Materials

33. Ceramic Selection by Component

33.1 Mechanical Seals

Recommended materials:

• SiC

• Al₂O₃

• ZrO₂

Selection criteria:

• High wear and high speed: SiC

• General corrosion resistance: Al₂O₃

• Impact resistance and precision contact: ZrO₂

33.2 Ceramic Bearings

Recommended materials:

• Si₃N₄

• ZrO₂

• Al₂O₃

Selection criteria:

• High speed and low weight: Si₃N₄

• High toughness and low noise: ZrO₂

• Cost and electrical insulation: Al₂O₃

33.3 Insulation Components

Recommended materials:

• Al₂O₃

• AlN

• BN

• Glass

Selection criteria:

• General insulation: Al₂O₃

• Heat dissipation and insulation: AlN

• High temperature and machinability: BN

• Transparent insulation: Glass

33.4 High-Temperature Structural Components

Recommended materials:

• SiC

• Si₃N₄

• Al₂O₃

• Zirconia

Selection criteria:

• Highest thermal conductivity and oxidation resistance: SiC

• Thermal shock and toughness: Si₃N₄

• Economical high-temperature insulation: Al₂O₃

• Thermal insulation and toughness: ZrO₂

33.5 Cutting Tools

Recommended materials:

• Al₂O₃

• Si₃N₄

• cBN

• WC-Co

• TiCN cermet

34. Drawing and Material-Specification Designation

It is not sufficient to specify only “ceramic” or “alumina.”

The following should also be specified.

34.1 Material Name and Purity

Examples:

• Al₂O₃ 99.5%

• 3Y-TZP

• Si₃N₄, gas-pressure sintered

• SiC, pressureless sintered

• AlN 99%

34.2 Sintering Method

• Pressureless sintering

• Reaction bonding

• Hot pressing

• Hot isostatic pressing

• Recrystallization

• Liquid-phase sintering

34.3 Density and Porosity

Examples:

• Relative density: 98% minimum

• Open porosity: 0.1% maximum

• Porosity: 30 ±5%

34.4 Mechanical Properties

• Flexural strength

• Compressive strength

• Fracture toughness

• Hardness

• Elastic modulus

• Weibull modulus

34.5 Thermal Properties

• Thermal conductivity

• Coefficient of thermal expansion

• Maximum service temperature

• Thermal-shock condition

34.6 Surface and Dimensional Requirements

• Surface roughness

• Flatness

• Parallelism

• Edge chamfer

• Crack-acceptance criteria

• Surface-defect criteria

34.7 Inspection Requirements

• Fluorescent penetrant inspection

• Ultrasonic inspection

• X-ray inspection

• Microscopic inspection

• Dimensional inspection

• Leak testing

35. Ceramic Material Selection Procedure

35.1 Confirm the Function

• Is it a wear-resistant component?

• Is it an electrical-insulation component?

• Is heat dissipation required?

• Is it a high-temperature structural component?

• Is piezoelectric or magnetic functionality required?

• Is it exposed to chemical corrosion?

• Does it contact food?

35.2 Confirm the Load

• Compression

• Tension

• Bending

• Impact

• Cyclic loading

• Contact loading

• Thermal stress

35.3 Confirm the Temperature

• Continuous service temperature

• Maximum temperature

• Rate of temperature change

• Thermal shock

• Oxidizing atmosphere

• Vacuum

• Thermal cycling

35.4 Confirm the Environment

• Water

• Steam

• Acids

• Alkalis

• Molten metals

• Plasma

• Vacuum

• Radiation

• Abrasive particles

35.5 Confirm Manufacturability and Cost

• Forming method

• Sintering shrinkage

• Post-machining

• Required precision

• Available stock shape

• Minimum order quantity

• Inspection cost

• Expected breakage rate

35.6 Confirm Reliability

• Defect distribution

• Weibull modulus

• Surface damage

• Slow crack growth

• Fatigue

• Thermal cycling

• Life testing

36. Final Summary

Ceramic materials are not simply hard and brittle materials.

Their actual performance is determined by a combination of:

• Chemical composition

• Relative proportion of ionic and covalent bonding

• Raw-material purity

• Particle size

• Grain size

• Porosity

• Sintering aids

• Grain-boundary phases

• Sintering method

• Surface finishing

• Defect distribution

• Service temperature and environment

Alumina is a representative structural ceramic that offers a good balance of hardness, wear resistance, electrical insulation, and cost.

Zirconia achieves high fracture toughness and flexural strength through transformation toughening and is suitable for precision machinery and medical components.

Silicon carbide provides extremely high hardness, thermal conductivity, low thermal expansion, and excellent corrosion resistance and is widely used in mechanical seals and high-temperature components.

Silicon nitride obtains high toughness and thermal-shock resistance through its elongated grain structure and is suitable for high-speed bearings and high-temperature rotating components.

Aluminum nitride simultaneously provides high thermal conductivity and electrical insulation, making it suitable for semiconductor thermal-management substrates.

Boron nitride provides high-temperature lubricity and machinability through its layered structure, while cubic boron nitride is used in ultra-hard cutting tools.

Boron carbide has extremely high hardness and low density but low fracture toughness and is therefore used selectively in armor and wear-resistant nozzles.

Barium titanate and PZT are functional ceramics that respond to electric fields and are used in capacitors, sensors, ultrasonic elements, and actuators.

Ferrites provide high electrical resistance and magnetic functionality and are used in high-frequency cores and electronic components.

Ceramic composites improve the brittleness of monolithic ceramics through crack deflection, crack bridging, and transformation toughening.

Therefore, ceramic selection should not be based only on the generic material name.

The following must be reviewed together:

• Purity and chemical composition

• Crystal phase

• Stabilizers and sintering aids

• Density and porosity

• Fracture toughness

• Flexural strength

• Thermal conductivity and thermal expansion

• Thermal-shock resistance

• Surface roughness and defects

• Manufacturing process

• Joining method

• Fluid and chemical environment

• Reliability and failure probability

• Supplier grade

Selecting a ceramic material is not the process of choosing the hardest available material.

It is a design process that selects the appropriate composition and microstructure for the required load, temperature, wear, chemical environment, electrical function, thermal function, and fracture reliability.