Chapter 15: Ceramics & Glass

Ceramics

Solid compounds with elements bonded together in a fixed atomic ratio by ionic and/or covalent bonds that comprise:

• at least one metal and one nonmetallic element solids or other nonmetal

• a combo of at least 2 nonmetallic element solids (MgO, ZrO₂) OR

• a combo of at least 2 nonmetallic elements solids and another nonmetal

Solid compounds that are formed by the application of heat or by a combination of heat and pressure

General Characteristics of Ceramics

• High melting temperature (withstand a lot of heat without losing viscosity)

• High compressive strength, but low tensile strength (b/c of low fracture toughness, ceramics are porous)

• Hard and wear-resistant (strong atomic bond)

• Brittle with low fracture toughness (little to no plastic deformation before fracture

• Prone to thermal shock

• Generally electrically insulating

• Generally thermally insulating

• Generally non-magnetic

• Chemically stables and oxidation-resistant (ceramics are already compounds of metals with oxygen, nitrogen or carbon)

Do crystallines have long or short range order?

Long range

What are the 3 types of crystalline ceramics?

1. Oxides → Al₂O₃ (alumina), SiO₂ (silica), FeO, Fe₂O₃

2. Carbides → SiC (silica carbide), WC (tungsten carbide), Fe₃C (cementite), Al₄C₃ (aluminum carbide)

3. Nitrides → BN (boron nitride), Si₃N₄ (silicon nitride), TiN (titanium nitride), all have high hardness

Low fracture toughness in ceramics means…

high sensitivity to cracks

Tensile strengths are always ______ to _______ of compressive in ceramics

1/10, 1/100

What does it mean if a material is closer to the 1:1 line on a Compressive vs Tensile Strength graph?

The material’s compressive and tensile strength are somewhat equal

Why are engineered ceramics better?

They have a refined microstructure with less porosity, fewer pores will prevent the voids from behaving as crack initiators in tension.

What are examples of Natural Ceramics?

• granite

• brick

• concrete

What are examples of engineered ceramics?

• alumina

• zirconia

• SiC

Why can’t ceramics be melted and cast into molds?

Due to their high melting temperature

What is the sequence of processing ceramics?

1. Synthesis of ceramic powders

2. Ball milling, blending, spray drying of powders using processing additives

3. Shaping of powders into useful shapes using pressing, slip casting, tape casting, etc.

4. Consolidation into a dense, monolithic object using sintering or firing

5. Secondary processing (grinding, cutting, polishing)

6. Final sintered ceramic product

What is spray drying?

small quantities liquid mixture containing ceramic powder is sprayed

What are some examples of processing additives?

• binders

• lubricants

• plasticizers (makes it easier to shape)

• dispersants (prevents clumping)

What is “Green powder” shaping?

shaping of powders into useful shapes (green ceramics)

Does glass have a crystalline or non-crystalline structure?

non-crystalline, short-range order of molecules but no long-range order, amorphous

Glass is usually based on _____

SiO₂ (silica) → windows, bottles, optical fibres, etc.

What are some properties of glass?

• strong ionic and covalent bonding

• brittle

• non-conducting

• susceptible to thermal shock

Why can’t glasses be used at very high temperatures?

They become more viscous

Are glasses as chemically stable as most ceramics?

No

What does it mean that glass is metastable?

• It will crystallize given the right combination of circumstances (slow cooling, long holding time, presence of nucleation sites)

• It is not in its most thermodynamically stable structure

Metastability

Not enough time for transformations to fully occur, phase transformation within microstructure not given enough time, not thermodynamic enough, unfavourable

Describe what is happening in the figure

• When silica crystallizes on cooling, an abrupt change in the density is observed

• For glassy silica silica, the change in slope at the glass temp indicates the formation of a glass from the undercooled liquid

Does glass have a fixed T_m or T_g?

No

Do crystalline materials have a fixed T_m?

Yes

Do crystalline materials have a T_g?

No

Annealing

Placing specimen in oven until glowing red

What does annealing do?

Removes residual stresses

What does tempering do?

Intentionally creates beneficial surface compressive stresses

How can you improve the properties/increase toughness of a material?

• Impose residual compressive stresses that cancel out some of the tensile forces imposed

• Substitutional additives can also increase surface compressive stress