Ceramics + processes*

Define Ceramics

Compounds composed of metallic and non-metallic elements.

Usually very strong compressively but brittle

E.g glass, clay etc.

Ceramic materials tree diagram

What happens if u change the clay-to-water ratio?

More water = easier to work with but can lead to shrinking once dried and fired - once shrunk may crack.

What is the clay-paste as we know it made of? + what happens to it

Clay

Water

Raw materials such as silica that help reduce shrinkage but reduce plasticity of paste

Fluxes that vitrify/melt during firing but encourage sintering of paste.

Mechanical properties of ceramics and why we don’t tense tensile loads

Brittle as mentioned and there is no tensile test for it:

1. Difficult to grip brittle objects For testing

2. Difficult to manufacture the required geometry needed for tensile tests

3. They necessitate perfect allignment to test tensility.

We do transverse loading/flexure tests instead.

Stress at fracture during flexure tests, similar to the ultimate tensile strength is called the flexural strength, modulus of rupture fracture strength or bend strength.

NOTE: the d in the equations for this value is the HEIGHT OF THE CROSS SECTIONAL AREA (b = base still)

ALSO NOTE: THIS IS NOT A MATERIAL PROPERTY as seen by the eq it depends on the size of the sample - more volume = higher chance of ‘weakest link’ somewhere and the whole sample is as strong as its weakest link therefore the whole structure is more likely to be weaker.

Is glass amorphous or crystalline or both? How can it be changed? + name and advbantages of changed mat

Glass is typically amorphous but can be changed to crystalline by heat treatment. AKA CRYSTALLIZATION

Product of this is called glass-ceramic.

Advantages include:

1. Easy to make

2. Strong

3. Good temperature operation

4. Biologically safe

Clay advantages

Naturally found

Abundant .:. Cheap

Good compressive strength as for all ceramics

Refactory ceramics advantages

1. VERY high temperature operation capabilities

2. Chemically inert even at said temperatures

3. Durable/last long

E.g. used in furnace lining.

Name e.g. = silica or fire-clay

Abrasive ceramics and how they’re used

They themselves must be resistant to wear in order to wear down other materials.

They must be resistant to high temperatures caused by friction.

Name e.g. = silica sand or diamond

Used by:

1. Bonded to grinder wheels

2. Coated to paper/cloth

3. Lapping = slurry of water+ LOOSE GRAIN pressed into work piece via rotating lap plate.

Cements examples, types and key words

Name e.g. = cement, plaster of Paris, lime

Two types - hydraulic and non-hydraulic - definitions self-exp.

Calcination = heat cement

Clinker = heated cement

Gypsum = additive to make it harden

Advanced ceramics

Ceramics with unique and modern properties such as electrical, magnetic and optical.

E.g. optical fibres =super high purity silica

E.g. piezoelectric = mechanical and electrical energy conversion.

Define green part and sintering

A shaped object formed from raw powder and binder before it is heated. - not the same as cement ones

Sintering/FIRING = process of compacting and solidifying due to pressure and or heat treatment - occurs in a KILN- not same as calcination

Main processing technique

1. Obtain powder

2. Obtain binder - usually water, and combine and shape into desired object

3. DRY - must dry before firing because if wet it will cause cracks.

4. Firing - this removes the tiny gaps between dry clay particles.

Main preparation technique

Via communititon: Turning rocky lumps into fine powder

2 categories: crushing and grinding

Communition includes:

1. Impact = part of grinding

2. Compression = part of crushing

3. Attrition = part of grinding

Crushing how its done and processes

done through compression of material
4 methods:

jaw crushing - oscillatory motion to compress rocks against rigid surface

Gyratory crushing - gyrating cone compresses rocks against rigid surface.

Roll crushing - rotary axle compresses rockets against surrounding rigid surface.

Hammer milling - rotary axle compresses rocks against OTHER oppositely rotating axle.

Grinding how its done and processes

done via impact and abrasion of material
3 types:

Ball mill - rollers rotate causing the container of STOCK and hard balls to rotate and grind over eachother continuously.

Roller mill - 2 grind rollers rotate causing in place whilst the table of stock also rotates below it - grinding the stock between.

Impact grinding - stock funnelled into an air tube which projects the stock making it it a hard surface and fall to the ground.

Shaping processes depending on water content of clay and their link to pressure.

Slip casting for 25-40% (least pressure for most water) (pourous plastic moulds allow water to seep through)

Plastic forming for 15-25% (by hand)

Semi-Dry pressing for 10-15% (press down into shape - the presser is removed and the base pushes the ceramic out)

Dry pressing for <10% (most pressure for least water) (same process and all positives but binders needed + simple geo)

Glazing

The application of a coating to a ceramic coating to make it water proof and aesthetic.

NOTE: MUST FIRE TWICE