Ceramics Flashcards
What are characteristics of ceramics?
Brittle (since atoms cannot move relative to each other without breaking bonds).
High melting points.
Poor conductors.
Low fracture toughness
Atoms in the crystal structure may vary greatly in size.
What bonding can be present in ceramics?
Ionic and covalent
Ceramics exist in what structural forms?
SiO2 (quartz, tridymite or crystibalite)
Non-crystalline ceramics are also possible, known as glasses, have an amorphous structure by adding impurities.
(Na+, Mg2+, Ca2+, Al3+)
What are the manufacturing steps of ceramics?
Raw materials - ball mill, roller mill or impact grinding. With additives….
Blending/mixing
Forming/shaping
Drying
Sintering
Finishing - machining, grinding or lapping
What are the additives for the raw material and what do they do?
Binders - substances that improve the mechanical strength of green ceramic bodies so they can pass through production steps, before sintering, without breakage.
Lubricants - added to decrease friction in pressing process, so there is uniform friction of the ceramic as it is pressed, reducing cracking.
Wetting agents - added to reduce the surface tension of liquids, improving the wetting of the particles, leads to more uniform packing density.
Plasticizers - added to soften the binder, increases the flexibility of the green body.
Deflocculant - a substance which when added causes a reduction in apparent viscosity.
What is the difference between blending and mixing?
Blending refers to when powders of the same chemical composition but possibly different particle sizes are intermingled. Different particle sizes are often blended to reduce porosity.
Mixing refers to powders of different chemistries being combined.
What are the different types of forming/shaping?
Isostatic - pressing of the material in ALL DIRECTIONS
Unidirectional - material is pressed from both sides of the same axis.
(Hydro) Plastic forming - carried out by extrusion, injection moulding or jiggering. The moisture content is adjusted so that they behave plastically enough to extrude without cracking.
Slip casting - summarized as followed:
1. A suspension (or slip) of particles in a liquid (often water) is formed.
- The suspension is poured into a shaped mould with porous walls.
- The liquid is absorbed from suspension into the mould wall leaving a dewatered “sheel”.
- When the semi-rigid shell is thick enough the excess suspension is poured off and after additional drying time the mould is opened and the product is removed.
What types of water are removed when drying?
- Water of suspension - either decanted or absorbed into the mould walls.
- Inter-particle water - Occurs mostly in clay like materials with layer type structures. When this water is removed shrinkage is readily apparent.
- Pore water - this water is in the interstices between the particles. Removal of this water leads to very little, if any, shrinkage.
- Absorbed water - this water isa very thin layer, usually only a few molecules thick, held on the surface of particles be electrostatic surface forces.
What is the sintering process?
- Preheat - in which lubricants and binders are burned off
- Sinter
- Cool down - slowly, prevents cracking and warping
Sintering increases strength hardness of a ceramic
What happens due to sintering?
Final pore removal.
Centres of particles will therefore move closer together as ions (or atoms) move to surfaces to create new contact and bonding between particles, this produces shrinkage.
What influences shrinkage and what may unequal shrinkage occur due to?
Greater initial porosity leads to greater shrinkage. This can be a problem where the production processes used may lead to non uniform densification.
Unequal shrinkage may also occur due to:
1. Unequal temperature distribution during sintering.
- Different lengths of time at sintering temperature, e.g. thin and thick regions in the same component.
- Poor mixing / homogenisation of particles
- Anisotropy in particles
What influences grain boundaries in cermaics?
Grain boundaries and particle surfaces.
More importantly, pores.
They represent discontinuities in the material and can therefore act as stress raisers in an intrinsically brittle material.
Pores are of what types and measured in what 2 ways?
Isolated pore
Dead-end pore
Cross-linked pores
Throat pores
- Apparent porosity - this measures the inter-connected porosity and determines the permeability of the material.
- True porosity - this is a measure of the total porosity, i.e. interconnected and closed.
