Ceramics

Question 1

How does the crystalline structure of ceramics vary?
Do all ceramic display a uniform structure?
What sort of chemical bonding occurs within ceramic materials?

Answer 1

The structure varies dependent on the arrangement of the molecule.

SiO2 displays different structures:
Quartz, tridymite or crystobalite (polymorphs).

The addition of other elements such as Na or Mg into SiO2 can produce an amorphous crystal - which is a crystal not displaying long range order.

The chemical bonding occuring can involve covalent bonding (sharing of electrons) or ionic bonding (donation of electrons).

Question 2

What are some of the additives that can be applied to ceramics and what are their roles?

Answer 2

Binders: improve the mechanical strength for firing.

Lubricant: decrease friction in the pressing process, uniform friction as the ceramic is pressed to reduce cracking.

Wetting agent: reduce the surface tension of liquids, improving the wetting of the particles. Leads to more uniform packing density.

Plasticizers: soften the binder, increases the flexibility of the green body.

Deflocculant: causes a reduction in apparent viscosity.

Question 3

How is the uniformity of the particulates feed ensured?
How does this improve on the final form of the final ceramic piece?

Answer 3

Ball milling, roll milling, and impact grinding can grind the powdered materials and improve their uniformity.
The density of the final piece can be influenced by anisotropy between particles forming voiding.
Voids contribute to the shrinkage of the material during sintering and can induce cracking.

Question 4

What is mixing or blending in relation to particulates and why is it applied?

Answer 4

Blending is applied for the particles of the same composition.
Mixing is applied for particles of different chemical composition.
This improves the homogeneity of the mix, reducing the occurance of voiding.

Question 5

What is the difference between isostatic pressing and unidirectional pressing? How do these processing techniques differ?

Answer 5

Isostatic pressing applies pressure uniformly for all directions where as unidirectional is one direction (or two, along the same axis).
The isostatic press requires the use of water to induce the pressures in a flexible mould.
Unidirectional applies a steel die and punch.

Question 6

What 3 defects are common in ceramic components and how do they influence the performance of the part?

Answer 6

As a group, ceramics are generally very brittle, particularly at low temperatures.
This problem is heightened by the presence of flaws such as cracks, porosity or inclusions.

Question 7

What ionic purities can be added to silicon to form its amorphous structure

Answer 7

Silicon can form with oxygen at high temperature to form silica which can then contain the following impurities to generate an amorphous structure.

Na+ Mg2+ Ca2+ Al3+

Question 8

Provide the names and some explanation of the first 5 main production processes involved in the production of ceramics

Answer 8

1. Raw materials chosen including additives which consist of binders, lubricants, wetting agents, plasticisers and deflocculents. The grinding of said material occurs.
2. Blending and mixing occurs.
   Blending is the stirring of powders of the same composition but different size.
   Mixing is the stirring of powders of different chemistries.
3. Forming and shaping.
   Involves the pressing of the ceramic powder under pressure.
   Types of pressure include isostatic, unidirectional, plastic forming and slip casting.
4. Drying the ceramic piece.
   Hydro plastically deformed or slip cast items will have high moisture content.
   Pressure formed items have lower moisture content but lubricant/binder residues will need removed.
5. Sintering the pieces to impart the solid state diffusion to increase the density of the material by reducing pore sizes.
6. Surface finishing.