Processing of Ceramics / Cermets Flashcards
What is the starting material form of a ceramic?
Powder
What is the basic process of creating ceramics?
Wet it down
Shape it
Get rid of the wettness
What are traditional ceramics wet down with? How is the wettness removed?
Water
Evaporation
What is comminution?
Reducing particle size so that paste is even. Usually crushing or grinding.
What are the main ingredients of a ceramic paste?
Clay (hydrous aluminum silicate)
Water
Alumina / Silica (to reduce shrinkage)
Fluxes and wetting agents
What are the four shaping processes
Slip casting (clay water mixture is a slurry) Plastic forming (clay is plastic) Semi dry pressing (clay is moist but with low plasticity) Dry pressing (clay is dry <5% water, no plasticity)
The wetter it is, the less pressure it requires
When does plastic forming use manual methods? Mechanical?
Manual - more water
Mechanical - less water (stiffer)
What are the four plastic forming methods?
Hand modeling
Jiggering (pressed radially)
Plastic pressing (pressed top and bottom)
Extrusion
Why is shrinkage a problem n ceramic processes?
Because the water adds volume that is removed with the water.
What are the two stages of drying?
Stage 1: rapid drying rate as water evaporates from surface and water from inside replaces it (volumetric shrinkage occurs here)
Stage 2: ceramic grains are in contact, no more shrinking
Why is firing a thing?
Develops bonds between ceramic grains, reduces porosity (additional shrinkage)
What is glazing?
Adding a surface coating to make ceramic impervious to water and enhance appearance
What makes preparation of new ceramics different than traditional?
Starting powders must be smaller and more uniform.
What traditional shaping methods are also used for new ceramics?
Slip casting
Extrusion
Dry Pressing
What is hot pressing?
Dry pressing at elevated temps so that sintering happens while pressing (no firing required)
Gets higher densities and finer grain size, but die life is reduced.
What is isostatic pressing?
Using hydrostatic pressure to compact ceramic - prevents non-uniform density
Is drying needed for new ceramics?
Not usually, because no water
Why is finishing sometimes required, and how is it done
Increase dimensional accuracy
Improve surface finish
Minor changes to part geometry
Uses abrasive processes
What kinds of materials are used for the grinding?
Diamond, for cutting off hardened ceramic
How are cemented carbides made?
Carbide and binder are mixed into a sludge, then dried and compacted and sintered. WC melts into the Co.
Does WC need a binder?
No, but the binder decreases porosity
