Ceramics Flashcards
Give examples of the use of ceramics
- Brakes
- Seals in gas turbine engines
- High Mach number nose cones + leading edges
- Thrusters
Give the three ways that ceramic pre-form is made
- Slip casting
- Dry pressing
- Cold isostatic pressing
Describe the method of slip casting (mainly for silica)
Give uses
- Powdered silica is made into a slip by mixing with water and a surfactant
- Poured in to a porous mould
- Water diffuses
- Solid layer formed
- Removed and ‘fired’ to sinter it
uses:
toilet seat
nose cones for missiles
Describe the method of dry pressing
Give uses
- Powder is pressed between dies
- Powder densifies
- A self-supporting body is obtained
- Body is sintered
uses: more precise than slip casting
Describe the method of ceramic cold isostatic pressing
- Powder is placed in a flexible mould
- It is contained in a pressure vessel
- A fluid is used to isostatically press the part to consolidate the pre-form
- Part is removed and sintered
Describe the method of ceramic injection moulding
- Powder is mixed with organic binder (polymer)
- Conventially it is injection moulded
- Polymer allows ceramic powder to flow in to the mould
- Polymer is pyrolysed (burned out)
- Ceramic part is sintered (loses a lot of size)
Describe the method of ceramic gelcasting
- Powder is mixed with a small amount of liquid monomer and a cross linker (so jelly like)
- Gel is formed
- Part is moulded and cross-linked
- Can then be machined
- Finally sintered to densify the component
Describe sintering
- Ceramic part is heated and the grain boundaries directly diffuse and fuse together making the part more dense and increase its strength
- Some ceramics required additives to facilitate this process
Describe the method of hot isostatic pressing
Same as cold, but uses heat during isostatic pressing. Part therefore does not require sintering afterwards
List the three methods to create ceramic matrix composites
- Deposition from gas phase
- Pyrolysis of a polymer
- Chemical reaction
Describe the general method of creating a ceramic matrix composite
- Produce a fibre pre-form
- Infiltrate he pre-form with a ceramic material (in gas/liquid form)
- Then try densify further (burn off polymer)
What are the two methods of joining metal and ceramic. Describe them.
Brazing
- Activation of the surface by metallisation
- Using a active braze that reacts with the surface
What ceramic can be used for sensors
Piezo-electric crystals
List 4 types of methods to manufacture ceramic coatings
- High velocity oxy-fuel (HVOF)
- Plasma spray
- Electron beam physical vapour deposition
- Chemical vapour deposition
Describe the method of High velocity oxy-fuel (HVOGF) for ceramic coating
- Coating material is sprayed from a nozzle at Mach 3
- A flame engulfs spray and heats it
- As it hits the surface, it melts both due to thermal + kinetic energy
- Particles fuse together to form dense coating
Describe the method of Plasma spray for ceramic coating
- Used for materials that melt at very high temp
- Versatile method and can coat with any material
1. Powders are softened in a plasma jet (16500deg) and sprayed at Mach 2 on to the surface
2. Surface of the substrate does not experience much heat build-up
Describe the method of Electron beam physical vapour deposition for ceramic coating
- Electron beam bombards a target of the coating material
- Atoms from the coating are vaporised
- They are deposited on everything in line of sight from the target
- Target rotated for uniformity
- Must be in a vacuum
Describe the method of chemical vapour deposition for ceramic coating
- Heated substrate is exposed to a volatile precursor material
- Precursor reacts on substrate forming a coat
- Normally plasma assisted to dissociate the reactants and form the coating
List some types of ceramic sensors
- Health monitoring
- Flight control: acceleration, gyroscopic effect
What is special about a piezoelectric material
- Develop an electric charge when they experience a mechanical stress, or visa versa (i.e. bending)
