Ceramics and Glass Flashcards
When are ceramics used in aerospace?
When high temperature performance is critical
High mach nosecones, brakes, seals, rocket nozzles, turbine coatings
Describe the slip casting process
- Powdered ceramic is mixed with water and surfactant
- Slip is poured into a porous mould
- Water diffuses out though the mould, forming a solid part
- Part is then removed from the mould and sintered
Describe the dry pressing process
- Dry powder is pressed between dies
- Powder densifies until a self supporting structure forms and the part is then sintered
Describe the cold isostatic pressing process
- Dry powder placed between flexible bladders
- Bladders filled, pressing powder from all sides
- Powder densifies and is then sintered
Describe the injection moulding process for forming a green body
- Ceramic powder mixed with an organic (polymer) binder (binder allows powder to flow)
- Conventionally injection moulded
- Part is heated so that binder pyrolyzes
- Part is then sintered
Describe the gelcasting process
- Powder is mixed with a liquid monomer and cross-linker to form a gel
- Part is moulded and cross-linked
- Part can then be machined before sintering
- This is used to form carbon-carbon brake disks
Why is it important that a green body is net shape before sintering?
Ceramic are hard to machine, so the shape cannot be modified after sintering
What happens during the sintering process?
Powder grains fuse directly to form a densified part
Describe the hot isostatic pressing process
- Part is placed between fluid bladders
- Bladders are filled with hot fluid and temperature and pressure applied to part
- Part is densified and sintered in one step
What are Ceramic Matrix Composites
- A fibre reinforcement in a densified ceramic body
- Fibre pre-form is produced as for a composite, then infiltrated by ceramic and sintered
Describe the Gas Phase Deposition process for producing ceramic matrix composites
- Carbon source gas and argon are introduced to the fibres at high temperature and pressure
- Gas decomposes and carbon deposits build up on fibres
Describe the pyrolysis of polymer method for producing ceramic matrix composites
- Polymer containing carbon infiltrates the fibre pre-form
- Polymer is pyrolyzed to form the ceramic
Describe the Chemical Reaction method for producing ceramic matrix composites
- Porous carbon preform is produces
- Liquid Silicon infiltrates the preform and reacts to form silicon carbide
- This method has the lowest porosity of all CMC methods
How are cooling ducts in turbine blades produced?
- A ceramic core is produced in the shape of the cooling ducts
- This is surrounded in wax in the shape of the turbine blade
- The blade is then investment cast from superalloy
- Ceramic core is dissolved out leaving a cavity in the shape of the cooling ducts
How are ceramic and metal joined?
- Brazing process
- Metal coating applied to ceramic, then braze applied that reacts with the ceramic surface
How do ceramics seal turbines?
- Turbine casing coated with abrasive ceramic
- As blades turn, ceramic is worn away just enough to form a seal with the turbine blades
How are ceramics used as thermal barriers?
- Metal is oxidised
- Ni-Al coat applied to surface to ensure coating adheres
- Alumina layer is grown from this bond coat
- Zirconia is then deposited on to the surface
Describe the HVOF (High Velocity Oxy-Fuel) process for manufacturing ceramic coatings
- Coating material is sprayed from a nozzle at a high mach number
- Flame engulfs spray and heats it
- Ceramic melts as it hits the surface
- Particles fuse to form a dense coating
Describe the Plasma Spray process for manufacturing ceramic coatings
- Used for coatings with a high melting point
- Powder is softened in a plasma jet and sprayed onto the surface at a high mach number
- Surface is not heated
- Powder fuses on incidence with the surface
Describe the Electron Beam Vapour Deposition process for manufacturing ceramic coatings
- An electron beam bombards a target of the coating material
- This causes the material to atomise, and is deposited on everything in line of site of the target
- Spray solidifies on incidence with the surface
Describe the Chemical Vapour Deposition process for manufacturing ceramic coatings
- Heated substrate is exposed to a volatile precursor material
- Precursor reacts on the surface to form the coating
- Often assisted by plasma to dissociate the reactants to form the coating
How are ceramics used in sensors
Piezoelectrics are ceramic materials, used in accelerometers and gyroscopes
- They produce electric current when they are stressed
- They also deform when electric current is applied to them so can be used as actuators
How is tempered glass safer than normal glass?
Tempered glass forms smaller blunt pieces when it breaks
It is also harder to break because the surface is under compression so it is harder for cracks to propagate through the glass
What is the thermal tempering process for glass?
- Glass is heated above its annealing temperature
- Surface is quenched above with cool air while the centre remains hot
- The surface contracts, putting it under compression
