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MCE Jon Ceramic Properties 3 - Coggle Diagram
MCE Jon Ceramic Properties 3
Chemical resistance
Determined by:
Strength of atomic bonding
Reaction kinetics between ceramic and environment at exposure temperature
Weakly bonded ceramics have low resistance e.g. NaCl attacked by water
Strongly ionic and covalent bonded are more stable
Gas Solid Reactions
Non-oxides are not stable at high temps
For Si-bearing ceramics, at low O2 partial pressures SiO gas forms -
active oxidation
Oxidation is linear and continuous, component can be actively consumed
Conditions are not common, but can occur in non-oxygen environments e.g. gas turbines or nuclear reactors where O2 is picked up as an impurity
If O2 partial pressure is >1mm Hg then sufficient O2 is present to form a protective SiO2 layer at the surface
Passive oxidation, oxidation rate increases at higher temperatures
Formation of SiO2 is initally rapid but decreases as thickness increases
Passive oxidation kinetics
Assuming after time t, thickness of oxide is s
Impurities can substantially affect oxidation
Materials with volatile contrituents should never be heated in a furnace with SiC or MoSi2 heating element
Oxidation can change strength
Low temp oxidation or short sharp high temp oxidation can :arrow_up: strength by blunting surface cracks
Prolonged high temp oxidation goes beyond and forms surface puts which :arrow_down: strength
Etched using hydrofluoric acid
needs either high temp or long time
grain boundaries etch first
Jet Engines
TBCs
Zirconia topcoat has low thermal conductivity and provides insulation
Bond coat provides oxidation and corrosion resistance and matches thermal properties and hence stress
Metal :arrow_right: CMCs
CMCs in combustor and exhaust nozzle, SiC/SiC in turbine blade and shroud
EBCs
Thin layer of SiO2 forms on surface of SiC
Exposure to moisture leads to recession
Non-porous barrier layer (Environmental Barrier Coating) forms
Balance between good and cost
Fibre reinforced CMCs
Fail gradually and give warning
Mechanisms
Weak interface composites
Used lots
Low adhesion coating applied to fibres before matrix, coating used is vulnerable to oxidation at high temp
Weak matrix composites
Used lots
Porous matrix = whole composite is weaker
Matrix is porous, reduced stiffness and damage tolerant. Worse than WIC as leads to permeability and bad properties
Fugitive Coating
Not really used
Fugitive means burns off
Matrix/reinforcement bond strength must be weak to allow relaxation of stress field at crack tip
