High Temp. Oxidation Protection (Coatings) Flashcards
What are the 4 complications?
- Alloy constituents will have different affinities for O2
- Complex oxides (ternary or higher) are possible
- Alloy constituents will have different diffusivities in the alloy
- Internal oxidation may be possible
Why are protective coatings very important for super alloys?
They are designed for strength and structural performance at high temperatures (1000C+)
Development is driven largely by jet engine development
Modern engines run hotter
To retain required mechanical properties Cr levels are often kept low
Why are protective coatings important for refractory metals (Mo, Nb, W and Ta)?
Retention of strength at temperature
High melting point metals (Nb ~ 2500C)
Not oxidation resistant at high temperatures (to be sacrificed)
Used in rockets and propulsion systems involving hot gases
The purpose of a coating is to prevent oxidation of the substrate material giving maximum reliability and maximising the time intervals between maintenance or replacement
In refractory metals, what do the high temperatures involved mean?
That the coating and substrate are not in equilibrium, there will be inter-diffusion between then two and the thermal properties, such as the coefficient of thermal expansion, will not match.
In addition to the oxidation, the coating must resist hot corrosion, particularly from sulphur compounds that could originate from fuel or sea salt. There can also be the effects thermal cycling and erosive processes.
Coating systems tend to be based on the selective oxidation of elements such as Al and/or Si. The coating contains the elements to be sacrificially oxidized.
What are the two generic types of coatings?
- Diffusion coatings
- Overlay coatings
Describe diffusion coatings.
One or more elements are diffused into the surface of the substrate to create an oxidation resistant layer.
A reservoir with a very high concentration of the element that should be enriched is created at the surface either in the gas phase or in a solid/liquid state.
The protective oxide coating should prevent (or strongly inhibit) attack.
Silicides tend to be preferred on refractory metals as they have a better thermal expansion match. The coatings can also be modified, with various additions, to promote “self healing” if the coating is damaged.
In super alloys, aluminides (i.e. NiAl or CoAl), are usually produced and subsequent oxidation produces a spall resistant Al2O3 coating.
How does the majority diffusion differ?
If the majority of the diffusion is from the coating to the substrate it is described as “net inward”
If the majority of the diffusion is from the substrate to the coating it is described as “net outward”
Describe overlay coatings.
Does not rely on diffusion as primary method for producing a coating (some inter-diffusion takes place)
Produced by PVD, thermal spraying process or overlay welding (cladding)
What is the main type of overlay coatings? Summarize each part?
“M”-Cr-Al-Y (where M = Ni or Co)
M is the matrix of the coating, the matrix is ductile to provide better resistance to thermal fatigue.
The Cr content is high to provide corrosion resistance.
The Al is included for Al2O3 formation.
The Y (Yttrium) is included to enhance the oxide adherence.
How does overlay coating thickness effect the coating?
Coating are relatively thick (~50-150um) compared to diffusion coating.
The protective life increases in proportion to the coating thickness.
Thicker coatings are more prone to cracking as the cyclically induced stresses are higher.
