About service temperatures of two different Ni-Al coatings
09-02-2015, 09:03 AM
|About service temperatures of two different Ni-Al coatings
Hi, every one:
I wonder why the service temperature of a Ni-Al coating sprayed using Metco 450 NS is 800 Celsius degree, while the coating using Metco 404NS, a spheroidal powders comprised of an aluminum core encapsulated
in a nickel shell, is designed to work at a lower temperature of upto 650 degree.
To my knowledge, the coating of Metco 450 ns is of a nickel solution crystalline microstructure with a melting temperature lower than 1400 degree, while the coating of Metco 404NS is composed of NiAl intermetallic compound phase with higher melting temperature of 1638 degree, and it is more oxidation resistant than the 450 ns coating, therefore the service temperature of the 404NS coating should be higher than the 450NS coating.
09-02-2015, 09:38 AM
|RE: About service temperatures of two different Ni-Al coatings
(09-02-2015 09:03 AM)fhyuan Wrote: Hi, every one:
It's no wonder that the Ni-Al-composition with 20% Al (Metco 404ns) is oxidized at lower temperatures, as Ni-Al-composition with 4% Al (Metco 450ns). During coating forming not nickel aluminides: Nickel remains nickel and aluminum is aluminum. Aluminum from Metco 404ns oxidized well below 600°C and thus breaks the whole layer (650°C is far too optimistic and only for short-term applications). In Metco 450ns Aluminum oxidizes also completely by 600-650°C, but the layer is not full broken; only after the oxidation of nickel (< 750°C, briefly up to 800°C), the layer is oxidized through.
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