difference between Ni clad Al and Al clad Ni

[clover]

Hello,everyone! Recently I was puzzled by one problem, what's the difference between Ni clad Al and Al clad Ni since we often employ them as bond coats.Any answers will be appreciated in advace!

[Gordon]

Hi clover

to the Surface Engineering Forum.

Well the difference, typically the Ni clad Al powders (Metco 404 types) are ~ 80/20Ni/Al. Aluminium particle plated in nickel. When sprayed produces exothermic reaction between Al and Ni and no doubt exothermic burning of aluminium. Coating bonds very well to many substrates forming coating containing many phases including nickel aluminides and oxides.

Al clad Ni (Metco 450 types) are typically ~ 95/5 Ni/Al. Composite powder of nickel particles with smaller particles of aluminium organically glue to the surface. Again when sprayed produces exothermic reaction, this time purely due to aluminium burning. Again coating bonds very well forming a coating with aluminium oxide inclusions and with little evidence of any nickel aluminides.

One the whole Al clad Ni (and NiAl alloys) are now favoured over Ni clad Al in most applications, claiming higher bond strength, higher coating strength, higher toughness, higher oxidation and corrosion resistance, less smoke when sprayed. Ni clad Al is a little harder and contains aluminides so may see benefit in wear resistance applications. There may still be some aerospace specs calling for Ni clad powders in which case you have no choice anyway.

[Gordon]

Moving thread to "Surface Engineering Threads"

[clover]

thank you for your detail expalation.

[operator]

I'd read many use metco 405 as bond coat and there will be another layer as top coat. Can I use metco 405 as top coat instead?
What is the purpose of bond coat, is it same as grit blasting to produce surface roughess for coating to take place?

[Gordon]

Hi operator

I'd read many use metco 405 as bond coat and there will be another layer as top coat. Can I use metco 405 as top coat instead?

Yes, certainly possible. Coatings obviously bond very well and are very rough as sprayed, resistant to oxidation, high temperatures and thermal shock. Coatings are fairly hard and wear resistant, though not particularly good against aqueous corrosion. Though, usually better/cheaper alternative exist for top coats/one-coats for most applications, so this primarily gets used as a bond coat

What is the purpose of bond coat, is it same as grit blasting to produce surface roughess for coating to take place?

Yes to some extent. It will bond to clean polished/machined surfaces when grit blasting is not possible, bond coats are always best applied to a grit blasted surface though when possible. Grit blasted/bond coated/top coated systems are more reliable than systems omitting the bond coat. Apart from providing high bonds to substrate and to top coat, it can also act as a buffer layer reducing the effects of stress.