Coating freak Off From Aluminium

[Eagleman]

Hi,

I'm new to this forum, thru reading this forum know that there are many expert on thermal spraying in this forum.

Recently i pick up a project on plasma spraying Aluminium band, the problem that i face was the coating keep freaking off from the aluminum like peeling an orange(no bonding), that i really can't understand. Any one can advise, below is the spraying detail.

1) Powder used- base coat 450ns, top coat 52c-Ns
2) Total coating thickness come around- 0.025"-0.040"
3) 60 grit was used for surface praperation with Pressure of 80psi

4) During spraying no preheat was used, 3 airjet was used for part cooling to minimise part expension, thickness per pass buildup around 0.001".


Any expert can advise which area should i improve on. Thank...


Rgds

Eagleman

[LEN WOOD]

Hello 'Eagleman',

I understand that the component is made of Aluminium? Special considerations need to be made for Aluminium component processing.

Are you spraying the ID or OD? What diamater and wall thickness is it?
What system and gun are you using?

Are you working to PWA or?

Send me an email (see below) and I can help you directly as we are in 'similar' time zones.

[derek]

There are two things you need to think about:
1, pre-heat the part will reduce the expansion different between Al and coating.
2, After blast the part, spray it immediately. If you let the part stand in air too long time, the surface will become deactivate.

[LEN WOOD]

The thing is that while preheating is predominantly conducive to bonding with Aluminium it accelerates oxidization of the blasted surface and therefore compromises the activated blast key. However, it may be necessary due to pre-machining stresses, etc. If it is required then it should be done indirectly on a surface away from the blast prepared surface.

While preheating does minimise the expansion difference but the part does cool after spraying and the coating band will remain at the 'expanded' dimension and that's when you have delamination/separation. Obviously the extent of which depends on part size, wall thickness, geometries, cooling, fixturing, coating thickness, etc.

Correct cooling and positioning of siphons is essential in some cases.

I think that you have probably 'overblasted' the part as the pressure is extreme for Aluminium.
As Derek pointed out Aluminium MUST be sprayed with minimal delay.

As with many TS issues there may be a handful of contributing failure factures.

'Eagleman' you must tell us more about the component and application. Does the coating come off in one piece?

[Eagleman]

Thank for the comment, it's really benifit me. Below is more info of the part

1) Repair area was an ID, but there is three repair close to each other with around two inch apart. Full case size was 67 inch.

2) Wall thickness for two of the repair was around 2 inch link to the case. and the other repair about 0.5 inch.

3) Spraying system used was Metco system with SG100 gun.

4) The turntable rpm use was 12 rpm with robot TS of 0.95.

5) The coating peel off as a ring ( continues)

6) The test history that I have done on this part before was as follow:
- direct prheat the part before spray.
- 60 grit with 60 psi.
- with and without cooling air.
- part clamp direct on turn table
- maximum spray pass given was 3 passes

[Gordon]

Hi Eagleman

to the Surface Engineering Forum.

It is very difficult to comment without observing process in person. But a few pointers in addition to those from Derek and Len.

With ID spraying, pre-heating and expanding substrate before spraying may be beneficial. But with aluminium substrate it is important not to apply heat direct to surface to be coated. Apply heat indirectly say from the OD, try and get a good even bulk temperature not above about 90 C. If situation allows, oven pre-heating before grit blasting, followed immediately by spraying can work well.

Grit seems fine and blasting pressure high. Possibly try coarser grit and lower pressure.

Venting all the crap and dust that inevitably flies around with ID work is very important. You will need to experiment with your set-up to reduce this as much as possible. This can have quite an impact on coating and bonding quality.