Question on abradable coatings : manufacturing and tests
05-18-2007, 04:47 PM,
#1
Question on abradable coatings : manufacturing and tests
Hi!

I have questions about the use of polymer in thermal sprayed abradable materials (TSAM) .

1) I would like to know if abradable coatings are always made of polymer.

Some TSAM are mentionned to have polymer after the thermal spray process like the AlSi/polyester and other don't : AlSi/graphite, Ni/grahite, MCrAlY and ceramics... are not mentionned with polymer phase. But I read that the polymer was a mean to controle the porosity rate in MCrAlYs because when you burn, it evaporated and become a porosity.

So, do the the AlSi/graphite, Ni/graphite, Metal matrix+release agent family, ceramic family, use this mean to control their porosity rate or is it only by tuning the therml spraying parameters ?

And do they (+MCrAlYs) still contain polymer after all the manufacturing process ?

2) I have read the list of abradable coatings on your website :
* Aluminium silicon alloy/polymer composites
* Aluminium silicon alloy/graphite composites
* Nickel/graphite composites
* Aluminium bronze/polymer composites
* Nickel chromium aluminium/boron nitride composites
* Nickel chromium aluminium/bentonite composites
* Nickel/aluminium composite sprayed porous
* Nickel chromium aluminium composite sprayed porous
* MCrAlY/BN/Polyester composites
* YSZ ceramic/Polyester composites

but I don't understand
* Nickel/aluminium composite sprayed porous
* Nickel chromium aluminium composite sprayed porous
Can you explain me a bit what do "metal composite sprayed porous" mean.


Thanks in advance.
Reply
05-18-2007, 07:06 PM,
#2
RE: Question on abradable coatings : manufacturing and tests
Hi karusade

Sign0016 to the Surface Engineering Forum.

1) No, in fact most abradable coatings don't employ polymers. Polymers are used in some coatings, being burnt out in post heat treatment to create a porous structure (not necessarily just for abradable coatings though). AlSi/polyester abradables are usually used with the polyester intact as it improves lubricity of the coating.

Abradable coatings containing graphite, bentonite, boron nitride, polymers, etc. tend not to be particularly porous. They rely on these additive materials to weaken the coating structure in a controlled manner and add lubricity.

MCrAlY based abradable coatings are design for use at temperatures far higher than polymers can survive, so if polymers were used in construction of these coatings, the polymer would be burnt out prior to or during service.

2) Some abradable coatings with no additives just rely on special spraying parameters to produce a structure with relatively poor inter particle cohesion and porosity (metal composite sprayed porous).

Abradable coatings normally have conflicting requirements; not to damage conterface material and to be readily cut to form seals, while being structurally sound and erosion resistant. In other words a carefully designed compromise between the weak and the strong.
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05-18-2007, 07:53 PM,
#3
RE: Question on abradable coatings : manufacturing and tests
Thanks a lot for your answer, Gordon : it is much more clearer Cool .

So, I misinterpreted the names, e.g. * MCrAlY/BN/Polyester composites or * YSZ ceramic/Polyester composites....
I understand now : it is the composition of the powder, right?

I have also another questions concernign tests.

3) I have to perform some tests to characterize abradable material in the next weeks and I would like to know how people do tensile tests on abradable ? I've read that Rolls Royce sprayed a mold with some AlSi/polyester to make a specimen or probe (I don't know the right word as I am not English). Is it always like that or is there other technics ? With the porosity won't it lead to non repeatable tensile curbs ? Is it possible to spray a surface and then to cut it into the shape of an usual tensil test specimen/probe.

Thanks a lot for having done all this : forum+website.Big Grin
I am really happy to have found this website : I ve gone on it very often this past month.

Regards
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05-19-2007, 02:07 PM,
#4
RE: Question on abradable coatings : manufacturing and tests
Hi karusade

Yes, that is the powder composition or make up of the coating in the as-sprayed condition only. Post heat treatment or high temperature service will modify the coating. It looks like it may be wise to make that point clearer on the abradable web page.

1) Testing tensile strength of the coating (parallel direction with reference to substrate) can be done using a number of different methods. Two I can think of are as follows:

2) Take a suitable piece of material in tube form. Cut in to 2 pieces halfway along its length. Prepare the tube ends so that they but up against each other very snugly with no measurable gap. Hold in place by using long bolt and nut. Spray desired coating over the join area. Remove securing bolt and perform tensile test. This will only give you the tensile strength value as there is no gauge length of unsupported coating to use an extensometer to assess elongation.

Take a flat tensile testing specimen. Spray coating on one side. Carefully, machine/grind off the original substrate material around the gauge length area of the test piece and conduct tensile testing.


Tensile bond strength testing (ASTM C633-69) essentially for testing bond strength, can on rare occasions give the tensile strength of the coating (perpendicular direction with reference to the substrate) when failure occurs within the coating and not at the substrate coating interface. It also indicates a minimum strength when failure occurs outside of the coating.

Remember that thermal spray coatings can show very different test values depending on the direction of testing. The structure is very far from being isotropic.

Hope that helps.
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03-18-2008, 05:01 PM,
#5
RE: Question on abradable coatings : manufacturing and tests
Tensile testing of pourous coatings such as abradables is a crap shoot at best due to epoxy penetration into the structure of the coating. Even with film type adhesives (FM-1000) you still get penetration.

A more applicapble test for application in turbines is thermal spallation resistance for obvious reasons.
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03-19-2008, 04:27 PM,
#6
RE: Question on abradable coatings : manufacturing and tests
Hi Jerrythespritzer

I would agree with your comments reference Tensile bond strength testing porous abradables, but not tensile testing coating as no adhesives are involved. Important point though and one that sometimes seems to be overlooked. Epoxy adhesives can penetrate coatings, strengthening both coating and bond giving false high readings. Using thick coatings and or sealing with a low strength sealer can help reduce this effect though.

Got a feeling that karusade is looking at this from an academic study view point rather than practical day to day testing.
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10-24-2008, 02:55 PM,
#7
RE: Question on abradable coatings : manufacturing and tests
Hi Karusade

The testing you mention is described in the paper entitled 'Freestanding abradable coating manufacture and tensile test development'.

If you have access to Science Direct then you can follow this link:

https://www.sciencedirect.com/science/ar...via%3Dihub

Hope this helps, if infact you are still working on this!!
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10-24-2008, 08:12 PM,
#8
RE: Question on abradable coatings : manufacturing and tests
Hi Richard

Sign0016 to the Surface Engineering Forum and thanks for your input.
Reply
10-27-2008, 05:19 PM,
#9
RE: Question on abradable coatings : manufacturing and tests
(05-18-2007, 07:53 PM)karusade Wrote: ...
3) I have to perform some tests to characterize abradable material in the next weeks and I would like to know how people do tensile tests on abradable ?

As I know most use ASTM 633 , and use FM1000 adhesive, with it no important penetration modify result (I have use it on 50% porosity coating, with no problem, notice that thickness is generally > 500 micrometers). I know that on some specifications they use epoxy adhesive that have a very high penetration but result is wrong and specification is wrong!
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