TBC Separation.

[steve]

Hi everyone,
I'm looking for advice regarding TBC top coat separation from an aluminized bond coat. Chipping can occur on both parts and samples.
The parts are APS and the sample is prepared with epoxy, cold mounted in a pressure pot.
The top coat bonds well to just bond coat only.
The aluminde is brittle which may be the problem but i'm not certain.
Any help would be greatly appreciated.

[Gordon]

Hi Steve

Could you describe the aluminising process for the bond coat? how does this effect bond coat surface texture?

[steve]

Hello Gordon,
The aluminizing process is a Nial "APVS" process using Cr-al donor with aluminium flouride activator. It's a chemical vapour deposition process. The base material is a nickel superalloy with a NiCraly bond coat. Aluminide coating thickness is approx 0.015" with an aluminium concentration of about 27% at the surface. The surface texture is similar to just bond coat with usual peaks and valleys.
I've attached some photos to hopefully better explain
   
   
   
   
best rgds,
Steve

[Gordon]

Hi Steve

Nice looking photomicrographs

Out of interest, you say cold mounted epoxy in pressure pot. Could you expand on that? I notice from photos the epoxy seems to be reinforced. Also interested in the pressure pot. The old fashioned metallographer coming out

The photomicrographs do seem to indicate a reasonable texture for bonding, though I was thinking along the idea that aluminising may be smoothing off the original texture of the bond coat. Well I'm not so sure now, but I will keep the option open.

Have you done any bond strength testing to back-up your metallographic findings on interface separation? Has this aluminised bond coat system proved itself ok before or is this just a hick-up?

The aluminides will be brittle, but then so will the ceramic top coat. I don't see much evidence of failure within the aluminised side of the interface to suggest this is the reason for failure.

[mehongbo]

Agree Gordon's input. I wish my TBC polishing is like yours-nice impregnated epoxy in pores yield to true porosity. But a lot of pressure made it I guess . then most likely this delaminiation (sample or part if delamination observed from matlab) comes from the mounting pressure.

It is too difficult to judge your base coating roughness from the photo, area too small. according to the photo to me the entire coating process is like APS MCrAlY bondcoat (10-15 micron)+Aluminizing (about 5 micron inch)+APS TBC (Sorry I may be wrong, not much experience on Aluminizing), then this 5 micron inch aluminishing should not smooth off your APS basecoat. Your option is to increase the MCrAlY particle size.

Hong

Hi Steve

Nice looking photomicrographs

Out of interest, you say cold mounted epoxy in pressure pot. Could you expand on that? I notice from photos the epoxy seems to be reinforced. Also interested in the pressure pot. The old fashioned metallographer coming out

The photomicrographs do seem to indicate a reasonable texture for bonding, though I was thinking along the idea that aluminising may be smoothing off the original texture of the bond coat. Well I'm not so sure now, but I will keep the option open.

Have you done any bond strength testing to back-up your metallographic findings on interface separation? Has this aluminised bond coat system proved itself ok before or is this just a hick-up?

The aluminides will be brittle, but then so will the ceramic top coat. I don't see much evidence of failure within the aluminised side of the interface to suggest this is the reason for failure.