metallographic preparation

[volf]

Good evening, everybody.
Can somebody advise me?
My question is about thermal spray coating metallographic specimen preparation.
Same samples were prepared by different manner, (it was Ni-Al Metco 450 NS, plasma coating).
One- by Bakelite hot molding, another by Epoxy (Epofix- of Struers) cold casting.
After the same grinding and polishing steps Bakelite molded sample demonstrated
microstructure without any defect, but epoxy molded sample revealed badly integrated structure with local interface separation and not sufficient bonding between particles.
Reason, that I see- is:
1- stresses, generating in the coating, caused by epoxy solidification- very big contact surface epoxy- coating.
From the other side, may be it is a real structure, and the nice and dense structure of Bakelite molded sample is the result of pressure, applied at the time of molding by press (Simplimet 2000). It is about 20 Mpa.
Thank you for any help.

volf

[Gordon]

Hi Volf

You are really highlighting the finickiness nature of thermal spray coatings during preparation for metallography. Many do not appreciate, that when viewing a coating microstructure it can be very difficult to know whether what we are seeing is a true representation of the structure or one that has been modified or distorted by the preparation process.

A common problem is that of pull-out, making the structure look more porous than it really is and exaggeration of faults. Metallographers will go to some lengths to avoid this problem, but another common problem can also occur where the structure looks too good to be true, where smearing occurs covering and hiding faults. A serious problem for the metallographer preparing thermal spray coatings, but made far worse by those that see no problem.

It is a bit difficult to comment on your particular problem, without actually being there and seeing. Generally I have not noticed any real problem with either mounting procedures with these types of coating. Epoxy cold mount procedures are favoured when dealing with the more structurally sensitive coatings, with the belief that it imparts less thermal and mechanical stress.

It is possible that your bakelite mounted sample is showing a slightly ?too good to be true? view, while your epoxy mounted sample may be showing the opposite effect. The true view lying somewhere in between.

One advantage of epoxy resin is it's ability to impregnate some of the porosity in the coating. Highly recommend vacuum impregnation. Impregnated samples tend to suffer very much less with smearing and pull-out as the sensitive areas are given support by the epoxy resin. Also, true porosity/cracks can easily be identified by the fact that they are filled with epoxy resin. Porosity/cracks not filled with epoxy resin could be interpreted with suspicion.

Tip: When making bakelite mounts, heat coating sample ~100 ?C, place tiny amount of epoxy resin (Epofix works well) onto coating. Epofix becomes very fluid and searching when heated. Quickly place sample into preheated mounting press and bring up to pressure without delay. Epofix cures quickly at mounting temperature. The pressure during mounting helps further with pushing of the resin into the coating. This method has served me well and seems to impregnate the coating almost as well as vacuum impregnation (possibly even better). Only problem is it can foul up your mounting press so regular cleaning is important.

[jonini]

I have metallographic preparation procedure for HVOF wc coating and this gives me <1% porosity, but my worries is...is this correct coz I've search on the net about the preparation and it seems that I'm the only one who uses this for final preparation for polishing w/c I'm doing manual polishing using 6,3,1 microns diamond cloth..... is this correct???