Hi Gordon,

Just read your reply today.

I hope you don't mind my continuing the discussion on the abradable coatings.

I want to ask you the possible causes of getting inconsistent hardness values from time to time.
I mean what critical factor can give significant contribution to this inconsistency.
We spray the specimens of Ni 75 - Graphite 25 coating as an abradable layer for compressor rotor of aircraft engine.
Although we always obtain hardness values within the range as recommended by the OEM and/or the material manufacturer (i.e. 30 - 50 HR15Y ) , there is still an inconsistency in the test results.
Sometimes they are all above 40 HR15Y , but lately they are all in the lower range i.e. 30 - 40 HR15Y only.
This makes us difficult to release the test result as the representative of the sprayed part, in a way that we would only find out the actual quality of our thermal spray process result after the part is installed and tested as a complete engine.
If the part has not released with top quality, we are afraid it will abrade easily and during engine testing (high pressure, high temperature) will affect other parts which are very costly.

Hope you or others can help me again this time and share the knowledge.

Thank you.

Hi Madila

These nickel/graphite coatings are sensitive to changes in spraying parameters and the slight variation in powder properties you get from different batches of powder (thorough tumbling of powder before use is also important). This reflects significantly on the hardness and abradability properties of the coatings. It is generally accepted that regular hardness testing should be done to first establish the optimum spraying parameters to achieve the desired hardness level. Usually done by adjusting the powder feed rate only (Metco 6P gun, higher gives softer, lower gives harder) or adjusting oxygen and fuel flows simultaneously only (Metco 5P guns, lower gives softer, higher gives harder). Hardness testing and "tweaking" parameters does need to be done on a regular basis and particularly when any changes are made like powder batch or equipment settings.

Another area that may be worth looking at is the hardness testing itself. Make sure the recommended procedures are carried out correctly. Coating thickness and preparing the coating for testing are important factors.

Hope this helps, but suspect you probably already know this anyway.

Hi again Gordon,

Thank you very much for the reply...
Because of your reply I remember to review the technical bulletin for this powder in order to know the general guidance from the material manufacturer.
I found the exact same recommendation from the manufacturer that also recommend to make the powder flow adjustments one-half pound per hour at a time and do hardness test after each change.
That's what I should try.. and I will.

Thanks for thought