tensile bond strength and met examination

[TurbineRepair]

Hi all,

Since we a spraying aero engine compounds our coating need to meet defined quality specs. Currently we are mainly testing for tensile bond strength and general metallographic make up. I would like to move away from testing bond strength on every part to every batch of powder to save money. We have not had a bond strength test fail so far which leads me to hypothesize that if the test piece passes metallographic evaluation it will pass tensile bond strength. Would people agree that this is indeed a reasonable assumption or is it likely that although the met examination is good the bond strength could be below par?

Thanks,

Jim

[erick212]

Hi all,

Since we a spraying aero engine compounds our coating need to meet defined quality specs. Currently we are mainly testing for tensile bond strength and general metallographic make up. I would like to move away from testing bond strength on every part to every batch of powder to save money. We have not had a bond strength test fail so far which leads me to hypothesize that if the test piece passes metallographic evaluation it will pass tensile bond strength. Would people agree that this is indeed a reasonable assumption or is it likely that although the met examination is good the bond strength could be below par?

Thanks,

Jim

That is not a correct assumption, even if the micro passes it does not mean that the tensile will. Take for instance a GE coating that I was developing..... the micro looked great, passed both Rockwell and Vickers hardness but, when it came to tensile, it failed really bad. The min requirement was 8500psi and I was failing @ about 6000psi. This is just one out of many times this has happened to me. Are you doing your testing in-house or are you sending it out? If you are sending it out you may want to look into bringing it in-house... in the long run you may save some time and money.


Erick


Erick

[TurbineRepair]

Hi Erick,
Thanks for your input.
We are currently sending the test buttons out for testing, around �400 for 3 buttons. I agree it would be nice to have the capability in house, �10K for the machine ..........
We are actually working to GE specs at the minute and the spec does say that 100% testing is not required once we have a proven history of successful passes. As I said we haven't had one fail yet but we haven't been spraying for very long.

Jim

[erick212]

I don't know what spec you are working to but, some Ge coating require a Round Robin correlation then.. 16 runs which means you need to run 16 coupons for micro, 16 samples for Rockwell, and here is the big one..... 16 sets of 3 bond bonttons for initial Qual... Now that's were it gets expensive sending them out. After doing qual on a few coatings you payed for the machine + ... As for finding out the minimum bond test required you can call your GE rep or the customer you are doing the coating for and find out from them. Always get things in writing... very inportant. I just would not want to get caught not doing all the requirements during an audit or if there is a coating failure after it leaves your facitlity.


Erick

[drramc]

Hi,
Unlike solid materials where microstructure can be correlated with strength properties, in thermal sprayed deposit it is not correct to rely on micro for tensile bond strength. In sprayed deposit mcro is used to find the porosities,oxide particles etc,and thus for eliminiting the negative effect on bonding.Also it is a purely mechanical bond and thus metallugical properties like microstructure has very little role to play in the bond strength.
ram chattopdhyay

[TurbineRepair]

Hi Ram,

Thank you for your comment, though it's not what I'd like to hear
As part of the micro examination we are looking at the coating interface. If we can clearly see a good bond with no signs of delamination would that not lead to a good tensile strength at the bond?

Erick,

We have all the appropriate manuals and standard practices for the coating acceptances so getting the info is not a problem. Why do you say 16 as number of samples?

Thanks

Jim

[Gordon]

Hi Jim

I would have to agree that you can not predict the bond strength from metallographic evaluation. Evaluation of the substrate/coating interface for profile, contamination and intimacy are important indicators to bond strength, but not enough on their own to predict strength. I have experienced situations where from the microstructure, I have predicted bond strength test failure and then been surprised that they passed and also the reverse situation where microstructure looks good, but bond strength test fails. You need to also consider that you are testing different test pieces, which may in fact not be exactly the same. This also applies to the job you are coating. These test pieces are more a system for checking process set-up and consistency rather than the actual coating quality on the treated components.

[TurbineRepair]

Thanks for that Gordon. Something to strengthen my case for buying the machine....