Measurement Variation in a Coating Thickness Meter

[GURU SDI]

Dear All,

We are getting Molybdenum coated parts from one of our suppliers.They are doing 100% inspection with coating thickness measurement meter and confirming the products. The same parts are received at our end & our inspection team doing the sampling inspection with coating thickness meter and raising Non-conformance. The drawing specification is 100 to 150 Microns. But the variation ranges 20 to 30 microns. Kindly provide your inputs for this measurement variation?

Thanks in advance..!

Gururaghavendran R

 

[Bev D]

Re: Measurement variation in Coating Thickness meter

You will need to do method comparison of the your system and your supplier's system. First ensure that both are calibrated. Then test multiple parts at the supplier twice and at your location twice. You will want to specify the exact location of the measurements on each part as the coating may vary across the part...

You will find a description of the approach and the the math in the attachment in this thread, post 7

You will find a spreadsheet that does the math and graphs for you here.

 

[Miner]

Re: Measurement variation in Coating Thickness meter

Good suggestions from Bev. What type of thickness testers are you and your supplier using?

 

[Ron Rompen]

Re: Measurement variation in Coating Thickness meter

I have found coating thickness meters (I am assuming you are using an ultrasound-type meter) to be notoriously inaccurate and not repeatable.
If you have the option open to you, I would urge you to use another method of evaluation (cross-section, mount and measure in a microscope).
What method does your supplier use for evaluation of their process?
If this is not feasible (for whatever reason) then I agree with Bev - a correlation between yourself and your supplier is the first place to start. Where are they measuring the coating thickness, and by what method?

 

[JeantheBigone]

Re: Measurement variation in Coating Thickness meter

Instruments for measuring coating thickness are generally sensitive to the exact chemical composition of the actual coating, which is why the calibration needs to be checked frequently.

I would urge you to use the mounted and polished cross-section method as the "gold standard" to calibrate both your instrument and that of your supplier using at least two coated samples with differing thickness. In fact, I would use the exact same samples used to make the metallographic mounts and have them measured both at your supplier facility and at your facility. If there are any differences, you know where to "dig."

Do be sure to measure coating thickness in the optical microscope using a stage micrometer traceable to NIST.