Thickness Measurement of Sprayed Monel

[wrjordan]

Gentlemen, any recommendations for a guage / procedure to get an accurate mesurement of the deposited thickness of Monel (70%Ni / 28% Cu) deposited by Twin wire arc spray ? We have an inquiry for a coating up to 2mm thick, and while we can build the thickness, measuring the coating is an issue as we cannot physically measure the customers part we will be spraying. (But we of course can with out test piece.)

[Gordon]

Hi wrjordan

May be possible using portable electronic thickness measuring devices , the ones using both magnetic and eddy-current methods. But it will be very much dependent on what substrate material supports the coating.

Links to some previous posts related to this topic:
coating-thickness-measurement-t-66.html
thickness-testing-of-316-l-coating-t-135.html
coating-thickness-measurement-t-202.html
coating-thickness-measurement-t-502.html
instrument-check-coating-thickness-thermal-barrier-coatings-t-399.html

[wrjordan]

Many thanks. I had searched for Monel but you are correct - we have the same issue for spraying other materials like 316.
The substrate is Steel, which compounds the problem. We have tried Elcometer and Positester magnetic guages - with little sucess. As you suggest in some of the linked posts above, we could "calibrate" them against a known thickness of Monel / 316 and use that for our basis, is this a repeatable method, or more of a "best guess" approach?

I am not familiar with the eddy current guages - do have any reference I could review? It was suggested that I also look at UT guages, but from what I know, the coating is very thin to determine with Ultrasonics.

Appreciate any advise.

Bill Jordan

[Gordon]

Hi Bill

Quote:Many thanks. I had searched for Monel but you are correct - we have the same issue for spraying other materials like 316.
The substrate is Steel, which compounds the problem. We have tried Elcometer and Positester magnetic guages - with little sucess. As you suggest in some of the linked posts above, we could "calibrate" them against a known thickness of Monel / 316 and use that for our basis, is this a repeatable method, or more of a "best guess" approach?

First, I must say I can't remember ever measuring monel coating thickness other than by physical means (micrometer/vernier), so my comments are not based on practical experience.

Monel and probably TS monel coating, I believe can be ferromagnetic. Though this effect should be weaker than the steel substrate, it will cause problems with calibrating magnetic gauges (you can't rely on non-magnetic brass or plastic shims). Really, you will need to calibrate gauge using actual coatings/substrate of known thicknesses. To complicate matters further (or may be not) the ferromagnetic curie point for monel - nickel copper alloys is at relatively low temperatures ~ 25 -100 C, so standardising on measurement temperature would also appear to be critical. May be heating part say above 100 C may make monel coating loose all ferromagnetic properties and you can calibrate with say brass shims (assuming instrument is happy measuring at these temperatures).

I think really it will be down to a bit of experimentation/trial and error to find your way. I would have a preference for making up some test blocks from actual substrate material and spraying different (known) thicknesses of coating covering your desired max and min limits.

I doubt whether eddy-current testing would be any good, but may be worth a try if you have the option on your measuring device. Similar calibration problems will still apply.

Quote:I am not familiar with the eddy current guages - do have any reference I could review? It was suggested that I also look at UT guages, but from what I know, the coating is very thin to determine with Ultrasonics.

Ultrasonic gauges I believe are good for measuring thickness like tube walls etc., not sure on suitablity for coatings. If you have not already done so, look over sites like Elcometer and the like, may also be worth contacting them direct for advice.

Good luck

[wrjordan]

Gordon,

Much appreciated. I have browsed Elcometer etc, and as you sugggest not a lot of off the shelf solutions. Will look at making our own "calibration blocks".

Thanks
Bill Jordan

[HayeCious]

I am just a little confused about plastic shims.,Can you add more information about this thing.,I just want to know more about this thing.,

_________________
Structural Foam

[k09]

Hi everyone,

I have a similar problem here... I want to measure the coating thickness of WC-Co on MS substrate. The problem here is the diameter of the job is 1m+ and therefore micrometer does not give accurate readings (because of size of micrometer and handling it).

I have already searched elcometer, Fischer, Electrophysiks and Defelsko with no luck...

If any one here finds a suitable digital instrument for this purpose, Kindly let all of us know.

[Gordon]

Hi k09

I think the typical electronic thickness gauges used for thermal spray coatings may be ok, but for accurate measurements you will need to calibrate instrument with actual coating and substrate and not rely on supplied calibration shims.

You could measure roll circumference using steel tapes. I think this method is fairly standard for large rolls, though I think it requires more skill than using a micrometer and more attention to factors such tape positioning/tensioning and effects of temperature.

[k09]

Thanks for your reply Gordon,

Well i never tried calibrating it seperately on the WC sprayed samples as i thought that it would not help. Because, Magnetic coating(Co in Powder) on magnetic substrate would anyways not be compatible with the principle of magnetic induction..right...but i surely give it a try...

And I wanted to confirm if you meant-
Get circumference-->Get Dia (C=3.14xD) ....like you said, it does sound more difficult especially when you have +20microns tolerance on as sprayed surface.

Well...i am surprised that in spite of such vast developmemts and WC itself having a wide range of applications, there are no digital instruments for measuring Ferrous on Ferrous coating thickness...

[Gordon]

Hi k09

Quote:Well i never tried calibrating it seperately on the WC sprayed samples as i thought that it would not help. Because, Magnetic coating(Co in Powder) on magnetic substrate would anyways not be compatible with the principle of magnetic induction..rightQuestion...but i surely give it a try...

Yes, but there may sufficient differences in magnetic properties between coating and steel substrate for it to work. That's why calibration with actual coating is important. A lot of the electronic thickness gauges come with dual functions - magnetic and eddy current. Eddy-current relies on differences in electrical conductivity. I would try both to see which is best.

Quote:And I wanted to confirm if you meant-
Get circumference-->Get Dia (C=3.14xD) ....like you said, it does sound more difficult especially when you have +20microns tolerance on as sprayed surface.

Yes I assume you can get special made tape gauges specific for this purpose. I have no hands on experience, just observed people using them. Not sure on degree of accuracy.

Quote:Well...i am surprised that in spite of such vast developmemts and WC itself having a wide range of applications, there are no digital instruments for measuring Ferrous on Ferrous coating thickness...Exclamation

Unfortunately, these gauges require the coating to have significant differences in either magnetic or electrical conductivity properties to that of the substrate to work properly.

[wrjordan]

Gentlemen, I asked DeFelsko about suitable guages for this application and revieved the following reply from tech support
Question - Would any of your Eddy current or Ultrasonic thickness coating gauges be appropriate for measuring the thickness of a 0.5 ~ 2mm thick semi magnetic coating (Monel / 361 Stainless steel) over a base steel layer ?
Answer: Eddy current instruments are ideal for measuring non-conductive coatings on non-ferrous metals. Monel/ stainless is conductive and therefore can not be measured with eddy current.

Measuring this application with ultrasonic technology is challenging as well. The sound wave travels too fast through the coating for the instrument to measure accurately.

We do not have an instrument designed for measuring your particular application."

Looks like we are out of luck!

[k09]

tough luck!.. Even I had tried Defelsko..got the same response..

I had even tried Helmut Fischer. The instrument somehow looked more sophisticated and it had both measuring principles(Magnetic Induction and eddy current) in one unit. But unfortunately it wasnt compatible with the coating-substrate combination. in my case it is WC-Co on MS.

But in the end the technical guy at Fischer said that if I send them some samples they will try and check if they can somehow calibrate the instrument for that particular coating.. If I do manage to get some samples coated, I will send it to them.. If I get any positive results, I will surely update..

In the meantime, I also have an Electrophysiks, eddy current type CTG with me.. As Gordon suggested earlier, I will try and see if I can calibrate the same for WC-Co coatings..

regards

K09

[rayudugirish]

Hi,
I faced a similar problem of measuring thickness of monel over SS using eddy current tester. I tried out numerous makes of gauges made by the likes of Elcometer, Physic etc but no success.

Later I realized during the coating process monel changes its magnetic properties, leading to erratic readings. Well to solve the same issue i had to resort to direct reading using micrometer and later on testing on test coupons by metallography.

For WC-Co or WC-Co-Cr no such problem was encountered and fairly good results were obtained within a range of 10 20 microns variations.

Girish

[k09]

Hi Girish,

Thanks for your inputs.

Can u please tell us which gauge you used for measuring WC-Co Coatings. I also have elektrophysik gauge but I did not get correct readings.. I can say this because i verified it with micrometer and the readings had a vast difference...

Also, Cobalt being magnetic, it causes erratic readings especially when the substrate is also ferro-magnetic (in my case MS)...

I have searched a million websites just to get some principle or instrument to measure thickness as in this case... but with no luck.. If i cudnt find nething on google then there is a major possibility that such instrument doesnot exist.....but i will keep trying..

Regards

K09

[dcrawford]

Hi,
Seems like you could use a laser device to measure the build up of the coating as it was being sprayed. I have some experience using them to profile weld depositions, but not with thermal spray. It could be as simple as pointing the instrument at one point on the part and measuring the build up as the torch makes multiple passes to as complicated as scanning a 3D profile of the part as it is being sprayed and having the computer adjust the robot's program to ensure consistent coating over the entire part.

Dan

[k09]

Hi Dan,

Thanx for the info. Well, I have never heard about laser coating measuring instruments. Can you please provide us Website links of the instruments that you have.

perhaps the website will have some info regarding the working principle of the instrument. it wud give us some idea whether it would work for thermal spraying applications..

Thanks & Regards

K09