pencil hardness and tukon hardness test

[transit]

Hi,

Newbie here. I have this long-bugging question of how to correlate the traditional pencil hardness test to tukon hardness test for painted surface.

For example, a HB hardness from pencil could meant a 6 KHN from tukon and so on.

Thank you,
Eric from Univ. of Michigan

[Gordon]

Hi Eric

to the Surface Engineering Forum.

I have not come across any equivalence tables for these hardness test methods. That does not mean they don't exist. There will be some correlation between pencil hardness and Knoop hardness, but I suspect like with some other hardness scale conversion will not be accurate or consistent, particularly when applied to different coating types.

Not sure, but may be worth reading: (link dead)

The only reliable way of establishing a correlation is to actually do comparative testing on the same coatings of a specific coating type. The correlation will only be reliably valid for that specific coating type under similar conditions as to the testing.

Pencil hardness test will be a good/quick/simple test for gauging the state of a coating, but in the case of stating whether a coating complies with say a specification in KHN, only a Knoop indentation hardness test can answer that, converting from a pencil hardness would certainly not be good practice. The same really applies to all different hardness testing methods and scales, their relationship is not consistent. Hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure.

Hope that helps

[transit]

Thank you very much for the warm welcome and great reply.

I understand that the pencil hardness test is more of qualitative analysis. That's one of the main reasons that my company is in the process of trying to switch over to Tukon since it's more scientific and "reliable".

Thank you very much once again.

[Gordon]

Hi Eric

Your response is much appreciated

In my view both tests are valid and very useful as long as they are considered as separate tests. Pencil test is a bit like the Mohs hardness test, a comparative test of resistance to scratching/gauging , while KHN is measure of indention size with respect to a specific indenter and force. Hardness is really quite a vague term, we can say comparatively one material is harder than another, but a numerical hardness value is strictly down to the specific test procedure. It is unlike measuring mass or length. Banging on again sorry comments are not specifically aimed at just you, but similar questions are often asked.

[transit]

Hi Gordon,

would you say that it's possible to correlate the two test methods if I perform the same test method/parameter for Tukon test on a list of material with different hardness, then use pencil test to find out the hardness trend between two methods?

Thank you,
Eric

[Gordon]

Hi Eric

Yes, ideal situation really. Having access to both methods and running tests in parallel on same parts should enable you to define correlation/equivalence with a sense of the accuracy and limitations and increased level of confidence. Certainly, a great deal better than relying on a generic conversion table. To be able to define say for example; a coating that lies between HB and 3H (from known calibrated pencil set) will (with high probability) fall within specification KHN limits of a certain coating, could be very useful. A quick and cheerful two pencil test could be used on the shop floor to check upper and lower limits giving confidence that further down the line the coatings will more than likely pass the KHN tests.

[bruisjem]

Pencil hardness test is only one of many tests to evaluate coating. There is obviously more than hardness of any coating to be considered,so do not judge a finish entirely on the basis of the test.