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