How accurate is the scratch hardness test?

The pencil as a measuring instrument

When testing scratch hardness according to Wolff-Wilborn, pencils with different degrees of hardness are dragged over the surface to be tested. Pencil hardness is the designation of the two pencil hardness grades, of which the softer one just leaves a writing mark on the lacquer film, while the next hardest one creates a perceptible indentation on the film.

The pencil scratch hardness test according to Wolff-Wilborn is still commonly used by many users.

The advantages: Fast and uncomplicated

This proven method offers the advantages that it can be carried out quickly and easily - and that the necessary equipment is easy to procure.
All that is needed are pencils in graduated degrees of hardness and a holding block with the help of which the operator guides the pencils at a 45° angle over the surface to be tested. For these reasons, the method not only has a long tradition, but apparently also prospects for the future: in the Asian markets, pencil scratch hardness testing according to Wolff-Wilborn is of continuously growing importance.

The disadvantages: Reproducibility and nuance are lacking

Nevertheless, every user is well advised to scrutinise the testing methods he uses from time to time in order to take advantage of optimisation opportunities. Here, the pencil hardness test indeed offers some potential. For although ISO standard 15184 (Coating materials - Determination of hardness of coatings by pencil testing/pencil hardness) specifies a clear procedure, some other methods of testing with pencils are also in use, some of which are no longer valid, but users are often unaware of this. This means that in some cases measurement results are compared that are not comparable at all. For example, there are different specifications for the length of the graphite lead to be exposed. The specifications vary between 3 mm and 6 +/-1 mm. In some test guidelines, the pencil is guided at an angle of 30° instead of 45°, and the weight block is pushed or pulled. There are different opinions on the geometry of the test-relevant end of the graphite lead tip - "flat" or "rounded" - as well as on the type of abrasive paper used to prepare the graphite lead. The same applies to the test load, which varies from 300 g to 7.5 N.

Not all pencils are the same

There is also a factor that many users of pencil scratch hardness testing are not aware of: There is no standard for the hardness gradation of pencils. Instead, each manufacturer has its own factory-specific hardness gradations, and this removes the basis for any reproducibility of the procedure: a pencil lead of hardness "3H" from manufacturer X will almost certainly not correspond exactly to the hardness "3H" from manufacturer Y. In fact, even a single deviation from the working method specified by ISO 15184 can lead to undesirable inaccuracies in the test results, which may not be recognised as such. With some of these variants presented here, the test result even changes considerably. In addition, there is a suspicion that the stability of the properties of pencils is limited in time. This means that age- or storage-dependent differences could occur within one and the same batch, which of course also influence the measurement results.

Uncertainties in evaluation

Uncertainties also exist in the evaluation of the scratch test. It is not quite easy to interpret and communicate the test trace left by the pencil, if any. The pencil lead can leave a plastic deformation on the surface to be tested; on the other hand, it can also injure the surface. A trace of abraded graphite is also possible, the clarity of which is influenced by even the finest structures on the surface to be tested. The scepticism towards the pencil test method is all the more appropriate because in many industries the demands on the surface quality of components are becoming ever higher. A wide-ranging and finely graduated display of even small differences/tendencies/nuances is therefore becoming increasingly important, and from this point of view the "pencil method" is certainly not the method of choice for the reasons described.

Results of a scratch test are not always easy to interpret.

Alternatives exist

In view of these uncertainties, it might seem reasonable to look for alternatives - and these alternatives do exist. The interested user has various other standard-compliant test methods at his disposal, of which ideally at least one is particularly well suited to precisely record the product properties to be tested and to present quality differences in a wide range.

With other test methods, much more accurate and reproducible results are achieved.

An example of this is the „Scratch Hardness Tester 413“ from Erichsen. With this device, the sample is fixed on a turntable. The testing tool is attached to a load arm with an adjustable weight that can be used to regulate the force of the tool on the sample. The resistance of the specimen to this action is visually assessed by the scratch mark. Thanks to a large selection of test probes, the device covers a wide range of specimens and degrees of hardness. The fine gradation of the test force in 0.01 N steps creates the prerequisite for very precise measurement results, which can be objectively represented as quality differences in numerical values. With such a "scratch hardness tester", more reproducible results can be generated that are easier to communicate. Therefore, in many cases it is worthwhile for the user to convince his customers and suppliers to come to a new synchronisation of the test method through joint agreement and thus to unambiguous, objectifiable and perfectly communicable results.