Overview
Pencil hardness tester is a simple and effective technique widely used in the surface hardness evaluation of coatings, inks, plastics and various industrial coatings. This method uses a set of pencils of known hardness to draw over the surface of the coating film under constant load and angle, and the hardness grade of the coating film is evaluated based on whether it causes visible scratches. The principle is based on the ability of the material surface to resist plastic deformation, and the results are expressed in pencil hardness scales, from the softest 6B to the hardest 9H. This method has strong standard operability and low equipment cost, which has important practical value in quality control and production sites.
Test Principle:
The core principle of the test is to simulate the scratching behavior of sharp objects on the surface of the coating film in actual use. The instrument typically consists of a gripper that holds the pencil, a guide that ensures that the test pen barrel is at 45° (90° in some cases) to the sample surface, and a weight that applies a specified load (usually 750g or 1000g). During the test, the pencil lead is carved out of a cylindrical tip and smoothed, which is swiped across the coating film under a uniform push. By trying different pencils from soft to hard until you find the highest hardness pencil that does not scratch the surface of the coating film, the pencil designation is defined as the hardness of the pencil coating.
Pencil hardness grading
Pencil hardness grading follows a uniform designation system. The system is divided according to the ratio of graphite to clay in the pencil lead: the higher the graphite ratio, the softer the refill; The higher the clay ratio, the harder the refill. The grading order from soft to hard is: 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, 9H. Among them, B stands for black, H means hard, HB and F are intermediate hardness. In coating testing, common ranges typically cover from B to 9H.
Testing process
Testing should be performed in a standard environment, and samples should be flat, clean, and fully cured. Key steps include: selecting a drawing pencil that meets standards (e.g., ASTM D3363 or ISO 15184); Use a special sharpener to expose the refill by about 5-6 mm and grind vertically on 400 mesh sandpaper to create a flat, sharp-edged cylindrical surface; Load the pencil into the instrument and apply the specified load; Push the instrument forward at a constant speed of about 1cm/s for at least 7mm distance; Observe the scratches at an angle of about 45 degrees in good light. Each hardness scale should be tested at least three times and at different locations on the sample.
Grading criteria
It is necessary to distinguish between "scratches" and "indentations". Scratch refers to the appearance of visible, continuous linear grooves or material removal on the surface of the coating film; Indentation is only surface deformation, light reflection changes but no material is removed. The pencil hardness value of the coating film is defined as the hardness of the coating film if at least two pencils with the same designation do not scratch the coating film in a series of pencils with hardness designations under the specified test conditions. If only one is not scratched, the lower grade is used as the reported value. The specific classification correspondence is described in the table below.
| Hardness designation range | A description of the typical coating type or condition |
| 6B to 2B | Extremely soft coatings, such as some uncured or highly elastic systems |
| B to F | Soft coatings, such as partial decorative topcoats |
| HB to 2H | Medium hardness, commonly found in conventional industrial baking paints or well-cured solvent-based coatings |
| 3H to 6H | High hardness coatings, such as high-performance UV-curable coatings, certain ceramic coatings |
| 7H to 9H | Very hard coatings, only found in special cross-linking systems or hard overlays |
Notes:
Test results are influenced by a variety of factors. Coating factors include resin type, cross-linking density, pigment volume concentration, curing degree and thickness. Environmental factors include temperature, humidity. Instrument operating factors include load accuracy, nib grinding quality, push speed and angle. To ensure comparable results, all provisions of the chosen standard method must be strictly followed. It should be noted that the hardness of the pencil mainly reflects the scratch resistance of the coating surface, and there is no direct conversion relationship with wear resistance, impact resistance and other properties. This method may not be suitable for textured surfaces or soft substrates.
Pencil hardness testing falls under the category of scratch hardness method. Compared with other methods such as pendulum damping hardness (measuring the viscoelasticity of the coating) and indentation hardness (such as Buckholz indentation), the response mechanism is different, and the results do not have a direct mathematical conversion formula. However, in engineering practice, empirical associations can be established through a large number of comparative data. For example, for the same type of coating system, the hardness of the pencil and the hardness of the pendulum rod often show a positive correlation, but the specific relationship needs to be determined by experiments. When choosing a test method, it should be based on the type of damage that may be sustained in the end use scenario of the product.
Epilogue
Pencil hardness meter assays provide a standardized means of quickly and intuitively assessing the scratch resistance of coating surfaces. Its clear grading criteria provide a practical basis for coating development, process control, and product acceptance. Understanding its principles, strictly implementing standard operations, and interpreting the results are of practical significance to ensure that the quality and performance of coating products meet specific application requirements. With the emergence of new materials, the application scope and interpretation of this method also need to be analyzed in combination with specific systems.
References
ASTM D3363-20 Standard Test Method for Film Hardness by Pencil Test.
ISO 15184:2020 Paints and varnishes — Determination of film hardness by pencil test.
GB/T 6739-2006 Colored paints and varnishes - Determination of paint film hardness by pencil method.
Coating Process Editorial Board. Coating Process (4th Edition). Chemical Industry Press.