Pencil hardness tester rapidly evaluates scratch resistance of paint film.

Introduction

In the production of industrial coatings, electronic packaging or automotive interior parts, the scratch resistance of paint films or coatings directly affects the durability and service life of products. Pencil hardness testers have long been used to quickly assess the resistance of coating surfaces to scratches as a simple, repeatable semi-quantitative testing tool. Based on domestic and foreign standards and industry practices, this paper systematically describes the standard process, result interpretation and precautions of pencil hardness tester in the scratch resistance evaluation of paint film, aiming to help inspectors obtain stable and reliable test data.

Test Principle:

The pencil hardness tester uses a set of standard pencils (from 6B to 9H) to scratch the surface of the coating under a fixed load and angle, and determines the surface hardness by observing the coating for visible scratches. Its essence is to simulate the shear stress exerted on the coating by hard objects in daily use, and the unit is expressed as a series of pencil hardness. Key parameters during testing include the angle between the pencil and the coating (usually 45°), the normal force applied to the pencil tip (750 g or 1 kg in common), and the way the scratch is interpreted (visual or microscopic).

Instruments and materials

A complete pencil hardness tester consists of the following parts: a load-bearing platform (for applying constant pressure), a pencil gripper (adjustable angle), a standard pencil set (range 6B to 9H), sandpaper for calibration (#400或更细), and a magnifying glass (10x more). Before testing, sandpaper should be used to sand the tip of the pencil flattened and form a rectangular section about 1mm wide to ensure even pressure distribution. The recommended temperature (23±2)°C and relative humidity (50±5)°C in the operating environment avoid temperature and humidity fluctuations interfering with the mechanical properties of the coating.

Operation process

The testing steps can be summarized into four stages:
1. Sample preparation: The test plate coated with dry paint film should be fixed on the water platform, and the thickness of the paint film should be controlled within the range of (25±5) μm, and there should be no visible defects on the surface.
2. Pencil Sharpening: Smooth the pencil tip with sandpaper before each use to remove the wear layer and ensure the rectangular contours of the section.
3. Scratch operation: Insert the pencil into the fixture so that the tip is at a 45° angle to the paint surface, and advance forward at least 25mm in length at a speed of about 5mm/s, and the application direction should be perpendicular to the coating surface. Each pencil is repeated three times on the same specimen, each time replacing the newly polished pencil tip.
4. Record the results: After gently wiping the scratched area with a dry soft cloth, observe whether continuous scratches (more than 3mm in length) appear in a well-lit area. Record the hardness level of the pencil that just does not cause scratches (i.e., the pencil number that is hardest and does not damage the coating).

Result interpretation and standards

The interpretation criteria are based on an international method: the pencil is tested sequentially from hard to soft to find the first pencil hardness level that does not cause visible damage at least twice out of three scratches. For example, if two of the three scratches of an H-grade pencil do not produce scratches, and the 2H-grade has scratches all three times, the final result is denoted as H. For improved consistency, it is recommended to wipe the scratched area with a cloth dipped in isopropyl alcohol before interpretation to eliminate debris interference. Common reference standards include ISO 15184, ASTM D3363, and GB/T 6739, and the correspondence between the three to the pencil hardness grade is shown in the following table:

Influencing factor control

To ensure test repeatability, focus on controlling for the following variables:
- Pencil Tip Condition: It should be used immediately after sanding to prevent the tip from deforming or oxidizing the passivation layer formed by increasing the test value.
- Apply pressure: The load-bearing platform should be calibrated regularly to ensure that the vertical force value deviation is within ±5%.
- Scratch speed: Excessive speed may cause dynamic elastic deformation, resulting in a hard result; Too slow increases the risk of creep. Constant mechanical propulsion is recommended rather than manual.
- Coating thickness and substrate: Thicker coatings (greater than 40 μm) may mask true hardness performance due to increased toughness, so it is recommended to verify at standard thicknesses; Test results for rigid substrates (e.g., metals) and flexible substrates (e.g., plastics) are not interchangeable.

Common deviations and corrections

The following deviations and corresponding corrections may occur in the test:
If the test results of the same pencil are inconsistent multiple times, first check whether the surface of the sample is flat or dust pollution; Clean and dry with isopropyl alcohol before use. If the edges of the scratch are jagged, it indicates that the tip of the pencil is not smoothed or the elasticity of the coating is too strong, and the pencil should be re-sharpened and the scratching speed should be reduced. For soft coatings (e.g., acrylics), consider adjusting the load to 500g to mitigate the crushing effect and indicate the correction conditions in the report.

Data Recording Essentials

Each report should include the following information: sample name and lot, paint thickness and curing conditions, ambient temperature and humidity, pencil durometer model and weight value, test orientation (straight or horizontal), criteria (e.g., visual or microscopic confirmation), and final hardness level. It is recommended to take photos and archive each scratch, especially the scratch image corresponding to the critical level, so as to facilitate subsequent comparative analysis.