The scratch hardness of paint film is a key indicator to evaluate the ability of coatings to resist scratches from sharp objects, and is widely used in automobiles, furniture, electronic equipment and industrial protection. This test provides a quantitative basis for product durability assessment by simulating the scratch damage that the coating may encounter in actual use. Although the principle of the test method is intuitive, the actual operation needs to comprehensively consider the instrument type, load application method, and result judgment criteria to ensure the comparability and accuracy of the data.
Test Principle:
The basic principle of scratch testing is to make the scribing needle scratch the surface of the paint film under a controlled load, and evaluate the hardness by observing whether there are visible scratches on the coating or determining the minimum load required to scratch the coating. Commonly used instruments can be divided into three categories: one is manual scratch meter, which is loaded step by step by weight; the second is the automatic scratch meter, which can continuously and linearly increase the load and monitor the acoustic emission or friction signal in real time; The third is a pencil hardness tester, which uses a standard hardness pencil to scratch the surface at a fixed angle. Different instruments are suitable for different scenarios, and the selection should match the expected use conditions of the coating.
Influencing factors
The test results are affected by multiple parameters. The radius of curvature of the needle tip and the material (usually diamond or carbide) directly affect the stress concentration. The load application rate and holding time determine the dynamic effect of loading. Ambient temperature and humidity may alter the viscoelasticity of the coating. The rigidity of the substrate also interferes with the scratch morphology through the support action. In addition, coating thickness, curing degree and substrate treatment status are all intrinsic variables that cannot be ignored. Before testing, the environment and the specimen should be fully balanced according to relevant standards (such as ISO 1518, ASTM D3363).
Comparison of common test methods
| Method Name | Core features: |
| Progressive loading method | The load increases linearly, and the critical load can be measured, which is suitable for scientific research |
| Fixed load method | Single or multi-point constant load, suitable for rapid screening of production lines |
| Pencil hardness method | Easy to operate, low cost, and relatively rough results |
Result judgment and data analysis
The result is usually determined by combining visual observation with instrumental measurement. Visual judgment is based on whether there are continuous scratches, whether the coating is peeling off, and whether the substrate is exposed. The instrument method is to measure the width and depth of the scratch by an optical microscope or profiler, and use the formula to calculate the hardness value. For example, for some plastic coatings, the scratch hardness is HsIt can be approximated as: Hs = k · P / w2where P is the load, w is the width of the scratch, and k is the constant related to the geometry of the scribing needle. When analyzing the data, attention should be paid to distinguishing between different failure modes such as elastic recovery, plastic deformation and brittle cracking.
Considerations in the test
To ensure test reliability, the following measures are recommended: the scribing tip needs to be regularly checked for wear and calibrated; The specimen should be flat and fixed to avoid vibration; Each sample is tested at least three different locations to take an average; Record environmental conditions and correlate them with test reports. For multi-layer coating systems, it is recommended to test in layers or account for scratch penetration depth. When the test results are abnormally discrete, the uniformity of the coating, the roughness of the substrate, or the alignment of the instrument should be checked.
Epilogue
Paint film scratch hardness testing is an important bridge between coating performance and end application. With the development of new coating materials and processes, testing methods need to be continuously optimized, such as introducing nano-scratch technology to evaluate micro-area performance or combining digital image analysis to achieve automatic fault identification. Practice shows that clear test objectives, strict adherence to standard processes, and comprehensive documentation of test conditions are the basis for obtaining effective data. In the future, further harmonization across industry standards will help improve the versatility and reference value of test results.
References
ISO 1518-1:2019, Coloured paints and varnishes — Scratch tests — Part 1: Constant load method
ASTM D3363-20, Standard Test Method for Determining Paint Film Hardness by Pencil Test
Technical Handbook of Physical Properties Test of Paint Film, Chemical Industry Press
Advances in Coatings Characterization: Scratch and Mar Resistance, Journal of Coatings Technology Research