Adhesion evaluation method
The adhesion between the coating and the substrate is one of the key indicators to evaluate the performance of the coating system. In the field of industrial testing, the circle method, the grid method and the pull method are three widely used adhesion testing methods. Each method is based on different mechanical principles and failure modes, and the test results differ in numerical and characteristic dimensions, but there is a certain physical correlation between them. Understanding the relevance of the results of these methods can help to rationally select test methods in practical applications and form a more comprehensive evaluation of coating adhesion properties.
Test Principle:
The circle scribing method uses a fixed load of a scribe needle to make a continuous circle movement on the coating surface, and is rated according to the number of turns the coating is scratched and the peeling condition. The grid method uses a cutting tool to form a grid on the coating, which is peeled off by tape and rated according to the proportion of the peeling area of the coating in the grid area. The pull-a-pull method measures the maximum stress when the coating is pulled apart by bonding a specific fixture to the surface of the coating and applying a tensile force vertically using a tensile machine, and directly obtains the adhesion strength value (usually in MPa).
Result expression and rating system
The output form of the results varies depending on the method. The results of the circle method and the grid method are usually graded (such as level 1 to 5, the smaller the number, the better the adhesion), which is a semi-quantitative evaluation. The pull-off rule provides a quantitative value of strength. This fundamental difference makes direct numerical conversion difficult, but through a large number of comparative experiments, it is possible to establish an empirical association between different ratings and intensity ranges.
Common adhesion test method results correspond to reference
Grid Rating (ISO/ASTM) - Approximate range of pull-open strength
Level 0 - >5 MPa
Level 1 - 3.5 - 5 MPa
Level 2 - 2.5 - 3.5 MPa
Level 3 - 1.5 - 2.5 MPa
Level 4 - 0.5 - 1.5 MPa
Level 5 - <0.5 MPa
Correlation analysis
The correlation between the three methods is mainly based on the cohesive strength of the coating, the bonding strength of the coating-substrate interface, and the stress state induced by the test. The pull-apart method measures the vertical interfacial bonding strength. The grid method and the circle method introduce shear and combined failure stresses, and the results are not only affected by the interface adhesion, but also closely related to the cohesive strength, flexibility and cutting edge effect of the coating itself. Therefore, for coatings with high cohesion strength but average interfacial adhesion, the results of the grid/circle method may be better than those of the pull-apart method. On the other hand, for brittle coatings, the pull-open method may measure higher values, but the grid method does not perform well.
This relationship can be simplified with the help of models. The adhesion strength of the interface is set to σi, the cohesive strength of the coating is σc。 The success of the grid method depends on whether the stress concentration at the edge of the grid after cutting causes the coating to peel off, which is σiWith σccompound function. The rough empirical relationship can be expressed as: the grid level and min(σi, k·σc) is negatively correlated, where k is the coefficient related to the plasticity of the coating and the geometry of the tool.
Method selection
In actual quality control or research work, the selection method needs to consider the coating system, substrate type and focus on the method. The pull-open method provides the most direct quantitative mechanical data for accurate comparison and performance tracking. The grid method is easy to operate and has a small destruction area, which is suitable for on-site rapid inspection and horizontal comparison of different batches of products. The circle method is more sensitive to coating flexibility and is often used to evaluate multi-layer coating systems.
Combined testing is recommended for critical evaluation: for example, the baseline strength data is obtained by the pull-apart method, supplemented by the grid method to evaluate the local uniformity and shear resistance of the coating. When the results of the two methods diverge significantly, the coating failure mode (cohesive failure, interfacial failure, or hybrid failure) should be analyzed in depth, which often reveals the deep performance information of the coating system.
Summary
There is a certain correlation between the test results of the circle method, the grid method and the pulling method, but it is not a simple linear correspondence. This correlation is constrained by the mechanical properties of the coating material, the interfacial state and the test principle. By understanding the physical basis and failure mechanism of each method, the test results of different methods can be correlated and interfered, so as to form a multi-dimensional and more reliable evaluation of coating adhesion. In practical applications, the leading test method should be selected according to specific needs, and can be verified and deepened by complementarity testing.
References
ISO 2409: Coloured paints and varnishes – Grid test.
ISO 4624: Colored paints and varnishes – Pull-off test for adhesion.
ASTM D3359: Standard Test Method for Testing Adhesion of Coatings by Tape.
ASTM D4541: Standard Test Method for Pull-Off Adhesion Testing of Coatings Using a Portable Adhesion Tester.
A review of related coating mechanics and adhesion testing techniques.