In the application of multi-layer structural materials such as composite materials, coatings, films, printed circuit boards, etc., the bonding strength between layers (that is, interlayer bonding force) is an important index to determine their overall performance and service life. Insufficient interlayer bonding force may lead to delamination, peeling and falling off of materials during use, thus affecting their functions and safety. Therefore, it is of great significance to test the strength of interlayer bonding force scientifically and accurately.

First, the definition and influencing factors of interlayer binding force

Interlayer adhesion refers to the ability of different material layers to adhere to each other through physical or chemical interaction. Its strength is affected by many factors, including material type, surface treatment method, bonding process (such as temperature, pressure, curing time), environmental conditions (humidity, temperature change) and so on.

Second, the commonly used test methods of interlayer adhesion

At present, the common test methods of interlayer adhesion mainly include the following:

1. Peel Test (peel test)

Peeling test is one of the most commonly used test methods to evaluate the force required for interlayer separation. According to the different peeling angles, it can be divided into two forms: 180 peeling and 90 peeling. When testing, one layer of material is fixed, and the other layer is peeled off at a constant speed by stretching equipment, and the required force value is recorded, so as to calculate the peeling strength per unit width.

2. Pull-off Test

This method is mainly used to test the bonding strength between coating or adhesive and substrate. The coating surface bonded with the test head was pulled away from the substrate by a special drawing instrument, and its maximum pull-off force was measured. This method is easy to operate and suitable for on-site detection.
　　3. Shear Test

Shear test is to measure the shear resistance between layers by applying shear force parallel to the interface. This method is suitable for evaluating the bonding properties of structural adhesive, welding layer and other materials.
　　4. Cross-cut Test
　　Commonly used for qualitative test of coating adhesion. After drawing a grid with a certain distance on the surface of the coating, use adhesive tape to attach it and peel it off quickly, observe the degree of coating peeling off, and evaluate it according to the standard rating (such as ISO 2409).

5. Nano-indentation and micro-area testing technology
　　With the development of micro-nano manufacturing technology, micro-testing methods such as nano-indentation are also used to evaluate the interlayer bonding performance of micro-areas, which are suitable for high-precision products such as thin films and semiconductor materials.

Third, the evaluation and standard of test results

The forms and units of data obtained by different test methods are different, which need to be selected and compared according to specific application scenarios. Generally, the unit of peeling strength is N/mm or kN/m, and the unit of drawing strength is MPa. There are related international standards, such as ASTM D903 (peel test), ISO 4624 (pull-out test) and ISO 2409 (ruled method), which provide a unified technical basis and evaluation system for testing.

Fourth, summary

As a key parameter of material interface performance, the accurate test of interlayer adhesion is very important for product quality control and performance optimization. Through scientific testing methods and standardized operation procedures, the bonding quality between layers in multi-layer structure can be effectively evaluated, which provides strong support for the research and development of new materials, process improvement and failure analysis. With the continuous progress of testing technology, the quantitative analysis of interlayer bonding force will be more accurate and efficient in the future.