Coating adhesion testing is a crucial process for evaluating the bonding strength between a coating and its substrate, and it is directly related to the coating’s durability and protective performance. Below are common test methods and key operational points:

1. Common Test Methods

1.1 Cross-Cut Test

● Principle: Use a cutter to score a grid pattern on the coating surface, then evaluate adhesion based on the peeled-off area after tape removal.

● Applicable Scenarios: Thin coatings (generally ≤250μm), such as paints and powder coatings.

● Standards: ASTM D3359, ISO 2409.

● Steps: Score 1×1mm or 2×2mm squares on the coating (depth reaching the substrate) using a cross-cut cutter.

○ Adhere tape to the grid and peel it off quickly.

○ Observe the peeling grade (Grade 0 = no peeling, Grade 5 = severe peeling).

1.2 Pull-Off Test

● Principle: Use a tensile testing machine to pull a bonded stud of a specific area away from the coating surface, and record the maximum tensile force.

● Applicable Scenarios: Thick coatings and high-adhesion coatings (e.g., metal platings, industrial anti-corrosion coatings).

● Standards: ASTM D4541, ISO 4624.

● Steps: Secure an aluminum or steel cylindrical bonded stud to the coating surface.

○ Pull the stud vertically using a tensile tester, and record the force at failure (unit: MPa).

○ Observe the failure mode (coating cohesive failure, interfacial peeling, or mixed failure).

1.3 Scratch Test

● Principle: Use a stylus to scratch the coating surface, and evaluate adhesion based on the critical load (the pressure at which the coating starts to peel).

● Applicable Scenarios: Hard coatings (e.g., ceramic coatings, PVD/CVD coatings).

● Standards: ASTM C1624, ISO 20502.

● Steps: Move the stylus at a constant speed while gradually increasing the load.

○ Detect the failure point using a microscope or acoustic emission sensor.

1.4 Bend Test

● Principle: Bend the coated sample to a specific angle and check if the coating cracks or peels.

● Applicable Scenarios: Flexible substrates (e.g., plastics, thin metal sheets).

● Standards: ASTM D522.

● Steps: Secure the sample on a bend testing machine.

○ Bend the sample to the specified radius or angle.

○ Inspect the coating for cracks.

1.5 Impact Test

● Principle: Impact the coating surface with a falling ball or impact hammer to evaluate the coating’s impact resistance and adhesion.

● Applicable Scenarios: Coatings requiring high impact resistance, such as those used in automobiles and ships.

● Standards: ASTM D2794.

● Steps: Set the impact energy (e.g., 1kg·m).

○ Observe if the coating cracks or peels.

2. Key Test Points

2.1 Substrate Surface Preparation

● Ensure the substrate is clean, dry, and free of oil, oxide layers, or impurities before testing.

● For rough surfaces (e.g., sandblasted surfaces), record the surface roughness (Ra value).

2.2 Coating Condition Control

● The coating must be fully cured (meeting the specified drying time or baking conditions).

● The test environment’s temperature and humidity should comply with standards (e.g., 23±2℃, 50±5%RH).

2.3 Instrument Calibration

● Equipment such as tensile testers and cross-cut cutters must be calibrated regularly to avoid errors.

● The tip radius and angle of the stylus used in the scratch test must meet standard requirements.

2.4 Result Analysis

● Distinguish between failure modes: Cohesive failure (internal cracking of the coating): Adhesion is greater than the coating’s own strength.

○ Interfacial failure (separation between coating and substrate): Insufficient adhesion.

● Quantitative data (e.g., MPa values from the pull-off test) must be comprehensively evaluated alongside the failure mode.

2.5 Handling of Special Cases

● For multi-layer coatings, conduct layered testing or specify the test layer.

● For flexible substrates, avoid false peeling caused by substrate deformation.

3. Precautions

● Method Selection: Choose the appropriate method based on coating thickness, substrate hardness, and application scenarios (e.g., prioritize the cross-cut test for thin coatings, and the pull-off test for high-adhesion coatings).

● Sample Representativeness: Test multiple areas (edges, center, etc.) to avoid result bias caused by local defects.

● Safety Protection: Wear protective gear when using cutters or impact equipment.

Through standardized operation and accurate analysis, coating adhesion testing can provide reliable basis for product development, quality control, and failure analysis.