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Coating Adhesion & Cross Hatch Test governed by ASTM D3359

Wed May 3, 2023

Coating adhesion is a key factor in determining the quality and durability of electric motor laminations used in a wide variety of industries. Coatings are applied to the laminations to provide electrical insulation and reduce loss due to eddy currents. However, if the coating is not properly adhering to the laminations, it can lead to significant problems, including reduced motor lifespan and performance, increased maintenance costs, as well as potential safety hazards. Therefore, it is crucial to evaluate the coating adhesion using reliable and accurate testing methods.

One of the most widely used methods for evaluating coating adhesion is the cross hatch tape test governed by ASTM D3359. This test involves applying a grid pattern by scratching into the surface of the metal and then applying a piece of tape over the scratch. The tape is then removed swiftly, and the surface is inspected for any signs of metal that sticks to the tape or any removal of the coating from the lamination. The adhesion loss is then determined based on the range set forth in ASTM D3359.

The cross hatch tape test is a relatively simple and straightforward method of evaluating coating adhesion, but it is essential to follow the ASTM D3359 standard procedures carefully to ensure accurate and reliable results. The standard provides guidelines for preparing the surface, applying and removing the tape, and evaluating the adhesion loss. The following are the main steps involved in performing the cross hatch tape test:

Preparation of the Surface

Before applying the tape, the surface must be thoroughly cleaned and dried to remove any dirt, dust, oil, or other contaminants that may affect the adhesion of the coating. The ASTM standard recommends using a clean, dry cloth or compressed air to clean the surface. Use of solvents that can dissolve or damage the coating should be avoided.

Application of the Grid Pattern

The next step is to apply the grid pattern to the surface using a cross hatch cutting tool. The standard specifies that the grid pattern should consist of ten or more evenly spaced cuts in each direction, forming squares of approximately 1 mm on the surface. The cuts should be deep enough to reach the substrate but not so deep as to damage the substrate or the coating.

Application of the Tape

After the grid pattern is applied, the tape is carefully applied to the surface, covering the entire grid pattern. The tape should be pressed firmly onto the surface to ensure good contact with the coating. The standard recommends using a 3M tape or equivalent with a peel strength of at least 7 N/cm.

Removal of the Tape

Once the tape is properly applied, it is removed swiftly by pulling it back over itself at a 180-degree angle. The standard specifies that the removal of the tape should be done as quickly as possible, but not so quickly that it causes the tape to break or tear. 

Evaluation of the Adhesion Loss

After the tape is removed, the surface is inspected for any signs of metal stuck on the tape or removal of the coating from the surface. The adhesion loss is then evaluated based on the range set forth in ASTM D3359. The standard provides six levels of adhesion loss, ranging from 0 to 5, based on the percentage of the grid area affected by the loss of adhesion. A level 0 means no loss of adhesion, while a level 5 indicates complete loss of adhesion.

The ASTM D3359 cross hatch tape test is a widely used method for evaluating coating adhesion to motor laminations. Manufacturers can quickly and easily determine whether their coatings are properly adhered to the substrate. While this test has some limitations, it remains a valuable tool in ensuring the quality and durability of coatings used in a wide range of applications.