Bend Testing

A key step in the electroplating process is determining the level of coating adhesion. There are many tests used to determine if the adhesion of an electroplated coating is sufficient. A bend test can ensure proper adhesion has occurred, and the finished product meets any applicable welding codes. The purpose of this adhesion testing method is to verify the base metal (substrate) and plated coating are properly fused, and the heat-affected zone demonstrates acceptable mechanical properties.

Proper Test Specimen Preparation

The sample used for this adhesion testing method should be flat and measure 10 mm wide and at least 150 mm long. The sample should be cut from the part at a distance of at least 25 mm from the edge. Guillotine shears are regarded as the most effective tool for cutting the sample, although any conventional cutting method will suffice. Long edges of the sample piece should be grinded or filed to achieve appropriate rounding.

In some situations, the shape of the plated part may make it difficult to obtain an appropriate test sample, if not impossible. In these cases, it is still possible to perform suitable adhesion testing. A test panel can be prepared by plating a large section of the base metal with the metal coating, then cutting appropriately sized samples from the panel. The test panel should be large enough to yield several samples.

Bend Test Process and Procedure

The bend test requires using a mandrel with a diameter measuring at least four times the thickness of the sample. The sample with the coated surface is placed in a vise with the coating facing outward and bent over the mandrel until its two legs are parallel. Bend the specimen back and forth repeatedly through an angle of 180 degrees. The bending should continue until base metal failure occurs. The specimen should maintain contact with the top of the mandrel throughout the process. Slow, steady pressure should be applied during bending.

Examine the deformed area of the sample under 4X magnification to see if the coating is peeling or flaking away from the substrate. A knife or sharp blade can be used to lift off the coating if fracturing or blistering occurs. Cracks alone are not necessarily an indication of insufficient adhesion unless the coating can be peeled away with relative ease. Cracking is not unusual, particularly when extremely hard or brittle coatings undergo adhesion testing.