The adhesion of an electrodeposited coating to its substrate is as important as its thickness and selection of the plated layer to the overall performance of the finishing system. The ability to properly test for adhesion of plated layers to a product is of utmost importance. Unfortunately, practical adhesion tests are generally qualitative and difficult to relate to the end application of a product. Although quantitative tests exist – i.e., tests that attempt to express the force necessary to separate the coating from the substrate in numerical terms – they are not suitable for routine use are generally reserved for research purposes.
Several common adhesion tests are provide below along with a basic description of how the test is performed. ASTM B571 is generally considered one of the better specifications for defining adhesion testing and many of the tests listed below are cited in ASTM B571. If the size and shape of the item to be tested does not permit the use of one of these tests, a test piece may be used. However, the test piece must be of the same material and preparation as the product and ideally of a similar configuration such that it can be plated along with the subject parts. In addition, if the plated product is very valuable, the use of test piece may be necessary.
Bend Test: Bend the part with the coated surface away, over a mandrel until its two legs are parallel; the diameter of the mandrel should be 4 times the thickness of the sample. Examine the deformed area under a low magnification (4X) for peeling or flaking of the coating from the substrate. If the coating fractures or blisters, a sharp blade may be used to attempt to lift off the coating. Brittle coatings may crack under this test, but cracks are not evidence of poor adhesion unless the coating can be peeled with a sharp instrument.
Burnishing Test: Rub a coated area, about 5 cm2, with a smooth-ended tool for about 15 seconds. The pressure imparted should be sufficient to burnish the coating but not to dig into it. Blisters, lifting, or peeling should not develop as a result of the burnishing. Note: This test is not suitable for thick coatings.
Chisel-knife Test: Use a sharp cold chisel to penetrate the coating, or at a coating-substrate interface exposed by sectioning the specimen. If it is possible to remove the deposit, the adhesion is not satisfactory. Note: This is not applicable to soft or thin coatings.
File Test: Saw off a piece of the coated specimen and inspect it for detachment at the deposit-substrate interface. Apply a coarse mill file across the sawed edge from the substrate toward the coating so as to raise it, using an angle of about 45 degrees to the coating surface. Note: This test is not suitable for soft or thin coatings.
Grind-saw Test: Hold the coated article against a rough emery wheel such that the wheel cuts roughly from the substrate toward the deposit. Often an aluminum oxide grinding wheel is used for this test. Note: This test is not suitable for thin or soft coatings.
Heat-quench Test: The article is heated in an oven to a temperature prescribed for the coating-substrate combination, then the specimen is quenched in water at room temperature. After quenching the test sample is inspected for any blistering or flaking. Note that the heating may actually improve adhesion by diffusion alloying, or it may form a brittle alloy layer at the interface. These effects limit the applicability of the test.
Impact Test: This test consists merely of hammering the specimen severely to see whether blistering or exfoliation occurs. The exact details of the impacts can be specified as desired by a customer.
Peel Test: A strip of steel or brass is bonded to the specimen by solder or a suitable adhesive; at an angle of 90 degrees the strip is pulled off the specimen. Failure at the deposit-substrate interface evidences poor adhesion.
Push Test: Drill a blind hole 7.5 mm in diameter from the underside of the specimen until the point of the drill tip comes within about 1.5 mm of the deposit-substrate interface on the opposite side. Support the specimen on a ring about 25 mm in diameter and apply steady pressure over the blind hole, using a hardened steel punch 6 mm in diameter, until a button sample is pushed out. Exfoliation or peeling of the coating in the button or crater area is evidence of poor adhesion. Note: This is not suitable for soft, thin or very ductile deposits.
Scribe-grind Test: Scribe two or more parallel lines or a rectangular grid pattern on the article using a hardened steel tool. The distance between the scribed lines should be about 10 times the nominal thickness of the coating, with a minimum of 0.4 mm. Cut through the coating to the substrate in a single stroke. If any portion of the coating between the lines breaks away from the substrate, adhesion is inadequate.
If not explicitly stated, in all of these tests peeling, flaking, blistering or exfoliation of the coating is evidence of poor adhesion.