Scratch indentation, a simple adhesion test method for thin films on polymeric supports

We have utilized scratch indentation as a test method for evaluating the strength and adhesion of polymeric substrates coated with metal and diamond-like carbon (DLC) films. Metallized films were prepared by coating poly(ethylene terephthalate) with copper, using a proprietary electroless deposition process. The copper adhesion was evaluated using a standard tape-pull test, a special inverse peel test in addition to scratch indentation, and a good correlation was observed among the methods. The most critical experimental parameter in the scratch indentation method was the stylus radius. Good differentiation of adhesion among samples was much more evident using an 800 μm tip radius as opposed to 200 μm. We concluded that a simple reliable measurement of practical adhesion could be based on commercially available scratch indentation equipment. The DLC coatings are hard and brittle compared to the ductile copper. The scratch indentation tests were quite sensitive to the strength of the DLC coatings. Adhesion was not directly measured but was inferred from optical analysis of the scratch trace. One of the instruments contained an acoustic emission detection channel and noise burst were recorded that could be directly associated with the cracking of the DLC coating when tip styli of 200 and 800 μm were used. Another instrument of quite different design utilized a stylus tip radius of 1 μm and could be operated in displacement control. Distinct drops in the normal force curve were observed when the stylus broke through the DLC coating. The critical load, Lc, was measured as a function of the coating thickness and could be fitted to a classical physical model based on the breaking strength of a thin disk.