Measurement of the local displacement field produced by a microindentation using speckle interferometry. Its application to analyse coating adhesion

ANDRÉS E. DOLINKO; GUILLERMO H. KAUFMANN

Nürtingen, Alemania

Congreso; Fringe 09’, the 6th International Workshop on Advanced Optical Metrology; 2009

Institut für Technische Optik

The application of a thin coating to a material increases the lifetime of many mechanical components made in conventional form by modifying their surface properties. Even though coating failure in service can be due to several causes, adhesion is usually the most important one. For this reason, it is essential to have suitable test methods to guarantee the adhesion of a given coating-substrate system. As the definition of adhesion is not a simple issue, the characterization of this property has also generated considerable debate in the technical literature. Although there are several test methods that can be used to evaluate the adhesion of a given coating-substrate combination, many of them are just qualitative or semi-quantitative. The indentation techniques are the most widely used approaches to characterise thin coating adhesion, but the interpretation of the obtained results is a quite complex problem. It is known that the introduction of a conical microindentation on the coated surface of a specimen generates a buckling. As the load is increased, a circular delamination in the coating-substrate interface is usually generated. It can also be demonstrated that the extension of the delaminated region is directly related to the strain energy release rate, which can be used to evaluate the adhesion of the coating to the substrate. In this paper we present a technique to investigate coating adhesion based on a digital speckle pattern interferometry (DSPI) system to measure the out-of-plane deflections generated by the buckling due to the introduction of a microindentation. As the indentation device was located outside of the optical bench, the specimen was repositioned using a specimen holder with high stability and no moving parts. One of the main problems to study adhesion problems is the difficulty of preparing specimens in which this property can be varied over a wide range. Therefore, as coating we decided to use a thin bronze sheet which was glued to one of the surfaces of each steel specimen by means of an epoxy resin. This methodology allows us to prepare specimens in which the adhesion can be changed through the smoothness variation of the coated surface of the substrate and also by increasing the adhesive curing time. The experiments performed in coated specimens with different interface conditions demonstrated that DSPI enables the measurement of the tiny buckling generated in the coating after the introduction of a microindentation. It was also demonstrated that the proposed technique allows the determination of the radius of the circular buckled region produced by different substrate-coating interface conditions. The change in adhesion could be successfully monitored as a function of the adhesive curing time. As it was expected, the experiments confirmed that shorter delamination radii were obtained when the time between the coating adhesion and the introduction of the indentation was increased. Finally, the evaluation of the radius of the buckled region as a function of the indentation load was in good agreement with the predictions of linear dependence given by a theoretical model.