Adhesion Characterization by SAM


Characterization of the bonding by measuring bond compliances is one of the real future challenges to acoustic microscopy. Modern microscopes can distinguish two states of bonding: a perfect bond and full delaminations or disbonding. It has recently been proven that the acoustic microscope distinguishes regions of interfacial degradation at the earliest stages while optical microscopy remains insensitive (Zinin, P. and W. Weise, Theory and applications of acoustic microscopy, in T. Kundu ed., Ultrasonic Nondestructive Evaluation: Engineering and Biological Material Characterization, CRC Press, Boca Raton, chapter 11, 654-724, 2003). Acoustic microscopy provides an opportunity not only to detect delaminations but also to investigate the structure and shape of the disbonding. We demonstrate this capability by describing SAM studies on adhesive bonding as is widely used in the aircraft industry. In order to increase adhesion between aluminum and epoxy, a special surface pretreatment used on aluminum adherends in aerospace applications produces a honeycomb-like oxide structure on the adherend surface. The adhesive penetrates some distance into the honeycomb cells to form a "micro-composite." The overall thickness of this "interlayer" between the bulk adhesive and the bulk adherend in a typical aluminum-epoxy joint is of the order of 1 m only. Degradation of the boundary between the interlayer and epoxy can be observed only under severe degradation conditions.   


 Acoustical micrograph (OXSAM) 15 mm epoxy/1 mm OAA/Al: frequency was 300 MHz: 22 days in water; 90o C; z = 0 Ám.


Acoustical micrograph (OXSAM) 15 mm epoxy/1 mm OAA/Al: frequency was 300 MHz: 22 days in water; 90o C; z = -65 Ám.


 We monitored degradation in epoxy coated samples using the scanning acoustic microscope. Figures above illustrates the imaging of defects located at the interface between aluminum and a 15 Ám epoxy layer. The surface image of the epoxy layer (left image) shows several defects as dark and bright spots. A comparison of images of the epoxy surface (left image) and of the epoxy-aluminum interface (right image) shows that most of the defects (voids) are located near the interface. The contrast of the defects gets brighter as the SAM focus moves closer to the interface and more defects become visible. Degradation has been seen at discrete sites, rather than uniformly over the surface. Degradation has only been detected when it has resulted in a small void/delamination. Recently, it has been found that combining the time-of-flight technique with acoustic microscopy is a powerful tool for investigating adhesion problems (Zeller, Kinloch, Cawley, Zinin, Briggs, et al., Review of Progress in Quantitative Nondestructive Evaluation. Vol.16B. Ed. by D.O. Thompson and D. E. Chimenti. Plenum Press. New York. 1997. pp. 1237)..