Nanoindentation to evaluate the mechanical properties of adhesive layers

Objectives: Understanding the mechanical properties of polymers is necessary to predict their behavior under stress. The objective of this study was to use a new nanoindentation technique to characterize the in-plane linear viscoelastic properties of dental adhesive layers. Materials and Methods: The dental adhesives used were Single Bond (SIB) and Single Bond Plus (SBP) made by 3M, ESPE, and Clearfil SE Bond (CSE) and Clearfil Tri-S Bond (CTS) by Kuraray Medical. A thin film of each adhesive was made by light curing the resin between two micro-glass slides, and was then tested on a nanoindenter system (ENT 1100, Elionix) with a Berkovich indenter at a constant loading rate of 0.1 mN/sec up to a maximum load of 1.8 mN. The load-displacement data of the loading segment were fitted to a curve (correlation coefficient R>0.9999), which was then used to calculate the creep compliance in shear (using the generalized Kelvin model), Young's relaxation modulus and Young's modulus. The results were compared to the values calculated by the nanoindentation device. Results:        

Conclusion: The new technique could be successfully applied to measure the properties of adhesives with a reasonably good accuracy. The traditional nanoindentation techniques, developed for time-independent materials, ignore viscoelastic behavior and may significantly over-estimate the Young's modulus of polymerized adhesive resins and thus are not appropriate for these materials.