Variation of adhesive force at the interface of Pd and SrTiO₃ as a consequence of residual stresses

Abstract

Initially, Pd thin films were deposited over a hard substrate using electron beam physical vapor deposition. The growth and the surface roughness of the films were analyzed and their effects upon conventional indentation test were discussed.Afterwards, an experimental method is described which can measure the critical fracture force in thin films using oscillating indentation. Initially, repetitive contacts at a single point with the purpose of identifying the fracture time provide the fracture force versus fracture time plot. Non-linear curve fitting of the data reveals the theoretical fracture force by a single indentation, which is called critical fracture force. Arguments are put forth to show the relation between piling up height and applied force. Discrepancies were observed in the plot of the ratio between total indentation depth and pilling up height versus applied force when higher loads than critical fracture force were applied.Discrepancies appear as a result of indenting the substrate. Nanoscratch test facilitated the possibility to measure adhesion strength and adhesion energy of the films considering measured critical fracture force as maximum applied force. The relation between residual compressive stresses, adhesion strength, plastic deformation, and pilling up area was discussed using dislocation theories. Indentation with high applied load leaves behind large plastic deformation and reduces the accuracy and reliability of the test results. Hence, lower loads (in the order of nN) were applied using Atomic Force Microscopy in the friction mode. Pulling off force was mapped in each thickness Confidential: not for distribution. Submitted to IOP Publishing for peer review 28 July 2010 of Pd films. The results confirm that the area around a hillock exhibits higher pulling off force due to the local stress relaxation as a consequence of hillock formation. By repeating the mapping process over different area with various applying forces, the plot of the pulling off force versus applied load was drawn representing discrepancies in the results at higher loads. This phenomenon is associated with the plastic deformation in the films.