Unambiguous detection of the adhesive failure of metal films in the microscratch test by waveform analysis of the friction signal

The vibrational microscratch tester monitors electrically the frictional response of a scratch-stylus which is forced to oscillate on the film surface. This tester is so sensitive that it is able to detect the fracture of ceramic films less than 30 nm thick in situ by catching an irregular jump of the friction. The adhesive failure of films of ductile materials, however, does not necessarily yield a sufficient fracture signal. In the present study, the waveform of the frictional signal for an oscillation period is decomposed into the frequency regime by Fourier expansion. The signal due to the solid friction is found to appear only in odd harmonics of the fundamental excitation frequency. By integrating the component of even harmonics of the observed signal, the critical failure of the adhesion has been made to observe more sensitively. The observation of the critical failure of thin copper films is demonstrated. q1999 Published by Elsevier Science B.V. All rights reserved