Measurement of local internal friction in metallic glasses

Abstract

Atomic force acoustic microscopy (AFAM), an advanced scanning probe microscopy technique, has been used to measure local elastic properties with a spatial resolution given by the tip-sample contact radius. AFAM is based on inducing out-of-plane vibrations in the specimen. The vibrations are sensed by the AFM cantilever from by the photodiode signal when its tip is in contact with the material under test. To measure local damping, the inverse quality factor Q−1 of the resonance curve is usually evaluated. Here, from the contact-resonance spectra obtained, we determine the real and imaginary part of the contact stiffness k* and from these two quantities the local damping factor Qloc−1 is obtained which is proportional to the imaginary part γ of the contact stiffness. The evaluation of the data is based on the cantilever's mass distribution with damped flexural modes and not on an effective point-mass approximation for the cantilever’s motion. The given equation is simple to use and has been employed to study the local Qloc−1 of amorphous PdCuSi metallic glass and its crystalline counterpart as a function of position of the AFM tip on the surface. The width of the distribution changes dramatically from the amorphous to the crystalline state as expected from the consequences of the potential-energy landscape picture. The center value of the distribution curve for Qloc−1 coincides very well with published data, based on global ultrasonic or internal friction measurements. This is compared to Qloc−1 measured in crystalline SrTiO3, which exhibits a narrow distribution, as expected.