Abstract
Zr-based metallic-glass coatings with micrometer-scale thickness are prepared by the radio-frequency magnetron-sputtering technique on silicon substrates. Using the load- and displacement-sensing nanoindentation technique, we have examined the dependence of their deformation behavior, especially the indentation hardness, on the strain rate and maximum indentation depth. For shallow indentation in which the substrate effect can be neglected, the increase of the penetration rate leads to the decrease of the hardness. This seemingly "negative" strain-rate-sensitivity is actually a result of the dependence of the degree of elastic deformation on the effective strain rate. The coating interface will block the shear-band propagation and promote the shear-band multiplication, so that the plastic flow is much easier to occur as the increase of the maximum penetration depth from a few percent of, to that comparable to, the coating thickness. We use a power-law viscoplastic constitutive relationship to illustrate key issues related to the indentation response of rate-dependent materials, while a phenomenological viscoplastic model with strain softening behavior is used to understand the unique features of the inhomogeneous deformation in metallic glasses. The scanning electron microscopy and atomic force microscopy are used to examine the shear bands and pileup around the indents.
