Viscoelastic Characterization of Fine Grained Nickel/Zirconia Composites

A mechanical spectroscopy study has been made on fine-grained Ni-TZP (ZrO2-3 mol%Y2O3) composites in an attempt to assess the following micromechanical prediction in a previous paper: a dual-phase material with two different fine-grained constituents, each deforming by Coble-type boundary-diffusion creep, exhibits viscoelastic (anelastic plus viscous) behavior in consequence of differential creep between the phases. Two composites with 10 and 20 vol% TZP, fabricated by powder processing, are examined. Despite the metal-rich compositions, either “metal-matrix” or “ceramic-matrix” microstructure is produced, which is characterized by essentially equiaxed, mostly single-grained, domains of one constituent in a fine-grained matrix of the other. Dynamic Young’s modulus, Eω, and internal friction, tanφ, are measured over a temperature range of 25 to 800°C at frequencies of 0.01, 0.05 and 0.1 Hz using a specially designed tension-compression apparatus; the data obtained are analyzed in terms of loss compliance, J2≅tanφ⁄Eω. Two relaxation peaks are observed in both composites; also, an exponential rise with temperature is a trend characterizing the background profile especially for the metal-matrix composite. The activation energies and pre-exponentials involved in the peak and background components are determined and, by making a further analysis based on the previous micromechanical formulation and also taking the well-known relaxation due to viscous grain-boundary sliding into account, the implications of these quantities are discussed in terms of constituent material parameters (boundary diffusivity, grain size, etc.).