Micromechanical characterization of microwave dielectric ceramic BaO–Sm ₂O ₃–5TiO ₂ by indentation and scratch methods

Abstract

Mechanical characterization of dielectric ceramics, which have drawn extensive attention in wireless communication, remains challenging. Micromechanical properties with microstructures of dielectric ceramic BaO-Sm2O3-5TiO2 (BST) were assessed by nanoindentation, microhardness, and microscratch tests under different indenters, along with X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. Accurate determination of elastic modulus (i.e., 260 GPa) and indentation hardness (i.e., J u s t A c c e p t e d 16.2 GPa) of brittle BST ceramic by instrumented indentation technique requires low loads with little indentation-induced damage. Elastic modulus and indentation hardness were analyzed by different methodologies such as energybased and displacement-based approaches, and elastic recovery of Knoop imprint. Consistent values (about 3.1 MPa·m 1/2 ) of fracture toughness of BST ceramic were obtained by different methods such as Vickers indenter-induced cracking method, energy-based nanoindentation approaches, and linear elastic fracture mechanics-based scratch approach with a spherical indenter, demonstrating successful applications of indentation and scratch methods in characterizing fracture properties of brittle solids. The deterioration of elastic modulus or indentation hardness with the increase in indentation load is caused by indentation-induced damage, and can be used to determine fracture toughness of material by energy-based nanoindentation approaches, and the critical void volume fraction is 0.27 (or 0.18) if elastic modulus (or indentation hardness) of the brittle BST ceramic is used. The fracture work at the critical load corresponding to the initial decrease in elastic modulus or indentation hardness can also be used to assess fracture toughness of brittle solids, opening new venues of application of nanoindentation test as a means to characterize fracture toughness of brittle ceramics.