Percolation effects on fracture in ductile-phase toughened oxide coatings

Abstract

The toughness and damage behaviors of ductile-phase toughened oxide coatings were characterized as the reinforcement volume fraction varied across the percolation threshold. The coatings, consisting of Ni particles in a borate glass-ceramic matrix, showed a rising resistance curve, with the extent of stable crack growth increasing with Ni content. While initiation toughness was relatively insensitive to reinforcement topology, peak toughness increased sharply once the Ni reinforcement percolated, reaching a maximum value of ~ 160 J/m2 in an interpenetrating composite coating with 35 vol% Ni. This toughness is sufficiently high to resist failure in the target application of rocket engine turbomachinery, where coatings must withstand rapid thermal transients upon engine startup and shutdown. Characterization of the crack path confirmed that this toughening increment corresponded to a transition from crack deflection to crack bridging as the dominant toughening mechanism. The implications of these results on design of ductile-phase toughened coatings are discussed. Graphical abstract Double-cantilever beam specimens with the ductile-phase toughened oxide coating as an interlayer between the two beams