Toughness of Ni/Al₂O₃ interfaces as dependent on micron-scale plasticity and atomistic-scale separation

Abstract

Ceramic/metal interfaces were studied that fail by atomistic separation accompanied by plastic dissipation in the metal. The macroscopic toughness of the specific Ni alloy/Al₂O₃ interface considered is typically on the order of ten times the atomistic work of separation in mode I and even higher if combinations of mode I and mode II act on the interface. Inputs to the computational model of interface toughness are: (i) strain gradient plasticity applied to the Ni alloy with a length parameter determined by an indentation test, and (ii) a potential characterizing mixed mode separation of the interface fit to atomistic results. The roles of the several length parameters in the strain gradient plasticity are determined for indentation and crack growth. One of the parameters is shown to be of dominant importance, thus establishing that indentation can be used to measure the relevant length parameter. Recent results for separation of Ni/Al₂O₃ interfaces computed by atomistic methods are reviewed, including a set of results computed for mixed mode separation. An approximate potential fit to these results is characterized by the work of separation, the peak separation stress for normal separation and the traction–displacement relation in pure shearing of the interface. With these inputs, the model for steady-state crack growth is used to compute the toughness of the interface under mode I and under the full range of mode mix. The effect of interface strength and the work of separation on macroscopic toughness is computed. Fundamental implications for plasticity-enhanced toughness emerge.

Keywords:

• fracture
• plasticity
• work of separation
• atomistic separation
• ceramic-metal interface

Acknowledgements

This work was supported in part by AFSOR under the MEANS-2 Program, in part by the School of Engineering and Applied Sciences, Harvard University and in part by NSFC through Grants 10672163 and 10721202 to the Mechanics Institute of the Chinese Academy of Sciences. The authors thank Y. Jiang and J.R. Smith for supplying the atomistic results and A.G. Evans and K.J. Hemker for extensive interactions.