Mechanics and mechanisms of crack growth at or near ceramic-metal interfaces : interface engineering strategies for promoting toughness

The strength and toughness of ceramic-metal joints is often controlled by the propagation path selected by stress-induced cracks. Against a background of recent linear elastic mechanics studies, experimental results from fracture tests on ceramic/metal/ceramic sandwich geometries are described which determine both the selection of crack path and the corresponding crack extension rates. It is found that crack path selection is controlled by the path of low microstructural resistance and the driving force directionality, which itself is a function of the far-field loading and the elastic compliance mismatch across the ceramic-metal interface. However, there are instances where the compliance mismatch takes the crack off the weak microstructural path, or where cracking occurs at, or near, both interfaces (crack jumping). Such cracking configurations can be tortuous and high toughness joints result. This paper discusses the potential for predicting and engineering, interfaces with enhanced toughness.