Strategies for damage tolerance enhancement in metal/ceramic thin films: Lessons learned from Ti/TiN

Most functional microelectronic devices as well as hard coatings use brittle ceramics like Titanium Nitride (TiN) in nanostructured, thin film form. Damage tolerance is critical to their deployment in service, and life extension. In this study, we explore multilayering to enhance the damage tolerance of such material systems. Ti/TiN is a model metal/ceramic system with a strong interface, where elastic-plastic mismatch could potentially be used to modify the crack driving force. We carry out systematic numerical simulations of crack driving force in Ti/TiN multilayers with changing layer spacing. Micro-cantilever experiments are then carried out on a selected set of multilayers to determine the fracture toughness as a function of the number of interfaces. The 50 layer multilayer exhibits a fracture toughness that is 82% higher than the single layer TiN while maintaining a comparable hardness to the latter. The weak intercolumnar boundaries of sputtered films are found to be a limitation in fully exploiting the advantage of the shielding effect due to alternating stiff and compliant layers. The results are discussed in the context of design parameters for multilayering in metal/ceramic thin film systems in general.