Mechanical failures of interfacial regions of ceramic-coating/metal-adhesion-layer/substrate systems were measured quantitatively and observed concurrently through instrumented microscale mechanical testing in-situ a scanning electron microscope (SEM). Failure of the interfacial regions of coating/interlayer/substrate systems was observed in micro-pillar specimens in-situ under different loading conditions, including shear, compression, and tension.
Under shear loading, shear failure of the interfacial region was observed to occur in two stages: an initial uniform shear plastic deformation of the entire metal interlayer followed by an unstable shear-off close to the metal/ceramic interface. Additional testing under compression loading conditions suggests that the unstable shear-off is concomitant with the metal/ceramic interface going from being “locked”, with no relative displacement between materials on the two sides of the interface, to being “unlocked”, with significant relative displacements. Failure of the interfacial region was also observed under tensile loading conditions.
Density functional theory (DFT) and molecular dynamics (MD) studies on one particular metal/ceramic interface, namely Ti/TiN, showed that a weak interaction plane exists in the metal layer near the chemical interface in a coherent Ti/TiN structure. Consequently, the free energy and theoretical shear strength of the semi-coherent Ti/TiN interface is found to depend on the physical location of the misfit dislocation network (MDN). The minimum energy and strength of the interface occur when the MDN is near, but not at the chemical interface. The present work gives new insight into the nature of mechanical failure of metal/ceramic interfaces, is relevant to materials-based engineering of metal/ceramic interfaces, and has applications to engineering of ceramic coating/substrate systems.