Molecular dynamic simulation of crack growth in Ti/TiN multilayer coatings

Three-dimensional molecular dynamics (MD) simulations were performed to investigate the crack growth in nanoscale multilayer Ti/TiN. The interatomic bonds were characterized using the second nearest-neighbor modified embedded-atom method. The models were built with an initial crack perpendicular to the interface in the ceramic layer under the uniaxial constant strain rate. First, simulations were conducted with two layer-numbers of Ti/TiN multilayers, including Ti(4 nm)/TiN(6 nm) and Ti(2 nm)/TiN(3 nm)/Ti(2 nm)/TiN(3 nm). The results showed that the crack in the TiN layer grew and reached the interface. The misorientation of the interface and the plastic deformation of the Ti layers can act as a barrier to crack growth and blunt crack tip at the interface. Also, Young's modulus and required stress for crack growth decreased by increasing the Ti layer thickness ratio at constant bilayer thickness. Furthermore, a decrease in the total thickness of the structure from 10 nm to 6 nm led to the inverse Hall-Petch effect. Increasing the strain rate has been shown to increase critical stress for crack growth, while Young's modulus remains unchanged. Also, at a higher strain rate, the structure toughness decreases. The results also revealed that increasing the temperature from 300 K to 700 K leads to a reduction in Young's modulus and critical stress.