Anisotropic deformation and fracture mechanisms of physical vapor deposited TiN/ZrN multilayers

Multilayers such as ceramic/ceramic multilayers have exhibited the great potential of achieving exceptional mechanical properties since they are made of dissimilar materials. Typically, these materials can be strengthened by tuning the laminated microstructure to enhance their properties. TiN/ZrN is a typical example of the ceramic/ceramic multilayer which is widely used in various fields owing to its extraordinary mechanical properties. Just like most multilayers, the strength of the TiN/ZrN multilayer depends on the layer thickness and can be improved by reducing the layer thickness to a few nanometers. However, the TiN/ZrN multilayers are susceptible to damages induced by low fracture resistance and cracks. As such, it is important to understand the deformation and fracture mechanism of TiN/ZrN multilayers.

Considering the parallel orientation of the TiN/ZrN interface to the surface, the deformation of TiN/ZrN multilayers have been widely speculated to be anisotropic, depending on the direction of loading. Recent research on a similar-structured Al/SiC system at parallel, perpendicular and inclined directions revealed useful insights on the anisotropic deformation that could be replicated in other multilayers. Motivated by the results of the metal/ceramic systems, a team of researchers: Professor Lingwei Yang from Hypervelocity Aerodynamic Institute, Dr. Yunsheng Chen from the Chinese Academy of Science, Dr. Jiao Chen from Xi’an Jiaotong University, Professor Chuanyun Wang from Northwestern Polytechnic University, and Dr. Guangyu He from Air Force Engineering University, investigated the anisotropic deformation and fracture mechanism of TiN/ZrN multilayers using a combination of nanoindentation and micropillar compression techniques. Their work is currently published in the journal, Ceramics International.

In their approach, a conventional physical vapor deposition method was used to prepare the TiN/ZrN multilayers. Nanoindentation was carried out at parallel (0°) and perpendicular (90°) directions to achieve the desired Young’s modulus and hardness. The anisotropic hardness of the multiplayer at the inclined direction was predicted through a finite element modelling of the nanoindentation. Additionally, micropillar compressions were carried out in both directions to explore the fracture mechanism and quantify the compression strength. The authors also used a combination of scanning electron microscopy and transmission electron microscopy to characterize the deformation and fracture mechanisms.

Results showed a stronger nanoindentation response of the multilayer when loaded in the perpendicular direction to the layer orientation, and the deformation was majorly controlled by the plasticity of the Zr layers. In the parallel orientation, however, a decrease in the hardness attributed to the pronounced kinking and cracking was observed. The modeling results suggested a minimized hardness response in the inclined orientation due to maximum shear along the layers. Furthermore, the micropillars oriented in the parallel and perpendicular directions exhibited brittle nature. However, the fracture strain at 90° was higher due to crack termination mechanism in this orientation.

In summary, the study explored the anisotropic deformation and fracture mechanism of physical vapor deposited TiN/ZrN multilayers in different orientations. Compared to the parallel orientation, perpendicular loading produced stronger nanoindentation response. The finite element modeling produced reliable predictions of the strength of the inclined orientations. Altogether, the study provided useful insights that would pave the way for advanced research on the anisotropic deformation and fracture mechanism of different multilayers. This would allow the development of multilayers with extraordinary mechanical properties.