Nanostructure of Mo2BC thin films and the effect on mechanical properties

Abstract

The demand for materials in structural and functional applications requires nowadays more and more combined properties like high stiffness and simultaneously good ductility. The investigation and understanding of these unique properties is very important regarding a continuing optimizing of the materials and finally taking benefit from the outstanding properties in the future. Mo 2 BC is a promising candidate in this field as it combines properties preventing crack formation and also ensuring long lifetime. Density functional theory calculations predicated a high stiffness and a moderate ductility for Mo 2 BC by determining the electronic structure and the mechanical properties.[1, 2] A possible application of this material is the use as a hard coating for cutting tools protection. In our work, we investigated Mo 2 BC thin films on silicon substrates which were synthesized by high‐power pulsed magnetron sputtering deposition technique.[3] Transmission electron microscopy (TEM) methods (bright‐field, high‐resolution TEM, selected area diffraction, electron energy loss spectroscopy) were used to compare the nanostructure of several Mo 2 BC films which were deposited at different substrate temperatures ranging from 380 °C to 630 °C. TEM samples were prepared conventionally using grinding and Ar + ion beam milling as well as by focused ion beam fabrication of thin lamellae extracted from the Mo 2 BC films. TEM investigations were performed at 300 kV using a FEI Titan Themis 60‐300. In Figure 1, TEM cross‐sectional micrographs of the Mo 2 BC film synthesized at the highest substrate temperature are shown exemplarily. The coating is fully crystalline and exhibits nanocrystalline columnar grains with a diameter of 10‐20 nm.[4] Furthermore, X‐ray diffraction experiments and micromechanical tests were performed in order to correlate the mechanical properties of the films with their nanostructure. We detected an increasing crystallinity and hardness but a decreasing fracture toughness with increasing substrate temperature.