Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography

Stephan Gleich,Rafael Soler,Jan-Ole Achenbach,Gerhard Dehm,Konda Gokuldoss Pradeep,Hamid Bolvardi,Jochen M Schneider,Christina Scheu,Hanna Fager

doi:10.1063/1.4999304

Abstract

In this work, the nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties. The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 °C. Transmission electron microscopy investigations revealed structural features at various length scales: bundles (30 nm to networks of several micrometers) consisting of columnar grains (∼10 nm in diameter), grain boundary regions with a less ordered atomic arrangement, and defects including disordered clusters (∼1.5 nm in diameter) as well as stacking faults within the grains. The most prominent defect with a volume fraction of ∼0.5% is the disordered clusters, which were investigated in detail by electron energy loss spectroscopy and atom probe tomography. The results provide conclusive evidence that Ar is incorporated into the Mo2BC film as disordered Ar-rich Mo-B-C clusters of approximately 1.5 nm in diameter. Hardness values of 28 ± 1 GPa were obtained by nanoindentation tests. The Young's modulus of the Mo2BC coating exhibits a value of 462 ± 9 GPa, which is consistent with ab initio calculations for crystalline and defect free Mo2BC and measurements of combinatorically deposited Mo2BC thin films at a substrate temperature of 900 °C. We conclude that a reduction of the substrate temperature of 270 °C has no significant influence on hardness and Young's modulus of the Mo2BC hard coating, even if its nanostructure exhibits defects.

Highlights

The nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties
The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 C
We focus on a Mo2BC coating deposited at lower substrate temperature (Ts 1⁄4 630 C) using an industrial growth system

Summary

Introduction

Density functional theory (DFT) calculations, performed by Emmerlich and co-workers, showed high electron densities between Mo-C and Mo-B, indicating a covalent or ionic bonding character between these atoms. Djaziri et al. have studied the cracking behavior of Mo2BC in comparison to TiAlN. Both coatings were deposited onto Cu substrates and subjected to a tensile strain of $11%. The industrial coating material TiAlN exhibited a 1.9 times denser crack network than the Mo2BC coating. These results support the DFT predictions where a positive Cauchy pressure and Pugh’s criterion are indicative of moderate ductility

Methods

Results

Discussion

Conclusion