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Mn+1AXn phase materials, commonly shortened to MAX, have garnered a lot of attention recently, mainly due to their outstanding mechanical, corrosion, stability and conductivity properties. From theoretical calculations, these materials have been shown to possess nanoscale 3-Dimensional (3D) defects near the phase-transformation regions.
Vanadium and titanium-based MAX materials have been widely studied but now a team of researchers have studied the defects in chromium-based Cr2AlC thin films.
MAX phase materials are composed of a transition metal (M), group III or IV elements (A) and carbon or nitrogen (X), and possess excellent mechanical strength, corrosion resistance, thermal stability and high electrical and thermal conductivity.
The other theoretical properties in Cr2AlC phase materials have been extensively studied, with researchers predicting their defects, energetics, electronic structure, growth phenomena, tribological, thermal, mechanical and magnetic properties.
Out of the many properties studied, the presence of defects in chromium (and other) MAX materials has gathered a lot of interest. Defects currently found through computational studies have included point defects from intrinsic and extrinsic impurities.
Intrinsic defects have implied antisite and interstitial incorporation, whereas extrinsic defects have been proffered to oxygen atoms. However, it has been thought that the presence of oxygen atoms, as defects, can allow both chromium and other MAX materials to exhibit self-healing behaviours.
Researchers have provided evidence of several hundred nanometer defects in other MAX materials, predominantly titanium-based films and bulk MAX materials, but have not been studied to the point where they could be identified through atomic resolution methods. As such, this was something that the researchers focused on.
The team produced the chromium thin films through high power pulse magnetron sputtering (HPPMS) in an industrial system (CC800/9, CemeCon AG). The researchers also used a scanning transmission electron microscopy with high angle annular dark field (STEM-HAADF) (FEI 80–300) instrument to analyse the interplanar distances around the 3D defects.
A Gatan post column energy filter system, FEI Helios 660 microscope, Omniprobe 5-post copper grid, Fischione Nano-Mill and in-house software (Dr. Probe) were also used during the experiments.
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The STEM-HAADF procedure was performed in the  crystal orientation. The researchers observed 3D nanoscale defects near the boundary of the Cr2AlC and the disordered solid solution (CrAl)xCy regions. The researchers were able to image the defects through the STEM system.
Lattice distortions were discovered for the first time on the atomic scale in these materials, where the researchers found a shortening of the Cr-Cr interplanar distance and an elongation of the Cr-Al distance around the defects. This is compared to the strain-free regions of the material.
The researchers also employed ab-intio density functional theory (DFT) calculations alongside their experimental results. The researchers analysed the density of states (DOS) of bulk Cr2AlC against the strained and unstrained regions of the (0001) surface of the Cr2AlC films.
The researchers discovered that the Cr-C bonds are stronger than Cr-Al bonds in the bulk material, but in the unstrained Cr2AlC film both bond types are weakened.
However, under strain, the Cr-C bonds recovered their bulk strength whilst the Cr-Al bonds only made a partial recovery and were still weaker than their bulk counterparts. The strain induced bond strengthening in Cr2AlC(0001) was also found to be larger for Cr d – C p bonds than for Cr d – Al p bonds.
The researchers found that both the experimental results and the theoretical calculations were consistent with each other, showing that such distortions and elongation of the Cr-Al distance occur within the vicinity of nanoscale 3D defects in Cr2AlC thin films.
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The presence and confirmation of such defects is of vital importance for MAX materials. Defects in general can act as nucleation sites for oxide formation during oxidation at elevated temperatures, which is of fundamental importance for self-healing behaviours in MAX phases. The fact that these previously-overlooked 3D defects occur in MAX phases shows that more properties, including self-healing, could be a reality and not just a theoretical prediction.
Sources and Further Reading
- “Nanometre-scale 3D defects in Cr2AlC thin films”- Chen Y. T., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-01196-3