Nanometre-scale 3D defects in Cr2AlC thin films

Abstract

MAX-phase Cr2AlC containing thin films were synthesized by magnetron sputtering in an industrial system. Nanometre-scale 3D defects are observed near the boundary between regions of Cr2AlC and of the disordered solid solution (CrAl)xCy. Shrinkage of the Cr-Cr interplanar distance and elongation of the Cr-Al distance in the vicinity of the defects are detected using transmission electron microscopy. The here observed deformation surrounding the defects was described using density functional theory by comparing the DOS of bulk Cr2AlC with the DOS of a strained and unstrained Cr2AlC(0001) surface. From the partial density of states analysis, it can be learned that Cr-C bonds are stronger than Cr-Al bonds in bulk Cr2AlC. Upon Cr2AlC(0001) surface formation, both bonds are weakened. While the Cr-C bonds recover their bulk strength as Cr2AlC(0001) is strained, the Cr-Al bonds experience only a partial recovery, still being weaker than their bulk counterparts. Hence, the strain induced bond strengthening in Cr2AlC(0001) is larger for Cr d – C p bonds than for Cr d – Al p bonds. The here observed changes in bonding due to the formation of a strained surface are consistent with the experimentally observed elongation of the Cr-Al distance in the vicinity of nm-scale 3D defects in Cr2AlC thin films.

Defects in an orthorhombic MoAlB MAB phase thin film grown at moderate synthesis temperature

“…A HAADF STEM image and corresponding line profile, showing the presence of an Al single layer in the vicinity of a 3D cluster, are available in the ESI (Fig.S6 †). Similar 3D cluster defects were observed in a Mo 2 BC/Cr 2 AlC coating 40,41. They might be caused by the bombardment with ions of the starting materials and Ar ions in the DC magnetron sputtering chamber.EELS studies were carried out on these various structural defects to compare changes in their fine features with the pristine MoAlB area in Fig.5(e).…”

“…Similar 3D cluster defects were observed in a Mo 2 BC/Cr 2 AlC coating. 40,41 They might be caused by the bombardment with ions of the starting materials and Ar ions in the DC magnetron sputtering chamber.…”

Incipient low-temperature formation of MAX phase in Cr–Al–C films

“…Due to its nano-laminate structure, Cr 2 AlC has low hardness, high Young's modulus, and good compression strength; it is a good electrical and thermal conductor and has very good oxidation resistance. Among possible applications of Cr 2 AlC, one can count protective coatings for turbine propellers in avionics [2] or self-healing materials [19]. Berger et al [10] have studied the influence of the substrate temperature on the formation of the Cr 2 AlC stable phase; however, their coatings had a mixed phase structure composed of (Cr,Al) 2 C x solid solution and ordered Cr 2 AlC.…”