Materials Chemistry, RWTH Aachen University, Kopernikusstr. 10, 52074, Aachen, Germany.
Central Facility for Electron Microscopy, RWTH Aachen University, 52056, Aachen, Germany.
Sci Rep. 2017 Apr 20;7(1):984. doi: 10.1038/s41598-017-01196-3.
MAX-phase CrAlC containing thin films were synthesized by magnetron sputtering in an industrial system. Nanometre-scale 3D defects are observed near the boundary between regions of CrAlC and of the disordered solid solution (CrAl)C. 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 CrAlC with the DOS of a strained and unstrained CrAlC(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 CrAlC. Upon CrAlC(0001) surface formation, both bonds are weakened. While the Cr-C bonds recover their bulk strength as CrAlC(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 CrAlC(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 CrAlC thin films.
采用磁控溅射在工业系统中合成了含 MAX 相 CrAlC 的薄膜。在 CrAlC 区域和无序固溶体 (CrAl)C 区域的边界附近观察到纳米级 3D 缺陷。使用透射电子显微镜检测到缺陷附近 Cr-Cr 面间距的收缩和 Cr-Al 距离的伸长。通过将体相 CrAlC 的 DOS 与应变和未应变的 CrAlC(0001)表面的 DOS 进行比较，用密度泛函理论描述了缺陷周围的这种变形。从部分态密度分析可以得知，在体相 CrAlC 中，Cr-C 键比 Cr-Al 键强。在 CrAlC(0001)表面形成时，两个键都被削弱。虽然 CrAlC(0001)在应变时 Cr-C 键恢复到体相强度，但 Cr-Al 键仅部分恢复，仍然比体相弱。因此，在 CrAlC(0001)中，应变诱导的键强化对于 Cr d - C p 键比 Cr d - Al p 键更大。由于形成应变表面而导致的键合变化与在 CrAlC 薄膜中纳米级 3D 缺陷附近观察到的 Cr-Al 距离伸长一致。
