Controlling microstructure, preferred orientation, and mechanical properties of Cr-Al-N by bombardment and alloying with Ta

Abstract

Recent ab initio studies showed that the inherent ductility of cubic structured Cr1-xAlxN coatings (as compared with similar hard coatings) significantly increases when alloyed with Ta. As there is only little experimental and theoretical information available, we have performed a combined experimental and ab initio based study on the influence of Ta additions (0, 2, 6, 12, and 26 at. % on the metal sublattice) on structure and mechanical properties of arc evaporated Cr1-x-yAlxTayN coatings with Al/(Cr + Al) ratios >0.61. With increasing Ta-content, the droplet number density decreases and the coating surface smoothens, which is much more pronounced as with increasing the bias potential from -40 to -120 V. Simultaneously, the columnar structure observed for Ta-free Cr0.37Al0.63N significantly changes into a fine-grained structure (crystallite size ∼5 nm) with clearly reduced columnar character. Increasing the Ta content also favors the formation of a preferred 200 growth orientation resulting in a reduction of the indentation moduli E from ∼500 to ∼375 GPa, which is in agreement with ab initio calculations. As the hardness H remains between 34 and 41 GPa, an increased resistance against brittle fracture is indicated with increasing Ta.