Hardness, stress relaxation and microstructure of TiBN multilayer coatings

Abstract

Multilayer coatings of a TiBN layer sequence have been prepared by sequential sputtering from a Ti cathode and a h-BN cathode. These layers were subjected to thermal treatment at 400°C by which a hardening process as consequence of interdiffusion was observed. Hardness values of up to 60 GPawere obtained. In contrast to most other hard coatings these TiBN coatings display only small macroscopic stress of the order of ±1 GPa as measured by the substrate bending method. Tensile or compressive stress can be engineered by correctly choosing the repeat thickness and the individual layer ratio. TEM investigations suggest that these low stress values are due to an internal stress relaxation process. The individual multilayers are subjected to a high microscopic internal stress resulting in fragmentation into very small pieces of about 50 nm. This is supported by the strong bending of these fragments as observed in the TEM micrographs.