Influence of the N-stoichiometry on the phase evolution of arc evaporated Ta and Si alloyed Ti-Al-N coatings

Abstract

Ti1-xAlxN is known for its excellent mechanical and thermal stability, especially as a protective coating for high-stress components. Since these applications are associated with high temperature environments, the prospect of increasing the operating temperature and realizing higher efficiencies and lifetimes drives the motivation for further enhancements with respect to thermal stability. In this study, a combination of two accepted approaches to improve mechanical and thermal properties was chosen. In a first step, the alloying of Ti1-xAlxN with Ta and Si, which has already been extensively researched and shown excellent results, is used to provide a delay of thermally activated decomposition processes and phase transformations, as well as a simultaneous improvement of the mechanical properties. Secondly, for the base system Ti1-xAlxN, the manipulation of the nitrogen content and thus the number of nitrogen vacancies in the coating has shown an interesting and effective possibility to retard decomposition processes. Therefore, in the present work, cathodic arc evaporation is used to deposit a series of Ti1-xAlxN, Ti1-x-yAlxTayN and Ti1-x-zAlxSizN coatings with the aim of tuned Nitrogen stoichiometries. Therefore, the a strong focus was set on the nitrogen supply during the deposition process and an optimization (pressure-controlled and flow-controlled). Throughout the nitrogen variations the phase evolution, morphology, and mechanical properties has been tracked. For all three material systems, the best results regarding the stoichiometry variation were obtained by the flow-controlled process using Ar/N2 mixtures. However, only a limited process window allows for the deposition single phased c-Ti1-xAlxN structures. An in-depth variation for the Ti1-x-yAlxTayN system, revealed that the pressure-controlled processes led to very slight differences in the N-content, whereas the flow-controlled processes with an Ar/N2 mixture produced a N-variation from 35.4 at% to 46.4 at%. Here a transition from multi-phased to the desired single phased cubic phase constitution is apparent. In terms of mechanical properties, the hardness varies from 19.9 ± 1.1 GPa to 36.8 ± 0.9 GPa for the different phase constitutions and alloying elements.