Developing Ti-Al-Ta-N based coatings: Thermal stability, oxidation resistance, machining performance and adaptive behavior under extreme tribological conditions

Abstract

Application of hard coatings was found to be a promising method in response to industry’s growing demand for improved productivity and avoiding high temperature in cutting zone. This study presents comprehensive investigations on the effect of Ta addition into TiAlN coating on mechanical properties, oxidation resistance, thermal stability, and ultra-high speed machining performance. The thermal stability of the coatings was studied using DSC, heat treatment between 900 and 1400 °C, and X-ray diffraction (XRD). Isothermal oxidation testing was performed at 1000 °C for 0.5 and 20 h in synthetic air followed by cross-section examination using field emission scanning electron microscopy (FE-SEM). The features of the chip formation were studied using SEM and electron backscattered diffraction (EBSD). Evidence of spinodal decomposition was found in the XRD patterns. However, the age-hardening effect was different in its degree of significance for monolayered and multilayered TiAlTaN coatings. Monolayered TiAlTaN coatings exhibited the best oxidation resistance, while Ta-free multilayered coatings fully oxidized after 20 h of exposure to 1000 °C temperatures. Tool life measurements showed a ~25% improvement for monolayered TiAlTaN coating compared to TiAlN under intense machining conditions and indicated that the addition of Ta in the coating greatly delayed the progression of crater wear and subsequent substrate exposure. This is attributed to the formation of protective tribo-oxide layers on the friction surface, the tool with monolayer TiAlTaN coating demonstrated optimal chip properties, resulting in improved metal flow at the chip’s undersurface. Ultimately, a lower fraction of recrystallization was found in EBSD images of chip cross-sections, indicating lower heat transfer and plastic deformation. This study showed a significant improvement in mechanical properties, thermal stability, oxidation resistance along with more than 25% improvement in high-speed machining performance induced by Ta addition into TiAlN coatings.