Tribological performance of different alloyed DLC and AlTiSiN coatings when sliding against Inconel 718 alloy for demanding applications

Machining Ni alloys such as Inconel 718 is challenging due to its good mechanical properties, low thermal conductivity and low elastic modulus, which causes high cutting temperatures at the chip-tool interface. The use of self-lubricating coatings, such as DLCs coatings has potential to improve significantly the machinability of Ni alloys. This research work aims to explore how different DLC, DLC-Si, DLC-W coatings behave tribologically when sliding against Inconel 718. The results will be compared with an industrial AlTiSiN coating used to protect the cutting tools. The morphology, mechanical properties and chemical composition of the coatings were analysed. To evaluate the tribological behaviour alternative sliding tests were performed under different conditions (room temperature – RT and 200 °C in dry condition and at RT with lubrication, using CUT-MAX S 50259-1 oil. The DLC coatings showed remarkable self-lubricating efficiency, with low coefficients of friction even in dry conditions, demonstrating that their self-lubricating capability is effective without the need for external lubricants. However, at a temperature of 200 °C, a significant increase in wear rate was observed for all DLC coatings, being 3 to 4 times higher as compared to that at room temperature. This is mainly caused by the conversion of sp 3 to sp 2 bonds coating oxidation. On the other hand, AlTiSiN demonstrated consistent specific wear rate, despite of the unfavourable circumstances, highlighting its suitability to use in extreme environments. These results show the limitations in film performance and underline the importance of balancing the strength of the coating with its impact on the opposing surface. Furthermore, it highlights the need to improve the thermal stability of DLC coatings for application in high temperature environments and dry conditions. • Alternative sliding of doped DLC and AlTiSiN films against Inconel 718 at RT, 200°C and lubricated is tested. • Although Inconel is aggressive in sliding, DLC and AlTiSiN films reduce COF and effectively resist wear, respectively. • AlTiSiN exhibited the highest coefficient of friction (COF), lowest wear rate at all the tested conditions. • DLC, DLC-Si and DLC-W coatings demonstrated excellent self-lubricating behaviour with low COFs. • The DLC coatings and their doped variants showed a significant increase in wear rate when subjected to 200 °C.