Control of tribofilm formation in dry machining of hardened AISI D2 steel by tuning the cutting speed

This study is focused on the investigation of nano-scale tribological effects formed during the machining of hardened material. Tribofilm formation was associated with the transfer of workpiece material to the cutting tool surface with further tribo-oxidation occurring through interaction with the environment. In addition to wear evaluation studies, a number of advanced surface analytical techniques were used to characterize surface phenomena, including X-ray photoelectron spectroscopy (XPS) and Auger Electron Spectroscopy (AES) (including AES imaging). This was combined with Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM/EDX) studies. Wear performance of uncoated ceramic (mixed alumina and TiCN) tools was studied in detail at different constant cutting speeds (50; 80; 100 and 120 m/min) and tuned through two-step and three-step cutting speed adjustments (120-100 and 120-100-80 m/min) during hard dry turning of AISI D2 tool steel. The tuning of the cutting speed was performed in order to enhance the generation of favourable tribofilms and obtain an extended tool life. As a result of these tuned cutting approaches, tool life was increased by 36% at 120-100 m/min and improved 99% at 120-100-80 m/min when tested separately. Both the two-step and three step speed adjustment involved enhanced formation of beneficial tribofilms on the tool surface under a higher cutting speed, with a subsequent lower speed preventing the wearing out of these films and thus sustaining the improved tribological conditions. This research provides a novel strategy to enhancing the efficiency of machining processes which operate under extreme tribological conditions of dry machining of hardened tool steels.