4. ConclusionsSeveral novel observations were noted in the dry high-speed hard turning of AISI 4340 usingalumina-based ceramic tools. At cutting speeds of 150 and 250 m/min, the pure alumina ceramic toolswith the addition of ZrO2 have higher wear resistance than the mixed alumina tools. XPS analysiscarried out on the worn area of the pure alumina ceramic tool at 250 m/min showed the formation of aprotective alumina tribo-film with sapphire crystal structure as well as a ZrO2 tribo-film that decreasedthe coefficient of friction at the tool-chip interface. This was indicated by the chip compression ratioresults and the chip underside micrographs. Increasing cutting speed to 700 m/min diminishes theprotective effect of the ZrO2 tribo-film when using the pure alumina tools as indicated by the wearcurve and the chip compression ratio at this speed. A higher amount of sapphire formed at the wornedge of the mixed alumina ceramic tool at 700 m/min. Moreover, this tool has a higher hardness andthermal conductivity than the pure alumina ceramic tool. Taking these factors into account, the mixedalumina ceramic tool outperforms the pure alumina one at 700 and 1000 m/min. The increase of thecutting speed from 700 to 1000 m/min demonstrated a reduction in the radial force component by 20%and the tangential force component by 12%, which could potentially improve the workpiece accuracy.The value of the cutting force components and chip compression ratio, indicated a decrease of thecoefficient of friction at the tool-chip interface upon applying HSM.