Suppression of built-up edge and machining damage in pure iron cutting using Lt-cBN tool

During ultra-precision cutting (UPC) of pure iron, notable chemical reactions between diamond and iron resulted in the formation of a built-up edge (BUE) and subsequent machining damage. Binderless cubic boron nitride (CBN), which exhibits high hardness and superior chemical inertness, has been identified as a promising alternative tool material for suppressing BUE formation. However, its fracture toughness was insufficient to prevent cutting tool breakage and ensure satisfactory surface quality. A recently developed binderless layered BN-toughened CBN (Lt-cBN) demonstrated superior fracture toughness compared to other commercial binderless CBN, indicating substantial potential for the UPC of pure iron. Therefore, this paper presented an analysis and evaluation of the performance of the Lt-cBN tool in suppressing BUE formation and machining damage, in comparison with the single crystal diamond (SCD) tool. Firstly, UPC experiments using both the Lt-cBN and SCD tools were conducted on pure iron. Meanwhile, the UPC performances were explored by analyzing the chip formation, cutting force, and tool wear. Then, the machined damage mechanisms of pure iron were thoroughly investigated based on observations of surface defects and subsurface microstructures. Finally, to further elucidate the influence mechanism of tool wear and BUE formation on cutting damage for both the Lt-cBN and SCD tools, finite element (FE) simulations were employed to analyze the pure iron cutting. The experimental results indicated that, at a cutting distance of 300 m, the wear rate and cutting force of Lt-cBN tool decreased by 16.8 % and 85.1 %, respectively, compared to those of the SCD tool. The Lt-cBN tool effectively inhibited the adhesion of pure iron, thereby avoiding the formation of a BUE. Accordingly, an ultra-smooth surface with roughness Sa < 10 nm across the entire workpiece was achieved using the Lt-cBN tool. Furthermore, compared to the SCD tool, the pure iron machined by Lt-cBN tool presented a thinner subsurface damage layer, not exceeding 2.5 μm in depth. This subsurface layer consisted of an ultrafine-grain layer of around 0 – 0.2 μm generated by dynamic recrystallization (RX) and a high plastic deformation layer approximately 0.2 – 2.5 μm in depth. However, pure iron machined by the SCD tool suffered significant damage due to the formation of BUE. The FE simulations indicated that the SCD tool exhibited a higher tendency to induce subsurface damage compared to the Lt-cBN tool, primarily due to its greater induction of thermal and plastic deformation.