Subsurface amorphization-induced ultrasmooth and ultrahard surface during the diamond turning of polycrystalline pure copper

• Amorphization phenomenon in diamond turning of pure copper is reported. • Hexagonal close-packed and monoclinic lattices of pure copper were experimentally observed in diamond turning. • Amorphization of pure copper results in an ultrasmooth surface finish of Sa 0.61 nm. • Amorphous pure copper achieves a remarkable 5.0 GPa nanohardness. • High shear strain leads to the amorphization and lattice transformation of pure copper. Previous simulations indicated that pure metals can be amorphized during diamond turning. However, no pure metal has been successfully amorphized during this procedure to date. Herein, a comprehensive study on the surface formation mechanism was performed by examining the surface finish and subsurface variation characteristics. Polycrystalline pure copper was selected as the working material. In this study, the amorphization phenomenon and lattice structure transformations (from face-centred cubic to hexagonal close-packed and monoclinic structures) of pure copper were observed. The amorphization and phase transformation inside the working material depended on the tool nose radius being large, the cutting edge being sharp, and the feed rate being low. By using a large-nose-radius (10 mm) diamond tool with a sharp cutting edge (10.3 nm) at a low feed rate (0.5 μm/rev), an ultrasmooth and ultrahard surface with a 0.61 nm roughness Sa and 5.0 GPa nanohardness was achieved. Molecular dynamics simulations revealed that the high shear strain was responsible for the amorphization and lattice structure transformation during pure copper diamond turning. The findings of this study were considered critical for generating an amorphous subsurface layer in a pure metal and for achieving a ultrasmooth and ultrahard surface using single-point diamond turning.