Thermal modulation of the water-induced surface ordering effect for tunable machining performance in copper micro-cutting

The water-induced surface ordering (WISO) effect, i.e., water molecules ordering the surface atoms of copper, has been shown to dramatically reduce the cutting force in micro-cutting of copper. However, its modulation beyond a simple "on-off" state remains unexplored. In this study, experiments are designed to investigate the regulation of WISO by qualitatively controlling the quantity of surface-bound water molecules via thermal means. Orthogonal micro-cutting was conducted on polycrystalline copper under varying depths of cut (doc) and surface temperatures. The experiments reveal that the cutting force reduction ratio (CFRR), defined as the percentage reduction of cutting force in the region with H2O compared to that without, demonstrates distinct consistency across doc and thermal sensitivity. Specifically, while cutting forces rise with increasing doc, the CFRR stabilizes at 40%-44% within the range of 15 μm–30 μm doc, which confirms that the effectiveness of WISO remains stable within a specific range of doc. In contrast, as surface temperature increases from 20 °C to 80 °C, the CFRR decreases from 70.72% to 57.67%. Since the increase in actual doc caused by thermal expansion plays a negligible role in the CFRR, this decay is mainly attributed to the thermally induced reduction of effective surface-bound water molecules. This is also demonstrated by the reduced difference in surface roughness Sa between regions with and without H2O at higher surface temperatures, as the Sa in regions with H2O remains lower than in regions without H2O. Furthermore, chip morphology changes from short, thick shapes in regions without H2O to long, thin ones in regions with H2O. Ultimately, this study reveals that the quantity of surface-bound water molecules is a crucial factor influencing the WISO effect, and modulating it through thermal means shows potential for controlling the machining performance in copper micro-cutting.