The sub-nanometer finishing surface evolution of tungsten carbide in shear thickening polishing and the effect of the impurities

After single-point diamond turning (SPDT), a considerable number of turning marks are typically generated on tungsten carbide molds surface, which can adversely affect their service performance. To eradicate these marks and achieve a sub-nanometer surface roughness, shear thickening polishing (STP) technology is employed to finish the tungsten carbide surface after SPDT process. Three polishing slurries with different abrasives were employed: 5 wt% SiO2 slurry with average particle size of 80 nm, 1 wt% polycrystalline diamond slurry with average particle size of 0.5 mu m, and 1 wt% monocrystalline diamond slurry with average particle size of 0.5 mu m. The influence of these slurries on the polishing performance was comprehensively evaluated, covering parameters including material removal rate (MRR), surface roughness (S-z), surface roughness (S-a), and surface topography. All experiments and measurements in this study were repeated three times, and the average values were taken as the final results to ensure the accuracy of the experiments. The results indicated that SiO2 exhibited the lowest polishing efficiency and yielded the poorest surface roughness. Monocrystalline diamond performed better, while polycrystalline diamond not only demonstrated superior efficiency but also achieved sub-nanometer finishing. During the shear thickening polishing process, the solid phase particles in the polishing slurry gradually encapsulate the abrasive particles, forming a "flexible fixed abrasive" which effectively removes the turning marks and reduces the surface roughness (S-a) to a sub-nanometer level of 0.9 nm. Through the selection of appropriate polishing angles, the S-a was further reduced to 0.8 nm. The materials removal mechanism in STP process was elucidated from two perspectives: the wear mechanism of abrasive and the influence of the impurities. Finally, STP process with selected conditions was applied to finish an curve surface of optical mold, and the surface roughness (S-a) reduces from 7.7 nm to 1.6 nm. This study provides a novel approach for the ultraprecesion finishing for optical molds with complex surface.