Plasma Polishing and Finishing of CVD-Diamond Coated WC (Co) Dies for Dry Stamping

The CVD-diamond coated WC (Co) dies have been high-lighted as a tool to make dry stamping of stainless sheets, aluminium alloys and cold-rolled steel sheets. The bare CVD-diamond film had higher roughness with steep asperities than 1.5 Rz so that the friction coefficient became 0.3 to 0.5. These asperities of rough diamond crystals ploughed and worn the counter material surface. Manual and mechanical polishing processes by using the diamond powders and oxidizers required for long, tedious time; in addition, the curved surfaces of diamond coated die shoulders are difficult to be homogeneously polished and finished. On the other hand, the nano-optical polishing was only effective to remove the nano-sized residuals on the polished surfaces. In the present paper, the plasma oxidation polishing and finishing process was proposed as a non-traditional method to reduce the original maximum surface roughness down to 0.1 μm in Rz or less than. The plasma-density intensifying device was designed and used to control the plasma sheath on the diamond films toward the homogeneous polishing in large surface area. Two types of test-pieces were prepared for polishing experiments; e.g., the diamond-coated flat and curved WC (Co). The former specimen was employed to prove the low friction and wearing state should be preserved in the tribo-testing. The latter one was used to demonstrate that the diamond film on the deep-drawing dies should be homogeneously polished and finished.