Plasma nitriding of cone-shaped protrusions formed by sputter etching of AISI 420 stainless steel and their application to impression punch to form micro-holes on polymer sheets

Abstract

Argon ion sputter-etching was applied to disk specimens of AISI type 420 martensitic stainless steel using a radio-frequency magnetron sputter-apparatus, and sharp cone-shaped protrusions with vertex angle of about 40° and base diameters less than 35 μm were formed. In order to increase compressive strength of protrusion tips without loss of sharpness, plasma-nitriding was applied to the as-sputter-etched specimens using nitrogen gas of 0.53 Pa mixed with argon gas of 0.67 Pa at a power of 50 W for 1.8 ks. When the plasma-nitrided (PN-) and as-sputter-etched (AS-) specimens were statically impressed on polyethylene (PE) and polyvinylchloride (PVC) sheets by increasing the nominal stress step by step from 0.624 to 12.5 MPa, the protrusion tips of PN-specimen did not show bending deformation even at 12.5 MPa whereas those of AS-specimen revealed bending already at 2.2 MPa. When the PN-protrusions were impressed statically on a polycarbonate (PC) sheet, a part of tall protrusion tips were flattened, but the shape of the holes on the sheet was not distorted. The maximum hole-diameter on the PVC sheet is smaller than that of the PE and PC sheets due to a larger viscoelastic recovery of the PVC sheet. When the PN-protrusions were impressed repeatedly on the PE and PVC sheets one thousand times with nominal impression stress of 0.078 MPa, the tips did not show bending and very dense micro-holes were formed. On the other hand, a large bending of the AS-protrusion tips occurred by the repeated impression on the PVC sheet. Thus, the PN-specimen can be applied to an impression punch to form fine and dense holes on polymer sheets.