Formation of Fine Protrusions by Sputter Etching of Stainless Steels and Evaluation of Gripping Ability of Fiber Sheets

Argon ion sputter etching of AISI type 420 stainless steels was carried out at a power of 250 W for 14.4 ks to form cone-shaped protrusions with bottom diameters of 10–20 µm by using a radio-frequency magnetron sputter apparatus. The sputter etching was also applied to AISI type 316 stainless steel at 250 W for 10.8 ks, and fine column-shaped protrusions with diameters smaller than 500 nm were formed. In addition, plasma-nitriding was applied to the protrusions of type 420 steel 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. The gripping ability of protrusions was measured by friction tests for three types of fiber sheets. For a cotton cloth, the friction coefficients of the as-sputter-etched and plasma-nitrided type 420 steel specimens were 1.3 and 1.7 at a nominal compressive stress of 0.8 kPa, and that of the as-sputter-etched type 316 steel specimen was 1.0. These are much larger than 0.8 and 0.5 of the specimens polished with #100 emery paper and buff-polished with alumina powders. Also for microfiber and polyester fiber sheets, the specimen with protrusions showed a higher gripping ability than those of the mechanically polished specimens. The reason for the large gripping ability of protrusion specimens is due to piercing of the protrusions between and into weaving yarns.