Effect of plasma nitriding on the strength of fine protrusions formed by sputter etching of AISI type 420 stainless steel

Abstract

Argon ion sputter-etching of AISI type 420 martensitic stainless steel was carried out to form conical protrusions with bottom diameter of 10–30μm on the specimen surface by using a radio-frequency magnetron sputter-apparatus. Plasma-nitriding was applied to the protrusions with various mixing rate of nitrogen and argon gas. The shape of the protrusions was examined by using a scanning electron microscope, and the nitrides formed on the protrusions were identified by means of an X-ray diffraction analysis. Micro-Vickers hardness tests and nano-scratch tests were used to evaluate the deformation resistance of the protrusions. By plasma-nitriding at a nitrogen pressure of 130Pa and a nitriding power of 50W, about 1.3 times larger indentation resistance than that of the as-sputter-etched protrusions was obtained after a long nitriding time of 14ks. When plasma-nitriding was carried out at a lower nitrogen pressure of 1.2Pa and a higher nitriding power of 200W for 7.2ks, indentation and scratch resistances of the protrusions largely increased to the values almost three times as large as those of the as-sputter-etched protrusions. However, the protrusions were brittle due to the formation of thick nitride layers and the sharpness of the protrusions was lost. Reduction of the nitriding power to 50W recovered the sharpness with small amount of surface nitride layer, but the indentation resistances were only 1.4 times larger values than those of the as-sputter-etched protrusions. On the other hand, plasma-nitriding using a mixture of nitrogen gas of 0.53Pa and argon gas of 0.67Pa at the power of 50W produced almost twice larger indentation and scratch resistances than those of the as-sputter-etched protrusions within a short nitriding time of 1.8ks, retaining the sharpness of protrusions without forming brittle nitrided layers.