Effect of plasma nitriding on the compressive strength of multi-functional fine protrusions formed by sputter-etching of stainless steels

Abstract

Argon ion sputter-etching of SUS420J2 martensitic stainless steel and SUS316 austenitic stainless steel were carried out to form fine protrusions on the specimen surface by using a radio-frequency magnetron sputter-apparatus. Plasma-nitriding was applied to the protrusions by 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. Micro-Vickers hardness tests on cone-shaped protrusions with diameter of about 20 μm formed on the SUS420J2 steel showed that the compressive strength of protrusion tip of the plasma-nitrided specimen is almost four times larger than that of the as-sputter-etched specimen. Nanoindentation tests on quasi-columnar protrusions with sizes smaller than 500 nm formed on the SUS316 steel showed not only much scatter of indentation force vs depth curves but also irregular changes of gradient of the curves. This means that an inhomogeneous contact of the indenter on the protrusions with different heights, sizes and spaces as well as a discontinuous deformation of protrusions occur with progress of indentation. Although the true compressive strength of one protrusion is not obtained directly by usual hardness tests, a measure of the deformation resistance of a group of protrusions can be obtained from the hardness that is extrapolated to zero indentation area on the indentation area vs hardness curve. The compressive resistance of the plasma-nitrided protrusions is almost six times as large as that of the as-sputter-etched ones.