Langmuir probe characterization of nitrogen plasma for surface nitriding of AISI-4140 steel

Langmuir probe measurements are carried out to characterize a 50 Hz ac generated nitrogen plasma as a function of filling pressure (2–5 mbar) under constant power level. Plasma nitriding of AISI-4140 steel is executed for different time durations (2, 6, 10, and 14 h) at filling pressures of 2 and 3 mbar, keeping the input power at 100 W and substrate temperature at 300 °C. The plasma nitrided samples are characterized for their surface properties by using X-ray diffractometer (XRD) and Vickers microhardness tester. Results demonstrate the formation of nitrogen infused ferrite phase (αN-Fe) owing to the inclusion of nitrogen in the original iron lattice as interstitial solid solution. Significant increase in the surface hardness is observed when samples are treated for 14 h at a filling pressure of 2 mbar. The rapid decrease in the hardness value with increase in imposed load suggests the formation of the skinny modified layer.