The samples were plasma-etched in argon at 800 Pa, 350 °C for 30 min.
Nitriding was performed for 5 h in a 80%-N2:20%-H2 gas mixture, flow rate of 20 sccm, and pressure of 800 Pa to maintain the sample temperature at 450 °C, the voltage was adjusted to 347 V and the current to 287 mA.