The glow-discharge chamber was pumped down to a base pressure of 3 Pa. The samples were plasma-etched in argon at 800 Pa, 350 °C for 30 min. Nitriding was performed for 5 h in a 80%-N
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. The sample holder was designed such as to decrease edge effects during the treatment, allowing the insertion of three samples in circular troughs, leveling the surfaces exposed to the plasma.