Plasma electrolytic nitriding of alpha- and beta-titanium alloy in ammonia-based electrolyte

The structure of alpha- and beta-titanium alloy, its microhardness, surface roughness, wear and corrosion resistance after anodic plasma electrolytic nitriding (PEN) in electrolyte containing ammonia were investigated. An X-ray diffractometer and a scanning electron microscopy (SEM) were used to characterize the phase composition of the modified layer and its surface morphology. Tribological properties of nitrided titanium alloy and their corrosion resistance are investigated. It was shown that the electrolyte containing ammonia and ammonium chloride provided the saturation of alloy with nitrogen and oxygen and formation TiO2 with rutile structure and nitrogen solid solution in titanium. The anodic PEN at 800 °C during 5 min results in an increase in microhardness to 520 HV and a decrease in surface roughness by 4 times owing to anode dissolution of titanium. Friction coefficient of nitrided samples can be 4.4 times reduced through sliding with speed of 0.49 m/s and load of 208.6 N. These regimes enable to diminish wear rate of titanium alloy samples after their wear testing by 4 orders. The decrease in the sliding speed and load leads to the rise of friction coefficient and wear rate.