Tribocorrosion behavior of plasma nitrided Ti-6Al-4V alloy in neutral NaCl solution

The tribocorrosion behavior of plasma nitrided Ti-6A1-4V alloy is investigated in a 0.9 M NaCl solution at room temperature by means of a sliding wear tribometer, utilizing alumina balls as the counterface material. Tests were performed at open circuit potential and under anodic polarization and the electrochemical parameters, namely anodic current, potential and galvanic current were continuously monitored. The plasma nitriding treatments were performed at two different temperatures, 973 K and 1173 K, and the resulting microstructures and phases were characterized. At 973 K a thin TiN-Ti2N layer was formed while at the higher temperature this compound layer was thicker and an inner hardened sublayer was present. In the absence of such an inner layer the sliding of the alumina pins promoted the nucleation and growth of microcracks in the nitride layer, leading to quick failure of the coating. The samples treated at higher temperature showed a more uniform wear process of the nitride layer, without crack formation, and even after its removal, the hardened sublayer did not suffer from the typical delamination wear of untreated Ti-6A1-4V. The damaging or removal of the protective passive film due to rubbing leads to the formation of a galvanic couple between the anodic wear track and the outside surface. The fraction of electrochemically removed material with respect to the total wear volume was approximately 0.2 for the untreated alloy and 0.5 for the nitride layer. © 2001 Elsevier Science B.V. All rights reserved.