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

The tribocorrosion behavior of plasma nitrided Ti-6Al-4V alloy is investiga ted 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 wer e 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 p erformed at two different temperatures, 973 K and 1173 K, and the resulting microstructures and phases were characterized. At 973 K a thin TiN-Ti2N la yer was formed while at the higher temperature this compound layer was thic ker and an inner hardened sublayer was present. In the absence of such an i nner layer the sliding of the alumina pins promoted the nucleation and grow th of microcracks in the nitride layer, leading to quick failure of the coa ting. The samples treated at higher temperature showed a more uniform wear process of the nitride layer, without crack formation, and even after its r emoval, the hardened sublayer did not suffer from the typical delamination wear of untreated Ti-6Al-4V. The damaging or removal of the protective pass ive film due to rubbing leads to the formation of a galvanic couple between the anodic wear track and the outside surface. The fraction of electrochem ically removed material with respect to the total wear volume was approxima tely 0.2 for the untreated alloy and 0.5 for the nitride layer. (C) 2001 El sevier Science B.V. All rights reserved.