F.P. Galliano

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

Abstract The tribocorrosion behavior of plasma nitrided Ti–6Al–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–Ti 2 N 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–6Al–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.

XRD and XPS study on reactive plasma sprayed titanium–titanium nitride coatings

Reactive plasma spraying of titanium in a nitrogen-containing plasma gas allows the formation of hard titanium composite coatings reinforced by in situ formed titanium nitrides (TiN and Ti2N). In the present study, the formation of titanium nitrides during the reactive spraying of titanium was studied by means of two different analysis techniques: X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The diffusion and solidification sequence predicted by the Ti-N phase diagram was consistent with the experimental results. The nitrogen retained in the coating was found mainly as titanium nitrides and not as interstitial nitrogen in the titanium lattice. It is hypothesised that the formation and direct solidification of TiN occurs during the flight of the sprayed particles. The presence of solid TiN constrains the thermal contraction of titanium splats upon solidification. The observed cell distortion of the residual α-Ti is thus attributed to tensile stresses arising from the solidification process.

Reactive plasma spraying of titanium in nitrogen containing plasma gas

Abstract The low hardness and the poor tribologic characteristics of titanium can be positively modified by means of reactive plasma spraying. The present study describes experimental results concerning reactive low pressure plasma spraying of pure titanium powders in an Ar/N 2 plasma gas in order to obtain thick titanium composite coatings with a high percentage of hard titanium nitrides (TiN and Ti 2 N). As measured by means of X-ray diffraction, the volume fraction of nitrides in the coating with increases increasing nitrogen content in the plasma gas mixture and with increased spraying distance. The nitrogen content of the coating shows a linear relationship with the microhardness of the coating. By varying the Ar/N 2 plasma gas ratio, adherent hard composite coatings, up to Knoop 1200 hardness and with low porosity are obtained. Such characteristics make them particularly suitable in all those technological applications where good corrosion resistance of titanium and high tribologic properties are required.

Corrosion resistance properties of plasma nitrided Ti–6Al–4V alloy in hydrochloric acid solutions

Abstract The corrosion properties of TiN layers obtained by glow discharge on Ti–6Al–4V samples were investigated by polarisation curves, cyclic voltammetry and loss weight tests, in different HCl solutions. The results were compared with those obtained on the untreated alloy tested in the same conditions. The corrosion resistance of nitrided samples depends on the quality of the film and increases as the thickness increases. At potential lower than 1000 mV (SSE) nitrided samples show passive behaviour even in concentrated HCl solution (4M). If the potential is higher than a threshold, which depends on the acid concentration, it is observed a high anodic current related to the TiN oxidation. This enhanced oxide growth promotes cracking of the oxide layer and the exposure of fresh TiN surface.

Air treatment of pure titanium by furnace and glow-discharge processes

Abstract The high oxygen affinity of titanium can be used to improve the surface hardness and wear resistance of titanium and titanium alloy components by means of thermal treatments. In the present paper a comparison of the effects of glow-discharge and furnace processes, carried out using air as treatment atmosphere, on the microstructure and mechanical properties of commercially pure titanium samples is reported. Both treatment types, performed at 973 and 1173 K for 0.5, 2 and 4 h, produce modified surface layers: the outer layer (compound layer) consists essentially of TiO 2 and small amounts of TiN x O y , while the inner layer (diffusion layer) consists of α-Ti crystals rich in interstitial atoms. The plasma treatment is shown to be more efficient in hardening titanium surface layers than the furnace process: by using the same treatment temperature and time, plasma treated samples have a thicker hardened layer, with higher hardness values than the furnace treated ones. A crystallographic characterisation of the α-Ti solid solution has also been performed and the a , c lattice parameters have been evaluated. Plasma treated samples show larger lattice parameters and c / a values than the furnace treated samples, suggesting a higher concentration of interstitial atoms due to the plasma treatment.

Corrosion resistance properties of reactive plasma-sprayed titanium composite coatings

Abstract Among thermal spraying methods, an attractive technical possibility lies in the fabrication of protective coatings or free-standing components by means of reactive plasma spraying (RPS) techniques. Using reactive gases, such as nitrogen or methane, it is possible to synthesize hard nitride or carbide phases in reactive metals like Ti, Cr or Al. In this investigation composite titanium-nitrides/titanium coatings produced by RPS through a controlled atmosphere plasma spray system (CAPS), were electrochemically tested to evaluate their corrosion behaviour. Two environments were selected: a neutral (0.5 M NaCl) and an acid aqueous solution (0.5 M NaCl+1 M HCl). The influence of porosity and nitrogen content on the corrosion resistance has been investigated. Polarization curves of coated samples, detached coatings, AISI304 substrate and commercially pure titanium (grade 2), are also reported and discussed. The corrosion resistance of coated samples was found to be mainly dependent on porosity values, thus optimization of plasma spraying parameters assumes a fundamental role to obtain wear and corrosion resistant deposits.

Wear resistance of Ti–6Al–4V alloy treated by means of glow-discharge and furnace treatments

Abstract The contemporary glow-discharge oxidising and nitriding treatment has shown to produce modified surface layers with enhanced hardness properties, in comparison with the conventional ion-nitriding process. The aim of the present study was to evaluate the tribological properties of ion-oxinitrided samples and to compare their wear behaviour with the one of furnace oxinitrided and ion-nitrided samples. At low coupling loads (50 N) the wear volumes of the treated samples result small and comparable for all the tested velocities. On the other hand, when high coupling loads are used (100 N), the wear of the ion-nitrided samples is higher than that of the oxinitrided ones, this effect becoming more remarkable as sliding velocity increases; moreover, the ion-oxinitriding treatment allows to achieve lower wear volumes than the ones obtained by means of the furnace oxinitriding process.