Nitrogen High Temperature Plasma Based Ion Implantation for the Treatment of Metal and Metal Alloys for Aerospace Application

Plasma based ion implantation (PBII), a well suited technique for the surface treatment of materials, was successfully employed to improve mechanical and tribological properties of Ti6Al4V, NiTi, Mo and Nb. This non-line-of-sight ion implantation method, when associated with the simultaneous heating of the substrate, is able to modify thick layers of the material, up to tens of micrometers, since implanted ions can easily diffuse to innermost layers. An inherent advantage of the method is that the treatment does not produce a film, avoiding problems of stress and delamination, an important issue due to the high temperature of operation that most materials for aerospace applications are submitted. In the current experiments the treated substrates were heated by both, ions and electrons, provided by the own plasma discharge and by an oxide cathode, allowing to establish a very stable temperature for each substrate: 500 °C for NiTi, 800 – 900 °C for Ti-6-Al-4V, 1000 °C for Mo and 1250 °C for Nb. Negative high voltage pulses varying from 7 kV to 12 kV, with duration of 40 microseconds and frequency of 400 Hz were employed for ion implantation. A myriad of new nitride phases were detected by X-ray diffraction, while changes of the microstructure were viewed by scanning electron microscopy. Glow discharge optical emission spectroscopy was used to measure the elemental depth profiles, which detected nitrogen diffusing up to 20 micrometers for Ti-6Al-4V.The significant hardening and the much lower wear rates of the treated surfaces, as well as the reduction of the friction coefficient were measured by Vickers hardness and by a tribometer, respectively. Constant load creep tests indicated significant reductions of the steady-state creep rates for treated martensitic Ti-6Al-4V, reaching minimum creep rates of 0.0318 h^-1 in comparison with 0.1938 h^-1 for untreated sample.