Nanostructured surfaces

Deformated nanostructuring of surface layers of Ti–5Al–5Mo–5V–1.5Cu–1Fe titanium alloy for intensification gas nitriding
Lavrys S.M., Pohrelyuk I.M.
1 Material Science Fundamentals of Surface Engineering Department, Karpenko Physico-Mechanical Institute of the NAS of Ukraine. Naukova str., 5, Lviv-79060, Ukraine.
E-mail: lavrys92@gmail.com

In domestic aircraft industry is widespread use of high-strength two-phase Ti5Al5Mo5V1.5CuFe titanium alloys due to their excellent combination of mechanical and corrosion properties. However, titanium alloys have important disadvantage: low wear resistance surface hardness. To eliminate this disadvantage, the working surface of titanium alloy protects against wear by thermo-chemical treatment, namely gas nitriding. However, the treatment temperature for the conventional gas nitriding of the titanium alloys in the range 750°C...950°C with a time exposure of more than ten hours to obtain a deep hardened layer, which leads to grain growth of material core and, consequently, a decrease of the mechanical characteristics and high energy consumption.
In this research, we proposed the effectiveness carrying out the previous surface nanostructuring by ball burnishing with subsequent gas nitriding. Such a combination of methods of surface hardening is justified by the fact that the preliminary nanostructuring of the surface layer provides an increase in the grain boundary area, accumulation of linear, point defects and compressive residual stresses in the surface layer, which in aggregate intensifies the processes of nitride-formation and gas-saturation during gas nitriding. This makes it possible to reduce the time and temperature and energy consumption of gas nitriding. 
According to the results it is found that the ball burnishing pretreatment introduced a 10 µm thick severe plastic deformation nanostructured surface layer. Also, the nanostructuring pre-treatment enhanced nitrogen diffusion and promoted nitriding kinetics, resulting in the formation of deeper diffusion zone and a nanocrystalline TiN surface layer compared to gas nitriding without nanostructuring pre-treatment. Such combined treatment of titanium alloy provides an increase of the tribotechnical characteristics of the tribo-pair Ti–5Al–5Mo–5V–1.5Cu–1Fe titanium alloy with Cu–10Al–4Fe–4Ni bronze alloy compared to gas nitriding without nanostructuring pre-treatment.