Effects of hydrogen exposure on the mechanical and tribological properties of α-titanium surfaces

• Mechanical properties of surface micro/sub-microvolumes of titanium and its tribological behavior have been investigated under the influence of electrolytic hydrogenation.
• Titanium hardening quantitative characteristics after hydrogenation were determined with application of the nanoindentation method.
• Titanium hydrogenation activates cracks formation and brittle fracture of microirregularities at the friction surface.
• The mechanism of hydrogen effect on tribological behavior of titanium has been proposed.

The influence of electrolytic hydrogenation on the mechanical properties of the near-surface microstructures of titanium and the resulting tribological behavior (scratch tests and reciprocating ball-on-plane tests) was investigated. Hardening effects of hydrogenation were revealed by nano-indentation methods. Additional quantitative characteristics, such as micro-indentation hardness, Young's modulus, and the work of elastic and plastic deformation, have also been determined. Plastic deformation of surface micro-asperities dominated during the sliding friction of non-hydrogenated titanium. By contrast, brittle fracture and crack formation were observed on hydrogenated surfaces. In a limited number of experiments, the coefficient of friction was reduced by as much as a factor of three after hydrogenation.A mechanism by which hydrogen affects the tribological behavior of titanium has been proposed. Hydrogen, localized in the dislocation cores, blocks their movement, which leads to hardening of titanium and a reduction in its ductility. During sliding, the periodic external and internal stresses overlap at the contact points, and that leads to micro-cracking and brittle fracture. Evidence for this mechanism remains to be demonstrated in future work.