Fibre laser nitriding of titanium and its alloy in open atmosphere for orthopaedic implant applications: Investigations on surface quality, microstructure and tribological properties

Laser nitriding is known to be an effective method to improve the surface hardness and wear resistance of titaniumand its alloys. However, the process requires a gas chamber and this greatly limits the practicability fortreating orthopaedic implants which involve complex-shaped parts or curved surfaces, such as the tapered surfacein a femoral stemor the ball-shaped surface in a femoral head.

To tackle this problem, a direct laser nitridingprocess in open atmosphere was performed on commercially pure titanium (grade 2, TiG2) and Ti6Al4V alloy(grade 5, TiG5) using a continuous-wave (CW) fibre laser. The effects of varying process parameters, for instancelaser power and nitrogen pressure on the surface quality, namely discolouration were quantified using ImageJanalysis.

The optimised process parameters to produce the gold-coloured nitride surfaces were also identified:40W(laser power), 25mm/s (scanning speed), 1.5mm(standoff distance) and 5 bar (N2 pressure). Particularly,N2 pressure at 5 barwas found to be the threshold above which significant discolouration will occur.

The surfacemorphology, composition, microstructure, micro-hardness, and tribological properties, particularly hydrodynamicsize distribution of wear debris, were carefully characterized and compared. The experimental resultsshowed that TiG2 and TiG5 reacted differently with the laser radiation at 1.06 μm wavelength in laser nitridingas evidenced by substantial differences in the microstructure, and surface colour and morphology.

Furthermore,both friction andwear properties were strongly affected by the hardness and microstructure of titaniumsamplesand direct laser nitriding led to substantial improvements in their wear resistant properties. Between the twotypes of titanium samples, bare TiG2 showed higher friction forces and wear rates, but this trend was reversedafter laser nitriding treatments.