Enhancement of CP-titanum wear resistance using a two-step CO2 laser-sustained plasma nitriding process

In this paper, a method of forming hard, wide-area, crack-free, and wear-resistant nitrided cases on commercially-pure titanium using a 3.5 kW CO2 laser-sustained plasma is described. This surface hardening method was comprised of two steps: (1) a laser-sustained nitrogen plasma was first used to nitride the titanium substrate; and (2) a laser-sustained argon plasma was then employed to remelt the nitrided layer deposited in the first step. Previous research using single laser trail experiments had shown that the (second) remelting step can eliminate cracks formed during the (first) nitriding step and homogenize the nitrided layer. In this work, the two-step nitriding-remelting process was extended to wider surface areas by depositing multiple overlapping trails at four different nitriding speeds and a constant remelting speed. The hardened layer was characterized using x-ray diffraction (XRD), optical metallography, and hardness testing. Reciprocating ball-on-flat wear tests were conducted to assess the wear resistance of the nitrided case, with the wear scar being characterized using scanning electron microscopy (SEM) and optical profilometry. Crack-free, hard cases of depths up to 600 μm and average hardness values up to 641 ± 86 HV0.3 were observed. The LSP nitriding-remelting treatment was found to improve the wear resistance of the base metal (CP-Ti) by up to 80%.