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Laser-sustained plasma (LSP) and CCD imaging of reactant species were employed to investigate the role of near-surface plasma in CO2 laser nitriding of titanium in open atmosphere. Insights were gained regarding the role of plasma processes and the role of reactive nitriding species in the nitriding process. Studies of single nitrided trails have identified the following regimes, as a function of LSP off-focal distance and beam translation speed, characterized by (1) the formation of heavily oxidized surfaces, (2) the formation of titanium nitride (TiN) nanoparticulate, (3) nitride formation in the absence of a surface-struck or LSP and (4) the formation of near-stoichiometric, oxide-free TiN surfaces with a LSP. For the first time it will be shown that the LSP can access nitriding conditions beyond those achieved with surface-struck plasma (or in the absence of plasma) to produce uniform, near-stoichiometric, titanium nitride coatings.

An analytical relationship was derived to describe the amount of energy absorbed within preplaced powder during the laser deposition process. The relationship, which reflects an exponential decay of Beer–Lambert, may be used to define internal absorption due to scattering within the powder layer regardless of the beam shape and energy distribution if the attenuation coefficient and bulk absorption are known. Experiments were conducted to estimate the attenuation coefficients for pure iron and pure copper powders, representing three powder size distributions, during CO2 and Nd:yttrium-aluminum-garnet (YAG) laser irradiation. Qualitative observations and trends of the experimental data indicated that greater beam penetration, accompanied by a decrease in the estimated attenuation coefficients, was associated with the larger powder particles, the lower wavelength of the Nd:YAG laser, and the copper powder. Attenuation coefficients were determined for the original powder size distributions and a larger size distribution obtained by sieving. The estimated attenuation coefficients representing the original powder size distributions were found experimentally to be 0.0144 μm−1 for the iron powder and 0.0113 μm−1 for the copper powder. These values compared to those of the materials in the solid state indicate that the attenuation coefficients of the powders are four orders of magnitude less and represent significant penetration of irradiation into the powder layer.

Titanium and its alloys possess several attractive properties that include a high strength-to-weight ratio, biocompatibility, and good corrosion resistance. However, due to their poor wear resistance, titanium components need to undergo surface hardening treatments before being used in applications involving high contact stresses. Laser nitriding is a thermochemical method of enhancing the surface hardness and wear resistance of titanium. This technique entails scanning the titanium substrate under a laser beam near its focal plane in the presence of nitrogen gas flow. At processing conditions characterized by low scan speeds, high laser powers, and small off-focal distances, a nitrogen plasma can be struck near the surface of the titanium substrate. When the substrate is removed, this plasma can be sustained indefinitely and away from any potentially interacting surfaces, by the laser power and a cascade ionization process. This paper presents a critical review of the literature pertaining to the laser nitriding of titanium in the presence of a laser-sustained plasma, with the ultimate objective of forming wide-area, deep, crack-free, wear-resistant nitrided cases on commercially pure titanium substrates.

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