Abstract
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.
Highlights
Lasers are characterized by their monochromatic, coherent, polarized, narrow, and low-divergence beams.Focused laser beams can deliver large quantities of energy a narrowLasers are characterized bytherefore their monochromatic, coherent, polarized, narrow,toand lowspot on can the surface of the substrate, to high energy(kilowatts) intensities divergenceFocused laser beams deliver large leading quantities of energy10 –1012 W/m2 during welding (compared to 106 –108 W/m2 for flux-shielded or of athe orderspot of 10(usually to narrow a fraction of a millimeter) on the surface of the substrate, leading to high 12 energy2 forof gas-shielded arcs [52]).this is absorbed within the firstto few layers the energy intensities of theFurthermore, order of 1010–10W/m2 during welding
This paper reviews the laser-sustained plasma (LSP) nitriding process and its potential to achieve deep-case hardening of titanium and is structured as follows: Section 2 discusses various surface hardening treatments that are commonly employed to enhance the tribological properties of titanium surfaces; Section 3 presents a detailed review of the laser nitriding process including the role of near-surface plasma as discussed in the literature spanning the past three decades; Section 4 describes recent research conducted at the Pennsylvania State University to study the unique properties of a LSP and its effects on the laser nitriding process, followed by the development of a novel two-step “LSP nitriding-remelting” method to form homogenous, wide-area, deep, crack-free, and wear-resistant nitrided cases on commercially-pure titanium (CP-Ti) substrates
Black et al [111] studied the interaction between a nitrogen LSP and a commerciallypure pureThe titanium (CP-Ti) substrate by from Nassar et al.’s [108] research (Figure 6a) where the substrate was normal to the laser beam
Summary
Advanced Production Engineering Group, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. Applied Research Laboratory, P.O. Box 30, N.
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