An Exploration of Laser-sustained Plasma Interactions with Titanium Substrates During Nitriding Without Direct Irradiation by the Laser

Open Access
Author:
Black, Amber N
Graduate Program:
Engineering Science
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 08, 2013
Committee Members:
  • Judith Todd Copley, Dissertation Advisor
  • Stephen M Copley, Committee Member
  • James Hansell Adair, Committee Member
  • Albert Eliot Segall, Committee Member
  • Dr Vladimir V Semak, Committee Member
Keywords:
  • laser
  • plasma
  • nitriding
  • titanium
  • laser-sustained plasma
  • laser material processing
  • laser nitriding
Abstract:
Laser-sustained plasma (LSP) is plasma which can be sustained indefinitely by a laser beam away from any potentially interacting surfaces. LSPs can be sustained at steady state by balancing power input through inverse bremsstrahlung absorption with loss through radiation (continuous and line), convection, and conduction. For many years, plasma has been considered a negative influence in laser materials processing, disrupting the beam path and distorting radiation prior to the beam reaching the surface. New research indicates that LSP can be an opportunity for metallurgical surface treatments and the deposition of coatings with an improvement in properties over conventional coating methods. For the first time, the LSP was used to nitride surfaces independently of the associated laser beam and the resulting specimens were examined to gain new insights into the effects of laser plasmas on surface modification processes. A titanium plate was placed parallel to and at a radial distance from an LSP, rather than perpendicular to it, as is the typical geometry for laser processing. During the exposure of the substrate to the LSP, the process was observed via a charge-coupled device (CCD) camera. The processed substrates were then examined visually, by scanning electron microscopy, energy dispersive x-ray spectroscopy, focused ion beam, transmission electron microscopy, and x-ray diffraction to elucidate the morphological and microstructural features that are characteristic of this processing method. Results indicated that an LSP is a powerful tool for heating surfaces and simultaneously introducing activated gas species into the melt. The nitrided surfaces exhibited complex and uncommon morphologies, including faceted titanium nitride crystals, which had not been produced by conventional laser nitriding. The underlying microstructure demonstrated that LSP can generate layers similar to those produced by conventional laser nitriding, but to a much greater depth. This characteristic structure exhibited four distinct layers with gradated nitrogen contents ranging from titanium nitride at the surface to the base titanium metal. A timeline for the evolution of these metallurgical transformations was developed based on process monitoring and the materials characterization. These observations also led to a new understanding of the influence of plasma on the laser nitriding process. New methods for observing LSP-material interactions without additional contributions from the laser beam itself were effectively employed. The viability of LSP nitriding as a surface modification process and as a tool for future research on plasma-substrate interactions is discussed.