Investigation of laser-sustained plasma and the role of plasma in carbon dioxide laser nitriding of titanium

Open Access
Author:
Nassar, Abdalla Ramadan
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 03, 2012
Committee Members:
  • Judith Todd Copley, Dissertation Advisor
  • Judith Todd Copley, Committee Chair
  • Stephen M Copley, Special Member
  • John David Mathews, Committee Member
  • Albert Eliot Segall, Committee Member
  • Vladimir V Semak, Committee Member
Keywords:
  • Laser-Sustained Plasma
  • LSP
  • Optical Discharge
  • Plasma
  • Nitriding
  • Laser Processing
  • Laser nitriding
  • Spectroscopy
  • CFD
Abstract:
Applications of plasma discharges to materials processing can be seen in everything from circuit boards to drill bits. However, plasma formed near the focus of a continuous laser beam, laser-sustained plasma (LSP), has hardly been considered for materials processing applications. This despite several remarkable properties: very high maximum electron temperatures (~17,000 K); high electron densities (on the order of 10^23 m^-3); and, the ability to be remotely moved simply by moving the focal point of the laser beam. In this work, several aspects of LSP were investigated. The electron temperature and electron density within an argon LSP were measured. These measurements were compared to the predictions of a computational fluid dynamics model, taking into account, for the first time, the effects of different laser beam modes. A nitrogen LSP was employed to gain new insights into the role of near-surface plasma in laser nitriding of titanium, a process in which nitrogen is introduced into a melted region formed by a scanning laser beam to produce a functionally-graded, hardened layer. Employing these insights, the production of large-area, crack-free, deep and oxygen-free laser-nitrided layers was demonstrated. The plasma temperature was measured via the relative intensities of excited atomic emission lines. This method, also known as a Boltzmann plot technique, relied on the assumption of local-thermodynamic equilibrium. The validity of this assumption was verified by measuring the spatially-resolved electron density using Stark broadening of the hydrogen-alpha emission line. However, reabsorption of plasma line emissions made temperature measurements using relative line intensities near the laser focus unreliable. That is, the region near the laser focus was found to be optically thick. An axi-symmetric model of a LSP sustained in a flow of argon was also developed under the assumption of local-thermodynamic equilibrium. The electron temperature and total beam absorption predicted by the model compared well with values from the literature and with the experimental results obtained using optical emission spectroscopy. The effect of several realistic laser beam modes were tested in the model using a "light pipe" technique. The validity of the technique was verified experimentally. A major part of this work was the application of LSP to investigate laser nitriding of titnaium. Interactions of the LSP with a surface were distinguished from interactions of the laser beam with a surface under conditions where the laser beam did not result in plasma formation. By combining this approach with optical emission spectroscopy, it was possible to decouple the effect of plasma and the laser beam during nitriding of titanium. In this way, new light was shed on the role of near-surface plasma during CO2 laser nitriding of titanium. Additionally, a new processing regime for near-stoichiometric, oxygen-free LSP nitriding of titanium was discovered. The work presented here culminates by demonstrating large-area, crack-free and dendrite-free nitrided layers as thick as 600 microns. These layers were shown to have a hardness profile which is functionally graded. The effect of using a nitrogen-argon gas mixture, in place of pure nitrogen, on cracking, hardness, and microstructure was elucidated. In this work, several aspects of LSP were investigated. The electron temperature and electron density within an argon LSP were measured. These measurements were compared to the predictions of a computational fluid dynamics model, taking into account, for the first time, the effects of different laser beam modes. A nitrogen LSP was employed to gain new insights into the role of near-surface plasma in laser nitriding of titanium, a process in which nitrogen is introduced into a melted region formed by a scanning laser beam to produce a functionally-graded, hardened layer. Employing these insights, the production of large-area, crack-free, deep and oxygen-free laser-nitrided layers was demonstrated. The plasma temperature was measured via the relative intensities of excited atomic emission lines. This method, also known as a Boltzmann plot technique, relied on the assumption of local-thermodynamic equilibrium. The validity of this assumption was verified by measuring the spatially-resolved electron density using Stark broadening of the hydrogen-alpha emission line. However, reabsorption of plasma line emissions made temperature measurements using relative line intensities near the laser focus unreliable. That is, the region near the laser focus was found to be optically thick. An axi-symmetric model of a LSP sustained in a flow of argon was also developed under the assumption of local-thermodynamic equilibrium. The electron temperature and total beam absorption predicted by the model compared well with values from the literature and with the experimental results obtained using optical emission spectroscopy. The effect of several realistic laser beam modes were tested in the model using a ``light pipe'' technique. The validity of the technique was verified experimentally. A major part of this work was the application of LSP to investigate laser nitriding of titnaium. Interactions of the LSP with a surface were distinguished from interactions of the laser beam with a surface under conditions where the laser beam did not result in plasma formation. By combining this approach with optical emission spectroscopy, it was possible to decouple the effect of plasma and the laser beam during nitriding of titanium. In this way, new light was shed on the role of near-surface plasma during CO2 laser nitriding of titanium. Additionally, a new processing regime for near-stoichiometric, oxygen-free LSP nitriding of titanium was discovered. The work presented here culminates by demonstrating large-area, crack-free and dendrite-free nitrided layers as thick as 600 microns. These layers were shown to have a hardness profile which is functionally graded. The effect of using a nitrogen-argon gas mixture, in place of pure nitrogen, on cracking, hardness, and microstructure was elucidated.