Date of Award

1997

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Science (Interdepartmental Program)

First Advisor

Efstathios I. Meletis

Abstract

An investigation of the plasma nitriding mechanism under intensified conditions (average particle energy up to 1200 eV) was carried out using a triode configuration. The significance of the different physical processes taking place during intensified plasma nitriding, such as sputtering, implantation, defect generation, redeposition and diffusion were investigated experimentally, while theoretical predictions were obtained by using the TRIM (transport of ions in matter) code. A titanium-base alloy was selected as the substrate material. Sputtering yield in an inert atmosphere was found to exhibit three regimes as a function of incoming particle energy. The low energy and high energy regimes were found to be in agreement with the theoretical predictions while the intermediate regime showed a reduced level of sputtering yield independent of energy. This was the first time that such reduced sputtering yield was observed and was attributed to the onset of other physical processes such as generation of bulk defects (vacancies). Sputtering during nitriding was overshadowed by the development of the nitride layer. Implantation studies showed that, N atoms can be implanted to the very near-surface region ($$200 eV. Theoretical predictions showed that vacancy generation initiates at particle energies $>\sim$300 eV and this process can exercise significant effects on N diffusion at energies $>$440 eV. The present results suggest that the primary role in the intensified plasma process is played by the ions since they possess higher energies and their effect is complimented by the energetic neutrals. Enhanced diffusivities (five orders of magnitude higher) were realized under intensified processing conditions over conventional processing. In deposition experiments, a threshold energy was identified ($\sim$150 eV) that is required to produce grain refinement and the formation of a denser and stable nitride structure at the surface. A model was developed on the premise that N diffusivity is proportional to the vacancy concentration and was consistent with the experimental evidence. An assessment of important engineering properties showed that significant improvements can be achieved by intensified plasma nitriding.

ISBN

9780591723748

Pages

190

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