Year

1995

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

The plasma nitriding response of a titanium alloy (Ti-6A1-4V) has been investigated as a function of the mode of plasma excitation, i.e. DC or Microwave. The Microwave Induced Plasma (MIP) was generated in a TM012 cylindrical cavity using a 1.5 kW power supply operating at 2.45 GHz. DC plasma nitriding was conducted in a 40 kW commercial unit. Nitriding experiments carried out in the MIP, in nitrogen-hydrogen gas mixtures at 65 to 100 mbar pressure, established that MIP provides an excellent mass transfer medium for nitriding of the titanium alloy. Furthermore, significant advantages over DC plasma have been discerned, regarding the surface appearance (uniformity) and colour of the nitrided Ti-6A1-4V alloy. The most striking feature of the MIP nitrided titanium samples is the uniform surface appearance from the edge to the centre. Except for a very narrow band around the edge, the rest of the surface displays a uniform golden colour. In stark contrast, DC plasma nitriding results in uneven surfaces, manifested as grey/silvery rings around the periphery of the specimen, and a dark brown colour in the central part. These differences have been correlated to the most pertinent plasma parameters of the two processes. MIP is an electrodeless plasma characterised by high operating pressure (65 to 100 mbar), a relatively high electron (or particle) density (of the order of 10^^ cm"^) and low electron/ion energies, typically in the range of 3-4 eV. DC plasma, on the other hand, operates at moderate pressures (3-5 mbar) and results in a lower ionisation efficiency. Further, in the DC plasma the specimen is at the cathode potential and is subjected to high energy ion bombardment which results in significant sputtering from the surface. It is argued that the severity of sputtering, especially around the edges of the sample, is primarily responsible for the non-uniformity of treatment in DC plasma nitriding. Excessive sputtering also prevents the build up of nitrogen in the surface of titanium in the early stages of DC plasma nitriding. Consequendy, the predominant layer forming on the surface is Ti2N which retards the diffusion of nitrogen. The accumulation of nitrogen on the surface as nitriding progresses results in the formation of superstoichiometric TiN exhibiting a dark brownish/reddish colour. For MIP nitriding, stoichiometric TiN, displaying a yellow golden colour, is the major constituent of the compound layer.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.