ICMCTF1999 Session B2-2: Plasma Assisted Thermochemical Treatments

As a case hardening process, the attractiveness of nitriding lies in its low treatment temperature. There are some alloys and applications, however, for which even the normal nitriding temperatures are too high. This paper reports developments, specifically the use of low pressure plasmas and high ion energy bombardment, which can extend the benefits of nitriding to lower temperatures.

At low pressures, plasma containing a large number of active species is generated by a source, separate from the workpiece, and diffuses throughout the treatment chamber. An arbitrary bias can be applied to the workpiece, resulting in energetic ion bombardment that can enhance the nitriding efficiency. Ion and neutral fluxes are generally lower than those at higher pressures so the temperature can be kept low.

At high negative biases, nitrogen mass transfer is not just by thermochemical absorption but also by implantation to depths of 0.1 µm. This process, which is generally called Plasma Immersion Ion Implantation (PIII or PI³), allows "nitriding" temperatures to be brought down further, even below 200°C where the very shallow case is still capable of producing improvements in the wear performance of some components. Since high concentrations of nitrogen can be produced in the surface of a metal, independently of thermochemical absorption processes, PI³ is being applied to a range of alloys at temperatures between 200° and 500°C. The important questions to be resolved are how high does the ion energy really have to be and how much can be achieved with the low pressure plasma alone?

We will discuss our investigations in the use of low pressure radiofrequency plasmas for nitriding and the effect of ion bombardment at a range of energies. Results will be presented for treatment of low alloy steels, austenitic stainless steels and tool steels, in order to assess the advantages and limitations of low pressure plasmas and high ion energies in industrial nitriding situations.

Alumina is an attractive material as insulator layer in thin-film sensors for aeroengines components ; indeed, compared to silicon oxide, this material has an higher electrical resistivity for temperature >400°C, and a thermal expansion coefficient closer to those of metallic alloys. For this application, coatings have been obtained by R.F. sputtering, which present high electrical qualities and good thermomechanical behaviour in the temperature range 20-1100°C. However, the low deposition rates and the difficulties to have acceptable thickness uniformity on three dimensional substrates are disadvantages for industrial development of the method, and reasons for the interest in plasma enhanced chemical vapour deposition.

This paper describes an approach to deposit insulating aluminium oxide layers on metallic substrates in the afterglow region of a microwave oxygen plasma (RMPECVD). The chosen precursor is an organometallic, commonly used because of its high vapour pressure and reactivity. The technology offers high deposition rates, simplified reaction pathways, and control of ion bombardment by means of substrate R.F. biasing. The experimental set-up is described, and the influence of the deposition parameters on deposition rate, physical and chemical properties of the coatings is presented.

Microwave power and pressure ranges are limited by the emergence of powder produced by reactions in gaseous phase for high values. Weight gain, which illustrates the deposition rate, is strongly dependent on the precursor flow rate. Density and chemical composition are improved by deposition temperature. The main impurities are CH_x groups. High compressive stresses and electrical resistivity are good points according to the previous use of coatings at high temperature on metallic blades.

An inductively coupled plasma (ICP) source for deposition of ZrO₂ and thin films by Plasma Assisted CVD from T-butoxide Zr precursor diluted in Ar and O₂ gas mixture, has been designed and assembled. A careful control of the substrate deposition conditions has been obtained by means of an independent RF power supply. Characterization of the plasma source by electrostatic probe indicates that plasma density up to 10¹²cm^-3 can be obtained. Zirconia coatings, with thickness up to 4 microns were deposited under different plasma conditions, on Si (100) polished wafers.

Microstructural characterization of the films has been carried out by means of XRD, EDX-SEM, EPMA, GDOES, AES and XPS, whilst mechanical properties of the coatings has been investigated by microindentation and nanoindentation techniques.

Correlation between deposition parameters, and microstructure has been established. It is shown that that the ion bombardment has a large influence on the coating characteristics. In particular, the possibility of tailoring mechanical properties of the films by controlling the applied DC bias voltage is discussed. Topics requiring further investigation will be also identified and discussed.

M. Nunogaki et al. ¹ have reported that very low dosage, moderate energy ion implantation of Mo into certain aluminum alloys leads to dramatic increases in the kinetics of their plasma-source nitriding and have reported the formation of case depths on the order of 1 mm in thickness following only 4 h at a temperature of 325°C. This process was reported to result in a case hardness of approximately 3X that of the core, but the microstructural and microchemical characteristics of the processed layer were not determined or reported.

The current authors have set out to replicate the implantation experiments into both commercial purity Al and high purity Ti, believing that the implanted Mo, residing in the extreme outer surface layer, may act as a catalyst to prevent the recombination of dissociated and ionized N₂, thereby enhancing the kinetics of nitriding. Following Mo implantation at an energy of 25 KV and a dosage of 3 x 10¹⁵ cm^-2, conventional diode and intensified triode plasma nitriding experiments have been performed at current densities ranging from 0.5 to 3.5 mA/cm². Post-nitriding characterization experiments to identify the phases present and the nitriding kinetics include x-ray diffraction, SEM, x-ray probe microanalysis, and nanoindentation. The results of this study will be presented and discussed in light of the authors' proposed catalytic implantation effect.

¹M. Nunogaki, H. Suezawa, Y. Kuratomi, and K. Miyazaki, Vacuum, 39 (1989) 281.