T. Czerwiec

Low-temperature plasma-assisted nitriding

Plasma-assisted nitriding is an attractive surface treatment for metallurgical surface modification to improve wear, hardness and fatigue resistance of ferrous and non-ferrous materials. For this purpose, ion nitriding by a d.c. glow discharge is generally efficient for numerous materials. However, for some metals and alloys, the processing temperature, dominated by the discharge parameters, is too high and cannot be controlled independently from the plasma reactivity. This paper reviews the following solutions for low-temperature plasma-assisted nitriding: pulsed d.c. discharge, thermionically assisted d.c. triode arrangements, plasma implantation, electron cyclotron resonance systems and thermionic arc discharges. We focus on metallurgical results obtained by these techniques on austenitic stainless steel and aluminium.

Progress in the analysis of the mechanisms of ion nitriding

Abstract In this paper the mechanisms of ion nitriding are discussed, particular attention being paid to d.c. diode nitriding. Recent trends in the analysis of the mechanisms of d.c. diode nitriding are reviewed, emphasizing the importance of the cathode fall region. Diagnostics of active species and calculation of their densities in the plasma are presented and related to modelling. The hydrogen effect and plasma-solid interaction are also discussed. New developments and alternatives to d.c. diode nitriding such as triode nitriding are highlighted.

Low-pressure, high-density plasma nitriding: mechanisms, technology and results

This paper reviews the low-pressure (<10 Pa), high-density plasma-assisted nitriding processes recently developed for metallurgical surface modification to improve wear, hardness and fatigue resistance of ferrous and non-ferrous materials. For that purpose, plasma generation is most frequently ensured by d.c. glow discharges at relatively high pressure (100–1000 Pa) with the underlying limitations associated with this technology. Nevertheless, more flexibility and control are required for plasma nitriding of promising non-ferrous materials such as titanium, aluminium and their alloys. These requirements are fulfilled by the recently developed enhanced or intensified plasma nitriding processes that operate at lower pressures (<10 Pa) such as: thermionically assisted d.c. triode arrangements (TAT), plasma immersion ion implantation (PIII) or plasma source ion implantation (PSII), electron cyclotron resonance (ECR) systems and thermionic arc discharges (TAD). The purpose of this paper is to review these new nitriding processes from both technological and fundamental points of view. Plasma parameters and plasma–surface interactions are considered for these processes.

Low temperature nitriding of AISI 316L stainless steel and titanium in a low pressure arc discharge

AISI 316L stainless steel (SS) and titanium nitriding were studied in a low pressure arc-assisted nitriding process where the substrate temperature and the plasma parameters are uncoupled. Lower nitriding temperature limits were explored for constant plasma parameters in Ar–N2 gas mixtures and substrates at floating potential. Nitrogen superficial concentration, layer thicknesses and X-ray diffraction analyses were performed on SS specimens nitrided at two temperatures (580 and 680 K) for different times and titanium nitriding was studied in the temperature range 750–1025 K. At low temperature, the nitriding performances are limited by a plasma–surface phenomenon that probably involves recombination of nitrogen atoms.

Plasma diagnostic by emission spectroscopy during vacuum arc remelting

The plasma produced during vacuum arc remelting of a Zircaloy4 electrode has been investigated by optical emission spectroscopy. Spatial variations of plasma emission along the arc axis has been measured with a specific apparatus consisting of nine aligned optic fibres. The plasma consists of zirconium atoms, of singly and doubly charged zirconium ions and of chromium atoms. The non-observation of emissions of tin and iron particles, which are, with chromium, the three main alloy components of Zircaloy4, suggests that the concentrations of these two species in the plasma are negligibly small. Distribution temperatures of atomic and ionic species of the order of 1 eV and high ionization degree of the plasma (greater than 70%) have been determined. The similar decay of the line intensities of the various species with increasing axial distance from the cathode surface indicates that the plasma composition remains approximately unchanged within the interelectrode region. Synthesis of the spectroscopic results has shown that the emission of vapour into the plasma cannot be accounted for by a mechanism of metal volatilization from the cathodic and anodic liquid surfaces only. It also involves emission mechanisms occurring in the cathode spot region, like the expulsion of metal droplets which volatilize or the ejection of particles.

Plasma assisted nitriding of Inconel 690

Abstract Inconel 690 is a nickel base alloy with a broad range of application such as nuclear reactor technology. A low temperature plasma assisted nitriding treatment is expected to improve the tribological properties without changing the corrosion resistance of this alloy. The nitrided case is constituted of two or three distinct layers depending on the plasma reactivity. These layers are, respectively, associated to three different metastable f.c.c. nitrogen solid solutions denoted γN1, γN2 and γN3. A parabolic rate constant and an apparent action energy are determined and compared to other values taken from the literature. Dislocation and stacking fault generated in the nitrided layer are expected to play a key role in the nitriding mechanisms of Inconel 690.

Diagnostic of arc discharges for plasma nitriding by optical emission spectroscopy

A new high current (100–300 A), low voltage (25–45 V) and low pressure (0.4–1 Pa) arc discharge plasma used for steel nitriding is presented and characterized by optical emission spectroscopy (OES). In this process, the nitriding rate is high even if the workpieces are nitrided at floating potential without hydrogen in Ar−N2 gas mixtures. With H2, there is no appreciable gain in nitrogen content or in growth rate of the diffusion layer. In this paper, the OES diagnostic technique has been used to characterize the different excitation processes of both neutral and ionic argon species in Ar−N2, Ar−H2 and Ar−N2−H2 gas mixtures. The reactivity of this process is evaluated by using AISI 316 L austenitic stainless steel substrates nitrided for different treatment times at 690 K.

Detection by emission spectroscopy of active species in plasma–surface processes

The present paper is a review of recent works relating to the optical emission spectroscopy applied to the determination of the ground state densities of N. O and B atoms in Ar-N 2 , Ar-O 2 and Ar-BCl 3 flowing post-discharges. New results on the B-atom destruction probability ( γ = 0.4-0.5) on quartz walls are presented. The effect of small quantities of H 2 into Ar-N2 and Ar-O 2 discharge on N and O atoms densities within the respective post-discharge has been analysed. Then, the authors demonstrate that the optical emission of Na in impurity may be applied to the determination of the vibrational temperature of N 2 (X,v) states at the usual temperatures for nitriding of metals in Ar-N 2 post-discharges. From the diagnostic of Ar-N 2 post-discharges, it is clearly specified that the nitriding of iron base alloys in a flowing post-discharge reactor is coming from N atoms, especially as a few H 2 is admixed into the Ar-N 2 discharge. Finally, correlation between N-atom density and the thickness of iron nitrided layers are given as H 2 is introduced into the Ar-N 2 discharge.

Sputtering of Al-Cr and Al-Ti composite targets in pure Ar and in reactive Ar-N2 plasmas

Al-Cr-(N ) and Al-Ti-(N) coatings were deposited on glass slides by reactive magnetron sputtering of composite concentric Al-Cr or Al-Ti targets in different Ar-N 2 gaseous mixtures (N 2 mass flow <2 sccm). When sputtering occurs in pure argon, the chromium or titanium content increases with the insert diameter of chromium or titanium. For a given insert diameter, the introduction of nitrogen into the reactor yields changes in the Ti/Al or Cr/Al content ratio in the coatings. Both plasma diagnostics, performed by optical emission spectroscopy, and computer simulation allow description of the mechanisms of matter transfer. The composition changes observed can clearly be attributed to a combined effect of differential nitriding of the target with respect to its current density distribution and to the nitride-to-metal sputtering yield ratios involved. However, preferential nitriding of the outer zones of the targets was also observed when the crown was aluminium (with titanium insert) or titanium (with aluminium insert). This behaviour can be related to the sputtering wind (wind effect), which has also been simulated by considering a sinusoidal attenuation of the impinging flux of reactive species from the external zone to the centre of the target.

A way to decrease the nitriding temperature of aluminium : the low-pressure arc-assisted nitriding process

Abstract For the purpose of applications in mechanics, nitriding of aluminium has been performed in a high-current (300 A), low-voltage (20–45 V) and low-pressure (0.8 Pa) thermionic arc. Although nitriding of ferrous materials is efficient in this arc-assisted nitriding process even for unbiased workpieces, an additional negative substrate bias voltage is necessary to process aluminium. Ion bombardment is necessary not only for ion cleaning in an Ar–H 2 gas mixture but also for the nitriding treatment in Ar–N 2 . Under these conditions, a compact and continuous aluminium nitride layer with hexagonal AlN phase is formed on pure aluminium at 450°C. The kinetics of aluminium nitride formation at low temperature (between 340 and 460 °C) is characterized by a two-stage mechanism comprising first the nucleation and growth of nodular AlN grains, followed by the formation of a continuous AlN layer. The growth rate of the aluminium nitride layer seems to be controlled by the rate of the chemical reaction to form AlN, rather than the rate of nitrogen diffusion. Some tribological tests performed on the aluminium nitride layers are also reported in order to evaluate the improvement in friction and wear behaviour.