H. Michel

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.

Optimization of duplex coatings built from nitriding and ion plating with continuous and discontinuous operation for construction and hot working steels

Abstract Multifunctional coatings combining a nitriding treatment and physical vapour deposition allow the performance of cutting and forging tools to be boosted, since the improved mechanical support of the coating makes them withstand higher loads. This treatment can also be used for wear parts made from construction steels to increase their fatigue and wear resistance. Hard coatings applied on nitrided layers can replace or enhance the e or γ′ layers currently used. These treatments can be made in a discontinuous mode using a dedicated equipment for the nitriding and coating treatment or in a continuous mode, i.e. directly in the coating reactor. These treatments were applied and optimized for a construction steel 35NCD16 and a hot working steel Z38CDV5-1. Coating conditions have a decisive impact on the thermal stability of the iron nitride layers. This aspect was studied in detail and several technical solutions have been identified. Finally, it will be shown that in contradiction to previous findings the coatings have only a negligible influence on the stress intensity in the nitrided zone.

Major damage mechanisms during scratch and wear testing of hard coatings on hard substrates

Abstract Scratch testing and wear testing of hard, brittle TiN coatings on high-speed steel substrates has been done by means of a modified commercial tester. Three-dimensional surface mapping is shown to be a powerful tool for coating failure analysis. Scratch testing damage probably starts by tensile-type crack nucleation behind the trailing edge of the diamond stylus. In a multi-pass scratch-wear operation surface damage is dominated by delamination along the coating/ substrate interface. Fracture mechanics is suggested to model such brittle failure. Sufficient coating ductility to accommodate subsurface plastic strain is shown to be a necessary mechanical property with respect to wear resistance.

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.

Thermodynamic and structural studies on nitrided Fe-1.62%Mn and Fe-0.56%V alloys

Abstract Precipitation in the diffusion zone of Fe–1.62wt.%Mn and Fe–0.56wt.%V alloys nitrided at 843 K was studied by TEM and XRD analyses. Recent thermodynamic data have been used to interpret the different results in both systems. In the manganese containing alloy, the thermodynamically stable θ-Mn6N5 phase precipitates in the diffusion zone. Around the maximum hardness, in a narrow zone between 10 and 150 μm below the γ′/α interface, needle shaped precipitates are also present oriented along three mutually orthogonal directions. This phase, designated θ′, belongs to one of the three space groups P 4 m 2 c 2 c , P 4 n 2 n 2 c or P 4 n 2 b 2 m . X-ray diffraction depth profiles show that the presence of theses precipitates is closely related to the macrostress in the diffusion layer. In the case of the vanadium containing alloy, nitriding simultaneously produces VN precipitates and a tweed structure which annealing experiments reveal to be metastable. EPMA studies show that excess nitrogen is present in the Fe–V alloy. Sidebands are observed only on the (200)α peaks and are attributed to the microstrain gradient at a given nitrogen concentration, due to the periodicity of the tweed structure. The tweed contrast is due to a tetragonal distortion of the matrix, and not to a spinodal microstructure.

Modeling of nitrogen atom recombination on Pyrex: Influence of the vibrationally excited N2 molecules on the loss probability of N

A numerical modeling of the surface recombination of nitrogen atoms on Pyrex is developed to account for the temperature dependence of the loss probability γ on the temperature in the range 290–611.5 K. Nitrogen atom recombination is a first-order Langmuir–Hinshelwood mechanism at a temperature below 400 K where both the Langmuir–Hinshelwood and the Eley–Rideal mechanisms are taking place above. Calculated results are compared to experimental measurements obtained in an Ar–N2 Lewis–Rayleigh afterglow. Satisfactory agreement is obtained. The activation energy of the recombination processes is close to 20 kJ/mol. The weak difference between the theory and the experiment is due to the value of the model parameters and not to the influence of the vibrationally excited molecules, which is very weak. It is suggested that the rate constant for vibrational de-excitation of N2(X,v) by an adsorption–phonon mechanism is underestimated. The recombination rate of nitrogen atom on the wall of a cylindrical Pyrex tube i...