On the fundamental mechanisms of active screen plasma nitriding

The nitriding mechanisms of conventional DC plasma treatments have been extensively studied and discussed, but no general agreement has been reached thus far. The sputtering and redeposition theory is among the most accepted ones but, even though this mechanism is feasible, its contribution to the nitriding effect is under question. Furthermore, the novel active screen plasma nitriding technique has been successful in treating samples left at floating potential, where sputtering can not be considered to play a major role. Therefore, it has been proposed that the material sputtered from the cathodic mesh of the active screen furnace (auxiliary cathode) and deposited onto the treated specimens is involved in the mass transfer of nitrogen. The contribution made by this transferred material is the focus of attention of the present study. The hardening effect on the treated specimens showed considerable correlation with the deposition layer, and the XRD analysis of this deposited material yielded possible FeN and FexN peaks. This finding supports the deposition of iron nitrides and their subsequent decomposition on the treated substrate as a mechanism of significance to the plasma nitriding treatments conducted in active screen experimental settings.

New insights into the mechanism of low-temperature active-screen plasma nitriding of austenitic stainless steel

Low-temperature active-screen plasma nitriding is an effective surface engineering technology to improve the wear and corrosion resistance of austenitic stainless steel through the formation of expanded austenite. The material sputtered from the active screen and redeposited on the specimens has been suggested to play an important role in the nitriding mechanism involved. This paper reports a patterned deposition layer, which is in correlation with the grain orientation of polycrystalline specimens. This has provided new insights into the nitriding mechanism. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Study of active screen plasma processing conditions for carburising and nitriding austenitic stainless steel

Active screen (AS) is an advanced technology for plasma surface engineering, which offers some advantages over conventional direct current (DC) plasma treatments. Such surface defects and process instabilities as arcing, edge and hollow cathode effects can be minimised or completely eliminated by the AS technique, with consequent improvements in surface quality and material properties. However, the lack of information and thorough understanding of the process mechanisms generate scepticism in industrial practitioners. In this project, AISI 316 specimens were plasma carburised and plasma nitrided at low temperature in AS and DC furnaces, and the treated samples were comparatively analysed. Two diagnostic techniques were used to study the plasma: optical fibre assisted optical emission spectroscopy, and a planar electrostatic probe. Optimum windows of treatment conditions for AS plasma nitriding and AS plasma carburising of austenitic stainless steel were identified and some evidence was obtained on the working principles of AS furnaces. These include the sputtering of material from the cathodic mesh and its deposition on the worktable, the generation of additional active species, and the electrostatic confinement of the plasma within the operative volume of the furnace.

Electrostatically confined plasma in segmented hollow cathode geometries for surface engineering

A segmented hollow cathode (SHC) geometry was used for electrostatic confinement of plasma, and surface engineering treatments were conducted in this arrangement. The assessed processes included plasma nitriding, reactive deposition of sputtered material, and deposition of carbonaceous films by plasma-enhanced chemical vapor deposition with a bipolar pulsed-dc power supply on steel substrates. The treated specimens exhibited uniform surface morphology and deposition layers. Characterization techniques included optical microscopy, scanning electron microscopy with energy dispersive X-ray capability, and X-ray diffraction. The advantages and potential applications of the SHC arrangement are discussed in view of these results.

Premature thermal fatigue failure of aluminium injection dies with duplex surface treatment

The premature failure of an aluminium injection die with a duplex surface treatment (plasma nitriding and physical vapor deposition coating) was investigated, in an effort to identify the causes of such premature failure of the component. The manufacturing and the operating conditions were documented. Analytical tools were used, including scanning electron microscopy with energy dispersive X-ray capability, X-ray diffraction, and instrumented microhardness testing. Preliminary observations showed a microstructure of coarse tempered martensite, and a considerably rough surface with porosity and cracks. A detailed analysis of crack initiation sites identified sulfur inclusions in the subsurface, underneath the coating. A further revision of the processing conditions revealed that a sulfur-impregnated grinding stone had been used to polish the die. The chemical composition of such grinding stone matched that of the inclusions found in the subsurface of the failed component. Thus, searched causes of premature failure could be discussed on the lights of the present findings.

Reducing the macroparticle content of cathodic arc evaporated TiN coatings

Cathodic arc evaporation (CAE) is a widely used technique for generating highly ionised plasma from which hard, wear-resistant PVD coatings can be deposited. A major drawback of this technique is the emission of micrometer-sized droplets of cathode material from the arc spot, which are commonly referred to as ‘macroparticles’. In this study, the effect of cathode poisoning was investigated as a method to reduce the number of macroparticles in PVD coatings. While the study focuses on the reduction of macroparticles in titanium nitride coatings, the outcomes and key findings can be broadly applied to the cathodic arc process, in particular, for the reduction of macroparticles in more advanced CAE coatings. The results support earlier findings that have shown that poisoning of the cathode can reduce the number of macroparticles emitted from the arc spot. The results of glow discharge optical emission spectroscopy (GD-OES) showed that the titanium content of the coatings varied little between the respective coatings despite changes in the deposition pressure from 0.1 to 1.2 Pa. The GD-OES results also showed the presence of oxide contamination at the surface of the coatings, which was significantly reduced with increasing deposition pressure. The coatings were also deposited onto high-speed steel twist drills to compare the metal-cutting performance when dry drilling a workpiece of cast iron. The results of the drill tests showed that tool life increased with a reduction in the number of macroparticles.

Effects of cage netting colour and density on the skin pigmentation and stress response of Australian snapper Pagrus auratus (Bloch & Schneider, 1801)

The unnaturally dark pigmentation of cultured Australian snapper Pagrus auratus can be improved through dietary astaxanthin supplementation and by holding fish in tanks with a white background. The practical application of these  laboratory-based findings was examined with two experiments to establish if the advantages of transferring fish to light coloured tanks before harvest could be achieved on-farm using white cages and to determine the effects of fish density on skin colour. For the first experiment, snapper (mean TL=29.7 cm) were transferred from a commercial snapper sea cage to black or white netted cages and fed diets supplemented with unesterified astaxanthin (supplied as Lucantin^® Pink, BASF) at 0 or 39 mg kg⁻¹ for 42 days. Skin colour was measured using the CIE L* (black–white), a* (green–red), b* (blue–yellow) colour scale. Snapper held in white netting cages became significantly lighter (higher L* ) than snapper held in black cages; however, values were not as high as previous laboratory-based studies in which snapper were held in white plastic-lined cages. Snapper fed astaxanthin displayed significantly greater a*and b* values, and total carotenoid concentrations after 42 days. In addition, total carotenoids were higher in fish from black than white cages. The second experiment was designed to investigate whether density reduced the improvements in skin colour achieved by holding fish in white coloured cages and whether cage colour affected stress. Snapper (mean weight=435 g) were acclimated to black cages and fed 39 mg kg⁻¹ astaxanthin for 44 days before transferring to black or white plastic-lined cages at 14 (low), 29 (mid) or 45 (high) kg m⁻³ for 7 days after which time skin colour, plasma cortisol and plasma glucose concentrations were measured. Skin lightness (L* ) was greater in snapper transferred to white plastic-lined cages with the lightest coloured fish obtained from the lowest density after 7 days. Density had no effect on plasma cortisol or glucose levels after 7 days, although plasma cortisol was elevated in snapper from black cages. For improved skin colouration we recommend feeding unesterified astaxanthin at 39 mg kg⁻¹ for approximately 6 weeks and transferring snapper to white plastic-lined cages or similar at low densities for short periods before harvest rather than producing fish in white netting sea cages subject to biofouling.

Wear mechanisms and microstructure of pulsed plasma nitrided AISI H13 tool steel

AISI H13 tool steel discs were pulsed plasma nitrided during different times at a constant temperature of 400 °C. Wear tests were performed in order to study the acting wear mechanisms. The samples were characterized by X-ray diffraction, scanning electron microscopy and hardness measurements. The results showed that longer nitriding times reduce the wear volumes. The friction coefficient was 0.20 ± 0.05 for all tested conditions and depends strongly on the presence of debris. After wear tests, the wear tracks were characterized by optical and scanning electron microscopy and the wear mechanisms were observed to change from low cycle fatigue or plastic shakedown to long cycle fatigue. These mechanisms were correlated to the microstructure and hardness of the nitrided layer.