Silvio Francisco Brunatto

DC Plasma Technology Applied to Powder Metallurgy: an Overview

DC plasma is a very promising technology for processing different materials, and is becoming especially interesting when low environmental impact and high-performance treatments are needed. Some of the intrinsic characteristics of DC plasma technology, which make it suitable for powder metallurgy (PM) and powder injection molding (PIM) parts production, are low-pressure processing and plasma environment high reactivity. Moreover it can be considered as a highly competitive green technology. In this work, an overview of some of the important DC plasma techniques applied to PM and PIM parts processing is presented. Emphasis is given to the descriptions of the main characteristics and the technique potentials of plasma-assisted nitriding, plasma-assisted thermal debinding, plasma-assisted sintering, and simultaneously plasma-assisted sintering and surface alloying. The aspects presented and discussed in this paper indicate that DC plasma processes are promising and competitive techniques for PM and PIM parts processing.

Influence of the gas mixture flow on the processing parameters of hollow cathode discharge iron sintering

A study was made to verify the influence of the gas mixture flow on the iron sintering process with simultaneous surface enrichment of alloying elements by hollow cathode discharge. In this process, two independent cathodes formed an annular discharge: (1) a pressed cylindrical sample of iron powder, acting as the central cathode, was placed concentrically inside an external (hollow) cathode; (2) the external cathode, machined from a AISI 310 steel bar, acted both to confine the geometry of the plasma and as a source of alloying elements (Cr and Ni). The sintering was carried out at 1423 K, for a period of 7.2 × 103 s, under a gas mixture of 80% Ar + 20% H2 and a pressure of 399 Pa, at flow rates of 2 × 10−6, 5 × 10−6, and 8 × 10−6 m3 s−1, with an inter-cathode radial space of 5.8 mm. The discharge was generated using a pulsed voltage power source with a 200 µs period. The gas mixture flow plays an important role both in the cleanliness of the sintering atmosphere (reflected in the electric power utilized to maintain the samples temperature) and in the amount of metallic atoms deposited on the samples surface (as a result of the sputtering and the oxidation/reduction process on the cathode surfaces).

Low-temperature plasma nitriding of sintered PIM 316L austenitic stainless steel

This work reports experimental results on sintered PIM 316L stainless steel low-temperature plasma nitriding. The effect of treatment temperature and time on process kinetics, microstructure and surface characteristics of the nitrided samples were investigated. Nitriding was carried out at temperatures of 350, 380, 410 and 440 oC , and times of 4, 8 and 16 h, using a gas mixture composed by 60% N2 + 20% H2 + 20% Ar, at a gas flow rate of 5.00 × 10-6 Nm3s-1, and a pressure of 800 Pa. The treated samples were characterized by scanning electron microscopy, X-ray diffractometry and microhardness measurements. Results indicate that low-temperature plasma nitriding is a diffusion controlled process. The calculated activation energy for nitrided layer growth was 111.4 kJmol-1. Apparently precipitation-free layers were produced in this study. It was also observed that the higher the treatment temperature and time the higher is the obtained surface hardness. Hardness up to 1343 HV0.025 was verified for samples nitrided at 440 oC. Finally, the characterization of the treated surface indicates the formation of cracks, which were observed in regions adjacent to the original pores after the treatment.

Solid-state mechanochemical activation of clay minerals and soluble phosphate mixtures to obtain slow-release fertilizers

Abstract This work describes the development of potential multi-element slow-release fertilizers obtained by mechanochemical activation of mixtures of kaolinite and ammonium or potassium monohydrogen phosphates. Preliminary results of talc amorphization have also been included. The methodology consists of milling the materials in a high-energy ball mill, where the influence of rotation and time of milling were investigated. The samples were characterized by XRD, FTIR, TGA/DTA, SEM and MAS-NMR. The experimental results explain the slow-release behaviour of the amorphous nanostructured materials in aqueous suspensions, especially the MASNMR spectra, which showed the changes in the chemical environment of the elements analysed. The materials displayed slow-release behaviour for phosphates probably because the aluminium ions in the mineral structure interact more thoroughly with phosphate than potassium or ammonium. Nevertheless, in general, all of the nutrients were released slowly.

Surface modification of iron sintered in hollow cathode discharge using an external stainless steel cathode

Metallurgical aspects of unalloyed iron sintered in a hollow cathode discharge were studied, with special emphasis on the chemical composition of the samples surface, which was modified by sputtering at the cathode. Two independent cathodes formed an annular hollow cathode discharge, and a pressed cylindrical sample of iron powder (99.75% pure), functioning as the central cathode, was placed concentrically inside an external cathode machined from a cylindrical AISI 310 stainless steel bar. In addition to confining the plasma, the outer cathode also acted as a source of alloying elements (Cr and Ni). The inter-cathodes radial space was 5.8 mm. Sintering was carried out at 1423 K (1150°C) for 30, 60, 120 and 240 min, under a gas mixture of 80% Ar and 20% H2 flowing at 5 × 10−6 m3 s−1. The pressure of the gas mixture was kept constant at 400 Pa (3 Torr). The discharge was generated using a pulsed voltage power supply with a total pulse period of 200 µs and the pulse duration ranging from 44 to 39 µs. Sintering time, which played an important role in the surface characteristics of the samples, was reflected in the amount of alloying element deposited on the surfaces of the samples. The atoms deposited by diffusion during sintering formed a layer containing chromium and nickel elements. A microprobe characterization of the surface showed the presence of up to 3.3 at% Cr (3.1 wt% Cr) and 2.5 at% Ni (2.6 wt% Ni). The Cr and Ni concentration profiles were verified to depths of up to 47.5 µm and 22.5 µm, respectively. The surface enrichment by alloying elements was attributed to the sputtering of atoms from the outer cathode.

Hollow cathode discharge: application of a deposition treatment in the iron sintering

The influence of a previous deposition treatment on the final amount of alloying elements (Cr and Ni) deposited and diffused into the surface of iron parts sintered in hollow cathode discharge (HCD) was studied. Cylindrical pure iron pressed samples, being a central cathode, were placed concentrically in the interior of an AISI 310 steel machine-made outer cathode, resulting in a 6 mm inter-cathode radial spacing. The study was divided in two steps: a) deposition treatment with the outer cathode acting as target and the iron sample acting as substrate (1123K -850 oC- and 60 minutes deposition temperature and time, respectively); and b) deposition treatment plus HCD sintering (1423K -1150 oC- and 60 minutes sintering temperature and time, respectively). The electrical discharge was generated using a pulsed voltage power source. The results indicate the presence of 6.5 at.% Cr and 6.9 at.% Ni on the samples surface. The concentration profiles were mathematically treated to quantify the actual amounts of Cr and Ni deposited on and diffused into the samples, and the integration of the fitted equations yielded the calculated areas of 133 (µm × at.% Cr) and 105 (µm × at.% Ni), respectively.

Sintering Iron Using a Hollow Cathode Discharge with External Ti Cathode

Unalloyed iron samples were sintered in the presence of a hollow cathode glow discharge, generated in a gas mixture of 80% Ar + 20% H 2 at a pressure of 400 Pa. The sample, which worked as the inner cathode of the discharge, was heated by the bombardment of strongly accelerated ions and fast neutrals created in the cathode sheath. The outer cathode of the hollow geometry consisted of a titanium cylinder. The samples were sintered at different temperature, 1050, 1150 and 1250 °C for 60 minutes. The temperature was adjusted by varying the time on/off of the pulsed power supply used to generates the discharge. Microstructural results are presented and show that samples may be successfully sintered in a hollow cathode discharge. In addition, Ti atoms sputtered from the outer cathode were deposited onto the sample surface and by diffusion, during sintering, resulted in the formation of a layer approximately 100 μm thick containing 1 at.% Ti.

Low-temperature Thermochemical Treatments of Stainless Steels – An Introduction

Plasma technology used to perform thermochemical treatments is well established for the majority of steels, but it is not the case for the different stainless steel classes. Thus, im‐ portant scientific and technological achievements can be expected in the coming years re‐ garding plasma-assisted thermochemical treatment of such steels. The metallurgical aspects as well as the application cost-efficiency of stainless steels impose specific re‐ quirements for the thermochemical treatment, such as easy native chromium-rich oxide layer removal and surface activation at low temperature, which do not appear for other steel classes (plain, low-alloy, and tool steels). Thus, due to the highly reactive physico‐ chemical environment created by the plasma, plasma-assisted technology presents ad‐ vantages over other “conventional” technologies like those performed in gas or liquid environments. Low temperature is needed to avoid the reduction of corrosion resistance of stainless steels, by suppressing chromium carbide/nitride precipitation, and, in this case, good surface properties are achieved by the formation of treated layers containing metastable phases. Such attributes make the low-temperature plasma thermochemical treatments of stainless steels an important R&D field in the domain of plasma technology and surface treatments, and the goal of this chapter is to introduce the reader to this im‐ portant topic.

Low-temperature Plasma Assisted Thermochemical Treatments of AISI 420 Steel: Comparative Study of Obtained Layers

Formation of metastable C–, N–, or even Formation of metastable C–, N–, or even C/N– expanded phases can be observed for typical non-equilibrium conditions attained at the plasma assisted thermochemical treatments when temperatures relatively low are used. In present work, kinetics data are considered in a comparative study comprising low-temperature plasma assisted carburizing, nitriding and nitrocarburizing of AISI 420 martensitic stainless steel samples treated at 350, 400, and 450 °C, aiming to put in evidence the main metallurgical differences of the obtained layers. Microstructural characterization and hardness measurement results for untreated and treated sample surfaces show significant difference for the carburized layer growth in relation to that verified for the nitrided and nitrocarburized layers. While the carburized layer is constituted of a thin outer layer and a deep diffusion layer, just the opposite was observed for the other two treatments, id est., formation of thicker outer layers and thinner diffusion layers. Finally, carbide-/nitride-precipitation-free layers were supposedly obtained for samples carburized, nitrided, and nitrocarburized at 350 °C temperature.

Application of Direct Current Plasma Sintering Process in Powder Metallurgy

Direct current (dc) plasma-assisted sintering of metal parts is a promising and relatively new research and development field in powder metallurgy (PM). In the present entry, it is intended to introduce the reader to the main applications of the dc plasma sintering process in PM. To achieve this goal, the present entry is divided in a brief introduction and sections in which the bases of the dc plasma abnormal glow discharge regime and its influence in the sintering process are carefully treated. In this case, a clear language is purposely used to didactically introduce the reader to this “fascinating glow world”, the dc plasma-assisted sintering of metal parts, aiming to put in evidence the main points on physicochemical aspects of the plasma environment, basic knowledge of the plasma heating, and surface-related phenomena during dc plasma sintering of parts. All these aspects are treated considering the main techniques of the dc plasma-assisted sintering process applied to PM. Finally, some results on DC plasma heating, sintering and surface modification are presented.