Harry Podlesak

In situ and ex situ examination of plasma-assisted nitriding of aluminium alloys

Abstract Pre-sputtering and plasma-assisted nitriding of pure aluminium and 2024 aluminium alloy were studied in situ by plasma monitoring and ex situ by several surface analysis methods. The influence of the process parameters, time, temperature, pressure and gas composition on mass and energy distributions of ions, as well as on topography and chemical composition, was examined. The chronology of the sputtering process could thus be clarified. An oxide film which is relatively thick compared to the native oxide layer and enriched with magnesium is formed at the initial stage of the sputtering treatment, followed by the removal of this film with a subsequent roughening of the surface. The cleaning effect of the plasma can be enhanced by increasing the argon pressure or by an intermixture of hydrogen. Due to a supplementary chemical etching, hydrogen admixture reduces the surface roughness and effectively decreases the oxygen content of the surface. During the nitriding treatment, nanocrystalline hexagonal aluminium nitride AlN is formed. Growth rate and nitrogen content of the nitride layer were found to strongly depend on the gas composition during sputtering and nitriding. The positive effect of addition of argon and hydrogen, respectively, arises from an intensification of the nitrogen plasma, the production of nitrogen–hydrogen molecular ions and an increase of the ion energy. The nitrogen pressure affects the layer thickness due to higher nitrogen activity and ion energies. The chemical composition of the cathode material influences discharge current and, as a result, the ion energies.

High-resolution microstructural investigations of interfaces between light metal alloy substrates and cold gas-sprayed coatings

Interfaces between light metal alloys, aluminum AA7022, and magnesium AZ91, and optimized cold gassprayed zinc-based coatings are characterized. The analyses include scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). Investigations by SEM show a seam with intensive mixing of the substrate and coating material, which is indicated by different values of gray due to element contrast. In energy-dispersive spectroscopy analyses, increased zinc concentrations compared with the substrate material are detected in <1 μm thick vortexes inside the seam. The TEM investigations prove that these areas consist of a homogeneous solid solution and submicron-sized or nanosized intermetallic phases with different concentrations of aluminum, zinc, and magnesium. Because diffusion processes cannot result in the observed microstructure. local melting followed by precipitation of intermetallic phases is concluded as the consequence of the intensive mechanical interaction at the substrate-coating interface during particle impact during the cold gas spraying of zinc on magnesium or aluminum substrates.

Corrosion characteristics of an ultrafine-grained Al-Mg-Si alloy (AA6082)

The corrosion behaviour of the aluminium alloy, AA6082, processed by equal-channel angular pressing (ECAP) after different passes (route E, room temperature) was studied in comparison to the coarse-grained counterpart. The results of the electrochemical investigations (cyclovoltammetry; electrochemical impedance spectroscopy, EIS) are presented in correlation with the microstructure before and after the corrosion examinations. Both, chemical (precipitations, phases) and physical (dislocations, high-angle grain boundaries, grain size, low-angle grain boundaries) inhomogeneities characterize the microstructure of this commercially used Al-Mg-Si alloy. Results indicate an improved resistance against pitting of the ECAP material expressed by a reduced pitting density of up to 50 % and lower pit depths. EIS measurements and microstructural examinations (scanning electron microscopy, transmission electron microscopy, 3D topography measurement) confirm that ECAP modifies the number, size and distribution of these inhomogeneities, which leads to a more favourable corrosion behaviour.

Powder Metallurgy of Particle-Reinforced Aluminium Matrix Composites (AMC) by Means of High-Energy Ball Milling

This paper deals with the production of aluminium matrix composites through high-energy milling, hot isostatic pressing and extrusion. Spherical powder of the aluminium alloy AA2017 (grain fraction > 100 μm) was used as matrix material. SiC and Al2O3 powders of submicron and micron grain size (< 2 μm) where chosen as reinforcement particles with contents between 5 and 15 vol.% respectively. The high-energy milling process was realised in a Simoloyer mill (Zoz). The milling time was about 4 hours. Hot isostatic pressing (HIP) was used to convert the compound powder into compact material. The extrusion process realises semi-finished products with different geometrical shapes.

Protection of Ti6Al4V surfaces by laser dispersion of diborides

In this work, one- and two-step laser dispersing of Ti6Al4V surfaces by use of elemental boron (B) as well as TiB2, ZrB2, and CrB2 was carried out with CO2 and Nd:YAG lasers using an adapted apparatus to provide inert conditions. Depending on the laser system, melt pool depths between 200 µm and more than 1000 µm were achieved, and the boride precipitates allowed an increase of the surface hardness from 350 HV0.05 in the initial state to more than 600 HV0.05. The modified surface areas were characterized by means of optical microscopy, scanning electron microscopy, and EDXS. Oscillating and cavitation erosion wear tests were carried out. For reinforcement of component surfaces with complex shape, a two-step laser deposition process and a technology for predeposition of diboride layers with defined thickness is required. The applicability of vacuum plasma spraying for predeposition is discussed.

Anodisation of Aluminium Alloys by Micro-Capillary Technique as a Tool for Reliable, Cost-Efficient, and Quick Process Parameter Determination

Anodisation is essential for improving surface properties of aluminium alloys and composites regarding wear and corrosion behaviour. Optimisation of the anodising process depends on microstructural constituents contained in aluminium alloys and represents a key task, consisting of the control of process parameters and electrolyte formulation. We applied the micro-capillary technique known from corrosion studies and modified it to form anodic aluminium oxide films on high-strength aluminium alloys in comparison to pure aluminium in sulphuric acid. A glass capillary with an opening of 800 μm in diameter was utilized. Corresponding electrochemical measurements during potentiodynamic and potentiostatic anodisation revealed anodic current responses similar to conventional anodisation. The measurement of film thickness was adapted to the thin anodised spots using ellipsometry and energy dispersive X-ray analysis. Cross sections prepared by focused ion beam milling confirm the thickness results and show the behaviour of intermetallic phases depending on the anodising potential. Consequently, micro-capillary anodising proved to be an effective tool for developing appropriate anodisation conditions for aluminium alloys and composites because it allows quick variation of electrolyte composition by applying low electrolyte volumes and rapid film formation due to short process durations at small areas and more flexible variation of process parameters due to the used set-up.

Synthesis, characterisation and photocatalytic activity of Co–doped ZnO nanoparticles

Co–doped ZnO nanoparticles were synthesised by combustion method using polyvinyl alcohol and zinc nitrate. Both X–ray diffraction (XRD) and selected area electron diffraction (SAED) analyses showed the single phase of hexagonal structure (PDF2 card: 01–070–8072) in all the samples. Transmission electron microscopy (TEM) and HRTEM images show that most of the particles have spherical shape and their grain size is ranging from 10 nm to 60 nm in diameter, which depended on both the Co ratio and the annealing temperatures. Energy–dispersive X–ray (EDX) spectrum analysis showed the proportion of elements in the nanoparticles in accordance with the original composition. The photocatalytic activity of samples was studied by measuring the decomposition of methylene blue in aqueous solution using UV–VIS spectroscopy. The results showed that the photocatalytic ability to decompose methylene blue reached 70% with ZnO and 98% with Zn0.97Co0.03O samples after 150 min of irradiation.

Influence of Milling Atmosphere on the High-Energy Ball-Milling Process of Producing Particle-Reinforced Aluminum Matrix Composites

High-energy ball milling (HEM) with subsequent consolidation is a suitable method to create particle-reinforced aluminum materials. In addition to other parameters, the used PCA (process control agent) as well as the atmosphere significantly influence the milling procedure. The present article deals with the influence of different milling atmospheres (air, argon, nitrogen) on the high-energy ball-milling process when milling an Al alloy with SiC particles. The investigations show that the reaction of the ground material with air, when rinsed with air, changes the milling behavior of the aluminum powder significantly. Unlike with inert atmospheres, the use of a process control agent (PCA) is therefore no longer necessary.