Self-lubricating,low-friction,wear-resistant Al-based quasicrystalline coatings

After gas atomization, a quasicrystalline powder based on aluminium was used to prepare a thick coating by high-velocity oxygen-fuel flame torch spraying. This layer was deposited on top of a bond-coat layer on a steel plate. A post-spraying annealing treatment turned the two layers to their stable state, a γ-brass crystal and an icosahedral quasicrystal, respectively. The projection parameters were selected in such a way that the coating behaved like a self-lubricating material, which offered very good wear resistance (duration of pin-on-disk tests superior to 5 km with negligible material loss) and low friction (µ ≤ 6% against sintered tungsten carbide), in contrast to the state of the art. This property was achieved thanks to, on the one hand, excellent bonding to the substrate via the bound coat, and on the other hand, presence at the boundaries between quasicrystalline flakes of a mixture of both threefold and fourfold coordinated carbon originating from spray processing. Application to hard materials used in mechanical devices is appealing, especially because soft, lubricating additives may not be needed, thus considerably increasing the lifetime of the devices and reducing waste of materials.     

Solution-based synthesis of AgI coatings for low-friction applications

Thin films of AgI have been synthesized from Ag surfaces and elemental I₂ using a rapid and simple solution-based method. The effect of using ultrasound during the synthesis was studied, as well as the influence of the nature of the solvent, the I₂ concentration, the time, the temperature, and the sonication power. The films were characterized using X-ray diffraction, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy, and found to consist of β-AgI, possibly along with some γ-AgI. It was found that sonication increases the film thickness and grain size. The nature of the solvent has a profound effect on the film growth, with mixtures of water and ethanol leading to thicker coatings than films synthesized using either component in its pure form. Selected coatings were tribologically tested, and the AgI coating was seen to lower the friction coefficient significantly compared to a reference Ag surface under otherwise identical conditions. Long lifetimes (over 30000 cycles) were seen against a Ag counter surface. Tracks and wear scars were studied using SEM and Raman spectroscopy, and it was found that the friction level remains low as long as there is AgI in the points of contact. This method is found to be a simple and fast way to deposit AgI on Ag with large possibilities of tuning the thickness and grains sizes of the resulting films, thereby optimizing it for the desired use.     

Quasicrystalline materials

The report in late 1984 of phases in rapidly cooled aluminium alloys whose electron diffraction patterns exhibit icosahedral symmetry has stimulated intensive research into the structure and properties of quasicrystalline materials. Here I review our current understanding of quasicrystalline materials and quasiperiodic tilings, and in particular I discuss in detail the interpretation of transmission electron microscopy observations of quasicrystalline phases.     

含准晶相Al－Cu－Fe合金的球磨非晶化研究

应用球磨方法使含准晶相的Ａｌ６５Ｃｕ２０Ｆｅ１５合金转变为非晶相。利用Ｘ射线结构分析和扫描电镜研究了经不同时间球磨的合金粉末的相结构及形貌。结果表明，Ａｌ－Ｃｕ－Ｆｅ二十面体准晶相比合金中的少量晶体相更易于非晶化，且在非晶化过程中没有中间相生成。应用高温差热分析仪（ＤＴＡ）和Ｘ射线衍射技术（ＸＲＤ），分别研究了该非晶粉末的晶化特性。     

Sputter deposited low-friction and tough Cr-Si3N4 nanocomposite coatings on plasma nitrided M2 steel

Nanocomposite coatings of Cr-Si₃N₄ exhibiting low friction and high toughness were prepared on plasma nitrided AISI M2 steel substrates using an unbalanced magnetron sputtering system. The surface morphology and cross-sectional microstructure of the Cr-Si₃N₄ nanocomposite coatings were studied using field emission scanning electron microscopy (FESEM) techniques. Cr-Si₃N₄ nanocomposite coatings prepared at 48 at.% Cr exhibited a dense microstructure with nanoindentation hardness and toughness values of 18 GPa and 2.0 MPam^½, respectively. Nanoscratch measurements indicated that Cr-Si₃N₄ nanocomposite coatings exhibited good adhesion with a maximum critical load of 150 mN. Ball-on-disc reciprocating tests at a load of 2 N showed that Cr-Si₃N₄ nanocomposite coatings prepared at 48 at.% Cr exhibited an average friction coefficient of 0.30. FESEM studies of the wear tracks indicated that there was no significant wear loss and the Cr-Si₃N₄ nanocomposite coatings exhibited only mild wear due to oxidation.     

Influence of Processing on the Microstructure of Alloys Which Contain Quasicrystalline Phases

In this investigation a microstructural characterization of the phases obtained in ternary and quaternary alloys was undertaken. One of the main objectives of this study was to explore the possibility of quasicrystalline phases formation under normal casting conditions from the liquid melt. The alloys investigated, were melted in an induction furnace and subsequently cast into a wedge shaped copper mould, which resulted in different cooling velocities in the same ingot. The thick and thin sections of the ingots were characterized by: Optical microscopy, x-ray diffractometry, chemical analysis using EDS and atomic absorption spectroscopy.     

准晶薄膜与涂层的制备、性能和应用

本文综述了近年来准晶薄膜的制备方法、性能及其应用等方面的研究工作。准晶薄膜的制备方法主要有物理汽相沉积法和热喷涂法两大类。准晶薄膜具有优异的力学、热学与光学等性能,已经应用于不粘锅涂层、热障涂层和太阳能吸收器等。     

Low Temperature Electronic Transport in Al–Cu–Ru Quasicrystalline Alloys

Electrical resistivities of several quasicrystalline (QC) icosahedral Al–Cu–Ru alloys have been measured between room temperature and mK temperatures. None of the Al–Cu–Ru samples exhibited “insulating” behaviors in their resistivities. One sample had a large resistivity of ≈49,000 μΩ cm at room temperature. Its resistivity increased with decreasing temperatures by a factor of 3.2 and exhibited a maximum around 2 K. Fitting discrepancies between the experimental magnetoconductivity data and the weak localization and electron–electron interaction theories suggest that the weak localization theory poorly describes the electronic conduction behavior of this highly resistive quasicrystalline sample.     

Quasicrystalline phase formation during heat treatment in mechanically alloyed Al65Cu20Fe15 alloy

Abstract

Quasicrystalline phase formation during heat treatment in the mechanically alloyed Al₆₅Cu₂₀Fe₁₅ powders was studied by X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The mechanical alloying was performed at speed of 300 rev min^–1 for times up to 70 h. It was found that mechanical alloying of the Al₆₅Cu₂₀Fe₁₅ powders did not result in the quasicrystalline (QC) icosahedral phase (i-phase) formation. The long time milling resulted in the formation of a cubic Al (Cu,Fe) solid solution phase (β-phase). The cubic Al (Cu,Fe) solid solution identified as β-phase was observed to be present as one of the major phases in the Al₆₅Cu₂₀Fe₁₅ alloy. The formation of the quasicrystalline icosahedral phase (i-phase) was only observed for short time milled powders after additional annealing at temperature above 500°C. The present investigation also showed that a tetragonal Al₂Cu phase (θ-phase) forms with short time milling. The tetragonal Al₇Cu₂Fe₁ phase (w-phase) was observed after heat treatment of the short time milled and unmilled powders. The present investigation indicated that an effective process to prepare the quasicrystalline materials was using a combination of short time milling and subsequent annealing.     

Characterization of coatings

The variety of physical and chemical properties of coatings is determined by their thickness, structure and chemical composition. A fundamental understanding of coating properties, as well as of their reproducibility, therefore requires a good knowledge of these parameters.During the last few years great progress has been made in the field of chemical analysis (including depth distribution) of thin films and coatings. This progress is mainly the result of the combination of recently developed surface analytical techniques such as Auger electron spectroscopy (AES), photoelectron spectroscopy (UPS, XPS), ion scattering spectroscopy (ISS) and secondary ion mass spectroscopy (SIMS), on one hand, and controlled simultaneous surface etching by sputtering on the other.With these surface analytical techniques the chemical composition of the uppermost monolayers is detected by energy or mass analysis of ion- (ISS, SIMS), electron- (AES) or photon- (UPS, XPS) induced emission of ions (ISS, SIMS) or electron (AES, UPS, XPS), respectively. By combining these techniques with sputtering, the depth distribution of elements and compounds can also be determined with a lateral resolution of some microns.In order to recognize the capabilities and limitations of these techniques for coating analysis, the fundamental emission processes as they appear in the various analytical techniques, as well as the details of the sputtering process, have to be taken into account. The main features such as detection limits, isotope sensitivity, detection of compounds etc. of these techniques will be compared for some typical examples.Other methods, such as high energy ion backscattering and the detection of sputtered particles in the gas phase, will also be considered.     

Vacuum-deposited carbon coatings

In recent years, highly favorable results have been obtained using low temperature isotropic pyrolytic carbons in prosthetic devices requiring a high degree of thromboresistance. The development of vacuum-deposited carbon coatings was undertaken to extend the application of carbon to geometries and configurations that cannot be fabricated from low temperature isotropic carbon. Vacuum-deposited coatings have been produced on a variety of metallic and polymeric substrates.The different vacuum deposition processes which have been investigated include electron beam gun evaporation using high vacuum, gas scattering and ion- plating conditions. In addition, sputtering processes using ion beams and magnetically confined plasmas were studied.The surface morphology, structure and preferred orientation of the coatings produced by the different processes were characterized by scanning and transmission electron microscopy. Film purity and interfacial characteristics were examined by Auger electron spectroscopy.The scanning electron microscopy study shows that thin carbon films generally have a smooth and featureless surface morphology. However, other surface morphology features are obtained in thicker films, depending on the processing conditions. The transmission electron micrographs show the absence of structure and growth features. Electron diffraction indicates that the films consist of a turbostratic phase and a non-crystalline phase. The apparent crystallite sizes are small, and there is no three-dimensional ordering. Generally, the films are isotropic and consist of relatively pure carbon, with the degree of disorder dependent on the process conditions.     

Cerium-based oxide coatings

Cerium-based oxide compounds are known for their wide range of applications in catalysis, corrosion prevention, electrochemical cells, photocatalysis, UV absorbers, biomaterials, microelectronics, optical devices, thermal coatings, and glass abrasives. The technological applications of these materials are possible due to a combination of the electronic structure of Ce and the size effects at the nanoscale. In particular, reversible transformation between the Ce(III) and Ce(IV) oxidation states on the surface of cerium oxides is critical to the functionality and potential uses of the materials. In this paper, the main technological applications of cerium-based oxide coatings are reviewed based on the work done to date. Special interest is placed on the emerging trends.     

Structured selective coatings

Nickel-impregnated anodized Al₂O₃ films on aluminium substrates were investigated as possible selective coatings for the photothermal conversion of solar energy. A purely chemical dip process for producing these spectrally selective coatings was developed as an alternative to the electrochemical process reported earlier. The process parameters were optimized to give α = 0.92 and ε = 0.19. These coatings are cheap, efficient and durable.X-ray diffraction, scanning electron microscopy and electron spectroscopy for chemical analysis investigations were performed at various stages during the formation of these coatings and their structure was determined from the data obtained. The proposed structure agrees with that reported earlier. Maxwell Garnett theory and the concept of the equivalent homogeneous medium were used to determine the theoretical optical properties of these films as a function of (1) the thickness and porosity of the Al₂O₃ layer and (2) the thickness of nickel deposited in the pores. The optimum values of the theoretical parameters are in good agreement with those determined experimentally.