Progress in Surface Treatment II

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Authors: Qiang Wang, Ming Xing Zhang
Abstract: Cold spray (CS) is an emerging revolutionary technology for deposition of metal or composite powders at a low temperature. High quality deposits can be produced without heating related defects, such as oxidation, phase transformation and residual tensile stress due to the shrinkage during fast cooling. The present contribution demonstrates the state of the art of CS process. Since nozzle is a key component in the CS equipment to prompt the efficiency of particle acceleration, the progress of its design is summarized. Major issues regarding to the successful adhesion of particles and corresponding interaction with substrates and existing deposits are discussed, in terms of numerical simulation and experimental observation. Current implemented CS applications are presented, and potential industrial applications are discussed. Keywords: cold spray; coatings; rapid manufacturing; particle impact; bonding mechanism; nozzle design
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Authors: T. Hussain
Abstract: Cold gas dynamic spraying (CGDS) is a relatively new branch of surface engineering that involves modification of the surface of substrates to provide specific engineering advantages, which the substrate alone cannot provide. Cold spraying, as a metal deposition technique, involves spraying of typically 10-40 μm particles which are accelerated by a propellant gas to 300-1200 m/s at a temperature well below the melting point of material, and upon impact deform and adhere to the substrate. The deposition process in cold spraying occurs in a solid state which results in reduced oxidation and absence of phase changes; whereas, in thermal spraying deposition occurs of molten or semi molten particles. Over the last decade the interest in cold spraying has increased substantially. Considerable effort has been invested in process developments and optimization of coatings like copper. However, bonding in cold spraying is still a matter of some debate. The most prevalent theory is that when a particle travels at a minimum required velocity the particle deforms at a very high strain rate upon impact and during this deformation thermal softening dominates over work hardening in impact zone and a material jet is produced. This material jet removes oxides from the surface of the materials and the metal-to-metal contact is established between the freshly exposed surfaces. However, precisely how this high strain rate deformation behaviour of material promotes bonding is still unclear and requires further investigations. This article provides a comprehensive review of the current theories of bonding in cold spraying based on numerical modelling of impact and experimental work. The numerical modelling of the impact section reviews adiabatic shear instability phenomena, critical velocity, critical particle diameter, window of deposition of particles, particle impact on various substrates and the role of adhesion and rebound energy. The review of the experimental section describes the shear lip formation, crater formation on the substrates, role of surface oxides, characterization of bond formation, role of substrate preparations, coating build up mechanisms and contributions of mechanical and metallurgical components in bonding. Cold spraying of copper and aluminium has been widely explored in the last decade, now it is of growing interest to the scientific and engineering communities to explore the potential of titanium and its alloys. Titanium and its alloys are widely utilized in many demanding environments such as aerospace, petrochemical, biomedical etc. Titanium components are very expensive to manufacture because of the costly extraction process of titanium and their difficult to machine properties. Therefore, additive manufacturing from powder and repair of titanium components are of great interest to the aerospace industry using technologies such as cold gas spraying. Titanium coating as a barrier layer has a great potential for corrosion resistant applications. Cold spraying has a great potential to produce oxygen-sensitive materials, such as titanium, without significant chemical degradation of the powder. In-flight oxidation of materials can be avoided to a great extent in cold spraying unlike thermal spraying. This review article provides a critical overview of deposition efficiency of titanium powder particles, critical velocity, bond strength, porosity, microhardness, microstructural features including microstrain and residual stress, mechanical properties reported by various research groups. A summary of the competitor warm sprayed titanium coating is also presented in this article.
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Authors: Andrei Manzat, A. Killinger, R. Gadow
Abstract: Rising demands for ecologically friendly automotive engines require a significant decrease in fuel consumption and emissions. Also the recent trend of downsizing engines demands for high performance materials for internal combustion engine applications. Tribologically functional coatings applied by supersonic flame spraying help in boosting the engine efficiency by reducing the internal friction and improving the durability and wear resistance of the cylinder running surface much-needed for engine downsizing tasks together with a high corrosion resistance enabling the use of bio fuels. In addition, the tailored surface topography of the thermal spray coatings help in supporting advantageous friction states and thereby show the benefit of reducing the oil consumption resulting in reduced emissions. The thermally sprayed coatings were applied using HVOF and HVSFS processes together with a specially designed spray gun trajectory in order to achieve a fast and cost efficient coating procedure. Several different coating materials, including novel nanostructured powders, have been investigated and compared to state-of-the-art cylinder liners. The performance of the coated cylinder liners regarding wear and corrosion resistance, friction coefficient and effects of the surface topography have been investigated in various test setups including engine tests.
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Authors: A. Killinger, R. Gadow
Abstract: Thermal Spraying of submicron and nanoscaled powder materials require the application of agglomerated powders that can be processed with standard HVOLF (High Velocity Oxy Liquid Fuel Flame Spraying) equipment, or the conversion of these powders into a finely dispersed suspension together with an organic solvent, appropriate for suspension spraying (HVSFS: High Velocity Suspension Flame Spraying). Both methods are suitable for the manufacturing of finely structured and dense coatings and offer new possibilities in functional coating development for new application fields. Due to their refined microstructure, mechanical and physical coating properties can differ significantly from their conventionally sprayed counterparts. The paper gives an overview of HVOLF sprayed WC-Co cermets containing submicron and nanocarbides and HVSF sprayed nanooxide ceramics used in numerous technical applications.
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Authors: Miriam Floristán, Andreas Killinger, Rainer Gadow
Abstract: For diverse applications in optical, electronic and consumer industries, the use of glass and glass ceramics as substrates for functional coatings is becoming of outstanding interest in order to develop advanced composites. Atmospheric Plasma Spraying (APS) is an adequate technology for the deposition of a wide variety of materials on glasses. Glass and glass ceramics are characterised by their specific thermo physical properties like low or even negative CTE, low heat conductivity and high dimensional stability. Consequently, modified production processes in comparison to the established coating operations on metal surfaces are required regarding the substrate activation methods or a more accurate heat transfer guidance to the substrate by optimized robot trajectories. This paper aims to give an overview of the investigations carried out at the IMTCCC for the development of plasma sprayed layer composites on borosilicate glass and glass ceramic substrates.
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Authors: A. Rempp, M. Widmann, A. Killinger, R. Gadow
Abstract: For the reduction or complete compensation of grease and lubricants in mechanical engineering, special combined coatings for machine components with extended tribological and chemical properties have been developed. Ceramic or metal polymer coating systems are a combination of thermally sprayed hard materials with polymers containing solid lubricants of inorganic and fluorpolymeric origin (i.e. lubricant lacquers). The thermally sprayed hard material guarantees hardness and wear resistance and the lubricant lacquer causes low friction coefficients and smooth gliding performance. In this paper, results of different polymer and multicoating systems on steel substrates are presented. Molybdenum, a cermet based on tungsten carbide, a copper based alloy and chromia were used as wear resistant materials and deposited by means of High Velocity Flame Spraying (HVOF/HVLF). On these surfaces, lubricant lacquers of some μm thickness were deposited by air spraying. The life-time and the friction coefficient of the resulting layers depending on surface texture of the hard material or substrate and lacquer composition were determined using a twin disc tester under dry sliding conditions.
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Authors: Jing Li, Xi Ping Guo
Abstract: The microstructure and high-temperature oxidation-resistant performance of several silicide coatings on an Nb-Ti-Si based alloy were revealed in the present work. These silicide coatings were prepared respectively at 1250°C for 8 h by pack siliconizing process, Si-Y co-deposition process and Si-Al-Y co-deposition process (with different Al contents in the packs). The results showed that the purely siliconized coating was composed of a (Ti,Nb)5Si3 ouer layer, a (Nb,X)Si2 (X represents Ti, Cr and Hf elements) middle layer and a (Ti,Nb)5Si4 inner layer. A thicker and more compact double-layer structure including a (Nb,X)Si2 outer layer and a (Ti,Nb)5Si4 inner layer was observed in the Si-Y co-deposition coating. In addition, a higher Y content (about 0.34 at. %) in the outer layer of the Si-Y co-deposition coating was obtained, while the Y content was only about 0.06 at. % in the purely siliconized coating. The Si-Al-Y co-deposition coating possessed a (Nb,X)Si2 outer layer, a (Ti,Nb)5Si4 middle layer and an Al, Cr-rich inner layer. A suitable addition of Al powders (5 wt. %) in the packs was beneficial to thicken the (Nb,X)Si2 outer layer, while a sharp reduction in the coating thickness was found when excess Al powders (10 wt. %) was added in the packs. However, compared with the former coating, the later coating prepared with more Al powders in the packs resulted in a slight increase in the content of Al and Y in the (Nb,X)Si2 outer layer from about 0.21 and 0.54 at. % to 0.87 and 0.79 at. % respectively. The thickness and microstructure of the scales formed on above four coatings upon oxidation at 1250°C for either 5 h or 100 h were comparatively investigated. The oxidation resistance of these silicide-type coatings was notably enhanced by the addition of Y and Al. The Si-Al-Y co-deposition coating, which was prepared with 5 wt. % Al powders in the pack, possessed the best oxidation resistance due to its optimum dense and continuous scale and compact coating remained. Keywords: Nb-Ti-Si based alloy; coating; microstructure; oxidation-resistant perfor-mance *Corresponding author. Tel./fax: +86 29 88494873. E-mail address: xpguo@nwpu.edu.cn (X. Guo).
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Authors: Nahed El Mahallawy, Mohamed Harhash
Abstract: In the present study, coating of some magnesium alloys including AZ31 and AZ91 were studied using different techniques namely anodizing, electroless and hot press cladding. AZ91 alloy was coated using anodizing process using three types of environmental friendly electrolytes; the first based on sodium silicate, the second based on sodium hydroxide-boric acid-borax and the third on sodium silicate-potassium hydroxide-sodium carbonate-sodium tetra borate. Characterization of the anodizing layer was achieved by determination of surface morphology, microstructure, phase analysis, coat thickness, adhesion and corrosion resistance. It was found that the anodic film thickness increases with increasing the current density, anodizing voltage and deposition time until the deposition stops due to the formation of a thick anodic film. The range of the anodic film thickness is 28 42 µm.Optimization of the anodizing conditions - current density and deposition time was determined for each electrolyte. A corrosion efficiency ranging from 94% to 97% was reached; the highest value corresponding to the third electrolyte. Another study is the electroless Ni plating technique with zinc pre-treatment applied on several magnesium alloys and the effect of pre-treatment and post heat treatment on the coat characteristics. The surface morphology, surface roughness, thickness of the layer, EDX analysis, adhesion, hardness and corrosion resistance are covered in this part. The electroless layer thickness is about 6 µm. The results showed good bond quality of the coat maintaining good corrosion behaviour of electroless Ni-P based on potentiodynamic polarization tests in chloride solution where it was improved after heat treatment process. On the other hand, AZ31 was covered by a commercial pure aluminum sheet by hot pressing. The influence of the applied pressure, holding time and temperature on the bond characteristics was studied. The experimental investigation has revealed a good bond quality due to the effective mutual diffusion of Mg and Al. The phase analysis resulted in the formation of two equilibrium phases namely; Mg17Al12 and Mg2Al3. The corrosion resistance of AZ31 is enhanced as a results of this process by 98.6%. Further points will be covered. Keywords: Magnesium alloys; Coating; Anodizing; Electroless; Hot press cladding; environmental friendly electrolytes; corrosion
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Authors: Adele Carradò
Abstract: Bioactive coatings are currently manufacturing using plasma-sprayed technique on metal implant surfaces in order to optimize bone-implant interactions. Nevertheless, some problems exist with coating process, e.g. poor interfacial adhesion, modification of coating properties, and the lack of an existing coating standard. In order to overcome some of the problems with the plasma-spraying process, researchers are investigating other experimental coating methods to enhance the adhesion and to control the coating properties. This paper will discuss the advantages and disadvantages of plasma spraying and the experimental coating processes as pulsed laser deposition as well as spin-coated sol-gel.
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