Thermal Sprayed Coatings Used Against Corrosion and Corrosive Wear

International audienceCoatings have historically been developed to provide protection against corrosion and erosion that is to protect the material from chemical and physical interaction with its environment. Corrosion and wear problems are still of great relevance in a wide range of industrial applications and products as they result in the degradation and eventual failure of components and systems both in the processing and manufacturing industries and in the service life of many components. Various technologies can be used to deposit the appropriate surface protection that can resist under specific conditions. They are usually distinguished by coating thickness: deposition of thin films (below 10 to 20 μm according to authors) and deposition of thick films. The latter, mostly produced at atmospheric pressure have a thickness over 30 μm, up to several millimeters and are used when the functional performance and life of component depend on the protective layer thickness. Both coating technology can also be divided into two distinct categories: "wet" and " dry " coating methods, the crucial difference being the medium in which the deposited material is processed. The former group mainly involves electroplating, electroless plating and hot-dip galvanizing while the second includes, among others methods, vapor deposition, thermal spray techniques, brazing, or weld overlays. This chapter deals with coatings deposited by thermal spraying. It is defined by Hermanek (2001) as follows , "Thermal spraying comprises a group of coating processes in which finely divided metallic or non-metallic materials are deposited in a molten or semi-molten condition to form a coating". The processes comprise: direct current (d.c.) arcs or radio frequency (r.f.) discharges-generated plasmas, plasma transferred arcs (PTA), wire arcs, flames, high velocity oxy-fuel flames (HVOF), high velocity air-fuel flames (HVAF), detonation guns (D-gun). Another spray technology has emerged recently ; it is called cold gas-dynamic spray technology, or Cold Spray (CS). It is not really a thermal spray technology as the high energy gas flow is produced by a compressed relatively cold gas (T < 800°C) expanding in a nozzle and will not be included in this presentation

Microstructural Evolution of Solid Oxide Fuel Cell: Modeling and Optimization

The solid oxide fuel cell (SOFC) with up to 60 % energy efficiency and a life expectancy of 40,000 hours has emerged as an ideal candidate to meet the energy challenges of the modern world. However, the commercialization of SOFC is hindered due to its high operating temperature, high manufacturing cost, and lack of structural reliability. Previous studies have investigated the development of functionally graded electrodes to improve SOFCs performance; however, further investigation needs to be done on the cell-level optimization for functionally graded electrodes. In addition, the performance of functionally graded SOFCs is influenced by the SOFC microstructural evolution at elevated operating temperature. Microstructural evolution cause performance degradation and crack formation at elevated operating temperate. It is generally believed that microstructural evolution caused by the coarsening of Ni particles of Ni-yttria stabilized Zirconia (YSZ) in the anode of a SOFC, which leads to the performance degradation and crack formation. Ni particle coarsening is believed to be controlled by the interface diffusion due to the minimization of total free energy of the anode system. High operating temperature of SOFC leads to the enhanced interface diffusion of Ni particle. In this work, a multi-scale electrode polarization model of SOFCs has been expanded and developed into a cell-level model using various nonlinear particle size and porosity graded microstructures. The cell-level SOFCs model has been utilized to reveal the complex relationship among the transport phenomena, which include the transports of electron, ion, and gas molecules through the electrode. The performance of functionally graded electrodes has been investigated to understand the effects of tailored electrode microstructures on cell power output, as well as microstructural evolution using diffuse-interface theory as well as phase-field method. The developed microstructural evolution framework is capable of exploring the quantitative effect of Ni particle coarsening by tracking the effective properties (e.g., particle size, particle size ratio, TPB area) on the performance of SOFC. The TPB is found to be affected by microstructural evolution and the accumulation of pores is discovered to be responsible for crack formation. The work advances the understanding of the cell performance with graded microstructures and the effect of microstructural evolution on the performance of SOFC

Predictive Modelling and Multiscale NDE Methods in Failure Assessment of Thermal Barrier Coatings

The use of thermal barrier coatings (TBCs) allows advanced gas turbine engines to operate at a temperature higher than the incipient melt temperature of the superalloy from which the engine components are made, thereby enhancing the performance and efficiency of the engine. However, delamination cracks initiated in these coatings during service limit their applications. This investigation analysed the effects of thermal cycling on the structure, thermo-mechanical properties and lifetime of Ni-based superalloy samples coated with a TBC. The results indicate that the coating system exhibits substantial changes during its life, with the thermo-mechanical properties of the TBC layers being highly sensitive to temperature and cyclic oxidation. The current study also presents a new finite element model that describes the evolution of the stress state within a thermal barrier coating subjected to thermal cycling loading. This computational framework was used to identify the optimal design parameters through a newly proposed sensitivity index, so that TBCs can be engineered with improved lifetime. Photoluminescence piezo-spectroscopy has been used to identify non-destructively the onset of microcracks and monitor their propagation through a proposed local damage factor that combines spectral shape evolution with peak shift. The computational spectral simulation was based on coupling the finite element model for the calculation of stress with an external routine for the prediction of luminescence spectra. A new non-destructive multi-sensor diagnostics procedure based on the combination of imaging- and laser-based techniques was presented. It has been demonstrated that it can accurately determine the remaining life of high-temperature coatings, and therefore it represents an important development direction for improving the reliability of TBCs. It is concluded that the results obtained in this research were quite satisfactory, which suggests that further model development and field testing of the non-destructive methodology are warranted for predictive failure assessment of thermal barrier coating systems. Keywords Thermal barrier coating Material properties Finite element modeling Photoluminescence piezo-spectroscopy Non-destructive evaluatio

Numerical investigations of thermal spray coating processes: combustion, supersonic flow, droplet injection, and substrate impingement phenomena

The aim of this thesis is to apply CFD methods to investigate the system characteristics of high speed thermal spray coating processes in order facilitate technological development. Supersonic flow phenomena, combustion, discrete droplet and particle migration with heating, phase change and disintegration, and particle impingement phenomena at the substrate are studied. Each published set of results provide an individual understanding of the underlying physics which control different aspects of thermal spray systems.A wide range of parametric studies have been carried out for HVOF, warm spray, and cold spay systems in order to build a better understanding of process design requirements. These parameters include: nozzle cross-section shape, particle size, processing gas type, nozzle throat diameter, and combustion chamber size. Detailed descriptions of the gas phase characteristics through liquid fuelled HVOF, warm spray, and cold spray systems are built and the interrelations between the gas and powder particle phases are discussed. A further study looks in detail at the disintegration of discrete phase water droplets, providing a new insight to the mechanisms which control droplet disintegration, and serves as a fundamental reference for future developments of liquid feedstock devices.In parallel with these gas-particle-droplet simulations, the impingement of molten and semi-molten powder droplets at the substrate is investigated and the models applied simulate the impingement, spreading and solidification. The results obtained shed light on the break-up phenomena on impact and describe in detail how the solidification process varies with an increasing impact velocity. The results obtained also visually describe the freezing induced break-up phenomenon at the splat periphery

Optimal Design of Functionally Graded Parts

Several additive manufacturing processes are capable of fabricating three-dimensional parts with complex distribution of material composition to achieve desired local properties and functions. This unique advantage could be exploited by developing and implementing methodologies capable of optimizing the distribution of material composition for one-, two-, and three-dimensional parts. This paper is the first effort to review the research works on developing these methods. The underlying components (i.e., building blocks) in all of these methods include the homogenization approach, material representation technique, finite element analysis approach, and the choice of optimization algorithm. The overall performance of each method mainly depends on these components and how they work together. For instance, if a simple one-dimensional analytical equation is used to represent the material composition distribution, the finite element analysis and optimization would be straightforward, but it does not have the versatility of a method which uses an advanced representation technique. In this paper, evolution of these methods is followed; noteworthy homogenization approaches, representation techniques, finite element analysis approaches, and optimization algorithms used/developed in these studies are described; and most powerful design methods are identified, explained, and compared against each other. Also, manufacturing techniques, capable of producing functionally graded materials with complex material distribution, are reviewed; and future research directions are discussed

Materials for Solar Thermal Chemical Reactor Applications

An extremely important component of concentrated solar energy development is designing robust reactors. ... Many different reactor designs have been proposed in recent literature. Transferring the solar radiation to process reactants can often be difficult. For this study a reactor design was created that uses multiple absorbing tubes. The absorber tubes are housed within a cavity that reflects spilled or emitted radiation to increase solar utilization efficiency. The reactor has been designed to fit the High Flux Solar Furnace at the National Renewable Energy Laboratory. Solar concentrating capabilities of the HFSF facility have been modeled using the ray-trace program SOLTRACE. Absorber tube positions were optimized to intercept a large fraction of the incident radiation. An attempt was also made to more evenly distribute the flux across multiple tubes. The outer cavity was fabricated out of polished, reflective, aluminum to reduce the systems thermal mass and shorten heating and cooling times during testing. The HFSF facility was qualified through black body calorimetery and flux versus inlet attenuation was mapped. Reactor performance was validated by installing absorber tubes and measuring temperature distributions. Maximum temperature differences between the central and surrounding alumina tubes were less than 350 K, at ~1673 K central tube maximum. This temperature difference decreased with higher absorptivity tube materials. Radial distance from the central tube seemed to have the largest effect on the maximum tube temperature as the tubes were farther from the focal point of the incident radiation. Cavity wall temperatures were kept below 50 K, at maximum absorber temperatures, which indicate excellent heat reflection and dissipation. ... Once thermal stresses were quantified an effort was made to find high temperature materials that are better suited to withstand thermal shock. Graphite is a thermal shock resistant high temperature material that would be well suited for solar thermal applications. However, graphite oxidizes above 773 K. By coating graphite with an oxygen barrier material, thermal shock resistant composites could help in a wide range of applications. Graphite powder was coated with alumina via atomic layer deposition (ALD). The powders had a marginal increase in oxidation resistance but coated powders showed a marked improvement in dispersability. Sedimentation and isoelectric tests showed a change in particle-particle interactions which was also validated by decreased particle size distributions of coated particles. Alumina-graphite composites showed enhanced thermal properties such as thermal diffusivity, thermal conductivity, and thermal expansion when compared to uncoated composites. This research provides a method to enhance bulk material properties of composites specifically using hard to disperse additives such as graphite and potentially carbon nanotubes. The work in this thesis represents a broad investigation into the feasibility of using concentrated solar thermal technology for renewable fuels production. Research pertaining to materials robustness, reactor design, and fuel production cycles has advanced the state of the art. Rudimentary experimentation with this technology was conducted decades ago. New research is needed to deal with the engineering challenges that are hindering large scale adaptation of concentrated solar energy today

The Thirteenth Annual Conference YUCOMAT 2011: Programme and the Book of Abstracts

The First Conference on materials science and engineering, including physics, physical chemistry, condensed matter chemistry, and technology in general, was held in September 1995, in Herceg Novi. An initiative to establish Yugoslav Materials Research Society was born at the conference and, similar to other MR societies in the world, the programme was made and objectives determined. The Yugoslav Materials Research Society (Yu-MRS), a nongovernment and non-profit scientific association, was founded in 1997 to promote multidisciplinary goal-oriented research in materials science and engineering. The main task and objective of the Society has been to encourage creativity in materials research and engineering to reach a harmonic coordination between achievements in this field in our country and analogous activities in the world with an aim to include our country into global international projects. Until 2003, Conferences were held every second year and then they grew into Annual Conferences that were traditionally held in Herceg Novi in September of every year. In 2007 Yu-MRS formed two new MRS: MRS-Serbia (official successor of Yu-MRS) and MRS-Montenegro (in founding). In 2008, MRS – Serbia became a member of FEMS (Federation of European Materials Societies)

Experimental and analytical studies of a CO2 laser-based flexible fabrication method for dies and molds

Laser-based flexible fabrication (LBFF), a solid freeform fabrication (SFF) method based on laser-cladding process, was developed as an alternative to conventional machining methods for producing dies and molds. LBFF is similar to processes such as LENS with additional features including shaped beam profile, quasi-coaxial powder delivery, and functionally graded materials. It uses a high-power continuous wave (CW) CO2 laser to fabricate functional tooling, dies and molds, of low surface roughness and high dimensional uniformity. It offers flexibility in designing parts with tailored materials, and in producing parts of complicated geometry.;Functionally graded molds of TiC/Ni-alloy and TiC/Ni-alloy/H13, and functional dies of H13 steel were built up by use of LBFF. Test studies on mold relief ability, strength, and dimensional stability at elevated temperatures were conducted and compared with bench mark H13 mold in gravity casting, in injection molding, and in thermal fatigue environment, respectively. Dies were also tested in aluminum extrusion under laboratory conditions. Results showed that dies and molds fabricated by LBFF had nearly full density, smooth surface (Ra \u3c 25 mum), and improved performance; the functionally graded molds had gradual change in elemental compositions in the transitional regions between distinct layers. In addition to experiments, analytical and finite element modeling of temperature distributions was performed to justify the use of shaped beam profiles in LBFF