Papers by Author: Jiří Matějíček

Abstract: Tungsten is currently considered as the most suitable plasma facing material for the first wall of a nuclear fusion reactor. First wall will be subjected to harsh conditions that will gradually deteriorate properties of the wall material. Some studies point out that fine-grained tungsten could be more resistant to the structure and property changes than coarse-grained tungsten. However, tailoring of tungsten microstructure is very laborious. Due to its high melting point, tungsten is very often processed mechanically and subsequently sintered into a compact body. In this study, preparation of ultrafine-grained tungsten by mechanical processing in a planetary ball mill was examined. Three types of tungsten samples were compared. One was made from coarse grained tungsten powder consolidated by SPS (spark plasma sintering). Other two samples were prepared from the powder processed in a planetary ball mill with and without addition of Y₂O₃. After ball milling, the powders were consolidated by SPS, i.e. fast sintering process that allows preserving fine-grained structure of the powder material. Properties of the samples such as hardness and thermal conductivity were examined and correlated with the processing history and microstructure.

Abstract: The paper participates in a development of composites. A composite tungsten-steel is studied as materials suitable for a first wall of tokamak, a composite FeAl + Al₂O₃ is a possible material for fourth generation of a nuclear power plant and a composite Al₂O₃ + YSZ is a potential implant material. The focus of our study is change of material properties near the interface and a determination of area size which is influenced by the adjacent material. Material properties are investigated by nanoindentation. The task is simulated using finite element method. Simulated specimen is composed of a tungsten part and a steel part. The sharp boundary between materials is a plane which is located parallel to the loading force direction. Elastic modulus is determined in dependence of a distance between the interface and a tip of the indenter. The simulated results are verified experimentally.

Abstract: The paper serves as an introduction to investigation of mechanical properties of functionally graded materials and deals with elastic nanoindentation numerical models. The models were based on the finite element method. Young's moduli were estimated by Oliver-Pharr method. The indenter geometry for which numerical solutions were accomplished was a rounded cone indenter. The effect of tip sharpness was examined by applying an increasing spherical tip radius. The results show that the apparent Young's modulus and the hardness increase linearly with increasing radius of the tip. The effect of approaching interface between two elastic materials on the apparent hardness and indentation modulus was identified in 3D model. The specimen consisted of two materials. First, the interface was linear and parallel to the direction of indentation, so that the Young's modulus changed suddenly. Second, the Young's modulus was continuously changing. The dependence on various boundary conditions of the specimen was also considered.

Abstract: This study was aimed to investigate the effect of the substrate preheating temperature on the overall quality of the coating/substrate interface. The coatings (stainless steel) were deposited using a water stabilized plasma torch (Institute of Plasma Physics, Prague, i.e. IPP, Czech Republic) on steel substrates. Three sets of samples were prepared under identical spraying and grit blasting condition; substrate preheating temperature was the only parameter which was varied, i.e. preheating to 150°C, 250°C and 350°C. Higher preheating temperature led to a significant increase in the coating adhesion.

Abstract: Boriding of highly alloyed steels done with the aim of increasing their wear resistance faces several issues connected with the microstructure of the base material and restraints during the diffusion of boron. The aim of the performed analyses was to ascertain whether significant increase of boriding time can enhance the surface hardness, contribute to creation of more compact microstructure and even lead to beneficial state of residual stresses in the borided layer. Using combination of X-ray diffraction and electro-chemical polishing, residual stress depth distributions in few tens of micrometres thick borided layers were obtained.

Abstract: In this work, application of the in-situ curvature method on plasma sprayed composite and graded coatings is presented. First, uniform composites of different W/Cu ratio, were sprayed by water stabilized plasma. By continuous monitoring of the curvature of a flat specimen during spraying, the stress evolution throughout the entire history of coating formation was traced. By a simultaneous monitoring of curvature and temperature during post-deposition cooling, Youngs moduli of the coatings were determined. Second, a 5-layer stepwise functionally graded material (FGM) was sprayed. With the knowledge of each layers properties, the complex evolution of deposition, thermal and residual stresses in the FGM could be determined. The ability to determine the stresses and mechanical properties with a spatial resolution comparable to the thickness of one spray pass is demonstrated.

Abstract: Stellite 6 Co-Cr-W-C coatings were sprayed by HVOF while systematically varying the spraying parameters, namely the equivalent ratio and combustion pressure. During spraying, the in-flight particle temperature and velocity were measured. Deposition, thermal and residual stresses were determined by in-situ curvature monitoring of the sprayed samples. Young's moduli and hardness of the coatings were determined by instrumented indentation. The relationship between spraying parameters, in-flight particle characteristics and mechanical properties is discussed.

Abstract: Thermonuclear fusion is a potential source of cleaner and safer energy for the future. Its technological realization depends on the development of materials able to survive and function in extreme conditions, often involving a variety of loading – thermal, mechanical, chemical and irradiation. Single bulk materials are often at the edge of their properties limits; therefore, composites and coatings are intensely studied. This article reviews the development, characteristics and applications of coatings for fusion reactor materials. First, the technological objectives and material-environment interactions are briefly summarized, together with materials requirements and the role of coatings. Then, specific applications in different areas of a fusion device are reviewed, namely the plasma facing components, electrical insulation, diffusion and permeation barriers. Various coating fabrication methods are mentioned and the respective coating characteristics are compared. Selected case studies are presented, with particular focus given to ITER and to ceramic coatings.

Abstract: A range of different spraying techniques can be used to coat the surfaces of engineering components. These techniques are based on different principles and can involve high temperature (plasma spray), high kinetic energy (cold spray) or both (HVOF spray – High-Velocity Oxi-Fuel). Resultant residual stress in such coatings, being a characteristic of the spraying process, can reveal details of the stress formation mechanism. When its dependence on the physical parameters and conditions of the spraying process is established, this knowledge can be used for the prediction and control of stress that occurs in applications. Neutron diffraction is a suitable method for obtaining stress distribution in such coatings. Residual stresses in two-phase Cu+W coatings made by water stabilized plasma spraying were studied. Two-phase coatings develop both significant microstress (inter-phase stress) and the stress dependence on phase content of the coating constituents. Through-thickness residual stress profiles have been measured by neutron diffraction with spatial resolution of 0.5 mm for a series of Cu+W coatings with varying volume fractions. Measurements were made in both phases in order to separate micro- and macro-stresses. Comprehensive sample characterization, measurements of the residual stresses, mechanical and thermal properties of the composite coatings enabled quantitative modeling and interpretation of the experimental data.

Abstract: Tungsten is the main candidate material for the armor of plasma facing components for ITER and future fusion devices [1]. Plasma spraying is an alternative method for manufacturing tungsten-based coatings, including composites and graded layers, having a number of advantageous features [2]. On the other hand, the main limitation to application of these coatings on high heat flux components, is their low thermal conductivity, originating in the layered structure [3]. This paper is focused on four methods of improving the coatings’ thermal conductivity. First method consists in modification of the basic spraying parameters, which have a direct influence on the coating structure and therefore properties. The other three methods involve post-processing of the coatings: molten copper infiltration, laser remelting and densification by HIPping. The latter encompasses also tungsten-copper composites of various compositions. Experimental results, including structural and thermal characterization, are presented for each method. Finally, the applicability of these methods, from the point of view of manufacturing the plasma facing components, is discussed.