Zdenek Pala

Influence of Microstructure on Thermal Properties of Axial Suspension Plasma-Sprayed YSZ Thermal Barrier Coatings

Suspension plasma spraying is a relatively new thermal spaying technique to produce advanced thermal barrier coatings (TBCs) and enables production of coatings with a variety of structures—highly dense, highly porous, segmented, or columnar. This work investigates suspension plasma-sprayed TBCs produced using axial injection with different process parameters. The influence of coating microstructure on thermal properties was of specific interest. Tests carried out included microstructural analysis, phase analysis, determination of porosity, and pore size distribution, as well as thermal diffusivity/conductivity measurements. Results showed that axial suspension plasma spraying process makes it possible to produce various columnar-type coatings under different processing conditions. Significant influence of microstructural features on thermal properties of the coatings was noted. In particular, the process parameter-dependent microstructural attributes, such as porosity, column density, and crystallite size, were shown to govern the thermal diffusivity and thermal conductivity of the coating.

Optimization of High Porosity Thermal Barrier Coatings Generated with a Porosity Former

Yttria-stabilized zirconia thermal barrier coatings are extensively used in turbine industry; however, increasing performance requirements have begun to make conventional air plasma sprayed coatings insufficient for future needs. Since the thermal conductivity of bulk material cannot be lowered easily; the design of highly porous coatings may be the most efficient way to achieve coatings with low thermal conductivity. Thus the approach of fabrication of coatings with a high porosity level based on plasma spraying of ceramic particles of dysprosia-stabilized zirconia mixed with polymer particles, has been tested. Both polymer and ceramic particles melt in plasma and after impact onto a substrate they form a coating. When the coating is subjected to heat treatment, polymer burns out and a complex structure of pores and cracks is formed. In order to obtain desired porosity level and microstructural features in coatings; a design of experiments, based on changes in spray distance, powder feeding rate, and plasma-forming atmosphere, was performed. Acquired coatings were evaluated for thermal conductivity and thermo-cyclic fatigue, and their morphology was assessed using scanning electron microscopy. It was shown that porosity level can be controlled by appropriate changes in spraying parameters.

Dielectric properties of CaTiO3 coatings prepared by plasma spraying

Abstract This paper presents a study of dielectric properties, namely the relative permittivity and loss factor dependence on the frequency of a weak electric field. Perovskite CaTiO3 was studied in the form of coatings and self-supporting plates made by plasma spraying. A conventional gas stabilised plasma gun (GSP) as well as a water stabilised plasma gun (WSP) were employed. It was observed that plasma sprayed titanates exhibit a strong relaxation of permittivity and loss factor decrease with a frequency rise. These properties are influenced by spray technique and spraying parameters, but the relaxation character in general is preserved in all cases. The volume resistivity of the samples was studied as well. Several aspects of the structural features of plasma deposits, especially the phase composition, porosity character, and their influence on dielectric properties are discussed.

Evaluation of surface, microstructure and phase modifications on various tungsten grades induced by pulsed plasma loading

Progress in the field of nuclear fusion requires the development of a new generation of tungsten materials that are expected to meet specific property, lifetime and safety requirements. Pursuing this goal, the new materials must be properly tested in a wide range of conditions including cases where material is brought to the molten stage, such as with large fusion plasma instabilities. In this study, two prospective candidates from the family of dispersion strengthened (DS) tungsten materials, i.e., W-1%Y2O3 and W-2.5%TiC, were subjected to extreme heat loading exerted by the deuterium plasma generator PF6. The study focuses on the interaction of the tungsten matrix with the dispersed particles during material melting. The materials underwent significant changes in microstructure and phase content. Among the most serious was the loss of TiC particles and void formation in W-2.5%TiC and phase change of polymorphic Y2O3 particles in W-1% Y2O3.

Photocatalytic and electrochemical properties of single- and multi-layer sub-stoichiometric titanium oxide coatings prepared by atmospheric plasma spraying

We studied the photocatalytic and electrochemical impedance properties of two different sub-stoichiometric titania powders deposited using plasma spraying. Two different commercial powders with markedly diverse mean size as well as size distribution were chosen. Thermal oxidation of these as-received powders was carried out to restore the O/Ti ratio to be 2.0. By this way, another two kinds of feedstock powders were obtained. Total of four kinds of feedstock powders were used in the experiments. All powders were sprayed using identical plasma spraying parameters. For some spraying runs, carbon steel served as the substrate, and for others, the previously sprayed titania coating from a different kind of feedstock. Combinations of single- and double-layer coatings were studied. Porosity, microstructure, phase composition, chemical composition, band gap based on reflectance measurement, and photocatalytic activity were examined. Electrochemical impedance spectroscopy demonstrated substantial differences between samples sprayed from the fine and coarse powders. Coatings from oxidized powders were slightly more photocatalytic. High spraying distance used for several of them was good for obtaining low band gap, whereas surface roughness and phase composition were not substantially different compared to standard spraying distance coatings.

Influence of Microstructure on Thermal Properties of Columnar Axial Suspension Plasma Sprayed Thermal Barrier Coatings

Suspension Plasma Spraying is a relatively new thermal spraying technique to produce advanced thermal barrier coatings. This technique enables the production of a variety of structures from highly ...

Fabrication and microstrain evolution of Al-TiB2 composite coating by cold spray deposition

This paper investigates the microstructure evolution of Al-TiB2 coatings prepared by cold spraying. In situ Al-TiB2 composite powders containing uniformly distributed titanium diboride (TiB2) particles with a size range of 5–100 nm in the Al matrix and Al/Al-TiB2 blended powders were used as the cold spray feedstock for coating fabrication on aluminium alloy substrates. The microstructures of the feedstock powders and as-deposited coatings were characterised using scanning electron microscopy with energy dispersive X-ray analysis and X-ray diffraction. Al/Al-TiB2 blended powder coatings, compromising closely packed powder particles, were sprayed to an approximate thickness of 500 µm. Al-TiB2 composite coatings (approximately 50 µm thick) were obtained retaining the microstructure of the composite powders being sprayed and no evidence of detrimental phase transformation was found. However, micro-cracks were found to exist in the Al-TiB2 coating due to the hardly deformable powder particles. Little or no microstrain was revealed in the as-sprayed Al-TiB2 coating, indicating that annealing may have occurred due to the localised adiabatic heating during the spraying process. It is demonstrated that it is possible to fabricate the Al-TiB2 composite coating by cold spray deposition but further improvements to eliminate coating cracking are required.

The Influence of Spraying Parameters on Stresses and Mechanical Properties of HVOF-Sprayed Co-Cr-W-C Coatings

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. Youngs 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.

Photocatalytic Activity of Titanium Oxide – Iron Oxide Coatings Prepared by Plasma Spraying

This study examines the photocatalytic activity of coatings produced by atmospheric plasma spraying (APS). The spraying tool used is a water -stabilized plasma gun WSP. The TiO2 �Fe2O 3powder withand withoutNa 2SiO3additive was agglomerated to build p articles suitable for feeding into the plasma jet and spraying. The coatings are analyzed by scanning electron microscopy, X -ray fluorescence and Xray diffraction. Photocatalytic degradation of acetone as well as the growth kinetics of the products -carbon dioxide and carbon monoxide -is quantified for both coating types and compared with a pure TiO 2 coating. The coatings show a lamellar structure, as is typical for this technological process. Howe ver the porosity is rather high. Anatase titania from the feedstock powder is converted to rutile phase whereas a presence of FeTiO 3is detected in the coating without Na 2SiO3. The coating from powder withNa 2SiO3 admixture is amorphous. Both coatings aremore photocatalytically active than the reference TiO 2coating.

Study of residual stresses, microstructure, and hardness in FeB and Fe 2 B ultra-hard layers

Boriding is a thermochemical diffusion-based process of achieving ultra-hard surface on metals. Two distinct crystalline phases, i.e. tetragonal Fe 2 B and orthorhombic FeB can exist in the surface layer penetrated by boron ions. In our contribution, we have studied the microstructure, the hardness, and the spatial distributions of both phase composition and residual stresses (RS) in samples exhibiting either single-phase Fe 2 B or duplex Fe 2 B-cum-FeB character. The indispensable knowledge of the elastic constants used in the stresses calculations from the measured strains by X-ray diffraction were gained from the refined lattice parameters of both iron borides employing density functional theory implemented in CASTEP software by Materials Studio. In the studied case, there is only minor occurrence of preferred orientation in the Fe 2 B phase and the evaluated RS have compressive character gradually decreasing from its maximum value on the very surface.