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1-11 of 11
T. Chráska
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Proceedings Papers
ITSC 2021, Thermal Spray 2021: Proceedings from the International Thermal Spray Conference, 131-138, May 24–28, 2021,
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Diamond-reinforced composites prepared by cold spray are emerging materials simultaneously featuring outstanding thermal conductivity and wear resistance. Their mechanical and fatigue properties relevant to perspective engineering applications were investigated using miniature bending specimens. Cold sprayed specimens with two different mass concentrations of diamond 20% and 50% in two metallic matrices (Al – lighter than diamond, Cu – heavier than diamond) were compared with the respective pure metal deposits. These pure metal coatings showed rather limited ductility. The diamond addition slightly improved ductility and fracture toughness of the Cu-based composites, having a small effect also on the fatigue crack growth resistance. In case of the Al composites, the ductility as well as fatigue crack growth resistance and fracture toughness have improved significantly. The static and fatigue failure mechanisms were fractographically analyzed and related to the microstructure of the coatings, observing that particle decohesion is the primary failure mechanism for both static and fatigue fracture.
Proceedings Papers
ITSC 2017, Thermal Spray 2017: Proceedings from the International Thermal Spray Conference, 36-41, June 7–9, 2017,
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In this paper, yttria-stabilized zirconia (YSZ) coatings were prepared by plasma spraying of ready-to-spray suspensions provided by three different manufacturers. High-enthalpy hybrid water-argon plasma torch WSPH 500 was successfully used for deposition of coatings with porous and columnar microstructure consisting of tetragonal non-transformable phase. Sensitivity of the deposition process to variation of deposition conditions was also evaluated by the change of suspension injection point position. Slight differences in the microstructures of the deposited coatings (in particular character of porosity and mutual bonding of the microsplats) were reflected in slight but measurable differences in hardness and wear resistance of the coatings indicating changes in the coating cohesion. Tensile adhesion/cohesion strength of the coatings was found to be in the range of 9 to 15 MPa. High coating porosity desirable for low thermal conductivity combined with high suspension feed rate (from about 100 to 120 ml/min in this study) makes the WSP-H coatings promising for further development for example in thermal barrier coatings applications.
Proceedings Papers
ITSC2016, Thermal Spray 2016: Proceedings from the International Thermal Spray Conference, 267-272, May 10–12, 2016,
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This paper presents the results of three experiments in which coatings are produced by suspension plasma spraying using a water-stabilized hybrid torch. Shadowgraphy is used to optimize the injection of the suspension and visualize liquid fragmentation in the plasma jet. Deposition efficiency is evaluated and the production of coatings with different microstructures is demonstrated for YSZ, YAG, and Al 2 O 3 .
Proceedings Papers
ITSC2014, Thermal Spray 2014: Proceedings from the International Thermal Spray Conference, 617-622, May 21–23, 2014,
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Alumina-zirconia ceramic material has been plasma sprayed using a water stabilized plasma torch (WSP) to produce free standing coatings. The as-sprayed coatings have very low porosity and are mostly amorphous. The amorphous material crystallizes at temperatures above 900 °C. A spark plasma sintering apparatus has been used to heat the as-sprayed samples to temperatures above 900 °C to induce crystallization while at the same time a uniaxial pressure of 80 GPa has been applied to the their surface. After such post-treatment, the ceramic samples are crystalline and exhibit very low open porosity. The as-sprayed amorphous materials also exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated samples whose microstructure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix.
Proceedings Papers
ITSC2014, Thermal Spray 2014: Proceedings from the International Thermal Spray Conference, 680-685, May 21–23, 2014,
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Dysprosia stabilized zirconia coatings with large globular pores have good potential as TBC topcoats. In previous work, such coatings have been produced by air plasma spraying with the aid of a polymer pore former. The aim of this work is to optimize the spraying parameters. A design of experiments approach was used to create a two-level full factorial test matrix based on spray distance, powder feed rate, and hydrogen flow. An agglomerated and sintered dysprosia stabilized zirconia (DySZ) powder mixed with polymer particles was sprayed on Hastelloy X substrates that had been prepared with NiCoCrAlY bond coats. The coatings obtained were evaluated based on thermal conductivity, thermocyclic fatigue life, and morphology, which are shown to correlate with spray parameters and in-flight particle properties.
Proceedings Papers
ITSC 2008, Thermal Spray 2008: Proceedings from the International Thermal Spray Conference, 435-439, June 2–4, 2008,
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An alternative method to produce bulk nanocrystalline materials and avoid the powder compaction step is to produce amorphous material by rapid solidification followed by controlled heat treatment to introduce nanocrystalline structure. The extremely high cooling rates in plasma sprayed particles give rise to formation of nonequilibrium phases, which may become amorphous for certain materials. Five different materials studied in this work are based on near-eutectic mixtures of alumina, zirconia and silica. The powder feedstock materials have been plasma sprayed using water stabilized plasma torch (WSP) and subsequently heat-treated to prepare nanocomposite materials with varying nanocrystallite size. The as-sprayed materials have very low open porosity and are mostly amorphous. The as-sprayed amorphous materials crystallize at temperatures around 950°C with an associated volume shrinkage of 1-2%. The resulting structure is best described as nanocomposite with very small crystallites (12 nm on average) embedded in inter-crystallite network. Role of the silica compound on phase composition, microstructure, and mechanical properties of the as-sprayed and annealed materials is discussed. Elastic properties were measured for the nanocrystalline materials. The as-sprayed amorphous materials exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat treated nanocrystalline samples.
Proceedings Papers
ITSC 2007, Thermal Spray 2007: Proceedings from the International Thermal Spray Conference, 627-632, May 14–16, 2007,
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Al 2 O 3 -ZrO 2 composite coatings were deposited by suspension thermal spraying of submicron feedstock powders. The suspensions were injected internally into a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1150 Hv 0.3 , as well as the highest abrasion wear resistance (ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia alloyed matrix. The HVOF microstructures also led to low thermal diffusivity, due to high amorphous phase content and limited particle bonding.
Proceedings Papers
ITSC 2005, Thermal Spray 2005: Proceedings from the International Thermal Spray Conference, 830-835, May 2–4, 2005,
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A novel material has been used for plasma spraying by WSP. The material is composed of three main phases, namely corundum (aluminum oxide), baddeleyite (zirconium oxide), and glassy phase (silicon oxide). The material is a refractory and exhibits very high hardness, extremely high abrasion resistance, and chemical resistance. Conventionally, the material is fabricated by melt casting and machining. Cast tiles of the material were ground and sieved to obtain the right powder cut size for plasma spraying by water stabilized plasma torch (WSP). Both dense coatings and free standing parts were achieved with the new material, which sprays very well with WSP. Spraying parameters were varied and molten particles were monitored in flight by DPV 2000. The coatings exhibit very low porosity and high hardness. The as-sprayed material is mostly amorphous with some nanocrystalline grains of aluminum and zirconium oxide present. The phase composition of the as-sprayed material is thus different from that of the feedstock material, which is mostly crystalline with a small fraction of amorphous silica glass. The microstructure of the newly sprayed material was studied by electron microscopy (SEM, TEM) and is very complex. Upon annealing, the as-sprayed material crystallizes around 950ºC. This result and other thermal properties were measured by TMA and DTA. The ease of plasma spraying and the coating properties make this material a suitable candidate for many industrial applications.
Proceedings Papers
ITSC 2005, Thermal Spray 2005: Proceedings from the International Thermal Spray Conference, 1235-1241, May 2–4, 2005,
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Basalt, an abundant and inexpensive natural raw material, is a glass-ceramic with good abrasion-wear resistance and chemical stability. Traditionally cast-shaped into flag stones, pipe linings and even fibrous composites, basalt can be processed by thermal spraying, potentially yielding highly dense coatings with few defects. Such overlays can seal base materials for wear applications in corrosive environments. Basalt coatings are produced by a number of common thermal spray techniques, including water-stabilized plasma spraying (WSP), high-velocity oxy-fuel (HVOF) and conventional air plasma spraying (APS). In-flight particle temperature and velocities are monitored with a particle diagnostic system (DPV 2000). Using different feedstock size cuts, the attainable ranges of particle states are delineated. Spray parameters are selected for each of the processes, based on deposition efficiency and porosity criteria. For typical conditions, particle velocities vary from 100 m/sec for WSP to 800 m/sec with HVOF. The microstructure and composition of the coatings are evaluated by scanning electron microscopy (SEM) and EDS-SEM. Crystal phase analysis is performed by X-ray diffraction (XRD). Abrasion resistance (ASTM G-65) and hardness (Vickers) of the as-sprayed coatings are compared. The microstructures and tribological properties are related to the particle size, temperature and velocity distributions, which are distinctly different for each process.
Proceedings Papers
ITSC 2005, Thermal Spray 2005: Proceedings from the International Thermal Spray Conference, 1292-1297, May 2–4, 2005,
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Individual particles of various ceramic materials sprayed by water stabilized plasma torch (WSP) were characterized in flight by an optical sensing device DPV-2000. Temperature, velocity, and diameter of individual particles were measured at the center of particle plume and maps of the particle plume cross-section were acquired as well. Plasma jet and consecutively particle plume of the WSP torch is much larger compared to gas stabilized torches and even larger than the maximum span of the DPV-2000 sensing head. In summary, temperatures of particles varied from 2000 to 2600°C and their velocities from 60 to 140 m/s depending on the powder feedstock cut size, particle density, feeding distance, spraying distance, and feed rate. The last three parameters were varied to study their effect on the particle states and their distribution in the plasma jet and to correlate these results with selected properties of the corresponding coating. Some of the spraying parameter effects are difficult to interpret, nevertheless, general trends have been established.
Proceedings Papers
ITSC 2005, Thermal Spray 2005: Proceedings from the International Thermal Spray Conference, 1434-1440, May 2–4, 2005,
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Nanostructured coatings of Al 2 O 3 and ZrO 2 (1.5% Y 2 O 5 ) are produced by suspension plasma spraying, introducing the liquid feedstock internally into the central part of three converging plasma jets of a Mettech Axial III torch (Northwest Mettech Corp.). Spraying nanosized ceramic powders in a liquid carrier can yield thinner coatings with more refined microstructures than conventional plasma spraying. In-flight particle states are measured for a number of plasma conditions of varying torch current, gas flow rates and compositions (Ar, H 2 , N 2 ), and related to the resulting microstructure and phase composition in the coatings, as determined by EDS, SEM and XRD. Results show that particle velocities up to 600 m/sec can be reached, yielding high impact velocities and cooling rates. Some comparison is made to previous work performed using external injection of a suspension droplet stream into a conventional dc plasma flame. At optimized conditions, dense γ-Al 2 O 3 deposits with an average crystallite size below 30 nm are obtained at deposition efficiency above 80%, which was not previously attainable. Eutectic alumina-zirconia composite coatings with a finely layered lamellar structure are also realized. The laminates have potential advantages as thermal barrier coatings with beneficial properties arising from nano-composite components.