Papers by Author: Miroslav Hnatko

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Authors: Ján Dusza, Monika Kašiarová, Lucia Hegedűsová, Miroslav Hnatko, Pavol Šajgalík
Abstract: Bending and contact strength of a carbon derived in-situ reinforced Si3N4-SiC micro/nanocomposite have been investigated. Four-point bending strength was measured using specimens with different effective volume and Weibull statistical analysis has been used for characterization of the strength values in the form of characteristic strength and Weibull modulus. The characteristic strength values of the investigated composite were σ0 = 675 MPa and σ0 = 832 MPa with the Weibull modulus of 6,4 and 8,6 for the specimens with effective volume 15,9 mm3 and 1,28 mm3 respectively. Contact strength of the investigated material was carried out using sphere on sphere and roller on roller methods and the obtained results are σ0S = 1997 MPa, σ0R = 1242 MPa, and mS = 17,1 mR = 6. Fracture origins in the specimens with effective volume of 15.9 mm3 tested in four-point bending were surface and volume located technological defects with dimensions from 10 μm to 180 μm, mainly in the form of clusters of pores and large SiC grains. Fracture mirror sizes were measured and the mirror constant was calculated. Fracture origins in the specimens with effective volume of 1.28 mm3 tested in four-point bending were surface (subsurface) defects with the same type as for the specimens with a higher effective volume. The fracture during the contact strength test „roller/roller“ has been initiated under the surface of the specimens and during the contact strength test „sphere/sphere“ by creation and growth of the cone cracks to critical size.
Authors: Dagmar Galusková, Pavol Šajgalík, Dušan Galusek, Miroslav Hnatko
Abstract: Two alumina based ceramics with 99.99 % and 95 % of Al2O3 were evaluated after dissolution in an aqueous solution containing 0.5 mol/l NaCl at temperatures of 150 and 200 °C. The weight loss of liquid phase sintered alumina was mainly attributed to congruent dissolution of SiO2 and CaO from grain-boundary amorphous film, which is accelerated at higher temperature, accompanied by precipitation of silicaceous phases from oversaturated solution at 200 °C. Pure polycrystalline alumina corroded by loss of alumina grains, which do not dissolve in the corrosion media.
Authors: Pavol Šajgalík, Miroslav Hnatko, Zoltán Lenčéš, Ján Dusza
Authors: Zuzana Vilčeková, Monika Kašiarová, Magdaléna Domanická, Miroslav Hnatko, Pavol Šajgalík
Abstract: Local mechanical properties, particularly the hardness and Youngs modulus of highly porous silicon nitride based foams were studied in this work. Silicon nitride foams were prepared using polyurethane foam replication method to obtain appropriate cellular structure suitable for bio-application. Two types of the polyurethane foams were used (with average pore size 0.48 mm and 0.62 mm). Some of these samples were prepared by single or multiple infiltrations. The effects of structures, temperature of calcination, volume fraction of Si3N4 powder and number of the infiltrations on the local mechanical properties were investigated. The Youngs modulus of studied samples range from 12 to 46 GPa at the macroscopic scale measured by resonant frequency technique and from 10 to 28 GPa at the microscopic scale measured by instrumented indentation. Results showed increase of the hardness and Youngs modulus with increasing of the calcination temperature, with increasing of the number of infiltrations and also with increasing of volume fraction of Si3N4 powder in suspension. The results obtained from nanoindentation carry out lower values in comparison with the values measured by resonant frequency technique.
Authors: Miroslav Balog, Pavol Šajgalík, Zoltán Lenčéš, Miroslav Hnatko, Jozef Kečkéš
Abstract: Nano- and macro-hardness of SiC and Si3N4 based ceramic materials prepared by liquid phase sintering were evaluated. The applied loads were 3.5 mN and 9.81 N, respectively. The measurements showed that the nano-hardness of both ceramics is substantially higher compared to the macro-hardness. The influence of solid solutions and grain boundary composition on the hardness of SiC-based ceramics was studied. The macro-hardness is strongly dependent on the grain boundary composition while the nano-hardness was nearly the same for all tested samples with different Re2O3-AlN additives. In the case of Si3N4 based ceramics the SiC nano-inclusions content was varied. As a source of SiC nanoinclusions and grain boundary phase modifierSiNC polymer precursor has been used. Nano- as well as micro-hardness increased with increasing SiC content. Present paper deals with the explanation of both results.
Authors: D. Galusková, Miroslav Hnatko, D. Galusek, Pavol Šajgalík
Abstract: Corrosion resistance of solid state (SSS) and liquid phase sintered (LPS) alumina under hydrothermal conditions (subcritical water and sodium chloride solution at temperatures up to 290 °C, and pressures up to 7 MPa) was evaluated. The influence of sintering additives as well as the conditions of the test (temperature, corrosion medium) on dissolution of ceramics was studied. For evaluation of the corrosion mechanisms special attention was paid to determination of the eluate chemistry combined with the chemical and phase analysis of corroded surfaces. Corrosion of LPS alumina in both media was controlled by dissolution of calcium aluminosilicate grain boundary glass. Corrosion rates of SSS ceramics were several times lower and were largely controlled by dissolution of highly resistant alumina matrix. Corrosion of the ceramics was moderately faster in sodium chloride solution. The data are supplemented by calculations of phase equilibria in the corrosion solutions.
Authors: Dagmar Galusková, Miroslav Hnatko, Jozef Kraxner, Dušan Galusek, Pavol Šajgalík
Abstract: The corrosion resistance of liquid phase sintered (LPS) alumina ceramics in aqueous environments strongly depends on composition and chemistry of grain boundary glass formed during sintering. The chemical durability of model alumino-silicate glasses with various contents of CaO in aqueous solutions was therefore evaluated. Prepared glasses were corroded under hydrothermal conditions in deionized water under static conditions. The examination of surface morphology of corroded specimens after the contact with deionized water, together with the analysis of corrosion solution provided information on mechanism of dissolution of grain boundary glasses in LPS aluminas and confirmed that dissolution process is hindered due to saturation of solution with respect to leached elements. The initial dissolution rates for studied glasses were determined. The results are applicable for optimization and enhancement of corrosion resistance of LPS alumina under hydrothermal conditions.
Authors: Miroslav Hnatko, Štefánia Lojanová, Helena Pálková, Zoltán Lenčéš, Pavol Šajgalík
Abstract: Si3N4/SiC nanocomposite material with yttria as sintering additive was prepared by hot pressing method. SiC nanoinclusions were produced by in situ reaction between SiO2 and C during the sintering process. The homogenous microstructure of hot pressed samples contained fine Si3N4 grains with average diameter of 200 nm. The post-sintering treatment at 1750°C for 26 and 70 hours significantly changed the microstructure of Si3N4/SiC composites. The difference between the average grain size of hot pressed sample and sample treated for 70 hours is approx. 300 %. In sake of this fact the mechanical properties (KIC, HV1) were not changed significantly. The largest differences between the mechanical properties of hot pressed and annealed samples were only 8 %.
Authors: Monika Kašiarová, Ján Dusza, Miroslav Hnatko, Pavol Šajgalík
Abstract: Four-point bending strength and Weibull distribution of the strength values of a carbon derived Si3N4-SiC nanocomposite have been investigated. The fracture origins were identified and characterized and a “labeled Weibull plot” was constructed. The fracture origins are surface, subsurface and volume located technological defects with a dimension from 10 µm to 180 µm, mainly in the form of cluster of pores and large SiC grains.
Authors: Zuzana Pramuková Vilčeková, Monika Kašiarová, Magdaléna Precnerová Domanická, Miroslav Hnatko, Pavol Šajgalík
Abstract: The study deals with the development of highly porous undegradable ceramics based on silicon nitride as potential replacement of trabecular bone. These materials were produced using replication method with polyurethane foams as pore-forming agents to achieve similar porous structure to trabecular bone. Prepared porous ceramics had a bimodal pore structure with macro-pores larger than 200 μm and micro-pores smaller than 1 μm in diameter, which are necessary for tissue ingrowths, cell adhesion, adsorption of biological metabolites and nutrition delivery in organism. The microstructure and local mechanical properties (Young’s modulus and Yield strength) were evaluated and compared with human trabecular bone. Results showed that studied porous materials have satisfactory porosity and pore sizes for trabecular bone replacement. Young’s modulus of bone was 12.6 ± 2.23 GPa and porous silicon nitride samples ranged from 10.9 ± 3.38 GPa to 12.9 ± 1.13 GPa. The values of Yield strength of trabecular bone was determined as 493 ± 30.7 MPa and the values of porous samples varied from 250 ± 19.3 MPa to 558 ± 36.5 MPa. Young’s modulus and Yield strength increase with increasing of the pre-sintering temperature and multiple infiltrations.
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