Papers by Author: Pavol Šajgalík

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.

Abstract: The present work studies the corrosion of three most widely used types of structural ceramics – silicon nitride, solid state sintered alumina and liquid phase sintered alumina – in 3 % aqueous solutions of sodium chloride at temperatures up to 290 °C and pressures up to 7 MPa. The corrosion of silicon nitride was controlled by attack of Si3N4 matrix grains, while yttrium oxynitride amorphous grain boundary phase was corrosion resistant. Corrosion of Si3N4 in reference media -distilled water - at 290 °C was characteristic by formation of passivation layer, which hindered further dissolution of silicon nitride matrix. The presence of sodium chloride resulted in formation of discontinuous layer of corrosion products, resulting in more severe corrosion than in distilled water. The corrosion of liquid phase sintered alumina was mainly attributed to congruent dissolution of SiO2 and CaO from grain-boundary amorphous film, which was accelerated at higher temperature, and accompanied by precipitation of siliceous phases from oversaturated solution at 200 °C. Pure polycrystalline alumina corroded by loss of alumina grains, which did not dissolve in the corrosion media. The corrosion impaired significantly the fracture strength of silicon nitride, creating new, corrosion related defects at the surface, while the influence of corrosion on fracture strength of polycrystalline aluminas was negligible.

Abstract: β-sialons prepared from pyrophyllite resp. kaoline were used for investigation of corrosion resistance in aluminium, NaF-AlF3 and NaCl-KCl mixture. The results were compared with β-sialon of similar composition, prepared from commercial powders. The differences in corrosion process of β-sialon prepared from natural hydrosilicates and synthetic powders are discussed.

Abstract: Mouth rim is a tableware quality issue perceived by customer in a very sensitive way. Investigation and evaluation of importance of main technological aspects were made in order to find optimal process set up ensuring high mouth rim efficiency. Process data, light microscopy and glass rim resistance against mechanical load were applied as methods allowing analysis of rim impact strength. Planned experiments were proposed to quantify the effect of parameters like glass wall thickness, set up of cracking off, edge melting and toughening process. Influence of glass fining process causing reboil visible only by light microscopy was found as one of the most important issues. Optimal process setup was proposed in both directions (i) technological set up and (ii) statistical process control.

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.

Abstract: The corrosion resistance of sialons made from commercial powders (AlN, Al2O3 and Si3N4) and from powder precursor produced by carbothermal reduction and nitridation of raw aluminosilicate (pyrophyllite) in molten steel were investigated. The corroded zone in sialon made from raw pyrophyllite (P1) is more then two times deeper compared to the corroded zone of sialon made from commercial powders (C1). The corrosion zone of sample P1 is on the average 610 μm deep, while in sample C1 it is only 260 μm. The main corrosion products are γ-Al2O3 and iron silicides. The phase compositions were estimated by neutron Rietveld refinement.

Abstract: In this study, effect of seed addition on the microstructural development of Y-Sm α-SiAlON system has been investigated. Besides using pre-produced rod-like α-SiAlON seeds, growth in the glassy phase and harvested by chemical treatment, powdered Y-Sm/α-SiAlON was also used as nuclei. Microstructural results of both systems were compared with un-seeded samples. It was found that the powdered α-SiAlON addition has also caused the acicular α-SiAlON grain formation, which indicates the dominant effect of thermodynamic stability on the preferential growth of α-SiAlON grains in the c direction.

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 %.

Abstract: Reaction bonded MgSiN2 (RBMSN) was prepared by direct nitridation of a Si/Mg2Si/Mg/Si3N4 powder compact in a temperature range of 1350-1550°C. The oxygen content of MgSiN2 was in the range of 0.4 – 0.6 wt%. A thermal stability examination showed that MgSiN2 is stable up to 1400°C at 0.1 MPa N2 pressure. The activation energy of decomposition calculated from the temperature dependence of weight loss is H = 383 kJ⋅mol-1. The time dependence and nitrogen pressure dependence of MgSiN2 decomposition was also investigated at constant temperature. MgSiN2 is stable at 1560°C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs free energy of formation of MgSiN2 was calculated in a temperature range 300-2500 K. Dense MgSiN2 ceramics or MgSiN2/Si3N4 composites with fluorine-based additives were prepared by hot pressing. The composite materials had a 4-point bending strength of 427 MPa and Vickers hardness (HV1) of 20.8 GPa, respectively. The indentation fracture toughness was 5.3 MPa.m1/2, due to the presence of elongated β-Si3N4 grains. The dielectric constant of dense reaction bonded MgSiN2 at 100 kHz was 9.5-10, while that of MgSiN2/Si3N4 composite in a wide range 50 – 6000, depending on composition and heat treatment.