Papers by Author: Dušan Galusek

Abstract: The decrease of the mechanical properties – hardness and reduced elastic modulus after corrosion in white wine was measured. Under static corrosion conditions no significant decrease was observed up to 8 hours of corrosion. Dynamic corrosion conditions cause detrimental decrease of properties (one order of magnitude) compare to the results of static corrosion test. This is due to the removal of the harder outer layer of the enamel during polishing. To obtain a relevant data concerning corrosion test, natural surface of a tooth should be investigated and tested.

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: The work reports on the preparation and properties of binary alumina-rare-earth oxide glass microspheres with high contents of aluminium oxide. Because Al2O3 is not a typical glass former, preparation of these glasses in bulk is difficult due to high melting temperatures and high tendency to crystallisation, which requires high cooling rates. One of the possibilities is preparation of glass microspheres by flame synthesis and rapid quenching of microspheres by spraying them with water. Microspheres with dimensions ~10 µm and with various compositions from the systems RE2O3-Al2O3 (RE = Y, Yb, La) have been prepared with the use of an in-house built equipment. The prepared microspheres have been characterised by SEM-EDX, IR, XRD and DTA.

Abstract: The paper gives a brief overview of polycrystalline alumina ceramics, including the solid-state sintered and ultrafine grained materials, and the liquid phase sintered aluminas. The influence of glass-forming sintering additives of commercial interest (MgO, CaO, SiO2) and processing conditions on microstructure development of polycrystalline alumina ceramics are discussed in more detail. The influence of grain size, the presence and composition of grain boundary glass, and of secondary crystalline phases in partially crystallized triple pockets on stress state in alumina is discussed.

Abstract: The Hertzian indentation technique has been evaluated as a possible means of assessing the surface fracture behaviour of a set of qualitatively similar liquid phase sintered polycrystalline aluminas with systematically varied composition, and a range of grain sizes. The method provides a better correlation between microstructure characteristics and surface fracture behaviour than does Vickers indentation. The fracture toughness determined from Hertzian indentations was found to be a function of grain size, irrespective of the composition of the material. The cumulative probabilities of formation of ring cracks at specific loads during indentation with a spherical indenter were determined for each material from the set of 25 measurements, and the parameters of distribution of the probability of fracture were calculated. A statistical correlation between the failure probability distribution and the size distribution of alumina grains was detected.

Abstract: The microstructure evolution and mechanical properties of alumina doped with rare earth oxide and rare earth-aluminosilicate glass has been investigated in order to understand better the impact of rare earth addition on hardness and fracture toughness. The aims of the present work are to examine the possibly beneficial effects of selected rare earth oxide added to alumina by two different routes in enhancing mechanical properties. The specific systems considered in this work were as follows: liquid phase sintered alumina containing RE2O3 (RE = La, Y) and RE2O3 – Al2O3 – SiO2 glassy phase up to 5 wt. %. The Vickers hardness and fracture toughness have been compared with the undoped alumina.