Papers by Author: Peter Tatarko

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: Influence of rare-earth oxide additives on the strength, fracture toughness and tribological behaviour of hot-pressed Si3N4 and Si3N4/SiC micro/nano-composites has been investigated. Four-point bending mode and ball on disc methods have been used for strength and wear tests and Single-Edge V-Notched Beam, Chevron Notched Beam, Indentation Strength and Indentation Fracture techniques for fracture toughness measurement. Fractography has been used to characterize strength limiting defects, fracture micromechanisms and damage mechanisms during the wear test. The strength values were strongly influenced by the present processing flaws. Wear behavior is significantly influenced by the chemical composition and by the microstructure of the materials.

Abstract: Microstructure and mechanical properties of Si3N4 and Si3N4 + SiC nanocomposites sintered with rare-earth oxide additives (La2O3, Y2O3, Yb2O3 and Lu2O3) have been investigated. The composites exhibited smaller grain diameter compared to that of monolithic materials. The aspect ratio of β-Si3N4 grains increased with a decreasing ionic radius of rare-earth elements in the Si3N4 monoliths as well as in the Si3N4-SiC nanocomposites. The hardness of both systems increased with a decreasing ionic radius of rare-earth element. The fracture toughness of the materials with coarser microstructure and higher aspect ratio was higher due to the more frequent toughening mechanisms. No significant difference between strength values of monoliths and composites was observed and the strength in the composites was determined mainly by the present processing flaws. Significantly improved creep resistance was observed in the case of composites and for materials with smaller ionic radius of RE3+.

Abstract: Fracture toughness of hot-pressed silicon nitride and Si3N4+SiC nanocomposites prepared with different rare-earth oxides (La2O3, Sm2O3, Y2O3, Yb2O3, Lu2O3) sintering additives have been investigated by Chevron Notched Beam, Indentation Strength and Indentation Fracture techniques. The fracture toughness values of composites were lower due to the finer microstructures and the lack of toughening mechanisms. In the Si3N4 with higher aspect ratio (Lu or Yb additives) crack deflection occurred more frequently compared to the Si3N4 doped with La or Y, which was responsible for the higher fracture toughness.