Papers by Author: Jaroslav Sedláček

Abstract: The work deals with the observation of microstructure and crack propagation from macro-indentation in a set of newly developed materials. Complex composites based on SiC matrix with 30, 40 and 50 wt. % of additives (Ti and NbC) were hot pressed at 1960 oC in air atmosphere under 30 MPa pressure for 1.5 hour. The microstructure and chemical composition were studied by SEM equipped with EDX analyzer. Hardness of the composites was evaluated by means of classic Vickers macro-indentation and fracture toughness was determined by the Anstis method. Indentation cracks were observed and their propagation was analyzed. The electrical conductivity as function of volume fraction of additives was determined.

Abstract: SiC based composite with 50 % of additives (Ti and NbC with ratio of 9:16) has been prepared. The microstructure, porosity, and chemical composition were studied using SEM equipped with EDS analyser. Local mechanical properties such a hardness and elastic modulus of individual components of the composite were investigated by nanoindentation using Berkovich indenter tip. Hardness and fracture toughness of studied material as a whole was evaluated by means of classic Vickers macroindentation. Indentation cracks were observed and their propagation was analyzed. It was shown that the present phases were distributed uniformly. Moreover, final density was satisfactory with porosity lower than 1 %. The individual constituents shown similar elasticity modulus (550 - 590 GPa). Hardness (H_IT) exhibited very pronounced load-size effect. At 10 mN load, hardness was 42.33 GPa ± 1.1 GPa for SiC and 35.73 GPa ± 0.9 GPa for TiNbC, while at 500 mN the composite hardness was 27.61 GPa ± 0.505 GPa. It is in good agreement with macrohardness values, when 27.6 GPa and 25 GPa has been measured for 1 and 10 kg loads, respectively. Indentation fracture toughness was 3.3 MPa.m^1/2 ± 0.22 MPa.m^1/2. Electrical conductivity was measured by four point probes method and its value was 8.8×10⁴ ± 0.3×10⁴ Sm^-1.

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.