Papers by Author: Ján Dusza

Abstract: The influence of 1 wt.% and 2 wt.% of graphene platelets (GPLs) addition on indentation fracture toughness (IF) of aluminium oxide (Al₂O₃) and silicon nitride (Si₃N₄) based composites has been investigated and compared to the monoliths. Ceramic composites reinforced with GPLs were prepared using hot-press processing technology. Microstructures were observed at fracture surfaces by scanning electron microscopy (SEM). Crack type identification was performed by gradually polishing of the indentation surface and mechanical properties of both systems were measured. Indentation fracture toughness was calculated by various methods and R-curves were prepared. The main activated toughening mechanisms, responsible for the increased fracture toughness are crack deflection, crack branching and crack bridging in the forms of graphene sheet pull-out or graphene necking.

Abstract: The scratch resistance of the silicon nitride with the addition of 1 and 7 wt% of graphene multiplatelets prepared by hot press sintering has been studied. The scratch resistance behaviour of Si₃N₄-GNP composites were investigated using a Rockwell indenter for normal applied loads ranging from 1-150 N. Si₃N₄-7-wt%GNP composite behaved differently during the scratch test depending on the normal applied load. The coefficient of friction changed dramatically at higher load and extensive crack propagation resulting in the chipping is observed.

Abstract: The influence of microstructural variations on the tribological properties and nanohardness of liquid phase sintered silicon carbide (LPS SiC) has been observed. In order to modify the microstructures samples were further heat treated at 1650°C and 1850°C for 5 hours to promote grain growth. The depth-sensing indentation tests of SiC materials were performed at several peak loads in the range 10-400 mN. The pin-on-flat dry sliding friction and wear experiments have been made on SiC ceramics in contact with Al₂O₃ ceramic ball at 10-50 N loads in an ambient environment. The nanohardness of samples with plate-like microstructure was about 34 GPa i.e. 3 GPa higher than nanohardness of SiC with fine globular microstructure. The SiC materials with coarser plate-like microstructure had similar COF (0.4-0.55) and better wear resistance (one order of magnitude at normal forces 10-20N) than SiC materials with fine globular microstructure.

Abstract: The room temperature elastic and plastic properties of the WC grains of WC-9%Co hardmetal were investigated by nanoindentation and atomic force microscope (AFM). Two easily distinguishable crystallographic planes (using EBSD analyses) were investigated, namely the basal and prismatic planes, on which nanoindentation tests were performed with different applied loads from 1 mN to 50 mN to determine the hardness and reduced-modulus, respectively. The results deriving from nanoindentation show significantly higher indentation hardness on basal planes (H_IT=28.9±0.1 GPa) than on prismatic ones (H_IT=21.9±0.1 GPa) over 10 mN load. For loads below that the results were inconsistent. The corresponding indents were checked by AFM and correct values of hardness were found. The discrepancies indicate the inaccuracy of the built-in evaluation procedure (Oliver-Pharr method) in this low load, or more precisely in the low indentation depth range. It is pointed out that below 50 nm contact depth the applied built-in contact area-contact depth function is not appropriate and a new correct contact area-contact depth function is proposed, thus the resulting recalculated hardness values are in good agreement with the AFM measurements.

Abstract: The nanohardness of WC – Co hardmetals has been investigated using instrumented indentation and Berkovich tip indenter. The nanohardness, HIT, and indentation modulus, EIT, of Co phase and individual WC grains and the influence of their crystalographic orientation have been studied. SEM, AFM and EBSD methods were used for the characterization of the microstructures and indents and for the identification of crystallographic orientation of WC grains, respectively. Strong indentation load-size effect and significant influence of the crystallographic orientation of WC crystals on HIT and EIT have been found. The nanohardness of Co binder was approximately 10 GPa and that of WC grains varied between 25 and 50 GPa, depending on the grain orientation and load. The nanohardness values of the basal and prismatic planes of individual WC grains at load of 10 mN were 40.4 ± 1.6 GPa and 32.8 ± 2.0 GPa, respectively.

Abstract: As the hardest and one of the most durable load bearing tissues of the body, enamel has attracted considerable interest from both material scientists and clinical practitioners due to its excellent mechanical properties. The aim of this investigation is to determine the influence of different loading conditions on the deformation behavior of human enamel using instrumented indentation and Berkovich indenter. The used samples were fresh intact human premolars, extracted due to orthodontic reasons. Hardness tests were performed with different loading regimes. To study the influence of loading rates tree different loading rates have been used with 10, 100 and 1000 mN/min. at maximum applied load of 200 mN. The indentation size effect (ISE) was studied using loads from 5 mN to 400 mN without holding time. The indents have been studied using atomoc force microscopy and scanning electron microscopy.

Abstract: Mechanical properties of porous silicon nitride prepared by two different processing routes have been studied. Depth sensing methods was used to measure the hardness and elastic modulus of experimental materials. The results were compared with the hardness and elastic modulus of trabecular bone in order to find out porous ceramics with properties close to that of trabecular bone. Material prepared by infiltration of polyurethane sponge exhibited properties close to the properties of bone and it is the potential material for further investigation in the bioapplication field.

Abstract: Tribological behavior of ZrO2 and Si3N4 based nanocomposites with addition of carbon nanofibres and nanotubes has been studied by the pin-on-disc technique. Friction coefficients were measured and recorded, wear rates were calculated in terms of material volume loss per load and sliding distance. The wear damage was studied using optical and electron microscopy and its mechanisms were identified. In monolithic materials the dominant wear mechanism was abrasion, in composites with CNF and with higher volume fraction of CNTs (5 and 10%) fiber pull-out and lubricating by the carbon phases occurred.

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+.