Papers by Author: Pavol Hvizdoš

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: Several bone cements were prepared in two ways of mixing (manual and vacuum bowl). Wear behavior, friction coefficient were studied by ball on disc method. Nano-hardness and Young`s modulus was studied by instrumented indentation. Obtained results were summarized by taking into account their way preparation, antibiotics content and testing conditions. There was found no significant time dependence of saline acting on Young`s modulus and nano-hardness values. Friction coefficient in saline was less than half in compare to dry sliding conditions

Abstract: Four materials Ti_xTa_1-xCN-20%Co of two chemical composition (x=0.9 and 0.95) and two high energy milling methods have been prepared. Nanoardness and elastic modulus for microstructure as a whole and both phases (matrix and hardmetal grain) were obtained (Tab. 1). Instrumented indentation was cerried out on the nanoindentation equipment TTX NHT (CSM instruments). Single load mode was used. Maximum applied loads of 20 and 50 mN for individual phases and 300 mN and 400 mN for microstructure as a whole were used.

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: Various bone cements with different compositions were prepared in several model configurations which differed in volume (i.e. setting temperature profile) and mode of mixing (manual and vacuum bowl). Local mechanical properties (nanohardness, elasticity modulus) of all experimental states were measured by nanoindentation technique and compared. Role of the preparation route as well as influence of composition, particularly of presence of antibiotics, on the measured properties was investigated.

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: Local mechanical properties of high Cr-alloyed sintered and cast steels with the same chemical composition were investigated using instrumented indentation method. Standard loading/unloading mode was applied, the measurements were done in load range 1 – 500 mN. Load size effect was observed and its parameters were evaluated. Indentation hardness and elastic modulus were found slightly higher for the sintered material. Differences in indentation parameters were explained based on microstructure of materials.

Abstract: Basic mechanical and wear properties of a commercial copper based composite Glidcop were studied. A Glidcop AL-60 grade (with 1.1 wt.% Al2O3) was used as the initial material. It was further treated by the Equal Channel Angular Pressing (ECAP) process in order to induce massive plastic deformation and to achieve very fine grained microstructure. Both, as-received and ECAP-ed materials were then characterized and the results compared. Hardness and elasticity modulus of the experimental materials were measured by instrumented indentation. Tribological properties were studied by pin-on-disk technique in dry sliding against a steel ball at a various temperatures from room temperature up to 600 °C. For all systems the coefficient of friction and specific wear rates were evaluated. Worn surfaces were studied by scanning electron microscopy and level of oxidation was measured using EDX spectrometry. It was found that above 200 °C the coefficient of friction decreased by about 50 %. The wear resistance with increasing temperature increased due to formation of harder oxide rich surface layer. Damage mechanisms were identified and their relationship with structural characteristics was inferred.

Abstract: Alumina and both tetragonal and cubic zirconia based composites with various volume fractions of constituents as well as with addition of carbon nanofibers were prepared by EPD. Mechanical properties (hardness, Youngs modulus) were measured by depth sensing indentation methods and related to chemical composition. Tribological behavior was studied using pin-on-disc technique at room temperatures in air at dry sliding. Coefficient of friction and wear rates were measured, the types of wear regimes were observed and damage micromechanisms identified.