Papers by Author: Miriam Kupková

Abstract: Fe-Mn alloys represent promising degradable biomaterials for temporary implants. To investigate the effect of corrosion on their mechanical properties, iron powders were mixed with 25, 30 and 35 wt.% of a manganese powder, compressed into prismatic bars and sintered. Sintered bars were immersed in Hank's solution for 8 weeks. The bending stiffness of each bar before and after its exposure to electrolyte was examined. The higher the porosity of a bar was, the higher relative reduction in bending stiffness the bar exhibited. A likely explanation was that in a more porous bar Hank’s solution penetrated deeper and affected larger volume fraction of bar’s material.

Abstract: Powders comprised of Fe particles and 25, 30, 35wt.% of Mn particles were mixed, compacted and sintered to investigate the effect of Mn on the properties of sintered Fe-Mn alloys. It was found that the sample’s swelling, microstructure and distribution of local hardness values were strongly affected by the Mn content. The particles in Fe-25Mn and Fe-30Mn samples exhibited a distinct onion-like structure causing a considerable variability in local properties, while the particles in Fe-35Mn samples were at a glance more homogeneous, with a large volume fraction occupied by a nearly uniform material with almost constant properties.

Abstract: Iron was considered a good material candidate for temporary implants in cardiovascular and orthopedic surgery. Mechanical properties of iron are attractive, however, a higher degradation rate is required. The contribution deals with the effect of silver content on microstructure and corrosion behavior of materials prepared from Ag coated iron powders. Using electroless deposition, Fe-powders with 0.29 and 2.1 wt.% of silver were prepared. Cylindrical specimens compacted at a pressure of 200 MPa were isothermally sintered at 1120°C for 60 min. The microstructure of the sintered specimens consisted of iron matrix with Ag-precipitates. The corrosion behaviour of sintered compacts was studied using the potentiodynamic polarization technique in Hank’s solution and complemented with SEM analysis. It was found out that corrosion resistance of material decreased with an increase in silver content.

Abstract: A simple oxidative polymerization of pyrrole (Py) directly onto the surface of iron (Fe) microparticles was applied to increase the content of carbon in resulting material. Detection and quantification of the PPy (polypyrrole) coatings obtained were performed by means of pyrolysis gas chromatography (Py-GC). A powder consisting of such particles was compacted. The effect of PPy coating on the compressibility of coated iron powder was analysed. Namely, a set of specimens was uniaxially pressed in a steel die. Compaction pressures ranged from 50 MPa up to 600 MPa. It was found that PPy coating has a positive effect on the compaction behaviour of iron powders in the low to moderate pressure region. At higher pressures, the brittleness of PPy coating adversely affected the compressibility. Both the light and the scanning electron microscopy (LM, SEM) were used to characterize the morphology of coated powders and the microstructure of pressed samples.

Abstract: When the depth-sensing (nano)indentation is applied to sintered samples, measured properties, which are expected to represent the material of an individual grain, seem to depend on the overall porosity of the macroscopic sample. To understand such a result, it is assumed that while the nanoindenter penetrates into the surface grain and probes the properties of its material, the grain itself serves as another, larger indenter indenting the rest of sample and probing the properties that represent the bulk of material rather than individual grains. Load vs. displacement curve reflects the synergetic response of these two “indenters” and so it contains information about the sample’s mechanical properties at both microscopic and macroscopic scales. Obtained theoretical results agree qualitatively with the experimental data (the dependence of the indentation modulus on the porosity of sample; the indentation size effect).

Abstract: The differences in mechanical properties of pressed and sintered specimens made from mixtures of iron and copper powders or from copper-coated iron powders, produced by a cementation process, were studied. For this purpose different copper contents were used (3, 8 or 12wt%) and the oxygen content of the coated powders was measured. After sintering at 1120°C for 60 minutes in hydrogen flow, microgradient structures were observed. The samples were investigated by light optical microscopy and tested under bending and tensile loads. Young´s Moduli were calculated from resonance frequencies. The copper-coating of the iron powder results in an improvement of all properties, owing to a more homogeneous copper distribution and the absence of large secondary pores, compared to specimens made from mixtures of iron and copper powders. In the case of Fe-12Cu (coated), all determined properties tend to result in a maximum: highest sintered density (7,33 g/cm³), tensile strength (489 MPa), transverse rupture strength (1098 MPa) and apparent hardness (162 HV10). The Young´s Modulus (150 GPa) of coated Fe-12Cu is nearly the same as that of sintered iron (154 GPa).

Abstract: Within a model considered, each of bonds between contacting grains is treated as a two-state system and represented by a binary variable. Its two values refer to the two possible states of bond – intact or broken. A Monte Carlo simulation of fracture is carried out on a set of binary variables arranged to a cubic lattice. The transition from one configuration of broken bonds to another is governed by a Griffith-like energy associated with each of configurations. The results demonstrate i) the capability of the model to provide a useful information (e.g. the increase in roughness of fracture surface with increasing temperature, that is the transition from “brittle” to “plastic” failure), and ii) the advantage of simulation by using the graphics processing unit (saving of a computational time).

Abstract: The existing density-pressure laws for a powder undergoing die compaction fit experimental data very well except for the low-pressure end of curves for some materials (SiC, Al2O3). To improve this, density - pressure relationship was modified by considering the internal friction and cohesion. For constant internal friction and vanishing cohesion, the modified relationship became mathematically equivalent to the existing one. For density-dependent friction, the modified and original relationships were able to provide nearly the same densities at medium and higher pressures but slightly different values at the low-pressure region.

Abstract: Irregular evolution of friction coefficient, recorded during the ball-on-disc test on Si3N4 based ceramic materials, was analysed by means of fractal geometry methods. Tests were carried out at room temperature, in air and without any lubricant. It was proven that the friction coefficient trace, considered as a geometric object, has the property of a fractal curve. The fractal dimension of this curve increased with increasing wear rate measured in a corresponding wear test. This could indicate the possible correlation between the wear rate and the fractal dimension of friction coefficient as a function of sliding distance (time).