Papers by Author: Martin Kupka

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: 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).