Papers by Author: Michal Besterci

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: Equal channel angular pressing (ECAP) is a material processing method for developing an ultrafine-grained (UFG) structure by introducing severe plastic deformation (SPD) in a bulk material with no changes in its cross-section. Numerous analytical and numerical studies on equal channel angular pressing have been performed in recent years. The present work focuses on the effects of die geometry width is defined by the angle between two channels Φ, angle on outer corner of die Ψ (or radius R) and angle within internal corner (or radius r) of die on average effective strain after one pass route. Next, there are analyses of strength properties, plastic properties, fracture mechanism, as well as analyses of Cu structure evolution after SPD by ECAP technology, in the paper. The sixteen passes through the ECAP matrix were realized using route C. The following experimental results and their analyses, the biggest increase of strength and microhardness was proved already after 4th pass. Valuation of fracture surfaces shows that after 12th pass plastic fracture is transformed from transcrystalline ductile mixed fracture. After 4th pass, the avarage grain size decreased from initial approximate size 7 µm to 200 nm, whereby the average grain size was changeless after subsequent deformations. Possible mechanism of high-angle boundary nanograins evolution consists of formation of cell structure, subgrains that transform with the increase of deformation into nanograins with big-angle misorientation.