Ivan DERMENDZHIEV’s research while affiliated with Angel Kanchev University of Ruse and other places

The analysis of nanomechanical properties of the materials for biological applications has become an increasingly useful tool on investigation of these materials. Although the biocompatibility of Ti has been confirmed, it is difficult to meet all the requirements, such as antibacterial ability, osseointegration and mechanical properties. Titanium alloys have a high friction coefficient while interacting with bone or tissue, which can cause wear debris pain and loosening of implants.This situation could be avoided using alloy surface modification by a durable, mechanically stable, wear resistant (Ti,Al,V)N coating with a low friction coefficient as a base for an overlaying nanostruc-tured ТiOх. This article represents the research on the influence of the process of vacuum arc deposition of (TiAlV)N coatings and next vacuum oxidation on their mechanical properties and morphology. The microstructure, mechanical properties and applicability of the investigated (Ti,Al,V)N/TiO x coatings for biocompatible titanium implants are discussed.

Abstract: An optimal way of heat treatment for plastic mould tool steels with higher heat resistance is proposed. The combination of the tempering process and nitriding or oxy-nitrocarburizing process that enhance surface properties contributes to high hardness, wear resistance and tough strength. Key words: tool steels, heat treatment, nitriding,oxy-nitrocarburizing.

Abstract. The vacuum oxy-nitrocarburizing (ONC) technique was used for carbonitriding tool steels in order to compare the effect of the reinforced thermal treatment (TT) to the properties of the formed layers. This method is more economical than others because it provides for faster nitrogen, carbon and oxygen diffusion at low pressure condition, which in turn allows for lower processing gas consumption and/or shorter treatment times with satisfactory results. The samples was air cooled in the vacuum chamber in N2 atmosphere and after that they were surface induction quenched in oil and low temperature tempered. The steels were characterized by optical microscopy, X-ray diffraction (XRD) and microhardness indentation. Our results showed that it is possible to change the microstructural, microhardness and phase profiles of the oxy-nitrocarburized steels by using different modes of thermo-chemical and/or thermal treatment. Thus the fatigue limit and surface wear resistance properties could be enhanced. Keywords: reinforced thermal treatment, oxy-nitrocarburizing, tool steel, microstructure, microhardness, wear resistance, fatigue limit.

Аbstract: Surface treatments used in daily manufacturing of parts are selected to serve functional and decorative requirements achieved by mass production. Within the last decade, improved and new deposition techniques were developed in PVD and thermochemical heat treatment. Generating optimized surfaces for different types of substrate materials (e.g. Al-alloys, case hardened steels, etc.) and geometries (e.g. bores) also impacts the running costs. That’s why in the present study the appearance, surface roughness and some properties of oxy-nitrocarburized and PVD-coated 9XC alloyed tool steel are discussed and compared. Keywords: PVD; vacuum oxy-nitrocarburizing; hardness; wear-resistance; friction; decoration

Abstract: In this study the changes in HU values in dependence with process parameters and the layers’ sequence deposition – carbide and nitride, are studied. The multilayered coatings are condensates on 9XC and X12M alloyed tool steels’ substrates. The coating kinds are TiN/TiC/…and TiC/TiN/…. The latter are prepared in vacuum chamber by magnetron sputtering using VT3-1 target and consecutive introducing CH4 и N2 reactive gases. A computerized Nanoindentor is used for the hardness measurements.

This study deals with the microstructure and lattice parameters of vacuum oxynitrocarborized pure electrolytic iron. The vacuum oxynitrocarborization treatment is performed to improve the wearing-out and corrosion resistance of pure electrolytic iron and AISI 1045 (C45, DIN1.1191) steel as a reference. The vacuum oxynitrocarborization has been carried out for a period of 7 hours at 550°С in ammonia and carbon dioxide atmosphere. The pressure in the vacuum chamber has been 8.104 Ра for the first 5 hours and 1.105 Pa during the last 2 hours. The micro-structural and micro-hardness changes in the iron and in the steel аre studied. Compound and diffusion layers in the oxynitrocarborized electrolytic iron and AISI 1045 steel are observed. The phase composition and structure of the both zones are discussed. A higher diffusion activity of the penetrating elements in the iron as compared to that of the steel is found out. The amount of the Fe4N phase in the surface layers of the iron is proved by X-ray diffraction analysis to be increased as compared to that in the steel. The ε-Fe2-3(N,C) phase formation is not influenced by the carbon content in the AISI 1045 steel. The presence of impurities could not cause alteration in α- and ε-phase parameters or it could not be detected by the X-ray diffraction analysis due to their low concentration. The iron hardness measured on the surface remains unchanged after the process (before oxynitrocarborization HV1=600, after - HV1=630), while the steel hardness remains medium. However, iron gains a corrosion resistance.