Vanadyum ve niyobyum borürlerin öğütme destekli katı hal sentezleme yöntemleri ve sinterleme teknikleri ile üretimi

The development of high technological boron products has attracted an increasing interest in the last half-century due to the demand for new materials having unique properties for special applications. Transition metal borides are the class of high-tech boron materials with attractive properties for diverse engineering applications where conventional materials cannot possess the rising requirements. The borides of vanadium and niobium have been characterized by the combination of high melting point, high hardness, high strength, good electrical and thermal conductivity, high chemical and thermal stability, low work function, good thermal shock resistance and corrosion resistance. This combination of excellent properties permits their applications in many areas such as mechanical industry, chemistry and microelectronic fields. They are recognized as the candidates for high-temperature structural applications including surface and thermal protection, refractory crucibles and plasma arc electrodes. Vanadium borides as coating materials on various steels and as wear resistant materials have been reported in many studies. Vanadium borides are promising for use as the anode material for air batteries due to their exceptionally high discharge capacity. Niobium borides are known as refractory coating materials that have excellent oxidation stability at high temperatures. It was also reported as an electrode for use in the refining of aluminium and as a current-conducting element in contact with molten aluminium in electrolytic cells. Non-stoichiometric compounds of niobium boride have been discussed in many reports as potential superconductors. Many production techniques have been applied in the preparation of vanadium and niobium borides. They have been fabricated conventionally by high temperature methods such as borothermal/carbothermal reduction processes, direct solid-phase reactions, arc melting or chemical vapour deposition. The recent techniques developed for preparing vanadium and niobium borides are self-propagating high-temperature synthesis (SHS), low-temperature synthesis in an autoclave and ball milling and mechanochemical synthesis. Even though many studies are available for the fabrication of vanadium and niobium boride powders, sintering processes of them have not been a well discussed topic in the literature. Recent studies are mainly focused on the sintering of niobium borides using various techniques including high pressure sintering and spark plasma sintering, however, these studies have been mostly carried out in order to investigate superconducting behaviour of niobium borides. Only a few studies reported the consolidation behaviour and mechanical properties of the sintered vanadium and niobium boride and their composites. However, it is significant to improve an effective fabrication technique of the sintered boride bodies and to investigate their consolidation behaviour and mechanical properties for determining their application areas. On the other hand, the use of high temperatures, expensive starting materials (elemental V/Nb or B) and/or complicated equipment may restrict the fabrication process to obtain vanadium and niobium borides for the demanded applications. Conventional techniques have some general restrictions such as high temperature operation, the formation of by-products and grain growth during processing. One of the major aims of this dissertation is to find an effective technique for preparing metal borides that could overcome the limitations of conventional productions and represent the advantages of time and energy savings, simplicity, low-cost raw materials and high-purity products. Milling-assisted solid-state methods utilized in this dissertation involve inducing chemical reactions in powder blends at room temperature or lower temperatures than thermodynamically required. The primary purpose of this dissertation is the preparation of vanadium and niobium boride and niobium boride-based composite powders in high-purity through economical ways from their oxide raw materials (V2O5, Nb2O5) and native boron source (B2O3) in presence of reducing agents (Mg, C). The powder preparations were utilized using two different techniques including mechanochemical reactions using oxide raw materials/Mg reductant and milling-assisted carbothermal reactions using oxide raw materials/C reductant. The consolidation of the obtained powders by different techniques was also intended in order to investigate the sintering behaviours, microstructural and mechanical properties of the final samples. In overall, detailed characterization investigations of the obtained boride and boride-based powders and their sintered products were carried out by means of various analysis techniques. Firstly, the fabrication of vanadium boride powders was investigated through the mechanochemical reactions of V2O5-B2O3-Mg powder blends in terms of varying milling times and B2O3 or Mg contents. Nano-sized vanadium boride powders having VB2-VB-V3B4 phases were obtained in high purity without any intermediate phase and any impurity. Secondly, the fabrication of niobium boride powders was investigated through the mechanochemical reactions of Nb2O5-B2O3-Mg powder blends in terms of varying milling times, considering thermal behaviours and applying an additional annealing process. Niobium boride powders having NbB, NbB2 and Nb3B4 phases in various amounts and single phase NbB powders were successfully synthesized with high-purity and nano-size. Thirdly, the fabrication of niobium boride-niobium carbide composite powders was investigated by mechanical milling of Nb2O5-B2O3-C powder blends and subsequent annealing processes in terms of varying milling times, annealing temperature and C content. NbB2-based composite powders comprising various amounts of NbC were fabricated via a carbothermal route in high-purity at reduced temperatures due to the effect of mechanical milling of stoichiometric powder blends. Finally, sintering of the obtained vanadium and niobium boride powders in high qualities was carried out by cold pressing/pressureless sintering (without or with metallic Co addition) and spark plasma sintering (SPS) techniques. The results were discussed regarding the effect of different production conditions and sintering techniques on the consolidation behaviour, microstructure and mechanical properties of the bulk samples. Bulk vanadium boride and niobium boride products fabricated by SPS and bulk niobium boride products fabricated by activated pressureless sintering exhibited high density, high hardness, high wear resistance and high friction coefficients. Furthermore, high elastic modulus and high fracture toughness values were obtained for the samples fabricated by activated pressureless sintering. This dissertation work contributed the first results to the literature on the preparation of pure vanadium and niobium boride powders via mechanochemical route of oxide starting materials. This work also revealed the first scientific data on the in-situ formation of NbB2-NbC composite powders via milling-assisted carbothermal route from oxide starting materials at decreased temperatures due to the effect of mechanical milling. Furthermore, the consolidation and characterization studies of the sintered vanadium and niobium borides also contributed to the literature in terms of sintering behaviour and mechanical properties. In conclusion, the significant outputs of this dissertation were obtaining the high-technology boron products from low-cost raw materials and creating of added value by using the native boron sources.