Synthesis and characterization of hafnium boride-based ceramic powders

Hafnium boride is a strategic material as one of the leading members of ultra-high temperature ceramics. In recent years, there has been an increasing research interest in transition metal diborides to meet the need for high temperature materials that can withstand extreme environments. These extreme conditions can include the effects of high temperature, oxidation, mechanical stress, radiation and wear. Hafnium and zirconium diborides are identified as the most promising candidates for high temperature applications due to their relatively good oxidation resistance compared to other refractory ceramics. Hafnium diboride has a melting point of 3380 °C, which is higher than many ceramic materials and also higher than few advanced ceramics whose melting temperatures are over 3000 °C. Additionally, it has remarkable properties including very high hardness, low thermal expansion coefficient, chemical inertness, high thermal and electrical conductivities. It exhibits a high oxidation resistance as a consequence of the stability of the HfO2 layer formed on its surface by oxidation at high working temperatures. Some potential applications for hafnium diboride include furnace elements, plasma arc electrodes, refractory linings, molten metal containments, wear resistant coatings, control rods and heating shields in nuclear reactors. However, majority of recent research has been stimulated by unmet material needs for thermal protection systems in extreme environments especially for sharp leading edges of high velocity flights or atmospheric re-entry vehicles. Hafnium diboride powders have been prepared by a variety of production methods. The conventional synthesis methods of hafnium diboride include high temperature borothermal/carbothermal reduction routes over 1500 ºC. Apart from the conventional methods, diverse production techniques including reaction between elemental powders of Hf and B, solid-state boron carbide reduction methods, self-propagating high-temperature synthesis (SHS), sol-gel method, hydrothermal/solvothermal synthesis, chemical vapor deposition (CVD) and milling-assisted processes have been applied. The conventional borothermal and borothermal/carbothermal reduction methods of HfO2 in which amorphous boron and B4C are used as boron and boron/carbon sources, are generally carried out at elevated temperatures above 1500ºC. Expensive equipment requirements and relatively high working temperatures are main drawbacks of these methods. In addition, large particle sizes induced by high synthesis temperature constitute some drawbacks during subsequent consolidation operations. Elemental powders have also been used to produce bulk HfB2 ceramics by spark plasma sintering (SPS) and dynamical consolidation techniques. Although, the reaction between elemental hafnium and boron is thermodynamically more favorable, it is not a cost-effective method due to the use of expensive raw materials. Also, solution-based methods have been applied to synthesize nano-sized HfB2 powders with low oxygen content and they offer homogeneous dispersion of different raw materials. However, high temperatures are still needed to ensure following carbothermal reduction. Furthermore, the CVD methods are generally employed with the aim of obtaining hard surface coatings rather than powder synthesis in order to improve the surface hardness and tribological properties of desired materials. Additionally, single crystals of HfB2 were prepared by floating zone method by means of radio frequency (RF) heating. Furthermore, some attempts were made by milling-assisted annealing processes: HfO2-HfB2 composite powders were produced by high-energy ball milling of HfO2-B2O3-Mg powder blends. In addition, preparation of HfB2 nanorods from HfCl4-B and HfCl4-B-Mg powder mixtures by mechanical activation and isothermal annealing were suggested. The lowest synthesis temperature was reported as 600 ºC in the case of utilizing a hydrothermal method in an autoclave. However, usage of relatively few amounts of reactants (in order of mmol) and small capacity equipment restrict the application of the process. In these studies, HfO2 and HfCl4 have been used as the most common hafnium sources depending on their properties in different synthesis methods. The HfCl4 has been extensively utilized in recent production methods such as low temperature hydrothermal/solvothermal synthesis, sol-gel and CVD methods with the aim of benefiting from its low boiling point and higher reactivity over HfO2. Moreover, most commonly used boron sources in various production methods are amorphous B, B2O3, H3BO3, NaBH4 powders and BCl3 gas. In this study, it was aimed to prepare hafnium diboride powders with novel methods provide several advantages such as usage of economical raw materials, utilization of simple equipment, reduced or low reaction temperature and scalable processes. The mechanically activated borothermal reduction route was applied to synthesize high purity and submicron size HfB2 powders from HfCl4-B powder blends. In addition, HfB2-HfO2 and HfB2 powders were synthesized by mechanochemical synthesis at room temperature in nano-scale from HfCl4-B2O3-Mg and HfCl4-B-Mg powder blends for the first time in the literature according to the best of our knowledge. In addition, mechanical activation was applied to HfCl4-B2O3-Mg precursors to enable the reaction at lower temperatures during the autoclave synthesis. Mechanical activation induced by high-energy ball milling enables to reduce synthesis temperature during subsequent heat treatment by the formation of active reactant particles and their homogeneous distribution throughout the microstructure. Thus, in the borothermal reduction and autoclave synthesis routes, mechanical activation is applied to reduce the synthesis temperatures and particle size of the final powders, and to increase the process efficiency. Also, low temperature autoclave processing was utilized to synthesize high purity and nano-scale HfB2 powders from starting powders blends of HfCl4-NaBH4-Mg. Although solvothermal autoclave synthesis methods are quite new methods for preparation of the boride compounds, they provide some advantageous which are control of size and morphology and preparing nanopowders in rod, cube or sheet morphologies Moreover, autoclave synthesis is considered as an environmentally friendly method because the process proceeds in a closed/isolated system and it saves energy by low synthesis temperature. In addition, the utilized powder systems were thermodynamically interpreted by the FactSageTM 6.2 thermochemical software to predict the reaction mechanisms. The detailed characterization of synthesized powders were performed using X-ray diffractometer (XRD), particle size analyzer (PSA), stereomicroscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS) and transmission electron microscope (TEM). The synthesized hafnium diboride-based powders were consolidated by two different sintering techniques. The cold pressing/pressureless sintering (without and with Co addition) and spark plasma sintering techniques were applied to selected powders which were obtained by optimum process parameters. The densification behaviour and microstructural characterization of the sintered samples were investigated using density measurement by Archimedes method and XRD, SM, SEM/EDX techniques. Mechanical properties of consolidated products were determined in terms of microhardness and wear properties. Finaly, I would be honoured if this dissertation can make a contribution to convert our boron sources into technological products with high added value.