Mechanochemical synthesis and characterization investigations of some refractory metal silicides

In recent years, metal silicides have attracted growing interest due to their unique properties such as high melting temperature, high temperature oxidation stability, high temperature strength and excellent diffusion-barrier characteristics. In regard of these properties, they have gained the attention of researchers for mainly two different usages as coating and high temperature structural materials. Good thermal stability, relatively low electrical resistivity and being good contact material for silicon-based materials make metal silicides important for coating materials in microelectronic applications. Additionally, high melting temperature, relatively low density, high temperature strength and good chemical and oxidation resistance make them potential candidates for high-temperature structural applications such as turbine engines and heating elements. The first commercial application of metal silicides was heating elements of MoSi2 based composites which was patented in 1956 by Kanthal. After that, researchers have mainly focused on molybdenum silicides and their composites among metal silicides. Tungsten silicide is the most resembling silicide to MoSi2 in terms of its crystal structure. However, its properties are slightly different. WSi2 can also be regarded as a promising material highlighted by high strength at elevated temperatures accompanying with its high melting point, its ability to form a SiO2 layer on its surface at high temperatures and its great corrosion resistance. In addition to these silicides, niobium silicide exhibits encouraging properties for many applications which require high temperature oxidation resistance under severe conditions. The most common preparation method for transition metal silicides is solid-state reactions resulting from directly heating of elemental powders at high temperatures. This process generally requires prolonged dwell times and high temperatures like 1200 ºC. Other methods successfully utilized for producing metal silicides are mechanical alloying (MA) and self-propagating high temperature synthesis (SHS). MA, as an advanced powder metallurgy technique, is a relatively simple and a cost effective mass production method at room temperature. Moreover, MA can be an efficient route to produce metal silicides by means of lower temperatures and shorter processing times than commercial methods due to repeated fragmentation and re-welding of powder particles. When the MA process is terminated with exchange/reduction reactions of the initial powders, the process is also termed as mechanochemical synthesis (MCS). In the literature, the synthesis of metal silicides usually conducted by MA of elemental powders which requires long ball milling times. In almost all studies on the fabrication of metal silicides, elemental powders were used as the initial materials. MCS process may reduce the milling time for reactions and provide more efficient production route in terms of using relatively cheaper starting materials (powders) compared to elemental powders. It is also known that MCS process usually yields to nano-sized particles. Therefore, synthesis of metal silicides by an MCS method may give promising results for their applications both at elevated temperatures and in microelectronic industries. In this dissertation, tungsten, molybdenum and niobium silicide powders were fabricated using a combined method of MCS and leaching process. The consolidation and sintering of selected powder compositions after MCS and purification were conducted by spark plasma sintering (SPS). The primary goal of this dissertation work was to synthesize tungsten, molybdenum and niobium silicide powders from their oxide starting powders (WO3, MoO3 and Nb2O5), silica (SiO2) and a reducing metal (Mg). The thermodynamic predictions of desired reactions for metal silicide formations were also studied and discussed together with experimental results to have a better understanding of the constituted phases and their reaction mechanisms. Different milling durations and excess initial additions were used as process parameters. The synthesis studies of tungsten silicide powders were also performed using other starting systems to achieve selected initial powder efficiency. Detailed characterizations of synthesized powders were carried out using an X-ray diffractometer (XRD), stereomicroscope (SM), differential scanning calorimeter (DSC), scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (SEM/EDS), particle size analyser (PSA), transmission electron microscope with energy-dispersive X-ray spectroscopy (TEM/EDS). The consolidation behaviours of synthesized metal silicides were also studied using different sintering temperatures. The microstructure and mechanical properties of the bulk silicide samples were characterized via optical microscopy (OM), XRD, SEM techniques, and calculations of Archimedes density, Vickers microhardness and fracture toughness values. Firstly, the mechanochemical synthesis studies of tungsten silicide powders were investigated in the WO3-SiO2-Mg starting powder system in terms of various milling time and excess reactant amounts. After MCS (milling of 20 min for reaction), the stoichiometric initial batch resulted in WSi2, W5Si3, W, MgO and Mg2SiO4 phases. Optimizing excess initial additions provided to synthesize mainly WSi2 structure in addition of small amounts of W5Si3 after MCS and leaching. The TEM investigations revealed that the average particle size of synthesized tungsten silicide powders was 97 mm and each tungsten silicide particle had coated with a SiO2 layer. In addition, the MCS of tungsten silicides were also investigated using, WO3-Si-Mg, W-SiO2-Mg and W-Si systems to compare their efficiency and resultant phases. The excess Mg added WO3-Si-Mg initials resulted in WSi2, W5Si3 and W phases after MCS (milled for 2 min) and leaching. In contrast to this system, the W-SiO2-Mg system could not be mechanical alloyed to yield tungsten silicide structures. In addition, W-Si system resulted in WSi2 formation after long milling duration like 15 h. Evaluation of all starting powder systems, the most efficient was WO3-SiO2-Mg system. Secondly, the molybdenum silicide powders from the MoO3-SiO2-Mg initial system were mechanochemically synthesized (20 min milling for reaction) and leached. The resultant powders comprised of α-MoSi2, β-MoSi2 and Mo5Si3 phases. The TEM images showed that particle sizes of synthesized molybdenum silicide were below 200 nm and encapsulated with SiO2 layers. Thirdly, the mechanochemically synthesis of niobium silicide was investigated using the Nb2O5-SiO2-Mg initial system. Two different Nb2O5 crystal structures (orthorhombic and monoclinic) was used to compare the initial crystal structure, and only the initial powder batch containing orthorhombic Nb2O5 resulted in MCS after 5 h of milling. The synthesized and leached niobium silicide powders had mainly the NbSi2 phase with traces of Nb5Si3 and SiO2. The TEM investigations revealed that the average particle size of synthesized powder was 61 nm and each particle had a SiO2 encapsulation layer. Finally, the selected compositions of synthesized metal silicide powders were consolidated at different temperatures by SPS. The most promising sintering result was obtained for the molybdenum silicide sample with a relative density of 94 %, Vickers microhardness of 15 GPa and fracture toughness of 3.5 MPa√m. This dissertation reveals the production results of tungsten, molybdenum and niobium silicides from their oxides by mechanochemical synthesis for the first time in the universal literature. Detailed characterizations showed that metal silicide with high purity powders had particle sizes below 200 nm. Moreover, a SiO2 encapsulation layer was detected in all synthesized metal silicide particles. In addition, SPS sintering studies of the mechanochemically synthesized metal silicide powders also contributed to the literature. To sum up, as important advanced ceramic materials, metal silicides, were produced from relatively inexpensive initial materials with a simple procedure at room temperature.