Production of ZrB2-TiB2 composite ceramic powders by self heating synthesis process

ZrB2 and TiB2 are one of the most significant metal borides. They are being used in various industrial areas from space technology to nuclear industry owing to their physical properties such as high melting point, high thermal/electrical conductivity and low density. Present study was conducted in two main stages: Self-propagating high-temperature synthesis (SHS) reactions and leaching. ZrO2, TiO2, B2O3 were used as starting material and Mg used as a reductant to produce ZrB2-TiB2 powders via self propagating high temperature synthesis. Therefore, it was aimed to determine production parameters, leaching conditions and comprehensive examination of final product features. The samples were characterized by using chemical analysis (AAS), XRD, XRF and SEM/EDS techniques. As known, Turkey has the biggest reserve of boron minerals minerals. This potential must be manipulated to produce valuable high technology ceramic materials. Apparently, ZrB2 and TiB2 which are called as high-tech ceramics have auspicious potential for industrial usage. The reason is that why ZrB2 and TiB2 is chosen as a research topic. Metal borides generally have a metallic (grey colored) appearance. The most remarkable properties of the binary metal borides are high melting point and very high hardness. Other important properties of binary metal borides are high thermal shock resistance, resistance to corrosion, high wear resistance and chemical stability at high temperatures. On the other hand, like other advanced ceramic materials, metal borides are brittle. They become a more ductile as temperature increases but at the same time their hardness decreases. Zirconium and titanium are transition metal,so ZrB2 and TiB2 are transition metal borides. Its atomic distribution is one zirconium and two boron atoms. ZrB2 consists of two dimensional networks of boron atoms. It has a melting point temperature near 3250°C. Titanium diboride has hexagonal crystal structure. . It has a melting point temperature approximately 3225°C. It is covalently bonded atomic structure provides the hardness values as high as 45 GPa. There are many usage area of metal borides in industry such as, impact resistant armors, cutting tools, aluminium evaporation crucibles, wear resistant coatings and aluminium electrolysis cathodes. SHS process can be used for the production of advanced ceramics, Intermetallics, Organic and inorganic compounds, Oxygen free single crystals, Polymers. Reaction is initiated from the surface of the mixture with heat flux, such as heated wire or laser. After initiation reaction becomes self sustaining and propagates into reaction mixture. All of the mixture was reacted and SHS products obtained. The SHS process has some important advantages such as a very short processing time, Less energy requirement, Simple processing, Sub-micron sized particles, High purity products without carbon content. Specific heat heat is the main process parameter; it is to estimate whether a SHS reaction process is self sustaining or not. The Specific Heat is the amount of heat required to change a unit mass of a substance by one degree in temperature. Furthermore, for the main reaction in the present study specific heat was calculated and it is between the value of 2250-4500 J/g. Another main process parameter is adiabatic temperature and also it shows to reaction is self-sustaining or not. Accorging to adiabatic temperature the ZrB2 – TiB2 production reaction is high enough for self sustaining. Adiabatic temperature was calculated by using FactSage Thermodynamic Databases. Effects of initial mixture on SHS products was also investigated by using Factsage programme. The first step of the study is the self-propagating high-temperature synthesis process. Boron oxide, the technical grade of ZrO2 , TiO2 and Mg powder as raw materials. 100 gram specimens of the powder mixtures prepared with different molar ratios were examined during the SHS process. Nine different sample of initial mixture had been created. The powder mixture was charged into the crucible and the W resistance wire was placed at the top of the mixture. A cover was used to close the crucible and inert argon gas was purged into to the system. The reaction was realized by passing electricity through the resistance wire. By the end of the SHS experiments, a black and spongy solid form had been obtained. Second step is the leaching process where the MgO and Mg-borates were tried to remove from ZrB2 by HCl leaching process due to this exothermal reaction. The theoretical HCl acid concentration required was calculated. All the leaching experiments were investigated at a 1/5 S/L ratio and 400 rpm stirring rate with 20 gram specimens. After the S/L separation ZrB2 – TiB2 based filter cakes were obtained. The XRD analysis of the SHS products obtained from the reaction given in the literature is given. The main phases were determined as ZrB2 ,TiB2 and MgO. Unreacted ZrO2 and TiO2 phases were determined also. These phases can not be removed by HCl leaching. Also some Mg-borates phases have formed. The XRD analysis of the leach-cake products we observed that approximately MgO and Mg-borates phases was eliminated. Optimum condition of leaching was defined as at 80°C using HCl acid and during 60 minutes. The result of chemical analyses which were conducted to leaching solution and XRD analyses of leaching cake, it is observed that ZrB2-TiB2 composite ceramic powder was formed and within the determined nine different sample it can be seen that MgO and Mg-borates phases mostly eliminated. Consequently, standart zirconium diboride and titanium diboride composite ceramic powders has been produced. Also high technologic TiB2-ZrB2 ceramic composite powders has been produced by one stage. This is an ongoing study and we aim to sintering this powders and make target for thin film applications, and determine this materials, hardness, densification and other mechanical properties.