Production and characterization of B4C based composites from B4C/Me starting powders by using spark plasma sintering (SPS) method

B4C exhibits high wear resistance, high chemical resistance and high neutron absorption capacity and having low density and known to be the third hardest material after diamond and cubic boron nitride. Having superior properties B4C ceramics exhibit a great range of application areas from aeronautic-space applications to ballistic and nuclear applications. Due to its covalently bonded crystal structure of reaching up to 2450 °C melting temperature and low self-diffusivity, B4C materials with superior properties mentioned above can only be produced by high temperature and pressure applied techniques. The most important mechanical weakness of B4C even if it is sintered to high relative densities is its low fracture toughness. In many reseaches it is emphasised that the disadvantages of B4C can be overcomed by either adding a new secondary phase or nano particles. In this study, as for the defense industry structure, fracture toughness of B4C is tried to be increased by metallic additives having different melting temperatures; moreover, for the energy industry gamma radiation resistance and neutron absorbtion capability of B4C is tried to be increased further. Spark plasma sintering, as a relatively new method is used for sintering B4C specimens in this study. SPS method have many advantages over other traditional and currently widespread other sintering methods. These advantages may be expressed as; very fast sintering rate, lower sintering temperatures and less energy consumption leading to prevention of grain coarsening, and increasing the physical and mechanical properties of the materials. Typically, ceramic-metal composites containing high amount of ceramic, are initially obtained by pressureless sintering methods to obtain porous ceramic and porous metal structures and futher produced by infiltrating the metal at a temperature above the metals melting temperature. However, lack of a homogeneous porous structure, wetting difficulties encountered in the structure of B4C results in long and non- homogeneous infiltration which becomes the most important disadvantage of this process. Spark Plasma Sintering of this study on the basis of a new production technique, different boron carbide-metal powder mixtures, high-specification B4C-metal composites with the short-process duration and low-temperature advantage of single-stage production is used in order to minimize intermediate phases and unwanted by products that would cause the fracture toughness to decrease. The original aspects of this study is to have a systematical investigation of the effect of metal additions (Al, Si and Ti) having low density and different melting temperatures on spark plasma sintering parameters and material characteristics of the produced materials, and investigate and compare the possible phases that might occur during spark plasma sintering throughout the experiments and to make their thermodynamic evaluation by Fact Sage software and to compare the experimental data with B4C-metal containing structures in the end product. In the SPS experiments, phases expected to occur in different temperatures are determined by Fact Sage software and compared with the XRD analysis results and further analysis is made regarding the status of the system's thermodynamic sintering temperature. In addition, the correct size and square geometry appropriate to the end products will be achieved for the first time. Starting from the physical and chemical characterization of powders, powder mixtures of B4C-metal in appropriate compositions are weighed and mixed in ball milling with sufficient Merck quality ethanol. After granulating, dried powders are loaded into the square graphite molds and is spark plasma sintered at various temperatures, pressures and soaking times. Samples were produced by SPS apparatus with 20000 A capacity SPS 7.40 MKVII, SPS Syntex Inc. instrument established in Chemical and Metallurgical Engineering Faculty/Materials and Metallurgical Engineering Department of Istanbul Technical University. After cleaning the surfaces of the samples extracted from the graphite moulds by sand blasting; for physical, mechanical and nuclear characterization they are cut by diamond wheel in order to achieve appropriate dimensions for characterization. The density of the samples are measured by using Archimedes' principle. Hardness and fracture toughness values of the samples are measured by indentation method, after polishing the sample pieces using diamond suspensions down to 1µm. All phase transformations of the samples are investigated by X-ray diffractometer. Field emmisive scanning electron microscopy (FE-SEM) is used for microstructure analysis. B4C-Al, B4C-Si and B4C-Ti composite materials produced by spark plasma sintering system is subjected to neutron and gamma radiation in order to determine their shielding properties. Radiation testing and characterization of monolithic B4C samples, B4C-Al, B4C-Si and B4C-Ti samples are carried out by Istanbul Technical University, Institute of Energy, Radiation Protection and Radioecology Research Group. For B4C- Al composites various temperature values between 1370-1450 °C and spark plasma sintering time as 0, 2, 4 and 8 min. are chosen whereas the heating rate is selected to be 100 and 200 °C/min. The relative density values reached upto 100% for B4C-vol.5% Al mixture spark plasma sintered at 1450 °C for 4 min. with a heating rate of 100 °C/min. under 40 MPa applied pressure. When microstructures of the spark plasma sintered B4C-5%Al composites are investigated, it is observed that low SPS temperetures and short sintering durations would cause the structure to stay porous whereas at 1450 °C the microstructures observed to have no pore at all. Hardness and fracture toughness of the mentioned samples are measured to be 29.7 GPa and 3.36 MPa.m1/2 respectively. During spark plasma sintering, there are local smelting within the sample as Al content increases, therfore, sintering temperatures are decreased to 1410 °C. Relative density values are measured to be 97.9%, 97.9%, and 98.3% for the 10%, 15% and 20%Al including B4C samples spark plasma sintered with a heating rate of 100 °C/min., at 1410 °C SPS temperature, for 4 min. soaking time under 40 MPa applied pressure. As Al content increased the hardness values varied between 29.6 GPa to 28.9 GPa and fracture toughness values reached to values up to 4.45 MPa.m1/2. The decrease in hardness values are caused due to the increased amount of AlB12 and graphite phases occurred from the reaction of B4C with Al. For B4C-Si composites as the spark plasma sintering temperature increases, liquidified Si, taking C from graphite dies, B4C and using excess C, forms SiC. Various spark plasma sintering temperatures such as 1300 , 1400 and 1500 °C for B4C-%5 Si composites; 0 and 4 minutes soaking times and 40 and 60 MPa applied pressure is used. Si content is increased to 10%, 15% and 20% in vol. for the starting powder at 1500 °C under 40 MPa and decreased to 1450 °C since for the 15% and 20% Si containing composites the samples are smelted and sticked to the inner surfaces of the moulds. As the Si amount increased, edge-center relative density differences are decreased and the density values are increased itself. Hardness values are obtained to be up to 28 GPa at the centers that high relative densities are obtained to be 22 GPa for edges. Fracture toughness values varied from 3,42 to 3,32 MPa.m1/2. As the Si amount increases in the composites SiC is formed in the final structure in the grain boundaries. As cracks reach the B4C-SiC interface, they do not extend along the interface but cross B4C-SiC particles since fracture toughness of SiC is higher that B4C. The toughness of the composite is higher than that of single B4C because the transgranular cracks formation can consume more energy. Relative density values in samples with 5% Ti by volume, spark plasma sintered at 1450 °C increased from 83.1% up to 98,6 % when the Ti amount increased to 20%. Similar to the effect of Ti amount on relative density, temperature increase acts for the favor of relative density as well. Increasing temperature and Ti amount in B4C composites leads to homogenity. During spark plasma sintering, since the melting temperature of Ti is much less than the melting tempereture of B4C as the main matrix, Ti metal starts to melt and fill the voids between B4C grains and melt while transfering to the liquid phase and reacts with B4C and takes B to its structure to constitude TiB2 phase. Since the B4C phases gets poorer on behalf of B as the B from B4C phase reacts with Ti to form TiB2; graphite phase is envealed. This newly formed TiB2 grains, due to Ti getting into liquid phase and new grains are nucleated on the surface of B4C grains, they take B4C phases as the main phases and form as large TiB2 grains as B4C matrix grains. This process further goes on until all of the coarser Ti grains are melted and transformed into finer TiB2 grains. In the middle of this newly formed TiB2 agglomerated grains there occurs graphite phase as lamel like structure. When B4C-Ti micrographs obtained from the fructure surfaces are observed B4C grains are bonded strongly with spark plasma sintering as a result, the crack passes through the B4C grains (transgrannular) and when it reaches to TiB2 grains (TiB2 agglomerate grain group) the crack passes along the weakly bonded TiB2 grain boundaries (intergrannular). Depending on the increasing Ti content from 5% Ti to 20% Ti, hardness decreased from 29.68 down to 25.32 GPa. As the TiB2 amount increases in the structure, since the hardness of TiB2 (24-29 GPa), is much lower than the hardness of B4C (32-36 GPa), the total hardness of the composite show a tendency to decrease. In the B4C-TiB2 composite samples, as Ti amount increases TiB2 amount increases in the bulk material as well leading to an increase in the fracture toughness values. The fracture toughness value increases to 5.4 MPa.m1/2 at 1550 °C and the highest fracture toughness value reached is 5.9 MPa.m1/2 for the 20% Ti included samples at 1500 °C. This increase in fracture toughness, is caused by the main matrix B4C having a different thermal expansion coefficient than newly formed TiB2 and stress caused by thermal expansion differences during the cooling phase leads to micro cracks and furthermore changes the direction of cracks and thus absorb the energy of the crack during long path way. When gamma radiation shielding properties are investigated B4C-Ti, B4C-Si and B4C-Al composites, titanium, silicon and aluminum addition into boron carbide lead to an increase in the density of the materials. Due to that sort of increase, against Cs-137 Co-60 gamma radiation there arises a better shielding opportunity with a lower thickness and volume compared to monolithic boron carbide. When the HVT (Half Value Thickness) values are investigated against thermal neutrons, for the B4C-Ti, B4C-Si and B4C-Al composites, for monolithic boron carbide it is measured as 0.973 cm; for the 20%Ti included composites it is measured as 1.674 cm, for the 20%Si included composites it is measured as 1.953 cm and for the 20% Al included composite it is measured as 1.868 cm. Since the molar mass of Ti atoms are greater than Si and Al atoms, electron density is higher in the environment due to the high electron number of Ti and as a result Ti contributes more to the gamma shielding properties of the composites. In contrast to gamma radiation shielding, the titanium, silicon or aluminum addition into boron carbide composite would increase the HVT values against thermal neutrons in other words metal additions would decrease the shielding capacity since increasing the metal content would decrease the ratio of B4C in the bulk structure causes a reduction in the boron-10 isotope ratio accordingly. When the three metal additives are compared within each other about their gamma shielding properties, Ti having a higher atomic number seems to be a more efficient additive, and the negative effect that it possesses on the material against neutron shielding is less than others are. In this study, spreading the application areas of boron products such as B4C-Al, B4C-Si and B4C-Ti composites exhibiting superior properties as well as the cost-effective production and use of new production technologies of new boron products have been the main goal. Physical, mechanical and radiation shielding properties of monolithic boron carbide has been improved in order to overcome many disadvantages due to production or content of the boron carbide ceramics. Thus 'Investigation of Boron products with the use of a variety of products that might create a synergy effect will allow the widespread use of national resources effectively and in the future will be used as a reference for the development of alternative studies. In such studies carried out with B4C-metal composite system in terms of nuclear radiation shielding, the use of boron-containing products will be promoted by the National sources.