Investigation of the role of impurities and sintering parameters on the microstructure and properties of dense titanium diboride ceramics and boron carbide–titanium diboride composites
Creators: name: Celik, Azmi
Publisher: name: No Publisher Supplied
Year: 2020

description: Titanium diboride (TiB2) is an important ultra-high temperature ceramic material due to its excellent mechanical, elastic, thermal, and chemical properties. It has received great attention for a variety of industrial and technological applications including refractory, cutting tools and aerospace applications. Boron carbide (B4C) is another significant non-oxide ceramic with favorable physical and chemical properties such as high hardness, high strength and good chemical stability. B4C is one of the most used ceramic materials as personal body armor plates, abrasives, wear-resistant components for grinding and cutting, and neutron absorbent material for nuclear reactors. Both TiB2 and B4C are strongly covalent and sintering of these ceramics is often challenging either with or without additives. Impurities in the starting powders such as TiO2 and B2O3 inhibit densification and the use of high temperatures cause exaggerated grain growth and spontaneous microcracking.The goals of this thesis were to synthesize phase-pure TiB2 powders, and process and characterize dense TiB2 ceramics as well as utilization of in-house synthesized powders in B4C–TiB2 composite samples. Densification experiments were carried out mainly using spark plasma sintering (SPS) method and hot pressing (HP) was used to fabricate industry-scale composite samples. This thesis examines the effects of starting powder quality and sintering parameters on the microstructure and mechanical properties of dense TiB2 ceramics and B4C–TiB2 composites. X-ray diffraction (XRD) was used to identify phases present in the powders and densified samples. Archimedes’ method was used to determine bulk density values of sintered monolithic and composite materials. The observations in the changes of microstructures were made using field-emission scanning electron microscopy (FESEM) equipped with an electron backscatter diffraction (EBSD) detector. Non-destructive ultrasonic evaluation (NDE) techniques were employed to measure elastic properties of the dense samples. Transmission electron microscopy (TEM) was utilized to reveal atomic structure of the grain boundaries of TiB2 specimens. Vickers micro-hardness testing and Berkovich nano-indentation experiments were performed on mechanically polished sample surfaces to measure hardness values.The results showed that synthesis of titanium diboride powders containing small amount of surface oxide impurity layers is of vital importance for good sinterability of the powders into dense monolithic TiB2 ceramics. The presence of residual surface oxide impurities (≥1.6 wt%) in commercial boride powders inhibited densification, therefore degraded the material properties. In-house synthesized TiB2 powders contained much lower oxygen content (≤0.5 wt%) and monolithic TiB2 ceramics with fine-grained microstructure (g = 4.6 ± 0.6 µm) were spark plasma sintered successfully up to relative density of 99.4% ρth using those powders. TEM observations revealed the existence, for the first time, of an amorphous BC-Si intergranular film (IGF) along the grain boundaries of monolithic TiB2 ceramics.B4C–TiB2 composite samples with up to 15 wt% TiB2 content were densified via spark plasma sintering (SPS) and hot pressing (HP) to investigate the effects of the TiB2 second phase addition on the microstructure and mechanical properties of boron carbide ceramics. Addition of 10 wt% TiB2 improved the Vickers hardness and fracture toughness of B4C by 15% and 25%, respectively. Microstructure evaluation through FESEM on hot pressed samples revealed that the rapid grain growth of boron carbide was inhibited as the average grain size of hot pressed B4C decreased from 16.99 ± 2.1 µm to 1.59 ± 0.5 µm and the number of planar defects was greatly decreased by TiB2 addition.