High efficiency and scalable pressure-less sintering of TiB2 for enhanced strength and hardness applications

Abstract

Titanium diboride (TiB2) is a promising material for commercial applications due to its excellent mechanical properties, melting point above 3000°C, wear resistance, and good electrical conductivity, similar to that of metals. However, its poor sinterability makes it difficult to fabricate fully dense components with good properties and mechanical performance. Sintering additives and different preparation methods can improve the sintering of TiB2, but the effectiveness varies greatly. In this thesis, various additives, preparation techniques, and sintering methods were systematically investigated to comprehensively analyze their impact. The main objective was to obtain a high-density material with good mechanical properties through pressure-less sintering, an easily scalable and very common technique at the industrial level. This work can be divided into four parts: In the first part, the impacts of different sintering aids (B4C, Si3N4, and MoSi2), preparation methods (ball-milling and high-energy-milling), and their combination on the densification of TiB2 were investigated through hot-pressing sintering. The shrinkage of the sample over time was recorded to study the densification rate and temperatures at which occurred. Residual porosity, microstructure, composition, and mechanical properties of the sintered material were measured to study how additives, preparation methods, and their possible synergy improved the densification of TiB2. MoSi2 added in amount of 5 vol% proved to be the best sintering aid, leading to complete densification due to the presence of a liquid phase. The second part of this work was focused on pressure-less sintering treatments. Several sets of samples were sintered at various temperatures (from 1900 to 2100°C) and dwell times (from 60 to 120 minutes), to identify the best conditions. The amount of sintering aid was then optimized and scale-up tests were conducted at various temperatures for the best compositions. Residual porosity, microstructure, composition, and mechanical properties of the sintered materials were measured and compared to those obtained through hot pressing. Higher densities and finer microstructures were achieved through high-energy-milling, both without and with sintering aids. In the case of MoSi2 addition, the additive and HEM worked together to create a synergistic effect that results in better densification (4.80 g/cm3) and a finer microstructure (1.2 ± 0.6 µm). The third part of the thesis explored the effect of high-energy milling (HEM) with WC-Co media on the densification of TiB2. Milling cycles of different durations were tested, and the resulting powders were characterized using XRD patterns, particle size analysis, SEM microscopy, and mass measurements. These powders were then pressure-less sintered, and the effects of milling time on densification, microstructure, and mechanical properties were evaluated. The reproducibility of the HEM process was investigated by repeating the same milling cycle one year later with more worn media. Finally, the role of WC was explored by preparing a mixture of TiB2 and WC powders (without Co) and ball-milling the mixture. From HEM powders analysis it was observed that while the introduction of WC-Co increased linearly with time, the average particle size decreased significantly within the first 10 minutes after which the rate of size reduction slowed down progressively. The fourth part of this work was dedicated to mechanical thermal and electrical testing of pressure-less sintered TiB2. Preliminary flexural strength tests were carried out in Faenza. Then, the mechanical, thermal, and electrical properties of specimens prepared at the Missouri University of Science and Technology were explored. Flexural strength and fracture toughness were measured at room and high temperature (1000-1600°C) to analyze the mechanical properties. Thermal conductivity tests were performed between 25 and 200°C, and between 200 and 2000°C, to obtain thermal properties. Electrical resistivity was measured for different currents at room temperature.