Sintering and characterization of high hardness/high toughness/high entropy structural ceramics for severe environments

Abstract

High-Tech ceramics offer a variety of outstanding physical and mechanical properties but their behaviour critically depend on the thermal treatments that consolidate their microstructure. Sintering of highly refractory ceramics is usually carried out at very high-temperature and application of mechanical pressures to obtain density close to the theoretical one. This limits their widespread application due to limitation in size and shape and excessive costs of production. The aim of this work is the achievement of bulk and composite materials characterized by high hardness, high toughness and high entropy values that can be efficiently sintered by different sintering techniques such as: hot pressing, pressureless sintering, gas pressure sintering or arc-melting. Materials under study include: - B4C and B4C-based composites, a class of ultra-hard materials specifically suited for ballistic applications. - Fiber-reinforced composites ceramic materials (UHT-CMC), a class of damage tolerant materials specifically suited for aerospace application. - Ultra high temperature ceramics (UHTC) with high entropy structure a novel class of advanced structural ceramic materials specifically suited for harsh environment. The PhD activity was divided into three parts: - The first part was focused on the development and characterization of light weight bulk material for the fabrication of ballistic protection systems. Sintering additives or others agents (α-SiC, nano-SiC, Si3N4, TiO2, WC) are used to improve the densification with conventional methods, avoiding if possible the application of mechanical pressure. In particular B4C-TiB2 composites were contaminated with WC to study the effect on densification, microstructure and properties. WC was introduced through a mild or a high energy milling with WC-6 wt% Co spheres or directly as sintering aid to 50 vol% B4C / 50 vol% TiB2 mixtures. High energy milling was very effective in improving the densification thanks to the synergistic action of WC impurities, acting as sintering aid, and size reduction of the starting B4C-TiB2 powders. As a result, the sintering temperature necessary for full densification decreased to 1860 °C and both strength and hardness benefited from the microstructure refinement, 860 ± 40 MPa and 28.5 ± 1.4 GPa respectively. High energy milling was then adopted for producing mixture of B4C-TiB2 that spanning from 100 v% B4C and 100 v% TiB2 studying the effect of different sintering technique such as hot pressing, pressureless sintering and gas pressure sintering. The B4C-rich composition showed the highest hardness and strength value in all sintering technique 30 GPa and 800 MPa respectively whilst the TiB2-rich composition showed the highest value of toughness, 5 MPa √m.

- The second part of this thesis was focused on fabrication of ultra-high temperature ceramic composites (UHT-CMCs) observing the influence of different coatings on carbon fibers through electrophoretic deposition technique. Different configurations of continuous carbon fiber-reinforced ultrahigh temperature ceramics (UHTCs), by combining coatings and matrix, were produced via electrophoretic deposition (EPD) and slurry infiltration. The toughening of non-periodic fiber distribution induced by the EPD process was investigated through work of fracture analysis. The results show that a non-periodic fiber distribution results in toughness increase from 8 MPa√m to 11 MPa√m with respect to a periodic fiber distribution. This toughness improvement does not strongly affect the flexural strength, which is mainly related to the fiber volumetric amount. It is shown that the assembling of carbon fibers into bundles (i.e. by dispersing the fibers with a non-periodic distribution) increases the crack propagation energy dissipated on the crack-wake from 0.5 kJ/m2 to 1 kJ/m2, which can be mainly ascribed to the fiber/bundle pull-out. On the other hand, the energy dissipated on the crack-tip (as fiber/matrix debonding) is fiber distribution-independent and increases from 0.3 kJ/m2 to 0.4 kJ/m2 with increasing the fiber amount from 33 vol% to 40 vol%. Finally, work of fracture (WoF) analysis is proposed as test to evaluate pull-out toughening instead of push-in and push-out tests. - The third part of this thesis was focused on the production of High entropy metal diboride (HEB). This novel class of ceramic materials represent a radically new approach to extend the chemical composition window of ultra-high temperature ceramics (UHTCs). In this work, arc-melting was used to produce dense HEBs starting from UHTC powders. In order to understand the influence of each individual diboride within the quinary system (HfB2, ZrB2, TiB2, TaB2 and CrB2), we investigated five quaternary equimolar solid solutions e.g. Hf-Zr-Ti-Ta, Hf-Zr-Ti-Cr, Hf-Zr-Ta-Cr, Hf-Ti-Ta-Cr, Zr-Ti-Ta-Cr and the overall quinary equimolar combination. Arc-melting allowed a rapid screening of favorable and unfavorable combinations. The produced HEBs were free from undesired oxides and characterized by linear variation of lattice parameters typical of diborides and binary solid solutions. Because of evaporation during arc melting, CrB2 was hardly found in the solid solution, suggesting that vapor pressure should be taken into account when designing HEB compositions especially for operating temperatures exceeding 2000 °C. Finally, Vickers microhardness ranged between the typical values of starting diborides.