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A tungsten carbide ceramic containing 5 vol% silicon carbide was hot pressed to full density at 1820 °C. A small amount of transient liquid phase based on W-C-Si-O facilitated oxide removal in the reducing environment and favoured the development of a bimodal microstructure containing sub-micrometric grains with square or rod-like morphologies. These microstructural features led to outstanding mechanical properties from room to elevated temperatures. For the first time, WC-materials were characterized up to 1500 °C exhibiting flexural strength over 1 GPa in the whole temperature range and fracture toughness from 7 to 15 MPa⋅√m. [ABSTRACT FROM AUTHOR]

Abstract An efficiency improvement of concentrating solar power systems relies on a significant increase of the operating temperatures, exceeding 600 °C. This goal can be achieved through the use of solar absorbers possessing high spectral selectivity and stability at such temperatures. Suitable alternatives to the largely used silicon carbide can be found in the ultra-high temperature ceramics class. This study focuses on the effect of processing, microstructure evolution and surface texture on the optical properties at room and high temperature. ZrB 2 -based ceramics are taken as case study to detect any correlation amongst composition, porosity, mean grain size, roughness and spectral selectivity. In addition, the effect of surface variation, induced by chemical etching or by exposure to oxidizing environment, thus simulating the actual operation conditions, are evaluated and compared to SiC optical properties. Absorbance and solar selectivity are discussed as a function of the microstructural and surface properties upon detailed roughness characterization. Advantages in the use of UHTCs as solar absorbers, strength and criticalities related to the use of these ceramics in comparison with SiC are discussed. Highlights • Effect of microstructure and finishing on the optical properties of diverse ZrB 2 materials. • Surface texturing by chemical etching led to higher solar absorbance and efficiency. • Oxidation significantly changes the borides properties which behave similar to SiC. • Superior durability and temperature stability enables UHTCs to operate above 500 °C. • UHTCs performances in solar absorbers overpass SiC-based materials ones. [ABSTRACT FROM AUTHOR]

ZrB 2 -based mixtures containing WC and SiC in various amounts were hot-pressed at 1930 °C, achieving final relative densities above 94%. The diboride matrix of the sintered ceramics was constituted by (Zr,W)B 2 solid solution shells grown onto original ZrB 2 grains. WC phase reacted during sintering leaving W-monoboride and mixed Zr,W-carbide as secondary phases. Room temperature flexure strength ranged from 540 to 630 MPa, but performances of major significance were determined at 1500 °C in air, with values exceeding 700 MPa. SiC was vital to improve the oxidation resistance at 1500 °C compared to SiC-free ZrB 2 -based ceramic. W, encased in the shell, retarded the inward diffusion of oxygen compared to the pure ZrB 2 . [ABSTRACT FROM AUTHOR]

This study reports the microstructural analysis and mechanical properties of a ZrB2 ceramic containing long BN-coated Hi-Nicalon SiC fibers. A composite was produced and thoroughly characterized by transmission electron microscopy to study the interfaces at the nanoscale level. Full densification was accomplished by hot pressing at 1450 °C. The fiber in the sintered material retained its pristine aspect, confirming that the coating was effective in preventing degradation due to interactions with the matrix. Pull-out was observed on fractured surfaces, but toughness values were about 4.5 MPa√m, which was comparable to those of ZrB2 materials with SiC additions in the form of particles or short fibers. However, the composites exhibited a controlled fracture behavior, as confirmed by a notably higher work of fracture, 140 J/m2, compared with 20-30 J/m² of unreinforced ZrB2 or ZrB2 containing chopped fibers. [ABSTRACT FROM AUTHOR]

The oxidation of ZrB 2 toughened with SiC short fibers has been evaluated in a thermo-gravimetric analyzer from 1200 to 1500 °C for 10 h and in a bottom-up loading furnace up to 1650 °C for 5–30 min. The microstructure has been investigated by SEM and TEM. The oxidation kinetics, after a transient stage, follows a linear behavior, indicating that the outer oxide layer is not protective above 1400 °C. Fibers notably oxidize above 1500 °C with formation of silica glass and a carbon-rich phase. Mechanisms of SiC fiber oxidation and formation of the various scales are proposed for the various temperature ranges. Comparisons with the oxidation of ZrB 2 containing SiC in form of particles, whiskers and previous generation fibers are also discussed. [ABSTRACT FROM AUTHOR]

ZrB 2 -ceramics containing 3rd generation SiC short fibers were hot pressed with different kinds of sintering agents in order to obtain an increase of fracture toughness. Several sintering temperatures were explored keeping fixed the amount of fiber to define the most suitable additive able to fully densify ZrB 2 with minimal fiber deterioration. In Part I of this article, the microstructure of the composites was examined through scanning electron microscope, to study the distribution of the secondary phases, and transmission electron microscope, to analyse the microstructure at nanoscale level, with particular attention to the evolution of the fiber morphology and to the study of the interfaces with the matrix in high resolution mode. For the first time the chemical stability of Tyranno SA3 fibers towards different sintering additives and ZrB 2 is presented. Comparison with Hi-Nicalon fiber is also discussed. [ABSTRACT FROM AUTHOR]

Abstract: This work aims at studying the interactions between Hi-Nicalon SiC fiber and HfB2 in presence of various sintering additive, such as Si3N4 or ZrSi2. Spark plasma sintering was used as suitable technique to interrupt the sintering at intermediate temperatures and thus analyze the microstructure evolution at the various densification stages. SEM investigations enabled to disclose the interaction mechanisms not only among fiber and matrix, but also revealed the curious mechanisms of formation of core–shell structures in HfB2 grains when ZrSi2 was employed. [Copyright &y& Elsevier]

Abstract: Ultra-refractory diborides are currently studied mainly as thermal protection materials for aerospace and military applications. However, their favorable properties (very high melting points and good thermo-mechanical properties at high temperatures) can be advantageously exploited to increase the operating temperature of thermodynamic solar plants in concentrating solar power systems. This paper reports on the spectral reflectance characterization of hafnium and zirconium diborides containing MoSi2 as secondary phase to evaluate their potential as novel solar absorbers. To assess the spectral selectivity properties, room-temperature hemispherical reflectance spectra were measured from the UV wavelength region to the mid-infrared, considering different levels of porosity for each system. Moreover, for zirconium diboride and hafnium diboride composites containing 10vol% of MoSi2 sintering aid, the thermal emittance was measured in the 1100–1400K temperature range. Room temperature spectral characteristics and high temperature emittance were compared to that of a monolithic silicon carbide. [Copyright &y& Elsevier]

Abstract: Borides and carbides of early transition metals are considered a class of promising materials for several applications, the most appealing ones being in the aerospace and energy sectors. The present work is mostly focused on toughening of UHTCs through incorporation of SiC chopped fibers. Mechanical properties of reinforced composites are compared to those of un-reinforced, whisker- and particle-reinforced materials and the effect of different kinds of sintering aids is studied. Addition of fibers allows toughness to be increased from 3–4MPam1/2 (for un-reinforced materials) to 5.0–6.2MPam1/2. The high temperature behavior is also investigated both in air furnace and in arc jet facility. Eventually, a paragraph is dedicated to potential of UHTCs as sunlight absorbers for future solar concentrating systems operating in the high temperature regime. [Copyright &y& Elsevier]

Abstract: In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB2 matrix in volumetric fraction of 10 and 20vol.%. The composites were hot-pressed between 1650 and 1730°C and their final relative densities were higher than 95%. Even at the lowest sintering temperature, the whiskers showed an evident degradation. On the other hand, the fibers maintained their initial shape and a strong interface formed between matrix and reinforcement. The fracture toughness of the composites increased from 30 to 50% compared to the baseline material, with the fibers showing a slightly higher toughening effect. In the whiskers-reinforced composites, the room-temperature strength increased when 10vol.% whiskers were added. In the fibers-reinforced composites, the room-temperature strength decreased regardless the amount of fibers added. The high-temperature strength of the composites was higher than that of the baseline material for both types of reinforcement. [Copyright &y& Elsevier]

Starting from a ZrB2 matrix, composites containing 10, 20 vol% of SiC whiskers and 20 vol% of SiC-chopped fibers were sintered by spark plasma sintering at 1500°C. The addition of whiskers allowed both strengthening (740–770 MPa) and toughening (5.1–5.7 MPa·m1/2) compared with the reference material. In the fiber-reinforced composite, the increase in fracture toughness (5.5 MPa·m1/2) was accompanied by a decrease of strength (370 MPa). Toughening mechanisms were explored through the analysis of crack propagation. Crack deflection, crack pinning, and thermal residual stresses were the most important mechanisms identified. The experimental toughness increase was successfully compared with the values predicted by theoretical models. Compared with the baseline material, the reinforced composites showed an increased strength at 1200°C in air. The highest value, 450 MPa, was for the fiber-reinforced composite. [ABSTRACT FROM AUTHOR]

Abstract: The aerothermal behaviour of a SiC fibre-reinforced ZrB2 composite machined in a sharp-shaped component with a tip curvature radius of 0.1mm was investigated using an arc-jet facility. The specimen underwent 4 thermal cycles and the emissivity was evaluated in the temperature range 1000–1700°C. The temperature profile along the specimen was calculated through computational fluid dynamic (CFD) modelling. The microstructural evolution was compared to a typical ZrB2 composite containing the same amount of SiC in particulate form, which underwent similar aerodynamic conditions. [Copyright &y& Elsevier]

ZrB2 composites containing SiC chopped fibers were hot pressed at 1600–1900°C using different sintering aids. The change of sintering aid strongly affected the densification temperature and the mechanical properties of the composites. Addition of MoSi2 was detrimental to the final microstructure and properties, due to the high temperature needed to reach a full densification. Addition of ZrSi2 decreased the sintering temperature to 1650°C, and the resulting composite containing 20vol.% fibers reached a toughness of 6.2MPam0.5. [Copyright &y& Elsevier]

Abstract: ZrB2-based composites were toughened by the simultaneous additions of SiC chopped fibers and SiC whiskers. The fracture toughness measured by Chevron Notched Beam in flexure was of the order of 6MPam1/2 and the 4-pt flexural strength around 500MPa. The values of mechanical properties are compared to those of unreinforced ZrB2 and composites reinforced by solely fibers or whiskers in order to understand whether a synergistic whiskers-fibers toughening action is present or not in these composites. The combination of two different reinforcing agents is a successful strategy to obtain a super reinforcement provided that densification, reinforcing agents chemical integrity and secondary phases composition and amount are properly controlled. [Copyright &y& Elsevier]

Abstract: Spark Plasma Sintering is used for the fabrication of highly dense HfB2 monolithic and HfB2–26vol.% SiCw composite. Reactive SPS from elemental reactants is preferred for the preparation of bulk HfB2 instead of classical sintering. The desired phase is rapidly formed through a solid–solid combustion synthesis mechanism, while full densification is achieved in 30min at 1350A when the applied pressure is switched from 20 to 50MPa after the synthesis reaction. A 99.4% dense whiskers-reinforced HfB2 ceramic matrix composite is also obtained in 30min by SPS (I =1350A, P =20MPa) using SHSed HfB2 powders and SiCw. Nevertheless, whiskers degradation into SiCp resulted under such conditions (temperature up to 1830°C). On the other hand, the presence of whiskers is clearly evidenced in 96% dense products obtained when the applied current was decreased down to 1200A (1700°C) while P was increased to 60MPa. [Copyright &y& Elsevier]