TaB2-based ceramics: microstructure, mechanical properties and oxidation resistance

Among ultra-high temperature ceramics, major attention has been devoted to zirconium and hafnium borides and carbides. Tantalum composites remain a less explored class of ceramics. In this contribution, TaB2-based ceramics were hot pressed with addition of 5-10 vol% MoSi2. Temperatures in the range of 1680-1780?C led to relative density around 90-95%. The microstructure was studied through X-ray diffraction, scanning and transmission electron microscopy and the results enabled the rebuilding of the densification mechanisms occurring upon sintering. The hardness was about 18 GPa, the fracture toughness 4.6 MPam1/2 and the room temperature flexural strength was around 630 MPa, but abruptly decreased to 220 MPa at 1200?C. The composite containing 10 vol% of MoSi2 was tested in a bottom-up furnace in the temperature range 1200-1700?C for 30 minutes. The microstructure appeared covered by a SiO2 layer, but the bulk remained unaltered up to 1600?C. At 1700?C the specimen vaporized. Nanoindentation was employed on the oxidized cross section to detect eventual mechanical properties modification associated to chemical/microstructural chang

Toughened ZrB2-based ceramics with addition of SiC whisker or short fiber

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 20 vol.%. 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 10 vol.% 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 reinforcemen

On the thermal shock resistance and mechanical properties of novel unidirectional UHTCMCs for extreme environments

Aerospace provides a strong driving force for technological development. Recently a novel class of composites for harsh environments, based on ultra-high temperature ceramic composites reinforced with continuous fibers (UHTCMC), is being developed. The goal of this work is to overcome the current data patchwork about their microstructural optimization and structural behavior, by showing a consistent mechanical characterization of well-defined and developed UHTCMCs based on ZrB2-matrix. The obtained composites have a density of 3.7 g/cm3 and porosity of less than 10%. The flexural strength increased from 360 to 550 MPa from room temperature to 1500 °C, showing a non-brittle behaviour. The composites were able to sustain a thermal shock severity as high as 1500 °C. The maximum decrease of strength at 1400 °C was 16% of the initial value, indicating that the samples could be shocked at even higher temperature. Flexural strength, Young’s modulus and coefficient of thermal expansions (CTE) of the composites were measured both along transverse and longitudinal direction and correlated to the microstructural features. The presented microstructural and mechanical characterization well defines the potentiality of the UHTCMCs and can be used as reference for the design and development of novel thermal protection systems and other structural components for harsh environments

Production and characterization of toughened UHTC

Ultra-high temperature ceramics are material candidate for application in aggressive environment, especially in sharp components of future generation of space vehicles, driven by the ultimate goal of major speed with new propulsion and hypersonic models. This class of materials is arising always more interest for wing leading edges and nose tips, as well as propulsion system elements. The most investigated system concerns the ZrB2-SiC ones, owing to a high strength, up to 1 GPa, high hardness, around 20 GPa, and oxidation resistance. The major weak point remains the low fracture toughness, 3 to 5 MPam1/2. It has recently been demonstrated that the introduction of elongated secondary phases and the choice of the proper sintering additive, can lead to almost twofold increase of the fracture toughness. This work presents the last development of toughened ZrB2 ceramics with addition of SiC whiskers or chopped fibers, or through the in-situ elongation of SiC particles. The effect of various sintering additives, MoSi2, Si3N4, ZrSi2 and WSi2, is investigated in relationship to the microstructure evolution upon sintering, to the interface between matrix and reinforcing element and to the high temperature behaviour. Flexural strength, with the 4-point method, and toughness, measured by the CNB technique, are compared to those of reference unreinforced materials. The addition of whiskers to ZrB2 -based materials allows both strengthening and toughening compared to the reference material, whilst the addition of fibers only leads to a toughness increase, but it is accompanied by a decrease of strength, due to a change of the defects population. The oxidation tests conducted in a bottom-up loading furnace at 1200, 1500 and 1700?C show that the reinforced composites behave similarly to the baseline ZrB2-SiC well known materia

Functionally graded ultra-high temperature ceramics: From thermo-elastic numerical analysis to damage tolerant composites

To maximize the toughening contributions due to fiber bridging and residual stresses upon layering, ultra-high temperature ceramics containing variable amounts of short carbon fiber in functionally graded stacking sequences were designed and characterized. Stress fields evaluated by finite element model on (AB)nA and more complex asymmetric architectures were compared to the experimental fracture toughness pointing to an effective toughness increment in those structures where the notch fell in zones of residual compression. For the best composite, toughness at room temperature achieved 7 MPa·m0.5 and further increased to 10 MPa·m0.5 when tested at 1500 °C within a light ZrB2-based composite with density below 4 g/cm3. According to the numerical simulations and the effective microstructural features of the composites, the main guidelines for the realization of ceramics with simultaneous failure tolerance and ablation resistance were established.Peer ReviewedPostprint (published version

Spark plasma sintering of TixTa1-xC0.5N0.5-based cermets: Effects of processing conditions on chemistry, microstructure and mechanical properties

Nanometric powdered TixTa1-xC0.5N0.5-based cermets were fabricated using a mechanically induced self-sustaining reaction and consolidated by spark plasma sintering. Highly dense cermets were obtained, and their chemistry, microstructure and mechanical properties were characterised by X-ray diffraction, scanning electron microscopy, image analysis, microindentation and nanoindentation. The microhardness was found to depend directly on the contiguity and size of the ceramic hard particles. The samples synthesised at the lowest temperature (1150°C) exhibited more homogeneous microstructures and smaller ceramic particles and the best combination of microhardness and fracture toughness.Gobierno de España MAT2011-2298

GRAWITA: VLT Survey Telescope observations of the gravitational wave sources GW150914 and GW151226

We report the results of deep optical follow-up surveys of the first two gravitational-wave sources, GW150914 and GW151226, done by the GRAvitationalWave Inaf TeAm Collaboration (GRAWITA). The VLT Survey Telescope (VST) responded promptly to the gravitational wave alerts sent by the LIGO and Virgo Collaborations, monitoring a region of 90 and 72 deg2 for GW150914 and GW151226, respectively, and repeated the observations over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the r band. The paper describes the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Several transients have been discovered but no candidates are recognized to be related to the gravitational wave events. Interestingly, among many contaminant supernovae, we find a possible correlation between the supernova VSTJ57.77559-59.13990 and GRB150827A detected by Fermi-GBM. The detection efficiency of VST observations for different types of electromagnetic counterparts of gravitational wave events is evaluated for the present and future follow-up surveys