Uncoated PAN-based carbon fibre-reinforced ultra-high temperature ceramic matrix composites via aqueous ZrB powder-based slurry impregnation coupled with mild polymer infiltration and pyrolysis, using allylhydrido polycarbosilane as source of amorphous SiC(O), were manufactured. To demonstrate the versatility of the process to realize tailored materials, unidirectional (UD), two dimensional (2D) and needle-punched (2.5D) cloths were impregnated. Microstructure and mechanical properties were investigated and correlated with fibre properties and architecture. The flexural strength was found over 250 MPa for unidirectional reinforced material, while the modulus exceeded 250 GPa for needle-punched one. The homogeneous distribution of UHTC phase around each single fibre and the weak fibre/matrix interface, due to the mild pyrolysis conditions, are the hallmark of this process and the key to improve durability and performance of materials for extreme environments without the application of expensive coating on fibres.

A ZrB2-based ceramic, containing short Hi-Nicalon SiC fibers, was fabricated with a Mo-impermeable buffer layer sandwiched between bulk and the outermost oxidation resistant ZrB2-MoSi2 layer, in order to prevent inward Mo diffusion and associated fiber degradation reactions. This additional layer consisted of ZrB2 doped with either Si3N4 or with the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. Scanning electron microscopy imaging and elemental mapping via energy-dispersive X-ray spectroscopy showed that this tailored sample geometry provides an effective diffusion barrier to prevent the SiC fibers from deterioration due to reactions with Mo or Mo-compounds. In contrast, the structure of the SiC fibers in a reference sample without buffer layer is strongly degraded by MoSi2 diffusion into the fiber core. The comparison of the three buffer-layer systems showed a moderate alteration of the fiber structure in the case of Si3N4 addition, whereas in the PDC-doped samples hardly any structural change within the fibers was observed. A stepwise reaction mechanism is deduced, based on the continuous progression of a reaction zone that propagates toward the ZrB2-MoSi2 top layer. The progression of such a reaction zone as a consequence of the different eutectic melts forming in the different layers, that is, first in the SiC-fiber-containing bulk, then in the buffer layer itself, and finally in the top layer at high temperature, allows for an effective separation of the ZrB2-MoSi2 top layer from the SiC fibers. Subsequent oxidation at 1500 degrees C and 1650 degrees C for 15 min did not affect the efficiency of all three buffer layers, since no structural changes regarding buffer layer and fibers were observed, as compared to the non-oxidized samples.

Oxyacetylene torch testing was performed at a range of angles of attack on C/ZrB-SiC-YO composites, from 10° to 90°. The ablation behaviour was studied in-situ with thermography, and the post-ablation morphologies investigated optically and with elemental analysis. Significantly lower surface temperatures were observed at oblique angles of attack leading to less damage in terms of both oxidation extent and material removal. Rudimentary modelling of a gas stream impinging on a perfect surface showed that the angular variation also led to a significant drop in pressure at more oblique angles of attack, with a commensurate increase in shear stress. The surface oxide formed during testing seemed mainly to correspond to the temperature distribution and was apparently more susceptible to damage from higher impinging pressure than shear stress. This study elucidated some interesting aspects of a modified ablation test and showed some new parameter ranges which may be useful in targeted material screening.

The effect of chromium diboride addition on the densification process and oxidation behavior of two ZrB2-MoSi2 and ZrB2-SiC baseline systems was studied. CrB2 was beneficial in lowering the sintering temperature owing to the tendency of its oxide to react with MoSi2 and SiC forming low-melting phases that helped the powder consolidation. Oxidation at 1500 °C induced the formation of further boron oxide as first consequence. In one case, when CrB2 was combined with MoSi2, an improved oxidation resistance was observed due to the stabilization of Cr-borides in the subscales saturated with B2O3. In the other case, when it was combined with SiC, the excessive low viscosity of the borosilicate glass facilitated the consumption of a thicker portion of materials as compared to the ZrB2-SiC reference.

This paper presents an extensive experimental campaign over the effect of SiC concentration on the aero-thermal behavior of ultra-high temperature ceramics in the hypersonic atmospheric re-entry environment. Four compositions made of ZrB2 with different amount of SiC from 5 to 18 vol.% were exposed to the supersonic plasma flow of an arc-heated plasma wind tunnel, at specific total enthalpies up to 20 MJ/kg, measuring their surface temperature by non-intrusive diagnostic equipment, including two-color pyrometers and an infrared thermo-camera. As SiC content increases, maximum steady-state temperature reached on the surface decreased and emissivity value are higher. During some tests, a spontaneous temperature jump in the order of 400 K was observed, which only occurred on the front surface of the sample. Surface temperatures over 2800 K were measured after a temperature jump. The composition which experienced the jump showed an external surface reaction made of only zirconia layer on the front surface, probably appearing upon complete removal of liquid borosilicate glass which forms during exposure to oxygen and is still present on the surface of samples with higher SiC content. The experiments demonstrated that the temperature jump, closely related to the overall thermal stability of the material, appears favoured by lower SiC amount, but it can be triggered also in case of larger SiC content (up to at least 15% vol.), as long as the flow total enthalpy and the exposure time are sufficiently high.

The excess carbon and oxygen contents of ZrC nano-powders was controlled, and the resultant effects on the densification of ZrC was analyzed. The particle size of the synthesized ZrC powder was about 200 non and its oxygen content was 0.49 wt%. The good results can be attributed to the rapid heating and cooling rate, the beneficial effects of current, and the relatively low synthesis temperature by using SPS (Spark plasma sintering) for the powder synthesis. The homogeneous distribution between reactants was an important factor to minimize the formation of excess carbon. With increasing the amount of carbon in the raw powder mixture, the oxygen content of ZrC powders decreased and the densification was suppressed. The decrease of excess carbon content, the presence of oxygen in the ZrC lattice, and the fine particle size promoted the sintering of ZrC ceramics without any additives at relatively low temperature and pressure (1750 degrees C, 40 MPa).

A new class of ZrB2 composites reinforced with 40 vol% C short fibers and containing 5 vol% SiC in combination with 5 vol% MoSi2, HfSi2 or WSi2 successfully withstood extreme conditions in a oxyacetylene torch. Different responses to the torch testing were recorded depending on which secondary phase was present; this was primarily a result of the final density which ranged between 83 and 94% of the theoretical value. The temperatures achieved on the surfaces of the samples tested also varied as a function of the residual porosity and ranged from 2080 to 2240 degrees C. HfSi2 additions offered the best performance and exceeded that of the baseline material that contained only SiC. It is believed that this was due to its ability to promote the elimination of porosity during densification and to the refractory nature of its oxide, HfO2. In contrast, MoSi2 and WSi2 formed highly volatile oxides on the surface, which did not offer better protection than the ZrO2-SiO2 scale that developed in the baseline.

Monolithic ZrB2 and TaB2 were produced starting from the precursors through Self-propagating High temperature Synthesis followed by Spark Plasma Sintering (SHS-SPS) and by means of Reactive Spark Plasma Sintering (RSPS). Both methods enabled to achieve almost fully dense ceramics with mean grain size typical of pure bulks. ZrB2 materials displayed significant differences in the final mean grain size of the products obtained by the two routes, while a satisfactory homogeneity was reached in both cases. This was confirmed by good mechanical strength values, about 400 MPa at room temperature, basically maintained up to 1200 degrees C. On the other hand, TaB2 sintered materials were quite diverse, in particular TaB2/RSPS showed a dual distribution of mean grain size, along with an almost no residual porosity, whilst TaB2/SHS-SPS had more homogeneous grain size distribution and diffused trapped porosity which corrupted the mechanical and oxidation performances. (C) 2015 Elsevier B.V. All rights reserved.

Ultra-high temperature boride ceramics have proved to show promising properties for novel solar receivers. The present work shows a further step towards their actual application, investigating how sintering technique and starting powders composition affect the properties of final materials. Thus we report on the comparative characterization of ZrB2, HIB2 and TaB2 produced by high pressure and pressureless techniques and with different aniounts of MoSi2 sintering aid. We investigate micro structural, mechanical and optical properties, in the perspective to assess the material potential for novel solar absorbers operating at higher temperatures than those currently available. Moreover, a systematic study has been carried out on ZrB2, producing with fixed high pressure sintering technique, a series of samples with MoSi2 compositions in the range 5-50 vol%. We show that the content of silicide and silicide-related secondary phases in the final pellets affects either the mechanical performance and the optical behavior. Thus, as far as the optical properties are concerned, the MoSi2 amount should be the lowest as possible to ensure a proper material consolidation whilst enhancing the absorbance/spectral selectivity. (C) 2016 Elsevier B.V. All rights reserved.

Besides ultra-refractoriness and favorable mechanical and chemical characteristics, carbides of early transition metals show intrinsic spectral selectivity, making them appealing for high-temperature solar absorber applications. However these kinds of ceramics can be produced using many processing methods resulting in different compositions, density and surface finishing. Thus the present work reports on the systematic study of microstructural, mechanical and optical properties of dense zirconium, hafnium and tantalum carbides as a function of the sintering method (high pressure or pressureless), implying use of 10 or 20 vol% of MoSi2 as sintering aid. The spectral hemispherical reflectance of Zr-, Hf- and Ta-carbides has been measured in the 0.25-16.5 mu m wavelength range and correlated to the surface microstructure and roughness. Room and high temperature fracture strength has been measured as well. (C) 2016 Elsevier Ltd. All rights reserved.