Self-propagating high-temperature synthesis of advanced ceramics MoSi2–HfB2–MoB

This study focuses on the investigation of the macrokinetic features of SHS (combustion synthesis) of elemental mixtures Mo–Hf–Si–B, in particular the mechanisms of structure and phase formation in the combustion front as well as the structure and properties of consolidated ceramics. Two routes for the fabrication of the composite SHS powder in system MoSi₂–HfB₂–MoB were used: (1) synthesis using Mo–Si–B and Hf–B mixtures followed by mixing of the combustion products and (2) synthesis using the four-component Mo–Hf–Si–B mixture. Dense ceramic samples with a homogeneous structure and low residual porosity (0.8–3.6%) were prepared by hot pressing of SHS powders. Although the particles size distribution and phase composition of SHS powders are similar for both synthesis routes, the structure and properties of both the composite SHS powders and hot-pressed ceramics differ considerably. Synthesis using the four-component Mo–Hf–Si–B mixture allows one to produce hierarchically ordered nanocomposite material with improved mechanical properties: hardness up to 17.6?GPa and fracture toughness up to 7.16?MPa?m^1/2.     

Effect of substrate rotation on structure, hardness and adhesion of magnetron sputtered TiB2 coating on high speed steel

Titanium diboride (TiB₂) coatings have been deposited on stationary and rotating high speed steel substrates by magnetron sputtering of a TiB₂ target. The structure and hardness of the coatings and the coating–substrate adhesion have been investigated by X-ray diffraction, field emission scanning electron microscopy, nanoindentation and microscratch tests. The results show that substrate rotation has a significant effect on these structural and properties features. It was found that, with substrate rotation, the TiB₂ coating exhibits a columnar structure with random orientation and relatively low hardness and coating–substrate adhesion. On the other hand, without substrate rotation, the TiB₂ coating shows a strong (001) texture with dense, equiaxed grain structure. The hardness and coating–substrate adhesion of the coatings deposited on stationary substrates are much higher than those deposited on rotating substrates. The observed phenomena are discussed in terms of the energy of the sputtered flux, which varies with the substrate–target distance during deposition.     

Optical properties of dense zirconium and tantalum diborides for solar thermal absorbers

Ultra-high temperature ceramics (UHTCs) are interesting materials for a large variety of applications under extreme conditions. This paper reports on the production and extensive characterization of highly dense, pure zirconium and tantalum diborides, with particular interest to their potential utilization in the thermal solar energy field. Monolithic bulk samples are produced by Spark Plasma Sintering starting from elemental reactants or using metal diboride powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). Microstructural and optical properties of products obtained by the two processing methods have been comparatively evaluated. We found that pure diborides show a good spectral selectivity, which is an appealing characteristic for solar absorber applications. No, or very small, differences in the optical properties have been evidenced when the two investigated processes adopted for the fabrication of dense TaB₂ and ZrB₂, respectively, are compared.     

Ignition and chemical mechanisms of volume combustion synthesis of titanium diboride

Reaction ignition and chemical mechanisms in volume combustion synthesis of TiB₂ via TiO₂–B₂O₃–Mg precursors were studied using in-situ differential thermal analysis, X-ray diffraction, scanning electron microscopy and thermochemical modeling. Mg–TiO₂ samples ignited at 607 °C through a sudden single step solid-solid reaction while Mg–B₂O₃ samples ignited at 810 °C after melting of magnesium. X-ray diffraction analysis revealed that reduction of TiO₂ occurs in multiple steps and forms intermediate compounds. Results showed that heat released from the first reaction between TiO₂ and Mg ignites the reactions between Mg, Ti and B₂O₃ resulting in the formation of TiB₂. Samples with larger TiO₂ particle size or a higher sample surface to volume ratio showed a two-step reaction behavior and the released heat in the first solid state reaction was insufficient for the propagation of the reaction throughout the sample. In addition, Mg₃B₂O₆ undesired by-product was formed as a result of this two-step reaction.     

Fusion welding of refractory metals and ZrB2-SiC-ZrC ceramics

Molybdenum and a molybdenum alloy were fusion welded to ZrB₂-based ceramics to determine if the electrical and thermal properties of the metals and ceramics affected their weldability. Commercial ceramic powders were hot pressed, machined into coupons, and preheated to 1600 °C before joining the ceramics to commercial metals using plasma arc welding. Weldability varied as indicated by the range of porosity observed within the fusion zones. Measured thermal and electrical properties appeared to have little to no effect on the weldability of metal-ceramic welds despite the large range of values measured across each property. Differences in melting temperatures between metal and ceramic coupons did affect weldability by changing the weld penetration depth into ceramic coupons. Future studies on metal-ceramic welds are suggested to investigate the effect that work function, melt viscosity, wetting, or other properties have on weldability.     

Effects of excess boron on combustion synthesis of alumina–tantalum boride composites

TaB- and TaB₂–Al₂O₃ in situ composites were fabricated by thermite-incorporated combustion synthesis from the powder mixtures of different combinations, including Ta₂O₅–Al–B, Ta₂O₅–Al–B₂O₃–B, and Ta₂O₅–B₂O₃–Al. Effects of excess boron were studied on the combustion dynamics and phase constituents of final products. For the B₂O₃-containing samples, the reaction was less exothermic and aluminothermic reduction of Ta₂O₅ and B₂O₃ was less complete, resulting in the deficiency of boron and the presence of TaO₂ and Ta. For the samples containing elemental boron, the occurrence of borothermic reduction of Ta₂O₅ also caused the loss of boron. Experimental evidence showed that boron in excess of the stoichiometric amount substantially enhanced the formation of tantalum borides, which in turn facilitated the reduction of Ta₂O₅ by Al. Consequently, the samples rich with boron in the molar proportions of Ta₂O₅:Al:B=3:10:9 and 3:10:16 (i.e., B/Ta=1.5 and 2.67) were found to be the optimum stoichiometries of producing TaB- and TaB₂–Al₂O₃ composites through a self-sustaining combustion process.     

Mechanical properties and grain orientation evolution of zirconium diboride-zirconium carbide ceramics

The effect of ZrC on the mechanical response of ZrB₂ ceramics has been evaluated from room temperature to 2000 °C. Zirconium diboride ceramics containing 10 vol% ZrC had higher strengths at all temperatures compared to previous reports for nominally pure ZrB₂. The addition of ZrC also increased fracture toughness from $～3\text{.5}\text{MPa}\sqrt{\text{m}}$ for nominally pure ZrB₂ to $～4\text{.3}\text{MPa}\sqrt{\text{m}}$ due to residual thermal stresses. The toughness was comparable with ZrB₂ up to 1600 °C, but increased to $4\text{.6}\text{MPa}\sqrt{\text{m}}$ at 1800 °C and 2000 °C. The increased toughness above 1600 °C was attributed to plasticity in the ZrC at elevated temperatures. Electron back-scattered diffraction analysis showed strong orientation of the ZrC grains along the [001] direction in the tensile region of specimens tested at 2000 °C, a phenomenon that has not been observed previously for fast fracture (crosshead displacement rate = 4.0 mm min^?1) in four point bending. It is believed that microstructural changes and plasticity at elevated temperature were the mechanisms behind the ultrafast reorientation of ZrC.     

Oxidation of ultrahigh temperature ceramics: kinetics,mechanisms, and applications

Materials capable of oxidizing in a protective manner at ultrahigh (>1700 °C) temperatures are needed to push beyond this barrier defined by SiC. Although possessing attractive mechanical properties and oxidation resistance, SiC-based materials are ultimately temperature limited by the melting point of SiO₂. The vast array of ultra-high and high temperature ceramic literature indicates the majority of these materials, like borides, carbides, MAX-phases, and high-entropy ceramics, fall woefully short regarding oxidation resistance. However, for specific applications, like low-orbit aeropropulsion, high ballistics coefficient atmospheric re-entry, and hypersonic cruise, there are a few promising materials. In the present review, oxidation criteria are gathered to build application specific heuristics and are then applied to a multitude of ultra-high temperature ceramics to gauge material efficacy. Discussion of oxidation kinetics, mechanisms and reaction products is offered for each material, identifying strengths, weaknesses, and the remaining gaps in our knowledge.     

原位合成多元硼化物对Al0.5CoCrCuFeNiBx高熵合金硬度与耐磨性能的影响

本文研究了Al0.5CoCrCuFeNiBx (x=0-1)的组织、相组成、硬度及耐磨性能。并预测了Al0.5CoCrCuFeNiBx (x=0-1)中简单固溶体形成规律。未添加硼元素的合金具有简单FCC固溶体结构。添加硼元素后,合金由简单FCC固溶体及多元硼化物组成。硼以硼化物形式析出,没有固溶到FCC固溶体中,因而添加硼对FCC固溶体的晶格常数无影响。硼化物的析出使合金的硬度提高,并且硬度随着硼含量的增加而呈线性增加。当硼含量x?0.4时,合多的磨耗阻变化不明显,但当硼含量x?0.6时,合金的磨耗阻抗随着硼含量增加而呈线性增加。 随着硼含量的增加,合金的磨损机制由粘着磨损转变为氧化磨损。合金硬度与耐磨性能的提高是高硬度的粗大硼化物与韧性的FCC固溶体基体共同作用的结果。     

Synthesis of single-crystalline lanthanum hexaboride nanowires by Au catalyst

Lanthanum hexaboride (LaB₆) nanowires have been successfully fabricated by the facile catalytic reaction of lanthanum (La) powders, and gas mixture of boron trichloride (BCl₃), hydrogen and argon, where Au was used as the catalyst. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED) were used to characterize the composition, morphology and structure of the samples. Single crystal column-shape LaB₆ nanowires were obtained. It is expected that LaB₆ nanowires can provide thermionic emission, field-induced emission, and thermal field-induced emission of electrons for TEM, SEM, flat panel displays, as well as many electronic devices that require high-performance electron source.