Phase composition and morphology of products of combustion of ferrosilicon in nitrogen

The results of a study of silicon nitride phase formation in combustion of ferrosilicon in gaseous nitrogen are reported. It was shown that formation of α-or β-modifications of silicon nitride is basically determined by the composition of the batch for self-propagating high-temperature synthesis. When ammonium chloride was added to the initial ferrosilicon, a combustion product with a high (up to 80%) α-Si₃N₄ content is formed, while dilution with the final product and magnesium fluoride results in predominant (more than 95%) formation of β-Si₃N₄. The particle size and shape are a function of the conditions of synthesis and are primarily determined by the temperature and the additives incorporated in the initial alloy. __________ Translated from Steklo i Keramika, No. 2, pp. 28–30, February, 2007.     

In situ fabrication and properties of 0.4MoB-0.1SiC-xMoSi2 composites by self-propagating synthesis and hot-press sintering

Mo, Si and B₄C powders were used to fabricate 0.4MoB-0.1SiC-xMoSi₂ composites by self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The effects of MoSi₂ content (x = 1, 0.75, 0.5 and 0.25) on phase composition, microstructure and properties of the composites were investigated. The results showed that the 0.4MoB-0.1SiC-xMoSi₂ composite exhibited Vickers hardness of 10.7–15.2 GPa, bending strength of 337–827 MPa and fracture toughness of 4.9–7.0 MPa m^1/2. The fracture toughness increased with the increasing volume fraction of MoB and SiC particles which were promoted by the toughening mechanisms, such as crack bridging, cracks deflection and crack branching. Moreover, the electrical resistivity showed an increasing trend with decreasing volume fraction of MoSi₂.     

In situ single-step reduction and silicidation of MoO3 to form MoSi2

Nanocrystalline molybdenum disilicide (MoSi₂) is synthesized in a specially designed autoclave at 900°C. The XRD results revealed that the formation of MoSi₂ is favorable with the blend of MoO₃, Si, and Mg powders. The HR-TEM and SAED patterns confirm the formation of MoSi₂ phase. The structural parameters (crystallite size, strain, stress, and deformation energy density) are calculated using the Williamson-Hall (W-H) analysis. The formation mechanism involved in the synthesis of MoSi₂ is proposed. The nonisothermal oxidation kinetics (~1200°C) of MoSi₂ phase is examined through the thermal analysis techniques. The activation energy is determined by the Kissinger-Akahira-Sunsose isoconversional kinetic model. Finally, the reaction mechanism involved during the oxidation of MoSi₂ phase is identified using the integral master-plots method.     

Influence of Powder Composition and Milling Media on the Formation of Molybdenum Disilicide by a Mechanically Induced Self-Propagating Reaction

The spontaneous formation of molybdenum disilicide (α-MoSi₂) during the mechanical alloying of a stoichiometric Mo-Si powder mixture occurred by a mechanically induced self-propagating reaction (MSR)—a mechanism similar to that of the thermally ignited self-propagating high-temperature synthesis (SHS). The bulk of α-MoSi₂ formation occurred in less than 15 min once the reaction commenced after 14 h, 15 min of milling. When the milling media was switched from zirconia to steel, however, the reaction rate became sluggish as both the α and β phases were formed. The mechanical alloying of the Si-rich and Si-poor compositions was also characterized by the gradual formation of α and β phases.     

Complex coatings for the protection of molybdenum

Conclusions The complex coating (MoSi₂+Al₂O₃) obtained by flame-spraying of aluminum oxide on the MoSi₂ layer on molybdenum has a higher oxidation and erosion resistance than a coating made up only of molybdenum disilicide. At 1200°C, mullite forms in the coating as a result of the interaction between amorphous silica and-Al₂O₃.Translated from Ogneupory, No. 6, pp. 53–54, June, 1968.     

Preparation of ZrC by self-propagating high-temperature synthesis

ZrC fine powder has been prepared by self-propagating high-temperature synthesis (SHS) based on exothermic reduction reaction of ZrO₂–C–Mg. The combustion temperature observed was 1979 K. The effects of Mg content and particle size on the combustion temperature and chemical composition of the product were investigated. The reducing agent Mg plays an important role on the purity of ZrC powder obtained by SHS process. Post-heat treatment was applied to decrease the oxygen content of the final product further.     

Preparation of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>–ZrO<Subscript>2</Subscript> ceramic composites by self-propagating high-temperature synthesis

Results are provided for a study of Si₃N₄–ZrO₂ composite ceramic material preparation by self-propagating high-temperature synthesis from ferrosilicon and zirconium concentrate. It is noted that as a result of high-temperature dissociation of ZrSiO₄ silicon dioxide is nitrided with formation of silicon oxynitride and it is condensed in surface layers of a specimen in the form of filamentary crystals.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

Cast silicides of molybdenum,tungsten, and niobium by combustion synthesis

Cast silicides of molybdenum, tungsten, and niobium were prepared by combustion synthesis under nitrogen pressure from powder mixtures of respective metal oxides with Al and Si. Upon variation in green composition, cast MoSi₂, WSi₂, NbSi₂, and their solid solutions with variable phase composition have been synthesized. NbSi₂ was obtained by combustion in the presence of added heat-generating CaO₂-Al powder mixture.     

Preparation and properties of MoSi2 based composites reinforced by carbon nanotubes

Molybdenum disilicide (MoSi₂) based composites with various contents of carbon nanotubes (CNTs) were made by sintering in vacuum at 1500 °C for 1 h. Mechanical properties of these composites at room temperature revealed the addition of CNTs to have good hardening and toughening effect on the matrix. Especially when adding 6.0% CNTs by volume, the hardness and fracture toughness were improved respectively by about 25.3% and 45.7% compared to pure MoSi₂. Phase identification and microstructure of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HTEM). Multi-walled CNTs were found in the powders synthesized by self-propagating high temperature synthesis (SHS) and SiC phase existed in the sintering samples. Fine grain and the favorable effect of dispersed SiC particles resulted in a high hardness of the CNTs/MoSi₂ composite. The toughening mechanisms for the CNTs/MoSi₂ composites included crack deflection, crack micro-bridging, crack branching, crack bowing and fine-grain pullout.     

Field-activated pressure-assisted combustion synthesis of MoSi<Subscript>2</Subscript>

The simultaneous synthesis and densification of MoSi₂ starting from elemental Mo and Si was carried out through a process of field-activated pressure-assisted combustion synthesis (FAPACS). After heating up to 922°C, the synthesis reaction occurs in the mode of thermal explosion but is ceased subsequently due to a sudden shock. The product consists of MoSi₂, residual Mo and amorphous Si with precipitation of very small MoSi₂ primary crystals containing. There is no transition phase layer between Mo and MoSi₂ as well as Si and MoSi₂. This indicates that metallic Mo reacts with molten Si directly forming MoSi₂ in the FAPACS process, without any intermediate reaction steps involved. The formation of MoSi₂ occurs via dissolution of Mo into Si melt followed by MoSi₂ precipitation from the melt.      

Macroscopic and microscopic aspects of product formation in the Fe2O3-TiO2-Al disperse system burning in the SHS regime

The macromorphology and micromorphology of the combustion products of a Fe₂O₃-TiO₂-Al powder mixture burning in the regime of self-propagating high-temperature synthesis were studied. Significant differences were found in the structure of the products with variation in the ratio of the mixture components. A mechanism for shape and phase formation was proposed that allows the observed differences to be explained from unified positions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 79–88, May–June, 2006.     

Synthesis of molybdenum silicide/mullite composites for high-temperature applications

Molybdenum silicide/mullite composite having about 31 wt. % MoSi₂ has been SHS-produced from a reactive blend composed of MoO₃, SiO₂, and Al. Additional amounts of silica were added to undergo mulHte formation reaction. The overall reaction involves both exothermic and endothermic ones. The endothermic mullite formation reaction is loaded onto the exothermic reaction of the reactive mixture. The effects of Al grain size (from 45 to 20 μm) and pre-heating temperature (from room temperature to 500°C) on the synthesis of the target composite were studied. Al grain size and preheating temperature were found to have decisive influence on the mullitization reaction. The sample containing 36-μm Al and ignited at 400°C was found to undergo complete mullitization reaction. The mechanism of the overall combustion reaction was postulated. In addition, oxidation resistance of this composite at 1100–1300°C in open atmosphere was determined.     

A novel sulfur-emission free route for preparing ultrafine MoSi2 powder by silicothermic reduction of MoS2

The extraction of metal from sulfide mineral is usually accompanied by the emission of sulfur-containing gas (e.g., SO₂), which will cause serious pollution to the environment. In this work, a sulfur-emission free route for preparing ultrafine molybdenum disilicide (MoSi₂) powder by silicothermic reduction of MoS₂ with the desulfurization agent of lime was proposed. The internal MoS₂-Si mixture is wrapped in an external desulfurization layer composed of lime. After the reaction is completed, the prepared MoSi₂ can be easily separated from the desulfurization layer by using a non-chemical method. In addition, when the reaction temperature is higher than 1000°C, almost all S in MoS₂ is transformed to sulfur-containing gas SiS, which can be fully captured by lime to generate CaS, Si, and CaSiO₃. For the raw material with a MoS₂:Si molar ratio of 1:4, after reacting at 1000, 1100, and 1200°C for 2 h, the average grain sizes of the obtained MoSi₂ powder are approximately 100, 300, and 800 nm, respectively. Moreover, when the reaction time is prolonged from 2 to 6 h at 1000°C, the average grain size of the acquired MoSi₂ powder is about 200 nm, and the residual sulfur content is about 0.12%. This work provides a new insight to extract metals or metal compounds from sulfide ores without releasing sulfur-containing gas.     

X-ray radiation in self-propagating high-temperature synthesis processes

Emission effects of heterogeneous combustion in the region of ionization radiation are studied. By an example of a Ti-B powder system, it is demonstrated that the processes of self-propagating high-temperature synthesis in the thermal explosion regime is accompanied by “soft” X-ray radiation with the quantum energy estimated as ≈5 keV. Key words: heterogeneous combustion, X-ray radiation. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 127–129, November–December, 2008.     

Combustion synthesis of MoSi2 based composite and selective laser sintering thereof

Additive manufacturing is gaining increasing attention as it provides cost-effective and waste-less production of materials with multi-axis geometries. Selective laser sintering of ceramics is very challenging in terms of poor sinterability caused by low thermal shock resistance and insufficient electron conductivity blocking absorption of laser beam energy.Here, we present a novel strategy for manufacturing dense, hierarchically structured ceramics, particularly, MoSi₂-based composites by selective laser sintering. MoSi₂-Si composite powders were prepared by combustion synthesis technique, where the ceramic grains were covered with different amount of Si. MoSi₂-Si powder was consolidated by selective laser sintering reaching 92% of density. The hardness of the manufactured samples varied with the amount of Si and applied laser current from 7.7–11.4?GPa. The maximum value of the compressive strength was determined to be 636?MPa. The manufactured MoSi₂-Si was subjected to nitridation, which resulted in the growth of Si₃N₄ fibres on the surface and pores of the samples.     

Ultrafine Si/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites obtained by combining methods of mechanical activation and self-propagating high-temperature synthesis

Mechanical activation of a mixture of silicon oxide and aluminum is studied by methods of infrared spectroscopy, differential thermal analysis, and x-ray diffraction. If a SiO₂/Al mechanocomposite is used as a precursor, then a Si/Al₂O₃ composite with a small grain size and a uniform distribution of the components can be obtained by the method of self-propagating high-temperature synthesis.     

Additively manufactured mesostructured MoSi2-Si3N4 ceramic lattice

Lattice structures, their shape, orientation, and density make the critical building blocks for macro-scale geometries during the AM process and, therefore, manipulation of the lattice structure extends to the overall quality of the final product. This work reports on manufacturing of MoSi₂-Si₃N₄ ceramic lattices through a selective laser melting (SLM) approach. The strategy first employs the production of core-shell structured MoSi₂/(10-13?wt%)Si composite powders of 3–10?μm particle size by combustion synthesis followed by SLM assembly of MoSi₂/Si lattices and their further nitridation to generate MoSi₂-Si₃N₄ mesostructures of designed geometry. Experimental results revealed that the volumetric energy density of SLM laser has remarkable influence on the cell parameters, strength, porosity and density of lattices. Under compressive test, samples sintered at a higher laser current demonstrated a higher strength value. Selective laser melting has shown its potential for production of cellular lattice mesostructures of ceramic-based composites with a low content of a binder metal, which can be subsequently converted into a ceramic phase to produce ceramic-ceramic structure.     

Effect of hot-dip siliconizing time on phase composition and microstructure of Mo–MoSi2 high temperature structural materials

Mo–MoSi₂ high temperature structural materials were obtained by hot-dip siliconizing method. The results show that the silicon melt and molybdenum substrate have good wettability, molten silicon reacts with molybdenum substrate to form a molybdenum silicide phase with a columnar structure, and a strong MoSi₂ crystallographic preferred orientation (CPO) on the (110) crystal face is characterized. While, the Mo–MoSi₂ high temperature structural materials consist mainly of silicon rich layer, MoSi₂ layer, interface layer and molybdenum substrate. The coating surfaces contain a high silicon concentration (55–65?wt%) than the MoSi₂ layer (30–35?wt%). Moreover, the thicknesses and the average grain sizes of siliconized coatings increase sharply with increasing siliconizing time. The coatings thicknesses are 20, 25, 30, and 50?µm, respectively. And the average grain sizes are 6.9, 9.3, 11.7, and 11.8?µm, respectively.     

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.