Application of self-propagating high-temperature synthesis and mechanical activation for obtaining nanocomposites

A new method is developed for obtaining powder nanocomposites consisting of a metallic or intermetallic matrix and ceramic nanometer particles as a reinforcing phase. This method involves the following consecutive processes: short-time mechanical activation of the mixture of powder reagents in a high-energy planetary ball mill, self-propagating high-temperature synthesis, and additional mechanical activation of the synthesis products. Specific features of the reinforcing phase synthesis in matrices are examined by an example of TiB₂-Cu and TiB₂-TiNi systems. Compaction conditions that allow obtaining volume nanostructural materials with high strength characteristics are found. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 58–71, March–April, 2007.     

AlMgB14–TiB2 composite materials obtained by self-propagating high-temperature synthesis and spark plasma sintering

In this work, AlMgB₁₄–TiB₂ composite materials were obtained by thermochemical-coupled self-propagating high-temperature synthesis (SHS) and subsequent spark plasma sintering. The mechanism was proposed for the formation of the composite materials in the thermochemical-coupled SHS mode. The phase composition, microstructure, and properties (density and Vickers hardness) of the dense AlMgB₁₄–TiB₂ materials were investigated. At a sintering temperature of 1470 °C, AlMgB₁₄ is decomposed into AlB₁₂ and Mg. The sample sintered at 1470 °C with a holding time of 5 min had a maximum average hardness of 32.1 GPa.     

Effect of reactant composition on the production of MoSi2 by self-propagating high-temperature synthesis

The effect of reactant composition, particle size of silicon, density of powdered compacts, and reaction atmosphere on the characteristics of molybdenum disilicide produced from molybdenum and silicon powders by self-propagating high-temperature synthesis, was studied in a pressurized reaction chamber at 1.5 bar. The atomic ratio of silicon to molybdenum (Si/Mo) was changed from 1.0 to 2.6 in order to investigate the effect of reactant composition on the characteristics of self-propagating high-temperature synthesis. Stable combustion was observed for the values of atomic ratios of silicon to molybdenum from 1.8 to 2.2 and SHS-produced material consisted of a uniform and single-phased MoSi₂. In the meantime unstable combustion such as oscillatory, spinning, and surface combustion was detected for the values of atomic ratios of silicon to molybdenum less than 1.8 or larger than 2.2. SHS-produced material under unstable combustion includes the impurities of Mo₅Si₃, Mo₃Si, unreacted Mo and Si resulting from the layered or reacted-on-surface structures, which give lower degree of reaction and possibly poor electrical properties of heating element MoSi₂. The value of criterion α suggested by Shkadinskii et al. to differentiate stable combustion from unstable one, is found to be 0.74 for producing molybdenum disilicide by self-propagating high-temperature synthesis. Stable combustion was detected for the values of α greater than 0.74 (α>0.74) to give the uniform and single-phased product while unstable combustion was observed for the values of α less than 0.74 (α<0.74) to result in a non-uniform and multiphase product. This critical value will help the industry to produce uniform and high-purity molybdenum disilicide by self-propagating high-temperature synthesis processes.     

Experimental and Calculation Analysis of the Behavior of Titanium Diboride-Boron Carbide Cermet under Impact Loading

A generalized mathematical model of the behavior of high-strength single-phase ceramics is proposed and analyzed by the example of titanium diboride TiB₂, boron carbide B₄C, and TiB₂ + B₄C cermet under impact loading. TiB₂-B₄C cermet with a metal binder is produced by self-propagating high-temperature synthesis and is characterized by experimentally confirmed high strength and impact resistance under dynamic loading. Piercing of an aluminum plate by a cermet striker and its deformation and fracture are investigated. A special feature of this interaction is the formation of a conical crater on impact. The mathematical modeling data and experimental results are compared and are found to be in satisfactory agreement.     

Production of TiB2 by SHS and HCl leaching at different temperatures: Characterization and investigation of sintering behavior by SPS

Sub-micron sized TiB₂ ceramic powders were prepared via self-propagating high-temperature synthesis (SHS) followed by HCl leaching at different temperatures. Purified powders obtained using optimum process parameters were consolidated by field assisted sintering technology/spark plasma sintering (FAST/SPS) technique. Phase and microstructural analyses of both the powder and sintered samples were carried out by X-ray diffractometer (XRD) and scanning electron microscope (SEM). The chemical analyses and particle size measurements of the specimen were conducted by inductively coupled plasma-mass spectrometry (ICP-MS) and dynamic light scattering (DLS) techniques. The final properties of the sintered sample were determined in terms of density and microhardness. The effects of different HCl leaching temperatures on the formation, microstructure, particle size, purity and sintering behavior of the SHS-produced TiB₂ powders were investigated. The SHS reaction of TiO₂-B₂O₃-Mg powders as a starting mixture yielded MgO, Mg₃(BO₃)₂ and Mg beside the desired phase TiB₂. All three magnesium containing by-products were completely removed by performing hot HCl leaching. TiB₂ powders after SHS reaction and leaching with 9.3 M HCl for 30 min at 80 °C revealed a minimum purity of 98.4% and a homogenous particle size distribution with an average particle size of 536 nm. In the ultimate SPS experiment which was conducted at 1500 °C for 5 min under a pressure of 50 MPa, a relative density of 94.9% and a micro-hardness value of 24.56 GPa were achieved.     

Effect of TiB2 particles on microstructure and mechanical properties of B4C–TiB2 ceramics prepared by hot pressing

B₄C-20 wt% TiB₂ ceramics were fabricated by hot pressing B₄C and ball-milled TiB₂ powder mixtures. The effects of the TiB₂ particle size on the microstructure and mechanical properties were investigated. The results showed that the TiB₂ particle size played an important role in the mechanical properties of the B₄C–TiB₂ ceramics. In addition, SiO₂ introduced by ball milling was beneficial for densification but detrimental to the mechanical properties of the B₄C–TiB₂ ceramics. The typical values of relative density, hardness, flexural strength, and fracture toughness of the ceramics were 99.20%, 35.22 GPa, 765 MPa, and 7.69 MPa m^1/2, respectively. The toughening mechanisms of the B₄C–TiB₂ ceramics were explained by crack deflection and crack branching. In this study, the effects of high pressure and temperature caused liquefying SiO₂ to migrate to the surface of B₄C–TiB₂ and react with diffused carbon source in the graphite foil to form a 30 μm thick SiC layered structure, which improved the high-temperature oxidation resistance of the material. The unique SiC layered structure overcame the insufficient oxidation resistance of B₄C and TiB₂, thereby improving the oxidation resistance of the ceramics under high-temperature service conditions.     

Enhancement on high-temperature microwave absorption properties of TiB2–MgO composites with multi-interfacial effects

TiB₂–MgO microwave absorbing materials with TiB₂ as the absorber, MgO as the matrix are prepared by spark plasma sintering (SPS). The influences of commercial TiB₂ content and sintering temperature on dielectric and microwave absorption (MA) properties are studied. Besides, to optimize the MA performance, TiB₂–MgO composite containing TiB₂ synthesized by the carbonthermal process is prepared. Meanwhile, its high-temperature dielectric and MA properties are investigated. Indeed, both the commercial TiB₂ content and sintering temperature play key roles in dielectric and MA properties, as they reaching 8 wt% and 1400 °C, the composite presents the optimal MA performance. For composite with synthetic TiB₂ as the absorber, the temperature has a positive effect on dielectric and MA properties. The enhanced high-temperature MA properties with minimum reflection loss (RL_min) of ?52.11 dB at 1.6 mm under 500 °C and effective absorption bandwidth (EAB, RL < ?5 dB) of 4.2 GHz at 1.4–1.6 mm under 800 °C are obtained, which is mainly attributed to the temperature-dependent interfacial polarization compared to the temperature-insensitive conductivity. The excellent mechanical properties (flexural strength = 212.48 MPa), thin absorbing layer (d < 2 mm), enhanced thermal stability and high-temperature MA properties indicate that the TiB₂–MgO composites can be considered as new candidates for high-temperature structure microwave absorbing materials.     

Some properties of composite materials in the SiC−TiB2 system

The possibility of obtaining dense compositions in the SiC−TiB₂ system with phase components of minimum size by hot quasihydrostatic pressing in a high-pressure chamber is studied. Some physicomechanical properties and the heat resistance of the materials obtained are described. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 7, pp. 2–4, July, 1997.     

Rotary kiln corrosion-erosion-resistant linings

The properties of magnesia refractories are studied with the aim of determining the possibility of their use for lining rotary kilns. Periclase-chromite refractory PKhPP has been developed with an increased Cr₂O₃ content, whose use combined with a laying cement, operating by the principle of self-propagating high-temperature synthesis, in a developed “slotted” single-layer lay-up of the furnace reaction zone, made it possible to increase the lining life of industrial rotary kilns by a factor of 1.5–1.8. __________ Translated from Novye Ogneupory, No. 4, pp. 13–17, April 2008     

Design of nanostructured cermet materials high-speed impact conditions

The means of production of nanostructured composite materials by the method of self-propagating high-temperature synthesis (SHS) with the application of pressure to the product of combustion and by the method of intensive plastic three-sided a,b,c deformation are considered. A composite material of combined structure is developed, which is a layer system, one of the layers of which is a product of the exothermic reaction of the Ti-B-Ni system in the form of an interlayer produced by pressing the heated SHS product, and the other layer is inert, the role of which is played by the metal layer. The properties of the developed materials, including the TiB₂ + B₄C-based ceramic metal, are studied under the conditions of high-speed impact.     

Synthesis and characterization of Al2O3 - ZrO2-based eutectic ceramic powder material dispersion-hardened with ZrB2 and WB particles prepared by SHS

Method of self-propagating high-temperature synthesis (SHS) was used to obtain ceramic powder material based on Al₂O₃-ZrO₂-WB/ZrB₂ system with size of particles ranging from 10 to 200?µm. The routes of chemical reactions taking place in the process of synthesis depending on the source mixture composition have been determined. It was shown that the formation of zirconium boride strengthening phase depends on the content of source components В and W and combustion temperature of mixture. The analysis of microstructure has shown that powder particles feature complex composite structure consisting of oxide eutectics of Al₂O₃-ZrO₂, individual phases of aluminum oxide and zirconium oxide as well as particles of WB and/or ZrB2. The technological properties of produced powders have been investigated.     

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.     

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.     

Long-sized rods of Al2O3–SiC–TiB2 ceramic composite material obtained by SHS-extrusion: Microstructure,X-ray analysis and properties

Long-sized rods of Al₂O₃–SiC–TiB₂ ceramic composite material were obtained by SHS-extrusion. The material was synthesized by self-propagating high-temperature synthesis (SHS) followed by high-temperature shear deformation. Ceramic samples app up to 465 mm in length and 5 mm in diameter were obtained. According to the results of XRD and SEM the obtained rods have a composite structure. The matrix is Al₂O₃ with distributed titanium diboride and silicon carbide particles. A uniform phase distribution was observed along the entire length of the rod. The microhardness of the matrix was 25–26 GPa, that of the dispersion-strengthening phases - 32–34 GPa. Heat resistance tests showed that during heat treatment at T = 1000 °C for 21 h, the sample specific weight gain and its real rate were 8.3 g/m² and 1 g/(m²∙h), respectively. The density, hardness and electrical resistivity of the samples obtained in this work were 3.27 g/cm³, 19.5 GPa, 3.1∙10⁻⁵ Ohm∙m, respectively.     

Ultra-STEM-EELS characterization of MgB<Subscript>2</Subscript> superconductor synthesized by different routes: Comparative study

We report a comparative study on the synthesis of MgB₂ by electrothermal explosion (ETE), self-propagating high temperature synthesis (SHS), and high-temperature sintering. Advantages of these methods are demonstrated by comparing the structural and electronic properties of final MgB₂ with a superconducting transition temperature of above 39 K. The structural properties of materials were studied by XRD and SEM/EDS (chemical cartography), while imaging and electronic properties were investigated by Ultra-STEM microscopy and electron energy loss spectroscopy (EELS), respectively. The results show that the synthesized bulk compounds are highly dense, with strong intergrain connections. The EEL spectra show the peaks of σ- and anti-bonding bonds, which is in good agreement with the theoretically calculated band structure, Fermi surface, and de Haas-van Alphen frequencies.     

Microstructural and mechanical characterization of SiC-submicron TiB2 composites

The aim of this work is to characterize ceramic composites SiC–TiB₂. After preparation of dense composites αSiC–TiB₂ (5, 10 and 15 vol% TiB₂) by reactive pressureless sintering, the materials have been characterized by their microstructure and their mechanical properties. The dispersion of TiB₂ particles is quite homogeneous, observed both by optical and scanning electron microscopies. Image analysis has revealed a majority of submicronic particles. Atomic force and transmission electron microscopies have shown the presence of nanometric TiB₂ particles. Concerning mechanical properties, toughness increases with the TiB₂ content, whereas hardness decreases when the TiB₂ content increases.     

The mechanism of phase and structure formation of the Ti-B-Fe system in a combustion wave

The mechanism of phase changes in the Ti-B-Fe system in a combustion wave for a mixture of Ti, B, and Fe powders and a ferroboron alloy-titanium mixture with the same proportion of elements is studied. It is found that the mechanism of structure formation depends significantly on the type of contact between the initial components. An x-ray phase and x-ray spectrum and structural microanalyses of the quenched layers of specimens show that the first contact melts occurring in the combustion wave are ferroboron (the first type of mixture) or ferrotitanium (the second type) melts. In the first case, the calculated high-melting compound TiB₂ forms as a result of the interaction between the two melts; in the second case, it forms as a result of the interaction of the melt with solid ferroboron, which, in turn, determines the different type of microstructure of the final combustion products. The highly disperse and more homogeneous structure of the products forms after combustion of the second-type mixture. A method of producing the Ti-B-Fe pore-free composite produced during the self-propagating high-temperature synthesis (SHS-composite) by combining the combustion with rolling of the synthesis products is considered. In properties, theresulting material is similar to tungsten-carbide materials. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 5, pp. 27–34, September–October, 2000. This work was supported by the Federal Program “Integration.”     

B4C‒TiB2 composite with modified microstructure and enhanced properties from optimal size coupling of raw powders

B₄C‒15 vol% TiB₂ composites were fabricated by in situ reactive spark plasma sintering with B₄C, TiC, and amorphous B powders as the raw materials. The size coupling of initial B₄C and TiC particles was optimized based on the reaction mechanism to derive B₄C‒TiB₂ composites with enhanced microstructure and properties. During the reactive sintering, fine B₄C–TiB₂ particles were firstly formed by an in situ reaction between TiC and B. Then, large B₄C particles tended to grow at the cost of small B₄C particles. The in situ TiB₂ grains gradually grew up and interconnect, distributing around the large B₄C grains to form an intergranular TiB₂ network. The results showed that the B₄C‒15 vol% TiB₂ composite prepared from 3.12 μm B₄C powder and 0.80 μm TiC powder had the optimal comprehensive properties, with a relative density of 99.50%, a Vickers hardness of 31.84 GPa, a flexural strength of 780 MPa, a fracture toughness of 5.77 MPa·m^1/2, as well as an electrical resistivity of 3.01 × 10⁻² Ω·cm.     

Effect of the solid precursors on the formation of nanosized TiBx powders in RF thermal plasma

Continuous synthesis of TiB_x (x≈0.5–2) nanoparticles from various low cost solid precursors such as titanium and titanium dioxide admixed with boron and/or carbon in radiofrequency thermal plasma was studied. Feasibility of TiB₂ formation was predicted by thermodynamic equilibrium calculations in the 500–5000 K temperature range. In all the investigated system high temperature reactions resulted in nanometer-sized TiB_x powders with a mean size varying between 13 and 83 nm. The yield of particular runs ranged from 38% to 97%. Among the synthesized products in addition to TiB_x, oxidized precursor residues were also found in smaller quantities. Although addition of carbon to the precursors could not completely prevent surface oxidation of boride particles, it contributed to the reduction of the mean particle size of the formed TiB₂.     

Shape-formation in the processes of self-propagating high-temperature synthesis of foams

Self-propagating high-temperature synthesis of cylindrical specimens of foam cermet with a relatively large length (l/D≫1) is performed. A Ti+0.6C mixture containing foaming agent Na₂B₂O₇·10H₂O was used. The initial compacts were enclosed in burning paper shells, which during self-propagating high-temperature synthesis made it possible to produce straight rods of foam ceramics, distinguished by an even surface. The dependence of the relative elongation of the specimens on the shell thickness was established. The possibility of controlling the shape of the foam SHS product by varying the slope of the combustion-front plane to the specimen axis was considered. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 121–122, May–June 1998.