Self-propagating High-temperature Synthesis Powders Research Articles

Effect of SHS powder processing on structure formation and optical transmittance of MgO–Y2O3 composite ceramic

Extending the transparency range of MgO–Y2O3 composite ceramic to wavelength less than 3 μm is highly desirable, as it broadens their potential applications in infrared (IR) optics. This study illustrates the effectiveness of deagglomerating a mixture of magnesium oxide and yttrium sesquioxide powders, derived from self-propagating high-temperature synthesis (SHS), in achieving this goal. The impact of powder deagglomeration on the microstructure and porosity defects of hot-pressed MgO–Y2O3 ceramic was analyzed using electron and IR microscopy. The results showed that the deagglomeration enhanced the homogeneity of the composite grain mixture, slowed down the grain growth, suppressed the formation of large grains, and significantly reduced the number of porosity microdefects in the material. The optical transmittance increased from 42 % to 77 % at a wavelength of 2 μm (for samples with a thickness of 1.2 mm) when deagglomerated powders were used, compared to non-deagglomerated ones.

Influence of partial titanium substitution by its hydride on structure and mechanical properties of TNM-B1 heat-resistant alloy, obtained by SHS powder hot isostatic pressing

This paper investigates the influence of partial substitution of titanium by its hydride on the microstructure and mechanical properties of TNM-B1 alloy obtained by powder metallurgy technology. The impact of the Ti:TiH2 ratio in the reaction mixture and heat treatment modes on the microstructure and mechanical properties of TNM-B1+1%Y2O3 alloy, obtained using high-energy ball milling (HEBM), selfpropagating high-temperature synthesis (SHS), and hot isostatic pressing (HIP) methods, has been examined. It was observed that a 10 % substitution of titanium with its hydride in the reaction mixtures reduces the oxygen content in SHS products from 1 % to 0.8 % due to the generation of a reducing atmosphere during the decomposition of TiH2 in the combustion wave. When the Ti : TiH2 ratio is 90 : 10, highest mechanical properties of TNM-B1+1%Y2O3 alloy were achieved: a compressive strength (σu) of 1200±15 MPa and a yield strength (YS) of 1030±25 MPa. An increase in the proportion of TiH2 results in a higher content of oxygen impurity, leading to the formation of Al2O3, which reduces the strength and ductility of the material. With additional heat treatment of TNM-B1+1%Y2O3 alloy, the globular structure transforms into a partially lamellar one, leading to an increase in σu by 50–300 MPa, depending on the TiH2 content. This attributed to a decrease in the average grain size and a reduction in dislocation mobility during deformation.

Densification mechanism during hot-pressing of single-phase zirconium boride powders with different dislocation density

ZrB2 powders with different dislocation density were synthesised by the self-propagating high-temperature synthesis (SHS) and carbothermal reduction (CTR) method. The defect structure was analysed by transmission electron microscopy (TEM), and the dislocation density was determined though the convolutional multiple whole profile fitting (CMWP-fit). The SHS process exhibited a very high rate of heating and cooling, which introduces a higher dislocation density (8.68 × 1015 m-2) due to thermal stresses caused by the volume change of crystal, and the crystal grew spirally. Comparably, the CTR process depicted a low rate of heating and cooling. The time was sufficient for crystal nucleation and growth, thus the dislocation density of crystal was lower (6.49 × 1011 m-2), and the crystal grew layer-by-layer. By mixing with the SHS and CTR powders, 5 kinds of powders with different dislocation density were prepared, followed by hot-pressing densification. The dislocation density of powder was calculated and result shows that with the increasing of the content of SHS powder from 0 to 1 the dislocation density increases from 6.49 × 1011 m-2 to 8.68 × 1015 m-2, meanwhile the relative density of ZrB2 ceramics increased from 82.1% to 98.1%. The hot-pressing densification can be divided into two stages. In the pre-sintering stage, with increasing of dislocation density of the powders, the densification mechanism changes from bulk diffusion to grain boundary diffusion, and all are of the dislocation-climb mechanism in the post-sintering stage.

On the Structure and Properties of AlMgB14-TiB2 Composites Obtained from SHS Powders by Spark Plasma Sintering.

AlMgB14-TiB2 composite materials were fabricated by self-propagating high-temperature synthesis (SHS) followed by spark plasma sintering of the obtained SHS products. It was found that, during the SHS, the AlMgB14 phase is formed at a donor (Ti + 2B) to acceptor (Al12Mg17-B) mass ratio of 3:7 and 4:6, respectively. The specimen sintered from the SHS powder with the donor:acceptor mass ratio of 5:5 at a temperature of 1470 °C has a uniform skeletal structure. The average hardness of the obtained specimen is 30.1 GPa.

Preparation of ZrB2-MoSi2 high oxygen resistant coating using nonequilibrium state powders by self-propagating high-temperature synthesis

To achieve high oxygen blocking structure of the ZrB2-MoSi2 coating applied on carbon structural material, ZrB2-MoSi2 coating was prepared by spark plasma sintering (SPS) method utilizing ZrB2-MoSi2 composite powders synthesized by self-propagating high-temperature synthesis (SHS) technique as raw materials. The oxygen blocking mechanism of the ZrB2-MoSi2 coatings at 1973 K was investigated. Compared with commercial powders, the coatings prepared by SHS powders exhibited superior density and inferior oxidation activity, which significantly heightened the structural oxygen blocking ability of the coatings in the active oxidation stage, thus characterizing higher oxidation protection efficiency. The rise of MoSi2 content facilitated the dispersion of transition metal oxide nanocrystals (5–20 nm) in the SiO2 glass layer and conduced to the increasing viscosity, thus strengthening the inerting impact of the compound glass layer in the inert oxidation stage. Nevertheless, the ZrB2-40 vol%MoSi2 coating sample prepared by SHS powders presented the lowest oxygen permeability of 0.3% and carbon loss rate of 0.29×10−6 g·cm−2·s−1. Owing to the gradient oxygen partial pressure inside the coatings, the Si-depleted layer was developed under the compound glass layer, which brought about acute oxygen erosion.

SHS-Derived Powders by Reactions' Coupling as Primary Products for Subsequent Consolidation.

The capability of self-propagating high-temperature synthesis (SHS) to produce powders that are characterized by a high sintering ability, owing to high heating and cooling rates inherent to the exothermic reaction, is of a special interest for the industry. In particular, SHS-derived powders comprise a significant defect concentration in order to effectively enhance the mass transfer processes during the sintering, which allows for the successful consolidation of difficult-to-sinter materials at relatively low sintering temperatures. From this perspective, the design of precursors suitable for sintering, synthesis in a controlled temperature regime and the optimization of geometrical and structural parameters of SHS powders as a potential feedstock for the consolidation is of key importance. Here, we report on the comparative studies concerning the SHS processing of composites for advanced powder metallurgy techniques. The synthesis and sintering peculiarities of the SHS through coupled reactions in the Me’O3(WO3,MoO3)-Me’’O(CuO,NiO)-Mg-C, Ti-B-Al12Mg17 systems are comparatively reviewed. The SHS coupling approach was used for the preparation of powders with a tuned degree of fineness (a high specific surface area of particles), a high-homogeneity and a controllable distribution of elements via both the regulation of the thermal regime of combustion in a wide range and the matching of the thermal and kinetic requirements of two interconnected reactions. Microstructural features of the powder feedstock greatly contributed to the subsequent consolidation process.

Structural and mechanical properties of Ti–Al–Nb–Mo–B alloy produced from the SHS powder subjected to high-energy ball milling

Ti–Al–Nb–Mo–B alloy was successfully produced via high-energy ball milling (HEBM), self-propagating high-temperature synthesis (SHS) and hot isostatic pressing (HIP) techniques. HEBM was used to prepare powder mixtures with different types of microstructure and homogeneous distribution of the components. After SHS in thermal explosion mode powder grains with a microgradient structure were synthesized: their central part consists of α2-Ti3Al and the peripheral part consists of γ-TiAl. Compacted samples with duplex fine-grained structure and high thermomechanical properties (σ0.2 = 1245 MPa and 524 at 20 °C and 800 °C, respectively) have been obtained using the HIP technology. It was found, that mechanical twinning is the preferential deformation mechanism of the studied Ti–Al–Nb–Mo–B alloy.

Structure, properties and oxidation resistance of prospective HfB<sub>2</sub>–SiC based ceramics

The paper focuses on obtaining a heterophase powdered and sintered ceramics based on hafnium diboride and silicon carbide by combined self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The structure of the synthesized SHS powder consists of hafnium diboride grains and agglomerated polyhedral 2–6 μm silicon carbide grains. The powders obtained had an average particle size of ~10 μm with a maximum value of 30 μm. Phase compositions were identical for the ceramics sintered by hot pressing and the synthesized powder. The resulting compact featured by a high degree of structural and chemical uniformity, porosity of 3.8 %, hardness of 19.8±0.4 GPa, strength of 597±59 MPa, and fracture toughness of 8.8±0.4 MPa·m1/2. Plasma torch testing (PTT) was carried out to determine the oxidation resistance under the influence of a high-enthalpy gas flow. The phase composition and surface microstructure of the compact after testing were investigated. The HP compact demonstrated an outstanding resistance to the high-temperature gas flow at 2150 °С and heat flow density of 5.6 MW/m2 for 300 s. A dense protective oxide layer 30–40 μm thick was formed on the surface of HfB2–SiC ceramics during the plasma torch testing. The layer consisted of a scaffold formed by HfO2 oxide grains with a space between them filled with SiO2–B2O3 amorphous borosilicate glass. The HfB2–SiC SHS composite powder was hot pressed to produce experimental samples of model bushings for the combustion chamber of a low thrust liquid rocket engine designed for PTT in the environment close to actual operating conditions.

Magnetic, electrical, and structural properties of Mg2+-doped nickel–zinc ferrite prepared by sol–gel–SHS method

Magnesium-substituted Ni–Zn ferrite (Ni0.5Zn0.5−xMgxFe2O4, x = 0–0.3, in steps of 0.05) has been successfully synthesized via a sol–gel–SHS (self-propagating high-temperature synthesis) method. The Fourier transform infrared spectra indicate the formation of spinel structure for Ni–Mg–Zn ferrites by sol–gel–SHS method. Thermogravimetric analysis reveals that the lowest decomposition temperature of SHS powder is approximately 400 °C. X-ray powder diffraction results signify a single-phase nature of cubic spinel structure for these samples without any detectable impurity phase. Lattice constant gradually decreases as the magnesium content continuously increases, while the average crystallite size increases except for undoped sample. Scanning electron microscopy images demonstrate that the samples consist of regular shaped grains without any phase segregation and agglomeration. Magnetization measurements indicate that all samples display a typical soft ferromagnetic characteristic with an abnormal saturation magnetization peak at x = 0.15. The frequency dependence of dielectric constant of the synthesized Ni–Mg–Zn ferrite exhibits a normal dielectric behavior, while the dielectric loss shows an abnormal peak behavior. The modulation mechanism for magnesium-substituted Ni–Zn ferrite on structure, electrical, and magnetic properties is systematically discussed.

Интервалы плавления гетерофазных систем Zr – Si – ZrB2 – ZrSi2 – MoSi2 и Zr – Si – HfB2 – HfSi2 – MoSi2

The work is devoted to studying the melting ranges of the base Zr – Si eutectic composition depending on the content of the heterophasic powder component in the ZrB2 – ZrSi2 – MoSi2 and HfB2 – HfSi2 – MoSi2 systems in an amount of 30 – 90 % obtained by the method of self-propagating high-temperature synthesis (SHS). The melting range of the mixture Zr – Si was 1420 – 1440 °C, while the addition of SHS-powders ZrB2 – ZrSi2 – MoSi2 led to an increase in the melting onset temperature Тmelt.onset to 1460 – 1560 °С and the complete melting temperature Tmelt.complete to 1480 – 1670 °C. The addition of HfB2 – HfSi2 – MoSi2 powders had a weak effect on the values of Тmelt.onset (1390 – 1430 °С), but led to an increase in the values of Tmelt.complete to 1510 – 1550 °С. X-ray phase analysis showed that the remelted samples contained ZrB2/HfB2, ZrSi2/HfSi2, MoSi2 phases and Si, with the number of phases being directly proportional to the content of SHS powders in the composition of the Zr – Si mixture. The ingots were characterized by a homogeneous structure consisting of a silicon matrix, ZrSi2/HfSi2, MoSi2 disilicide grains, with ZrB2/HfB2 diboride inclusions.

Synthesis and Electron Beam Facing of Titanium Monoboride–Titanium Matrix Composite Powders

Titanium monoboride–titanium composite powders with the titanium matrix content from 20 to 60% are obtained by self-propagating high temperature synthesis (SHS) in titanium and boron powder mixtures. The SHS powders are used for cladding of coatings with a thickness of up to 3 mm on a VT1-0 titanium sheet by electron beam facing. The phase composition and microstructure of the initial powders and cladded coatings are investigated via X-ray diffraction analysis and optical and scanning electron microscopy. On the basis of the microstructure investigation and microhardness profiles in the coating/base plate transition zone, a conclusion about effective adhesion of the coating to the base plate is made. The dependence of the hardness and abrasive wear resistance of the cladded coatings on the phase composition of the powders used for cladding is studied. It is found that the hardness of the coatings strengthened by needle-like titanium monoboride inclusions in the titanium matrix is 2.2-fold higher and abrasive wear is 3.7-fold higher when compared to the properties of VT1-0 titanium. Here, the effect of strengthening and increase in the wear resistance of the titanium matrix by titanium monoboride particles is much smaller than similar effects in the case of the use of disperse titanium carbide particles as the strengthening phase in cladded coatings.

Magnesiothermal Synthesis and Consolidation of the Multicomponent Powder Ceramics in the Zr–Si–Mo–B System

This work is devoted to the fabrication of ZrB2–ZrSi2–MoSi2-based composite powder ceramics by self-propagating high-temperature synthesis (SHS) according to the scheme of magnesium thermal reduction from oxide feedstock and its subsequent hot-pressing (HP) consolidation. The combustion of reaction mixtures has rather high adiabatic temperatures in a range of 2060–2120 K and burning rates in a range of 8.3–9.4 g/s. The end product yield of the magnesiothermal reduction is 34–38%. The resulting powder contains 13–47% ZrB2, 21–70% ZrSi2, 2–32% ZrSi, and 10–18% MoSi2, depending on the composition of the initial reaction mixture; has high structural homogeneity; and consists of composite polyhedral particles with an average size on the order of 8 μm. The structure of ceramics consolidated by HP from the SHS powder is homogeneous and includes needle ZrB2 grains distributed in the ZrSi2 matrix, MoSi2 inclusions of various morphologies, and inclusions of ZrSiO4 silicate distributed over the ZrSi2 grain boundaries. The HP-formed samples have a high degree of homogeneity of the chemical composition and residual porosity of 2.5–7.4%.

Heretophase Ceramics in the Hf–Si–Mo–B System Fabricated by the Combination of SHS and Hot Pressing Methods

This work is devoted to the fabrication of heterophase powder and consolidated ceramics based on hafnium and molybdenum borides and silicides by combining self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic HfB2–HfSi2–MoSi2 SHS powders are fabricated according to the magnesium-thermal reduction flowsheet from oxide raw materials, in which the combustion wave has temperatures of 1750–2119 K and rather high mss combustion rates of 8.4–9.3 g/s. The structure of synthesized SHS powders consists of relative coarse MoSi2 grains up to 10 μm in size, submicron elongated HfB2 grains mainly located inside the MoSi2 grains, and rounded Si precipitates. The composition with a lower boron concentration contains numerous polyhedral HfSi2 grains smaller than 10 μm in size. The resulting powders have an average particle size of ~6 μm with a maximal size up to 26 μm. The phase compositions of the ceramics consolidated by the HP method and synthesized SHS powders are identical. The microstructure of compact samples consists of faceted elongated HfB2 grains 0.5–10.0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size, and silicon interlayers. The consolidated ceramics have a high structural and chemical homogeneity, low residual porosity of 1.1–1.7%, high hardness of 11.7–12.6 GPa, and thermal conductivity of 62–87 W/(m K).

Heterophase ceramics in the Hf–Si–Mo–B system obtained by a combination of SHS and hot pressing methods

The paper focuses on obtaining heterophase powder ceramics and consolidated ceramics based on borides and silicides of hafnium and molybdenum by combining the methods of self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic SHS powders HfB2–HfSi2–MoSi2 were obtained according to the scheme of magnesium-thermal reduction from oxide raw materials where the combustion wave is characterized by temperatures of 1750–2119 K and high mass combustion rates of 8,4– 9,3 g/s. The structure of synthesized SHS powders consists of relatively large MoSi2 grains up to 10 μm in size and submicron elongated HfB2 grains located mainly inside the MoSi2 grains and rounded Si precipitates. The composition with a lower concentration of boron contains a large number of polyhedral HfSi2 grains with a size of less than 10 μm. The resulting powders are characterized by an average particle size of ~6 μm with a maximum size up to 26 μm. Phase compositions of ceramics consolidated by the HP method and SHS synthesized powders are identical. The microstructure of compact samples consists of faceted HfB2 elongated grains 0,5– 10,0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size and silicon interlayers. Consolidated ceramics has a high structural and chemical homogeneity, low residual porosity of 1,1–1,7 %, high hardness of 11,7–12,6 GPa and thermal conductivity of 62–87 W/(m·K).

Magnesiothermal synthesis and consolidation of multicomponent powder ceramics in the Zr–Si–Mo–B system

The work aims to obtain composite powder ceramics based on ZrB2–ZrSi2–MoSi2 by the self-propagating high-temperature synthesis (SHS) according to the scheme of magnesium thermal reduction from oxide raw materials, as well as its subsequent consolidation by hot pressing (HP). The combustion of the reaction mixtures is characterized by rather high adiabatic temperatures in the range of 2060 to 2120 K and burning rates in the range of 8,3 to 9,4 g/s. The yield of the end product with magnesiothermal reduction is 34–38 %. The resulting powder contains 13–47 % ZrB2, 21–70 % ZrSi2, 2–32 % ZrSi, and 10–18 % MoSi2 depending on the composition of the initial reaction mixture. It is characterized by high structural homogeneity and consists of composite particles of polyhedral shape with an average about 8 microns in size. The structure of ceramics consolidated by the HP method from SHS powder is homogeneous and includes ZrB2 needle grains distributed in a ZrSi2 matrix, MoSi2 inclusions of various morphology and ZrSiO4 silicate, distributed along the grain boundaries of ZrSi2. The samples obtained by HP are characterized by a high degree of homogeneity of the chemical composition and a residual porosity of 2,5–7,4 %.

Synthesis and electron beam facing of TiB – TiC – titanium matrix hybrid composite powders

TiB – TiC – titanium matrix composite powders have been synthesized by self-propagating high temperature synthesis (SHS) in titanium, boron and carbon reactive powder mixtures. A target volume content of the titanium matrix (binder) in the powders was 50%. The SHS powders were cladded on VT1-0 titanium sheet by electron beam facing. A thickness of the cladded coatings varied from 1 to 3 mm depending on the pass number. A phase composition and a structure of the SHS powders and of the cladded coatings were invetigated be X-ray diffraction, optical and scanning electron microscopy. According to structure investigation and hardness profiles in the “coating – titanium base plate” transition zone an adhesion of the coating to the base is high. The hardness and abrasive wear resistance tests of the cladded coatings were carried out depending on the powder used for cladding. The maximum hardness increase of the coatings strengthened by titanium monoboride and titanium carbide inclusions is 2.2 times and abrasive wear resistance – 4.3 times as compared with VТ1-0 base. According to authors’ earlier results enhance hardness and abrasive wear resistant effects of titanium matrix by titanium carbide particles and titanium monoboride is near to the wear resistance of coatings deposited with SHS TiB + Ti powder, but ~5 times less than the wear resistance of coatings surfaced with SHS powder TiC + Ti.

Synthesis and electron beam facing oftitanium monoboride – titanium matrix composite powders

Titanium monoboride – titanium matrix composite powders have been synthesized by self-propagating high temperature synthesis (SHS) in titanium and boron reactive powder mixtures. Titanium matrix (binder) content varied from 20 to 60%. The SHS powders were cladded on VT1-0 titanium sheet by electron beam facing. Cladded coatings’ thickness varied from 1 to 3 mm depending on the pass number. Phase composition and structure of powders and coatings were investigated by X-ray diffraction, optical and scanning electron microscopy. According to structure investigation and hardness profiles view in the “coating – titanium base plate” transition zone an adhesion of the coating to the base is high. The hardness and abrasive wear resistance tests of the cladded coatings were carried out depending on the powder used for cladding. The maximum hardness of the coatings strengthened by eagle-like titanium monoboride inclusions as compared with VТ1-0 base increases 2.2 times and abrasive wear resistance 3.7 times. According to previously obtained results hardening and abrasive wear resistance of titanium monoboride is much weaker than that of titanium carbide: hardness increases 1.7 times, wear resistance 5.8 times.

Investigation of SHS-products in titanium and carbon powder mixtures with excess of titanium content

The products of self-propagating high-temperature synthesis (SHS) in the reaction mixtures of titanium and carbon (carbon black) containing an excess of titanium content with the purpose of obtaining composite “titanium carbide–titanium binder” powders from the synthesis products were studied by X-ray diffraction analysis, optical and scanning electron microscopy. Based on the results of the investigation of the phase and elemental composition of SHS powders synthesized in argon, the formation of non-stoichiometric titanium carbide has been established which led to a decrease in the content of the titanium binder in composite powders in comparison with the target values. The results of the study of the morphology and structure dependencies of the carbide phase in composite powders on the content of titanium in reactive mixtures have been discussed in conjunction with the results of the determination of the thermo-kinetic characteristics of SHS in the wave combustion mode.

Novel processing route for the fabrication of bulk high-entropy metal diborides

A single high-entropy phase material with hexagonal structure is produced by a two-steps processing method. Elemental reactants are first remarkably converted by Self-propagating High-temperature Synthesis (SHS). The completion of the chemical transformation to the desired (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 phase and its concurrent consolidation up to 92.5% relative density is achieved by processing the SHS powders at 1950 °C via Spark Plasma Sintering. It is clearly evidenced that the use of the SHS technique is extremely beneficial to promote the formation of high-entropy ceramics, as compared to the time consuming ball milling treatment alternatively adopted.

Microstructure and Wear Resistance of Detonation Coatings Obtained from Titanium Carbide–Nichrome SHS Powders Having Different Particle Sizes

The size effect of particles (20–45, 45–56, 56–71, 71–90 μm) of TiC–NiCr n vol % (n = 30, 40, 50) self-propagating high-temperature synthesis (SHS) cermet powders synthesized in the direct combustion mode on the tribological characteristics of detonation coatings is studied. These powders are sprayed in a mode optimized previously using Cr3C2–NiCr 21 vol % commercial cermet powder (PraxAir 1375VM, USA) with particle sizes ranging from 20 to 45 μm.