Polyacrylonitrile derived carbon-fiber fabric and C/SiC core-shell fiber fabric have been examined to assess their applicability to the synthesis of fiber-reinforced Ti3SiC2 MAX phase ceramic composites using a method based on thermally initiated combustion reaction followed by an in-situ reactive melt infiltration process. The synthesis was conducted in a vacuum hot press furnace at 1500 degrees C under 2.5 MPa for 30 min. Microstructural analysis of the synthesis products revealed that the behavior of carbon fibers and C/SiC core-shell fibers toward Ti-rich melts differed significantly from each other, resulting in the different degrees of fiber fabric impregnation. In the case of the carbon fiber fabric, a large portion of the fibers were found to remain essentially unimpregnated. In the case of the C/SiC core-shell fiber fabric, all fabric layers were well infiltrated with matrix material, but the fibers themselves underwent severe structural degradation and were therefore recognized to be virtually incompatible with Ti-rich melts.

SiC particles coated with Ti-Si-C layer were prepared by the molten salt synthesis method using NaCl-KCl eutectic mixture as the reaction medium and Ti metal powder as the Ti source. The synthesis was conducted under static argon atmosphere at 900 degrees C for 1-3 h. The post-synthesis treatment included dissolution of salts in hot water, followed by ultrasonic dispersion and sedimentation procedures. The thickness of the Ti-Si-C layer varied from 0.1 to 0.9 mu m as the ratio of Ti to SiC components in the starting mixture changed from 0.05 to 0.4. The layer contained mainly nanocrystalline Ti5Si3, minor phases were TiC and Ti3SiC2. The as-prepared powders were hot pressed under 30 MPa at 1700 degrees C. The densification behavior of the samples during hot pressing as well as changes both in phase composition and in microstructure were studied. The flexural strength of the hot-pressed samples increased with an increase in the Ti:SiC molar ratio, giving the value as high as 213 +/- 20 MPa for the sample with Ti/SiC = 0.4.

The paper is devoted to the microstructural evidence of key reaction steps in the combustion synthesis of SiC particulate reinforced Ti3SiC2 ceramic matrix composites from multilayer stack of titanium foils and polymer films filled with SiC and TiC particles. It was found that the primary interaction step results in the formation of intermediate product layer, which exhibits a duplex microstructure consisting of a continuous Ti5Si3 matrix embedded with islands of TiC0.52 particles. Next, in the combustion step, a significant rise in the sample temperature leads to complete melting of the titanium foils. The molten titanium infiltrates then the area occupied by SiC and TiC particles. Finally, the peritectic reaction between Ti-rich melt and carbide particles results in the formation of a Ti3SiC2 MAX phase matrix. An excess quantity of SiC particles remains uniformly entrapped within the matrix, eventually giving a microstructure of particulate-reinforced ceramic matrix composite.

A novel 312-type MAX-phase solid solution series in the Zr-Ti-Si-C system has been synthesized by the vacuum carbosilicothermic reduction method using mixtures of TiO2, ZrO2, SiC, and Si powders as starting materials. The upper limit for Zr content in metal sublattice of the synthesized (Zr,Ti)3SiC2 MAX phase solid solutions was found to be as much as approximately 66 at%, closely corresponding to a hypothetical quaternary Zr2TiSiC2 MAX phase. A wide miscibility gap inside the interval of Zr content in metal sublattice ranging between 22 at% and 55 at% was found. Crystal structure of the synthesized MAX-phase solid solutions was studied by HR-STEM/HAADF and XRD Rietveld analyses. The lattice constants were determined to be linearly correlated with Zr content as predicted by Vegard's law. A significant inhomogeneity in distribution of metal atoms similar to that of out-of -plane ordered quaternary MAX phases has been established for both Ti-rich and Zr-rich MAX-phase solid solutions.

The authors have developed a three-stage technology for making dense Ti3SiC2–TiB2–(TiC)–SiC ceramic composites of a leucoxene concentrate being a product of previous treatment of titanium-containing sandstones. The first stage means the synthesis of agglomerated Ti3SiC2–TiB2–SiC powders which may significantly differ in SiC content. The synthesis proceeds by the method of the vacuum carbosilicothermic reduction of leucoxene concentrate using SiC as a reducing agent with addition of B4C as a solid boron-containing component. The second stage is etching the obtained powders with hydrofluoric acid in order to remove the by-products of silicide composition having been formed of impurities in leucoxene concentrate. At the final third stage, the purified Ti3SiC2–TiB2–(TiC)–SiC powders are hot-pressed in a graphite die under 30 MPa at a temperature of 1500-1550 °C. The end product is Ti3SiC2–TiB2–(TiC)–SiC ceramic composites with nearly absolute pore-free microstructure.

A novel quaternary 413-type Zr3TiSiC3 MAX phase has been synthesized in 97% yield by the vacuum carbosilicothermic reduction (VCSTR) method using mixtures of TiO2, ZrO2, SiC, and C powders as starting materials. The crystal structure of the synthesized Zr3TiSiC3 was studied by X-ray diffraction Rietveld analysis. The lattice constants of Zr3TiSiC3 MAX phase were a = 0.328292(5) nm and c = 2.39651(6) nm. It was revealed that the distribution of Zr and Ti atoms was fairly uniform in the basal plane of lattice, but was markedly inhomogeneous along the c-axis, with Zr atoms preferentially occupying the inner layers in the [M4X3] blocks rather than outer layers. The occupations of the outer and inner layers by Zr atoms were found to be approximately 67.4% and 82.6%, respectively, indicating a rather poor chemical ordering of Zr and Ti atoms in metal sublattice of Zr3TiSiC3 MAX phase.

This paper reports on vacuum carbosilicothermic reduction of zirconium(IV), hafnium(IV), vanadium(V), niobium(V), tantalum(V), and chromium(III) oxides at a temperature of 1600°C, using silicon carbide as a reducing agent. It has been shown that the only phase resulting from the reduction of hafnium oxide is hafnium carbide. The reduction of chromium and vanadium oxides yields only chromium silicide and vanadium silicides, respectively. The reduction of zirconium, niobium, and tantalum oxides leads to the formation of carbides and silicides of the respective metals. No ternary compounds similar to MAX phases have been obtained.

Dense Ti3SiC2-TiB2-(TiC)-SiC composites have been fabricated from leucoxene concentrate, a product of pre-liminary processing of titanium-containing sandstones, by a three-step technique. In the first step, Ti3SiC2-TiB2-SiC agglomerated powders substantially differing in SiC content were prepared by the method of the vacuum carbosilicothermic reduction (VCSTR) synthesis using SiC as a reductant and B4C additive as a solid boriding agent. The unwanted impurities were removed from the products of the VCSTR synthesis by leaching with hydrofluoric acid in the second step. In the third step, the purified Ti3SiC2-TiB2-SiC powders were hot -pressed in a graphite die under 30 MPa at 1500-1550 degrees C, resulting in nearly fully dense ceramics. The values of flexural strength and fracture toughness measured for the prepared leucoxene-derived ceramics fell in the ranges 450-600 MPa and 5.6-7.0 MPa m1/2, respectively, with the best results obtained for the sample with low SiC content. It was also shown that when SiC content was quite low, a part of Ti3SiC2 may have decomposed during the hot pressing step, resulting in the formation of the corresponding amount of TiC. These observations thus suggest that the presence of SiC particles in the composites play an ambivalent role, on the one hand, negatively affecting strength properties, but on the other hand, preventing the decomposition of Ti3SiC2 during the hot pressing step. According to the results obtained from analyses of microstructure and strength properties of the leucoxene-derived Ti3SiC2-TiB2-(TiC)-SiC composites fabricated through the VCSTR synthesis, it was concluded that these materials have high potential to be applied as a promising structural ceramics.

Novel C/SiC core-shell fibers have been synthesized through incomplete conversion of carbon fibers by their siliconization with SiO gas. The synthesis was performed in the laboratory-made semi-closed batch-type reactor at 1380 degrees C for 3 h using a 9:1 M ratio mixture of Si and SiO2 powders as a solid source of SiO gas. The conversion rate of carbon into SiC was 34.0%. All synthesized fibers had a distinct C/SiC core-shell composite structure. The fiber product was of fairly good uniformity in respect of the shell thickness which varied approximately from 0.6 mu m to 0.8 mu m depending on the location of fibers inside the reactor. It was revealed that the formation of the shell was the result of inward growth of the SiC product layer. The effectiveness of the proposed semi-closed reactor for the synthesis of C/SiC core-shell fibers has been demonstrated.

A new approach to the fabrication of C/SiC core–shell composite fibers through the high-temperature siliconization of carbon fibers with SiO gas in a laboratory-made semi-batch reactor using a granulated 9Si + SiO2 mixture as a solid-phase source of SiO reactive gas has been proposed. The flowsheet of the process provides a uniform siliconization of the material throughout the entire volume of the reactor. It has been shown that C/SiC composite fibers can thus be prepared. Carbon fiber conversion can vary over a wide range depending on the reactor load parameters. The core–shell composite structure is a result of the growth of the SiC layer inward the fiber, which can give rise to deep longitudinal–radial cracks when the SiC shell thickness exceeds 0.7–0.8 µm. At lower conversions, no cracking of the SiC shell occurs, and the produced C/SiC core–shell composite fibers retain their integrity.

Fully dense high-entropy carbide (HEC) ceramic has been prepared from a mixture of the group IV and V transition metal oxides by a two-step technique, which involved the vacuum carbosilicothermic reduction (VCSTR) synthesis of a composite powder containing 75 vol.% HEC, 20 vol.% (Nb1-xMex)Si2 (where Me = Ti, Zr, Hf, Ta), and 5 vol.% SiC followed by hot pressing of the as-synthesized product. It was found that the reaction between (Nb1-xMex)Si2 and HEC took place during hot pressing, thereby allowing effective sintering to occur. The mechanical properties of the obtained nearly single-phase HEC ceramic were comparable to or even slightly better than those of HEC ceramics prepared by other methods. The use of VCSTR synthesis as a key step in the preparation of fully dense HEC ceramic was concluded to be effective both in lowering the sintering temperature and in improving the mechanical properties.

— We examine general aspects of the formation of Ti 3 SiC 2 –TiB 2 –SiC ceramic composites during vacuum carbosilicothermic reduction of titanium oxide mineral raw materials (leucoxene concentrate from the Yaregskoe occurrence) in the presence of B 4 C as a boron-containing additive, with SiC as a reducing agent. The effect of starting-mixture composition on the phase composition of the reduction product and the formation of minor phases is analyzed. We demonstrate that, with increasing B 4 C concentration in the starting mixture, the fraction of the forming TiB 2 rises systematically, reaching 48 vol %.

— We have studied the desilicidation of Ti 3 SiC 2 MAX phase powder at 1600°C in a CO + x SiO ( x = 0, 1, 1.5, 2, 3) gas atmosphere. The required gaseous atmosphere was generated using granulated powder mixtures containing commercial carbon, TiO 2 , SiO 2 , and SiC, which released CO and SiO gases during heating as a result of oxide reduction reactions. Ti 3 SiC 2 desilicidation was sensitive to the gas phase composition and occurred only if the CO : SiO ratio exceeded 1 : 3 ( x < 3). The Ti 3 SiC 2 desilicidation process was shown to be reverse of TiC silicidation with SiO gas and could be described by the following chemical reaction scheme: Ti 3 SiC 2 + CO( g ) = 3TiC + SiO( g ).

A new approach to the processing of leucoxene concentrate through the vacuum silicothermic reduction of titanium and silicon oxides using technical silicon as a reducing agent is presented. It has been shown that the reactions occurring during the high-temperature process are accompanied by the formation of SiO gas resulting in desiliconization of the leucoxene concentrate. The highest desiliconization degree of 82 % was achieved when the content of silicon in the reactant mixture was 29 wt. %.

We report the results of our experiments on cyclic exposure of Ti3SiC2 ceramics to CO environment at high temperature. The growth of a two-layer scale composed of a dense outer layer containing beta-SiC and TiC microcrystals, and a porous inner layer of TiC microcrystals was observed. The corrosion was accompanied by the release of SiO gas, which then took part in the growth of beta-SiC microcrystals in the upper part of the outer layer by the reaction with CO gas. (C) 2019 Elsevier B.V. All rights reserved.

A new approach is presented for processing leucoxene concentrate through vacuum silicothermal reduction of titanium and silicon oxides using commercial-grade silicon as a reducing agent. It is shown that reactions occurring during a high-temperature process are accompanied by formation of SiO gas resulting in desiliconization of leucoxene concentrate. The best desiliconization degree of 82% is achieved when the content of silicon in the reaction mixture is 29 wt.%.

Dense Ti3SiC2-SiC, Ti4SiC3-SiC, and Ti3SiC2-Ti4SiC3-SiC ceramic composites were fabricated through carbosilicothermic reduction of TiO2 under vacuum, followed by hot pressing of the as-synthesized products under 25 MPa at 1600 degrees C. In the reduction step, SiC either alone or in combination with elemental Si was used as a reductant. A one-third excess of SiC was added in the reaction mixtures in order to ensure the presence of approximately 30 vol.% SiC in the products of synthesis. During the hot pressing step, the samples that contained Ti3SiC2 showed better densification compared to those containing Ti4SiC3. The obtained composites exhibited the strength properties typical of coarse-grained MAX-phase ceramics. The flexural strength values of 424 and 321 MPa were achieved in Ti3SiC2-SiC, and Ti3SiC2-Ti4SiC3-SiC composites, respectively. The fracture toughness values were 5.7 MPa.m(1/2).