Search Results (13)

This study presents the fabrication possibilities of ultra-high-temperature ceramics of ZrB₂-30 vol.%SiC and (ZrB₂-HfB₂)-30 vol.% SiC composition using the reaction spark plasma sintering of composite powders ZrB₂(HfB₂)-(SiO₂-C) under two-stage heating conditions. The phase composition and microstructure of the obtained ceramic materials have been subjected to detailed analysis, their electrical conductivity has been evaluated using the four-contact method, and the electron work function has been determined using Kelvin probe force microscopy. The thermal analysis in the air, as well as the calcination of the samples at temperatures of 800, 1000, and 1200 °C in the air, demonstrated a comparable behavior of the materials in general. However, based on the XRD data and mapping of the distribution of elements on the oxidized surface (EDX), a slightly higher oxidation resistance of the ceramics (ZrB₂-HfB₂)-30 vol.% SiC was observed. The I-V curves of the sample surfaces recorded with atomic force microscopy demonstrated that following oxidation in the air at 1200 °C, the surfaces of the materials exhibited a marked reduction in current conductivity due to the formation of a dielectric layer. However, data obtained from Kelvin probe force microscopy indicated that (ZrB₂-HfB₂)-30 vol.% SiC ceramics also demonstrated enhanced resistance to oxidation. Full article

High-entropy diboride-based (MeB₂-based) ceramics are promising high-temperature structural materials because of their excellent mechanical properties, high-temperature stability, and oxidation resistance. In order to achieve low-temperature sintering of the high-entropy ceramics, a novel preparation method of high-entropy (Ti,Zr,Nb,Mo,W)B₂-SiC ceramics based on reactive sintering of pre-alloyed solid-solution metals and nonmetals of Si, C, B₄C was conducted in the current work. Mechanical alloying behavior of the mixed metal powders, as well as the phase composition, microstructure, mechanical properties, and oxidation behavior of the as-sintered MeB₂-SiC ceramic were investigated. The XRD, SEM, and EPMA results indicated that the primary MeB₂ solid-solution and SiC phases could be successfully formed during reactive sintering at a relatively low temperature of 1650 °C. The as-sintered MeB₂-SiC ceramics had a high relative density of 97.8% and high mechanical properties (hardness of 19.74 ± 0.8 GPa, flexure strength of 533 ± 38 MPa, and fracture toughness of 6.01 ± 0.77 MPa·m^1/2). Combining the oxidation behavior and microstructure evolution of the oxidation layer, a continuous and relatively dense MeO_x-SiO₂ oxidation layer was gradually formed and covered on the external surface, leading to decelerating oxidation behavior after an oxidation exposure time of 10 min. Full article

New inorganic nanostructured matrices for fiber-reinforced composites with enhanced high-temperature stability were developed from alkali aluminosilicate polymers doped with different ultra-high-temperature ceramic (UHTC) particles. The alkali aluminosilicate matrices were synthesized at room temperature with a high SiO₂:Al₂O₃ ratio and then further functionalized by doping with 4–5 wt % of micrometric SiC, ZrB₂, ZrC, and HfC powders and finally thermally stabilized as glass–ceramics at 750 °C. The different UHTC-doped matrices were characterized according to their dimensional and microstructural changes after thermal cycling in air flux at 1000 °C. The first results showed that carbide-based UHTC powders improved the thermal stability of the matrices, preventing the excessive swelling of the material and the formation of detrimental voids that might result in the lack of adhesion with reinforcing fibers. Contrarily, the addition of ZrB₂ resulted in an excessive matrix swelling at high temperature, thus proving no efficacy compared to the undoped matrix. Impregnation tests carried out on C-fiber fabrics showed good processability, adhesion to the fibers, and fracture pull-out, especially for carbide-based matrices. Full article

With the use of electrolytic Cu powder, Zr powder, Si powder and nickel-coated B₄C powder as cladding powders, in-situ synthesized ZrB₂-SiC reinforced copper matrix composite coatings were prepared by laser cladding on the surface of the copper substrate to improve the surface hardness and wear resistance. Under the condition of a laser energy density at 60 kJ/cm², the macroscopic surface of the composite coating was continuously flat. The microstructure and phase of the cladding coating were analyzed by means of XRD and SEM. The reinforcements with nano-scale particle and micron-scale needle-like structures were in-situ synthesized in the cladding coating, and the content of the reinforcement phase decreased slightly from the coating surface to the substrate. The phase analysis results showed that the reinforcements included ZrB₂ and SiC. When the content of the reinforcement was increased to 30 wt%, microhardness also increased from 48 HV_0.2 to 309 HV_0.2, which was about 5.6 times that of the copper matrix. The wear resistance of the composite coatings was characterized by current-carrying wear tests. By keeping the sliding speed and load constant, the wear rate decreased with an increase in the reinforcement content, and the wear mechanism changed from adhesive wear to abrasive wear. The wear rate of the composite coating with the current was higher than that without the current due to its electric ablation and high temperature. Full article

Titanium metal matrix composites/TMMCs are reinforced ceramic reinforcements that have been developed and used in the automotive, biological, implants, and aerospace fields. At high temperatures, TMMCs can provide up to 50% weight reduction compared to monolithic super alloys while maintaining comparable quality or state of strength. The objective of this research was the analysis and evaluation of the effect/influence of different sintering temperatures, reinforcement size dependence of mechanical properties, and fortification mechanisms on the particle size distribution of B₄C, SiC, and ZrO₂ reinforced TMMCs that were produced and fabricated by powder metallurgy/PM. SEM, XRD, a Rockwell hardness tester, and the Archimedes principle were used in this analysis. The composites’ hardness, approximation, tensile, yielding, and ultimate strength were all increased. As the composite was reinforced with low-density ceramics material and particles, its density decreased. The volume and void content in all the synthesized specimens is below 1%; this is the result of good sample densification, mechanical properties and uniform distribution of the reinforced particle samples; 5% B₄C, 12.5% SiC, 7.5% ZrO₂, 75% Ti develop higher mechanical properties, such as higher hardness, approximation tensile, yielding, and ultimate strength and low porosity. Full article

The problem of creating and implementing high-temperature coatings for the protection of carbon–carbon (C/C) composites remains relevant due to the extremely low or insufficient heat resistance of C/C composites in an oxygen-containing environment. In the present work, detonation spraying was used for preparing new ZrB₂–35MoSi₂–10Al coatings on the surface of C/C composites without a sublayer. As a stabilizer of high-temperature modification of zirconia, and to increase the wettability of the surface of C/C composites, 5 wt.% Y₂O₃ and 10 wt.% Al were added to the initial powder mixture, respectively. The structure of the as-sprayed coating presents many lamellae piled up one upon another, and is composed of hexagonal ZrB₂ (h- ZrB₂), tetragonal MoSi₂ (t-MoSi₂), monoclinic ZrO₂ (m-ZrO₂), tetragonal ZrO₂ (t-ZrO₂), monoclinic SiO₂ (m-SiO₂), and cubic Al phases. The oxidation behavior and microstructural evolution of the ZrB₂–35MoSi₂–10Al composite coating were characterized from RT to 1400 °C in open air. During oxidation at 1400 °C, a continuous layer of silicate glass was formed on the coating surface. This layer contained cubic ZrO₂ (c-ZrO₂), m-ZrO₂, and small amounts of mullite and zircon. The results indicated that a new ZrB₂–35MoSi₂–10Al composite coating could be used on the surface of C/C composites as a protective layer from oxidation at elevated temperatures. Full article

In this article, the coatings of ZrB₂-xMoSi₂-Y₂O₃-yAl (x = 24, 35, 45 wt %; y = 10, 15, 20 wt %) were applied to the surface of a carbon/carbon composite to protect against high-temperature oxidation using a multi-chamber detonation accelerator. The kinetic analysis of the formation processes of a glass-forming layer during the oxidation of the initial components of the system ZrB₂-MoSi₂-Y₂O₃-Al in an air atmosphere at a temperature of 1400 °C was carried out and the kinetically significant stages of the heterogeneous reaction were determined. It is shown that the speed and density of the formation of a glassy matrix can be adjusted by fine-tuning the ratio of components in the initial powder. Full article

Aiming to provide key materials in order to improve the fracture toughness of ZrB₂ ceramics, ZrB₂-SiC composite powders with in situ grown SiC whiskers were successfully synthesized via a simple molten-salt-assisted ferrous-catalyzed carbothermal reduction method. Thermodynamic calculations on the ZrO₂-SiO₂-B₂O₃-C-Fe system were carried out. The effects of heating temperature and ferrous catalyst amount on the growth behavior of SiC whiskers in ZrB₂-SiC composite powders were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The results showed that the aspect ratio of SiC whiskers and the relative content of ZrB₂ particles increased with increasing heating temperature (1523–1723 K) and a molar ratio of Fe to ZrSiO₄ from 0:1 to 0.2:1. Phase-pure ZrB₂-SiC composite powders were obtained at 1723 K when the molar ratio of raw materials was 0.2:0.5:1:1.5:8.4 (Fe:NaCl:ZrSiO₄:B₂O₃:C). Single crystalline β-SiC whiskers with a mean diameter of 0.15 μm and an aspect ratio of 70–120 were homogeneously distributed in the final composite powders. A molten-salt-assisted iron-catalyzed vapor–solid mechanism was promoted for the growth mechanism of in situ grown SiC whiskers. Full article

Nataliakulikite, Ca₄Ti₂(Fe³⁺,Fe²⁺)(Si,Fe³⁺,Al)O₁₁, is a mineral intermediate between perovskite CaTiO₃ and brownmillerite Ca₂(Fe,Al)₂O₅. It was discovered as a minor mineral in a high-temperature pyrometamorphic larnite-gehlenite rock at the Nahal Morag Canyon of the Hatrurim Basin, Israel. Nataliakulikite is associated with larnite, flamite, gehlenite, magnesioferrite, Fe³⁺-rich perovskite, fluorapatite, barite, Hashemite, and retrograde phases (afwillite, hillebrandite, portlandite, calcite, ettringite, hydrogarnet, and other hydrated Ca-silicates). The mineral forms brown subhedral or prismatic grains (up to 20 µm) and their intergrowths (up to 50 μm). Its empirical formula (n = 47) is (Ca_3.992Sr_0.014U_0.004)(Ti_1.933Zr_0.030Nb_0.002) (Fe³⁺_0.610Fe²⁺_0.405Cr_0.005Mn_0.005)(Si_0.447Fe³⁺_0.337Al_0.216)O₁₁ and shows Si predominance in tetrahedral site. The unit-cell parameters (HRTEM data) and space group are: a = 5.254, b = 30.302, c = 5.488 Å, V = 873.7 ų, Pnma, Z = 4. These dimensions and Electron backscatter diffraction (EBSD) data strongly support the structural identity between nataliakulikite and synthetic Ca₄Ti₂Fe³⁺₂O₁₁ (2CaTiO₃∙Ca₂Fe³⁺₂O₅), an intermediate compound in the system CaTiO₃-Ca₂Fe³⁺₂O₅. In general, this mineral is a Si-Fe²⁺-rich natural analog of synthetic Ca₄Ti₂Fe³⁺₂O₁₁. The X-ray powder diffraction data (CuKα -radiation), calculated from unit-cell dimensions, show the strongest lines {d [Å], (I_calc)} at: 2.681(100), 1.898(30), 2.627(26), 2.744(23), 1.894(22), 15.151(19), 1.572(14), 3.795(8). The calculated density is 4.006 g/cm³. The crystal structure of nataliakulikite has not been refined because of small sizes of grains. The Raman spectrum shows strong bands at 128, 223, 274, 562, and 790 cm⁻¹. Nataliakulikite from the Hatrurim Basin crystallized under the conditions of combustion metamorphism at high temperatures (1160–1200 °C) and low pressures (HT-region of the spurrite-merwinite facies). Full article

To address the various shortcomings of a high material cost, energy-intensive temperature conditions and ultra-low efficiency of the conventional boro/carbothermal reduction method for the industrial preparation of ZrB₂-SiC powders, a novel molten-salt and microwave-modified boro/carbothermal reduction method (MSM-BCTR) was developed to synthesize ZrB₂-SiC powders. As a result, phase pure ZrB₂-SiC powders can be obtained by firing low-cost zircon (ZrSiO₄), amorphous carbon (C), and boron carbide (B₄C) at a reduced temperature of 1200 °C for only 20 min. Such processing conditions are remarkably milder than not only that required for conventional boro/carbothermal reduction method to prepare phase pure ZrB₂ or ZrB₂-SiC powders (firing temperature of above 1500 °C and dwelling time of at least several hours), but also that even with costly active metals (e.g., Mg and Al). More importantly, the as-obtained ZrB₂ particles had a single crystalline nature and well-defined plate-like morphology, which is believed to be favorable for enhancing the mechanical properties, especially toughness of their bulk counterpart. The achievement of a highly-efficient preparation of such high-quality ZrB₂-SiC powders at a reduced temperature should be mainly attributed to the specific molten-salt and microwave-modified boro/carbothermal reduction method. Full article

This work discusses microstructure evolution during ball milling and hot pressing of Ti-xZr-10Si-5B (x = 2 and 5 at. %) and Ti-xZr-20Si-10B (x = 5, 7, 10, 15 and 20 at. %) powder mixtures. Mechanical alloying was carried out in a ball mill using stainless steel balls and vials, 300 rpm and a ball-to-powder ratio of 10:1. Powders milled for 600 min were then hot-pressed (25 MPa) under vacuum at 1100 °C for 60 min. As-milled and hot-pressed samples were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and energy dispersive spectrometry (EDS). Peaks of Si and Zr disappeared in powders milled for 60 and 180 min, respectively, while the lattice parameters and cell volume of α-Ti were varied during ball milling up to 300 min indicating that supersaturated solid solutions were achieved. Ti₆Si₂B dissolving up to 10 at. % Zr was found in microstructure of hot-pressed Ti-xZr-10Si-5B (x = 2 and 5 at. %) and Ti-xZr-20Si-10B (x = 2, 5, 7 and 10 at. %) alloys. The amount of TiB and Ti₅Si₃ was preferentially increased whereas the Ti₃Si formed in microstructure of the hot-pressed Ti-15Zr-20Si-5B and Ti-20Zr-20Si-10B alloys. Full article

The aqueous dispersion behavior of ZrB₂, SiC powders with B₄C and C as sintering aids was investigated. Well co-dispersed suspension can be obtained in acidic solutions in presence of polyethyleneimine (PEI). The adsorption of PEI on the powder surface was measured by thermal gravimetric (TG) analysis. Rheological measurements displayed the effect of dispersant on the flow behavior of as-prepared slurries. An optimum condition was obtained with 1 wt % PEI. The viscosity of 40 vol % ZrB₂–SiC–B₄C–C (ZSBC) suspension at 100 s⁻¹ was as low as 0.74 Pa·s, which was suitable for aqueous processing. Full article

This work aims at studying the effect of the milling conditions on the microstructure and mechanical properties of a ZrB₂-5 vol% Si₃N₄ matrix reinforced with chopped Hi-Nicalon SiC fibers. Several composites were obtained using different milling conditions in terms of time, speed and type of milling media. The composites were prepared from commercial powders, ball milled, dried and shaped, and hot pressed at 1720 °C. Their relative bulk densities achieved values as high as 99%. For each material the fiber length distribution, the extent of reacted fiber area and matrix mean grain size were evaluated in order to ascertain the effects of milling time, milling speed and type of milling media. While the fracture toughness and hardness were statistically the same independently of the milling conditions, the flexural strength changed. From the results obtained, the best milling conditions for optimized mechanical properties were determined. Full article