The chemical and microstructural transformation of the surface of a 31.5 vol.% ZrB2-31.5 vol.% HfB2-27 vol.% SiC-10 vol.% CCNT ultrahigh-temperature ceramic sample (where CCNT refers to carbon nanotubes) was studied under the influence of a subsonic N2-plasma flow with the addition of 5 mol% methane, simulating aerodynamic heating in the atmosphere of Titan. As in the case of pure nitrogen flow, it was found that silicon carbide is removed from the surface. Zirconium and hafnium diborides are partially transformed into a Zr-Hf-B-C-N solid solution in the experiment conducted. XRD, Raman spectroscopy, and SEM-EDX analysis show that the presence of C2 in the N2-CH4 plasma flow leads to surface carbonization (formation of a graphite- and diamond-like coating with a high proportion of amorphous carbon), resulting in significant changes in the microstructure and emissivity, potentially affecting the catalytic properties of the surface.

The hydroxyapatite powders with co-doping of Ca2+ with Na+ and Ce3+ were obtained by aqueous precipitation. Co-doping was considered as a method of charge compensation in the cationic sublattice of the HA structure by means of the 2Ca2+-> Ce3+ + Me+ in order to maintain the stability of phase composition, improve the optical and microstructural properties of the HA ceramics. The content of dopants ranged from 0.25 to 1.0 mol. % relative to calcium atoms in HA. The ceramics were obtained by traditional sintering in an air atmosphere and by hot pressing under soft reducing conditions in order to trace Ce oxidizing in HA matrix. Phase and chemical composition of the obtained ceramics were investigated using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and atomic emission spectrometry with inductively coupled plasma (AES-ICP). Microstructure and microhardness were estimated in order to trace the dopant effect on the ceramic formation. Luminescence spectra were recorded to evaluate the content of cerium ions in the Ce3+ state and to reveal the dopant effect on structure features of the HA.

The behavior of ultrahigh-temperature ceramics of the composition (HfB2-30vol%SiC)-2vol%C(graphene) under the influence of subsonic flow of dissociated nitrogen has been studied. It is shown that at the chosen exposure parameters, at the steady-state surface temperature similar to 2170-2190 degrees & Scy;, surface degradation of the material is observed to a depth of about 10-20 mu m. This is expressed by the fact that, as a result of the ongoing reactions, silicon carbide is removed from the near surface zone and HfB2 is transformed into the hexagonal phase of low hafnium nitride Hf3N2. It is noted that the phase composition of the surface in this case is significantly different from that observed after impact on ceramics based on HfB2-SiC supersonic flow of air plasma. The data obtained may be useful for evaluating the prospects of using ceramic composites for the exploration of space bodies with atmospheres of high nitrogen content.

The effectiveness of the application of 15 vol

Ultrahigh-temperature ceramic composites based on hafnium diboride have a wide range of applications, including as components for high-speed aircraft and energy generation and storage devices. Consequently, developing methodologies for their fabrication and studying their properties are of paramount importance, in particular in using them as an electrode material for energy storage devices with increased oxidation resistance. This study investigates the behavior of ceramic composites based on the HfB2-HfO2-SiC system, obtained using 15 vol% Ti2AlC MAX-phase as a sintering component, under the influence of subsonic flow of dissociated air. It was determined that incorporating the modifying component (Ti2AlC) altered the composition of the silicate melt formed on the surface during ceramic oxidation. This modification led to the observation of a protective antioxidant function. Consequently, liquation was observed in the silicate melt layer, resulting in the formation of spherical phase inhomogeneities in its volume with increased content of titanium, aluminum, and hafnium. It is hypothesized that the increase in the high-temperature viscosity of this melt prevents it from being carried away in the form of drops, even at a surface temperature of ~1900–2000 °C. Despite the established temperature, there is no sharp increase in its values above 2400–2500 °C. This is due to the evaporation of silicate melt from the surface. In addition, the electrochemical behavior of the obtained material in a liquid electrolyte medium (KOH, 3 mol/L) was examined, and it was shown that according to the value of electrical conductivity and specific capacitance, it is a promising electrode material for supercapacitors.

The investigation of the behavior of ZrB2-SiC-based ultra-high temperature ceramic (UHTC) materials under high-velocity CO2 plasma flow is of significant importance and relevance for evaluating their prospective use in the exploration of planets such as Venus or Mars. Accordingly, the degradation process of a ZrB2-30 vol.% SiC ceramic composite, fabricated by hot-pressing at 1700 °C with a 15 vol.% Ti2AlC sintering aid, was examined using a high-frequency induction plasmatron. It was found that the modification of the ceramic’s elemental and phase composition during consolidation, resulting from the interaction between ZrB2 and Ti2AlC, leads to the formation of an approximately 400 µm-thick multi-layered oxidation zone following 15 min stepwise thermochemical exposure at surface temperatures reaching up to 1970 °C. This area consists of a lower layer depleted of silicon carbide and an upper layer containing large pores (up to 160–200 µm), where ZrO2 particles are distributed within a silicate melt. SEM analysis revealed that introduction of more refractory titanium and aluminum oxides into the melt upon oxidation, along with liquation within the melt, prevents the complete removal of this sealing melt from the sample surface. This effect remains even after 8 min exposure at an average temperature of ~1960–1970 °C.

In order to study the promising potential of HfB2–30 vol % SiC ultrahigh-temperature ceramic materials modified with low amounts of reduced graphene oxide for the creation of aerospace equipment intended for use in N2-based atmospheres, the effect of high-speed dissociated nitrogen flow on it has been investigated. It has been established that under the chosen conditions of exposure during the stepwise increase of the anode power supply of plasma torch and, accordingly, the influencing heat flux, at certain parameters there is a sharp increase in the surface temperature from ~1750 to 2000-2100°C. At the same time, further increase of the heat flux has no obvious and proportional effect on the temperature of the sample surface, which may indicate its high catalyticity with respect to the reactions of surface recombination of atomic nitrogen. It is shown that the surface layers of the material undergo chemical transformation (removal of silicon-containing substances, formation of a new phase based on HfN), which is accompanied by a significant change in the microstructure (formation of dendrite-like structures), which affects the optical and catalytic characteristics of the surface.

This study explores a novel method for fabricating Ca-alpha-sialon ceramics using a calcium aluminate oxide eutectic additive. Conventional sintering techniques for Ca-alpha-sialon ceramics employ nitrogen-containing additives to facilitate the Si-N bond substitution mechanism with Al-O and Al-N bonds. In this work, calcium aluminate additives are utilized, leading to the formation of Ca-alpha-sialon ceramics via carbothermal reduction processes and partial nitridation of aluminum oxide. The influence of varying calcium aluminate content (5, 10, 15, 30, and 40 wt%) on the phase composition of Ca-alpha-sialon ceramics, produced by hot pressing at 1650 degrees C in a nitrogen atmosphere, was investigated. Results indicate that increasing the calcium aluminate content leads to the formation of a beta-sialon phase, which subsequently transforms into Ca-alpha-sialon. At 40 wt% calcium aluminate, complete conversion of alpha-Si3N4 to Ca-alpha-sialon is achieved, with significant glassy phase formation at the grain boundaries. The study elucidates the relationship between calcium aluminate content, phase composition, lattice parameters, mechanical properties, oxidation resistance, and thermal expansion of the ceramics.

The effect of a high-velocity flow of dissociated nitrogen on a sample of HfB2–30 vol

Hydroxyapatite (HAp) biomaterials are intended for use as bone substitutes, target functional agents in theranostics or bioimaging due to affinity for hard tissues, biocompatibility, and bioactivity. This study is devoted to HAp doped with cerium, having distinctive feature to change the oxidation state (3+/4+), which affects the physico-chemical and optical properties of the obtained powders. Cerium-doped HAp nanopowders with Ce content up to 0.5 wt% were obtained through precipitation technique and then were heat treated. The following conditions for heat treating were used: (i) heating in air atmosphere, (ii) hot pressing, (iii) heating in a carbon filling. For characterization, the complex methodology was implemented, including X-ray diffraction (XRD) analysis and high temperature XRD, X-ray absorption near edge structure (XANES) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, inductively coupled plasma atomic emission spectrometry (ICP-AES), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy, and, finally, photoluminescence (PL) spectroscopy. It was shown that the monophase cerium-HAp is stable up to a temperature of 1000 degrees C in the oxidizing atmosphere, above 1000 degrees C cerium(4+) oxide forms. Using reducing atmosphere during heating prevents the transition of Ce 3+ to Ce 4+ . PL, EPR and XANES studding clarified obviously the Ce 3+ content in the materials obtained under different processing conditions. It was shown, that Ce-HAp powders produced by precipitation and hot pressing contain Ce 3+ and exhibit luminescence. The significant luminescence of hot pressed Ce-HAps was caused by a higher concentration of Ce 3+ ions and incorporation of carbonate ions as a result of the processing.

The short-term (5 min) exposure to the supersonic flow of carbon dioxide plasma on ultrahigh-temperature ceramics of HfB2-30vol.%SiC composition has been studied. It was shown that, when established on the surface at a temperature of 1615–1655 °C, the beginning of the formation of an oxidized layer takes place. Raman spectroscopy and scanning electron microscopy studies showed that the formation of a porous SiC-depleted region is not possible under the HfO2-SiO2 surface oxide layer. Numerical modeling based on the Navier–Stokes equations and experimental probe measurements of the test conditions were performed. The desirability of continuing systematic studies on the behavior of ultrahigh-temperature ZrB2/HfB2-SiC ceramics, including those doped with various components under the influence of high-enthalpy gas flows, was noted.

— Si 3 N 4 /TiN ceramic composites have been prepared by hot pressing in a nitrogen atmosphere using fine Si 3 N 4 and Ti powders and CaO–Al 2 O 3 sintering aids and investigated. The results demonstrate that hot pressing was accompanied by titanium nitridation, yielding titanium nitride with the composition TiN 0.9 . Reaction between silicon nitride and the sintering aid led to the formation of a Ca-α-SiAlON with the composition Ca 0.67 (Si 10 Al 2 )(N 15.3 O 0.7 ). In addition to the major phases, calcium aluminosilicate with the composition Ca 3 Al 2 Si 3 O 12 was identified. Increasing the percentage of titanium nitride in the composites prepared at a temperature of 1650°C led to an increase in their density and Vickers microhardness: from 3.18 ± 0.03 to 4.33 ± 0.03 g/cm 3 and from 17 ± 1.1 to 29.4 ± 0.9 GPa, respectively.

Abstract—The structure, the phase composition, the physicomechanical properties, and the corrosion resistance of a precipitation-hardened composite material fabricated by hot pressing (1050°C, 30 MPa, 30 min) of powders of high-alloy ferritic steel (Fe base alloy, 30

The features of oxidation of ultra-high-temperature ceramic material HfB2-30 vol.%SiC modified with 1 vol.% graphene as a result of supersonic flow of dissociated CO2 (generated with the use of high-frequency induction plasmatron), as well as under the influence of combined heating by high-speed CO2 jets and ytterbium laser radiation, were studied for the first time. It was found that the addition of laser radiation leads to local heating of the central region from ~1750 to ~2000–2200 °C; the observed temperature difference between the central region and the periphery of ~300–550 °C did not lead to cracking and destruction of the sample. Oxidized surfaces and cross sections of HfB2-SiC-CG ceramics with and without laser heating were investigated using X-ray phase analysis, Raman spectroscopy and scanning electron microscopy with local elemental analysis. During oxidation by supersonic flow of dissociated CO2, a multilayer near-surface region similar to that formed under the influence of high-speed dissociated air flows was formed. An increase in surface temperature with the addition of laser heating from 1750–1790 to 2000–2200 °C (short term, within 2 min) led to a two to threefold increase in the thickness of the degraded near-surface area of ceramics from 165 to 380 microns. The experimental results indicate promising applications of ceramic materials based on HfB2-SiC as part of high-speed flying vehicles in planetary atmospheres predominantly composed of CO2 (e.g., Venus and Mars).

We employed contact alloying in the range 1000–1860°С to study the reaction specifics between SiC and Al2O3−(t + m)ZrO2(Y2O3) oxide composition. Real-time experiments with photographic recording of the changing size and shape of the Al2O3−(t + m)ZrO2(Y2O3) sample on a SiC ceramic substrate showed that Al2O3−(t + m)ZrO2(Y2O3) compositions react with the silicon carbide substrate in the range 1720–1860°С to melt and penetrate into (impregnate) the substrate. X-ray powder diffraction patterns were measured for samples taken from the contact area of the oxide composition with SiC directly on the substrate and in a chipped-off 1-mm-deep near-surface layer. ZrС, Al2Y4O9, and Al3.21Si0.47 were formed in the contact area via redox reactions involving oxide melt, in addition to 6H-SiC, Si and Al2O3, t-ZrO2 phases, which are the initial components of the substrate and oxide composition, respectively.

We have studied the effect of sodium fluoride as a sintering aid for β-sialons on the phase composition and physicomechanical properties of Si5AlON7 and Si4Al2O2N6. Two-step high-temperature firing of the β-sialons in the presence of NaF under a nitrogen atmosphere has been shown to cause no significant changes in the phase composition of the materials. The density and microhardness of the materials prepared using 0.5 and 5.0 wt

We employed contact alloying in the range 1000–1860°С to study the reaction specifics between SiC and Al 2 O 3 −( t + m )ZrO 2 (Y 2 O 3 ) oxide composition. Real-time experiments with photographic recording of the changing size and shape of the Al 2 O 3 −( t + m )ZrO 2 (Y 2 O 3 ) sample on a SiC ceramic substrate showed that Al 2 O 3 −( t + m )ZrO 2 (Y 2 O 3 ) compositions react with the silicon carbide substrate in the range 1720–1860°С to melt and penetrate into (impregnate) the substrate. X-ray powder diffraction patterns were measured for samples taken from the contact area of the oxide composition with SiC directly on the substrate and in a chipped-off <1-mm-deep near-surface layer. ZrС, Al 2 Y 4 O 9 , and Al 3.21 Si 0.47 were formed in the contact area via redox reactions involving oxide melt, in addition to 6H-SiC, Si and Al 2 O 3 , t -ZrO 2 phases, which are the initial components of the substrate and oxide composition, respectively.

The hot press method was used to obtain the SiC composite from different morphology and partial size powder. The starting SiC powder was: 1) fragmentation particle shape industrial charge of silicon carbide obtained by the Acheson method with sintering additive (9 wt.% Y 2 O 3 − Al 2 O 3 ) by Saint Gobain, 2) spherical particles SiC obtained by SHS in a laboratory. Bending strength, critical stress intensity factor, density, and the friction coefficient of samples of the obtained ceramics were determined. It has been established that the properties of ceramics obtained from SHS silicon carbide powder (spherical particles with sizes of 100–400 nm), due to better compaction, were at least 10% higher than samples from Saint Gobain powder: bending strength (400 ± 22 MPa), density (3.23 ± 0.01 g/cm 3 ), critical stress intensity factor (К 1С = 4.8 ± 0.3 MPa∙m 1/2 ), friction coefficient (0.1126 ± 0.0031).