Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.     

Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.     

Performance investigation of resin bonded ferro-silicon nitride-corundum refractories after creep at 1300 °C

Novel tempered resin bonded ferro-silicon nitride-corundum refractories containing 0 wt%, 15 wt% and 25 wt% ferro-silicon nitride were prepared respectively. Creep tests were performed under a load of 0.2 MPa at a temperature of 1300 °C for 50 h in air. The results showed that creep performance was significantly improved by the addition of ferro-silicon nitride. Ferro-silicon nitride-corundum containing 15 wt% ferro-silicon nitride initially presented a steady-state stage and was able to remain stable from the beginning of the holding time until 50 h of creep testing. All the specimens exhibited cold crushing strength more than 100 MPa both before and after creep testing. Phase composition and microstructure were analyzed following the creep experiments. The results showed that Si₂N₂O and O’-sialon crystals formed in situ during creep testing, in addition to the conversion of α-Si₃N₄ to β-Si₃N₄. Liquid Fe₃Si from the ferro-silicon nitride component accelerated the formation of the O’-sialon and prolonged the growth of β-Si₃N₄, which improved the creep performance significantly. Fe₃Si liquid migrated into the pores, and some Fe₃Si coexisted with residual carbon from the resin, which filled a part of pores and protected the specimens from severe oxidation.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

Microstructure and mechanical properties of titanium aluminum carbides neutron irradiated at 400–700 °C

This work reports the first mechanical properties of Ti₃AlC₂-Ti₅Al₂C₃ materials neutron irradiated at ∼400, 630 and 700 °C at a fluence of 2 × 10²⁵ n m⁻² (E > 0.1 MeV) or a displacement dose of ∼2 dpa. After irradiation at ∼400 °C, anisotropic swelling and loss of 90% flexural strength was observed. After irradiation at ∼630–700 °C, properties were unchanged. Microcracking and kinking-delamination had occurred during irradiation at ∼630–700 °C. Further examination showed no cavities in Ti₃AlC₂ after irradiation at ∼630 °C, and MX and A lamellae were preserved. However, disturbance of (0004) reflections corresponding to M-A layers was observed, and the number density of line/planar defects was ∼10²³ m⁻³ of size 5–10 nm. HAADF identified these defects as antisite Ti_Al atoms. Ti₃AlC₂-Ti₅Al₂C₃ shows abrupt dynamic recovery of A-layers from ∼630 °C, but a higher temperature appears necessary for full recovery.     

HiPIMS induced high-purity Ti3AlC2 MAX phase coating at low-temperature of 700 °C

Ti₃AlC₂, one of Ti-Al-C MAX phases, has received extensive attention due to its unique nano-laminated structure and combined properties of metals and ceramics. However, ultra-high synthesis temperature exceeding 800 °C is a critical challenge for broad application of Ti₃AlC₂ coatings on temperature-sensitive substrates. In this study, Ti-Al-C coatings were deposited on Ti-6Al-4V substrates using high-power impulse magnetron sputtering (HiPIMS) and DC sputtering (DCMS) for comparison. Different from as-deposited amorphous Ti-Al-C coating by DCMS, nanocrystalline TiAl_x compound was achieved by HiPIMS deposition due to highly ionized plasma flux with high kinetic energy. Furthermore, HiPIMS promoted the generation of dense and smooth Ti₃AlC₂ phase coating after low-temperature annealing at 700 °C, while annealed DCMS coating only obtained Ti₂AlC. In-situ XRD demonstrated such Ti₃AlC₂ phase could be early involved in crystallization at 450 °C, lowest than synthesis temperature ever reported. The mechanical properties of Ti₃AlC₂ coating were also discussed in terms of structural evolution.     

Effect of micro-sized alumina powder on the hydration products of calcium aluminate cement at 40 °C

In this work, the effect of different micro-sized alumina powders on the hydration products of calcium aluminate cement (CAC) during hydration at 40 °C is studied. The cement hydration at the designated times is terminated by the freeze-vacuum method. The phase development and microstructure evolution during the cement hydration are investigated by XRD and DSC, and SEM, respectively. It is found that 3CaO·Al₂O₃·6H₂O (C₃AH₆) is the dominant product of the pure CAC after hydration at 40 °C for 3.5 h. But 2CaO·Al₂O₃·8H₂O (C₂AH₈) is the dominant hydrate and C₃AH₆ is not found in the mixtures of CAC and micro-sized alumina powder under the same condition. The results indicate that the addition of alumina powders promotes the formation of C₂AH₈ and retards the conversion of C₂AH₈ to the C₃AH₆ phase. Moreover, such phase development with alumina addition is discussed.     

Carbon nanotube growth at 420 °C using nickel/carbon composite thin films as catalyst supports

Nickel/carbon composite (Ni/C) thin films were used as catalyst supports for the growth of vertically aligned multiwalled carbon nanotubes (MWCNTs) at temperature as low as 420 °C. Nickel nanoparticles embedded within the carbon matrix of Ni/C films have served as catalysts for the synthesis of nanotubes by PECVD using acetylene/ammonia plasma. Two different nickel contents (40 at.% and 60 at.%) in the films were used. Analysis indicated a diffusion of nickel atoms in the form of nanoparticles to the film surface upon annealing. This diffusion depends on both annealing temperature and nickel concentration in the films and affects the MWCNT growth at low temperature. The MWCNT synthesis was tested at growth temperature ranging between 335 and 520 °C. The growth of MWCNTs at 420 °C was only achieved by using Ni/C films with a high nickel content (60 at.%). These MWCNTs did not present considerable loss in their growth rate and structural quality compared to MWCNTs grown on classical substrates (Ni catalysts deposited on TiN), at higher temperature (520–600 °C). The results suggest that carbon saturation at the surface and subsurface of nickel catalysts of the Ni/C films is responsible for the improvement of MWCNT growth at low temperature.     

High temperature oxidation behavior of porous MoAlB ceramic from 800 °C to 1100 °C in air

MoAlB possesses the characteristics of both metals and ceramic materials, which has attracted extensive attention due to its excellent high-temperature oxidation resistance. For this reason, porous MoAlB is considered applicable to the practice of filtration under harsh environment. In this study, the high-temperature oxidation behavior of porous MoAlB ceramics is systematically studied at the temperatures ranging from 800 to 1100 °C. According to the results, the porous MoAlB exhibits good oxidation resistance at a maximum temperature of 1000 °C. The oxidation kinetics of porous MoAlB can be divided into three stages, and the estimated activation energies of the three stages are 253.83 kJ·mol⁻¹, 367.48 kJ·mol⁻¹ and 317.84 kJ·mol⁻¹, respectively. In the stable stage at 1000 °C, the quadratic mass gain per unit area shows linearity over time, and the oxidation rate of porous MoAlB reaches 37.31 mg²·cm⁻⁴·h⁻¹. As revealed by the analysis of the composition and microstructure of oxide layers, the main components of the oxide layer include MoO₃, MoO₂, Al₂O₃, B₂O₃. With the extension of oxidation time, the content of Al₂O₃ in the oxide films increases. The average pore size, permeability and open pore porosity of porous MoAlB show a trend of first decreasing and then tending to be stable. In addition, a discussion is conducted on the high-temperature oxidation mechanism of porous MoAlB.     

Formation mechanism of AlN-SiC solid solution with multiple morphologies in Al-Si-SiC composites under flowing nitrogen at 1300 °C

The non-oxide-reinforced phase AlN-SiC solid solution with high performance was successfully synthesized in the resin-bonded Al-Si-SiC composites under flowing nitrogen at 1300 °C. The AlN-SiC solid solution was synthesized by three paths of liquid-solid, gas-solid and gas-gas reactions through modulation of Al/Si ratio, and controllable microstructure of AlN-SiC solid solution was attained. The phase composition and microstructure of the sintered samples were characterized by XRD and SEM, combined with thermodynamics, the formation mechanism of AlN-SiC solid solution was investigated and the reaction model was established. Al was not detected while Si was detested by XRD. Granular, short columnar and whisker-like AlN-SiC solid solution were generated and their positions varied. As the temperature increases, the partial pressure of oxygen decreases due to the oxidation of Al, Si and SiC on the surface of the sample, inside the sample, the active oxidation takes place, generating Al₂O(g), SiO(g) and CO(g). Due to the low oxygen partial pressure, Al is preferentially nitrided to form a thin AlN layer on its surface. The AlN layer is broken as the temperature increases, then liquid Al with carbon from resin begins to flow, leaving the residual shell of AlN in situ. When it flows to the surface of Si, Al-Si_alloy is formed locally inside the Si particles under the wetting effect of C, then hexagonal AlN-SiC solid solution is formed inside the Si shell. Part of SiO(g) + CO(g) diffuses into the interior of the AlN residual shell and reacts by aggregation to form a granular AlN-SiC solid solution in the shell wall; others diffuses into the pores of the sample for vapor deposition, and finally forms stacked hexagonal flaky whiskers. The in-situ generated of AlN-SiC solid solution with multiple morphologies in the composite plays a joint toughening effect, which can significantly enhance the comprehensive performance of the composite. In this experiment, the synthesis of AlN-SiC solid solution without sintering aids under normal pressure at a low temperature. It is expected to be applied to the blast furnace and to realize the longevity of blast furnace.     

Rhodium-incorporated faujasite prepared at 80 °C

Rhodium-incorporated zeolites were synthesized from orthosilicate, aluminum nitrate, and rhodium chloride at 80 °C. Crystal phase diagram (zeolite types as functions of rhodium feed ratio and reaction period) showed that pure faujasite was formed in a wide range, but prolonged reaction caused partial transformation from faujasite to cancrinite at the rhodium feed ratio Rh/(Al + Rh) ≤ 0.02 and to gismondine at Rh/(Al + Rh) ≥ 0.4. X-ray diffraction analysis illustrated that an increase in the rhodium feed ratio caused appreciable decreases in the d-spacings, suggesting that rhodium was incorporated into the faujasite framework. ICP-OES analysis for the products showed that the rhodium content increased almost linearly with the increase in the rhodium feed ratio with an upper limit of the analyzed Rh content Rh/(Al + Rh) = 0.183 at the Rh feed ratio of 0.3. The products were almost quantitatively ion-exchanged using an ammonium chloride aqueous solution to study the ammonia desorption profiles on TG-DTA/MS analysis in the heating process. Considerably sharp exothermic peaks were observed at 245 °C simultaneously in the heating process. These peaks are probably related to catalytic decomposition of ammonia and desorption of ammonia, nitrogen, and water in the zeolite. A hypothesis was proposed for this mechanism: rhodium can be abruptly eliminated at an elevated temperature from the zeolite framework toward the micropores to form its hydroxides or oxides, then they triggered a catalytic decomposition of the adsorbed ammonia. The effect of calcination on crystallinity of the products and catalytic reactivity for hydrogen peroxide decomposition supported the proposed mechanism.     

UHTC-based matrix as protection for Cf/C composites: Original manufacturing,microstructural characterisation and oxidation behaviour at temperature above 2000 °C

In space propulsion applications, the development of new ceramic matrix composites with improved resistance to oxidation and ablation at high temperature is needed and ultra-high temperature ceramics-based ones appear the most suitable. Combination of both powder impregnation (ZrB₂, C) and liquid silicon infiltration enabled manufacturing of UHTC based matrices in C_f/C preforms with less than 10 vol% open porosity and various proportions and homogeneous distribution of C, ZrB₂, SiC and Si. Oxidation behaviour was evaluated on composite structures using an oxyacetylene torch at temperatures higher than 2000 °C. Chemical analyses and microstructural observations before and after oxidation testing evidenced the protection ability of ZrB₂-SiC-Si matrices thanks to the formation of multi-oxide scales which resisted even tested durations of 6 min and pointed the unharmful presence of residual 12 vol% silicon on the composite for use at high temperature under high gas flows.     

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf_0.84Y_0.16O_1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m^-1 K^-1 at as-sprayed state to 1.6 W m^-1 K^-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.     

Direct microwave sintering of yttria-stabilized zirconia at 2·45 GHz

The feasibility of using direct MW heating—with 2·45 GHz radiation distributed in multimode applicators—for the obtainment of uncracked fully dense zirconia ceramics was studied. It was found that such a sintering approach can be used in the case of ZrO₂(Y₂O₃) powder compacts. Suitable correlation between target load mass, heating chamber architecture and forward power profile is the key to direct MW heating without thermal runaway. Sintered bulk densities close to the theoretical were obtained after firing cycles of about 2 h. Sintering rate enhancement in the MW furnace resulted in a reduction of ∼100°C in the minimal temperature required for full densification. Mechanical properties of MW and conventionally sintered specimens (fully dense state) were not significantly different. ©     

Preparation of high–strength closed–pore foamed ceramics from desalted sea sand at temperatures below 1000 °C

Based on high–temperature sintering with SiC as the foaming agent, the technical potential of preparing foamed ceramics (FCs) from desalted sea sand at temperatures below 1000 °C was studied. Rapid melting of the ceramic bodies at elevated temperatures helped to seal more foaming gas, resulting in a large foaming volume for the FCs. If the interior of the ceramic bodies melted quickly during sintering, the foaming gas was trapped in situ, resulting in a homogenous FC pore structure. By coordinating the borax content and sintering temperature of the green bodies, the melting characteristics of the ceramic bodies could be optimised during sintering, usually producing a large foaming volume and a homogeneous FC pore structure. The FCs sintered from the green bodies with 25–35 wt% of borax at 900–1000 °C obtained high total/closed porosities of (68–75)%/(65–72)%, a relatively dense surface, a homogenous pore structure, and a relatively high compressive strength of 8.1–11.2 MPa.     

Fracture behavior of a melt-infiltration-processed SiC/Si composite at 900 °C in argon,air, and steam

The fracture behavior of a melt-infiltration-processed SiC/Si composite, used to mimic the matrices of industrial fiber-reinforced ceramic composites, was examined in different atmospheres and temperatures. Specimens tested in four-point bending at 900 °C in oxygen-gettered argon, dry air, or steam-rich atmospheres exhibited higher average fracture strengths than specimens tested at 25 °C. Higher mean fracture strength values were obtained for specimens tested in dry air or in a steam-rich atmosphere at 900 °C than for specimens tested in high-purity, oxygen-gettered argon at this temperature. The increased fracture strengths obtained in air and in steam-rich atmospheres coincided with increased specimen oxidation and apparent oxide filling and blunting of flaws in these composites. A transition in the location of catastrophic failure, from sites of preexisting damage created by Vickers indentations for tests in argon to other locations for tests in air or steam-rich atmospheres, was also consistent with such apparent oxide filling/blunting of indentation-induced flaws.     

Demixing pressures of hydroxy-terminated poly(dimethylsiloxane)–carbon dioxide binary mixtures at 313.2 K, 323.2 K and 333.2 K

The phase behavior of PDMS(OH)–CO₂ binary mixtures was investigated. Two different molecular weight PDMS(OH) were utilized and the demixing pressures were determined at three temperatures for a wide composition range. Both of these polymers were found to form miscible mixtures with CO₂ at all compositions at pressures lower than 31 MPa in the temperature range 313.2–333.2 K. Depending on the composition of the binary mixtures, two types of phase separation was observed during depressurization; the bubble point and the cloud point. In addition, at specific weight fractions a color change was also observed which was attributed to the mixture critical point. The demixing pressures were observed to increase with temperature and decrease with increasing polymer weight fraction. In addition, higher demixing pressures were obtained for the higher molecular weight polymer mixtures. The bubble point data were modeled by using Sanchez–Lacombe equation of state (SLEoS) and the binary interaction parameters were regressed at the studied temperatures. It was observed that the binary interaction parameters decreased with increasing temperature.     

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B₄C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B₄C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B₄C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B₄C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.     

Improving the Cyclic-Oxidation Resistance of Ti3AlC2 at 550°C and 650°C by Preoxidation at 1100°C

Preoxidation of Ti₃AlC₂ at 1100°C for 2 h was conducted to improve its cyclic-oxidation resistance at the testing temperature of 550°C and 650°C in air. The cyclic oxidation of the preoxidized Ti₃AlC₂ was found to follow a parabolic rate law rather than the linear oxidation rate for that without preoxidation. Through the X-ray diffraction and SEM analysis, the remarkable improvement of the cyclic-oxidation resistance of preoxidation Ti₃AlC₂ is suggested due to the existence of protective α-Al₂O₃ layers formed during the preoxidation treatment, which inhibits the formation of amorphous Al₂O₃, which can result in larger thermal stress and stress-induced microcracks.