Fabrication and properties of Cf/Ta4HfC5-SiC composite via precursor infiltration and pyrolysis

In this study, a C_f/Ta₄HfC₅-SiC ultra-high-temperature ceramic matrix composite exhibiting a homogeneous phase distribution was successfully fabricated via precursor infiltration and pyrolysis processing. Initially, the pyrolysis and solid solution mechanisms exhibited by the Ta₄HfC₅ precursor were investigated and characterized through TG-MS and XRD analysis. The as-fabricated C_f/Ta₄HfC₅-SiC composite exhibited a density and open porosity of 2.84 g/cm³ and 10.62 vol%, respectively. It also exhibited outstanding mechanical properties, with a flexural strength of 339 ± 20 MPa and fracture toughness of 11.56 ± 0.77 MPa·m^1/2. The C_f/Ta₄HfC₅-SiC composite demonstrated strong ablation resistance under a heat flux of 5 MW/m² at ~2400℃, with corresponding linear and mass recession rates of 5.33 μm/s and 6.18 mg/s, respectively. The combination of strong mechanical properties and ablation resistance provides a solid basis for the use of the C_f/Ta₄HfC₅-SiC composite in a new generation of ultra-high-temperature materials.     

Novel Ti3C2Tx MXene/epoxy intumescent fire-retardant coatings for ancient wooden architectures

Most of the ancient buildings are made of inflammable wooden structures, which have serious potential safety hazards. Applying fire-retardant coating is one of the simplest and most effective means of fire prevention in ancient wooden buildings. In this work, we have demonstrated that the Ti₃C₂T_x transition metal carbide/carbonitride (MXene) was applied as the synergetic agent, waterborne epoxy resin as the film-forming agent, ammonium polyphosphate, dipentaerythritol, and melamine (P-C-N system) as the intumescent fire-retardant system to prepare Ti₃C₂T_x/epoxy intumescent fire-retardant coating (TEIFC). The results showed that MXene has significantly improved the fire-retardant performance of the coating. By incorporating 3 wt% Ti₃C₂T_x (TEIFC-3, with 62 wt% P-C-N system), the coating displayed UL-94 V-0 rating with the limiting oxygen index value of 38%. In addition, the combination of Ti₃C₂T_x and P-C-N system enhanced the Shore hardness of the coating to 95 SHD (TEIFC-3). Furthermore, TEIFC-3 presented high thermal stability with the T_HRI of 177.0°C and T_dmax of 380.5°C. This work provides a novel strategy for the design and preparation of intumescent fire-retardant coating, which will greatly broaden the industrial applications of MXene-based polymer composites in the field of fire prevention of ancient buildings.     

Structural design and ablation performance of ZrB2/MoSi2 laminated coating for SiC coated carbon/carbon composites

A protective coating alternated with ZrB₂ and MoSi₂ laminated layers was designed and prepared on carbon/carbon (C/C) composites with SiC inner layer by supersonic atmosphere plasma spraying. After ablated at a heat flux of 2.4 MW/m² for 30s, ZrB₂/MoSi₂ laminated coating was in good condition with a linear growth rate and mass gain rate of 1.67 μm/s and 0.44 mg/s, respectively. From the central region to the border region, the calculated residual thermal stress of ZrB₂/MoSi₂ laminated coating decreased at first and then increased rapidly, illustrating the size change of the generated laminated cracks. The alternate design of ZrB₂ layers for erosion and MoSi₂ layers for oxidation resulted in the laminated stress distribution and improved ablation resistance.     

Thermal properties and performance of carbon fiber-based ultra-high temperature ceramic matrix composites (Cf-UHTCMCs)

The thermophysical properties of carbon fiber-based ultra-high temperature ceramic matrix composites have been determined to aid designers who need these properties when considering using the composites in ultra-high temperature aerospace applications. The coefficient of thermal expansion (CTE) and thermal diffusivity of the composites were measured parallel and perpendicular to the ply direction; the thermal conductivity was measured using the laser-flash method and the heat capacity calculated from the relationship between the thermal diffusivity, density, and thermal conductivity. Both the CTE and thermal conductivity showed higher values across the ply and increased with increasing temperature as expected, whilst the thermal diffusivity showed higher values parallel to the ply and increased smoothly with temperature. In addition, two different but related oxyfuel torch tests, based on oxyacetylene and oxypropane, were used to evaluate the thermo-ablation behavior of the composites. The tests showed how good the composites were at withstanding the ultra-high temperatures, high heat fluxes, and gas velocities involved.     

Low-temperature sintered (Ti,Zr, Nb,Ta, Mo)C-based composites toughened with damage-free SiCw

The high sintering temperature would have a great tendency to damage the morphology and thus properties of the silicon carbide whisker (SiC_w) in high entropy carbide-silicon carbide whisker (HEC-SiC_w) composites, which, in turn, would impact the effectiveness of the operative toughening mechanisms. The objective of this study was to achieve full contributions to the toughening effects of SiC_w by preparing (Ti, Zr, Nb, Ta, Mo)C-SiC_w composites at low temperature (1600 ℃) using cobalt as additives. Results showed that the fracture toughness of the (Ti, Zr, Nb, Ta, Mo)C bulk reinforced with 20 vol% SiC_w and 5 vol% Co was 7.2 MPa?m^1/2, which was much higher than that of the (Ti, Zr, Nb, Ta, Mo)C bulk only sintered with 5 vol% Co (3.4 MPa?m^1/2). Meanwhile, it was also higher than that of the reported HEC-20 vol% SiC_w composite sintered at 2000 ℃ (4.3 MPa?m^1/2). For the fracture toughness of HEC-SiC_w composites, it was significantly increased by the introduction of damage-free SiC_w.     

Toughening of laminated ZrB2-SiC ceramics with residual surface compression

Laminated ZrB₂-SiC ceramics with residual surface compression were prepared by stacking layers with different SiC contents. The maximum apparent fracture toughness of these laminated ZrB₂-SiC ceramics was 10.4 MPam^1/2, which was much higher than that of monolithic ZrB₂-SiC ceramics. The theoretical predictions showed that the apparent fracture toughness was strongly dependent on the position of the notch tip, which was confirmed by the SENB tests. Moreover, laminated ceramics showed a higher fracture load when the notch tip located in the compressive layer, whereas showed a lower fracture load as the notch tip within the tensile layer. The toughening effect of residual compressive stresses was verified by the appearance of crack deflection and pop-in event. The influence of geometrical parameters on the apparent fracture toughness and residual stresses was analyzed. The results of theoretical calculation indicated that the highest residual compressive stress did not correspond to the highest apparent fracture toughness.     

Thermal and electrical properties of a high entropy carbide (Ta,Hf, Nb,Zr) at elevated temperatures

The thermal and electrical properties were measured for a high entropy carbide ceramic, consisting of (Hf, Ta, Zr, Nb)C. The ceramic was produced by spark plasma sintering a mixture of the monocarbides and had a relative density of more than 97.6%. The resulting ceramic was chemically homogeneous as a single-phase solid solution formed from the constituent carbides. The thermal diffusivity (0.045–0.087 cm²/s) and heat capacity (0.23–0.44 J/g•K) were measured from room temperature up to 2000°C. The thermal conductivity increased from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The phonon and electron contributions to the thermal conductivity were investigated, which showed that the increase in thermal conductivity was predominantly due to the electron contribution, while the phonon contribution was independent of temperature. The electrical resistivity increased from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800°C.     

Investigation of phase composition and microwave dielectric properties of MgO‐Ta2O5 ceramics with ultrahigh Qf value

Four MgO‐Ta₂O₅ ceramics with the MgO/Ta₂O₅ mole ratio x = 1, 2, 3, and 4 were prepared by traditional solid‐state reaction method, and the influence of x on the phase composition, microstructure, and dielectric properties (the dielectric constant ε_r, the temperature coefficient of resonant frequency τ_f and the quality factor Qf) of the materials was investigated using XRD, SEM, etc. The results indicated that the ceramics were composed of two crystalline phases MgTa₂O₆ and Mg₄Ta₂O₉ in the composition range studied, and that the dielectric properties ln ε, 1/Qf, and τ_f changed proportionally to the fraction of main crystal phases, which meet perfectly with the mixing model proposed in this study. It is obvious that the proportion of the two crystal phases could be precisely controlled by x, and thereby, the dielectric properties can be conveniently and precisely tailored. Our research provided a new microwave dielectric ceramic with the composition of 2MgO‐Ta₂O₅, which has an ultrahigh Qf value (211 000 GHz), low dielectric constant ε_r (19.9), and near zero temperature coefficient of resonant frequency τ_f (8 ppm/°C).     

Densification,mechanical, and tribological properties of ZrB2-ZrCx composites produced by reactive hot pressing

ZrB₂-ZrC_x composites were produced using Zr:B₄C powder mixtures in the molar ratios of 3:1, 3.5:1, 4:1, and 5:1 by reactive hot pressing (RHP) at 4-7 MPa, 1200°C for 60 minutes. X-ray diffraction analyses confirmed the formation of nonstoichiometric zirconium carbide (ZrC_x) with different lattice parameters and enhanced carbide formation by increasing the Zr mole fraction. An increase in applied pressure from 4 to 7 MPa was responsible for the improved relative density (RD) of 4Zr:B₄C composition from 86% to 99%. Microstructural studies on Zr-rich composites showed a reduction in unreacted B₄C particles and enriched elongated ZrB₂ platelets. Reaction and densification mechanism in 4Zr:B₄C composition were studied as a function of temperature increased from 600 to 1200°C at an applied constant pressure of 7 MPa. After 1000°C, <40 vol.% of unreacted Zr was observed during the densification process. Concurrently, low energies of carbon diffusion and carbon vacancy formation were found to enhance nonstoichiometric ZrC_x formation, which was found to be responsible for the completion of the reaction. The plastic deformation of unreacted Zr was responsible for the densification of the ZrB₂-ZrC_x composite. The results clearly showed that the applied pressure is five times lower than the reported values. Moreover, a temperature of 1200°C was sufficient to produce dense ZrB₂-ZrC_x composites. The improved microhardness, flexural strength, fracture toughness, and specific wear rate were 8.2-15 GPa, 265-590 MPa, 2.82-6.33 MPa.m^1/2, and 1.43-0.376 × 10⁻² mm²/N, respectively.     

A self-assemble strategy toward conductive 2D MXene reinforced ZrO2 composites with sensing performance

The 2D MXene material provides an opportunity for manufacturing ceramic composites with special structures and functions. However, the maintenance of a complete layered structure and controlled exfoliation of few-layered MXene sheets as well as homogeneous distribution should be carefully designed and carried out. Herein, we try to explore some essential questions in the system, including the evolution of phase and morphology of MXene as well as its influence on the microstructure, which is critical. We established a general self-assembly strategy for fabricating MXene-ZrO₂ hybrids by electrostatic interaction. After SPS, fine toughening and conductive network has been constructed in composites with improved mechanical properties and a percolation threshold as low as 1?2 wt.%. The conductivity related to spatial distribution of MXene sheets endows the composite with self-monitoring ability to sense stain and internal cracks in situ. Such composite may be a promising smart material for potential structural and functional applications.     

3DN C/SiC-MoS2 self-lubricating composites with high friction stability and excellent elevated-temperature lubrication

Aerospace lubrication system requires materials with stable service life and excellent elevated-temperature lubrication performance. Herein, MoS₂ lubricants were used to fill the pores of three-dimensional needled (3DN) C/SiC to prepare 3DN/SiC-MoS₂ self-lubricating composites. The effects of 3DN C/SiC-MoS₂ self-lubricating composites under different friction time and temperatures were investigated. Results reveals that the average friction coefficients of 3DN C/SiC-MoS₂ self-lubricating sample fluctuated between 0.118 and 0.202 under different friction time, and the wear rate maintained about 10⁻⁴ mm³/(N m). Especially, the sample under the long friction time of 7200s, with average friction coefficient of 0.161 and lowest wear rate of 3.57×10^-5 mm³/(N m), indicating that the self-lubricating sample has a stable service life. Additionally, the average friction coefficients of 3DN C/SiC-MoS₂ self-lubricating sample at 200℃ and 400℃ were lower than that of at 25℃, and the wear rate increased within the order of 10^-4 mm³/(N m) as the temperature increased.     

Ablation resistance of HfC-TaC/HfC-SiC alternate coating for SiC-coated carbon/carbon composites under cyclic ablation

HfC-TaC/HfC-SiC alternate coatings with different sublayer thicknesses were fabricated on SiC-coated carbon/carbon composites by supersonic atmosphere plasma spraying. Their ablation resistance was studied under oxyacetylene torch and compared with monolayered HfC-TaC coating. The alternate coating with 6 spray cycles of HfC-TaC and 3 spray cycles of HfC-SiC sublayers exhibited the best ablation performance as confirmed by the integral coating morphology and the lowest ablation rates. A dense oxide layer acting as an oxygen insulator and the release of thermal stress induced by the formation of dendritic cracks are thought to be responsible for its great ablation resistance. For the alternate coating with 4 spray cycles of HfC-TaC and 2 spray cycles of HfC-SiC sublayers, exfoliation occurred at the interface of two adjacent sublayers, leading to violent evaporation of exposed HfC-SiC sublayer.     

Strong and super tough: Layered ceramic‐polymer composites with bio‐inspired morphology

Bio‐inspired layered ceramic‐polymer composites with high strength and toughness were prepared from sintered aluminum oxide ceramic sheets and cationically curing epoxy resins toughened with poly(ε‐caprolactone) (PCL). The architecture of the composite is inspired by nacre but is arranged on a larger scale. Ceramic sheets with a nominal thickness of 250 μm were assembled into composite plates by adhesive layers with a nominal thickness of 20 μm. Before the manufacturing of the composites, the stress‐strain properties of the polymer component were tailored by the variation in the PCL content between 0 and 39 wt%. For composites with 4 and 15 ceramic layers, the bending strengths achieved 327 MPa and 376 MPa, which are higher than that of pure ceramic sheets. Moreover, composites with 15 ceramic layers show a 16 times higher toughness compared to that of the pure ceramic sheets. The results indicate that the toughness of the layered composites increases significantly with the number of layers. Inspired by the geometrical ratio of the natural sheet composite nacre, we have achieved a similar strength but a 2 times higher toughness than nacre by only adding up to 6 vol% of the polymer.     

Compositional disorder and sintering of entropy stabilized (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders

Entropy-stabilized (Hf,Nb,Ta,Ti,Zr)B₂ solid solution powders produced by a carbo/boro-thermal reduction followed by solid solution formation were first analysed by synchrotron radiation x-ray diffraction, and their long range periodicity (i.e. lattice parameters) as well as the micro-strain intended as lattice disorder were quantitatively determined. A model to describe the micro-strain was proposed. The as-synthesized (Hf,Nb,Ta,Ti,Zr)B₂ solid solution powders were then hot-pressed at 2200 K and 50 MPa until near full densification was achieved. The hot-pressed material had a residual micro-porosity of 1.3 vol.% and consisted of a (Hf,Nb,Ta,Ti,Zr)B₂ ceramic matrix, 0.3-1 μm grain size range, and of a residual 10 vol.% B₄C particulate component, grain size in the range 0.2-2 μm. B₄C was a side product of the former synthesis and, after hot-pressing, remained trapped along the grain boundaries of the primary (Hf,Nb,Ta,Ti,Zr)B₂ solid solution ceramic matrix. Micro-hardness HV_0.2 = 22.7 ± 1.9 GPa for 1.96 N applied force was measured.     

Influence of metallic properties on the fracture characteristics of Al2O3/Ti/Ni-laminated composites

The mutual confinement of ceramics and metals in laminated composites tends to change the original properties of ceramics and metals. In this study, two kinds of laminated composites, Al₂O₃/Ti and Al₂O₃/Ti/Ni, were prepared. Three-point bending experiments revealed that Al₂O₃/Ti underwent brittle fracture after elastic deformation. The fracture morphology analysis revealed that the Ti in Al₂O₃/Ti became brittle due to the formation of columnar crystals. The temperature gradient perpendicular to the direction of laminations during preparation was responsible for the formation of columnar crystals. The force–displacement curves of the Al₂O₃/Ti/Ni combine the properties of elastic deformation of ceramics and plastic deformation of metals. The reason why the Al₂O₃/Ti/Ni did not fracture completely in the bending experiments is that Ni maintained the toughness, and there is a good interfacial bond among Al₂O₃, Ti, and Ni. The indentation crack analysis revealed that cracks have long transverse propagation and short longitudinal propagation in both laminated composites. Finite element analysis revealed that this was due to compressive stress in the Al₂O₃ layer and tensile stress in the metal layer. This compressive stress consumes the crack energy in the longitudinal direction and stops the crack in the metal layer. The brittle to ductile gradient transition among Al₂O₃, Ti, and Ni, combined with the guidance of crack propagation direction by the interfacial layer, enhances the ability of Al₂O₃/Ti/Ni to resist damage.     

Preparation of B4C composites toughened by MoB2/Mo2B5-SiC interlocking structure via reactive hot pressing

B₄C composites toughened by MoB₂/Mo₂B₅-SiC interlocking structure were prepared via reactive hot pressing with B₄C and MoSi₂ as raw materials. The phase composition, microstructure, and mechanical properties of the fabricated B₄C composites were studied. The crack propagation and fracture surface were observed, and the toughening mechanism was analyzed. The results indicate that the interlocking structure of MoB₂/Mo₂B₅-SiC is formed in the obtained B₄C composites. The relative density, flexural strength, and fracture toughness of the B₄C composites reach 99.3%, 480 MPa, and 5.2 MPa·m^1/2, respectively, when the MoSi₂ content is 30 wt%. The hardness is 33 GPa when the MoSi₂ content is 20 wt%. The special laminar fracture of the interlocking structure of MoB₂/Mo₂B₅-SiC elongates the crack extending path and thus consumes more energy of crack extension. The phenomena of crack bridging and branching and the special laminar fracture of the interlocking structure have a synergistic effect on promoting the overall fracture toughness.     

Effect of the Y2O3 amount on the oxidation behavior of ZrB2-SiC-based coatings for carbon/carbon composites

In this paper, the effect of Y₂O₃ addition on the oxidation resistance of ZrB₂-SiC-Y₂O₃ coating for the SiC coated carbon/carbon composites was investigated. Results confirmed the great benefits of adding Y₂O₃ to the oxidation-resistant properties of the original coating from different aspects. The dense structure of surface and cross-section and formation of yttria stabilized zirconia, Y₂Si₂O₇, t-ZrO₂ phases were observed after adding Y₂O₃. Additionally, according to the TEM results, yttrium silicate existed in the form of nanoparticles, while yttria stabilized zirconia existed in the form of agglomeration within the SiO₂ liquid phase. This superior oxidation resistance was attributed to the following reasons: (i) the formed Zr-Si-Y-O glass barrier layer blocked the oxygen diffusion and healed the cracks; (ii) the reduced m-ZrO₂ content weakened the volume expansion of the coating and avoided spallation; (iii) Y₂Si₂O₇ served as a pinning phase which modified the stability of liquid SiO₂ at elevated temperatures.     

Effect of slurry and sol-gel introduce SiCnws on ablation and bending behaviors of modified SiCf/HfC-SiC composites

The slurry and sol-gel methods were used to introduce SiC nanowires (SiC_nws) into the SiC_f/HfC-SiC composites. The microstructures, ablation, and bending behaviors of the SiC_nws modified composites prepared by the two methods were compared. The bending strengths of the modified composites obtained by introducing SiC_nws by the slurry and sol-gel methods were 224 ± 19 and 154 ± 14 MPa, respectively. The results showed that SiC fibers with chemical corrosion and thermal damage during the sol-gel process decreased the bending strength of the SiC_nws-modified SiC_f/HfC-SiC composites. Meanwhile, the pyrolytic carbon interface accompanying corrosion damage in the sol-gel process led to the degradation of interface function, which hindered the interface debonding and fiber sliding of the composites during the bending test. After ablation, the bending strengths of the two composites were 188 ± 19 and 50 ± 7 MPa, respectively. The bending strength retention of the modified composites fabricated by the slurry method (83.9%) was higher than that (32.5%) of the composites fabricated by the sol-gel method after ablation. As the composites fabricated by the slurry method exhibited a good ablation resistance under the oxyacetylene flame (∼2350°C).     

Synthesis of Chl@Ti3C2 composites as an anode material for lithium storage

Two-dimensional (2D) titanium carbide MXene Ti₃C₂ has attracted significant research interest in energy storage applications. In this study, we prepared Chl@Ti₃C₂ composites by simply mixing a chlorophyll derivative (e.g., zinc methyl 3-devinyl-3-hydroxymethyl- pyropheophorbide a (Chl)) and Ti₃C₂ in tetrahydrofuran, where the Chl molecules were aggregated among the multi-layered Ti₃C₂ MXene or on its surface, increasing the interlayer space of Ti₃C₂. The as-prepared Chl@Ti₃C₂ was employed as the anode material in the lithium-ion battery (LIB) with lithium metal as the cathode. The resulting LIB exhibited a higher reversible capacity and longer cycle performance than those of LIB based on pure Ti₃C₂, and its specific discharge capacity continuously increased along with the increasing number of cycles, which can be attributed to the gradual activation of Chl@Ti₃C₂ accompanied by the electrochemical reactions. The discharge capacity of 1 wt-% Chl@Ti₃C₂ was recorded to be 325 mA·h·g^–1 at the current density of 50 mA·g^–1 with a Coulombic efficiency of 56% and a reversible discharge capacity of 173 mA·h·g^–1 at the current density of 500 mA·g^–1 after 800 cycles. This work provides a novel strategy for improving the energy storage performance of 2D MXene materials by expanding the layer distance with organic dye aggregates.     

Sintering of β.q.ss. and gahnite glass-ceramic/silicon carbide composites

The sintering of β.quartz solid solution, β.q._ss., and gahnite glass-ceramic/particulate SiC composites has been studied by two different sintering procedures. In one procedure, the composites were fired above the melting point of the crystalline phase, identified by DTA, at a very high heating rate (800 °C min⁻¹) in air, nitrogen and argon atmospheres, for 1–6 min. It was found that the reduction of ZnO constituent of the glass by SiC particles gives rise to Zn, CO, and SiO gaseous products preventing complete densification of composites. In the other procedure, sintering was done at about crystallization peak temperature of the glass phase, employing a low heating rate (40 °C min⁻¹) in air for 60 min. In this case, the circumferential tensile stress in the glass-ceramic matrix phase, caused by the presence of incompressible SiC particles, retards the densification of the composites. The maximum amount of SiC particles yielding a reasonably dense composite was found to be 9 vol.%.