Self-healing behavior in MoSi2/borosilicate glass composite

The self-healing behavior of MoSi₂/borosilicate glass composite was investigated by comparing the flexure strength of pre-scratched specimens before and after healing treatment. The post-healing strength partially recovered when healing temperature was higher than 800 °C, and the healing of the scratch was observed after healing treatment at 900 °C and 1400 °C. Two kinds of possible mechanisms were proposed on the basis of morphology and elemental analyses of healing areas. The specimen treated at 900 °C showed a porous healing area and the strength recovered 28.6% resulted from the oxidation of MoSi₂ into MoO₃ and SiO₂. By contrast, the specimen treated at 1400 °C had a dense healing area and the strength recovered 86.9% due to the viscous flow of borosilicate glass.     

The improvement of the self-healing ability of MoSi2 coatings at 900–1200 °C by introducing SiB6

The brittleness of MoSi₂ ceramic and the thermal mismatch between MoSi₂ coating and C / C composite lead to brittle cracking of the coating at 900−1200 °C. This problem has been overcome in this studyby introducing submicron-SiB₆ into the coating. The pre-fabricated cracks and a kinetics model of hot-pressed SiB₆-MoSi₂ ceramic could quantitatively predict the glass growth and crack healing. As expected, enhancing temperature and SiB₆ content increased the growth rate of the borosilicate glass and the crack healing ability of MoSi₂ ceramic, which was ascribed to the lower oxidation activation energy and larger specific surface area of submicron-SiB₆. For the plasma sprayed coating, SiB₆ with submicron structure was benefit for cracking inhibition and formation of borosilicate glass during oxidation, reducing the oxygen permeability and the consumption of inner coating. Hence, the 15 % SiB₆-MoSi₂ coatings raised the protection times to 84 and 120 h at 900 and 1200 °C respectively, presenting favorable oxidation protective performance.     

High-performance ZrB2-SiC-Cf composite prepared by low-temperature hot pressing using nanosized ZrB2 powder

High-performance ZrB₂-SiC-C_f composite was successfully prepared by low temperature (1450 °C) hot pressing using nanosized ZrB₂ powder. Such material exhibited a non-brittle fracture feature, high work of fracture (321 J/m²) and excellent thermal shock resistance as well as good oxidation resistance. Composite incorporating carbon fibers in which the degradation of the carbon fiber was effectively inhibited through low-temperature sintering displayed remarkably improved thermal shock resistance with a critical temperature difference of 754 °C, almost twice those of the reported ZrB₂-based ultra-high temperature ceramics. The thermal and chemical stability of the carbon fiber and ceramic matrix were further analyzed by thermodynamic calculation and HR-TEM analysis.     

Enhanced oxidation resistance of SiC/SiC minicomposites via slurry infiltration of oxide layers

SiC based composite materials commonly have protective silica surface in air. Under humid environments at high temperatures, like occur in jet engines, the silica surface layer reacts with water molecules to form volatile silicon hydroxide (Si(OH)₄) and the protection is reduced which cause jet engine degradation. An alternative approach to protect SiC based composites would be to infiltrate the SiC matrix via slurry with an oxide material that is resistant to the high-temperature and humid environment. As proof of concept, aqueous based mullite particle slurries were infiltrated by pressurized flow and by capillarity of the wetting slurry on the external surface of the porous SiC matrix of single-fiber-tow SiC/SiC minicomposites. Minicomposites were precracked at room temperature during tensile tests then tested in tensile creep in air at 1200 °C to study the degree of protection that the infiltrated mullite provided at high temperatures. Next, fracture surfaces were examined using SEM.     

Long-term oxidation behaviors and strength retention properties of self-healing SiCf/SiC-SiBCN composites

The self-healing SiC_f/SiC-SiBCN composites with various boron contents in SiBCN were prepared, and their long-term oxidation behaviors and strength retention properties were investigated. The 100 h oxidation at 1200–1350 °C leads to parabolic mass gain of the obtained composites. With the oxidation temperature increased from 1200 °C to 1350 °C, the oxidation rate constants increase from 5.91 × 10^?8 mg²/(mm⁴ h) to 9.31 × 10^?7 mg²/(mm⁴ h) for the boron-lean (3.14%) composites, and from 2.57 × 10^?7 mg²/(mm⁴ h) to 6.04 × 10^?7 mg²/(mm⁴ h) for the boron-rich (7.18 wt%) composites. Correspondingly, the oxidation activation energy decreases from 363 kJ/mol to 112 kJ/mol due to the low initial oxidation temperature of boron-rich SiBCN. All the composites exhibit the higher strength retention rates after 1350 °C oxidation due to the enhanced self-healing performance. The boron-rich composites show a high strength retention rate of up to 104% due to the good self-healing capacity of the boron-rich SiBCN as well as the high CVI-SiC content.     

Effect of oxidation on room temperature strength of ZrB2- and HfB2- based ultra-high temperature ceramics

The effect of oxidation on room temperature (RT) flexure strength degradation in SiC-reinforced ultra-high temperature ceramics (UHTCs) and La₂O₃-doped UHTCs has been characterised in the temperature range 1400–1600°C for oxidation times of up to 32?h. Flaw healing was identified for oxide scale thicknesses 50 μm. Two oxide scale configurations have been proposed to minimise RT strength degradation. The most promising is the scale with a porous layer containing non-interconnected porosity (85–90% dense) of either MeLa₂O₇ or MeO_xC_y (Me?=?Zr or Hf).     

Oxidation protective MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

A novel kind of dense MoSi₂-SiC-Si coating was prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. Mo-SiC-C precoating was fabricated via slurry dipping method, and then MoSi₂-SiC-Si coating with dense structure consisting of Si, MoSi₂ and SiC was obtained by vapor silicon infiltration process. The isothermal oxidation tests at temperatures from 800 to 1600 °C and TGA test from room temperature to 1500 °C were used to evaluate the oxidation resistance ability of the MoSi₂-SiC-Si coating. The experimental results indicate that the prepared coating has good oxidation protection ability at a wide temperature range from room temperature to 1600 °C. Meanwhile, the oxidation of the coated samples is a weight gain process at temperatures from 800 to 1500 °C due to the formed SiO₂ layer on the surface of coating. After oxidation for 220 h at 1600 °C, the weight loss of the coated sample was only 0.96%, which is considered to be the excessive consumption of the outer coating and the appearance of defects in the coating. Two layers can be observed in the coating after oxidation, namely, SiO₂ layer and MoSi₂-SiC-Si layer.     

Synergistic reinforcement effects of ZrB2 on the ultra-high temperature stability of Cf/SiC composite fabricated by liquid silicon infiltration

A carbon fiber-reinforced silicon carbide (C_f/SiC) composite was fabricated with ZrB₂ via the liquid silicon infiltration (LSI) method. A prepreg was prepared by impregnating the phenolic resin with the ZrB₂ powder. The as-LSIed composites were tested for 5 min with an oxyacetylene torch to evaluate their ablation and oxidation properties under an ultra-high temperature environment. The ZrB₂ powders and SiC matrix between carbon fiber bundles generated a dense ZrO₂-SiO₂ layer, which inhibited further oxygen diffusion into the composite and minimized the ablation and oxidation of the carbon fibers. Weight loss and linear ablation rate were further reduced with the addition of ZrB₂ to the C_f/SiC composite; moreover, the synergistic effect of ZrB₂ and SiC reinforced the ablation properties with increased ZrB₂ content. ZrB₂ also reduced the amount of residual silicon, which was detrimental to the mechanical properties of C_f/SiC composite.     

Additive manufacturing of silicon carbide by selective laser sintering of PA12 powders and polymer infiltration and pyrolysis

In this work, we propose a novel hybrid additive manufacturing technique, which combines selective laser sintering (SLS) of polyamide powders and subsequent preceramic polymer infiltration and pyrolysis to manufacture Silicon Carbide components for complex architectures. By controlling the porosity of the sintered polymeric preform we are able to control the shrinkage upon the first infiltration and pyrolysis. This enabled the manufacturing of smaller features than those achievable with other manufacturing techniques. The mechanical strength of the resulting ceramic increased with the number of reinfiltration cycles up to 24 MPa, inversely the residual porosity decreased to 10 vol%. The microstructure showed two distinct phases of SiOC and SiC. The first was attributed to the interaction between the porous polyamide and the ceramic precursor during the first infiltration. SiC derived from the pyrolysis of the preceramic precursor alone.     

Ultra-high temperature ceramics based on ZrB2 obtained by pressureless sintering with addition of Cr3C2, Mo2C,and WC

ZrB₂-MeC and ZrB₂-19 vol% SiC-Me_xC_y where Me=Cr, Mo, W were obtained by pressureless sintering. The capability to promote densification of ZrB₂ and ZrB₂-SiC matrices is the highest for WC and lowest for Cr₃C₂. The interaction between the components results in the formation of new phases, such as MeB (MoB, CrB, WB), a solid solution based on ZrC, and a solid solution based on ZrB₂. The addition of Cr₃C₂ decreases the mechanical properties. On the other hand, the addition of Mo₂C or WC to ZrB₂-19 vol% SiC composite ceramics leads increased mechanical properties. Long-term oxidation of ceramics at 1500 °C for 50 h showed that, in binary ZrB₂-Me_xC_y, a protective oxide scale does not form on the surface thus leading to the destruction of the composite. On the contrary, triple composites showed high oxidation resistance, due to the formation of dense oxide scale on the surface, with ZrB₂-SiC-Mo₂C displaying the best performance.     

Large-scale synthesis of hafnium carbide nanowires via a Ni-assisted polymer infiltration and pyrolysis

Hafnium carbide (HfC) nanowires were successfully synthesized on C/C composites via a Ni-assisted polymer infiltration and pyrolysis. Before synthesizing HfC nanowires, the composition and microstructure of the organic HfC precursor and its pyrolyzed products were characterized by Fourier transform infrared spectra, X-ray photoelectron spectroscopy, and X-ray diffraction. The effect of heat-treatment temperature on the morphology and microstructure of HfC nanowires were investigated by scanning electron microscopy and transmission electron microscopy. Results show that HfC nanowires exhibits three-layer core-shell structure, including HfC core, HfO₂ inner shell (~2 nm) and carbon nanosheet outer shell (~1 nm). The obtained HfC nanowire is typically grown along <01> with the diameters of ~100 nm and the length of several micrometers. The growth of HfC nanowire follows the combination of top-type vapor-liquid-solid and solid-liquid-solid mechanism.     

Evolution of microstructure and anti-oxidation ability of ZrB2 improved by a unique inert glass phase

Zirconium Boride is widely used as thermal protection materials. However, the characteristics of poor oxidation resistance of ZrB₂ at high temperatures limits its applications. In order to improve the anti-oxidation performance of ZrB₂, a novel method of doping LaF₃ is adopted in present study. In the synthesis process of ZrB₂ via carbothermal reduction method, LaF₃ was introduced to the initial materials. Motivated by the slightly higher formation temperature of ZrB₂ than the melting point of LaF₃, a liquid glass phase composed of F and La was deposited on the surface of ZrB₂, which naturally brought in oxygen starvation and contributed to improved oxidation resistance performance. The crystal lattice parameters and the anti-oxidation ability of ZrB₂ are closely dependent on the added LaF₃ content, which can ascribe to La³⁺/Zr²⁺ substitution and the amount of glass phase on the surface of ZrB₂.     

Oxidation resistance of ZrB2 and ZrB2-SiC ultrafine powders synthesized by a combined sol-gel and boro/carbothermal reduction method

ZrB₂ and ZrB₂-SiC powders were prepared by a combined sol-gel and boro/carbothermal reduction method, and their oxidation kinetics was studied by using a non-isothermal thermogravimetric technique. The results showed that the Mample power law (n=1) was the most probable mechanism function, and the incorporation of SiC into ZrB₂ greatly enhance the latter's oxidation resistance. The oxidation activation energy values of phase pure ZrB₂ and ZrB₂-SiC powders were respectively 249 and 308 kJ/mol.     

ZrB2对低碳镁碳材料抗氧化性能的影响

在以大结晶镁砂、天然鳞片石墨为主原料的低碳MgO-C材料(C含量6%)中,分别加入2%的Al或2%、4%、6%的ZrB2,检测了试样在氧气中经950℃、1150℃和1350℃氧化30min后的质量损失和脱碳层厚度,研究了氧化试验后试样的显微结构和相组成。结果表明:适量的ZrB2可以显著提高低碳MgO-C材料的抗氧化性能。其机理是ZrB氧化后生成的BO与MgO反应生成液相包裹在石墨周围,阻止了石墨的氧化。     

Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping

The oxidative degradation of ZrB₂ ceramics is the main challenge for its extensive application under high temperature condition. Here, we report an effective method for co-doping suitable compounds into ZrB₂ in order to significantly improve its anti-oxidation performance. The incorporation of SiC and WC into ZrB₂ matrix is achieved using spark plasma sintering (SPS) at 1800?°C. The oxidation behavior of ZrB₂-based ceramics is investigated in the temperature range of 1000?°C–1600?°C. The oxidation resistance of single SiC-doped ZrB₂ ceramics is improved due to the formation of silica layer on the surface of the ceramics. As for the WC-doped ZrB₂, a dense ZrO₂ layer is formed which enhances the oxidation resistance. Notably, the SiC and WC co-doped ZrB₂ ceramics with relative density of almost 100% exhibit the lowest oxidation weight gain in the process of oxidation treatment. Consequently, the co-doped ZrB₂ ceramics have the highest oxidation resistance among all the samples.     

High temperature oxidation resistance of Y2O3 modified ZrB2-SiC coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at high temperature, Y₂O₃ modified ZrB₂-SiC coating was fabricated on C/C composites by atmospheric plasma spraying. The microstructure and chemical composition of the coatings were characterized by SEM, EDS, and XRD. Experiment results showed that the coating with 10 wt% Y₂O₃ presented a relatively compact surface without evident holes and cracks. No peeling off occurred on the interface between the coating and substrate. The ZSY10 coating underwent oxidation at 1450 °C for 10 h with a mass loss of 5.77%, while that of ZS coating was as high as 16.79%. The existence of Y₂O₃ played an important role in inhibiting the phase transition of ZrO₂, thus avoiding the cracks caused by the volume expansion of the coating. Meanwhile, Y₂SiO₅ and ZrSiO₄ had a similar coefficient of thermal expansion (CTE), which could relieve the thermal stress inside the coating. The ceramic phases Y₂SiO₅, Y₂Si₂O₇ and ZrSiO₄ with high thermal stability and low oxygen permeability reduced the volatilization of SiO₂.     

Bubble phenomenon of ZrB2 based composites at high temperatures

Bubble phenomenon is common for ultra-high temperature ceramics (UHTCs) during oxidation or ablation processes, which will impair the oxidation/ablation resistant properties. This work is aiming to illuminate the formation and rupture processes of bubbles. In this work, ZrB₂-SiC-WB composite coatings were prepared via vacuum plasma spray technique and oxidized at 1500?°C for different durations. Obvious bubble phenomenon was observed. The morphology and distribution of bubbles were characterized. The formation mechanism of bubbles was calculated and analyzed based on thermal dynamics. The results showed that B₂O₃ gas played a key role in affecting the bubble behaviors. Bubbles tended to nucleate near the interface between the solid and liquid oxide layers. Small bubbles aggregated to large bubbles near the outmost liquid layer. Large bubbles near the surface were easy to rupture. The calculated results were consistent with the observed results.     

Mechanical and ablation properties of a C/C-HfB2-SiC composite prepared by high-solid-loading slurry impregnation combined with precursor infiltration and pyrolysis

An improved high-solid-loading slurry impregnation process was developed to introduce HfB₂ particles into a low-density C/C preform efficiently, and precursor infiltration and pyrolysis process was used for densification to obtain a C/C-HfB₂-SiC composite. The microstructure characterization revealed that HfB₂ particles uniformly filled the pores in the C/C preform, and SiC well densified the interstices between HfB₂ particles and the small pores in the carbon fiber bundles. After being tested, the C/C-HfB₂-SiC composite had a density of 4.07 g/cm³ and a bending strength of 344.8 MPa, and exhibited a non-brittle fracture behavior. After ablation with oxyacetylene flame at 2500 ℃ for 120 s, the mass ablation rate and linear ablation rate of the C/C-HfB₂-SiC composite were 0.5 mg/s and 0.415 μm/s, respectively. The good ablation performance is attributed to the hindering effect of the HfO₂ scale on oxygen diffusion at high temperature.     

Contact interaction and hot pressing of ZrB2-MoSi2 in CO/CO2 atmosphere

ZrB₂-15 vol% MoSi₂ ceramics were hot pressed in CO/CO₂ atmosphere in the 1700–1900^oС temperature range. During hot pressing, MoSi₂ decomposes into Mo and Si and the phase composition of the as-sintered ceramic results in ZrB₂, (Zr, Mo)B₂, SiC, SiO₂, and MoB. Contact melting between ZrB₂ and MoSi₂ was observed at 1800^oC, corresponding to the formation of (Zr, Mo)B₂. Ceramics obtained at1800–1850^oС had ∼ 500 МPа and 200 MPa strength at room at 1800^oC in vacuum, respectively. The thickness of the oxidized scales upon exposing the samples at 1600 ^oC for 120 min was 30–80 µm and depended on the amount of residual MoSi₂ and (Zr, Mo)B₂. The highest oxidation resistance was observed for the ceramic sintered at 1850 °C.     

Enhanced mechanical properties and thermal shock resistance of Cf/ZrB2-SiC composite via an efficient slurry injection combined with vibration-assisted vacuum infiltration

An efficient slurry injection combined with vibration-assisted vacuum infiltration process has been developed to fabricate 3D continuous carbon fiber reinforced ZrB₂-SiC ceramics. Homogenous distribution between carbon fiber and ceramic was achieved successfully, leading to an enhancement in mechanical properties. The C_f-PyC/ZrB₂-SiC composite exhibited a typical non-brittle fracture mode with a superior fracture toughness of 6.72 ± 0.21 MPa·m^1/2 and an extraordinary work of fracture of 2270 J/m², respectively, increasing by nearly 14.8 % and 36 % as compared with those of a parent composite fabricated by only slurry injection and slurry infiltration. The enhancement in fracture toughness and work of fracture were attributed to multiple toughening mechanism including crack deflection, PyC coated fiber bundles pull-out and fiber bridging. Moreover, a critical thermal shock temperature difference of 814 °C was achieved, higher than that of traditional ZrB₂-based ceramics. This work presents an efficient approach to fabricate high-performance C_f/UHTCs with uniform architecture.