Thermal properties of field-assisted-sintered SiCN–Y2O3 composites

Polymer-derived amorphous SiCN has excellent high-temperature stability and properties. To reduce the shrinkage during pyrolysis and to improve the high-temperature oxidation resistance, Y₂O₃ was added as a filler. In this study, polymer-derived SiCN–Y₂O₃ composites were fabricated by mixing a polymeric precursor of SiCN with Y₂O₃ submicron powders in different ratios. The mixtures were cross-linked and pyrolyzed in argon. SiCN–Y₂O₃ composites were processed using field-assisted sintering technology at 1350°C for 5 min under vacuum. Dense SiCN–Y₂O₃ composite pellets were successfully made with relative density higher than 98% and homogeneous microstructure. Due to low temperature and short time of the heat-treatment, the grain growth of Y₂O₃ was substantially inhibited. The Y₂O₃ grain size was ∼1 μm after sintering. The composites’ heat capacity, thermal diffusivity, and thermal expansion coefficients were characterized as a function of temperature. The thermal conductivity of the composites ceramics decreased as the amount of amorphous SiCN increased and the coefficient of thermal expansion (CTE) of the composites increased with Y₂O₃ content. However, the thermal conductivity and CTE did not follow the rule of mixture. This is likely due to the partial oxidation of SiCN and the resultant impurity phases such as Y₂SiO₅, Y₂Si₂O₇, and Y_4.67(SiO₄)₃O.     

Design and fabrication of Al2O3f/SiCN composite with excellent microwave absorbing and mechanical properties

A new kind of structural and functional integration ceramic matrix composite material was prepared from high-performance alumina (Al₂O₃) fibers and absorbing silicon carbonitride (SiCN) ceramics via a combination of polymer infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) methods. The Al₂O₃ fiber annealed at its cracked temperature had enhanced permittivity, because the sizing agent on the Al₂O₃ fiber surface was cracked into pyrolysis carbon. For PIP + CVI Al₂O_3f/SiCN composites, PIP SiCN matrix with low conductivity was used as the matching phase, while CVI SiCN matrix with medium permittivity and dielectric loss was regarded as the reinforcing phase distributed in porous PIP SiCN matrix and inter-bundles of Al₂O₃ fiber to improve their mechanical and microwave absorption properties. The fracture toughness and flexural strength of Al₂O_3f/SiCN composite were determined to be 9.4 ± 0.5 MPa m^1/2 and 279 ± 28 MPa, respectively. Based on the design principles for impedance matching, the Al₂O_3f/SiCN composites before and after oxidation were used as loss and impedance layers, respectively. It was found that the optimized composite had the lowest reflection coefficient (RC) of −70 dB and the effective absorption bandwidth covering the whole X-band. In conclusion, Al₂O_3f/SiCN composite can serve as a high-temperature structural material with excellent microwave absorption properties for aerospace applications.     

Oxidation/ablation behaviors of hafnium carbide-silicon carbonitride systems at 1500 and 2500 C

The oxide scales of hafnium carbide (HfC) typically exhibit a porous structure after oxidation/ablation due to the release of gas oxidation products, which allows oxygen penetration to promote the rapid oxidation of the HfC matrices. Here, we report that the oxidation/ablation resistance of HfC was enhanced by the incorporation of amorphous silicon carbonitride (SiCN). HfC-SiCN ceramics with 10 vol % SiCN showed a significant improvement in the oxidation/ablation resistance compared with pure HfC. The HfC-10 vol % SiCN ceramic has a higher density with good mechanical properties. After being oxidized at 1500 °C for 2 h, a dense and homogeneous HfO₂-HfSiO₄ layer with low oxygen permeability is formed. The ablation resistance of the HfC-10 vol % SiCN ceramic is improved due to the formation of the triple-layer structure oxide with good thermal stability and mechanical scouring resistance. After ablation under an oxyacetylene flame for 60 s, the mass and linear ablation rates of HfC-10 vol % SiCN ceramic are −0.019 mg cm⁻² s⁻¹ and -0.156 μm s⁻¹, respectively.     

Design and optimization of oxidation resistant coating for C/C aircraft brake materials

A SiCN/borosilicate glass anti-oxidation coating with double-layer structure was designed for C/C aircraft brake materials. The SiCN layer was introduced as transition layer to improve the wettability between borosilicate glass and C/C composites, and the microstructure results indicated that the coating with SiCN inner layer was dense and uniform. The oxidation resistance evaluation of the coated samples was conducted at 800 °C in air for 10 h. The weight loss of SiCN/borosilicate glass coated samples valued ~ 5.66% indicated that the oxidation resistant property of the simple SiCN/borosilicate glass coating was not good, which was mainly due to the relative large viscosity of borosilicate glass at 800 °C. B₄C was introduced to add into the outer glass coating to improve the self-healing ability of the coating. After oxidized at 800 °C in air for 10 h, the weight loss of the SiCN/borosilicate glass-B₄C coated samples was ~ 2.48%. B₄C could consume the oxygen diffused into the coating and the reacted product B₂O₃ with a better fluidity at 800 °C could effectively heal cracks and pores in the coating to improve the oxidation resistance property. The reaction of B₄C oxidized to B₂O₃ was accompanied with ~ 1.5 times volume expansion, which was also beneficial for the healing of defects.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

Oxidation resistance and thermal shock properties of self-healing SiCN/borosilicate glass-B4C-Al2O3 coatings for C/C aircraft brake materials

SiCN/borosilicate glass-B₄C-Al₂O₃ coating was deposited on carbon fiber-reinforced carbon matrix (C/C) brake materials to protect them from oxidation. Microstructural analysis revealed that the coating was dense and uniform. Fabricated coating showed excellent oxidation resistance and significantly low weight losses after oxidation in dry air for 10?h than SiCN/borosilicate glass-B₄C coated samples (ca. 0.12%, 0.51%, and 0.29% at 700, 800, and 900?°C, respectively). B₄C is believed to react with the oxygen diffused into the coating to produce B₂O₃, which could heal cracks of the coating and improve its self-sealing ability and oxidation resistance. The Al₂O₃ present in the outer glass layer is believed to inhibit volatilization of B₂O₃, thereby reducing weight losses in air. Fabricated coating also possessed excellent oxidation resistance under fresh and sea water conditions, with cracks and pores generated during oxidization process being effectively healed. Prepared coating materials showed excellent thermal shock resistances after 50 thermal shock cycles, with weight losses being as low as 0.23%.     

Dielectric and microwave absorption properties of polymer derived SiCN ceramics annealed in N2 atmosphere

Porous SiCN ceramics were successfully fabricated by pyrolysis of a kind of polysilazane. The effects of annealing temperature on the microstructure evolution, direct-current electrical conductivity, dielectric properties, and microwave absorption properties of SiCN in the frequency range 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, SiC, Si₃N₄ and free carbon nanodomains are gradually formed in the SiCN. Both the SiC and free carbon nanodomains lead to the increases of the complex relative permittivity and loss tangent of SiCN. With the increase of the annealing temperature, the average real permittivity, imaginary permittivity and loss tangent increase from 4.4, 0.2 and 0.05 to 13.8, 6.3 and 0.46, respectively. The minimum reflection coefficient and the frequency bandwidth below −10 dB for SiCN annealed at 1500 °C are −53 dB and 3.02 GHz, indicating good microwave absorption properties.     

Oxidation Behavior of a Fully Dense Polymer-Derived Amorphous Silicon Carbonitride Ceramic

The oxidation behavior of a polymer-derived amorphous silicon carbonitride (SiCN) ceramic was studied at temperature range of 900°–1200°C using fully dense samples, which were obtained using a novel pressure-assisted pyrolysis technique. The oxidation kinetics was investigated by measuring the thickness of oxide layers. The data were found to fit a typical parabolic kinetics. The measured oxidation rate constant and activation energy of the SiCN are close to those of CVD and single-crystal SiC. The results suggest that the oxidation mechanism of the SiCN is the same as that of SiC: oxygen diffusion through a silica layer.     

Polymer-derived Co2Si(Co)/SiCN composite ceramics with tunable microwave absorption properties

In this paper, Co₂Si(Co)/SiCN composite ceramics were synthesized by simple precursor-derived ceramics method. The phase composition, morphology, and microwave absorption properties of Co₂Si(Co)/SiCN composite ceramics at different pyrolysis temperatures (1000–1400°C) were studied. When pyrolysis temperature was 1300°C, carbon nanowires (CNWs), Co₂Si, Si₂N₂O, SiC and Si₃N₄ were in situ generated and the best electromagnetic wave (EMW) absorption performance was obtained. The minimum reflection loss reached−50.04 dB at 4.81 mm, and the effective absorption bandwidth broadened to 3.48 GHz (14.52–18 GHz) at 1.31 mm. The excellent EMW absorption performance mainly comes from the coexistence of multiple loss mechanisms, including the magnetic loss of Co₂Si, the conduction loss of CNWs, and the heterogeneous interfaces polarization between varieties of nanocrystals and amorphous ceramic matrix. By adjusting the sample thickness from 1 to 5 mm, the effective absorption of S1300 can cover the entire X and Ku bands, from 3.36 to 18 GHz. This study provides a simple way to synthesize high performance ceramic-based microwave absorbing materials.     

High-temperature microstructural evolution of SiCN fibers derived from polycarbosilane with different C/N ratios

Research into the high-temperature microstructural evolution of SiCN ceramic fibers is important for the aerospace application of advanced ceramic matrix composites in harsh environments. In this work, we studied the microstructural evolution of SiCN fibers with different C/N ratios that derived from polycarbosilane fibers at the annealing temperature range of 1400∼1600 °C. These results showed that the phase separation of SiC_xN_y phase and the two-dimension grain growth process of free carbon nanoclusters could be processed at the researched temperature range. As the annealing temperature increased to 1600 °C, the crystallization of amorphous SiC and Si₃N₄ could be detected. SEM and Raman analysis showed that the decomposition and carbothermal reduction of the Si₃N₄ phase at high temperatures played primary roles in contributing to the fiber strength degradation. Thus, a higher C/N ratio, which is beneficial for inhibiting the decomposition of amorphous Si₃N₄, helps SiCN fibers retain high tensile strength at high temperatures.     

Synthesis and pyrolysis of single-source precursor for HfCxN1-x-SiC ceramic with different SiC contents

A novel single-source precursor was synthesized to prepare HfC_xN_1-x/SiC multiphase ceramics by using hafnium chloride (HfCl₄), diallylamine (DAA) and polycarbosilane (PCS). We conducted an investigation of the synthesis process, polymer-to-ceramic conversion, as well as the microstructure and phase evolution of HfC_xN_1-x/SiC multiphase ceramics with different levels of SiC content. The results showed that the core-shell particles of HfC_xN_1-x-carbon were embedded homogeneously in the β-SiC matrix which is beneficial for preventing grain growth and improving oxidation resistance. Based on data from oxidation tests, the ceramics improved the oxidation temperature and remained stable at a high temperature (1500 °C) with oxidation layer formation on the surface. Due to the highly cross-linked structure without oxygen, high ceramic yield, homogeneous composition and excellent oxidation resistance of the pyrolysis product, the as-prepared precursor is a promising material for making high-performance composite ceramics.     

Effect of acrylic acid additive on electric conductivity of polymer-derived amorphous silicon carbonitride

The effect of acrylic acid additive on the electric conductivity of amorphous SiCN derived from polymeric precursor was studied. The conductivity showed to follow the Arrhenius dependence on pyrolysis temperature, but with much smaller activation energy, as compared to the unmodified SiCN. Structural analysis using Raman and XPS revealed that the size of the free-carbon clusters within the AC-modified SiCN changed with pyrolysis temperature, but the sp²-to-sp³ ratio remained almost the same. The reason for the effect of AC on the carbon cluster was speculated. The mechanisms governing the conductivity behavior of the AC-modified SiCN were discussed.     

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

A kind of chemical vapor infiltration (CVI) Si₃N₄–BN–SiCN composite ceramic with excellent electromagnetic wave (EMW) absorbing properties is obtained by CVI BN interface and SiCN matrix on porous Si₃N₄ ceramics, and then annealed at high temperatures (1200°C‐1500°C) in N₂ atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI SiCN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si₃N₄–BN–SiCN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient (RC) and maximum effective absorption bandwidth (EAB) of the composite ceramics annealed at 1300°C were ?47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m^1/2, respectively.     

Effect of Si content on the microstructure and properties of Ti–Si–C composite coatings prepared by reactive plasma spraying

In this study, Ti–Si–C composite coatings were synthesized via plasma spraying of agglomerated powders prepared by a spray drying/precursor pyrolysis technology using Ti, Si, and sucrose powders. The influence of Si content, ranging from 0 wt% to 24 wt%, on the microstructure, mechanical properties, and oxidation resistance of the composite coatings was investigated. Results show that the phase composition of the Ti–Si–C composite coatings changes with the increasing Si content. The coatings without Si addition consist of TiC and Ti₃O; the coatings with 6–18 wt% Si are composed of TiC, Ti₅Si₃, and Ti₃O; the coatings with Si content of 24 wt% form only TiC and Ti₅Si₃ phases. As the Si content increases, the hardness of the Ti–Si–C composite coatings increases first and then decreases, depending on the intrinsic hardness of the ceramic phases, the brittleness of Ti₅Si₃, and the defects such as pores and cracks. The Ti–Si–C composite coatings have high wear resistance due to the in-situ synthesized high-hardness TiC and Ti₅Si₃. Owing to the high brittleness of Ti₅Si₃, the increasing Si content leads to higher wear volume loss at room temperature, which can be partially improved in high-temperature wear tests. The oxidation resistance of Ti–Si–C composite coatings increases with the increase of Si content, and the higher the oxidation temperature, the more obvious the influence of the Si addition on oxidation resistance.     

Effect of pyrolysis temperature on the electric conductivity of polymer-derived silicoboron carbonitride

The electric conductivity of polymer-derived SiBCNs pyrolyzed at different temperatures was studied. We showed that the boron impeded the graphitization of the free-carbon phase in the SiBCN, leading to a higher characteristic temperature and activation energy as compared to the SiCN. Such an impeding effect is due to the interaction between h-BN and graphite phase. We also provided a credible evidence to show that the increase in the electric conductivity of the SiBCN with pyrolysis temperature is likely due to the increase in the conductivity of the free-carbon phase.     

Mechanical and microstructural characterisation of SiC- and SiBNC-fibre reinforced CMCs manufactured via PIP method before and after exposure to air

Non-oxide CMCs based on pyc-coated SiC-fibres (Tyranno SA3?) as well as novel amorphous ceramic fibres in the quaternary system Si–B–C–N (named SiBNC-fibres) were manufactured via polymer infiltration and pyrolysis process. Two different fibre architectures were applied: 0|90° unidirectional (UD) cross-ply and 0|90° plain weave fabric layer. UD cross-ply reinforced CMCs exhibit much more uniformly distributed filaments leading to better infiltration efficiency in resin transfer moulding process. Bending strength and fracture behaviour are strongly influenced by fibre architecture: UD cross-ply reinforced CMCs show higher bending strengths and less non-linear behaviour compared to plain weave fibre reinforcement. In tensile test there is no evidence of an influence of fibre architecture. Mechanical properties of unexposed SiC_pyc/SiCN and SiBNC_pyc/SiCN strongly correlate with fibre properties. After exposure to air (T = 1100 °C, 20 h), a significant decrease of mechanical properties could be observed, caused by complete oxidation of pyc-fibre coating interfered with silica formation.     

Oxidation resistance of multi-walled carbon nanotubes coated with polycarbosilane-derived SiCxOy ceramic

Multi-walled carbon nanotubes (MWCNTs) have been successfully coated with a thin SiC_xO_y coating when polycarbosilane (PCS) was used as precursor and pyrolyzed in a coke bed. Meanwhile, effect of PCS concentration on oxidation resistance of the coated MWCNTs is studied. The results showed that the pyrolysis products of PCS were composed of amorphous SiC_xO_y as the main phase, together with β-SiC and SiO₂ as the minor phases whose amount increased a little with the increase of temperature from 1000 °C to 1500 °C. The thickness of SiC_xO_y coating on the surface of MWCNTs increased a little from 1 wt.% to 5 wt.%, but decreased dramatically with PCS concentration in the range of 10-30 wt.%. The oxidation resistance of the coated MWCNTs was greatly improved in comparison with as-received ones. The oxidation peak temperature of the coated MWCNTs reached 783.7 °C, much higher than 652.2 °C for as-received ones.     

A novel SiCN@TiO2 core–shell ceramic microspheres derived from a polymeric precursor

A novel core–shell ceramic microspheres, composed of a SiCN inner core and TiO₂ nanoparticles outer shell, were prepared via emulsion technique and polymer-derived ceramics (PDCs) method. The forming process of SiCN@TiO₂ core–shell ceramic microspheres were controlled by adjusting the ratio of raw material, curing temperature and pyrolysis temperature. The morphology, chemical composition and phase transformation were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). PVSZ@TiO₂ microspheres with good spherical structure and uniform-dispersed TiO₂ surface were fabricated at 200 °C with raw material ratio of 25%. After pyrolyzed at 1400 °C, the obtained SiCN@TiO₂ core–shell ceramic microspheres retained spherical structure. The XRD showed that the products were mainly composed of rutile TiO₂, SiC and Si₃N₄ crystalline phase, which were generated by polyvinylsilazane.     

Polymer derived silicon oxycarbide ceramic monoliths: Microstructure development and associated materials properties

Polymer derived SiOC and SiCN ceramics (PDCs) are interesting candidates for additive manufacturing techniques to develop micro sized ceramics with the highest precision. PDCs are obtained by the pyrolysis of crosslinked polymer precursors at elevated temperatures. Within this work, we are investigating PDC SiOC ceramic monoliths synthesized from liquid polysiloxane precursor crosslinked with divinylbenzene for fabrication of conductive electromechanical devices. Microstructure of the final ceramics was found to be greatly influenced by the pyrolysis temperature. Crystallization in SiOC ceramics starts above 1200?°C due to the onset of carbothermal reduction leading to the formation of SiC and SiO₂ rich phases. Microstructural characterisation using ex-situ X-ray diffraction, FTIR, Raman spectra and microscopy imaging confirms the formation of nano crystalline SiC ceramics at 1400?°C. The electrical and mechanical properties of the ceramics are found to be significantly influenced by the phase separation with samples becoming more electrically conducting but with reduced strength at 1400?°C. A maximum electrical conductivity of 10¹ S?cm^?1 is observed for the 1400?°C samples due to enhancement in the ordering of the free carbon network. Mechanical testing using the ball on 3 balls (B3B) method revealed a characteristic flexural strength of 922?MPa for 1000?°C amorphous samples and at a higher pyrolysis temperature, materials become weaker with reduced strength.