Fabrication and characterization of biomorphic cellular C/SiC–ZrC composite ceramics from wood

Biomorphic cellular C/SiC–ZrC composite ceramics were fabricated from pine and oak wood precursors. Carbonaceous preforms were first prepared by wood pyrolysis and subsequently infiltrated with polyzirconobutanediol (PZC) and polycarbosilane (PCS) to form the composite ceramics. TGA/DTG and dilatometric analysis were used to study the pyrolysis of the wood precursor. XRD and SEM analyses were applied to characterize the microscopic properties of the resulting biomorphic cellular C/SiC–ZrC composite ceramics. Compared with oak, pine was preferable for preparation of cellular C/SiC–ZrC composite ceramics because of its unique microstructure. The SiC–ZrC phase distribution differed within the composite ceramics. In addition, the compression strengths of wood, charcoal, and cellular C/SiC–ZrC composite ceramics were measured in the axis direction. Results showed the improved compression strength of biomorphic cellular C/SiC–ZrC composite ceramics when the impregnation cycles were repeated.     

Improving the sinterability of ZrC–SiC composite powders by Mg addition

High-density ZrC–SiC composite ceramics are typically sintered under demanding conditions, specifically, high sintering pressures and high temperatures. However, the need for such conditions can be alleviated by the use of ZrC–SiC composite nanoparticles with a high sintering activity. In the present study, core-shell-structured hybrid ZrC–SiC composite nanoparticles were synthesised with the addition of Mg by using a sol-gel process combined with in-situ carbothermal reduction reactions. The synthesis route, characterisation, and sintering mechanism were investigated in detail. It was found that the addition of MgCl₂ to the precursors of ZrC–SiC can not only strengthen the network structure of ZrC–SiC gel but also lead to the formation of an amorphous Mg–Si–O oxide coating on the nanoparticle surfaces, which enhances the sinterability of ZrC–SiC nanoparticles. As a result, a compact ZrC–SiC composite ceramic with a higher relative density (up to 91.3%) than the contrast sample was successfully prepared by pressure-free sintering at 1700 °C.     

Fabrication and mechanical properties of laminated HfC–SiC/BN ceramics

Laminated HfC–SiC/BN ceramics were successfully fabricated by tape casting and hot pressing. Fully dense HfC–SiC ultra-high temperature ceramics with homogeneous structure were obtained. The introduction of the weak BN layer resulted in a slight decrease of the flexural strength but significantly improved the fracture toughness compared with monolithic HfC–SiC ceramics. The fracture toughness of laminated HfC–SiC/BN ceramics in the parallel direction peaked at 8.06 ± 0.46 MPa m^1/2, which increased by 115% than that of monolithic HfC–SiC ceramics. The composites showed non-catastrophic fracture behaviors in both parallel and perpendicular directions. It indicates that laminated structure design is a promising approach to obtain full density HfC–SiC ceramics with high fracture toughness.     

Ablation mechanism of C/C–SiC and C/C–SiC–ZrC composites in hypersonic oxygen-enriched environment

In this study, C/C–SiC and C/C–SiC–ZrC composites were prepared via chemical vapor infiltration and polymer infiltration pyrolysis, and the ablation mechanism under hypersonic oxygen-rich environmental conditions was investigated. The C/C–SiC composites demonstrate an excellent ablation resistance in a hypersonic oxygen-rich environment with a relatively low temperature and speed of approximately 1800 K and 1100 m/s, respectively. It is only in the ablation center area with higher temperatures that a certain degree of thermochemical ablation was observed. The mass and linear ablation rates of C/C–SiC composites (0.027 g/s and 0.117 mm/s, respectively) showed a significant increase in a hypersonic oxygen-rich environment with a temperature and velocity of approximately 2050 K and 2000 m/s, respectively. The high-temperature ablation resistance of ZrC-modified C/C–SiC–ZrC composites improved significantly. However, the ZrC ceramic component had a considerable impact on the ablation resistance of the material. The structural integrity of C/C–20SiC–30ZrC composites was relatively high in hypersonic oxygen-rich environments with a jet temperature and velocity of 2050 K and 2000 m/s, respectively, and mass and linear ablation rates were 0.012 g/s and 0.015 mm/s, respectively. When the ZrC content increased by 40%, the ablation resistance of the composite reduced significantly, whereas the mass and linear ablation rates increased to 0.043 g/s and 0.130 mm/s, respectively.     

Active cellular preform derived from stems of jute and sticks of cane and their suitability for bulk porous Si/SiC ceramics

Stems of Jute (Corchorus capsularis L.) and sticks of Cane (Calamus rotang L.), plants of immense economic importance in the Indian subcontinent, were converted into carbonaceous perform (C-preform) maintaining the circular cylindrical shapes in lengths of 0.02–0.05 m by controlled thermal processing. Plant material precursors were characterized by analysis of elemental (C, H, N) and molecular (cellulose, hemicellulose, lignin) compositions, by determination of Bulk Density (BD) and ash content and by optical microscopy and X-ray diffractometry (XRD). C-preforms were also characterized by measurement of BD and by Scanning Electron Microscopy (SEM) and XRD. The C-preforms were further subjected to infiltration with Si-melt (1823–1923 K) under vacuum. Spontaneous infiltration and reaction yielded composite ceramics preserving the morphology of native Jute Stem (JS) and Cane Stick (CS) precursors on macro and micro scale. The materials were found to be duplex composites with Si and β-SiC as crystalline phases. The end ceramics were characterized by measurement of BD, and also by SEM and by XRD. Measured mean BD of the Si/SiC composites derived from JS and CS were 2190 Kg m⁻³ and 2250 Kg m⁻³. The respective volume fractions of large diameter (>100 μm) bulk pores were 0.134 and 0.204, in the composites derived from JS and CS. Taking into account the measured volume fraction internal pores of 0.11 and 0.149, the volume fractions of SiC were calculated to be 0.136 and 0.307 in the composites derived from JS and CS respectively, closely tallying with those calculated from the C-preform bulk densities. The cellular Si/SiC ceramics derived from JS and CS having special morphologies with long and large porous channels and oriented growth of constituent phases are likely to be suitable for devices such as high temperature insulators, catalyst support structures for gas phase reactions at elevated temperatures, molten metal filters and others.     

Synthesis of ZrC–SiC Powders from Hybrid Liquid Precursors with Improved Oxidation Resistance

ZrC–SiC powders are synthesized by high‐temperature pyrolysis of hybrid liquid precursors, which are prepared from organic Zr‐containing precursor (PZC) and liquid polycarbosilane (LPCS). Due to the excellent miscibility between PZC and LPCS, the hybrid liquid precursors are formed by dissolving PZC into LPCS without adding organic solvent. The viscosity and elemental content of Zr and Si of the hybrid precursors are readily adjustable by controlling the LPCS/PZC mass ratio. SEM and TEM observations reveal that the ZrC–SiC powders pyrolyzed at 1550°C exhibit spherical morphology with characteristic dimension of less than 60 nm, and the two phases are uniformly distributed in composite powders. The advantage of the ZrC–SiC powders synthesized by this novel method is demonstrated by investigating the oxidation behavior of powders with different amount of SiC and ZrC. Below 700°C, ZrC quickly oxidizes to generate an almost nonprotective ZrO₂ scale, whereas at ~ 1000°C, dense and protective SiO₂ forms that improves the oxidation resistance of the ZrC–SiC composite powders.     

Dielectric and microwave-absorption properties of SiC nanoparticle/SiBCN composite ceramics

SiC nanoparticles with different contents (5–20 wt%) were mixed with liquid polyborosilazane. The compound was used to prepare SiC nanoparticle/polymer-derived SiBCN ceramics (SiC/PDCs-SiBCN). Thermal gravity tests (25–1400 °C) in air and helium atmosphere were used to investigate the thermal stability of SiC/PDCs-SiBCN. Dielectric and microwave-absorption properties of SiC/PDCs-SiBCN were determined at frequencies of 8.2–12.4 GHz by waveguide method. Results show that the addition of SiC nanoparticles increased the thermal stability of SiBCN ceramics. The permittivity, dielectric loss and absorption coefficient of ceramics increased as an elevated SiC content, resulting from the increase of carrier concentration. To understand the high-temperature dielectric property of SiC/PDCs-SiBCN, the permittivity of SiBCN ceramics with 15 wt% of SiC was measured at temperatures of 293–773 K. The composite ceramics were found to have a visible increase in the permittivity and dielectric loss, indicating their great potential as the high-temperature microwave absorption materials.     

Preparation and ablation properties of SiC nanowire-reinforced ZrC–SiC coating-matrix integrated C/C composites

A modification of the precursor infiltration pyrolysis (PIP) method was explored to prepare the integrated doped ceramic matrix and coating by the added SiC nanowires layer and shape-stabilization process. The epitaxial layer of SiC nanowires provided surficial attachments for the precursor. And the shape-stabilization process aggregated loose ceramic particles into a coating. Then the SiC nanowire-reinforced ZrC–SiC coating-matrix integrated C/C (S/SZ-CZ/C) composite was simply prepared by the modified PIP method. The bonding strength between the coating and matrix of the S/SZ-CZ/C composite was improved. Through the ablation test, the mass and linear ablation rate of the S/SZ-CZ/C composite were 0.46 mg/s and 0.67 μm/s, which were 60.34 % and 74.91 % lower than those of the SiC nanowire-reinforced C/C–ZrC (S/CZ/C) composite, respectively. The integration of the coating and matrix enabled the formation of a continuous oxide layer of molten SiO₂ and ZrO₂ in the ablation process, which helped to block the oxygen and heat during the ablation test. Thus the ablation resistance of the materials was systematically and effectively improved.     

Reactive Hot Pressing of ZrC–SiC Ceramics at Low Temperature

Composites of ZrC–SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC_1?x, SiC, and ZrSi. Low‐temperature densification of ZrC?SiC ceramics is attributed to the formed nonstoichiometric ZrC_1?x and Zr–Si liquid phase. Adding 5 wt% Si to ZrC, the three‐point bending strength of formed ZrC_0.8–13.4 vol%SiC ceramics reached 819 ± 102 MPa with hardness and toughness being 20.5 GPa and 3.3 MPa·m^1/2, respectively.     

Laser ablation behaviors of vacuum plasma sprayed ZrC-based coatings

ZrC, ZrC-30 vol% SiC, and ZrC-30 vol% TiC coatings were fabricated by vacuum plasma spray and the laser ablation behaviors were evaluated by a CO₂ laser beam under two heat fluxes (15.9 and 25.5 MW/m²). The phase compositions and microstructures of the coatings after ablation were investigated and the effect of SiC and TiC additives was analyzed. The results showed that the ZrC–SiC coating displayed better ablation resistance compared with the ZrC and ZrC–TiC coatings under 15.9 MW/m² heat flux. While the ZrC–TiC coating exhibited the improved ablation resistance under 25.5 MW/m² heat flux. The continuous and integral ZrO₂–SiO₂ scale provided protective effect for the ZrC–SiC coating. A liquid ZrO₂–TiO₂ layer which owned self-healing ability was formed for the ZrC–TiC coating in both heat fluxes. However, the state of the formed liquid, like amount, viscosity, evaporation, and decomposition, was influenced by the environment and was vital for the ablation resistance. This work might give a clue for designing ultrahigh-temperature ceramics as potential laser ablation–resistant coating materials.     

Selective laser reaction synthesis of SiC,Si3N4 and HfC/SiC composites for additive manufacturing

Selective laser reaction sintering techniques (SLRS) techniques were investigated for the production of near net-shape non-oxide ceramics including SiC, Si₃N₄, and HfC/SiC composites that might be compatible with prevailing powder bed fusion additive manufacturing processes. Reaction bonded layers of covalent ceramics were produced using in-situ reactions that occur during selective laser processing and layer formation. During SLRS, precursor materials composed of metal and/or metal oxide powders were fashioned into powder beds for conversion to non-oxide ceramic layers. Laser-processing was used to initiate simultaneous chemical conversion and local interparticle bonding of precursor particles in 100 vol% CH₄ or NH₃ gases. Several factors related to the reaction synthesis process—precursor chemistry, gas-solid and gas-liquid synthesis mechanisms, precursor vapor pressures—were investigated in relation to resulting microstructures and non-oxide yields. Results indicated that the volumetric changes which occurred during in-situ conversion of single component precursors negatively impacted the surface layer microstructure. To circumvent the internal stresses and cracking that accompanied the conversion of Si or Hf (that expands upon conversion) or SiO_x (that contracts during conversion), optimized ratios of the precursor constituents were used to produce near isovolumetric conversion to the product phase. Phase characterization indicated that precipitation of SiC from the Si/SiO₂ melt formed continuous, crack-free, and dense layers of 93.7 wt% SiC that were approximately 35 µm thick_, while sintered HfC/SiC composites (84.2 wt% yield) were produced from the laser-processing of Hf/SiO₂ in CH₄. By contrast, the SLRS of Si/SiO_x precursor materials used to produce Si₃N₄ resulted in whisker formation and materials vaporization due to the high temperatures required for conversion. The results demonstrate that under appropriate processing conditions and precursor selection, the formation of near net-shape SiC and SiC composites might be achieved through single-step AM-compatible techniques.     

Enhanced microwave-absorption properties of polymer-derived SiC/SiOC composite ceramics modified by carbon nanowires

Novel microwave-absorbing SiOC composite ceramics with dual nanowires (carbon nanowires (CNWs) and SiC nanowires) with high performances were fabricated by using the polymer-derivation method and heat treatment in Ar atmosphere. The introduction of CNWs in the amorphous SiOC ceramics promotes the ceramic crystallization into SiC nanoparticles and SiC nanowires at lower annealing temperatures, which leads to multi-phases and multiple nano heterogeneous interfaces. The distinctive architectures largely increase the interfacial and dipole polarizations of the composite ceramics. The CNWs/SiC/SiOC composite ceramics exhibit excellent microwave-absorption properties in the Ku band (12.4–18 GHz). The minimum reflection coefficient (RC) is -24.5 dB at a thickness of 1.8 mm, while the maximum effective absorption bandwidth (EAB, the corresponding frequency band in which RC is smaller than -10 dB) is 4.8 GHz at a thickness of 1.9 mm, which make the CNWs/SiC/SiOC composite ceramics promising electromagnetic-wave-absorbing materials.     

Synthesis of ZrC–SiC Powders by a Preceramic Solution Route

Homogenous liquid precursor for ZrC–SiC was prepared by blending of Zr(OC₄H₉)₄ and Poly[(methylsilylene)acetylene]. This precursor could be cured at 250°C and converted into binary ZrC–SiC composite ceramics upon heat treatment at 1700°C. The pyrolysis mechanism and optimal molar ratio of the precursor were investigated by XRD. The morphology and elements analyses were conducted by SEM and corresponding energy‐dispersive spectrometer. The evolution of carbon during ceramization was studied by Raman spectroscopy. The results showed that the precursor samples heat treated at 900°C consisted of t‐ZrO₂ (main phase) and m‐ZrO₂ (minor phase). The higher temperature induced phase transformation and t‐ZrO₂ converted into m‐ZrO₂. Further heating led to the formation of ZrC and SiC due to the carbothermal reduction, and the ceramic sample changed from compact to porous due to the generation of carbon oxides. With the increasing molar ratios of C/Zr, the residual oxides in 1700°C ceramic samples converted into ZrC and almost pure ZrC–SiC composite ceramics could be obtained in ZS‐3 sample. The Zr, Si, and C elements were well distributed in the obtained ceramics powders and particles with a distribution of 100 ~ 300 nm consisted of well‐crystallized ZrC and SiC phases.     

Synthesis of a meltable polyzirconosilane precursor for SiZrNC multinary ceramics

Polymeric precursors with refractory metal in the main chain while still possess meltable/soluble characteristics are highly desired for ultra-high temperature ceramics (UHTCs). Herein, we proposed a stepwise synthesis of polyzirconosilane (PZCS) with Zr-C-Si-N polymeric chain. The reaction mechanism was discussed based on the frontier molecular orbital theory and atom transfer radical polymerization, which was initiated and propagated by a low oxidation state active specie of Cp₂Zr(II). The PZCS precursor has excellent soluble and meltable properties with a softening point of 80.8 ~ 89.2 ^oC, which could be spun into green fibers by melt spinning technique. The ceramization process of PZCS was studied, and the resulted SiZrNC multinary ceramics were composed of ZrC/SiC nanocrystals embedded by graphitized carbon phase. The excellent mouldable properties, oxygen-free compositions and high Zr content of PZCS make it an ideal precursor for the preparation of UHTCs matrixes and fibers.     

Polymer-metal slurry reactive melt infiltration: A flexible and controllable ceramic modification strategy for irregular C/C components

Herein, we investigate the applicability of the polycarbosilane (PCS)–metal slurry reactive melt infiltration (RMI) process to various metals. The slurry exhibiting the best ceramized ability was used to examine the relationship between the ceramic thickness and reactive time, ceramic thickness and reactive temperature, and infiltration depth and slurry-coating thickness. The results show that the thickness of the ceramic layer increases with reactive time and temperature and the infiltration depth increases with the coating thickness. PCS–Si₉₀Zr₁₀ slurry RMI was selected to modify cylindrical nozzle C/C preforms, and dense C/C–SiC–ZrC composites with a density of ~2.05 g cm⁻³ were obtained. Owing to the good control of the PCS–Si₉₀Zr₁₀ slurry RMI on the interface, matrix, and carbon fiber of the as-received cylindrical composites, the bending strength of the C/C–SiC–ZrC composites was as high as 306.4 MPa, which is considerably higher than that of a C/C preforms (70.4 MPa). Considering the ablation resistance, the mass and linear ablation rates of the C/C–SiC–ZrC composite (~0.29 mg s⁻¹ and ~2.48 × 10⁻³ mm s⁻¹, respectively) were similar to those of the composites prepared using traditional RMI (~0.23 mg s⁻¹ and ~2.29 × 10⁻³ mm s⁻¹). The proposed polymer–metal RMI is more suitable for the modification of C/C preforms with thin-wall structures owing to its advantages including precise control of infiltration dose and flexible operation of slurry coating. Furthermore, it is suitable for the local modification of C/C components.     

Preparation,microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al₄SiC₄ powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al₄SiC₄ and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m^1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si⁵⁺ ions at 650 °C using a fluence of 1.1 × 10¹⁶ ions/cm², which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.     

Ablation behavior of a C/C-ZrC-SiC composite based on high-solid-loading slurry impregnation under oxyacetylene torch

A C/C-ZrC-SiC composite was successfully prepared by high-solid-loading slurry impregnation combined with polymer infiltration and pyrolysis. The microstructure and ablation behavior of the C/C–ZrC–SiC composite were investigated. ZrC particles were uniformly distributed in the matrix, and the obtained C/C–ZrC–SiC composite had a high density of 2.74 g/cm³. After exposure to oxyacetylene flame with a heat flux of 3.86 MW/m² for 120 s, the mass and linear ablation rates of the composite were 0.72 ± 0.11 mg/s and 0.52 ± 0.09 µm/s, respectively. The excellent ablation properties of the composite were attributed to the protection of the matrix by a three-layered oxide scale consisting of ZrO₂/SiO₂-rich/ZrO₂-SiO₂.     

Synthesis,Characterization, and Microstructure of ZrC/SiC Composite Ceramics via Liquid Precursor Conversion Method

The ceramic precursor for ZrC/SiC was prepared via solution‐based processing using polyzirconoxane, polycarbosilane, and divinylbenzene. The precursor could be transformed into ZrC/SiC ceramic powders at relative low temperature (1500°C). The cross‐linking process of precursor was studied by FT–IR. The conversion from precursor into ceramic was investigated by TGA, XRD. The ceramic compositions and microstructures were identified by element analysis, Raman spectra, SEM, and corresponding EDS. The results indicated that the ceramic samples remained amorphous below 1000°C and t–ZrO₂ initially generated at 1200°C. Further heating to 1400°C led to the formation of ZrC and SiC with the phase transformation of ZrO₂ and almost pure ZrC/SiC could be obtained upon heat‐treatment at 1500°C. During heat treatments, the ceramic sample changed from compact to porous due to carbothermal reduction. The ceramic powders with particle size of 100 nm~400 nm consisted of high crystalline degree ZrC and SiC phases, and Zr, Si, C were well distributed at the different sites in ceramic powders. The free carbon content was lowered to 1.60 wt% in final ZrC/SiC composite ceramics.     

Polymer precursor synthesis of novel ZrC–SiC ultrahigh-temperature ceramics and modulation of their molecular structure

In this study, ZrC–SiC composite ceramics were prepared with varying Zr/Si molar ratios using sol–gel method. Composites were characterized by Fourier-transform infrared spectroscopy (FT–IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDS). FT–IR analysis confirmed macromolecular network structure of composites, in which the precursor is composed of polyvinyl butyral (PVB) as main chain, silane molecules are interlinked via –OH moieties in PVB side chains, and Zr atoms are crosslinked with Si in corresponding proportion. Ceramic precursor begins to decompose at a temperature exceeding 1300 °C and is completely transformed into ZrC–SiC composite ceramics with corresponding Zr/Si molar ratio at 1600 °C. Raman spectroscopy and TEM results reveal that after annealing at 1600 °C, ZrC powder uniformly covers surface of SiC ceramics, and high-crystallinity graphite carbon covers ZrC powder.     

Polymeric liquid precursor of multi-component ZrC–SiC ceramic

Polymeric liquid ceramic precursors for the production of multi-component ZrC–SiC ceramics were prepared by reactive blending of polyzirconoxanesal, phenylacetylene-terminated polysilane and bisphenol-A type benzoxazine. The polymeric liquid precursors of ZrC–SiC ceramic have the processing capability of Precursor-Infiltration-and-Pyrolysis technique in ceramic composites fabrication. The thermal cure reactions included by the catalytic polymerisation of ethynyl groups, the ring opening polymerisation of benzoxazine rings, and the condensation of zirconate with phenolic hydroxyl and Si–H at 200–350°C. The monolithic ceramics were formed upon pyrolysis at 1000, 1200 and 1600°C in a yield of 65, 62 and 40%, respectively. X-ray diffraction and SEM–EDS results revealed that almost pure, elemental, uniformly distributed ZrC–SiC multi-component ceramic monolith was obtained through pyrolysis at 1600°C via carbothermal reduction of ZrO₂.