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.     

Synthesis,Ceramic Conversion and Microstructure Analysis of Zirconium Modified Polycarbosilane

Polymer derived ceramics have been widely being explored as high temperature structural components in aerospace as rocket nozzles, nose tip and leading edges of reusable launch vehicles. Polycarbosilane (PCS) was modified by a condensation reaction with zirconium acetylacetonate [Zr(acac)₄] to form polyzirconocarbosilane (PZrCS). A series of PZrCS were synthesized, which could be transformed into Si–Zr–C ceramic phases on pyrolysis. The ceramic yield of PCS was significantly improved by the introduction of zirconium into the system. The XRD patterns of the PZrCs show the characteristic peaks of ?SiC at 1300 °C and at 1500 °C the characteristic peaks of ZrC and ZrO₂ were observed. The carbothermal reaction in PZrCS was completed at 1650 °C and the resulting ceramic was non-oxide SiC/ZrC phase. The SEM images proved that the increase in concentration of zirconium in the final ceramic decreases the surface uniformity. HRTEM analysis of PZrCS heat treated at 1650 °C shows the evolution of oxide free ZrC/SiC phase with compatible grain boundaries without stacking fault. It could be concluded that the technique of introducing ultra-high temperature ceramic phases into the SiC matrix is an effective approach to improve the high-temperature performance of silicon based ceramics.     

In-situ synthesis of ultra-fine ZrB2–ZrC–SiC nanopowders by sol-gel method

ZrB₂–ZrC–SiC nanopowders with uniform phase distribution were prepared from cost-effective ZrOCl₂·8H₂O by a simple sol-gel method. The synthesis route, ceramization mechanism and morphology evolution of the nanopowders were investigated. ZrB₂–ZrC–SiC ceramic precursor can be successfully obtained through hydrolysis and condensation reactions between the raw materials. Pyrolysis of the precursor was completed at 650 °C, and it produced ZrO₂, SiO₂, B₂O₃ and amorphous carbon with a yield of 39% at 1300 °C. By heat-treated at 1500 °C for 2 h, highly crystallized ZrB₂–ZrC–SiC ceramics with narrow size distribution were obtained. With the holding time of 2 h, both the crystal size and the particle size can be refined. Further prolonging the holding time can lead to serious particles coarsening. Studies on the microstructure evolution of the generated carbon during the ceramic conversion demonstrates the negative effect of the ceramic formation on the structure order improvement of the carbon, due to the large amount of defects generated in it by the boro/carbothermal reduction reactions.     

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.     

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.     

Preparation and characterization of polymer-derived Zr/Si/C multiphase ceramics and microspheres with electromagnetic wave absorbing capabilities

Precursors for Zr/Si/C multiphase ceramics were synthesized by the reactions of dilithiozirconocene complex with dichlorodimethylsilane, methyltrichlorosilane and dichloromethylvinylsilane, respectively. The precursor-to-ceramic process of the precursor was investigated by TG-GC–MS and TG-FTIR analyses, confirming a complete transformation from organometallic polymers into ceramics below 800 °C. Annealing experiments of the derived ceramics at temperatures from 1000 °C to 2000 °C indicated the crystallization from ZrSiO₄, ZrO₂ to ZrC. Furthermore, micrometer-sized Zr/Si/C ceramic microspheres were successfully fabricated from the precursor at 1000 °C, showing surface morphology like wrinkled pea. According to the XRD, HRTEM and XPS analyses, such multiphase ceramic microspheres consist of ZrSiO₄, ZrO₂, and amorphous SiO_xC_y. Interestingly, the ceramic microspheres performed satisfactory electromagnetic wave absorbing capacity with the RL_max reaching −34 dB, which could be potential candidates for electromagnetic micro-devices.     

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.     

Microstructure and ablation behavior of SiC coated C/C–SiC–ZrC composites prepared by a hybrid infiltration process

In this study, C/C–SiC–ZrC composites coated with SiC were prepared by precursor infiltration pyrolysis combined with reactive melt infiltration. The pyrolysis behavior of the hybrid precursor was investigated using thermal gravimetric analysis-differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy techniques. The microstructure and ablation behavior of the composites were also investigated. The results indicate that the composites exhibit an interesting structure, wherein a ceramic coating composed of SiC and a small quantity of ZrC covers the exterior of the composites, and the SiC–ZrC hybrid ceramics are partially embedded in the matrix pores and distributed around the carbon fibers as well. The composites exhibit good ablation resistance with a surface temperature of over 2300 °C during ablation. After ablation for 120 s, the mass and linear ablation rates of the composites are 0.0026 g/s and 0.0037 mm/s, respectively. The great ablation resistance of the composites is attributed to the formation of a continuous phase of molten SiO₂ containing SiC and ZrO₂, which seals the pores of the composites during ablation.     

Mechanical,Thermal, and Oxidation Properties of Refractory Hafnium and zirconium Compounds

The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB₂, HfC_0·98, HfC_0·67, and HfN_0·92 ceramics. The oxidation resistance of ceramics in the ZrB₂–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB₂ exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC_0·98 to 1100°C for HfC_0·67 ceramics. The transition temperature of HfB₂ was 1100°C. The ZrB₂/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB₄, and C using displacement reactions. The ceramics with ZrB₂ as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB₂ and ZrC ceramics. The ceramics with ZrB₂/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.     

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.     

Synthesis and thermal performance of polymer precursor for ZrC ceramic

A novel ZrC preceramic precursor (PZC) was compounded via liquid phase chemical reaction without any organic solvent choosing ZrOCl₂·8H₂O and polyvinyl alcohol as Zr source and C source, respectively. The composition and structure of ZrC precursor were analysed through XRD, FT-IR, XPS and SEM. The results showed both Zr-O-C bonds and Zr-O bonds existed in the precursor. The results observed by SEM showed that many irregular particles were generated, whose particle sizes were mainly in the range of 0.2–3 μm. In addition, particle aggregation can be easily observed. Besides, the thermal property and pyrolysis process of PZC were studied. In accordance with XRD, the initial temperature of the earliest detection of ZrC in pyrolysis products of PZC was 1300 °C. Monoclinic ZrO₂ and tetragonal ZrO₂ can be observed at this temperature as well. Ulteriorly, when the pyrolysis temperature was risen up to 1500 °C, only ZrC ceramic can be found.     

Novel (Zr,Ti)(C,N)–SiC ceramics via reactive hot-pressing at low temperature

Novel (Zr, Ti)(C, N)–SiC ceramics were fabricated by reactive hot-pressing at 1500–1700 °C using ZrC, TiC_0.5N_0.5, and Si powders as raw materials for the first time. The effects of Si addition on the microstructures and mechanical properties were investigated. The reaction completely proceeded to generate SiC with an Si addition of 5 mol%. The grain refinement by SiC and solid solution strengthening of (Zr, Ti)(C, N) improve the mechanical properties. A high flexural strength of 522 ± 20 MPa was obtained in the Z5T5S ceramics (ZrC with 4.75 mol% TiC_0.5N_0.5 and 5 mol% Si additional). When the Si addition increases to 10 mol%, the residual Ti-stabilized β-ZrSi phase appears, which significantly reduces the flexural strength. The Vickers hardness and fracture toughness monotonically increase with increasing Si addition. The fine-grained microstructure without a residual phase and abundant intrinsic lattice defects in the Z5T5S composite endow it with the potential to obtain excellent radiation resistance.     

A meltable precursor for zirconium carbide ceramics and C/C-ZrC composites

For process simplification and rapid densification of ceramic composites, a meltable single-source ZrC precursor was prepared by condensing zirconium acetylacetonate (Zr(acac)₄) at 190?°C for 40–150?min. The preparation of ZrC precursor and the conversion from precursor to ceramics were investigated by using FTIR and NMR spectroscopies, GPC, DSC-TGA, XRD, SEM, EDS and TEM. The precursor had low viscosity (~ 10?mPa?s) and proper processing window (60?min) for precursor infiltration and pyrolysis (PIP). The ceramic yield at 1650?°C was 29.6%, and EDS revealed that the composition was (ZrC)_0.337(HfC)_0.0025(ZrO₂)_0.044C_0.1865. The ceramics were composed of 0.2–0.5?µm grains which aggregated to form a stacked structure surrounded by amorphous carbon. The preparation processes were designed, and C/C-ZrC composites with the density of 2.45?g/cm³ were successfully fabricated through 11 cycles of PIP with Zr(acac)₄. In conclusion, the synthetic method provides a simple and cheap route for precursors, and allows combined composite preparation with high efficiency.     

Structural characteristics and ablative behavior of C/C–ZrC–SiC composites reinforced with “Z-pins like” Zr–Si–B–C multiphase ceramic rods

In order to improve the ablation and oxidation resistance of C/C–ZrC–SiC composites in wide temperature domain, “Z-pins like” Zr–Si–B–C multiphase ceramic rods are prepared in the matrix. The influence of different sintering temperatures on the microstructure of ceramic rods and the ablative behavior of heterogeneous composites are studied. The results showed that the ZrB₂ and SiC phases are formed in the sintered matrix, and the increase of sintering temperature is beneficial to improve the density of the ceramic rods. The ablation properties of samples have been greatly improved. The mass and linear ablation rate are 0.8 mg/s and 3.85 μm/s, respectively, at an ablation temperature of 3000 °C and an ablation time of 60 s. After ablation, the matrix surface is covered with SiO₂ and ZrO₂ mixed oxide films. This is due to the preferential oxidation of “Z-pins like” Zr–Si–B–C multiphase ceramic rods in the ablation process, and B₂O₃ melt, SiO₂ melt, borosilicate glass, ZrSiO₄ melt and ZrO₂ oxide film can be generated successively from the low-temperature segment to the ultra-high temperature segment. These oxidation products can be used as compensation oxide melts for the healing of cracks and holes on the matrix surface in different temperature ranges and effectively prevent the external heat from spreading into the matrix. Therefore, C/C–ZrC–SiC composites with “Z-pins like” Zr–Si–B–C multiphase ceramic rods achieve ablation resistance in wide temperature domain.     

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

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

Ablation behaviors of SiC nanowire-reinforced ZrC-SiC coating-matrix integrated C/C composites with different ratios of precursors

A novel method to prepare a coating on the C/C composite is discussed. The precursor infiltration pyrolysis method is usually applied to prepare interior ceramic matrix, thus SiC nanowires that can absorb the surficial precursor are added to prepare surficial ceramics. The method accomplishes the integration of the coating and the matrix so that no coating peels off after ablation. Moreover, the material with a ZrC/SiC precursor ratio of 5:1 (Z5S1), whose mass and linear rates are 0.47 mg/s and 0.95 µm/s, exhibits the highest overall resistance to ablation. The results demonstrate that higher ZrC content and more uniform phase distribution are beneficial to keep ZrO₂ in solid and form a denser and firmer oxide layer, which is more effective in improving the ablation resistance of the C/C composite.     

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.     

Structure and energetics of ZrC(100)||c-ZrO2(001) interface: A combination of experiments,finite temperature molecular dynamics,periodic DFT and atomistic thermodynamic modeling

The oxidation process of ZrC is very important as it affects its initial excellent mechanical and physical properties. ZrC is an ultra-high temperature ceramic, but forms low refractory oxides at lower temperatures of 500–600 °C. To develop core/shell materials by coating the ZrC surface with another material that forms protective layers on ZrC and prevents it from oxidation (such as SiC), there is the need to study and characterize the oxidized layer surrounding ZrC particles. XPS, ToF-SIMS, TEM-ED and EDX analyses were used to investigate the covering oxidized layer, and polycrystalline ZrO₂(mainly cubic phase) was identified. Some traces of the tetragonal phase are observed to be present as shells around the ZrC particles with a thickness of about 4 nm on the average. Periodic DFT was subsequently used to characterize the interface formed between ZrC(100) and c-ZrO₂(001) phases. A strong interface was noticed mainly with charge transfer from Zr (c-ZrO₂ side) at the interface to O and C (ZrC side) atoms at the interface. The interfacial properties are local to only the first and second layers of ZrO₂, and not on the third and fourth layers of ZrO₂, as Bader charge analysis revealed substantial charge transfer at the interface region with no charge redistribution in the second ZrO₂ layer and subsequent bulk layers. The main physical quantity, ideal work of adhesion (W_ad), used to characterize the interface, remains quite constant for all ZrO₂ layers, and converges at three layers of ZrO₂. The interfacial bonds formed are observed to be stronger than the free surfaces in the corresponding ZrC and c-ZrO₂ used to generate the interface.     

High‐Strength ZrC Ceramics Doped with Aluminum

High‐strength ZrC ceramics with relative density above 98% were prepared by reactive hot pressing of ZrC and Al at 1900°C. The reaction between ZrC and Al resulted in the formation of ZrC_1?x, Zr₃Al₃C₅ and Zr–Al compound such as AlZr₃ and Al–C–Zr. The intermediate product AlZr₃ below 1600°C and remained Al–C–Zr phase could form liquid phase and promoted the first stage of densification process. The improvement in densification behavior at higher temperatures (1800°C–1900°C) could be attributed to the formation of nonstoichiometric ZrC_1?x. Adding 5 wt% and 7.5 wt% Al to ZrC, the formed ZrC_0.85–Zr₃Al₃C₅ and ZrC_0.80–Zr₃Al₃C₅ based ceramics had 3‐point bending strength as high as 757 ± 79 MPa and 967 ± 50 MPa, respectively, with hardness and fracture toughness being 16.2–18.3 GPa and 3.3–3.5 MPa m^1/2, respectively.     

On the oxidation behavior of ZrB2–SiC–VC composites

In this study, near-fully dense ZrB₂–SiC–VC (75-20-5 vol%) composite was manufactured through hot pressing at 1850°C under the pressure of 40 MPa for 60 min. Then the oxidation examination of the composite was carried out under different durations and temperatures. The microstructure and phase evolution after hot pressing and oxidation processes were examined by scanning electron microscopy, and X-ray diffractometry. The VC addition led to the formation of ZrC and VSi₂ phases, which assisted the densification of the composite by removing ZrO₂ from the particles’ surface. The oxides of ZrO₂, SiO₂, ZrSiO₄, V₂O₅, and VO₂ formed distinct layers on the sample during the oxidation at 1700°C for 4 h with a parabolic regimen and activation energy of 177.5 kJ/mol.