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.     

Preparation and properties of a novel precursor-derived Zr-C-B-N composite ceramic via zirconocene and borazine

A novel preceramic polymer polyzirconocenyborazane (PZCBN) was synthesized by the polymerization of Bis(cyclopentadienyl)zirconium divinyl and borazine, introducing Zr, B, C, N together. The formation and concentration of elements Zr, C, B, N in the precursor and ceramic were detected through FTIR NMR, XRD, SEM and TEM. From the analysis, the Cp₂Zr(CH?CH₂)₂ and borazine linked together via the addition reaction between C?C and B-H. And after pyrolysis at 1200 °C, the precursor turned to ZrC/ZrB₂/BN composite ceramics, with a yield of 52 wt%. EDX resulted showed that the elements were well dispersed in the ceramics. According to SEM and TEM, the ceramic had a relatively dense structure with nano crystalline areas of ZrC embedded in the amorphous Zr-C-B-N matrix. TGA in air demonstrated that the ceramic had a favorable property on oxidation resistance.     

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.     

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.     

Preparation,ablation behavior and mechanism of C/C-ZrC-SiC and C/C-SiC composites

ZrC precursor was synthesized by a solution approach using ZrOCl₂·8H₂O, acetylacetonate, glycerol and boron-modified phenolic resin. A ZrC yield of ~ 40.56 wt% was obtained at 1500 °C in the C/Zr molar ratio of 1:1. C/C-ZrC-SiC composites were fabricated by a combined processes of chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) using the synthesized ZrC precursor. For comparison, C/C-SiC composites were prepared by CVI. Thermogravimetric analysis showed that C/C-ZrC-SiC composites exhibited better oxidation resistance than C/C-SiC composites. After oxyacetylene torch ablation, the mass ablation rate of C/C-ZrC-SiC composites was 9.23% lower than that of C/C-SiC composites. The porous ZrO₂ skeleton in the ablation center was prone to be peeled off by the flame flow, resulting in the higher linear ablation rate of C/C-ZrC-SiC composites. The oxide layers of ZrO₂ and SiO₂ were formed on the transition and brim region of C/C-ZrC-SiC composites and acted as effective heat and oxygen barriers. For C/C-SiC composites, the C-SiC matrix was severely depleted in the ablation center and the formed SiO₂ layer in the brim region could protect the matrix against further ablation.     

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.     

Synthesis of a soluble preceramic polymer for ZrC using 2-hydroxybenzyl alcohol as carbon source

A preceramic polymer for ZrC was successfully synthesised by chemical reaction between zirconium oxychloride (ZrOCl₂·8H₂O) and 2-Hydroxybenzyl alcohol via a one-pot route. The molecular structure, thermal properties and pyrolysis behaviour of the precursor were investigated. The results indicated that the precursor might be Zr–O–Zr chain polymer with 2-Hydroxybenzyl alcohol as ligand. The precursor was air-stable and exhibited excellent solubility in common organic solvents. The conversions from precursor to ZrC powders were investigated by TG-DTA, X-ray diffraction, Scanning electron microscope, TEM and Raman spectrum. The precursor underwent a thermal decomposition in four steps, and ZrC powders were formed at 1300°C via carbothermal reduction reaction of ZrO₂ and carbon in argon with ceramic yield of 63.0%. The ZrC particles were fine and exhibited irregular polyhedron morphology with average size in the range of 100–300?nm.     

Chemical process and solid solution formation in reactive hot pressed ZrB2–SiC ceramics doped with W

ZrB₂–SiC doped with W was prepared from a mixture of Zr, Si, B₄C and W via reactive hot pressing. The fully dense ZrB₂–SiC–WB–ZrC ceramic was obtained at 1900°C for 60 min under 30?MPa in an argon atmosphere. Reaction path and solid solution characteristics of the starting powders were studied through a series of pressureless heat treatment at temperatures between 700 and 1500°C. The solid solution phases of (Zr, W)B₂, (W, Zr)B and (Zr, W)C were formed directly by reactions between the precursors. Homogeneous distribution of solute atoms in solution and the solid solubilities were also studied.     

A new precursor of liquid and curable polysiloxane for highly cost-efficient SiOC-based composites

Polymer-derived SiOC matrix composites are very promising structure ceramics in moderate temperature (<?1200?°C) application, in view of their outstanding comprehensive performance and satisfying costs. Herein, we developed a new precursor of liquid and curable polysiloxane (LC-PSO) with Si-H and vinyl side groups to meet the requirements of precursor infiltration and pyrolysis (PIP) route for preparing highly cost-efficient composites. It was found that the crosslink structure was completely built below 150?°C through hydrosilylation reaction, thus converting the liquid precursor into solid state and rendering the cured product with a high ceramic yield of 81.1%. The addition reaction was also greatly inhibited under 5?°C in order that the LC-PSO was able to store for long time. We revealed that although the SiOC ceramic underwent structural rearrangement and slow crystallization during pyrolysis, no mass loss was observed below 1400?°C before the carbothermal reaction was initiated. The prepared carbon fiber-reinforced SiOC (2D-Cf/SiOC) composite verified the feasibility of directly using LC-PSO as precursor for PIP process, and the mean flexure strength and modulus of the composite were 253.3?MPa and 33.3?GPa, respectively. Our work presents the great potential of LC-PSO in fabricating highly cost-efficient CMCs for moderate temperature application.     

Novel class of precursor-derived Zr–La–B–C(O) based ceramics containing nano-crystalline ultra-high temperature phases stable beyond 1600 °C

In this work, a novel ultra-high temperature resistant precursor-derived ceramic containing Zr, La, B, and C was synthesized through precursor modification of phenol formaldehyde resin. The thermal stability and resistance to crystallization of the ceramic at a temperature of 1600 °C was investigated and was found to be profoundly influenced by the boron content in the starting precursors. The ceramics remained amorphous at 1600 °C for 2 h in argon and upon sustained heat-treatment for up to 16 h resulted in nano-crystalline ultra-high temperature phases such as ZrB₂, ZrC, LaB₆ and La₂Zr₂O₇. Thermodynamic equilibrium phase calculations show that even longer durations of heat treatment may be required to achieve thermodynamic equilibrium. High-resolution transmission electron microscopy revealed encapsulation of nanocrystals (<5 nm) in an amorphous matrix surrounded by turbostratic layers of carbon inhibiting its growth. Spectrochemical techniques confirmed the presence of boron substituted carbon in the amorphous matrix of the ceramic. The unique nature of the amorphous matrix lends the ceramic resistance to crystallization and chemical degradation that can surpass the likes of classical silicon-based precursor-derived ceramics.     

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.     

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.     

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.     

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.     

In-situ pyrolyzed polymethylsilsesquioxane multi-walled carbon nanotubes derived ceramic nanocomposites for electromagnetic wave absorption

Iron acetylacetonate (Fe(acac)₃) modified polymethylsilsesquioxane (PMS), simplified as PMS(Fe), was firstly obtained from PMS and Fe(acac)₃ via the condensation reaction. Multi-walled carbon nanotubes (MWCNTs) were then introduced to fabricate the corresponding MWCNTs/SiC nanocrystals/amorphous SiOC ceramic composites via pyrolyzed process. Owing to the catalytic effect of iron and heterogeneous nucleation promoted by MWCNTs, SiC nanocrystals were separated from SiOC amorphous ceramic matrix under 1400?°C. When the mass fraction of MWCNTs was 9?wt%, the obtained MWCNTs/SiC nanocrystals/amorphous SiOC ceramic composite (C9) demonstrated high microwave-absorbing properties. The minimum reflection loss (RL_min) and effective absorption bandwidth (EBA) of the obtained C9 at X-band (8.2–12.4) reached ?61.8?dB and 2.6?GHz (a thickness of 2.19?mm), respectively. Compared with other polymer-derived ceramics (PDCs), the RL_min was higher and the required thickness was thinner. This excellent microwave-absorbing property was due to the interfacial polarization relaxation generated between nanocrystals (MWCNTs & SiC) and amorphous SiOC, and the formed complete conductive networks inside the ceramic composites.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.     

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.     

Densification and toughening mechanisms in spark plasma sintered ZrB2-based composites with zirconium and graphite additives

The densification behavior and toughening mechanisms of ZrB₂-based composites with in-situ formed ZrC were investigated. The composites were spark plasma sintered at 1700 °C for 7 min under the applied pressure of 40 MPa. Metallic zirconium and graphite flakes were used as precursors to achieve ZrC reinforcement. Microstructural and phase analyses as well as mechanical characterizations were carried out on the near fully-dense composite samples. Results indicated ZrC as the only secondary phase in composite with 5 vol% of metallic Zr and graphite flakes. However, higher volume fractions of precursor materials led to the formation of ZrO₂ as the dominant secondary phase. Whereas decreasing trend of the hardness number versus volume fraction of the precursors was observed, the highest indentation fracture toughness was achieved in sample with 15 vol% metallic Zr/graphite flakes. Finally, the formation of secondary phases and their effects on densification, and mechanical behavior of the composites were discussed.