Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

The porous SiC–Si₃N₄ composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si₃N₄ powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s⁻¹) and also suppressed the generation of NH₃ and N₂ from Si₃N₄ hydrolysis and reaction between Si₃N₄ and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si₃N₄ composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N₂ and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si₂N₂O fibers formed in situ by the reaction of Si₃N₄ and amorphous SiO₂ effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m^1/2. Compared with monolithic Si₃N₄ ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO₂ and Si₃N₄. The porous SiC–Si₃N₄ composite ceramics prepared by the unoxidized Si₃N₄ powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.     

Preparation,mechanical and thermal properties of Si3N4 ceramics by gelcasting using low–toxic DMAA gelling system and gas pressure sintering

In this work, Si₃N₄ ceramics were fabricated through an aqueous gelcasting method using a low–toxic monomer called N, N–dimethylacrylamide (DMAA) followed by gas pressure sintering at 1850 °C for 2 h under 6 MPa N₂ atmosphere. The effect of solid loading on performance of slurries, green and sintered bodies was investigated systematically. The results show that the slurries with a solid loading as high as 50 vol% (viscosity 0.17 Pa.s at 100 s^–1) were achieved. With the increase of solid loading (30–50 vol%), the green bodies exhibited a monotonically decreased, however high enough in general, flexural strength of 16.50–11.52 MPa, which was comparable to that of widely–used neurovirulent acrylamide (AM) gelling system. In regard to the sintered bodies, increasing solid loading significantly promoted sintering and improved mechanical properties and thermal conductivity as a result of the increased density, bimodal distribution structure, as well as suitable interfacial bonding strength. The best performance parameters of Si₃N₄ ceramics, bulk density of 3.25 g/cm³, apparent porosity of 0.67%, flexural strength of 898.92 MPa, fracture toughness of 6.42 MPa m^1/2, Vickers hardness of 2.81 GPa, and thermal conductivity of 34.69 W m^–1 K^–1, were obtained at 50 vol% solid loading. This work renders low–toxic DMAA gelling system promising prospect in preparation of high–performance Si₃N₄ ceramics by gelcasting.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.     

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.     

Fabrication of Si3N4 ceramics by post-reaction sintering using Si–Y2O3–Al2O3 nanocomposite particles prepared by mechanical treatment

Post-reaction sintering of a powder compact of Si and sintering aids is a useful technique for fabricating silicon nitride (Si₃N₄) ceramics at low costs. In order to inhibit the inhomogeneous and uncontrollable exothermic nitridation of Si in the powder compact, Si–Y₂O₃–Al₂O₃ nanocomposite particles are designed as an aid for post-reaction sintering. These Si–Y₂O₃–Al₂O₃ nanocomposite particles are prepared via mechanical treatment applying high shear stress. Scanning electron microscopy (SEM) observations show that Y₂O₃ and Al₂O₃ particles are homogenously dispersed, and fixed to the Si particles. A green compact prepared using the Si–Y₂O₃–Al₂O₃ nanocomposite particles results in lower electrical resistivity than that prepared using a powder mixed by wet ball-milling, which suggests that Si particles in the green compact prepared using the nanocomposite particles are isolated by Y₂O₃ and Al₂O₃ particles. The isolation of Si particles by the sintering aids successfully prevents the Si particles from melting and agglomerating during the nitridation process, resulting in a higher nitridation ratio and higher α-Si₃N₄ phase content due to the inhibition of rapid heat transfer caused by the exothermic reaction. The nitridation ratio also increases with the applied power during mechanical treatment. As a result of firing the homogeneously nitrided powder compacts at high temperatures, Si₃N₄ ceramics with homogeneous microstructure and improved density are successfully fabricated in this manner.     

High-strength and wave-transmitting Si3N4–Si2N2O-BN composites prepared using selective laser sintering

In this study, high-strength and wave-transmission silicon nitride (Si₃N₄) composites were successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding and before final sintering, and the optimal moulding process parameters for the SLS Si₃N₄ ceramics were determined. The effects of the sintering aids and secondary CIP on the bulk density, porosity, flexural strength, fracture toughness, and wave-transmitting properties of the Si₃N₄ composites were studied. The results showed that the increased CIP pressure was beneficial to the densification of SLS Si₃N₄ ceramics and improved their mechanical properties. However, the wave-transmitting performance decreased as the CIP pressure increased. The Si₃N₄ ceramics prepared by the moulding of sample S₁₁ were more in line with the performance requirements of the radomes. To obtain good comprehensive performance, an additional 3% of interparticle Y₂O₃ was added to the pre-printed mixed powder of granulated Si₃N₄ particles and resin and the secondary CIP pressure was adjusted to 280 MPa. After sintering, the bending strength, fracture toughness, and dielectric constant of the Si₃N₄ ceramics were 651 MPa, 6.0 MPa m^1/2, and 3.48 respectively. This study provides an important method for preparing of Si₃N₄ composite radomes using SLS process.     

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.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

Properties of Si3N4–TiN composites fabricated by spark plasma sintering by using a mixture of Si3N4 and Ti powders

Si₃N₄–TiN composites were successfully fabricated via planetary ball milling of 70 mass% Si₃N₄ and 30 mass% Ti powders, followed by spark plasma sintering (SPS) at 1250–1350 °C. The sintering mechanism for SPS was a hybrid of dissolution–reprecipitation and viscous flow. The electrical resistivity decreased with increasing sintering temperature up to a minimum at 1250 °C and then increased with the increasing sintering temperature. The composites prepared by SPS at 1250–1350 °C could be easily machined by electrical discharge machining. Composite prepared by SPS at 1300 °C showed a high hardness (17.78 GPa) and a good machinability.     

Thermal shock resistance of Si3N4 and Si3N4–SiC ceramics with rare-earth oxide sintering additives

The influence of various rare-earth oxide additives and the addition of SiC nanoparticles on the thermal shock resistance of the Si₃N₄ based materials was investigated. The location of SiC particles inside the Si₃N₄ grains contributed to a higher level of residual stresses, which caused a failure at the lower temperature difference compared to the composites with a preferential location of the SiC at the grain boundaries. A critical temperature difference increased with an increasing ionic radius of RE³⁺ for both the composites and the monoliths. The critical temperature difference for the composite (580 °C) and the monolith (680 °C) sintered with La₂O₃ was significantly higher compared to the composite and the monolith doped with Lu₂O₃ (430 °C). A good agreement was found between the results of the critical temperature difference estimated by the indentation quench test and that obtained by the strength retention method.     

Effect of TiB2 particles on microstructure and mechanical properties of B4C–TiB2 ceramics prepared by hot pressing

B₄C-20 wt% TiB₂ ceramics were fabricated by hot pressing B₄C and ball-milled TiB₂ powder mixtures. The effects of the TiB₂ particle size on the microstructure and mechanical properties were investigated. The results showed that the TiB₂ particle size played an important role in the mechanical properties of the B₄C–TiB₂ ceramics. In addition, SiO₂ introduced by ball milling was beneficial for densification but detrimental to the mechanical properties of the B₄C–TiB₂ ceramics. The typical values of relative density, hardness, flexural strength, and fracture toughness of the ceramics were 99.20%, 35.22 GPa, 765 MPa, and 7.69 MPa m^1/2, respectively. The toughening mechanisms of the B₄C–TiB₂ ceramics were explained by crack deflection and crack branching. In this study, the effects of high pressure and temperature caused liquefying SiO₂ to migrate to the surface of B₄C–TiB₂ and react with diffused carbon source in the graphite foil to form a 30 μm thick SiC layered structure, which improved the high-temperature oxidation resistance of the material. The unique SiC layered structure overcame the insufficient oxidation resistance of B₄C and TiB₂, thereby improving the oxidation resistance of the ceramics under high-temperature service conditions.     

A comparative study on dielectric and mechanical properties of porous β-Si3N4 ceramics by controlling porosity and microstructure

To ensure the porous Si₃N₄ ceramics with an excellent comprehensive performance for the application of radome, the commonly neglected effect of designed microstructures on dielectric and mechanical properties was investigated. Two typical porous Si₃N₄ ceramics with similar porosity but different microstructures (unidirectionally aligned and uniformly micropore structure with a different dimension of grains) were obtained by freeze casting and gel-casing process. The results indicate that the microstructure has a significant influence on the mechanical properties yet not an obvious effect on the dielectric properties, which means the dielectric constant can be reliably designed based on the porosity without consideration of different microstructures, meanwhile, the mechanical properties can be optimized by the microstructure. Besides, the results of the dielectric properties predicted by Finite Element Analysis show a high agreement with the experimental results. The study could be helpful to design a wave-transmitting component with integrated structure and function.     

Effect of nano- and micro-sized Si3N4 powder on phase formation,microstructure and properties of β′-SiAlON prepared by spark plasma sintering

The phase formation behavior of β′-SiAlON with the general formula Si_6-zAl_zO_zN_8-z was studied comprehensively for z values from 1 to 3 using spark plasma sintering (SPS) as the consolidation technique at synthesis temperatures from 1400 to 1700 °C. The samples were prepared close to the β′-SiAlON composition line: Si₃N₄ ? 4/3(AlN·Al₂O₃) in the phase diagram using (A) nano-sized amorphous Si₃N₄ and (B) micro-sized β-Si₃N₄ precursors. Field-emission scanning electron microscopy (FESEM) was used for microstructural analysis.Most compositions reached almost full density at all SPS temperatures. Compared with the micro-sized β-Si₃N₄ precursor, the nano-sized amorphous Si₃N₄ precursor accelerated the reaction kinetics, promoting the formation of dense β′-SiAlON + O′-SiAlON composites after SPS at synthesis temperatures of 1400–1500 °C. This resulted in very high values of Vickers hardness (Hv₁₀) = 18.2–19.2 GPa for the z = 1 composition related to the hardness of the O′-SiAlON component phase.In general, for samples synthesized from nano-sized amorphous Si₃N₄, which were almost fully dense, containing >95% β′-SiAlON, the hardness values were 13.4–13.8 GPa with a fracture toughness of 3.5–4.6 MPa m^1/2. For equivalent samples synthesized from micro-sized β-Si₃N₄, hardness was in the range 13.9–14.4 GPa with a fracture toughness of 4.3–4.5 MPa.m^1/2. These values are comparable with fully dense β′-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.     

Effect of Mo and W interlayers on microstructure and mechanical properties of Si3N4–nickel-base superalloy joints

Si₃N₄/nickel-base superalloy (Inconel-625) and Si₃N₄/Si₃N₄ joints with refractory metal (W and Mo) interlayers were vacuum brazed using a Ti-active braze Cu-ABA (92.75Cu–3Si–2Al–2.25Ti) at 1317 K for 30 min with the following interlayered arrangements: Si₃N₄/Mo/W/Inconel and Si₃N₄/Mo/W/Si₃N₄. The joints exhibited Ti segregation at the Si₃N₄/Cu-ABA interface, elemental interdiffusion across the joint interfaces, and sound metallurgical bonding. Knoop microhardness profiles revealed hardness gradients across the joints that mimicked the interlayered arrangement. The compressive shear strength of Si₃N₄/Si₃N₄ joints both with and without W and Mo layers was ∼142 MPa but the strength of Si₃N₄/Inconel joints increased from ∼9 MPa for directly bonded joints without interlayers to 53.5 MPa for joints with Mo and W interlayers.     

Microstructure evolution and mechanical properties of porous Si3N4 and dense Si3N4 joints bonded using CaO–Li2O-Al2O3–SiO2 glass-ceramic

A novel CaO-Li₂O-Al₂O₃-SiO₂ (CLAS) glass was developed for the joining of porous Si₃N₄ and dense Si₃N₄. A multiphase interlayer consisting of CaAl₂Si₂O₈, LiAlSi₂O₆ and CaSiO₃ phases was formed in joint, which possessed matched CTE with the Si₃N₄ substrates. In addition, the infiltrated layer with bilayer structure in the porous Si₃N₄ substrate was observed. The effects of joining temperature and cooling rate on microstructure, phase evolution and shear strength of joints were studied carefully. The results showed that the kinds of precipitated phases remained invariable with the joining temperature increased, but the crystallinity in the interlayer was improved remarkably as the cooling rate reduced. The maximum shear strength of 45 MPa was obtained when the joining temperature and cooling rate were 1100 °C and 5 °C/min, respectively. Moreover, fracture during the shear test occurred mainly within porous Si₃N₄ side, indicating superior joining of dense Si₃N₄/glass-ceramic/porous Si₃N₄.     

Correlation of boron content and high temperature stability in Si–B–C–N ceramics

The preparation and characterization of precursor derived Si–B–C–N ceramics with similar Si/C/N ratios but variable boron content are reported. The polymeric precursors were prepared via hydroboration of poly(methylvinylsilazane) using different BH₃·SMe₂/polymer stoichiometries. High temperature thermogravimetric analysis of as-pyrolysed ceramics as well as XRD studies of post-annealed samples display a retarding effect of boron on both crystallization of SiC and Si₃N₄ and stabilization of crystalline β-Si₃N₄.     

Tribological properties of Si3N4–graphene nanocomposites

Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt% of various types of graphene platelets were studied. The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at room temperature in dry sliding. Coefficient of friction and specific wear rates were calculated and related to the damage mechanisms observed in the wear tracks. The measured properties were then assessed with respect to the type and volume fraction of the graphene additives. It is shown that addition of such amounts of carbon phases does not lower the coefficient of friction. Graphene platelets seem to be integrated into the matrix very strongly and they do not participate in lubricating processes. The best performance offers materials with 3 wt% of larger sized graphene, which have the highest wear resistance.     

Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO₂–SiO₂ system was studied. Compositions consisting of CaO, ZrO₂, and SiO₂ were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO₃) and calcium zirconium silicate (Ca₂ZrSi₄O₁₂) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

C_f–Si₃N₄ sandwich composites were prepared by gelcasting using α-Si₃N₄ powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of C_f–Si₃N₄ sandwich composites were investigated. SEM and EDS analysis results reveal that the SiC interphase could effectively overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700 °C. The investigation of microwave absorbing property reveals that, compared with the Si₃N₄ ceramics, both the real (ε?$\epsilon ?$) and imaginary (ε??$\epsilon ??$) permittivity of C_f–Si₃N₄ sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the C_f–Si₃N₄ sandwich composites are significantly enhanced compared with pure Si₃N₄ ceramic, and the reflection loss gradually decreases from −3.5 dB to −14.4 dB with the increase of frequency, while the pure Si₃N₄ ceramic keeps at −0.1 dB. Particularly, the relationship between permittivity of C_f–Si₃N₄ sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both ε?$\epsilon ?$ and ωε??$\omega \epsilon ??$ are inversely proportional to the frequency square ω²${\omega }^2$, and the predicted results agree quite well with the measured data.     

Fabrication and properties of a dense SiC NWs-toughened α-Si3N4 composite coating on porous Si3N4 ceramics

A dense SiC nanowires-toughened α-Si₃N₄ coating was prepared using a two-step technique for protecting porous Si₃N₄ ceramic against mechanical damage, and effect of SiC nanowires content on microstructures and properties of the coating were investigated. XRD, SEM and TEM analysis results revealed that as-prepared coatings consisted of α-Si₃N₄, O'-Sialon, SiC nanowires and Y–Al–Si–O–N glass phase. Furthermore, Vickers hardness of the coated porous Si₃N₄ ceramics increased gradually with the increasing SiC nanowires content from 0 to 10 wt%, which is attributed to the gradual improvement in intrinsic elastic modulus (E), hardness (H) and H³/E² of the coatings. But, when the SiC nanowires content was 15 wt%, the thickness of the coating became relatively thinner, so that its protective ability was weakened and Vickers hardness started to decrease accordingly. Meanwhile, the assistance of SiC nanowires enhanced fracture toughness of the coatings obviously because SiC nanowires in the coatings can produce various toughening mechanisms during mechanical damage. When the SiC nanowires content was 10 wt%, its fracture toughness reached the maximum value, which was 6.27 ± 0.05 MPa·m^1/2.