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.     

Fabrication of continuously porous SiC–Si3N4 composite using SiC powder by extrusion process

Large amounts of waste SiC sludge containing small amounts of Si and organic lubricant were produced during the wire cutting process of single crystal silicon ingots. Waste SiC sludge was purified by washing it with organic solvent and purified SiC powder was used to fabricate the continuously porous SiC–Si₃N₄ composites, using an extrusion process, in which carbon, 6 wt% Y₂O₃ + 2 wt% Al₂O₃ and ethylene vinyl acetate were added as a pore-forming agent, sintering additives and binder, respectively. In the burning-out process, the binder and carbon were fully removed and continuously porous SiC–Si₃N₄ composites were successfully fabricated. The green bodies containing waste SiC, Si powder and sintering additives were nitrided at 1400 °C in a flowing N₂ + 10% H₂ gas mixture. The continuously porous composites contained SiC, α-Si₃N₄, β-Si₃N₄ and few Fe phases. The pore size of the second passed and third passed SiC–Si₃N₄ composites was 260 μm and 35 μm in diameter, respectively. The values of bending strength and hardness in the second passed and third passed samples were 62.97 MPa, 388 Hv and 77.82 MPa, 423 Hv, respectively.     

Effect of milling energy on mechanical activation of (Mo + Si3N4) powders during the synthesis of Si3N4–MoSi2 in situ composites

Attempts have been made to study the effect of milling energy and type of grinding media on the mechanical activation during the production of MoSi₂ from a reaction between Mo and Si₃N₄. Powder mixtures of Mo and Si₃N₄ in the molar ratios of 1:1, 1:2 and 1:3 were ball milled using WC, steel, and ZrO₂ grinding media for mechanical activation. In order to evaluate the results obtained after high-energy ball milling and pyrolysis of these milled powder mixture, milling parameters have been converted to two energy parameters, namely, impact energy of the ball and total energy of milling. The optimum impact energy of ball required for mechanical activation of Mo + xSi₃N₄ (x = 3, 2, 1) powder mixtures by WC grinding media was found to be in the range of 0.145–0.173 J, which leads to a reduction of pyrolysis temperature by 100–200 °C. Samples milled with higher impact energy than the optimum range led to formation of undesirable phases, which dilutes the effect of mechanical activation. Samples milled with both steel and ZrO₂ grinding media having lower impact energies than the optimum show the presence of enormous contamination during milling and phases like ZrSi₂, Fe₃Si and Fe₅Si₃ were observed after pyrolysis without any significant reduction in pyrolysis temperature required for MoSi₂ synthesis.     

Graphene nanoplatelet/silicon nitride composites with high electrical conductivity

Silicon nitride (Si₃N₄) processed with up to 25 vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40 Scm^?1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction.Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT < 0) for high GNP contents. A thin amorphous layer was identified at the Si₃N₄/GNPs interface that may affect the conduction for the parallel configuration.     

Diamond dispersed Si3N4 composites obtained by pulsed electric current sintering

30 vol.% 2 and 30 μm diamond dispersed Si₃N₄ matrix composites were prepared by pulsed electric current sintering (PECS) for 4 min at 100 MPa in the 1550–1750 °C range. The densification behaviour, microstructure, Si₃N₄ phase transformation and stiffness of the composites were assessed, as well as the thermal stability of the dispersed diamond phase. Monolithic Si₃N₄ with 4 wt% Al₂O₃ and 5 wt% Y₂O₃ sintering additives was fully densified at 1550 °C for 4 min and 60 MPa. The densification and α to β-Si₃N₄ transformation were substantially suppressed upon adding 30 vol.% diamond particles. Diamond graphitisation in the Si₃N₄ matrix was closely correlated to the sintering temperature and grit size. The dispersed coarse grained diamonds significantly improved the fracture toughness of the diamond composite, whereas the Vickers hardness was comparable to that of the Si₃N₄ matrix ceramic. The Elastic modulus measurements were found to be an excellent tool to assess diamond graphitisation in a Si₃N₄ matrix.     

Effects of PMMA on porous structure of pollucite

The effects of PMMA as a pore-forming reagent and the powder for Cs-deficient pollucite, Cs₉Al_0.9Si_2.1O₆, calcined at 1073 K, on the microstructure of the porous body of Cs_0.9Al_0.9Si_2.1O₆ were investigated. The Cs_0.9Al_0.9Si_2.1O₆ porous bodies were fabricated by sintering the green compacts of the calcined powder and PMMA adding 35 mass% to the calcined powder. When the green compact was heated at 873 K in air for 20 h, pores <1 μm were observed in the porous body, suggesting that the PMMA previously dissolved in acetone was uniformly distributed in the calcined powder by the ball milling. The pore size of the obtained porous structure increased with increasing the size of the aggregated particles and the pore size distribution was significantly related to the size of Al₂O₃ balls and the time for the ball milling for mixing the calcined powders and PMMA.     

Mechanical and dielectric properties of porous Si2N2O–Si3N4 in situ composites

Mechanical and dielectric properties of porous Si₂N₂O–Si₃N₄ in situ composites fabricated for use as radome by gel-casting process were investigated. The flexural strength of the Si₂N₂O–Si₃N₄ ceramics is 230.46 ± 13.24 MPa, the complex permittivity of the composites varies from 4.34 to 4.59 and the dissipation factor varies from 0.00053 to 0.00092 from room temperature to elevated temperature (1150 °C) at the X-band. In the porous regions, some Si₂N₂O fibers (50–100 nm in diameter) are observed which may improve the materials properties.     

In situ synthesis of Si2N2O/Si3N4 composite ceramics using polysilyloxycarbodiimide precursors

In situ synthesis of Si₂N₂O/Si₃N₄ composite ceramics was conducted via thermolysis of novel polysilyloxycarbodiimide ([SiOSi(NCN)₃]_n) precursors between 1000 and 1500 °C in nitrogen atmosphere. The relative structures of Si₂N₂O/Si₃N₄ composite ceramics were explained by the structural evolution observed by electron energy-loss spectroscopy but also by Fourier transform infrared and ²⁹Si-NMR spectrometry. An amorphous single-phase Si₂N₂O ceramic with porous structure with pore size of 10–20 μm in diameter was obtained via a pyrolyzed process at 1000 °C. After heat-treatment at 1400 °C, a composite ceramic was obtained composed of 53.2 wt.% Si₂N₂O and 46.8 wt.% Si₃N₄ phases. The amount of Si₂N₂O phase in the composite ceramic decreased further after heat-treatment at 1500 °C and a crystalline product containing 12.8 wt.% Si₂N₂O and 87.2 wt.% Si₃N₄ phases was obtained. In addition, it is interesting that residual carbon in the ceramic composite nearly disappeared and no SiC phase was observed in the final Si₂N₂O/Si₃N₄ composite.     

Microstructure and fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPLs) as filler. The influence of the addition of GPLs on the microstructure development and on the fracture toughness of Si₃N₄ + GPLs composites was investigated. The GPLs with thickness from 5 nm to 50 nm are relatively homogeneously distributed in the matrix of all composites, however overlapping/bundle formation of GPLs was found, containing 2–4 platelets as well. The single GPLs and overlapped GPLs are located at the boundaries of Si₃N₄, and hinder the grain growth and change the shape of the grains. The fracture toughness was significantly higher for all composites in comparison to the monolithic Si₃N₄ with the highest value of 9.9 MPa m^0.5 for the composite containing the GPLs with smallest dimension. The main toughening mechanisms originated from the presence of graphene platelets, and responsible for the increase in the fracture toughness values are crack deflection, crack branching and crack bridging.     

Enhanced sinterability of mechanical alloyed La9.33Si2Ge4O26 oxyapatite powders for IT-SOFC electrolytes

Dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared by electrical sintering at 1400 °C in static air for 1 h from dry milling La₂O₃, SiO₂ and GeO₂ powders, in adequate atomic proportions, at 350 rpm for 15 h, under controlled environmental conditions, in a planetary ball mill. The densification behaviour of apatite-type phase La_9.33Si₂Ge₄O₂₆ powders synthesized by mechanical alloying was investigated through microstructural evolution with sintering temperature by means of XRD and SEM/EDS analyses. The content of germanium in the sintered samples remained almost constant, suggesting that its incorporation in the apatite phase hinders the high temperature (>1250 °C) volatilization process.     

Si3N4/graphene nanocomposites for tribological application in aqueous environments prepared by attritor milling and hot pressing

Advanced ceramic materials have proved their superior wear resistance as well as mechanical and chemical properties in a wide range of industrial applications. Today there are standard materials for components and tools that are exposed to severe tribological, thermal or corrosive conditions. The main aim of this work is to develop novel, highly efficient tribological systems on the basis of ceramic/graphene nanocomposites as well as to prove their superior quality and to demonstrate their suitability for technical applications e.g. for slide bearings and face seals in aqueous media. Current research in the field of ceramic nanocomposites shows that is possible to make ceramic materials with improved mechanical and tribological properties by incorporating graphene into the Si₃N₄ structure. Multilayered graphene (MLG) was prepared by attritor milling at 10 h intensive milling of few micrometer sized graphite powders. The large quantity, very cheap and quick preparation process are a main strengths of our MLG. Si₃N₄/MLG nanocomposites were prepared by attritor milling and sintered by hot pressing (HP). The Si₃N₄ ceramics were produced with 1 wt%, 3 wt%, 5 wt% and 10 wt% content of MLG. Their structure was examined by transmission electron microscopy (TEM). The tribological behavior of composites in aqueous environment was investigated and showed the decreasing character of wear at increased MLG content. This new approach is very promising, since ceramic microstructures can be designed with high toughness and provide improved wear resistance at low friction.     

Interfacial microstructure of the Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was jointed with itself by active brazing with a Cu–Pd–Ti filler alloy. Interfacial microstructure of the Si₃N₄/Si₃N₄ joint was analyzed by EPMA, TEM and X-ray diffract meter. The results indicate that a TiN reaction layer with a thickness about 5 μm is formed at the interface between Si₃N₄ ceramic and filler alloy. The TiN reaction layer is composed of two zones: one next to the Si₃N₄ ceramic with grains of 100 nm and the other zone that connects with the filler alloy and has grains of 1 μm. The microstructure of the joint can be described as: Si₃N₄ ceramic/TiN layer with fine grains/TiN layer with coarsen grains/Cu[Pd] solid solution. Some new phases, such as Pd₂Si, PdTiSi, Ti₅Si₃ and TiN, were formed in the Cu[Pd] solid solution interlayer. With increasing brazing temperature from 1100 °C to 1200 °C, the thickness of the TiN reaction layer is not changed. Meanwhile, the amount and size of the TiN and Pd₂Si phases in the Cu[Pd] solid solution increase, while, the amount of the PdTiSi phase decreases.     

Microstructural and mechanical properties of porous Si3N4 carbon coated ceramic brazed with a cobalt-silicon filler

The Co_22.5Si_77.5 (at.%) braze was used to bond porous Si₃N₄ ceramics. The effects of brazing temperature on microstructure and the bonding strength of the joint were studied. The results reveal that no visible reaction layer was observed. The corresponding joint strength was low. In order to improve the joint strength a carbon coated modification of the porous Si₃N₄ substrate was suggested. The impact of this modification on the joint properties was examined. It was established that a SiC reaction layer with a thickness from ∼15 μm to ∼65 μm was formed at the interface and SiC nanowires were observed when the temperature increased from 1280 °C to 1340 °C. The maximum shear strength of the carbon coated and uncoated joints were 115 MPa and 44 MPa, respectively. The significant improvement of the joint strength was attributed to the SiC reaction layer and a strengthening by the presence of SiC nanowires. .     

Physical,thermal and ablative performance of CVI densified urethane-mimetic SiC preforms containing in situ grown Si3N4 whiskers

A two-step process has been developed for silicon carbide (SiC) coated polyurethane mimetic SiC preform containing silicon nitride (Si₃N₄) whiskers. SiC/Si₃N₄ preforms were prepared by pyrolysis/siliconization treatment at 1600 °C, of powder compacts containing rigid polyurethane, novolac and Si, forming a porous body with in situ grown Si₃N₄ whiskers. The properties were controlled by varying Si/C mole ratios such as 1–2.5. After densification using a chemical vapour infiltration, the resulting SiC/Si₃N₄/SiC composites showed excellent oxidation resistance, thermal conductivity of 4.32–6.62 Wm⁻¹ K⁻¹, ablation rate of 2.38 × 10⁻³ − 3.24 × 10⁻³ g cm⁻² s and a flexural strength 43.12–55.33 MPa for a final density of 1.39–1.62 gcm⁻³. The presence of a Si₃N₄ phase reduced the thermal expansion mismatch resulting in relatively small cracks and well-bonded layers even after ablation testing. This innovative two-step processing can provide opportunities for expanded design for using SiC/Si₃N₄/SiC composites being lightweight, inexpensive, homogeneous and isotropic for various high temperature applications.     

Extraordinary toughening enhancement and flexural strength in Si3N4 composites using graphene sheets

Two types of Si₃N₄ composites containing graphene nanostructures using two different graphene sources, pristine graphene nanoplatelets and graphene oxide layers were produced by Spark Plasma Sintering. The maximum toughness of 10.4 MPa m^1/2, measured by flexure testing of pre-cracked bars, was achieved for a composite (∼60β/40α-Si₃N₄, ∼300 nm grain size) with 4 vol.% of reduced graphene oxide, indicating a toughening enhancement of 135% when compared to a similar Si₃N₄. This was also accompanied by a 10% increase in flexure strength (1040 MPa). For the composites with thicker graphene nanoplateletes only a 40% of toughness increase (6.6 MPa m^1/2) without strength improvement was observed for the same filler content. The large difference in the maximum toughness values accomplished for both types of composites was attributed to variations in the graphene/Si₃N₄ interface characteristics and the extent of monolayer graphene exfoliation.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Catalytic nitridation preparation of high-performance Si3N4(w)-SiC composite using Fe2O3 nano-particle catalyst: Experimental and DFT studies

The complete conversion from Si into Si₃N₄ was achieved after 2 h nitridation at 1400 °C by using in-situ formed Fe₂O₃ nano-particles (NPs) as a catalyst. Such a synthesis condition was remarkably milder than that (>1450 °C for many hours) required by the conventional Si nitridation method. Density functional theory (DFT) calculations suggest that Fe₂O₃ catalyst accelerates the Si nitridation via weakening the bond strength of absorbed N₂ molecule. Furthermore, Si₃N_4(w)-SiC composites prepared by the present catalytic nitridation method showed excellent high-temperature properties including modulus of rupture (MOR of 29.9 MPa at 1400 °C), thermal shock resistance (residual MOR percentage of 50% at ΔT = 1300 °C), as well as good oxidation resistance and cryolite corrosion resistance against molten cryolite. It can be concluded that, Fe₂O₃ NPs not only greatly accelerated the Si nitridation and Si₃N₄ formation, but also facilitated the epitaxial growth of reinforcement phase of Si₃N₄ whisker in the Si₃N_4(w)-SiC composites.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Modeling for high-temperature dielectric behavior of multilayer Cf/Si3N4 composites in X-band

The high-temperature dielectric behavior of multilayer C_f/Si₃N₄ composites fabricated by gelcasting and pressuureless sintering was intensively investigated at temperatures coverage up to 800 °C in X-band (8.2–12.4 GHz). Experimental results have shown the permittivity of Si₃N₄ matrix exhibits excellent thermo-stability with temperature coefficient lower than 10⁻³ °C⁻¹. Besides, both the real and imaginary parts of permittivity of multilayer C_f/Si₃N₄ composites exhibit positive temperature coefficient characteristic which attributed to the enhancement of space charge polarization. Furthermore, temperature-dependent permittivity of C_f/Si₃N₄ composites is demonstrated to be well distributed on circular arcs with centers actually keep around the real ($\epsilon \text{'}$) axis in Cole-Cole plane. Finally, the relaxation time for multilayer C_f/Si₃N₄ composites gradually increases from 216.1 ps to 250.2 ps when heated from room temperature to 800 °C, and is almost twice as much as a single cycle for electromagnetic wave in X-band which leads to continuous decrease in permittivity with frequency.     

A MoSi2-SiOC-Si3N4/SiC anti-oxidation coating for C/C composites prepared at relatively low temperature

In this study, SiC coating for C/C composites was prepared by pack cementation method at 1773 K, and MoSi₂-SiOC-Si₃N₄ as an outer coating was successfully fabricated on the SiC coated samples by slurry method at 1273 K. The microstructure and phase composition of the coatings were analyzed. Results showed that a porous β-SiC inner coating and a crack-free MoSi₂-SiOC-Si₃N₄ coating are formed. Effect of Si₃N₄ content on the oxidation resistance of the coated C/C composites at 1773 K in air was also investigated. The weight loss curves revealed that introducing the appropriate proportion of Si₃N₄ could improve the oxidation resistance of coating. The MoSi₂-SiOC/SiC coated C/C sample had an accelerated weight loss after oxidation in air for 20 h. However, the coating containing 45% Si₃N₄ could protect C/C composition from oxidation for 100 h with a minute weight loss of 0.63%.