High-purity stoichiometric Si3N4 ceramics through trimethylsilyl-substituted polysilazanes

Trimethylsilyl-substituted polysilazanes were designed and successfully synthesized. They were used to fabricate high-purity stoichiometric Si₃N₄ ceramics through pyrolysis process. Trimethylsilyl groups improved the stability of polysilazanes and easily escaped during pyrolysis, which effectively reduced oxygen and carbon content in the final polymer-derived Si₃N₄. The C content of Si₃N₄ ceramic was below 0.06 wt%, and the O content was below 1.2 wt%. The Si₃N₄ ceramics remained amorphous up to 1400°C, yet they were completely transformed into α-Si₃N₄ at 1500°C. Synergistic effect from low oxygen and carbon content contributed to highly stable amorphous state of Si₃N₄ till high temperatures. This amorphous Si₃N₄ ceramics could be used in cutting-edge technology where high purity is compulsory.     

Inhibiting crystallization of fused silica ceramic at high temperature with addition of α-Si3N4

Fused silica (SiO₂) ceramic crucibles with α-Si₃N₄ coating are commonly used for smelting photovoltaic silicon (Si). However, SiO₂ ceramics will inevitably undergo crystallization and large volume change during the high-temperature service, which will lead to crucible cracking and deteriorate the quality and yield of the Si ingot. In this work, α-Si₃N₄/SiO₂ ceramics are fabricated by introducing α-Si₃N₄ into SiO₂ ceramics to inhibit crystallization. The results show that the introduction of α-Si₃N₄ can effectively inhibit crystallization of SiO₂ ceramics at temperature higher than 1450 °C. Only 5 wt% cristobalite form in SiO₂ ceramic with 20 wt% α-Si₃N₄ (heated at 1550 °C for 30min). The crystallization activation energy of SiO₂ ceramic containing 20 wt% α-Si₃N₄ increases by 2.27 times to 931.2kJ/mol compared with that of pure SiO₂ ceramic (409.6kJ/mol). The inhibition crystallization effect and increased activation energy derive from the in-situ formation of O–Si–N chemical bond and physical isolation of SiO₂ particles by α-Si₃N₄ powders.     

From design to characterization of zirconium nitride/silicon nitride nanocomposites

ZrN/Si₃N₄ nanocomposites have been prepared by chemically crosslinking two polysilazanes with a zirconium-based compound and subsequent heat-treatment at temperatures ranging from 1000 to 1600 °C. The polymer synthesis has been systematically investigated using FT-IR, solid-state NMR, and elemental analyses. Then, the pyrolysis under ammonia at 1000 °C trigering the thermo-chemical polymer-to-ceramic conversion was examined, leading to X-ray amorphous ceramics with yields governed by the chemistry of the neat polysilazane. Investigations of the structural evolution of the single-phase amorphous ceramic network above 1000 °C by X-ray diffraction and Raman spectroscopy pointed out that the ZrN phase already segregated at 1400 °C and formed highly crystalline ZrN/Si₃N₄ nanocomposites at 1600 °C. HRTEM investigations validated the unique nanostructural feature of the nanocomposites made of ZrN nanocrystals distributed in α- and β-Si₃N₄ phases. Our preliminary investigations of the optical properties showed that these structural changes allowed tuning the optical properties of ZrN/Si₃N₄ nanocomposites.     

Effects of oxygen on the thermal stability of silicon nitride fibers

Two types of Si₃N₄ fibers with different oxygen contents were annealed in a nitrogen atmosphere at 1500 °C for 1 h. After annealing, the fiber (SN-L) containing 0.5 wt% oxygen crystallized to α-Si₃N₄ but lost its strength, whereas the fiber (SN-H) containing 4.2 wt% oxygen was amorphous and retained 63.6% of its strength. The phase transition in these fibers was mainly influenced by the oxygen level. The lower oxygen content in the SN-L favored the precipitation of an almost stoichiometric composition of α-Si₃N₄ initially at ~1450 °C with an activation energy (Ea) of 663.357 kJ/mol. Nanopores existing naturally in the fiber promoted crystallization via heterogeneous nucleation. SN-H precipitated as an amorphous SiN_xO_y metaphase preferentially at ~1400 °C with an Ea of 440.434 kJ/mol, owing to the higher oxygen content approaching that of Si₂N₂O. SiN_xO_y inhibited the crystallization of α-Si₃N₄, making SN-H more thermally stable than SN-L at temperatures above 1500 °C.     

A comprehensive study on the influence of the polyorganosilazane chemistry and material shape on the high temperature behavior of titanium nitride/silicon nitride nanocomposites

The high temperature crystallization behavior of polytitanosilazane-derived amorphous SiTiN ceramics was investigated in a nitrogen atmosphere using XRD, Raman spectroscopy, TEM, SEM and BET. At 1400 °C, TiN is the first phase to nucleate in SiTiN ceramics forming nanocomposites with a homogeneous distribution of TiN nanocrystals within an amorphous Si₃N₄ matrix. Above 1400 °C, XRD indicates that the temperature at which Si₃N₄ crystallizes depends on the volume fraction of TiN present in nanocomposites. This is closely related to the chemistry of the polyorganosilazanes used to synthesize polytitanosilazanes. The use of perhydridopolysilazane, the most reactive polyorganosilazane, allows preparing TiN/Si₃N₄ nanocomposites with a remarkable stability of the amorphous matrix up to 1800 °C as mesoporous materials and powders. Dense monoliths crystallize earlier than the powder analogs because of the use of an ammonia pre-treatment before polymer warm-pressing.     

A novel method of strong and tough Si3N4 ceramic from the particle-stabilized foam

The brittleness of Si₃N₄ ceramics has always limited its wide application. In this paper, Si₃N₄ ceramics were prepared based on foam. Combining the unique honeycomb structure of the ceramic foams and the self-toughening mechanism of Si₃N₄, the strengthening and toughening of Si₃N₄ ceramics can be further achieved by adjusting the microstructure of Si₃N₄ ceramic foams. The powder particles are self-assembled by particle-stabilized foaming to form a foam body with a honeycomb structure. It was pretreated at different temperatures (1450–1750°C). The microstructure evolution of foamed ceramics at different pretreatment temperatures and the conversion rate of α-Si₃N₄ to β-Si₃N₄ at different pretreatment temperatures were explored. Then the foamed ceramics with different microstructures are hot-press sintered to prepare Si₃N₄ dense ceramics. The effects of different microstructures of foamed ceramics on the strength and toughness of Si₃N₄ ceramics were analyzed. The experimental results show that the relative density of Si₃N₄ ceramics prepared at a particle pretreatment temperature of 1500°C is 97.8%, and its flexural strength and fracture toughness are relatively the highest, which are 1089 ± 60 MPa and 12.9 ± 1.3 MPa m^1/2, respectively. Compared with the traditional powder hot-pressing sintering, the improvement is 21% and 33%, respectively. It is shown that this method of preparing Si₃N₄ ceramics based on foam has the potential to strengthen and toughen Si₃N₄ ceramics.     

A simple and efficient method for the synthesis of SiBNC ceramics with different Si/B atomic ratios

The high-temperature durability of SiBNC ceramics is significantly influenced by Si/B ratios and the synthetic procedures. Single-source synthetic routes can yield homogeneous ceramics at the atomic level, but the Si/B ratio cannot be efficiently adjusted. In this paper, a simple and efficient method for the synthesis of SiBNC precursor polyborosilazanes (PBSZs) with different Si/B ratios has been established via a one-pot reaction involving boron trichloride, dichloromethylsilane and hexamethyldisilazane in different molar ratios. The Si/B ratios of the derived SiBNC ceramics were consistent with that of the precursor PBSZs. When pyrolysed at 1000 °C, PBSZs with 0.52, 0.94 and 2.12 Si/B ratios transformed into SiB_2.6N₅C_2.2, SiB_0.9N_2.7C_1.3 and Si₂BN₃C_1.4 ceramics respectively. The polymer-to-ceramic process was also studied and featured ceramic yields of 43.2 wt%, 50.1 wt% and 62.2 wt%, respectively. The derived ceramic SiB_0.9N_2.7C_1.3 resisted crystallization up until 1700 °C, whereas the SiB_2.6N₅C_2.2 and Si₂BN₃C_1.4 could remain amorphous up to 1600 °C only. Using the precursor with 0.94 Si/B ratio, the SiBNC ceramic fibres were also obtained.     

Microstructure and mechanical properties of α/β-Si3N4 composite ceramics with novel ternary additives prepared via spark plasma sintering

In this study, α/β-Si₃N₄ composite ceramics with high hardness and toughness were fabricated by adopting two different novel ternary additives, ZrN–AlN–Al₂O₃/Y₂O₃, and spark plasma sintering at 1550 °C under 40 MPa. The phase composition, microstructure, grain distribution, crack propagation process and mechanical properties of sintered bulk were investigated. Results demonstrated that the sintered α/β-Si₃N₄ composite ceramics with ZrN–AlN–Al₂O₃ contained the most α phase, which resulted in a maximum Vickers hardness of 18.41 ± 0.31 GPa. In the α/β-Si₃N₄ composite ceramics with ZrN–AlN–Y₂O₃ additives, Zr₃AlN MAX-phase and ZrO phase were found and their formation mechanisms were explained. The fracture appearance presented coarser elongated β-Si₃N₄ grains and denser microstructure when 20 wt% TiC particles were mixed into Si₃N₄ matrix, meanwhile, exhibited maximum mean grain diameter of 0.98 ± 0.24 μm. As a result, the compact α/β-Si₃N₄ composite ceramics containing ZrN–AlN–Y₂O₃ additives and TiC particles displayed the optimal bending strength and fracture toughness of 822.63 ± 28.75 MPa and 8.53 ± 0.21 MPa?m^1/2, respectively. Moreover, the synergistic toughening of rod-like β-Si₃N₄ grains and TiC reinforced particles revealed the beneficial effect on the enhanced fracture toughness of Si₃N₄ ceramic matrix.     

A facile method to produce rodlike β-Si3N4 seed crystallites for bimodal structure controlling

β-Si₃N₄ rodlike seed crystallites were successfully produced by single-step heat treatment of commercial α-Si₃N₄ powder at 1900°C for 20 h under an N₂ gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α- ⇀ β-Si₃N₄ phase transformation proceeded mainly at 1900°C, and this temperature was lower than the theoretical α-Si₃N₄ dissociation temperature (1933°C) under N₂ gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α-Si₃N₄ crystal lattice was suggested as the driving force for the present oxide additive-free α- ⇀ β-Si₃N₄ phase transformation. β-Si₃N₄ ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd₂O₃. Starting α-Si₃N₄ powder with 10 vol% rodlike β-Si₃N₄ seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950°C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β-Si₃N₄ ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m^−1/2 and 109.3 ± 0.4 W m⁻¹ K⁻¹, respectively, retaining a high fracture strength of about 1 GPa.     

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.     

Fabrication,thermal shock resistance,and dielectric property of α-Si3N4-based ceramic coating on porous Si3N4 ceramics

A dense α-Si₃N₄-based ceramic protective coating was successfully prepared on porous Si₃N₄ ceramics by a liquid infiltration and filling method. The coating composed of a primary α-Si₃N₄ phase and secondary O'-Sialon, β-Sialon, and Y–Si–Al–O–N glass phase. After thermal shock at ΔT = 1000°C for five times, cracks were produced, but the tip of crack stopped inside the coating; so the coated porous Si₃N₄ ceramics still had a good waterproof ability and its water absorption was only 7%. During thermal shock, toughening mechanisms involving needle-like O'-Sialon particle bridging, crack deflection, and rough fracture, occurred within the cracks, contributing to thermal shock resistance of the coating. The dielectric constant of the coated porous Si₃N₄ ceramics showed a slow increase trend with increasing temperature, and it reached the maximum value of 3.57 at 1100°C at the frequency of 11 GHz. The dielectric loss increased slowly as the temperature increased from room temperature to 900°C, but it started to increase evidently when the temperature was over 900°C.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

Single-source-precursor synthesis and high-temperature evolution of novel mesoporous SiVN(O)-based ceramic nanocomposites

Mesoporous SiVN(O) ceramics were prepared from a mixture consisting of VO(acac)₂-modified perhydropolysilazane and polystyrene. The resulting amorphous single-phase SiVN(O) ceramics remained amorphous in nitrogen atmosphere up to 1400 °C. The as-prepared materials consist of nanoscaled vanadium nitride dispersed in amorphous Si₃N₄; exposure to 1600 °C leads to the crystallization of VN and Si₃N₄. The specific surface area (SSA) and the pore size of the SiVN(O)-based ceramics can be easily controlled by the temperature of thermal treatment and by the amount of polystyrene. The average pore size of the prepared SiVN(O) ceramics was 4–10 nm and their largest SSA values, 642 and 506 m²/g, were achieved upon ammonolysis at 800 and 1000 °C, respectively. The combination of metal-modified single-source precursors and encapsulated porogens provides a convenient one-pot synthesis process to prepare mesoporous ceramic nanocomposites with controllable phase compositions and morphology.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Enhancement of thermal shock resistance in β-Si3N4 coating with in situ synthesized β-Si3N4 nanowires/nanobelts on porous Si3N4 ceramics

A dense β-Si₃N₄ coating toughened by β-Si₃N₄ nanowires/nanobelts was prepared by a combined technique involving chemical vapor deposition and reactive melt infiltration to protect porous Si₃N₄ ceramics in this work. A porous β-Si₃N₄ nanowires/nanobelts layer was synthesized in situ on porous Si₃N₄ ceramics by chemical vapor deposition, and then Y–Si–Al–O–N silicate liquid was infiltrated into the porous layer by reactive melt infiltration to form a dense composite coating. The coating consisted of well-dispersion β-Si₃N₄ nanowires/nanobelts, fine β-Si₃N₄ particles and small amount of silicate glass. The testing results revealed that as-prepared coating displayed a relatively high fracture toughness, which was up to 7.9 ± 0.05 MPa m^1/2, and it is of great significance to improve thermal shock resistance of the coating. After thermal cycling for 15 times at ΔT = 1200 °C, the coated porous Si₃N₄ ceramics still had a high residual strength ratio of 82.2%, and its water absorption increased only to 6.21% from 3.47%. The results will be a solid foundation for the application of the coating in long-period extreme high temperature environment.     

The preparation of silicon nitride ceramics by gelcasting and pressureless sintering

A new gelling system based on the polymerization of hydantion epoxy resin and 3,3′-Diaminodipropylamine (DPTA) was successfully developed for fabricating silicon nitride (Si₃N₄) ceramics. The effects of pH value, the dispersant content, solid volume fraction and hydantion epoxy resin amount on the rheological properties of the Si₃N₄ slurries were investigated. The relative density of green body obtained from the solid loading of 52 vol% Si₃N₄ slurry reached up to 62.7%. As the concentration of hydantion epoxy resin increased from 5 wt% to 20 wt%, the flexural strength of Si₃N₄ green body enhanced from 5.3 MPa to 31.6 MPa. After pressureless sintering at 1780 °C for 80 min, the sintered samples exhibited the unique interlocking microstructure of elongated β-Si₃N₄ grains, which was beneficial to improve the mechanical properties of Si₃N₄ ceramics. The relative density, flexural strength and fracture toughness of Si₃N₄ ceramics reached 97.8%, 687 MPa and 6.5 MPa m^1/2, respectively.     

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.     

Non-additive sintering of Si2N2O ceramic with enhanced high-temperature strength,oxidation resistance,and dielectric properties

The high-temperature mechanical and dielectric properties of Si₂N₂O ceramics are often limited by the introduction of a sintering aid. Herein, dense Si₂N₂O was prepared at 1700 °C by hot-pressing oxidized amorphous Si₃N₄ powder without sintering additives. A homogeneous network with short-range order and a SiN₃O structure was formed in the oxidized amorphous Si₃N₄ powder during the hot-pressing process. Si₂N₂O crystals preferentially nucleated at positions within the SiN₃O structure and grew into rod-like and plate-like grains. Fully dense ceramics with mainly crystalline Si₂N₂O and some residual amorphous phases were obtained. The as-prepared Si₂N₂O possessed a good flexural strength of 311 ± 14.9 MPa at 1400 °C, oxidation resistance at 1500 °C, and a low dielectric loss tangent of less than 5 × 10⁻³ at 1000 °C.     

Cutting performance improvement of Si3N4 ceramic cutting tools by adding boride

Cutting performances of silicon nitride (Si₃N₄) ceramic cutting tools with and without boride additive (2.5 vol% ZrB₂ or TiB₂) prepared by hot-pressing at 1500°C were investigated. Due to the α- to β-Si₃N₄ phase transformation and low densification temperature, boride-containing Si₃N₄ ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si₃N₄–2.5 vol% TiB₂ ceramic (∼2480 m) and Si₃N₄–2.5 vol% ZrB₂ ceramic (∼2200 m) were higher than the monolithic Si₃N₄ ceramic (∼1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride-containing Si₃N₄-based inserts were improved by adding 2.5 vol% ZrB₂ or TiB₂. The improved cutting performance indicated that the boride-containing Si₃N₄ ceramics are expected to be used in the field of ceramic cutting tools.     

Effect of the diluent on combustion synthesis of silicon nitride

Si₃N₄ powders were prepared by combustion synthesis with 1- and 3-μm α-Si₃N₄, β-Si₃N₄ diluent and BN inert diluent. The maximum temperatures of samples with boron nitride (BN) as a diluent are about 1500–1600°C lower than that of samples with α-Si₃N₄ and β-Si₃N₄ as diluents are about 1600–1800°C. Moreover, the newly formed α-Si₃N₄ contents in the synthesized products with BN as diluent over 90 wt% are much higher than those with α-Si₃N₄ and β-Si₃N₄ as diluent about 20–40 wt%. The strip-like α-Si₃N₄, rod-like β-Si₃N₄ grains, and radiative shaped grains can be observed in the synthesized products. Finally, the effect of the diluent on the α-phase content of combustion synthesized Si₃N₄ is discussed, which provides key guidance for preparing Si₃N₄ powders with high α-phase content.