Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.     

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.     

Mechanism of improving the mechanical properties of Si3N4/TiC ceramic tool materials prepared by spark plasma sintering

Spark plasma sintering (SPS) is a new sintering method having shorter sintering time and higher densification speed than the traditional sintering methods. In this paper, the Si₃N₄/TiC ceramic tool material is sintered by SPS. The microstructure and mechanical properties of the material under different sintering parameters are compared. The sintering process of the material is then analyzed, and the best sintering parameters are obtained. Heat the material to 1600°C and keep the temperature for 15 min, then continue to heat to 1700°C and keep the temperature for 10 min, Si₃N₄/TiC ceramic tool material has high mechanical properties, its bending strength, fracture toughness, and Vickers hardness are 959 MPa, 8.61 MPa·m^1/2, and 15.21 GPa, respectively. The scanning electron microscope (SEM) analysis shows that under this condition, the sintering additives and Si₃N₄/TiC material form the liquid phase, which makes the Si₃N₄ particles rearrange, dissolve, precipitate, and transform into rod shape β-Si₃N₄. In addition, under the action of pulse current and external pressure, electric sparks are generated between TiC particles, which allows the material transfer and particle refinement. Therefore, the β-Si₃N₄ has uniform grain size, and it is vertically and horizontally arranged in the structure, which makes the material have excellent mechanical properties.     

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%.     

Effects of Gd2O3 and MgSiN2 sintering additives on the thermal conductivity and mechanical properties of Si3N4 ceramics

Si₃N₄ ceramics were prepared by gas pressure sintering at 1900°C for 12 h under a nitrogen pressure of 1 MPa using Gd₂O₃ and MgSiN₂ as sintering additives. The effects of the Gd₂O₃/MgSiN₂ ratio on the densification, microstructure, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were systematically investigated. It was found that a low Gd₂O₃/MgSiN₂ ratio facilitated the thermal diffusivity of Si₃N₄ ceramics while a high Gd₂O₃/MgSiN₂ ratio benefited the densification and mechanical properties. When the Gd₂O₃/MgSiN₂ ratio was 1:1, Si₃N₄ ceramics obtained an obvious exaggerated bimodal microstructure and the optimal properties. The thermal conductivity, flexural strength, and fracture toughness were 124 W·m⁻¹·k⁻¹, 648 MPa, and 9.12 MPa·m^1/2, respectively. Comparing with the results in the literature, it was shown that Gd₂O₃-MgSiN₂ was an effective additives system for obtaining Si₃N₄ ceramics with high thermal conductivity and superior mechanical properties.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Mechanical properties and thermal conductivity of dense β-SiAlON ceramics fabricated by two-stage spark plasma sintering with Al2O3-AlN-Y2O3 additives

Fully dense β-SiAlON ceramics with excellent mechanical properties and good thermal conductivity were fabricated by two-stage spark plasma sintering (SPS) processes without and with applying pressure respectively, using α-Si₃N₄ powder and 6 Al₂O₃-3 AlN-6 Y₂O₃ (in wt.%, label with 636), 424 and 422 additives. In the first stage SPS process without pressure, the relative dense β-SiAlON ceramics with interlock microstructures of elongated grains and density of 3.14˜3.18 g cm⁻³, hardness of 14.00˜14.82 GPa and fracture toughness of 6.00˜6.63 MPa m^1/2 were obtained by sintering at about 1600 °C for 20 min. In the second stage SPS process at about 1425 °C for 5 min under pressure of 24 MPa, the fully dese β-SiAlON ceramics with density of 3.22˜3.24 g cm⁻³, high hardness of 15.68˜15.95 GPa, high fracture toughness of 6.38˜7.03 MPa m^1/2 and thermal conductivity of 13.5˜19.6 Wm^-1K^-1 were obtained. The reaction between the samples and the graphite mold can be avoided in this fabrication method.     

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.     

Rapid fabrication of Si3N4 ceramics with gradient appearance and microstructure

Si₃N₄ ceramics with tailored gradient in color and microstructure were prepared by a rapid cost-effective one-step approach. The gradient microstructure was obtained by the manipulation of the dissolution-reprecipitation process, by controlling the sintering temperature and sintering additive content. In the Si₃N₄ ceramics, the β-phase content gradually changed from 84% to 11%. The Si₃N₄ ceramics exhibited white color on one side and showed a hardness of 19 GPa and fracture toughness of 7 MPa·m^1/2 and may be suitable for bio-implantation applications.     

Effects of discrete and simultaneous addition of SiC and Si3N4 on microstructural development of TiB2 ceramics

The impact of Si₃N₄ and SiC additives incorporation in the microstructure and sintering behavior of TiB₂-based composites were studied. Three ceramic composites including TiB₂–Si₃N₄, TiB₂–SiC, and TiB₂–SiC–Si₃N₄ were manufactured by spark plasma sintering (SPS) at 1950 °C for 8 min under 35 MPa. The acquired ceramics were analyzed by X-ray diffractometry and scanning electron microscopy. In addition, the sintering thermodynamic was investigated using the HSC Chemistry package. X-ray diffraction patterns of the prepared ceramics revealed the in-situ formation of graphite and boron nitride in the final composites initiated from SiC and Si₃N₄, respectively. The thermodynamic assessments proved the role of liquid phase sintering on the sinterability enhancement of all composite samples. Field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy verified the in-situ formation of both BN and graphite components in the sample containing SiC and Si₃N₄ additives. Finally, the fractographical investigations clarified the transgranular breakage as the main fracture mode in the TiB₂-based ceramics.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Thermal conductivity and mechanical properties of Si3N4 ceramics with binary fluoride sintering additives

Silicon nitride (Si₃N₄) ceramics were fabricated by gas pressure sintering (GPS) using four sintering additives: Y₂O₃–MgO, Y₂O₃–MgF₂, YF₃–MgO, and YF₃–MgF₂. The phase composition, grain growth kinetics, mechanical properties, and thermal conductivities of the Si₃N₄ ceramics were compared. The results indicated that the reduction of YF₃ on SiO₂, induced a high Y₂O₃/SiO₂ secondary phase ratio, which improved the thermal conductivity of the Si₃N₄ ceramics. The depolymerization of F atom reduces the diffusion energy barrier of solute atom and weakens the viscous resistance of anion group, which was beneficial to grain boundary migration. Besides exhibiting a lower grain growth exponent(n = 2.5)and growth activation energy (Q = 587.94 ± 15.35 kJ/mol), samples doped with binary fluorides showed excellent properties, including appreciable thermal conductivity (69 W m⁻¹ K⁻¹), hardness (14.63 ± 0.12 GPa), and fracture toughness (8.75 ± 0.18 MPa m^1/2), as well as desirable bending strength (751 ± 14 MPa).     

Influence of powder mixing processes on phase composition,microstructure, and mechanical properties of α/β-SiAlON ceramic tool materials

α/β-SiAlON composite ceramics using powders mixed through different processes were prepared via spark plasma sintering. Effects of different powder mixing methods (ball-milling and ultrasonic vibration with mechanical stirring), ball-milling media (Al₂O₃ and Si₃N₄ balls), and mixing times (12 and 24 h) on the components and morphology of mixed powders, phase composition, microstructure, and mechanical properties of α/β-SiAlON ceramics were studied. Results indicate that additional oxides are mixed to powders when ball-milled with Al₂O₃ balls, leading to an increase in the amount of amorphous glass and a decline in α/β ratio and mechanical properties. These effects were reduced when powders were ball-milled with Si₃N₄ balls, and α-SiAlON content and fracture toughness increased significantly. Moreover, α/β-SiAlON ceramics that underwent ultrasonic vibration with mechanical stirring exhibited the highest hardness and oxidation resistance at high temperature, and their α/β ratio was close to expected value.     

Mechanical properties of hybrid multilayer graphene/whisker-reinforced ceramic composites: Simulation and experimental investigation

The trial-and-error method used in ceramics research has certain limitations such as the high blindness of material component design. Moreover, calculations of the toughness of ceramics using the extended finite element method, which is the most broadly applied technique, are complicated. To overcome these issues, in this study, multilayer graphene (MLG)/Si₃N₄ whisker (Si₃N_4w)-reinforced Si₃N₄ ceramics (MWSCs) were used as the model material, and the modeling of MWSCs was conducted using Voronoi tessellation. Additionally, a more concise novel approach was applied for the prediction of the fracture toughness of MWSCs. Furthermore, the optimal MLG and Si₃N_4w contents were predicted, and then they were verified by fabricating MWSCs using spark plasma sintering (SPS). Simulation results indicated that the optimum MLG and Si₃N_4w contents to enable the toughness and hardness to reach the maximum values (9.87 MPa·m^1/2 and 23.19 GPa) were 1 wt% and 3 wt%, which were consistent with the experimental results. Consequently, the effectiveness of the proposed method was verified. Moreover, the experimental values of the maximum fracture toughness and hardness were 11.04 MPa·m^1/2 and 20.29 GPa, which were 47.20% and 12.10% higher than those of Si₃N₄ ceramics reinforced with 1 wt% MLG, respectively. The synergistic toughening effects of MLG and Si₃N_4w were significantly reflected. The load-bearing effect, bridging, and crack deflection induced by MLG and Si₃N_4w were the key reasons for the improvement in the mechanical properties of MWSCs.     

Influence of Sintering Aid on the Translucency of Spark Plasma‐Sintered Silicon Nitride Ceramics

Single additives (Y₂O₃, MgO, and Al₂O₃) were used as sintering aid of Si₃N₄. The density, crystal phase, microstructure, transmittance, hardness, and fracture toughness of sintered Si₃N₄ were investigated. Highly densified sintered bodies were obtained in single Y₂O₃ and MgO systems, but not in single Al₂O₃ system. The XRD results indicated that sintered bodies were composed of β‐Si₃N₄. The SEM images showed that all the sintered bodies had a fine‐grained microstructure with an average diameter of 0.29–0.37 μm. The thickness of grain boundary was changed with additive content. The transmittance (T) and the wavelength (λ) followed the relationship of T∝ exp(?λ^?2.3) due to the light scattering. The transmittance was mainly influenced by the refractive index of additives and the thickness of grain boundary phase. The hardness and fracture toughness of sintered ceramics were 12.6–15.5 GPa and 6.2–7.2 MPam^1/2, respectively.     

Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

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.     

Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si₃N₄ ceramics were prepared using novel two-step sintering method by mixing α-Si₃N₄ as raw material with nanoscale Y₂O₃–MgO via Y(NO₃)₃ and Mg(NO₃)₂ solutions. Si₃N₄ composite powders with in situ uniformly distributed Y₂O₃–MgO were obtained through solid–liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO₃ and densification at high temperature. Variations in O, Y, and Mg contents in Si₃N₄–Y₂O₃–MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO₂ on Si₃N₄ surface reacted with MgO to form low-melting-point MgSiO₃ compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y–Si–O–N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y₂Si₃O₃N₄ secondary phase occurred simultaneously. Two-step sintered Si₃N₄ ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si₃N₄ ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m·K), and fracture toughness (7.6 MPa?m^1/2) of Si₃N₄ ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si₃N₄ ceramics.     

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.