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.     

Effect of ZrB2 content on phase assemblage and mechanical properties of Si3N4–ZrB2 ceramics prepared at low temperature

Based on the previous work on Si₃N₄–ZrB₂ [Wu et al. J Eur Ceram Soc;2017,37:4217], the influence of ZrB₂ addition on the phase and microstructure evolution of Si₃N₄–ZrB₂ composites was emphatically investigated, and the mechanical properties were compared with pure Si₃N₄ ceramics. It was revealed that the ratio of β‐ to (α+β)‐Si₃N₄ significantly increased from 14.3% in pure Si₃N₄ ceramics to 39.8% in Si₃N₄ with 15 vol% ZrB₂ addition, indicating that the introduction of ZrB₂ promoted α‐ to β‐Si₃N₄ phase transformation. As a consequence, the microstructure of the composite showed the bimodal distribution, containing both elongated and equiaxed Si₃N₄ grains. For the pure Si₃N₄, Vickers hardness, fracture toughness and flexural strength was 22.8 GPa, 7.6 MPa m^1/2, and 334.5 MPa, respectively. In contrast, the composite of Si₃N₄–30 vol% ZrB₂ simultaneously possessed an excellent combination of mechanical properties: 19.5 GPa in hardness, 9.8 MPa m^1/2 in toughness and 702.0 MPa in strength. Present study suggested that Si₃N₄‐based ceramics with high hardness, high toughness, and high strength could be obtained by the combination of appropriate ZrB₂ content and low hot‐pressing temperature.     

导电 Si3N4 基复相陶瓷研究进展

Si_3N_4陶瓷具有优异的力学性能和导热性能,然而其固有的高硬度和脆性极大地限制了其加工性能。通过添加导电相改善Si3N4陶瓷的导电性能可实现对Si_3N_4陶瓷的电火花加工。添加的导电相主要包括钛基化合物(TiN、TiC、TiC N、TiB_2)、锆基化合物(Zr B_2、Zr N)和MoSi_2等导电陶瓷以及碳纳米管(CNT)、碳纳米纤维(CNF)、石墨烯纳米片(GNP)等导电碳基纳米材料。本论文详细回顾了Si_3N_4基导电陶瓷的研究进展,并对今后Si_3N_4基导电陶瓷的发展趋势进行了展望。     

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.     

Fabrication of highly dense Si3N4 via record low-content additive system for low-temperature pressureless sintering

Dense Si₃N₄ ceramics were fabricated by pressureless sintering at a low temperature of 1650°C with a short holding period of 1 h under a nitrogen atmosphere. The role of ternary oxide additives (Y₂O₃–MgO–Al₂O₃, Y₂O₃–MgO–SiO₂, Y₂O₃–MgO–ZrO₂) on the phase, microstructure, and mechanical properties of Si₃N₄ was examined. Only 5 wt.% of Y₂O₃–MgO–Al₂O₃ additive was sufficient to achieve >98% of theoretical density with remarkably high biaxial strength (∼1200 MPa) and prominent hardness (∼15.5 GPa). Among the three additives used, Y₂O₃–MgO–Al₂O₃ displayed the finest grain diameter (0.54 μm), whereas Y₂O₃–MgO–ZrO₂ produced the largest average grain diameter (∼0.95 μm); the influence was seen on their mechanical properties. The low additive content Si₃N₄ system is expected to have superior high-temperature properties compared to the system with high additive content. This study shows a cost-effective fabrication of highly dense Si₃N₄ with excellent mechanical properties.     

Zirconium Diboride with High Thermal Conductivity

Thermal properties were characterized for zirconium diboride produced by reactive hot pressing and compared to ZrB₂ ceramics that were hot pressed from commercial powders. No sintering additives were used in either process. Thermal conductivity was calculated from measured values of heat capacity, thermal diffusivity, and density for temperatures ranging from 298 to 2273 K. ZrB₂ produced by reactive hot pressing achieved near full density, but had a small volume fraction of ZrO₂, whereas hot‐pressed ZrB₂ contained porosity and carbon inclusions. Reactive hot pressing produced a ceramic with higher thermal diffusivity and heat capacity, resulting in thermal conductivities of 127 W·(m·K)^?1 at 298 K and 80 W·(m·K)^?1 at 2273 K, which were up to ~30% higher than typically reported for hot‐pressed ZrB₂.     

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.     

Elevated Temperature Strength Enhancement of ZrB2–30 vol% SiC Ceramics by Postsintering Thermal Annealing

The mechanical properties of dense, hot‐pressed ZrB₂–30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot‐pressed or after hot‐pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot‐pressed ZrB₂–SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot‐pressed ZrB₂–SiC. Neither ZrB₂ grain size nor SiC cluster size changed for these heat‐treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB₂‐based ceramics.     

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.     

Influence of intergranular phases on edge integrity of Si3N4/SiC wood cutting tools

Si₃N₄/SiC composites used for industrial wood cutting were processed by a near net shape route involving gas pressure sintering with sintering additives such as Al₂O₃, La₂O₃, Y₂O₃ and MgO. The cutting edge integrity of these knives was tested in a cutting trial and compared to knives made by a hot pressing route. It was found that the intergranular phase has a crucial influence on the cutting edge integrity. The boundary phase was analysed by EFTEM and EDX mapping on TEM samples: in gas pressure sintered composites the crystallisation of the apatite Y₅Si₃O₁₂N phase was identified. In the hot pressed composite the boundary phase consisted only of silicates. These composites showed better edge stability than cutting tools with a Y-N-apatite phase. The formation of the type of intergranular phase was found to be determined by the amount of MgO sintering aid and the temperature of the post sintering heat treatment.     

Effect of MgO/Al2O3 ratio on the crystallization behaviour of Li2O–MgO–Al2O3–SiO2 glass-ceramic and its wettability on Si3N4 ceramic

The composition governs the crystallization ability, the type and content of crystal phases of glass-ceramics. Glass-ceramic joining materials have generated more research interest in recent years. Here, we prepared a novel Li₂O–MgO–Al₂O₃–SiO₂ glass-ceramic for the application of joining Si₃N₄ ceramics. We investigated the influence of the MgO/Al₂O₃ composition ratio on microstructure and crystallization behaviour. The crystallization kinetics demonstrated that the glasses had excellent crystallization ability and high crystallinity. β-LiAlSi₂O₆ and Mg₂SiO₄ were precipitated from the glass-ceramics, and the increase of MgO concentration was conducive to the precipitation of Mg₂SiO₄. Among the glass-ceramic samples, the thermal expansion coefficient of LMAS2 glass-ceramic was 3.1 × 10^?6/°C, which was very close to that of Si₃N₄ ceramics. The wetting test showed that the final contact angle of the glass droplet on the Si₃N₄ ceramic surface was 32° and the interface was well bonded.     

Electrical resistivity of Si3N4 ceramics with Yb2O3 additive

Si₃N₄ ceramics with excellent mechanical properties are used for heat dissipation substrates and so on. In order to improve their reliability and expand their application fields, it is desirable to understand and control the electrical properties of Si₃N₄ ceramics. In this study, the electrical resistivity of Si₃N₄ ceramics with Yb₂O₃ additive was investigated by applying various voltages at temperatures ranging from 25°C to 300°C. When Yb₂O₃ was added as a sintering aid to Si₃N₄ ceramics, a crystalline J-phase (Yb₄Si₂O₇N₂) was formed and their electrical resistivity was significantly lower than that of Y₂O₃ additive. The electrical resistivity of the Yb₂O₃-added ceramics decreased with an increase in temperature and applied voltage. Yb existed in multiple valence states, Yb²⁺ and Yb³⁺, in the Si₃N₄ ceramics and the decrease in the electrical resistivity can be attributed hopping conduction through the J-phase. The J-phase in the Si₃N₄ ceramics was observed to be continuous, and percolation analysis suggested that the J-phase formed an infinite cluster. Therefore, the decrease in the electrical resistivity of the Yb₂O₃-added Si₃N₄ ceramics was found mainly to result from the formation of an infinite cluster of J-phase, which exhibits hopping conduction.     

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.     

Theoretical Study on the Relationship Between Crystal Chemistry and Properties of Quaternary Y–Si–O–N Oxynitrides

Y–Si–O–N quaternary oxynitrides (Y₅Si₃O₁₂N, Y₄Si₂O₇N₂, YSiO₂N, Y₂Si₃O₃N₄, and Y₃Si₅ON₉) are recognized as important secondary grain‐boundary phases in silicon nitride and believed to have important impacts on the high‐temperature mechanical properties and thermal conductivity of Si₃N₄ ceramic. In this work, equilibrium crystal structures, theoretical mechanical properties (second‐order elastic constants, polycrystalline bulk modulus, shear modulus, Young's modulus, and Vickers hardness) of the five quaternary phases are calculated using first‐principle total energy calculations. Meanwhile, temperature dependence of thermal conductivities of all five compounds is obtained based on Debye–Clarke model and Slack equation. On the basis of theoretical prediction, we establish the relationship between the componential (cation/anion or cation/cation ratios) and structural characteristics (bonding configurations) and mechanical/thermal properties. Our results are expected to provide helpful guidelines to improve the performances of Y–Si–O–N ceramics, and further guide the optimization of mechanical and thermal properties of Si₃N₄ by properly tailoring the secondary grain‐boundary phases.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

SiAlON–Al2O3 ceramics as potential biomaterials

When used as implants, Al₂O₃ is unable of directly achieving good chemical bonding with soft and hard tissues. To overcome this problem, SiAlON–Al₂O₃ ceramics were prepared in this study by direct nitridation. Phase composition, porosity, bulk density, and compression strengths were examined, and biological properties were evaluated by cell culture on ceramic surface. Major phase of SiAlON–Al₂O₃ ceramics was identified as Si₄Al₂O₂N₆, formed by reaction of Si, Al and Al₂O₃ under nitrogen atmosphere at high temperature. As Al₂O₃ content increased, porosity and compressive strength decreased. Therefore, Si₄Al₂O₂N₆ phase could improve sintering, leading to formation of composites with better properties. The porosity and compression strength were found suitable for requirement of biomaterials. Cell culture experiments showed that cells could proliferate and survival well on ceramic surface, indicating good biocompatibility of Si₄Al₂O₂N₆ phase in SiAlON–Al₂O₃ ceramics. Overall, these data look promising and might provide novel strategies for development of future SiAlON–Al₂O₃ bioceramics.     

Effects of Y2O3/MgO ratio on mechanical properties and thermal conductivity of silicon nitride ceramics

In this work, the effects of Y₂O₃/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y₂O₃/MgO. It was found that a low Y₂O₃/MgO ratio facilitated the densification of Si₃N₄ ceramics while a high Y₂O₃/MgO ratio benefited the phase transformation of Si₃N₄ ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m^1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y₂O₃/MgO ratio of 3:4 by sintering at 1900°C for 4 h.     

Lithium Disilicate Glass‐Ceramics by Heat Treatment of Lithium Metasilicate Glass‐Ceramics Obtained by Hot Pressing

Lithium disilicate glass‐ceramics are widely used as dental ceramics due to their machinability and translucency. In this study, lithium disilicate glass‐ceramic was fabricated through heat treatment of lithium metasilicate glass‐ceramics obtained by hot pressing of glass powder composed of SiO₂–Li₂O–P₂O₅–ZrO₂–Al₂O₃–K₂O–CeO₂ at low temperature. The crystalline phase, microstructure, and mechanical properties were investigated. The results indicated that lithium metasilicate glass‐ceramic with a relative density of higher than 99% was obtained after hot pressing, and glass‐ceramic with interlocked rod‐like Li₂Si₂O₅ crystals and good flexural strength (338 ± 20 MPa) was successfully obtained through heat treatment. The two‐step method was believed to be feasible in tailoring the microstructure and mechanical properties of lithium disilicate glass‐ceramics.     

Effect of in situ synthesized and additives on the thermal performance of mullite thermal storage ceramics

High-performance thermal storage ceramics can enable utilization of solar thermal power generation plants. In this work, in situ synthesis was used to prepare mullite thermal storage ceramics. Calcined bauxite, talc, and kaolin were used as raw materials. The effects of additives (e.g., SiC, Si₃N₄, TiC, and ZrB₂) on the density, mechanical durability, phase components, microstructure, and thermal performance of the mullite ceramics were studied. The results showed that the thermal expansion coefficient, thermal conductivity, and heat storage density of the mullite ceramics were affected by their phase components. SiC and Si₃N₄ did not decompose during the in situ syntheses, but TiC and ZrB₂ decomposed. With the addition of 10 wt% SiC, the thermal conductivity improved to 2.72 W (m K)^?1 (298 K). The heat storage density of this material was 688 kJ kg^?1 (273–1073 K). Consequently, the in situ synthesized mullite thermal storage ceramic with added SiC could be a promising candidate material for a compound latent-sensible heat storage system.     

Si3N4-ZrB2 ceramics prepared at low temperature with improved mechanical properties

Si₃N₄-ZrB₂ ceramics were hot-pressed at 1500 °C using self-synthesized fine ZrB₂ powders containing 2.0 wt% B₂O₃ together with MgO-Re₂O₃ (Re = Y, Yb) additives. Both Si₃N₄ and ZrB₂ grains in the hot-pressed ceramics were featured with elongated and equiaxed morphology. The presence of elongated Si₃N₄ and ZrB₂ grains led to the partial texture of the ceramics under the applied pressure. Vickers hardness and fracture toughness of Si₃N₄-ZrB₂ ceramics with MgO-Re₂O₃ additives prepared at low temperature were about 19–20 GPa and 9–11 MPa m^1/2, respectively, higher than the reported values of Si₃N₄-based ceramics prepared at high temperature (1800 °C or above) under the same test method.