Si3N4 Ceramics Research Articles

Multilayer gradient rare earth disilicate coating prepared by in-situ melt infiltration and sintering method with high thermal shock resistance on porous Si3N4 ceramics

Dense multilayer gradient rare earth disilicate (γ-Y2Si2O7 /β-Yb2Si2O7 /β-Lu2Si2O7) coatings were in-situ prepared by melt-infiltration /sintering procedure on porous Si3N4 ceramics for water resistance. Experimental and numerical simulation methods were used to study their thermal shock behavior. As a control, thermal shock behavior of pure γ-Y2Si2O7 coatings were also compared. FEM results showed that the gradient design of modulus in ceramic coating can effectively avoid the mismatch of mechanical properties between coating layer and internal substrate, reducing the transient thermal stress in each layer during thermal shock. All of the pure γ-Y2Si2O7 coatings were failed after thermal shock tests with ΔT ≈ 1200 ℃. However, when sintering temperature of multi-layer disilicate coatings were higher than 1400 ℃, the water absorption rates after thermal shock were all less than 5%, still showing good waterproof performance. The gradient design of modulus could effectively improve the structural stability of ceramic coatings.

Low-temperature fabrication of Si3N4 ceramics with high thermal conductivities using a single Mg2Si sintering additive

Silicon nitride ceramics were prepared by plasma-activated sintering at 1550 °C with 2−5 wt% Mg2Si as a sintering agent. The effects of the Mg2Si content on the phase, microstructure, and thermal conductivity of the ceramics were studied. Mg2Si reacts with SiO2 to form MgO and Si, benefiting to the transformation and densification of Si3N4. After heat treatment at 1700 °C for 6 h in a flowing N2, the residue Si reacted with N2 and lattice O to form Si3N4 and SiO, resulting in decreases in the grain-boundary phase and lattice O content. After the heat treatment, the sample prepared with 3 wt% Mg2Si achieved a relatively good balance between the thermal conductivity and strength, with values of 110 W/(m⋅K) and 705 ± 30 MPa, respectively.

W/Si3N4 composite ceramics reinforced with core‐shell W‐crystallized oxynitride phase

AbstractThe introduction of a reinforced phase is an effective way to toughen silicon nitride ceramics. However, the residual stress caused by thermal expansion mismatch between the reinforced phase and Si3N4 ceramics may generate interfacial defects and deteriorate the strength of silicon nitride ceramics. Therefore, a novel strategy for optimizing interface structure to reinforceW/Si3N4 composite ceramics has been proposed. After introducing (Y2O3, Al2O3, W) particles in the W/Si3N4 ceramics, the melted liquid phase encompasses the W particles at high temperature and in situ‐generates the core‐shell structural W‐crystallized oxynitride phase as the material cools. As a result, the interfacial defect was inhibited after the core‐shell W‐crystallized oxynitride phase was introduced. With the increase of W addition from 0 to 4 wt%, the flexural strength of the W/Si3N4 ceramics with introducing core‐shell structural W‐crystallized oxynitride phase increases from 985 to 1168 MPa, and the fracture toughness improves from 6.5 to 9.0 MPa m1/2.

Effects of MgO-RE2O3 and SiO2-RE2O3 (RE = La, Nd, Gd, Ho and Lu) complex sintering additives on the phase composition, microstructure and mechanical properties of dual-phase Si3N4 ceramics

Dual-phase (α+β)-Si3N4 ceramics with MgO-RE2O3 and SiO2-RE2O3 (RE = La, Nd, Gd, Ho and Lu) complex sintering additives were prepared by SPS at 1650 and 1700 °C. The effect of sintering additives on the phase transition, microstructure evolution and mechanical properties of the Si3N4 ceramics were studied. For the MgO-RE2O3 samples, as the RE radius increased, the phase transition was decelerated, the percentage of rod-like grains and fracture toughness decreased and the hardness showed a hill-shape curve. It is proposed that the effect of RE radius is connected with the segregation behavior of RE cations on the surface of Si3N4 grains. For the SiO2-RE2O3 samples, as the RE radius increased, the phase transition was accelerated and the percentage of rod-like grains increased. It is suggested that the effect of RE radius is related with the liquid phase viscosity and nitrogen solubility.

Synergistic Reinforcement of Si3N4 Based Ceramics Fabricated via Multiphase Strengthening under Low Temperature and Short Holding Time.

Si3N4 ceramic as a tool material shows promising application prospects in high-speed machining fields; however, the required high mechanical properties and low-cost preparation of Si3N4 ceramic tool materials restrict its application. Herein, synergistic reinforced Si3N4 ceramic tool materials were fabricated by adding β-Si3N4 seeds, inexpensive Si3N4 whiskers and TiC particles into coarse commercial Si3N4 powder (D50 = 1.5 μm), then sintering by hot-pressing with low temperature and short holding time (1600 °C-30 min-40 MPa). The phase assemblage, microstructure evolution and toughening mechanisms were investigated. The results reveal that the sintered Si3N4 ceramics with synergistic reinforcement, compared to those with individual reinforcement, present an enhancement in relative density (from 94.92% to 97.15%), flexural strength (from 467.56 ± 36.48 to 809.10 ± 45.59 MPa), and fracture toughness (from 8.38 ± 0.19 to 10.67 ± 0.16 MPa·m1/2), as well as a fine Vickers hardness of 16.86 ± 0.19 GPa. Additionally, the various reinforcement modes of Si3N4 ceramics including intergranular fracture, crack deflection, crack bridging and whiskers extraction were observed in crack propagation, arising from the contributions of the added β-Si3N4 seeds, Si3N4 whiskers and TiC particles. This work is expected to serve as a reference for the production of ceramic cutting tools.

Tribological properties of graphene/β-Si3N4 whisker reinforced Si3N4 ceramic composites

Exploring ceramic-based self-lubricating materials with low coefficient of friction (COF) has become one of the hot directions to advance the service performance and life of high-end equipment. In this work, the tribological properties of multilayer graphene (MLG)/β-Si3N4 whisker (β-Si3N4w)/Si3N4 ceramics were investigated. The effects of load and linear speed on the dry sliding COF were discussed. The friction-reducing and self-lubricating mechanisms of MLG and β-Si3N4w were deeply analyzed according to the wear morphology and mechanical properties of Si3N4-based ceramics. The friction frequency was proposed to analyze the wear mechanism. Results indicated that MLG and β-Si3N4w led to a breakthrough reduction in COF. At the load of 5–20 N, the COF reached an ultra-low level of 0.04–0.07, which was a further 89.86% lower than the reported studies. The comprehensive enhancement of mechanical properties and the existence of MLG lubricating film were the mainly responsible for the ultra-low COF. The COF of MLG/β-Si3N4w/Si3N4 ceramics raised to 0.24–0.31 at 50–200 mm/s, but it was still 33.88–47.64% lower than that of graphene/Si3N4 ceramics. The increase in COF was attributed to the high wear induced by the raised friction frequency. The furrow wear and adhesive wear were the main wear forms of MLG/β-Si3N4w/Si3N4 ceramics.

Effect of the ratio of Y2O3 and MgSiN2 sintering additives on the microstructure, thermal and mechanical properties of Si3N4 ceramics

To investigate the mechanism of the influence of the sintering additive ratio on the microstructure evolution of silicon nitride (Si3N4) ceramics, Si3N4 ceramics were prepared by gas pressure sintering using different ratios of Y2O3/MgSiN2 as additives. Varying the ratio of Y2O3/MgSiN2 changed the composition of the liquid phase, thereby affecting the densification process and phase transformation rate of Si3N4 ceramics, and ultimately changing their microstructure. The reduction in the Y2O3/MgSiN2 ratio raised the N/O and Si/O ratios in the system, which increased the liquid phase viscosity. The Si3N4 grain distribution varied from "normal to bimodal". In the case of increasing the proportion of MgSiN2 and prolonging the sintering time, more of the glass phase evaporated, which reduced the content of the low thermal conductivity phase and increased the continuity of Si3N4/Si3N4. Therefore, Si3N4 ceramics with 1 wt% Y2O3 and 5 wt% MgSiN2 exhibited excellent properties after sintering at 1900 °C for 12 h, with thermal conductivity, bending strength, and fracture toughness of 106.7 Wm−1K−1, 776 MPa, and 11.66 MPa m1/2, respectively.

Joining silicon nitride ceramics with ZrSi2-SiC composite filler by spark plasma sintering

A novel ZrSi2 + 10 wt% SiC composite filler was employed to join silicon nitride (Si3N4) ceramics by spark plasma sintering (SPS) method at temperatures ranging from 1200 ℃ to 1400 ℃ in Ar atmosphere. Results of XRD analyses showed that the interlayer was composed of SiC, ZrSi2, t-ZrO2, m-ZrO2, and SiO2 phase. Results of electronic microscopy examinations showed that a layer of ZrO2 with a thickness of about 3 µm was formed at the interface between the interlayer and the substrate when the joint was joined at 1400 °C. The optimal shear strength of 114.2 ± 3.6 MPa was obtained at a relatively low temperature of 1400 ℃ for 10 min. The use of ZrSi2, which exhibits excellent mechanical properties and oxidation resistance, could potentially provide the mechanically robust joints of Si3N4 ceramics to be applied in oxidative environments at elevated temperatures.

Effect of sintering temperature on hardness gradient of Si3N4 ceramics fabricated by spark plasma sintering

The SPS technique has broad prospects for application in the design of Si3N4 materials with customized microstructures and properties. The continuously symmetrical graded silicon nitride ceramics with hard edge and tough center were prepared by spark plasma sintering at different sintering temperatures. By means of experiment and COMSOL simulation, it was revealed that different sintering temperatures created different gradient temperature fields of the samples, which further affected the transformation amount of the internal α-Si3N4 phase. The hardness and fracture toughness values were distributed in gradient, and the gradient between the center and the edge increased first and then stabilized with the temperature rising. The hardness value of silicon nitride changed between 16.93 ± 0.11 GPa and 21.16 ± 0.13 GPa, and the fracture toughness value varied between 3.92 ± 0.12 MPa∙m1/2 and 6.94 ± 0.11 MPa∙m1/2.When the sintering temperature exceeded 1800 °C, the hardness gradient of the sample was reduced, possibly due to the oxidation of silicon nitride. Finally, the mechanism of the influence of sintering temperature on the preparation of gradient silicon nitride ceramic materials was proposed.

Influence of the ratio of sintering aids on the properties of porous Si3N4 ceramics fabricated by digital light processing

The sintering aids play an important role in affecting the properties of porous Si3N4 ceramics. However, there are few researches on the properties of porous Si3N4 ceramics fabricated by digital light processing (DLP) with different ratios of sintering aids. In this paper, porous Si3N4 ceramics with different ratios of sintering aids (Y2O3-Al2O3) were formed by DLP technology. The influence of Y2O3-Al2O3 ratios on the properties of Si3N4 slurry and porous ceramic was studied systematically. The ratio of Y2O3-Al2O3 had little effect on the rheology and cure depth of Si3N4 slurry due to the low addition of sintering aids. The increase of Y2O3-Al2O3 ratio promoted the anisotropic growth of β-Si3N4. When the ratio of Y2O3-Al2O3 was 9:1, the aspect ratio of β grains reached the maximum. As the ratio of Y2O3-Al2O3 powders increased, the linear shrinkage of porous Si3N4 ceramics showed an increasing and then decreasing trend in three directions. When the Y2O3-Al2O3 ratio was 3:7, the shrinkage rate in the length, width and height direction reached the maximum (27.03%, 30.27% and 40.02%, respectively). The bulk density and flexural strength exhibited an initial increase followed by a subsequent decrease, while the porosity showed the opposite trend. When the Y2O3-Al2O3 ratio was 9:1, the porosity reached a maximum of 28.1%. And the bulk density and flexural strength were 2.42 g/cm3 and 421.58 MPa, respectively. This study is of great significance as it lays the experimental foundation in the performance control of porous Si3N4 ceramics fabricated by DLP.

Rapid fabrication of Si3N4 ceramics with gradient appearance and microstructure

AbstractSi3N4 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·m1/2 and may be suitable for bio‐implantation applications.

A comparative study on tribological behavior of Al2O3, AlN, Si3N4 and ZrO2 ceramics sliding against polycrystalline diamond ball

Studying the tribological behavior between diamond materials and ceramics is crucial for understanding the grinding mechanism of diamond wheels on ceramics. To comprehend the tribological characteristics between diamond materials and various ceramics, this paper presents a comparative study of the tribological behaviors of Al2O3, AlN, Si3N4, and ZrO2 ceramics sliding against polycrystalline diamond (PCD) balls. The dependence of the coefficient of friction (COF), wear characteristics as well as the surface morphology of different ceramics and SiC-bonded PCD balls on load, sliding stage were examined. The effect of nanofluids on the COF and wear of different ceramics was also investigated. The results showed that the tribological characteristics of ceramic materials are strongly dependent on mechanical properties. Al2O3 and AlN ceramics with lower fracture toughness exhibit a significantly higher COF and wear volume than that of Si3N4 and ZrO2 ceramics. Al2O3 presents the highest COF (0.38) and wear volume, and Si3N4 owns the lowest COF (0.08) and wear volume under the load of 9.8 N. By examining the wear grooves of different ceramics, it is found that Al2O3 and AlN ceramics own larger debris than that of Si3N4 and ZrO2, which also causes more smearing on the ceramic and adhesion of wear debris to the SiC-bonded PCD ball for Al2O3 and AlN. Under lubricants, the COFs and the adhesion of wear debris are effectively reduced, and the wear rate of the ceramic is increased. There is little difference between deionized (DI) water and nanofluids on the effect of lubricants on COF for Al2O3 and AlN. However, for ceramics with higher fracture toughness, the nanofluids can reduce the COFs of Si3N4 and ZrO2 in comparison with DI water.

Sonocapillary and wetting mechanism during ultrasonic brazing porous Si3N4 ceramics in air

In this work, porous Si3N4 ceramics are directly brazed in air using ZnAl filler with the assistance of ultrasonic vibration. The results show that good wetting of ceramic is realized within 10 s at low temperature of 450 °C. Under ultrasonic, high pressure of 7.5 MPa occurs near the entrance of ceramic micro-hole, which helps to overcome the energy barrier (1.09–2.18 MPa) to initiate sonocapillary. The capillary action of the filler is rapid, and then the infiltrated filler tightly bonds with the ceramic. The microstructure analysis reveals that Al element of the filler and O element that pre-dissolved in the filler are the main wetting elements, and the bonding interface is amorphous. It is the high temperature (1500 K) and pressure (4.87 × 108 MPa) caused by the collapse of the cavitation bubbles that cause the rapid wetting of the ceramics, which is proved by simulation. Ultrasonic power higher than Mode II produces joint stronger than the ceramic substrate, showing the potential of ultrasonic brazing.

Mechanism of superior charge suppression behavior of Si3N4 ceramics at solid-gas interface

Recent study has found excellent bulk and surface insulation property of Si3N4 ceramic which make it potential to serve as key insulation component in various energy transmission apparatus. Charge accumulation at gas-solid interface is a critical insulation property but few relevant study on Si3N4 can be found. This contribution has investigated the surface charge property of Si3N4 ceramic compared to traditional insulation polymer under different electrical-thermal stress. Through its comparison with the electric field distribution, the main source of the surface charge accumulation is analyzed, indicating that the superior surface charge suppression performance of Si3N4 lies on its low carrier density and conduction barrier, which considerably prevent charge injection and suppress the normal electric field which drives the charge to the gas-solid interface. The proposed mechanism is also validated by the characterization on conducting and space charge property for Si3N4 as well as dynamic simulation on surface charge accumulation.

Effect of ZrB2 on promoting the phase transformation mechanism of Si3N4-based ceramics at low temperature

In this study, the mechanism of the effect of ZrB2 on phase transformation of Si3N4 at a low temperature and the influence of its content on Si3N4-based ceramics were investigated. Previous study has shown that oxide impurities, i.e., B2O3 and ZrO2 on ZrB2 particles, alone cannot contribute to phase transformation of Si3N4 at a low temperature. But, the introduction of 0.5 vol% ZrB2 into Si3N4 ceramics can promote the α-β phase transformation of Si3N4, which is confirmed to be the role of boron by comparison of the experimental results obtained from the addition of 0.5 vol% Zr and 0.5 vol% B. Increasing the ZrB2 content from 0 vol% to 2.5 vol% intensifies the α-β phase transformation while decreasing the α phase content of Si3N4-based ceramics, accompanied by a slight grain growth, leading to a decrease in hardness. At the same time, aspect ratio and the quantities of elongated grains per square micron increase, and thus the fracture toughness increases significantly. However, when the content of ZrB2 increases to 5 vol%, the Si3N4-based ceramics not only have a substantial decrease in hardness, but also the fracture toughness fails to be effectively improved due to high porosity and the decrease in aspect ratio and the quantity of elongated grains per square micron. The current study demonstrates that the dense Si3N4-based ceramics with high hardness and toughness (hardness ∼19.9 ± 0.2 GPa, toughness ∼6.27 ± 0.19 MPa m1/2) can be prepared successfully at 1600 °C by introducing 0.5 vol% ZrB2.

Influence of the content of polymethyl methacrylate on the properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si3N4 ceramics are highly regarded as ideal materials for radomes due to their unique characteristics. However, the slurry used for the preparation of porous Si3N4 ceramics suffers from a low cure depth, making it challenging to fabricate ceramic components using DLP technology. In this study, porous Si3N4 ceramics were prepared by combining DLP technology with pore-forming agent method. The addition of polymethyl methacrylate (PMMA) powders with lower refractive index than that of Si3N4 powders can improve the penetration depth of ultraviolet light in the Si3N4 slurry. A systematic study was conducted to investigate the influence of the addition of PMMA powders on the properties of Si3N4 slurries and porous Si3N4 ceramics. When PMMA powders were added at 10 wt%, the slurry with a lowest viscosity of 0.13 Pa s (the shear rate is 30 s−1) and cure depth of 40.0 μm (the exposure energy is 600 mJ/cm2) was obtained. With the increase of PMMA content, porous Si3N4 ceramics experienced a gradual decrease in both the flexural strength and bulk density, while the porosity increased from 14.41% to 27.62%. Specifically, when 20 wt% PMMA was added, the resulting porous Si3N4 ceramics had a lowest bulk density (2.41 g/cm3), a maximum porosity (27.62%), and a flexural strength (435.87 MPa). The study is of great significance in establishing an experimental foundation for fabricating porous Si3N4 ceramics by using DLP technology.

Densification Behavior, Microstructural Development and Mechanical Properties of Spark Plasma Sintered α/β-Silicon Nitride Ceramics with Different Rare Earth Oxide Additives

ABSTRACT Monolithic Si3N4 ceramics were fabricated by spark plasma sintering at 1750°C for 15 min under 50 M Pa applied pressure using α-Si3N4 powder with 2 wt% Y2O3, 2 wt% Dy2O3, 2 wt% Eu2O3 and 8 wt% Y2O3 as single rare earth oxide additives. It aimed to investigate the effects of the composition and content of rare earth oxide additives on densification, microstructural development, and mechanical properties of α/β-silicon nitride ceramics via XRD and SEM analyses, temperature and displacement curves obtained through spark plasma sintering and mechanical tests. Addition to 2 wt% Y2O3 as an additive into composition resulted in high relative density (98%), hardness (14.9 GPa), and fracture toughness (9.8 M Pa.m1/2). The use of Dy2O3 instead of Y2O3 decreased β-Si3N4 phase ratio and densification rate. This behavior continued further with Eu2O3 addition. The increment of Y2O3 content to 8 wt% provided the same relative density and fracture toughness (98% and 9.8 M Pa.m1/2), whereas hardness was reduced to 12.6 GPa. As a result, densification was achieved only by Y2O3 addition regardless of additive content, and both high hardness and fracture toughness were obtained using only 2 wt% Y2O3 as a single additive.

Enhancement of Si3N4 ceramics via evolution of grain interface between ZrO2 and Si3N4 under pressureless sintering

AbstractSilicon nitride (Si3N4) as a structural ceramic material, its strength, and toughness are the decisive characteristics of damage tolerance and reliability. In this work, Si3N4 ceramics is enhanced via evolution of grain interface between Si3N4 and ZrO2, which bending strength and fracture toughness reached 982.8 MPa and 9.81 MPa·m1/2, respectively. The interface evolution of the ZrO2 and Si3N4 grains were investigated via both first‐principles molecular dynamics simulation and experiment. The Si3N4/ZrO2 interface structure was observed by high‐resolution transmission electron microscopy, include: (i) glass phase film, (ii) Zr3N4 film. First‐principles molecular dynamics simulation reported here provide an atomic‐level description of the formation mechanisms of the interface structure. The Si‐O bond and Zr‐N bond was formed to provide the Si3N4/ZrO2 interface bond. Moreover, the embedded‐like composite structure between Si3N4 and ZrO2 was formed during the sintering process, which can deflect cracks and result in an increase in the fracture energy. Due to the strength difference of Si3N4 and ZrO2, cracks tend to propagate through the grains of ZrO2 and deflects when it encounters the rod‐like Si3N4 grains. In the same time, the ZrO2 plays a role with respect to refinement of the β‐Si3N4 grain size and decrease of the glass phase content.

Effects of β-Si3N4 Seeds on Microstructure and Performance of Si3N4 Ceramics in Semiconductor Package.

Among the various ceramic substrate materials, Si3N4 ceramics have demonstrated high thermal conductivity, good thermal shock resistance, and excellent corrosion resistance. As a result, they are well-suited for semiconductor substrates in high-power and harsh conditions encountered in automobiles, high-speed rail, aerospace, and wind power. In this work, Si3N4 ceramics with various ratios of α-Si3N4 and β-Si3N4 in raw powder form were prepared by spark plasma sintering (SPS) at 1650 °C for 30 min under 30 MPa. When the content of β-Si3N4 was lower than 20%, with the increase in β-Si3N4 content, the ceramic grain size changed gradually from 1.5 μm to 1 μm and finally resulted in 2 μm mixed grains. However, As the content of β-Si3N4 seed crystal increased from 20% to 50%, with the increase in β-Si3N4 content, the ceramic grain size changed gradually from 1 μm and 2 μm to 1.5 μm. Therefore, when the content of β-Si3N4 in the raw powder is 20%, the sintered ceramics exhibited a double-peak structure distribution and the best overall performance with a density of 97.5%, fracture toughness of 12.1 MPa·m1/2, and a Vickers hardness of 14.5 GPa. The results of this study are expected to provide a new way of studying the fracture toughness of silicon nitride ceramic substrates.

Application of polypropylene carbonate for the tape casting of silicon nitride ceramics

AbstractQPAC40 (polypropylene carbonate), with a little decomposition residue, is commonly used as a binder in aluminum nitride (AlN) tape casting. In this paper, we tried to explore its application in silicon nitride (Si3N4) tape casting. By studying the influence of dispersant, binder, plasticizer/binder ratio, and solid loading on slurry and green tape properties, the optimum formulation of the tape casting of Si3N4 slurry was determined, and the green tape with a uniform structure and relative density up to 63.16% was prepared. Si3N4 ceramics were obtained by debinding at 600°C for 1 h in vacuum and gas‐pressure sintering at 1830°C for 2 h in N2. The thermal conductivity and flexural strength of Si3N4 ceramics were 56.28 ± 1.21 W/(m·K) and 1130.67 ± 23.58 MPa, respectively. These results indicated that QPAC40 can be used to prepare Si3N4 sheets through tape casting.