Fracture Toughness Of Si3N4 Ceramics Research Articles

Fracture toughness evaluation of silicon nitride from microstructures via convolutional neural network

AbstractThe fracture toughness of silicon nitride (Si3N4) ceramics was evaluated directly from their microstructures via deep learning using convolutional neural network models. Totally 156 data sets containing microstructural images and relative densities were prepared from 45 types of Si3N4 samples as input feature quantities (IFQs) and were correlated to the fracture toughness as an objective variable. The data sets were divided into two groups. One was used for training, resulting in the creation of regression models for two kinds of IFQs: the microstructures only and a combination of the microstructures and the relative densities. The other group was used for testing the validity of the created models. As a result, the determination coefficient was approximately 0.8 even when using only the microstructures as the IFQs and was further improved when adding the relative densities. It was revealed that the fracture toughness of Si3N4 ceramics was well evaluated from their microstructures.

Preparation, microstructure, and properties of GPS silicon nitride ceramics with β‐Si 3 N 4 seeds and nanophase additives

Si3N4 ceramics with high thermal conductivity and outstanding mechanical properties were prepared by adding β-Si3N4 seeds and nanophase α-Si3N4 powders as modifiers. The introduction of β-Si3N4 seeds enhanced the growth of β-Si3N4 grains. Owing to the interlocked structure induced by the β-Si3N4 grains, the fracture toughness of Si3N4 ceramics reached a high value of 7.6 MPa·m1/2; also, the large-sized grains increased the contact possibility of Si3N4 grains, improving the thermal conductivity of Si3N4 ceramics (64 W/(m·K)). Because of the introduction of nanophase α-Si3N4, the flexural strength, fracture toughness, and thermal conductivity of the Si3N4 ceramics increased to 754 MPa, 7.2 MPa·m1/2, and 54 W/(m·K), respectively. According to the analysis of the growth kinetics of Si3N4 grains, the rapid growth of Si3N4 grains was ascribed to the reduction in the activation energy resulting from the introduction of β-Si3N4 seeds and nanophase α-Si3N4.

Effect of Mo and TiC addition on the microstructure and mechanical properties of spark plasma sintered Si3N4 composites

In this study, to improve the fracture toughness of Si3N4 ceramics and overcome the reduction in the hardness caused by toughening, the Si3N4-based composite ceramics were prepared by spark plasma sintering. Fixed amounts of Y2O3 (7 wt%) and TiC (8 wt%) together with Mo adjusted in the range of 0-8 wt% were incorporated into the Si3N4 matrix. Besides, Si3N4 composites were also prepared as control groups by excluding TiC and Mo. The results reveal that the addition of TiC improves the hardness of the Si3N4 composites by inhibiting the transformation from α-Si3N4 to β-Si3N4 and grain refinement. Meanwhile, the high hardness of the composites is maintained even after toughening with Mo. The addition of Mo promotes the nucleation and phase transformation of α-Si3N4 by promoting the transfer of the Si and N atoms. At the same time, molybdenum silicide is formed in-situ, improving fracture toughness through various toughening mechanisms. The Si3N4 composites with 8 wt% TiC and 8 wt% Mo sintered at 1700 °C exhibit the optimal properties, with Vickers hardness and fracture toughness determined to be 18.25 GPa and 10.2 MPa•m1/2, respectively.

Study of the mechanical properties and toughening mechanism of ZrO2 particles toughened Si3N4 ceramics

Si3N4 ceramic has excellent properties such as high temperature resistance, high hardness, and high thermal stability, but it has the disadvantages of high hardness and brittleness and difficulty in later processing. In this paper, ZrO2 was used as toughening phase, and ZrO2 toughened Si3N4 ceramics was prepared by injection molding. The effects of ZrO2 sintering temperature and content on the mechanical properties and fracture morphology were studied. Experiments show that when the ZrO2 content is 10 wt.% and the sintering temperature reaches 1650 °C, the bending strength and fracture toughness of Si3N4 ceramics reach the maximum at the same time, which are 767 MPa and 8.7 MPa·m1/2, respectively. The density is high. XRD analysis revealed that if the sintering temperature is too high, the ZrO2/Si3N4 system will generate a large number of ZrN impurity phases that cannot be phase-transformed, which ultimately affects the ceramic properties. According to fracture morphology, the toughening mechanism of ZrO2 is stress-induced phase transition.

Mechanical properties optimization of Si3N4 ceramics by in-situ introduction of core-shell structural W-Fe5Si3

It is one inevitable problem that optimizing properties of Si3N4 ceramics by introducing second phase would produce thermal mismatch stress, which may lead to interfacial bonding and strength deterioration. In this work, a novel strategy for regulating the residual thermal stress and inhibiting the interfacial debonding in Fe5Si3/Si3N4 ceramics was proposed, which results in the simultaneous increase of strength and toughness of Si3N4 ceramics. Instead of Fe5Si3 particle, the core-shell structural W-Fe5Si3 particles were in situ generated in the Si3N4 matrix by introducing a new WSi2/FeSi2 powder directly. The maximum principal stress, residual radial and tangential stress around the core-shell structural W-Fe5Si3 and Fe5Si3 particle were compared by using the finite element method. The W core inhibits the radial contraction of the Fe5Si3 shell during the cooling process and alters the residual thermal stress distribution. Both the Griffith fracture equation and Lange's energy model indicate that the core-shell structural W-Fe5Si3 inhibits the interfacial debonding, which is consistent with the experiment result. As a result, with the increase of WSi2/FeSi2 addition from 0 to 1 wt%, the fracture toughness of Si3N4 ceramics increases from 7.1 to 7.7 MPa m1/2, and the strength improves from 1016 to 1173 MPa.

YB2C2: A new additive for fabricating Si3N4 ceramics with superior mechanical properties and medium thermal conductivity

Si3N4 ceramics were hot-pressed with MgO and self-synthesized YB2C2 as sintering additives at 1800 °C for 2 h under the pressure of 60 MPa. The addition of YB2C2 played a significant role in the simultaneous achievement of superior mechanical properties and medium thermal conductivity of Si3N4 ceramics. Due to the existence of multilayer YB2C2 nanoplatelets, the flexural strength and fracture toughness of Si3N4 ceramics reached up to 1189.6 ± 43.7 MPa and 9.46 ± 0.14 MPa m1/2, respectively. Owing to the reaction between YB2C2 and native SiO2 on the surface of Si3N4 powders, the lattice oxygen was removed and the thermal conductivity of Si3N4 ceramics was about 77.0 W/(m·K).

The fabrication of tungsten reinforced silicon nitride ceramics by altering nitrogen pressure

Due to high ductility, high-temperature melting, low thermal expansion coefficient, etc., tungsten (W) might be considered to be an ideal reinforcement in toughening or strengthening Si3N4 ceramics. However, it is difficult to fabricate W/Si3N4 composites due to the possible reactions between W and Si3N4 during sintering process at the high temperature. In this work, a novel way to avoid the reactions and fabricate the W/Si3N4 composites was proposed by thermodynamic analysis and verified by experiment. Firstly, the phase equilibrium between W and Si3N4 as a function of temperature and nitrogen pressure was thermodynamically calculated, which indicates that one critical nitrogen pressure exists for reactions between W and Si3N4 at a certain temperature. As the nitrogen pressure is higher than the critical value, the reactions would be inhibited or adversely proceeded. Based on the results, W was innovatively in-situ introduced in the form of WSi2 after sintering at 1750 °C under 50 bar nitrogen pressure. Moreover, the fracture toughness of Si3N4 ceramics was enhanced from 7.1 ± 0.2 to 8.0 ± 0.4 MPa m1/2, which proposes a new reinforcement or method in toughening Si3N4 ceramics.

Stress distribution around Fe5Si3 and its effect on interface status and mechanical properties of Si3N4 ceramics

AbstractSi3N4 ceramics with different amount of Fe5Si3 were prepared by adding FeSi2. Residual thermal stress distribution and elastic energy around Fe5Si3 particles in various depths were calculated. The interface status between second phase particles and matrix was analyzed in terms of stress and energy. High tangential compressive stresses and low radial tensile stresses were generated along the surface of the ceramics. Elastic strain energy caused by unit interface was high around big particles in deep area of the ceramics. Microcracks are observed around the big Fe5Si3 particles. Furthermore, accord to our calculation, microcracks are easily generated around particles in superficial layer of matrix when second phase particles have lower thermal expansion coefficient than the matrix, while microcracks tend to be generated in deep layer of matrix preferentially when the thermal expansion coefficient of second phase particles is higher than matrix. Residual stresses and microcracks around Fe5Si3 particles greatly influenced mechanical properties. Fracture toughness of Si3N4 ceramics with similar Si3N4 particle size distribution increased with the amount of Fe5Si3, and fine Fe5Si3 particles could enhance the strength of Si3N4 ceramics. Si3N4 ceramics exceeding 1.2 GPa strength were prepared.

Silicon nitride-based composites reinforced with zirconia nanofibres

Silicon nitride (Si3N4) ceramics exhibit remarkable mechanical, chemical and thermal properties; however, fracture toughness is a decisive characteristic for these advanced ceramics. In this paper, an alternative route to improve the fracture toughness of Si3N4 ceramics is presented. In order to enhance the fracture behaviour, we used ZrO2 nanofibres to combine two types of toughening effects such as fibre toughening and phase transformation toughening. We characterized the effect of nanofibers on the mechanical properties using instrumented hardness, Young's modulus, fracture toughness and bending tests. In order to compare the effects of nanofibers, we also studied silicon-nitride ceramics containing ZrO2 particles. The results of fracture toughness and flexural strength measurements show a significant improvement: 15wt% ZrO2 fibre exhibits 10.05 ± 0.7MPam1/2 fracture toughness and 543 ± 19MPa flexural strength, which means 105% and 115% improvements compared to the reinforcement-free samples. We assume that ZrO2 nanofibres can increase the mechanical properties of the composites in a complex way, including phase transformation toughening and fibre-toughening mechanisms.

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 52vol% Si3N4 slurry reached up to 62.7%. As the concentration of hydantion epoxy resin increased from 5wt% to 20wt%, the flexural strength of Si3N4 green body enhanced from 5.3MPa to 31.6MPa. After pressureless sintering at 1780°C for 80min, 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%, 687MPa and 6.5MPam1/2, respectively.

Enhanced fracture toughness of silicon nitride ceramics at cryogenic temperatures

We reported, for the first time, the fracture toughness of Si3N4 ceramics at cryogenic temperatures. The fracture toughness increased obviously from 4.77 ± 0.39 to 6.78 ± 0.50 MPa m1/2 with decreasing temperature from 293 to 77 K. A higher fraction of intergranular fracture calculated from the observations of crack path microstructures is responsible for the improvement in toughness at 77 K. The effect of residual stresses on the fracture mode and thus the toughness is discussed on the basis of theoretical calculation and analysis.

Microstructure and Performance of the Hot-Pressed Silicon Nitride Ceramics with Lu<sub>2</sub>O<sub>3</sub> Additives

The influence of Lu2O3 on phase transformation and seeds morphology was investigated. The result showed that the β-Si3N4 seeds with up to 95% β phase content could be obtained with 2wt% Lu2O3 as the additive content under 1750°C for two hours. The microstructure and mechanical properties of hot-pressed Si3N4 ceramics, using 9wt.% of Lu2O3• additives were investigated by the means of MTS measurements and Vickers indentation crack size measurements, as well as XRD and SEM. It was known that the high fracture toughness of Si3N4 ceramics was attributed to the rodlike morphology of β-Si3N4 grains. And the reinforcement effect and mechanism of β-Si3N4 seed were studied. It was found that the grain size and its distribution influence the property and microstructure of Si3N4 ceramics, namely, the relative narrow distribution of grain diameter in some extent and relative wide range of bimodal distribution of grain aspect ratio could improve the property of Si3N4 ceramics. The improvement in the fracture toughness with the amount of additive was mainly attributed to elongated grain growth during the sintering process.The high temperature properties of self-reinforced Si3N4 with different additives were studied. By this method, self-reinforced Si3N4 ceramics with an increment of 10~20 percent of fracture toughness was successfully fabricated.

Influence of Additives and Hot-Press Sintering on Mechanical and Lipophilic Properties of Silicon Nitride Ceramics

Fe3O4 and Mo and were added to hot-press sintered Si3N4 ceramics to improve their lipophilic and mechanical properties. The bending strength, relative density, hardness and fracture toughness of Si3N4 ceramics with added Fe3O4 and Mo were improved by hot-press sintering compared with samples prepared with the pressureless process. In particular, the bending strength and relative density were improved by about 200 and 104%, respectively. Both the macro- and micro-lipophilicity of Fe3O4- and Mo-added ceramics were improved when prepared with the hot-press process, compared with those prepared with the pressureless process. This can be attributed to the addition of Fe3O4, and the formation of MoO3 and Si2N2O during hot-press sintering. The lower friction coefficient and high wear resistance of Fe3O4- and Mo-added ceramics have been achieved by applying the hot press process. The high wear resistance is considered to be due to the improvement of hardness and fracture toughness, which decreased the loss of particles during the wear test.

Microstructural Design and Control of Silicon Nitride Ceramics

The improvement of mechanical properties by microstructural control has been one of the main topics of interest in the development of silicon nitride ceramics. Toughening, by developing an in situ composite or self-reinforced microstructure, has attracted particular attention.Microstructural design is a key factor in the optimization of processing parameters. The microstructures of sintered materials are composed of silicon nitride grains and grain boundaries, which can be either crystalline, amorphous, or partially crystalline, depending on the composition, amount of sintering additives, and processing parameters. Silicon nitride ceramics have been fabricated with an addition of metal oxides and rare-earth oxides that form a liquid phase during sintering and accelerate grain boundary diffusion. The effect of composition of the glassy phase on the mechanical properties of ceramics is presented by Becher et al. and Hoffmann elsewhere in this issue. This article focuses specifically on the design and control of grain size.As it is well recognized, many processing parameters affect grain growth behavior and the resulting microstructure. During sintering, the α- to β-phase transformation leads to a self-reinforcing microstructure on account of the anisotropic grain growth of the stable hexagonal β- Si3N4 phase. Therefore, α-rich powders are widely used for starting materials. Phase transformation accelerates anisotropic grain growth, resulting in an increase in the fracture toughness of Si3N4 ceramics. Kang and Han discuss the effect of phase transformation on nucleation and grain growth in an article in this issue. The effect of the grain-size distribution on microstructural development is described in this article, based on studies conducted mostly with β-Si3N4 powders.

ＨＩＰ処理による無添加および添加Ｓｉ３Ｎ４の焼結と性質

Hot isostatic pressing of Si3N4 powders with and without additives was performed using a glass container, and also various kinds of pressure-less sintered Si3N4 were HIP'ed without a container. The effects of HIP treatment on density, microstructure, flexural strength, microhardness and fracture toughness of Si3N4 ceramics were studied. By HIPing of Si3N4 ceramics using a glass container, it was difficult to reach the theoretical density. The microhardness of HIP'ed Si3N4 without additives was low, and the fracture toughness of HIP'ed Si3N4 with and without additives was about 5MN/m3/2. Thermal conductivity of HIP'ed Si3N4 without additives was 22∼25W/m·K, and it decreased with increasing the amount of additives. The density, flexural strength and microhardness of pressure-less sintered Si3N4 which contained Al2O3 and Y2O3 as oxide additives were remarkably improved by HIP treatment using nitrogen as a pressure transmitting gas.It is very important to select the sintering conditions for fabricating the pre-sintered body of Si3N4 in order to improve the mechanical properties of Si3N4 ceramics by HIPing treatment.