Influence of processing on fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si₃N₄ grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9 MPa m^0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3 MPa m^0.5. The highest value of K_IC was 9.9 MPa m^0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5 MPa m^0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging.     

The effect of graphene nanoplatelet thickness on the fracture toughness of Si3N4 composites

Si₃N₄ composites with 3 and 5?wt% of graphene nanoplatelet (GNP) additions were prepared by spark plasma sintering. We used both commercially available GNPs and thinner few-layer graphene nanoplatelets (FL-GNPs) prepared by further exfoliation through ball milling with melamine addition. We found that by employing thinner FL-GNPs as filler material a 100% increase in the fracture toughness of Si₃N₄/3?wt% FL-GNP composites (10.5?±?0.2?MPa?m^1/2) can be achieved as compared to the monolithic Si₃N₄ samples (5.1?±?0.3?MPa?m^1/2), and 60% increase compared to conventional Si₃N₄/3?wt% GNP composites (6.6?±?0.4?MPa?m^1/2). For 5?wt% filler content the increase of the fracture toughness was near 50% for both GNP and FL-GNP fillers. The hardness of the composites decreased with increasing GNP content. However, composites reinforced with 5?wt% of FL-GNPs displayed 30% higher Vickers hardness (12.8?±?0.2?GPa) than their counterparts comprising conventional GNP fillers (9.8?±?0.2?GPa). We attribute the enhanced mechanical properties obtained with thinner FL-GNPs to their higher aspect ratio leading to a more homogeneous dispersion, higher interface area, as well as smaller pores in the ceramic matrix.     

Microstructure and mechanical properties of Si3N4f/Si3N4 composites with different coatings

The Si₃N₄ coating and Si₃N₄ coating with Si₃N₄ whiskers as reinforcement (Si₃N_4w-Si₃N₄) were prepared by chemical vapor deposition (CVD) on two-dimensional silicon nitride fiber reinforced silicon nitride ceramic matrix composites (2D Si₃N_4f/Si₃N₄ composites). The effects of process parameters of as-prepared coating including the preparation temperature and volume fraction of Si₃N_4w on the microstructure and mechanical properties of the composites were investigated. Compared with Si₃N₄ coating, Si₃N_4w-Si₃N₄ coating shows more significant effect on the strength and toughness of the composites, and both strengthening and toughening mechanism were analyzed.     

Microstructure,high-temperature mechanical and dielectric properties of novel Si3N4 f/SiNO wave-transparent composites

Continuous Si₃N₄ fiber reinforced SiNO matrix composites (Si₃N_4 f/SiNO composites) were innovatively prepared for long-time high-temperature resistant wave-transparent materials of hypersonic aircraft. The microstructure, high-temperature mechanical and dielectric properties of Si₃N_4 f/SiNO composites were investigated in detail. The as-fabricated Si₃N_4 f/SiNO composites have homogeneous SiNO matrix distribution for the special winding process, which is beneficial for the mechanical strength and wave-transparent properties. The average tensile strength and flexural strength at room temperature is 87.8 MPa and 171.2 MPa respectively, which suggests Si₃N_4 f/SiNO composites have excellent mechanical strength. The tensile strength value decreases to 54.6 MPa after heat-treated at 1000 ℃ for the surface reactions between the SiNO matrix and Si₃N₄ fibers. After heat-treated at 1550 ℃, the composites have the tensile strength value of 24.2 MPa for the high strength retention rate of Si₃N₄ fibers at this temperature. Si₃N_4 f/SiNO composites have excellent room temperature dielectric properties and excellent dielectric stability in different frequency bands (7–18 GHz). The dielectric constant values vary from 3.69 to 3.75 while the dielectric loss attains the order of 10^?3. The dielectric constants and dielectric loss of Si₃N_4 f/SiNO composites are relatively stable from RT to 800 ℃. The as-fabricated Si₃N_4 f/SiNO composites that have excellent high temperature resistance and dielectric properties are the ideal high temperature wave-transparent composites.     

Effect of the specimen size on strength of Si3N4 + SiC composite

Bending and contact strength of a recently developed carbon derived Si₃N₄ + SiC micro/nanocomposite have been investigated in four-point and opposite sphere modes using specimens with two different sizes of 3 mm × 4 mm × 45 mm and 1 mm × 2 mm × 20 mm. The fracture origins during bending test were clusters of pores and large SiC grains with different size, shape and location, which resulted in relatively low strength (675 MPa and 832 MPa) and Weibull moduli (6.4 and 8.6). The fracture origins in specimens tested in contact mode were cone cracks, originated and arised during the loading, and their similar size and shape, together with the preexisting stress field resulted in high strength (1167 MPa and 1997 MPa) and relative high Weibull moduli (17 and 15). In agreement with the theory, bending test specimens with smaller effective volume exhibited higher strength. In contact test, interaction of cone cracks occured in the small specimens at lower load as in larger one, which resulted in the lower strength of small specimens.     

Effect of milling energy on mechanical activation of (Mo + Si3N4) powders during the synthesis of Si3N4–MoSi2 in situ composites

Attempts have been made to study the effect of milling energy and type of grinding media on the mechanical activation during the production of MoSi₂ from a reaction between Mo and Si₃N₄. Powder mixtures of Mo and Si₃N₄ in the molar ratios of 1:1, 1:2 and 1:3 were ball milled using WC, steel, and ZrO₂ grinding media for mechanical activation. In order to evaluate the results obtained after high-energy ball milling and pyrolysis of these milled powder mixture, milling parameters have been converted to two energy parameters, namely, impact energy of the ball and total energy of milling. The optimum impact energy of ball required for mechanical activation of Mo + xSi₃N₄ (x = 3, 2, 1) powder mixtures by WC grinding media was found to be in the range of 0.145–0.173 J, which leads to a reduction of pyrolysis temperature by 100–200 °C. Samples milled with higher impact energy than the optimum range led to formation of undesirable phases, which dilutes the effect of mechanical activation. Samples milled with both steel and ZrO₂ grinding media having lower impact energies than the optimum show the presence of enormous contamination during milling and phases like ZrSi₂, Fe₃Si and Fe₅Si₃ were observed after pyrolysis without any significant reduction in pyrolysis temperature required for MoSi₂ synthesis.     

Synthesis of conducting graphene/Si3N4 composites by spark plasma sintering

Graphene/silicon nitride (Si₃N₄) composites with high fraction of few layered graphene are synthesized by an in situ reduction of graphene oxide (GO) during spark plasma sintering (SPS) of the GO/Si₃N₄ composites. The adequate intermixing of the GO layers and the ceramic powders is achieved in alcohol under sonication followed by blade mixing. The reduction of GO occurs together with the composite densification in SPS, thus avoiding the implementation of additional reduction steps. The materials are studied by X-ray photoelectron and micro-Raman spectroscopy, revealing a high level of recovery of graphene-like domains. The SPS graphene/Si₃N₄ composites exhibit relatively large electrical conductivity values caused by the presence of reduced graphene oxide (∼1 S cm⁻¹ for ∼4 vol.%, and ∼7 S cm⁻¹ for 7 vol.% of reduced-GO). This single-step process also prevents the formation of highly curved graphene sheets during the thermal treatment as the sheets are homogeneously embedded in the ceramic matrix. The uniform distribution of the reduced GO sheets in the composites also produces a noticeable grain refinement of the silicon nitride matrix.     

Formation mechanism of Si3N4 in reaction-bonded Si3N4-SiC composites

The formation mechanism and thermodynamics of Si₃N₄ in reaction-bonded Si₃N₄-SiC materials were analyzed. There are two kinds of Si₃N₄, fibroid α-Si₃N₄ and columnar β-Si₃N₄, which are formed by different processes in Si₃N₄-SiC materials. Silicon reacts with oxygen, forming gaseous SiO and reducing oxygen partial pressure. SiO(g) diffuses from central to peripheral sections of blocks and reacts with nitrogen, thus forming Si₃N₄, mainly in peripheral sections. The reaction between silicon and oxygen causes the consumption of oxygen and leads to low oxygen partial pressure in the sintering system, which allows silicon to react with nitrogen directly generating Si₃N₄in situ. SiO(g) reacts with nitrogen forming Si₃N₄ at both central and peripheral sections of block. The non-uniform distribution of Si₃N₄ and uneven microstructure is caused by the generation process, indicating that it is unavoidable in Si₃N₄-SiC composites.     

Extraordinary toughening enhancement and flexural strength in Si3N4 composites using graphene sheets

Two types of Si₃N₄ composites containing graphene nanostructures using two different graphene sources, pristine graphene nanoplatelets and graphene oxide layers were produced by Spark Plasma Sintering. The maximum toughness of 10.4 MPa m^1/2, measured by flexure testing of pre-cracked bars, was achieved for a composite (∼60β/40α-Si₃N₄, ∼300 nm grain size) with 4 vol.% of reduced graphene oxide, indicating a toughening enhancement of 135% when compared to a similar Si₃N₄. This was also accompanied by a 10% increase in flexure strength (1040 MPa). For the composites with thicker graphene nanoplateletes only a 40% of toughness increase (6.6 MPa m^1/2) without strength improvement was observed for the same filler content. The large difference in the maximum toughness values accomplished for both types of composites was attributed to variations in the graphene/Si₃N₄ interface characteristics and the extent of monolayer graphene exfoliation.     

Nano/macro-hardness and fracture resistance of Si3N4/SiC composites with up to 13 wt.% of SiC nano-particles

Relations between composition and mechanical properties of the Si₃N₄/SiC micro/nano-composites were studied by combination of nano-indentation and Vickers indentation techniques. The Si₃N₄/SiC composites were prepared from crystalline Si₃N₄ powder doped with SiNC amorphous precursor and yttria as the sintering aid. During sintering the SiNC precursor crystallised to yield both SiC and Si₃N₄. The in situ formed SiC particles were located both inter- and intra-granularly. The presence of SiC nano-particles enhanced the nano- and macro-hardness, and the fracture toughness of the composites. The nano-hardness of Si₃N₄/SiC composites ranged between 20 and 24 GPa, and depends on the volume fraction of SiC. The nano-hardness of individual Si₃N₄ grains exhibited large scatter as the consequence of the presence of intra-SiC inclusions, which directly influence the measured values as the harder phase, or by generating large thermal stresses within Si₃N₄ grains. Consequently the scatter of nano-hardness was much larger than in case of macro-hardness where the measured values are averaged over large area. The nano-indentation of grain boundaries indicates that the boundaries are much softer than the surrounding matrix phase. Apart of indentation size effect (ISE) this is believed to be an additional reason why the measured values of macro-hardness are lower than the nano-hardness. The maximum fracture toughness (5.8 MPa m^1/2) was achieved for the composite with the total amount of 8 wt.% SiC, where a percolating network of intergranular SiC particles was formed, as indicated by the measurement of electrical resistivity.     

Fracture toughness of concentric Si3N4-based laminated structures

A new concentric rectangular laminated structure was designed and fabricated by slip-casting method and densified by pressureless sintering process. One class of laminates consists of layers of Si₃N₄ with 7 wt.% Y₂O₃ and 3 wt.% Al₂O₃ as sintering aids, and of interlayers consisting of 50 wt.% BN and 50 wt.% Al₂O₃ designated as SN-(BN + Al₂O₃). The other class of laminates has the same Si₃N₄ layer composition but different interlayer composition of 90 wt.% BN and 10 wt.% Si₃N₄ designated as SN-(BN + SN). The objective of this paper is to investigate the effects of the number of layers and their thickness on apparent fracture toughness of these laminates. The interfacial layer composition was discussed in terms of its role in toughening of the laminates. For the SN-(BN + Al₂O₃) laminates the highest apparent fracture toughness of 22 MPa m^1/2 was found in the samples with 7 Si₃N₄ layers and for the SN-(BN + SN) laminates the highest apparent fracture toughness of 19.5 MPa m^1/2 was found in the samples with 4 Si₃N₄ layers.     

Biomimetic high toughness Si3N4 ceramics with inverse-bouligand structure

Inspired by the Bouligand structure in nature, a kind of Inverse-Bouligand structure was designed. Different from the bionic Bouligand structure of a one-dimensional material unit, the inverse-Bouligand structure is obtained by stacking and twisting the ceramic layer with a parallel groove structure. The grooves arranged in parallel are first etched on the ceramic green body and then sintered after stacking and twisting. After sintering, the groove structure can still be maintained, so the resulting laminated twisted groove structure in the bulk forms an inverse-Bouligand structure, which can promote crack deflection and improve the toughness of the material during the fracture process. After testing different twist angles, the results show that when the twist angle is 15°, the toughness of the material can reach 11.22 ± 1.77 MPa m^1/2, and high strength can be guaranteed. The synergistic effect of interlayer crack deflection and regional crack twist is the main mechanism for the improvement of material toughness.     

Preparation of Porous Si3N4 Ceramics with Controlled Porosity and Microstructure by Fibrous α‐Si3N4 Addition

Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α‐Si₃N₄ powder with a various content ratios. The effects of the contents of the fibrous α‐Si₃N₄ powder (0%–100%) on the microstructure and mechanical properties of porous Si₃N₄ ceramics were studied. As the increase of the fibrous α‐Si₃N₄ powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si₃N₄ particle. XRD analysis proved the complete formation of single β‐Si₃N₄ phase. SEM analysis revealed that the microstructure of the low content of fibrous α‐Si₃N₄ porous ceramics was almost composed of fine elongated β‐Si₃N₄ grains with high aspect ratio while numerous coarse elongated β‐Si₃N₄ grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α‐Si₃N₄ powder increased. With the increase in content of the fibrous α‐Si₃N₄ powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties.     

Microstructure,Mechanical, and Thermal Properties of (ZrB2 + ZrC)/Zr3[Al(Si)]4C6 Composite

The microstructure, mechanical, and thermal properties of in situ hot‐pressed 30 vol% (ZrB₂+ZrC)/Zr₃[Al(Si)]₄C₆ composite have been investigated and compared with monolithic Zr₃[Al(Si)]₄C₆ ceramic. The composite is composed of ZrB₂ and ZrC grains embedded in a Zr₃[Al(Si)]₄C₆ matrix. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 415 GPa), strength (bending strength of 621 MPa), and toughness (fracture toughness of 7.37 MPa·m^1/2) compared with monolithic Zr₃[Al(Si)]₄C₆. The composite retains high modulus of 357 GPa at 1430°C (86% of that at ambient temperature) due to clean grain boundaries with no glassy phase. In addition, the composite exhibits higher specific heat capacity and thermal conductivity but slightly lower coefficient of thermal expansion compared with monolithic Zr₃[Al(Si)]₄C₆. The calculation of the thermal stress fracture resistance parameter (R) predicts a much improved thermal shock resistance of the composite. Based on these results, (ZrB₂+ZrC)/Zr₃[Al(Si)]₄C₆ composites show promising potential for high‐temperature and ultra high‐temperature applications.     

Characterization of graphene nanoplatelets-Si3N4 composites by Raman spectroscopy

Raman spectroscopy is a very useful tool to study composites containing carbon phases and in particular graphene based structures. For a Si₃N₄ composite containing 3 wt.% of graphene nanoplateletes (GNPs) fabricated by spark plasma sintering (SPS), the main characteristics observed in the Raman spectra and the images obtained by mapping specific graphene peaks are presented. Raman images evidence differences in the relative intensity of typical D, G carbon peaks for two different specimen orientations – perpendicular and parallel to the SPS axis – attributed to the GNP orientation. An interesting difference between the Raman spectra of the GNP composite and the Si₃N₄ specimen similarly processed is the presence of free Si in the blank specimen, whereas in the composite the presence of SiC is observed. Both the Si and the SiC are imaged by using corresponding peaks indicating their homogenous and similar distributions in both materials.     

Boron carbide/graphene platelet ceramics with improved fracture toughness and electrical conductivity

Boron carbide/graphene platelet (B₄C/GPLs) composites have been prepared with a different weight percent of GPLs as sintering additive and reinforcing phase, hot pressed at 2100 °C in argon. The influence of the GPLs addition on fracture toughness (K_IC) and electrical conductivity was investigated. Single Edge V-Notched Beam (SEVNB) method was used for fracture toughness measurements and the four-point Van der Pauw method for electrical conductivity measurements. With increasing amount of GPLs additives, the fracture toughness increased due to the activated toughening mechanisms in the form of crack deflection, crack bridging, crack branching and graphene sheet pull-out. The highest fracture toughness of 4.48 MPa.m^1/2 was achieved at 10 wt.% of GPLs addition, which was ∼50% higher than the K_IC value of the reference material. The electrical conductivity increased with GPLs addition and reached the maximum value at 8 wt.% of GPLs, 1.526 × 10³ S/m in the perpendicular and 8.72 × 10² S/m in the parallel direction to the hot press direction, respectively.     

Microwave dielectric properties of BaxLa4Ti3 + xO12 + 3x (x = 0.0–1.0) ceramics

In the BaO–La₂O₃–TiO₂ system, the Ba_nLa₄Ti_3 + nO_12 + 3n homologous compounds exist on the tie line BaTiO₃–La₄Ti₃O₁₂ besides tungstenbronze-type like Ba_6 − 3xR_8 + 2xTi₁₈O₅₄ (R = rare earth) solid solutions. There are four kinds of compounds in the homologous series: n = 0, La₄Ti₃O₁₂; n = 1, BaLa₄Ti₄O₁₅; n = 2, Ba₂La₄Ti₅O₁₈; n = 4, Ba₄La₄Ti₇O₂₄. These compounds have the layered hexagonal perovskite-like structure, which has a common sub-structure in the crystal structure. These compounds have been investigated in our previous studies. In this study, we have investigated the phase relation and the microwave dielectric properties of Ba_xLa₄Ti_3 + xO_12 + 3x ceramics in the range of x between 0.2 and 1.0. With the increase in x, the dielectric constant ɛ_r locates around 45, the quality factor Q × f shows over 80,000 GHz at x = 0.2 and the minimum value of 30,000 GHz at x = 0.9, and the temperature coefficients of resonant frequency τ_f is improved from −17 to −12 ppm/°C. At x = 0.2, the ceramic composition obtained has dielectric constant ɛ_r = 42, the temperature coefficient of the resonant frequency τ_f  = −17 ppm/°C and a high Q × f of 86,000 GHz.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Micro-electrode discharge machining of TiN/Si3 N4 composites

Abstract

The feasibility of machining TiN/Si₃ N₄ composites by a micro-EDM method has been explored. Samples of composites containing two different sizes of TiN particles were prepared by hot pressing and the resulting materials characterised in terms of microstructure, strength, fracture toughness, and electrical resistivity. A low electrical resistivity of 1·25 × 10^-3Ω cm^-1 was obtained in a 40 vol.-% composition. Micropores of 700 μm depth and 70 μm dia. were successfully machined into TiN/Si₃ N₄ composites by the micro-EDM method. The results demonstrated the possibility of machining engineering ceramics by the micro-EDM method through the incorporation of conducting toughening phases. The size and content of TiN particles were found to have substantial effects on the strength, toughness, and electrical resistivity of the composites.