Toughening of silicon nitride ceramics by addition of multilayer graphene

Silicon nitride (Si₃N₄) ceramics have superior mechanical properties allowing their broad application in many technical fields. In this work, Si₃N₄-based composites with 1–5?wt% multilayer graphene (MLG) content were fabricated by spark plasma sintering at different temperatures and holding time in order to improve the fracture resistance of the Si₃N₄ ceramic. Our investigation focused on understanding the relationships between the microstructure and mechanical properties with special attention to the intergranular phases between Si₃N₄ matrix and MLG reinforcement.We have found that nanopores developed at the Si₃N₄-MLG interface due to a reaction between carbon and the oxygen available in the topmost layer of the Si₃N₄ particles. Interface porosity has an optimum for the toughening effect. In 1?wt% MLG/Si₃N₄ composites nanopores are local, but separated at the Si₃N₄-MLG interface, which promote the MLG pull-out mechanism imparting a significant toughening effect on the composite. Beyond the optimal 1?wt% MLG content, MLG platelets agglomerate and excessive porosity are developed at the Si₃N₄-MLG interfaces, leading to weaker matrix- graphene adhesion and thus lower fracture toughness.     

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.     

Preparation and properties of silicon nitride-phosphate composites for application in microwave furnace

Si₃N₄ ceramic is one of the most promising microwave metallurgy furnace materials because of the outstanding mechanical, relatively low dielectric properties and excellent thermal shock resistance. However, the difficult sintering of Si₃N₄ ceramics extremely restrict their large-scale application in the field of refractories for microwave metallurgy. In this work, silicon nitride-phosphate ceramics were fabricated by introducing aluminum phosphate or chromium phosphate aluminum into Si₃N₄ ceramics at 1500 °C. The effect of the amount of aluminum phosphate and chromium phosphate aluminum on sintering performance and dielectric properties was investigated. The results showed that the addition of aluminum phosphate or chromium phosphate aluminum could promote sintering, and the mechanical and dielectric properties of Si₃N₄ ceramics were efficiently improved. The Si₃N₄-aluminum phosphate composites exhibited better sintering performance (higher density and mechanical property) than that of Si₃N₄-chromium phosphate aluminum composites. Meanwhile, the dielectric constant and dielectric loss of Si₃N₄-chromium phosphate aluminum composites were better than Si₃N₄-chromium phosphate aluminum composites.     

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%.     

Enhanced toughness and reliability of Si3N4-SiCw composites under oscillatory pressure sintering

We reported an oscillatory pressure sintering (OPS) process to consolidate Si₃N₄-SiCw composites. For comparison, the composites were also prepared by hot pressing (HP) method. The specimen by OPS process reveals an accelerated rate of grain growth in the c axial direction and thus an increased average aspect ratio compared with the specimen by HP process. The oscillatory pressure also performs a positive impact on the Si₃N₄-SiCw composites as the bulk density of the OPS specimen increases to 3.270?g?cm^?3 accompanied with higher fracture strength and hardness of 1133?MPa and 16.1?GPa, respectively, compared with those of the HP specimen. Significantly, the increased fracture toughness and Weibull modulus found in the OPS specimen indicate toughening effects and material reliability are improved aided by the oscillatory pressure. Current results suggest OPS to be a promising technique for preparing highly densified Si₃N₄-SiCw composites with enhanced mechanical properties.     

Quantitative analysis of toughening effect of crack deflection on Si3N4/Si2N2O composites through improved indentation method

In this study, Si₃N₄/Si₂N₂O composite ceramics prepared by hot pressing were used as an example, and the material fracture morphology and fracture mechanism were analyzed. Based on the formula of fracture toughness measured by an indentation method, a quantitative computation method was proposed to determine the toughened effect of ceramic materials resulting from the crack deflection by the second phase. The grain size and sintering density are increased with the increase of sintering temperature. The toughening effects resulting from the crack deflection is increased, and the main mode of fracture is transformed into the transgranular fracture. The Si₂N₂O grains can play a role in the toughening process because these grains can hinder the cracks extending along the radial direction. However, when the cracks extend in the axial direction, the toughening effect of Si₂N₂O grains is not obvious because of the internal stacking faults in the axial direction. The improved indentation method can quantitatively analyze the toughening effect of the second phase of composite ceramics, and the validity of this method are verified by comparing the fracture toughness of Si₃N4/Si₂N₂O and fine grained β- Si₃N₄ ceramics.     

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.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

Preparation mullite/Si3N4 composites by reaction spark plasma sintering and their characterization

In the present study, in-situ mullite/Si₃N₄ composites were prepared successfully by reaction spark plasma sintering. For this purpose, 5, 10 and 15?wt% of Si₃N₄ were added to stoichiometric mullite made of mechanically milled mixture of alumina and kaolin clay to investigate the effect of reinforcement content on the final properties of the prepared composites. The sintering processes were performed at 1400?°C under the initial and final applied pressures of 10 and 30?MPa and the vacuum condition of 17?Pa. The XRD patterns revealed the mullite and Si₃N₄ peaks as the dominant crystalline phases. Microstructural investigations demonstrated a uniform distribution of Si₃N₄ inside mullite matrix for the composites containing 5 and 10?wt% of the reinforcement particles. Meanwhile, some agglomerates of Si₃N₄ were observed in the microstructure of the mullite-15?wt%Si₃N₄ composite. Moreover, no evidence of reaction between the starting materials was detected through XRD and FESEM analyses. The highest values of hardness, bending strength, and fracture toughness obtained for the composite containing 15?wt% of Si₃N₄ were 19.14?GPa, 481?MPa and 3.85?MPa?m^?1/2, respectively. The fracture toughness mechanisms were detected as crack branching, breaking and deflection, as well as particles pulling-out, all of which were observed in the mullite-15?wt%Si₃N₄ composite.     

Graphene nanoribbon ceramic composites

By unzipping multi-walled carbon nanotubes (MWCNTs) it is possible to obtain graphene nanoribbons (GNRs) that could then be used as fillers in ceramic composites. Here we report the fabrication of silicon nitride (Si₃N₄) ceramics with different contents of GNRs by spark plasma sintering. The GNR fillers confer electrical conductivity to the Si₃N₄ composites, following a semiconducting-like behavior at relatively low volume filler concentrations (0.04). In addition, a toughening effect, produced by GNRs bridging the cracks was observed. GNRs appear to be an efficient alternative to graphene-based composites, useful in the fabrication of novel multifunctional ceramic composites.     

Preceramic Polymer‐Derived SiAlON as Sintering Aid for Silicon Nitride

Two kinds of sintering additives based on the polysiloxanes or polysilazanes filled with nano‐sized powders as SiAlON precursors were tested for the densification of Si₃N₄‐based ceramics. The results showed that both systems can be successfully used as additives for the preparation of Si₃N₄ ceramics with favorable mechanical characteristics. The ceramics were sintered with 18 wt% of preceramic polymer‐based mixture, and good fracture resistance and high hardness values were obtained after sintering in optimized conditions (temperature, dwell time, nitrogen pressure). Higher densification temperatures and longer holding times were required for sintering of samples with polysilazane‐based precursors. The best toughness values were approximately 5 MPa·m^0.5, while the highest hardness was about 19 GPa. The differences in mechanical properties of the prepared composites can be related to the phase composition, microstructure and different chemical bonds present in the ceramic residue generated upon pyrolysis and final densification.     

A novel method of strong and tough Si3N4 ceramic from the particle-stabilized foam

The brittleness of Si₃N₄ ceramics has always limited its wide application. In this paper, Si₃N₄ ceramics were prepared based on foam. Combining the unique honeycomb structure of the ceramic foams and the self-toughening mechanism of Si₃N₄, the strengthening and toughening of Si₃N₄ ceramics can be further achieved by adjusting the microstructure of Si₃N₄ ceramic foams. The powder particles are self-assembled by particle-stabilized foaming to form a foam body with a honeycomb structure. It was pretreated at different temperatures (1450–1750°C). The microstructure evolution of foamed ceramics at different pretreatment temperatures and the conversion rate of α-Si₃N₄ to β-Si₃N₄ at different pretreatment temperatures were explored. Then the foamed ceramics with different microstructures are hot-press sintered to prepare Si₃N₄ dense ceramics. The effects of different microstructures of foamed ceramics on the strength and toughness of Si₃N₄ ceramics were analyzed. The experimental results show that the relative density of Si₃N₄ ceramics prepared at a particle pretreatment temperature of 1500°C is 97.8%, and its flexural strength and fracture toughness are relatively the highest, which are 1089 ± 60 MPa and 12.9 ± 1.3 MPa m^1/2, respectively. Compared with the traditional powder hot-pressing sintering, the improvement is 21% and 33%, respectively. It is shown that this method of preparing Si₃N₄ ceramics based on foam has the potential to strengthen and toughen Si₃N₄ ceramics.     

Spark plasma sintering: A powerful tool to develop new silicon nitride-based materials

Several key topics on the current assisted sintering of Si₃N₄-based materials are reviewed. First, different proposed mechanisms for spark plasma sintered (SPSed) ceramics are presented, discussing the electric field effect on the liquid phase behaviour and how it may influence the liquid phase sintering of Si₃N₄ ceramics. Next, we show that the SPS is a powerful tool to develop new Si₃N₄-based materials with tailored microstructures, such as functionally graded materials (FGMs) and carbon nanotubes (CNTs) containing Si₃N₄ matrix composites. Si₃N₄ FGMs are fabricated from a sole homogenous Si₃N₄ mixture just modifying the SPS system punches set-up, thus creating a temperature gradient through the specimen. Finally, the capability of SPS to get dense Si₃N₄/CNTs composites overcoming both constraint densification and nanotubes degradation is proved.     

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.     

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.     

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.     

Preparation and Performance Study of Sialon‐Si3N4‐SiC Composite Ceramics for Concentrated Solar Power

For lowering the sintering temperature of silicon carbide ceramics used for solar thermal energy storage technology, O'‐Sialon and silicon nitride were employed as composite phases to construct Sialon‐Si₃N₄‐SiC composite ceramics. The composite ceramics were synthesized using SiC, Si₃N₄, quartz, and different alumina sources as starting materials with noncontact graphite‐buried sintering method. Influences of alumina sources on the physical properties and thermal shock resistance of the composites were studied. The results revealed that the employment of O'‐Sialon and silicon nitride could decrease the sintering temperature greatly to 1540°C. The optimum formula G2 prepared from mullite as alumina source achieved the best performances: 66.7 MPa of bending strength, 10.0 W/(m·K) of thermal conductivity. The composition parameter x = 0.4 of O'‐Sialon decreased to 0.04 after 30 cycles thermal shock, and the bending strength increased with a rate of 11.0% due to the increase of O'‐Sialon grain size, and the optimization of microstructure caused by the transformation of O'‐Sialon grains and densification within the samples. The good thermal shock resistance makes the composites suitable for the use as thermal storage materials of concentrated solar power generation.     

Improving the electrical and microwave absorbing properties of Si3N4 ceramics with carbon nanotube fibers

Carbon nanotube fibers (CNTFs) reinforced Si₃N₄ ceramics has been prepared by incorporating CNTF preforms into ceramic precursor and followed by the sintering process. A SiC interface layer is formed due to the chemical reaction between Si and CNTs, leading to a good bonding between CNTs and Si₃N₄ matrix. Due to the ceramic deposition, the oxidation resistance is increased of 200 °C. Furthermore, the CNTs have well orientation and high-volume distribution (7 wt%) in the hybrid composites. Obvious improvements of the electrical conductivity (up to 103 S/m) and the microwave absorbing performance (up to 6 dB at 15 GHz) are obtained for the composites containing CNTFs. Our work provides a meaningful way to fabricate multifunctional ceramics possessing high electrical and microwave absorbind properties.     

Low-temperature Pr3Si2C2-assisted liquid-phase sintering of SiC with improved thermal conductivity

A novel Pr₃Si₂C₂ additive was uniformly coated on SiC particles using a molten-salt method to fabricate a high-density SiC ceramics via liquid-phase spark plasma sintering at a relatively low temperature (1400°C). According to the calculated Pr–Si–C-phase diagram, the liquid phase was formed at ∼1217°C, which effectively improved the sintering rate of SiC by the solution–reprecipitation process. When the sintering temperature increased from 1400 to 1600°C, the thermal conductivity of SiC increased from 84 to 126 W/(m K), as a consequence of the grain growth. However, an increasing amount of the sintering additive increased the interfacial thermal resistance, resulting in a decrease of thermal conductivity of the materials. The highest thermal conductivity of 141 W/(m K) was obtained for the material having the largest SiC grains and an optimized amount of the additive at the grain boundaries and triple junctions. The proposed Pr₃Si₂C₂-assisted liquid-phase sintering of SiC can be potentially used for the fabrication of SiC-based ceramic composites, where a low sintering temperature would inhibit the grain growth of SiC fibers.