Si3N4-based Ceramics Research Articles

Effect of load on the tribological properties of Si3N4-based composite with N-GQDs

Purpose The purpose of this paper is to improve the mechanical and tribological properties of Si3N4 ceramics and to make the application of Si3N4 ceramics as tribological materials more extensive. Design/methodology/approach Si3N4-based composite ceramics (SN-2L) containing nitrogen-doped graphene quantum dots (N-GQDs) were prepared by hot press sintering process through adding 2 Wt.% nanolignin as precursor to the Si3N4 matrix, and the dry friction and wear behaviors of Si3N4-based composite against TC4 disc were performed at the different loads by using pin-on-disc tester. Findings The friction coefficients and wear rates of SN-2L composite against TC4 were significantly lower than those of the single-phase Si3N4 against TC4 at the load range from 15 to 45 N. At higher load of 45 N, SN-2L/TC4 pair presented the lowest friction coefficient of 0.25, and the wear rates of the pins and discs were as low as 1.76 × 10−6 and 2.59 × 10−4mm3/N·m. The low friction and wear behavior could be attributed to the detachment of N-GQDs from the ceramic matrix to the worn surface at the load of 30 N or higher, and then an effective lubricating film containing N-GQDs, SiO2, TiO2 and Al2SiO5 formed in the worn surface. While, at the same test condition, the friction coefficient of the single-phase Si3N4 against TC4 was at a range from 0.45 to 0.58. The spalling and cracking morphology formed on the worn surface of single-phase Si3N4, and the wear mechanism was mainly dominated by adhesive and abrasive wear. Originality/value Overall, a high-performance green ceramic composite was prepared, and the composite had a good potential for application in engineering tribology fields (such as aerospace bearings). Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-05-2024-0161/

Fabrication of one-dimensional textured Si3N4-based ceramics with high hardness and toughness by low temperature hot extrusion

Aiming to achieve silicon nitride (Si3N4) ceramics with high hardness and high toughness, one-dimensional textured Si3N4-based ceramics, with and without 0.5 vol% ZrB2 additive, were prepared using spark plasma sintering combined with hot extrusion at temperatures of 1600 °C and 1800 °C. One-dimensional texture (fI = 0.89) was achieved at a hot extrusion temperature of 1600 °C for the Si3N4 ceramic with the introduction of 0.5 vol% ZrB2. The fracture toughness (7.49 ± 0.47 MPa m1/2) closely approached that of the Si3N4 sample processed at the hot extrusion temperature of 1800 °C (7.95 ± 0.27 MPa m1/2), while the hardness (18.55 ± 0.18 GPa) was significantly higher than that of the Si3N4 sample processed at a hot extrusion temperature of 1800 °C (16.48 ± 0.20 GPa). It was demonstrated that one-dimensional textured Si3N4-based ceramics with high hardness and toughness can be prepared by the introduction of ZrB2 combined with low temperature hot extrusion.

ZrO2 reinforced porous Si3N4-based ceramics with improved mechanical properties fabricated via vat photopolymerization (VPP)

Porous Si3N4 ceramics are widely applied in many fields owing to their excellent properties such as high electromagnetic wave permeability, oxidation resistance, and wear resistance. Besides, the vat photopolymerization (VPP) technique can fabricate porous Si3N4-based ceramics with complex shapes and high precision. Nevertheless, The high absorptivity and refractive index of Si3N4 to ultraviolet light make it difficult to form porous Si3N4-based ceramics with the VPP technique. In this work, the addition of ZrO2 simultaneously improved the printability of Si3N4 slurries and the performance of Si3N4 ceramics via VPP technology. Rheological behavior and curing properties of the slurries were investigated, and the influence of ZrO2 content on the mechanical properties of printed Si3N4-based ceramics was investigated systematically. The addition of ZrO2 decreased the UV absorptivity of composite powder, thus improving the curing depth from 25.2 ± 2.6 μm to 64.8 ± 3.7 μm. XRD analysis revealed that ZrO2 in the composite ceramics mainly exists as the phase of yttria-stabilized tetragonal zirconia (Zr0.92Y0.08O1.96), which could ensure the effect of phase transformation toughening. With the increase of ZrO2 content, flexural strength and fracture toughness of porous Si3N4-based ceramics exhibited a trend of increase first and then decrease. As ZrO2 content increased to 10 wt%, the flexural strength and fracture toughness of porous Si3N4-based ceramics were improved from 214.7 MPa and 4.23 MPa·m1/2 to 345.0 MPa and 5.69 MPa·m1/2, respectively. Therefore, this work provided an important reference for the fabrication of high-performance porous Si3N4-based ceramic components with complex structures.

Microwave absorption and infrared stealth capabilities of hot-pressed Si3N4/CNT ceramics tailored by CNT content

Current research on radar and infrared dual stealth in dense Si3N4-based ceramic materials is limited. This study investigates the influence of carbon nanotubes (CNTs) on the composition, microstructure, electromagnetic wave absorption (EMA) capabilities, and infrared emissivity of hot-pressed Si3N4-based ceramics. The addition of CNTs resulted in the formation of numerous heterogeneous interfaces and the CNTs conductive network, initially enhancing the ceramics’ EMA capabilities before gradually decreasing. Si3N4-based ceramics loaded with 0.5 wt% and 1 wt% CNTs exhibited significant EMA capabilities, with a minimum refection loss of −60.95 dB at thickness of 7.49 mm. However, with an increase in CNT content, the increasing heterogeneous interfaces and pores in ceramics enhanced the absorption of the electromagnetic wave in the infrared band, resulting in a rise in total infrared emissivity from 0.584 to 0.679. To optimize radar and infrared stealth performance, a two-layered structure can be designed, with Si3N4 and Si3N4/CNT ceramics serving as the outermost and second layers, respectively. This study provides a way to design high density Si3N4-based ceramics with infrared and radar stealth.

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.

Phase evolution and oxidation resistance of Si3N4/HfB2/HfBxCyN1–x–y ceramic nanocomposites prepared from tailored preceramic polymers

Single-source-precursor derived ceramics exhibit advantages for the preparation of Si-based ceramics with controllable phase composition and adjustable functional/mechanical properties, which has significant potential for (ultra)high temperature ceramic materials. Within the present work, a series of hafnium/boron-containing Si3N4-based ceramics (SiHfBCN) are prepared upon pyrolysis/annealing of the corresponding single-source-precursors in N2 atmosphere at different temperatures ranging from 1000 °C to 1700 °C. The high-temperature (micro)structural evolution with respect to the annealing temperatures and boron concentration was studied using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. The results show that the amorphous SiHfBCN ceramics convert upon crystallization into ceramic nanocomposites consisting of a α-Si3N4 matrix with embedded Si, HfB2, and HfBxCyN1–x–y. The formation and stability of HfBxCyN1–x–y solid solution were discussed in detail. Finally, the oxidation resistance of the obtained α-Si3N4/HfB2/HfBxCyN1–x–y ceramic nanocomposites was investigated by thermal gravimetric analysis in air.

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.

Synergistic effect of graphene and β-Si3N4 whisker enables Si3N4 ceramic composites to obtain ultra-low friction coefficient

This work aimed to further enhance the tribological properties of Si3N4-based ceramics by introducing β-Si3N4 whisker (β-Si3N4w) into multilayer graphene (MLG)/Si3N4 ceramics to inhibit the reduction in the hardness of ceramic matrix induced by MLG. Therefore, the Si3N4 ceramics containing 1 wt% MLG and 1–5 wt% β-Si3N4w were prepared. The tribological properties of specimens at room temperature were evaluated using a ball-disk rotating tribo-tester. The coefficient of friction (COF) and wear resistance of MLG/β-Si3N4w/Si3N4 ceramic composites (MwSNCC) were analyzed. The existence of MLG and β-Si3N4w enabled the dry sliding COF of MwSNCC to notably reduce to 0.04–0.07, which was 89.43–95.40% lower than that of monolithic Si3N4 ceramics, and the wear resistance was enhanced by 33–62.40%.

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.

The modeling and mechanical properties prediction of whisker-reinforced ceramic composites

Whiskers have been extensively used in ceramics as excellent reinforcing phases during the past decades. Despite in-depth investigation of whisker-reinforced ceramics (WRC), however, the time-consuming, environmentally unfriendly, content design blindness of experimental method as well as pathogenicity of whiskers force people to explore quick and green research approaches. Therefore, this work aims at developing a modeling approach of WRC and predicting the mechanical properties of WRC, thereby compensating for the disadvantages of traditional experimental method. Initially, a modeling approach of WRC based on Voronoi tessellation was presented. Then, taking β-Si3N4w and Si3N4 as research objects, the prediction models of mechanical properties of 0–6 wt% β-Si3N4w-reinforced Si3N4 ceramics (SWRSC) were established. Finally, the effect of β-Si3N4w content on mechanical properties of Si3N4-based ceramics was discussed, and the enhancement mechanism of β-Si3N4w was revealed. Results indicated that the flexural strength of SWRSC was significantly advanced with increasing β-Si3N4w content. Crack deflection and intergranular fracture induced by β-Si3N4w as well as load-bearing effect of β-Si3N4w were the mainly responsible for the reinforcement of Si3N4-based ceramics. The enhancement mechanisms, previously observed experimentally, were effectively simulated. The fracture toughness reached a maximum of 6.17 MPa m1/2 when β-Si3N4w amount was 1 wt%, which was approximately 36.50% higher than monolithic Si3N4 ceramic. The predicted value of β-Si3N4w optimal content was consistent with relevant experimental conclusions. When β-Si3N4w content was 3 wt%, the hardness of Si3N4-based ceramics reached the maximum value of 23.21 GPa. The effectiveness of prediction results and the presented modeling method were confirmed by comparing with related test values.

Effect of thermal annealing on dielectric property and thermal conductivity of Si3N4–BaTiO3 composite ceramics

Si3N4–BaTiO3 composite ceramics were prepared by hot pressing sintering. The effects of the amount of BaTiO3 and annealing process on the microstructure, phase composition, dielectric properties and thermal conductivity of Si3N4 composite ceramics were studied. With the increase of BaTiO3 content, the density and the dielectric performance of the Si3N4-based composite ceramics are improved, while the thermal conductivity is reduced. The annealing treatment can not only inhibit the growth of β-Si3N4, refine the grain, reduce the content of intercrystalline glass phase and porosity, but also promote the precipitation of hexagonal barium titanate, all of which are beneficial to increase the dielectric constant and decrease the dielectric loss, but will slightly reduce the thermal conductivity of Si3N4–BaTiO3 composite ceramic. The sample A2BT15 with 15% BaTiO3 sintered at 1750 °C for 2 h and annealed at 1300 °C exhibited the maximum dielectric constant (ε = 10.57), the minimum dielectric loss (tanδ = 9×10-4), a high relative density (98.8%) and an acceptable thermal conductivity (14.5 W·m-1·K-1). Therefore, the selection of appropriate BaTiO3 content and annealing process can regulate the dielectric property, thermal performance and mechanical property of Si3N4–BaTiO3 composite ceramics in a certain range.

Multilayer graphene and β-Si3N4 whisker-reinforced porous Si3N4 ceramics by spark plasma incomplete sintering

Combining the advantages of spark plasma sintering (SPS) and incomplete sintering (IS) in preparation of Si3N4 ceramics, the spark plasma incomplete sintering (SPIS) was presented to fabricate porous Si3N4 ceramics. Considering balancing the contradictory relationship between the porosity and toughness, 1 wt% multilayer graphene (MLG) was incorporated into porous Si3N4 ceramics. Meanwhile, for the sake of compensating for the decrease in the hardness of porous Si3N4 ceramics induced by MLG, 0–8 wt% β-Si3N4 whisker (β-Si3N4w) were synergistically added. The element distribution, phase constitutions and microstructures of MLG/β-Si3N4w-reinforced porous Si3N4 ceramics were characterized by energy dispersive spectroscopy (EDS), x-rays diffraction (XRD) and scanning electron microscope (SEM). The effects of β-Si3N4w content on porosity, hardness and fracture toughness of porous Si3N4-based ceramics were analyzed. Results indicated that the porosity of porous Si3N4-based ceramics reached 21%, and then decreased significantly after the β-Si3N4w content exceeded 4 wt%. The hardness of porous Si3N4-based ceramics obtained the maximum value 2.81 GPa when β-Si3N4w was added 8 wt%, which was 31.92% higher than monolithic porous Si3N4 ceramic. The mass transfer, diffusion and fusion induced by β-Si3N4w were considered to be the main responsible for the increase of hardness. The fracture toughness of specimen with 1 wt% MLG and 4 wt% β-Si3N4w was advanced to 3.14 MPa m1/2, which was 15.44% higher than monolithic porous Si3N4 ceramic. The markedly enhanced of the fracture toughness was attributed to the synergistic effects of crack deflection, pull-out and bridging resulted by MLG/β-Si3N4w.

A novel strategy for c-axis textured silicon nitride ceramics by hot extrusion

Highly c-axis textured β-silicon nitride (β-Si3N4) ceramic with fine grains was prepared by a new method of hot extrusion for the first time. The (002) pole figure on the section plane vertical to extruding direction showed a characteristic of center rotational symmetry. The average cline angle between elongated β-Si3N4 grains and hot extruding direction was about 14.4°. The degree of c-axis texturing by hot extrusion was comparable to that achieved by rotating magnetic field. The hardness and toughness anisotropy in different direction was apparent and relatively higher hardness was achieved in the present work mainly due to the finer grain size. Therefore, many different compositions of c-axis aligned Si3N4-based ceramics with tailored mechanical properties could be achieved by the strategy of hot extrusion.

Self-propagating high temperature synthesis (SHS) of porous Si3N4-based ceramics with considerable dimensions and study on mechanical properties and oxidation behavior

The aim of present work is to fabricate porous Si3N4 ceramics with considerable dimensions and homogeneous microstructure by self-propagating high temperature synthesis (SHS) using Si, Si3N4 diluent and Y2O3 as raw materials. The results indicate that Si3N4 diluent with coarse particle sizes and appropriate β-phase content is beneficial to obtain porous Si3N4 ceramics with homogeneous microstructure and excellent mechanical property by controlling the shrinkage inside the sample. The produced Si3N4 ceramics possessed excellent flexural strength of 168 MPa∼259 MPa, and high Weibull modulus of 11.0∼17.2. Additionally, BN and SiC are added as second phase to modify the properties of Si3N4-based ceramics. Optimum flexural strength of 170 MPa and 137 MPa were obtained with 10 wt.% addition of BN and SiC respectively. After oxidation at 1100 °C∼1300 °C, second phase-doped Si3N4 ceramics also presented higher residual strength than pure Si3N4 ceramics.

Formation of bionic surface textures composed by micro-channels using nanosecond laser on Si3N4-based ceramics

Bionic surface textures exhibit excellent properties in improving the friction and wear resistance. To obtain the micro-channel bionic textures with high-quality on Si3N4/TiC ceramic surface, the effect of laser process parameters by using a nanosecond laser with 1064 nm wavelength on micro-channel surface quality and geometry is studied. The surface morphology, depth and width of the micro-channels are examined by scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM). The surface compositions of heat-affected are detected by energy dispersive spectrometer (EDS). Results show that the surface morphology and geometry of the micro-channels are seriously influenced by the laser process parameters. Higher laser power, lower scanning speed, higher frequency and overscan are prone to produce larger channel depth and width due to more accumulated energies. The optimal laser process parameters of the micro-channels are obtained, correspondingly the micro-channel depth and width are about 40–45 μm and 55–60 μm, respectively; meanwhile, it is found that the oxidation occurs in the heat-affected zone. Finally, five kinds of bionic surface textures orderly composed by micro-channels (square, circle, diamond, hexagon and sector) are fabricated on Si3N4/TiC ceramic surface based on the optimal laser process parameters.

Effects of pore structures on the dielectric properties of silicon nitride-based ceramics

The pore structures are invariably ignored when evaluating the dielectric properties of silicon nitride (Si3N4) ceramics. In this work, the effects of pore structures on the dielectric properties were revealed using both theoretical models and experimental evaluation. Theoretical models illustrate that the pore geometry and average pore radius are the determinants of dielectric properties when porosity remains constant. Experimentally, Si3N4-based ceramics with different pore structures were achieved via adding different amounts of β-Si3N4 whiskers. Then the correlations between dielectric properties and pore structures were investigated. The marked variation between experimental dielectric constants and three homogeneous isometrical sphere models reveals that the pore structures have conspicuous effects on the dielectric properties, as well as the consistent loss tangents.

Study of the Mechanical Properties and Toughening Mechanism of Nano-NbC Particles Toughened Si₃N₄-Based Ceramics.

In this article, nano-NbC particles toughened Si₃N₄-based ceramics were prepared by injection moulding and their mechanical properties along with toughening mechanism were studied. An increase of nano-NbC content, gradually homogenizes microstructure of the Si₃N₄-based ceramics along with increase in its density. However, the fracture toughness and flexural strength first increases and then decreases. The Si₃N₄-based ceramics demonstrate good comprehensive properties at the 15 wt% nano-NbC content and sintering temperature of 1550 °C (where the density is 85.3%, the flexural strength is 845 MPa, and the fracture toughness is 9.3 MPa·m1/2), Backscattered electron imaging shows that nano-NbC particles can be well dispersed in the Si₃N₄ ceramic matrix by injection moulding and ceramics are toughened by crack deflection and microcracking effects. It was also found that increasing sintering temperature makes the β-Si₃N₄ grain distribution more uniform by reducing the porosity.

Investigation on Electromagnetic Wave Absorption of SiCw/Si₃N₄ Composites Exposed to Short-Time Oxidation.

As oxidation is inevitable during application of microwave absorbing materials (MAM) exposed to oxygen-containing atmosphere at elevated temperatures. The investigations concerning influences of oxidation on microstructure and microwave absorption (MA) properties are of great significance. Aiming to better regulating MA for Si₃N₄-based ceramics serviced in high temperature environment, the evolution of MA properties of SiCw/Si₃N₄ composites (SwSN) after oxidation at 1200 °C in ambient air for short time was studied. It was found that after oxidation for 7.5 h, both the real and imaginary permittivity of SwSN showed only slight decrease due to good oxidation protection provided by dense SiO₂ layer formed on the surface. Moreover, SwSN after short-time of oxidation also showed an almost negligible decline in MA properties. SwSN which has good oxidation resistance and stable microwave attenuation abilities was subjected to oxidation at 1200 °C for short time. The results showed its favorable and potential applications in high-temperature microwave absorption fields.

Porous Si3N4-based ceramics with uniform pore structure originated from single-shell hollow microspheres

Herein, the advantage of single-shell hollow microspheres on forming pores has been exploited to acquire porous ceramics with homogeneous microstructure, while the hollow microspheres also acted as reaction source. Dispersant reaction method has been applied to realize the perfect combination between microspheres and Si3N4 particles, which could be attributed to the repulsion between particles is weakened, particles agglomerate together and holding microspheres among them tightly. Owing to the normal distribution of hollow spheres and their single-shell structure, porous Si2N2O-Si3N4 ceramics with uniform pore distribution have been fabricated. The results show that the addition of silica hollow spheres contributes to the decrease in dielectric constant, since their porosity could be increased effectively and Si2N2O phase exhibiting low dielectric constant is generated. High-performance porous Si3N4 ceramics with porosity of 45.7% have been prepared through employing fly ash hollow microspheres, which possess flexural strength of 108.76 ± 6.25 MPa, fracture toughness of 1.78 ± 0.09 MPa m1/2 and dielectric constant of 3.53. This strategy is proved to be a convenient, eco-friendly and effective method to synthesize ideal candidates for radomes.

Facile fabrication of nanofibrous network reinforced hierarchical structured porous Si3N4-based ceramics based on Si-Si3N4 binary particle-stabilized foams

A facile method for preparation of SiC nanofibrous network reinforced hierarchical structured porous Si3N4 based ceramics via annealing Si-Si3N4 binary particle-stabilized foam in coke bed is reported in this work for the first time. The atmosphere created by coke bed enables the fabrication of Si3N4 based porous ceramics at 1450 °C without using sintering additives. As prepared specimens possess a relative high compressive strength due to the partial oxidation of Si3N4 particles. This novel structure with three levels of hierarchy characterized by flexible SiC nanofibrous network, cell walls and unit cells can be characterized. The vaporization of silicon powder results in formation of Si(g), SiO(g) and allows subsequent generation of SiC nanofibrous network, leading to a considerable number of micro-pores on cell walls simultaneously. As-prepared porous ceramics contained 76.7% porosity and had compressive strength of 10.81 MPa.