Si3N4 Samples Research Articles

Preparation of in-situ formed TiN0.3-Ti5Si3-TiN composites through reactive spark plasma sintering of Ti and Si3N4

In-situ intermetallic-ceramic composites have been prepared successfully via spark plasma sintering process using Ti and Si3N4 starting materials. 5, 10 and 15 wt% of nano-sized Si3N4 particles were mixed with Ti powders by milling in ethanol media for 2 h. Spark plasma sintering processes were performed at 1250 °C for each sample. It was observed that initial shrinkage of the sample with the lower amount of Si3N4 took place at the earlier time and the total shrinkage of this sample was higher than the other samples due to lower refractoriness of this composite. The phase formation was investigated by XRD and FESEM analyses. XRD analysis showed that the TiN0.3 and Ti5Si3 phases are formed for all samples after sintering process. It was also observed that in Ti-15%Si3N4 sample, the third phase of TiN was formed due to increasing of the N concentration in the composite and formation of a concentrated solid solution of N in Ti (50 at%). By increasing the Si3N4 content, the Ti5Si3 amount was increased due to increasing of Si concentration in the composite. Finally, the hardness and flexural strength of the composites were examined by Vickers hardness and 3-point bending strength tests. The results showed that increasing the content of Si3N4 in the composites leading to increasing of the hardness from 632 Hv to 1126 Hv due to formation of hard phases. Inversely, bending strength of the samples was decreased from 880 MPa to 710 MPa by increasing of Si3N4 content in the composites due to increasing of the composite brittleness.

Enhanced thermal conductivity in Si3N4 ceramic by addition of a small amount of carbon

In order to fabricate Si3N4 ceramic with enhanced thermal conductivity, 93 mol%α-Si3N4-2 mol%Yb2O3-5 mol%MgO powder mixture was doped with 5 mol% carbon, and sintered firstly at 1500 °C for 8 h and subsequently at 1900 °C for 12 h under 1 MPa nitrogen pressure. During the first-step sintering, the carbothermal reduction process significantly reduced the oxygen content and increased the N/O ratio of intergranular secondary phase, resulting in the precipitation of Yb2Si4O7N2 crystalline phase, higher β-Si3N4 content and larger rod-like β-Si3N4 grains in the semi-finished Si3N4 sample. After the second-step sintering, the final dense Si3N4 product acquired coarser elongated grains, lower lattice oxygen content, tighter Si3N4-Si3N4 interfaces and more devitrified intergranular phase due to the further carbothermal reduction of oxynitride secondary phase. Consequently, the addition of carbon enabled Si3N4 ceramic to gain a significant increase of ∼25.5% in thermal conductivity from 102 to 128 W∙m−1 K−1.

Effect of the oxidization of Si3N4 powder on the microstructural and mechanical properties of hot isostatic pressed silicon nitride

The effect of nanosized oxidized silicon nitride powder particles on the microstructural and mechanical properties of hot isostatic pressed silicon nitride was studied. The starting <alpha> Si3N4 powders were oxidized for 10 and 20 h at 1000 °C in ambient air environment. An amorphous oxide layer was observed on the surface of Si3N4 powder particles using high resolution transmission electron microscopy (HRTEM) whose thickness increased with the oxidation time. The powders were hot isostatic pressed at 1500 °C and 1700 °C in nitrogen gas environment under 20 MPa for 3 h. Complete <alpha> to <beta> transformation was observed in Si3N4 samples sintered at 1700 C under 20 MPa for 3 h compared to Si3N4 samples sintered at 1500 °C. Silicon oxynitride (Si2N2O) was found in sintered samples whose amount increased with the oxidation time. The <beta> phase decreased with the increase of Si2N2O phase in the sintered samples. The flexural strength of samples sintered at 1700 °C is higher due to the higher amount of <beta>-Si3N4 phase than for samples sintered at 1500 °C. Decreasing flexural strength was observed with the oxidation time. Higher amount of oxygen in the base powders resulted in a higher Si2N2O content in the sintered Si3N4.

Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freeze-drying

Porous Si3N4 ceramics with uniform pore distributions were fabricated by freeze drying with liquid N2 as refrigerant, and then sintered at 1700 °C under an N2 pressure of 0.3 MPa. The Si3N4 dispersant slurries were prepared by ball milling for 12 h and casting into a mold using liquid nitrogen as a freezing medium with the top surface of the sample exposed to air at room temperature. The temperature gradient contributed to bimodal peaks of pore size distribution. Effects of solid concentration on the porosity, mechanical, and dielectric properties of the Si3N4 sample were systematically investigated. As the solid content increased from 15 vol.% to 40 vol.%, the porosity decreased from 83.9% to 40.58%. Therefore, flexural strength and compressive strength increased from 0.1 MPa to 94.7 MPa, and 1.3 MPa–314 MPa, respectively. Moreover, the dielectric constant increased from 1.4 to 4.0. This study indicated that the freeze-drying process is a useful method for fabricating novel porous Si3N4 ceramics with unidirectional channels.

In vitro tribological and biocompatibility evaluation of sintered silicon nitride

In the present work, dense silicon nitride (Si3N4) was developed using pressureless sintering for biomedical applications. The Si3N4 samples with 15 wt% sintering additives (Al2O3 + Y2O3) were sintered at 1700 °C for 2 h followed by characterization in terms of phase constituents, mechanical, in vitro tribological and biological properties. In vitro tribological tests in simulated body fluid (SBF) revealed that the wear rate and coefficient of friction of Si3N4 reduced by one order of magnitude in the presence of 10% fetal bovine serum (FBS). The in vitro biocompatibility evaluations using human osteoblast-like cells (MG63) confirmed that the sintered Si3N4 samples were non-cytotoxic.

Effect of molten zone ablated by femtosecond lasers on fracture toughness of Si3N4 measured by SEVNB method

Femtosecond lasers were used to cut an ultra-sharp V-notch on the Si3N4 ceramic sample for fracture toughness testing. The molten zone on the V-notch surface ablated by femtosecond lasers was studied using SEM, EDS and Raman spectra. The thickness of the molten Si shell is about 2μm, however, the thickness of the molten shell at the V-notch tip is very thin, and it can be easily etched with hydrofluoric acid. The fracture toughness value of the Si3N4 sample with the molten Si shell is equal to that after etched. It is concluded that the molten zone on the V-notch surface have no influence on fracture toughness of liquid phase sintered Si3N4 ceramics.

Interfacial characterization in the brazing of silicon nitride to niobium joining using a double interlayer

Both the wetting and adherence of a metal foil interlayer used to join ceramics to metals have a direct effect on the interface formation during the brazing process. Sometimes, double interlayers are used to improve the strength of the joints and to reduce the stress resulting from the difference between the thermal expansion coefficients of the base materials. In this study, we investigated the brazing of silicon nitride, Si3N4 (ceramic material) to Nb (refractory metal) using a double interlayer. In order to understand the interfacial behavior of the Si3N4/Nb joint, cylindrical samples of Si3N4 with relative densities of 94.2% and 84.7% were used. Sandwich-like samples of Si3N4/Ag–Cu/Cu–Zn/Nb were joined at temperatures from 1000°C to 1100°C using different holding times under an inert atmosphere (argon). Analysis by field emission scanning electron microscopy (FESEM) revealed that the diffusion rate of silicon and the higher porosity of the ceramic specimen improved the spreading of the liquid metal through the interface at a lower joining temperature. Nb–Si compounds were observed on the nearby niobium substrate when a large dense ceramic was joined at 1100°C for 20min. This sample had a chemical composition close to the Nb5Si3 binary phase as confirmed by electron probe microanalysis (EPMA).

Effect of Si addition on the mechanical and thermal properties of sintered reaction bonded silicon nitride

Advanced silicon nitride (Si3N4) ceramics were fabricated using a mixture of Si3N4 and silicon (Si) powders via conventional processing and sintering method. These Si3N4 ceramics with sintering additives of ZrO2+Gd2O3+MgO were sintered at 1800°C and 0.1MPa in N2 atmosphere for 2h. The effects of added Si content on density, phases, microstructure, flexural strength, and thermal conductivity of the sintered Si3N4 samples were investigated in this study. The results showed that with the increase of Si content added, the density of the samples decreased from 3.39g/cm3 to 2.92g/cm3 except for the sample without initial Si3N4 powder addition, while the thermal diffusivity of the samples decreased slightly. This study suggested that addition of Si powder, which varied from 0 to 100%, in the starting materials might provide a promising route to fabricate cost-effective Si3N4 ceramics with a good combination of mechanical and thermal properties.

Silicon nitride surface chemistry: A potent regulator of mesenchymal progenitor cell activity in bone formation

Polycrystalline silicon nitride (Si3N4), sintered with the addition of minor fractions of yttrium and aluminum oxides (i.e., Y2O3 and Al2O3), possesses uniquely adjustable surface chemistry that results in improved cell metabolism and enhanced bone formation. Building upon previous in vitro mineralization studies using osteosarcoma cells, this study examined interactions between various chemically modulated Si3N4 surfaces and murine mesenchymal progenitor cells (KUSA-A1). It was discovered that various pressurized thermal-treatments coupled with adiabatic and non-adiabatic cooling of sintered Si3N4 samples resulted in partial or full coverage of their surfaces with different Si–Y–O–N compounds. Full coverage by mostly yttrium silicate (β-Y2Si2O7) was obtained by non-adiabatic cooling, whereas partial coverage with N-apatite (Y10(SiO4)6N2) occurred under adiabatic conditions. These peculiar phases were found to be particularly efficient in stimulating the in vitro differentiation of KUSA-A1 cells into osteoblasts, although according to different microscopic mechanisms. The final amount of bone formation was nearly identical for both phases. Cell differentiation was monitored by assessing the concentration of the osteogenic marker γ-carboxyglutamate (i.e., Gla-osteocalcin). It was found to be ∼45% higher for Si3N4 samples possessing the N-apatite phase than for biomedical titanium alloy controls tested under exactly the same conditions. Concurrent measurements of the bone resorption marker Glu-osteocalcin (i.e., an undercarboxylated form of γ-carboxyglutamate) showed significant inhibition of osteoclastogenesis on these surface-treated Si3N4 samples as compared to the controls. Bone formation was assessed using in situ Raman microprobe spectroscopy and ex situ laser microscopy. These two independent analytical techniques consistently found an increase of ∼80% in expressed hydroxyapatite when compared to a biomedical titanium alloy. This study suggests that surface-treated Si3N4 may have a powerful anabolic, differentiating, and antiapoptotic effect on osteoblasts in vitro, and a concurrent inhibitive action on osteoclastogenesis. Given additional research, Si3N4 may represent a new therapeutic solution for bone disorders and for engineered implants that physiologically regulate bone growth processes.

Fabrication and thermal shock resistance of β-Si3N4-based environmental barrier coating on porous Si3N4 ceramic

A dense β-Si3N4-based waterproof environmental barrier coating was fabricated using the liquid infiltration and filling method on porous Si3N4 ceramic. Microstructure and thermal shock resistance of the coating were investigated. The results showed that the coating consisted of β-Si3N4, Y10Al2Si3O18N4 and Y-Si-Al-O glass. As-prepared coating exhibited excellent thermal shock resistance. During thermal shock at ∆T=1200°C, cracks were formed within the coating, but they did not penetrate through the whole coating due to the toughening mechanisms of the coating, including β-Si3N4 particle bridging and crack branching. In addition, the coating had a good crack self-sealing ability during thermal cycling, which is mainly attributed to glass flow and oxidation of β-Si3N4 particles within the crack. After thermal cycling for 25 times, the coating still had a good waterproof ability, and the water absorption of the coated porous Si3N4 sample increased slightly from 4.5% to 9.3%.

Effect of the Si content on structure and mechanical properties in Al1-xSixN materials

Molecular dynamics simulations of Al1-xSixN samples with different Si content were carried out to investigate their structures, void distributions and mechanical properties. Such effects on other microscopic characteristics, such as bond-length distance, bond-angle and coordination number distribution have been observed. Based on the common neighbor analysis, we found that Si3N4 sample has amorphous state and AlN sample has major amorphous phase mixed with crystalline AlN. The Al1-xSixN samples show the structures of crystalline and amorphous AlN segregated with amorphous Si3N4. The crystalline phase contains major fcc-AlN and minor hcp-AlN. The AlN region is compressed by amorphous Si3N4 region and becomes more denser with increasing Si content. The mostly big voids locate in the Si3N4 regions. The volume of voids increases but the radii of voids decrease with increasing Si content. The strengthening enhancement is observed in the Al0.5Si0.5N sample with denser phases and the lowest ratio nhcp-AlN/nfcc-AlN.

Influence of graphene addition and sintering temperature on physical properties of Si3N4 matrix composites

The Si3N4-graphene composites (1wt% addition of graphene) were manufactured using powder metallurgy and consolidated using the Spark Plasma Sintering (SPS) method. In order to achieve composites with high mechanical properties, optimisation of a Si3N4 and Si3N4-graphene sintering temperatures were carried on for five and four different sintering temperatures respectively. Obtained samples were characterised by measurements of their density, hardness, fracture toughness and Young's modulus. Qualitative and quantitative phase composition was analysed by XRD diffraction. Microstructure analysis of sinters was also performed using Scanning Electron Microscopy (SEM). The highest relative density (99.31% and 99.32%) and fracture toughness (KIC=6.0MPa∗m0.5) was measured for samples sintered at 1650°C and 1700°C respectively, for Si3N4 and Si3N4–Gn composite. That shows no increase of fracture toughness between pure silicon nitride sinter and composite with 1% wt of graphene. Furthermore, XRD results showing 33% less of β grains in Si3N4–Gn composite compared to pure Si3N4 sample, proving that graphene stabilise the α phase in silicon nitride.

Effect of pH and monovalent cations on the Raman spectrum of water: Basics revisited and application to measure concentration gradients at water/solid interface in Si3N4 biomaterial

The effect of hydrogen carbonate (HCO3−) and cations (Na+, K+) solvated in water were revisited according to high spectrally resolved Raman measurements. Water solutions with different bicarbonate concentrations or added with increasing amounts of monovalent cations were examined with respect to their Raman spectra both in the bulk state and at the solid/liquid interface with a silicon nitride (Si3N4) bioceramic. Spectroscopic calibrations confirmed that the Raman emission from OH-stretching in water is sensitive to molarity variations (in the order of tens of mM). The concentration gradient developed at the solid/liquid interface in cation-added solutions interacting with a Si3N4 surface was measured and found to be peculiar to individual cations. Local variation in pH was detected in ionic solutions interacting with Si3N4 samples, which might represent a useful property for Si3N4 in a number of biomedical applications.

Influence of a passivation layer on strain relaxation and lattice disorder in thin nano-crystalline Pt films during in-situ annealing

In this work we compared the relaxation of strain and lattice disorder in thin nano-crystalline Pt films for samples covered with a Si3N4 layer and samples without a cover layer, respectively. We measured thickness and interplanar distance of the Pt film by X-ray reflectometry and X-ray diffractometry during in-situ annealing using synchrotron radiation. The results show that strain and lattice disorder relaxation are impeded if the Pt film is sealed with a cover layer to suppress the creation of vacancies at the Pt surface. This emphasizes the postulated important role of the generation of vacancies at the free surface of thin metal films for strain relaxation during isothermal annealing.

Synthesis and characterization of Si3N4 wires from binary carbonaceous silica aerogels

Silicon nitride ultrafine powders were synthesized from a binary sol–gel route, in which tetraethoxysilane (TEOS) and saccharose were used to prepare a carbonaceous silica xerogel, and ferric nitrate was employed as an additive. The influence of reaction temperature and mass ratio of C to Si (CSi=0.1, 0.5, 1) on the synthesis of Si3N4 wires was studied. After purification, the Si3N4 sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectrum and Fourier transform infrared spectroscopy. The results show that the Si3N4 was fully formed with two kinds of morphologies including globular and wire with a diameter of 100–500nm and a length of several microns at reaction temperature of 1400°C for 10h in nitrogen flow (200ml/min) by employing the mass ratio of CSi=0.5.

The Influence of Chromium Oxide on the Sintering Behavior of Silicon Nitride

In this study, pressureless sintering of silicon nitride via the addition of chromium oxide was investigated. Silicon nitride samples containing additives from the Cr–Cr2O3–SiO2 system were sintered under different conditions. The phase transformation, the degree of densification and the in situ reactions between Si3N4 and chromium compounds were investigated and the reactions were validated thermodynamically. It was found that Si3N4 reacts with Cr2O3 and Cr and these in situ reactions lead to Si3N4 decomposition and consequently formation of a series of chromium silicides including Cr3Si, Cr5Si, and CrSi2. Due to the presence of chromium silicides as liquid phases during sintering, the α‐Si3N4 to β‐Si3N4 phase transformation started around 1400°C and was completed around 1800°C. On the other hand, densification of Si3N4 samples with chromium oxide addition was not observed. The participation of Cr2O3 in these in situ reactions prevents sufficient formation of chromium silicate and leads to insufficient liquid phase with poor wettability during sintering, resulting in poor densification.

Microstructure, mechanical and dielectric properties of highly porous silicon nitride ceramics produced by a new water-based freeze casting

Novel graded porous Si3N4 ceramics with different solid content have been fabricated by a new water-based freeze-casting method. A 25μm-thick surface dense layer on the porous layer was successfully formed without any noticeable defects. A great number of fibrous grains protruding from the internal walls of the macroscopic aligned channels were observed in the porous Si3N4 samples with 20–25vol.% solid content, but little fibrous grains were formed in the sample with 15vol.% solid content at the similar processing route. As the solid content increasing from 15 to 25vol.%, the porosity decreased from 76.8% to 66.4%, while the flexural strength and dielectric constants at 10GHz increased from 14.3MPa to 33.5MPa and 1.78 to 2.64, respectively. The dense/porous Si3N4 ceramic is feasible in broadband radomes application at high temperature due to ultra-low dielectric constants and moderate strength.

Effect of reaction time in direct nitridation of Si powders

Si3N4 nanomaterials have been synthesised via direct nitridation of Si powders at 1300°C for different reaction time. The purified Si3N4 samples were characterised by Fourier transform infrared spectroscopy, X-ray diffraction and transmission electron microscopy. The results showed that the samples obtained for different reaction time are mixtures of α- and β-Si3N4 phases. The percentage of β-Si3N4 in the Si3N4 samples has obviously increased with the reaction time. The Si3N4 samples are mainly composed of nanobelts with different width.

Porous Si3N4 Fabricated by Phase Separation Method Using Benzoic Acid as Pore‐Forming Agent

Porous Si3N4 ceramics with interconnected pores were fabricated by the phase separation method using benzoic acid (BA) as a pore‐forming agent. The green bodies were prepared at room temperature under vacuum and pore‐forming agent was quickly removed by sublimation below 200°C. The resulting porous green body exhibits uniform distribution of rod‐like pores in the matrix. The porosity can be controlled by adjusting the BA content in Si3N4 slurry. The flexural strength and fracture toughness of the porous Si3N4 samples are in the range of 85–126 MPa and 1.08–1.7 MPa·m1/2 with the porosity range of 57%–65.8%.

Tribology Study of Silicon Nitride-Based Nanocomposites with Carbon Additions

Tribology tests were conducted on silicon nitride-based nanocomposites with various carbon additions to explore the effects of microstructure, the type and quantity of carbon additives and the preparation routes on the behavior. The nanocomposites consisted of Si3N4 and C in the proportions of 1 – 10 wt % carbon nanotube (CNT), or carbon black (CB), or graphite, or graphene. Specimens were produced by hot isostatic pressing. X-ray diffraction and scanning electron microscopy were used to reveal phase composition and microstructure. Unlubricated ball-on-disk tribology tests with silicon nitride counter face were carried out at room temperature in ambient atmosphere. Contact profilometer was used to profile the wear tracks. The friction coefficients of the pure Si3N4 and Si3N4 samples with 3% CNT varied between 0,77-0,81. Addition of 10% graphite and 3% CB to Si3N4 resulted in friction coefficients of 0,83 and 0,72 respectively. Si3N4 samples with 3% graphene showed distinctly lower friction levels of 0,52 and smaller scatter of the measured values. The wear track study revealed that high graphene content in the Si3N4 matrix caused relatively big wear particles and an uneven wear track.