Effects of nitrogen pressure on properties of sintered reaction-bonded silicon nitride

We prepared sintered reaction-bonded silicon nitride ceramics by using yttria and magnesia as sintering additives and evaluated the effects of nitrogen pressure (0.1–1.0 MPa) on their microstructure, bending strength, fracture toughness, and thermal conductivity. The ratio of β phase in the nitrided compacts varied with the pressure and increased with increasing it. Since many β grains in the nitrided compacts were formed and interlocked each other with a stable three-dimensional structure which restricted the shrinkage during the sintering procedure, many pores remained in the sintered body. Under the middle pressure (0.3–0.5 MPa), the grains grew large because the number of formed nuclei was small. On the other hand, under the high pressures (0.8–1.0 MPa), the grains were relatively fine and uniform because of a large number of nuclei. Since the porosity and grain length depended on the nitridation mechanism, which was affected by the nitrogen pressure, the properties largely varied accordingly. The nitridation at 0.1 MPa gave the best properties in this study.     

Effects of nitridation temperature on properties of sintered reaction-bonded silicon nitride

We prepared sintered reaction-bonded silicon nitride ceramics by using yttria and magnesia as sintering additives and evaluated effects of the nitridation temperature on their microstructure, bending strength, fracture toughness, and thermal conductivity. The effects of the nitridation temperature were large, but different depending on the property. The ratio of β-phase in the nitrided compacts significantly increased with increasing the nitridation temperature, whereas their microstructures had no clear difference. Although the bending strength varied, it maintains a high value of 800 MPa. Fracture toughness was almost constant regardless the temperature. The thermal conductivity improved as the β-phase in the nitrided compact increases. This resulted in a decrease of the lattice oxygen content and increase of the thermal conductivity. Therefore, elevating the nitridation temperature and consequently the β-phase ratio should be a promising strategy for achieving compatibly high strength and high thermal conductivity, which are generally known to be in a trade-off relationship.     

Effects of yttria and magnesia on densification and thermal conductivity of sintered reaction-bonded silicon nitrides

A variety of combinations of Y₂O₃ and MgO were used as additives in preparing Si₃N₄ ceramics by the sintering of reaction-bonded silicon nitride (SRBSN) method. By varying the amount of Y₂O₃ in the range of 0-5 mol% and that of MgO in the range of 0-8 mol%, the effects of Y₂O₃ and MgO additives on nitridation and sintering behaviors as well as thermal conductivity were studied. It was found that appropriate amount and combination of Y₂O₃ and MgO additives were essential for attaining full densification and achieving high thermal conductivity. The sample doped with 2.5 mol% of Y₂O₃ and 5 mol% of MgO attained a thermal conductivity of 128 Wm⁻¹K⁻¹ when sintered at 1900°C for 6 hours, and the sample doped with 2 mol% of Y₂O₃ and 4 mol% of MgO achieved a thermal conductivity of 156 Wm⁻¹K⁻¹ when sintered for 24 hours.     

Effect of microstructures on dielectric breakdown strength of sintered reaction-bonded silicon nitride ceramics

Silicon nitride (Si₃N₄) was prepared from silicon by a sintered reaction-bonded silicon nitride method using yttria and magnesia as sintering additives. Post-sintering (PS) of nitrided compacts was carried out at 1850°C under a nitrogen pressure of 1 MPa. Effect of PS time on microstructure and dielectric breakdown strength (DBS) of the prepared Si₃N₄ ceramics was evaluated. The DBS was measured using specimens with four different thicknesses (0.30, 0.20, 0.10, and 0.05 mm) in order to examine the thickness dependence. The porosity of the sintered Si₃N₄ decreased by prolonging the PS time, and the full density could be achieved at the PS time of over 6 h. After full densification, rod-like β-Si₃N₄ grains grew up, and their maximum grain size increased from 45.1 to 154.7 μm by prolonging the PS time from 6 to 48 h. The DBS of the thick Si₃N₄ substrates (0.30 mm) showed little variation from 35.4 to 47.0 kV/mm, regardless of the PS time. On the other hand, that of the thin ones (0.05 mm) dramatically decreased from 99.5 to 9.8 kV/mm with increased the PS time from 6 to 48 h. Because the DBS sharply decreased at the thin substrate sintered for longer time in which some large-elongated grains might span the substrate thickness-wise throughout, it was inferred that the interface between β-Si₃N₄ grains and grain boundary phase/intergranular glassy films might be a path of the dielectric breakdown.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Enhanced thermal conductivity and flexural strength of sintered reaction-bonded silicon nitride with addition of (Y0.96Eu0.04)2O3

Sintered reaction-bonded silicon nitride (SRBSN) with high thermal conductivity was obtained using (Y_0.96Eu_0.04)₂O₃ and MgO as sintering additives. Green compacts were nitrided at 1400°C for 4 h. Post-sintering was carried out at 1850 and 1900°C for 4 h, respectively. In reaction-bonded silicon nitride (RBSN) doped with Y₂O₃ and MgO, the β-Si₃N₄ content and nitridation degree were 51.1% and 93.8%, respectively. However, the β-Si₃N₄ content and nitridation degree were 72.6% and 96.7% in a nitrided compact doped with (Y_0.96Eu_0.04)₂O₃ and MgO. After post-sintering, the phase composition, microstructure, mechanical properties, and thermal conductivity were investigated. After sintering at 1900°C for 4 h, the thermal conductivity of SRBSN doped with (Y_0.96Eu_0.04)₂O₃ and MgO was increased by 16.5% compared to that of the samples doped with Y₂O₃ and MgO. The highest hardness of 1639 HV and the good flexural strength of 776.4 MPa were also achieved in the sample doped with 2-mol.% (Y_0.96Eu_0.04)₂O₃ and 5-mol.% MgO.     

Improved thermal conductivity of sintered reaction-bonded silicon nitride using a BN/graphite powder bed

Sintered reaction-bonded silicon nitride (SRBSN) with improved thermal conductivity was achieved after the green compact of submicron Si powder containing 4.22 wt% impurity oxygen and Y₂O₃-MgO additives was nitrided at 1400 °C for 6 h and then post-sintered at 1900 °C for 12 h using a BN/graphite powder bed. During nitridation, the BN/10 wt% C powder bed altered the chemistry of secondary phase by promoting the removal of SiO₂, which led to the formation of larger, purer and more elongated Si₃N₄ grains in RBSN sample. Moreover, it also enhanced the elimination of SiO₂ and residual Y₂Si₃O₃N₄ secondary phase during post-sintering, and thus induced larger elongated grains, decreased lattice oxygen content and increased Si₃N₄-Si₃N₄ contiguity in final SRBSN product. These characteristics enabled SRBSN to obtain significant increase (∼40.7%) in thermal conductivity from 86 to 121 W ∙ m⁻¹ ∙ K⁻¹ without obvious decrease in electrical resistivity after the use of BN/graphite instead of BN as powder bed.     

High thermal conductivity of spark plasma sintered silicon carbide ceramics with yttria and scandia

A fully dense SiC ceramic with a room‐temperature thermal conductivity of 262 W·(m·K)^?1 was obtained via spark plasma sintering β‐SiC powder containing 0.79 vol% Y₂O₃‐Sc₂O₃. High‐resolution transmission electron microscopy revealed two different SiC‐SiC boundaries, that is, amorphous and clean boundaries, in addition to a fully crystallized junction phase. A high thermal conductivity was attributed to a low lattice oxygen content and the presence of clean SiC‐SiC boundaries.     

Optimization of the tape casting process for the development of high performance silicon nitride substrate

In this paper, tape casting of Si₃N₄ substrate were investigated and optimized. The effects of dispersant content, binder, plasticizer/binder ratio, and solid loading on the green sheet properties were studied. An optimal formulation for the tape casting slurries was proposed, green tape with homogeneous microstructure and higher relative density of 56.08% was developed. After gas‐pressure sintering and annealing, Si₃N₄ substrate with a relative density of above 99% and thermal conductivity as 58 W/m/K was obtained. Results showed that the combination of tape casting and gas‐pressure sintering is feasible for the development of Si₃N₄ circuit substrates for power electronic devices.     

Investigation on low-temperature sintered AlN nanoceramics with high thermal conductivity

The aim of this research was to investigate the effect of sintering additive and relatively low-sintering temperature on the thermal conductivity of aluminum nitride nanoceramic. While using nanosized AlN powder and liquid-phase sintering additives, the various sintering processes were performed at temperatures 1400 and 1500°C for several hours. In the analysis methods, thermal conductivity (K) and thermal diffusivity (α) were measured using thermal conductivity analyzer (Hot Disk), scanning electron microscope (SEM) was used to observe the surface morphology of the microstructure, x-ray diffraction analyzer (XRD) to analyze the grain size and crystal structure, Raman spectroscopy (Raman) emission spectrum was analyzed to identify the material microstructure and the densities of AlN specimens were measured by Archimedes method. It was found that the thermal conductivity is related to the densification of nanosize low-temperature sintered material, which can be controlled by additives and sintering temperature. With Y₂O₃ sintering add, the densification of AlN for low-temperature sintering increased by the factor of ～5% to ～12%, and the thermal conductivity was enhanced by 25%. The relative density observed in this research is about 78%-84%, and the thermal conductivity measured is in the range of 9-14 W/mK.     

Effect of β-Si3N4 seed particles on the property of sintered reaction-bonded silicon nitride

The effect of seeding on the microstructural development and mechanical properties of sintered reaction-bonded silicon nitride was investigated by the use of β-Si₃N₄ particles. Seeding of an appropriate amount of the β-Si₃N₄ particles gave rise to the promotion of density and the resulting increase in fracture strength and hardness. The fracture toughness was also increased due to the development of elongated grains. On the other hand, addition of a large amount of seed particles showed lower strength and fracture toughness owing to the coalescence of large elongated grains. By seeding of 2 wt.% β-Si₃N₄ particles on Si powders of 7 μm, high fracture strength of 1100 MPa and fracture toughness of 7.2 MPa·m^1/2 were obtained in sintered reaction-bonded silicon nitride ceramics.     

氮化硅反应烧结的研究进展

氮化硅作为高温功能陶瓷性能优越，但将其制备成陶瓷零件比较困难，目前一般用反应烧结法制备氮化硅陶瓷零件。此外，反应烧结制备氮化硅陶瓷还具有成本低、烧结温度低、产品成型好、陶瓷高温性能好等优点。综述了氮化硅陶瓷反应烧结工艺流程和工艺的优缺点，着重介绍了氮化硅反应烧结在成型工艺、烧结工艺、原材料影响、后处理和陶瓷增韧等方面所取得的进展。     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

Development of High Thermal Conductivity Aluminum Nitride Ceramic

AIN ceramics with densities varying from 3.18 to 3.30 g/cm³ and room-temperature thermal conductivities varying from 88 to 193 W/m K were produced. Different sintering conditions, packing powders, AIN powder sources, carbon additive, and sintering times were evaluated, and the key processing parameters which cause the differences in density and thermal conductivity were identified. SEM, TEM, and EDS were used to characterize the correlation between thermal conductivity, microstructure, and processing parameters. The important parameters which control the thermal conductivity of AIN ceramics are discussed.     

Controllable wear behaviors of silicon nitride sliding against sintered polycrystalline diamond via altering humidity

The tribological behaviors of silicon nitride (Si₃N₄) sliding against sintered polycrystalline diamond (PCD) were investigated by varying the relative humidity (RH) in the testing atmosphere. The results indicated that higher RH corresponds to higher wear loss of Si₃N₄ and the wear loss of PCD almost fell close to zero. Especially in the case of 85% RH, both a maximum wear loss of Si₃N₄ and a maximum friction coefficient were achieved. In addition, this study revealed insights into the interface chemistry effects on the wear behavior of Si₃N₄ under humidity. When water molecules were introduced into the testing atmosphere, the hydrolysis reaction occurred on the Si₃N₄ surface with the formation of the Si‐O‐Si bond across the sliding interface. And then, the hydration reaction dominated the process, during which Si‐OH was formed through the bond fracture of the Si‐O‐Si. The X‐ray photoelectron spectroscopy results showed that the ratios of Si‐OH/Si‐O and Si‐N/Si‐OH+Si‐O bonds increased as the relative RH levels increased. As a consequence, the wear loss of Si₃N₄ significantly increased. Thus, due to the hydrolysis and hydration reactions, the tribological behaviors of Si₃N₄ against sintered polycrystalline diamond can be essentially controlled via varying RH levels.     

Effects of Y2O3/MgO ratio on mechanical properties and thermal conductivity of silicon nitride ceramics

In this work, the effects of Y₂O₃/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y₂O₃/MgO. It was found that a low Y₂O₃/MgO ratio facilitated the densification of Si₃N₄ ceramics while a high Y₂O₃/MgO ratio benefited the phase transformation of Si₃N₄ ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m^1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y₂O₃/MgO ratio of 3:4 by sintering at 1900°C for 4 h.     

Evaluation of tribological behavior of silicon carbide and silicon nitride ceramics under different conditions

A comparative analysis of the tribological behavior of commercially available sintered silicon carbide (SiC) and three different types of silicon nitride (Si₃N₄) ceramics have been carried out using the ball-on-disk method in dry and lubrication (deionized [DI] water and ethanol) environment. Scanning electron microscopy (SEM) was used to understand the morphology and chemical composition of the tribo-surfaces. Sintered SiC (Hexoloy-SA) had the highest friction coefficient during dry sliding with an average of ∼0.34. Deionized water showed a minor improvement in friction (∼0.27) while ethanol reduced the friction greatly to ∼0.18 compared to dry sliding. During dry sliding, the presence of an abrasive third body was responsible for the high wear rates (WRs) in these compositions. Hexoloy-SA showed a lower WR during ethanol and DI water lubrication due to the formation of stable tribofilms as well as higher hardness which resisted the formation of third bodies. In comparison, Si₃N₄ samples showed a lower WR in DI water and ethanol. The samples also showed composition-dependent behavior which indicates that grain structure and grain boundary chemistry are playing a vital role in the tribological process.     

The quantitative investigation of the lattice oxygen and grain edge oxygen on the thermal conductivity of aluminum nitride ceramics

The good thermal conductivity of AlN is essential for insulation and high heat dissipation applications. However, the influence of oxygen impurities at various locations (the lattice oxygen and grain edge oxygen) on the thermal resistivity of AlN ceramics is unclear. In this study, AlN ceramics with various oxygen distributions are prepared by different methods, and the oxygen contents of different regions are distinguished. The results indicate that the lattice oxygen is the main factor affecting thermal resistivity. Meanwhile, high-temperature annealing and pre-sintering processes can lower the lattice oxygen content from 0.061 wt% to 0.038 wt% and 0.036 wt%, respectively. Additionally, when grain edge phase volume is less than 4 vol%, it does not contribute significantly to thermal resistivity. The main formation of thermal resistance changes from phonon-defect scattering to phonon-phonon scattering with increasing temperature. These results may be informative for the microstructure design of AlN ceramics with high thermal conductivity.     

ZrSi2–MgO as novel additives for high thermal conductivity of β-Si3N4 ceramics

A novel ZrSi₂–MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900°C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi₂ in the range of 0-7 mol%, the effect of ZrSi₂ addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi₂–MgO were effective for the densification of Si₃N₄ ceramics. XRD observations demonstrated that ZrSi₂ reacted with native silica on the Si₃N₄ surface to generate ZrO₂ and β-Si₃N₄ grains. TEM and in situ dilatometry confirmed that the as formed ZrO₂ collaborated with MgO and Si₃N₄ to form Si–Zr–Mg–O–N liquid phase promoting the densification of Si₃N₄. Abnormal grain growth was promoted by in situ generated β-Si₃N₄ grains. Consequently, compared to ZrO₂-doped materials, the addition of ZrSi₂ led to enlarged grains, extremely thin grain boundary film and high contiguity of Si₃N₄–Si₃N₄ grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W·(m·K)⁻¹ when ZrO₂ was substituted by ZrSi₂.