Fabrication Of Si3N4 Ceramics Research Articles

Fabrication of Si3N4 ceramics with adjustable microstructure and mechanical properties via Y2O3 contents in ZrN–AlN–Y2O3 ternary sintering additives

To adapt to the different application environments, Si3N4 ceramics are required to have different mechanical properties. Therefore, in this study, Si3N4 ceramics with adjustable mechanical properties were fabricated via using ZrN–AlN–Y2O3 ternary sintering additives and hot-pressing at 1650 °C for 30 min. Subsequently, the effect of the Y2O3 concentration on the phase assemblage, microstructure, mechanical properties, and crack propagation of Si3N4 ceramics was systematically investigated. The results demonstrated that Si3N4 ceramics presented adjustable microstructure and mechanical properties with the Y2O3 contents added in the range of 0∼8 wt%. With the increase of Y2O3 contents, the average aspect ratio of Si3N4 grains had an increase of 61.77%, increasing from 3.95 to 6.39. Besides, the Si3N4 grains and the formed YAG intergranular phase exhibited a fairly good combination at the nano-scale. Si3N4 grains and intergranular phase were the main influence factors on the mechanical properties of Si3N4 ceramics. As a result, the bending strength was from 700.99 ± 38.72 to 961.13 ± 36.84 MPa. The fracture toughness exhibited a range of 4.87 ± 0.11 to 6.10 ± 0.23 MPa⋅m1/2, while Vickers hardness achieved a high level of 16.09 ± 0.16 to 19.98 ± 0.39 GPa.

Fabrication of Si3N4 ceramics with high thermal conductivity and flexural strength via novel two-step gas-pressure sintering

Si3N4 ceramic substrates serving as heat dissipater and supporting component are required to have excellent thermal and mechanical properties. To prepare Si3N4 with desirable properties, a novel two-step gas-pressure sintering route including a pre-sintering step followed by a high-temperature sintering step was devised. The effects of pre-sintering temperature (1500 – 1600 °C) on the phase transformation, microstructure, thermal and mechanical properties of the samples were studied. The pre-sintering temperature played an important role in adjusting the Si3N4 particles’ rearrangement and α→β transformation rate. Furthermore, the densification process for the Si3N4 ceramics prepared via the two-step gas-pressure sintering was revealed. After sintered at 1525 °C for 3 h followed by a high-temperature sintering at 1850 °C for another 3 h, the prepared Si3N4 compact with a bimodal microstructure presented the highest thermal conductivity and flexural strength of 79.42 W·m−1·K−1 and 801 MPa, respectively, which holds great application prospects as ceramic substrates.

Synergistic effect of binary fluoride sintering additives on the properties of silicon nitride ceramics

A variety of combinations of YF3 and MgF2 were used as sintering aids in the fabrication of Si3N4 ceramics via gas pressure sintering (GPS). The synergistic effects of YF3 and MgF2 on the liquid viscosity, mechanical properties, thermal conductivities, and grain growth kinetics of the Si3N4 ceramics were investigated. The results showed that appropriately adjusting the YF3/MgF2 ratio could decrease liquid viscosity, reducing the diffusion energy barrier of the solute atom and promoting mass transfer. Meanwhile, the chemical bonding strength in the grain boundary complexions formed by the metal cations also influenced grain boundary migration. Samples doped with 4 mol% YF3 and 2 mol% MgF2 achieved the lowest grain growth exponent (n = 2.9) and growth activation energy (Q = 616.7 ± 16.5 kJ mol−1) as well as the highest thermal conductivity (83 W m−1 K−1) and fracture toughness (8.82 ± 0.13 MPa m1/2).

Microstructural control of green bodies prepared from Si-based multi-component non-aqueous slurries and their effects on fabrication of Si3N4 ceramics through post-reaction sintering

A Si-based slurry containing Si particles covered with Y2O3 and MgO nanoparticles (NPs) has been successfully prepared and then applied to shape Si-based green compacts for the fabrication of silicon nitride (Si3N4) ceramics via post-reaction sintering. It was found that Y2O3 and MgO NPs modified with polyethyleneimine-oleic acid complex (PEI-OA) could be effectively attached to Si particles by simple mixing in dense toluene slurry. Field emission scanning electron microscopy observations confirmed the attachment of PEI-OA-modified sintering aids to Si particles without forming large NP agglomerates. The adsorption of the PEI-OA-modified sintering aids and PEI-OA on the surface of Si particles drastically improved the stability of the Si-based toluene slurry, which was subsequently molded through wet vacuum casting and dewaxed to fabricate a Si-based green body. The obtained green body was nitrided at 1375 °C for 4 h at a N2 pressure of 0.15 MPa and further sintered at 1850 °C for 2 h at a N2 pressure of 0.9 MPa. The adsorption of sintering aid particles on the Si surface reduced the number of contact points between Si particles in the green body, which effectively suppressed the Si melting process during nitriding and improved the characteristics of the produced nitride body such as the degree of nitriding and α/(α+β) ratio of Si3N4, leading to the successful fabrication of high-density Si3N4 ceramics during the subsequent densification step.

Fabrication of Si3N4 ceramics by post-reaction sintering using Si–Y2O3–Al2O3 nanocomposite particles prepared by mechanical treatment

Post-reaction sintering of a powder compact of Si and sintering aids is a useful technique for fabricating silicon nitride (Si3N4) ceramics at low costs. In order to inhibit the inhomogeneous and uncontrollable exothermic nitridation of Si in the powder compact, Si–Y2O3–Al2O3 nanocomposite particles are designed as an aid for post-reaction sintering. These Si–Y2O3–Al2O3 nanocomposite particles are prepared via mechanical treatment applying high shear stress. Scanning electron microscopy (SEM) observations show that Y2O3 and Al2O3 particles are homogenously dispersed, and fixed to the Si particles. A green compact prepared using the Si–Y2O3–Al2O3 nanocomposite particles results in lower electrical resistivity than that prepared using a powder mixed by wet ball-milling, which suggests that Si particles in the green compact prepared using the nanocomposite particles are isolated by Y2O3 and Al2O3 particles. The isolation of Si particles by the sintering aids successfully prevents the Si particles from melting and agglomerating during the nitridation process, resulting in a higher nitridation ratio and higher α-Si3N4 phase content due to the inhibition of rapid heat transfer caused by the exothermic reaction. The nitridation ratio also increases with the applied power during mechanical treatment. As a result of firing the homogeneously nitrided powder compacts at high temperatures, Si3N4 ceramics with homogeneous microstructure and improved density are successfully fabricated in this manner.

Fabrication of Si3N4 Ceramics Using Raw Materials Prepared by Bead Milling

Fabrication of Si3N4 Ceramics Using Raw Materials Prepared by Bead Milling

Effect of Diluents on Post-Reaction Sintering of Silicon Nitride Ceramics

Post-reaction sintering as a technique for the fabrication of Si3N4 ceramics has received much attention as a cost-effective process due to the use of cheap Si powder as a raw material. In this method, the rapid exothermic nitridation of Si results in local melting of Si to cause its agglomeration, which is expected to be a flaw after densification. Therefore, control of the exothermic reaction is needed to improve the reliability of post-reaction sintered Si3N4 ceramics. In this study, Si3N4 ceramics were fabricated by post-reaction sintering with Si3N4 or SiO2 powders in order to control the exothermic reaction. As a result, the microstructure and bending strength of Si3N4 ceramics was changed by adding these additives. In particular, the addition of SiO2 resulted in the high strength of Si3N4 ceramics. Consequently, it was found that Si3N4 and SiO2 particles played the role of diluents, and SiO2 was effective in post-reaction sintering as an oxygen donor.