Nonaqueous gel casting using multicomponent concentrated slurries through Michael additive reaction for fabricating silicon nitride dense ceramics

A novel gel casting system using nonaqueous, multicomponent, and concentrated slurries was designed for manufacturing Si₃N₄ dense ceramics with complex structures. The proposed process involves the dispersion of reagent particles in α-terpineol aided by the saturated adsorption of polyethyleneimine partially complexed with oleic acid (PEI-OA) as well as the addition of small amounts of multifunctional acrylates (MAs). The occurrence of the Michael additive reaction between the MAs and the remaining amines of PEI-OA fixed on the reagent particles resulted in a gradual increase in the viscosity of the slurry until its solidification. Using the Si₃N₄/α-terpineol slurry as a model system, the effects of processing parameters such as OA complex ratio, particle volume fraction, and MA structures on the solidification behaviors were systematically investigated. A slurry having higher particle concentration which was treated with PEI-OA comprised from lesser OA ratio found to effectively solidify within 2 h when mixed with MA having larger number of acryloyl group per molecule. The proposed gel casting process was then applied to the multicomponent Si₃N₄–Y₂O₃–Al₂O₃–TiO₂–AlN/α-terpineol system. The gel casted complex structured green compacts were successfully densified up to 98% by liquid phase sintering at 1750 °C under 0.9 MPa N₂ gas pressure.     

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.     

Preparation and characterization of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics prepared by compression molding and slip casting methods

Porous silicon nitride (Si₃N₄) ceramics were fabricated by compression molding and slip casting methods using petroleum coke as pore forming agent, and Y₂O₃-Al₂O₃ as sintering additives. Microstructure, mechanical properties and gas permeability of porous Si₃N₄ ceramics were investigated. The mechanical properties and microstructure of porous Si₃N₄ ceramics prepared by compression molding were better than those which were prepared by slip casting method, whereas slip casting method is suitable for the preparation of porous Si₃N₄ ceramics with higher porosity and excellent gas permeability.     

High-strength silicon nitride ceramics obtained by grain-boundary crystallization

A monolithic type of toughened silicon nitride ceramics has been developed from Si₃N₄-Y₂O₃ systems. However, because of the existence of a second phase, the fracture strength decreases at elevated temperatures. To improve the high-temperature strength of silicon nitride, some additional components were investigated. It was found that the addition of hafnia to the Si₃N₄-Y₂O₃-AIN system gave a greater high-temperature strength based on the promotion of grain-boundary phase crystallization: namely, 126 kg mm⁻² in 3-point bend strength at 1300 °C for the hot-pressed specimen, and 90 kg mm⁻² at 1300 °C for the pressureless sintered specimen. The role of the hafnia in crystallization is not yet clear, and is being characterized by electron microscopy and microanalysis.     

Effects of sintering aids on the microstructure and mechanical properties of laminated Si3N4/BN ceramics

Laminated Si₃N₄/BN ceramics with two types of sintering aids, MgO–Y₂O₃–Al₂O₃ (MYA) and La₂O₃–Y₂O₃–Al₂O₃ (LYA), were fabricated through roll compaction and hot-pressing. Sintering aids influence evidently the microstructure and mechanical properties of laminated Si₃N₄/BN ceramics. In comparison with La₂O₃–Y₂O₃–Al₂O₃, MgO–Y₂O₃–Al₂O₃ sintering aid is easier to form a glassy phase with lower viscosity and lower eutectic temperature, which is much easier to migrate into BN interlayers. This results in the denser interlayer microstructure and good bending strength of laminated Si₃N₄/BN ceramics at room temperature, but poor work of fracture (WOF) at room temperature, low strength and work of fracture at elevated temperature. In addition, the LYA sintering aid is good for forming elongated and interlocked β-Si₃N₄ grains and beneficial to the mechanical properties of the laminated Si₃N₄/BN ceramics.     

Oxidation of sintered silicon nitride

The influence of oxidation at 1200 °C in air for up to 1000 h on the mechanical properties of two Si₃N₄-Y₂O₃-Al₂O₃ materials with different Y₂O₃/Al₂O₃ ratios, Material A (Si₃N₄-13.9 wt% Y₂O₃-4.5 wt% Al₂O₃) and Material B (Si₃N₄-6.0 wt% Y₂O₃-12.4 wt% Al₂O₃), was investigated. The oxidation significantly improves the high-temperature strength and fracture toughness of both materials, but more for Material A. After oxidation, Material A at 1300 °C retains 93% of its room-temperature strength and 87% higher than that before the oxidation. The oxidation has a different effect on the room-temperature K _IC for the two materials. The room-temperature Weibull modulus of Material A decreased by more than half while the 1200 °C Weibull modulus decreased slightly after oxidation. The annealing treatment prior to oxidation had no effect on the high-temperature strengths of the materials after oxidation. The effect of oxidation on mechanical properties is discussed in terms of the microstructure change of the materials.     

Intermediate temperature oxidation in silicon nitride yttria ceramics

Silicon nitride billets with both 4% and 8% Y₂O₃ additives have been subjected to oxidation treatments for up to 300 h, in air, in the temperature range 700 to 1000° C. Flexure strength and weight gain measurements together with both scanning and transmission electron microscopy and X-ray diffraction studies were conducted on these billets in an effort to understand the oxidation process. It appears that the degradation phenomena is associated with both the formation of phases outside the Si₃N₄-Si₂ON₂-Y₂Si₂O₇ compatibility triangle of the system Si₃N₄-SiO₂-Y₂O₃ and with the decomposition of W-containing phases at and near the grain boundaries.     

Heat conductivity,physico-mechanical properties and interrelations of them and structures of pressureless sintered composites produced of Si3N4-Al2O3-Y2O3(-ZrO2) nanodispersed system

The microstructures and properties of high-density materials produced by a pressureless sintering of nanodispersed powders of composition Si₃N₄-Al₂O₃-Y₂O₃(-ZrO₂) have been studied. It has been found that these self-reinforced materials of low glass phase content exhibit the bending strength R _bm = 720–850 MPa, Vickers hardness = 15.3–15.7 GPa, fracture toughness = 6.0–6.1 MPa·m^1/2, and heat conductivity = 15–25 W(m·K). The factors affecting physico-mechanical properties and heat conductivity of silicon-nitride materials have been discussed.     

Fabrication of Mg- and Ti-Doped Submicron-Grained Alpha-Alumina-Based Ceramics

Nanocrystalline alumina ceramics were prepared by magnetic-pulse compaction followed by pressureless sintering. The relative density of green compacts exceeded 0.7. The medium in which the starting powders were classified (gas, water, or ethanol) was shown to have a significant effect on their sinterability. The effects of Mg and Ti additions on the sintering behavior of Al₂O₃ were studied under optimized heat-treatment conditions. Mg-doped -Al₂O₃ ceramics containing uniformly distributed MgAl₂O₄ (4 wt %) and offering hardness values of up to 24 GPa were prepared. In the course of sintering, the grain size of these ceramics increases from 110 to 300 nm, and their relative density rises from 0.94 to 0.98.     

Dynamic compaction of silicon nitride powder

Dynamic compaction experiments were carried out on fine Si₃N₄ powder, that contained no additives, using maximum pressures of from 20 to 77 GPa. With pressures of from 20 to 64 G Pa the relative densities of the resulting Si₃N₄ compacts were the same: 96% of the theoretical density, but their microhardness values differed significantly. The optimum shock pressure for the Si₃N₄ powder with an initial density of 60% was near 44 G Pa. At this pressure, sintered Si₃N₄ compacts with a density of 96% of the theoretical density and a microhardness of 21.2 G Pa were obtained. However, at 64 G Pa, -Si₃ N₄ was transformed to -Si₃N₄ as a result of the high temperatures experienced during the compaction process. Because of this transformation, the microhardness of the compacted Si₃N₄ was reduced significantly.     

Compressive creep behavior of hot-pressed Si3N4-CRE2O3-Al2O3 ceramics

In this work, yttrium-rare earth oxide solid solution, CRE₂O₃, produced at FAENQUIL-DEMAR at a cost of only 20% of pure commercial Y₂O₃, was used as sintering additive of hot-pressed Si₃N₄ ceramics. The objective of this work was to characterize and to investigate the creep behavior of these ceramics. The samples were sintered by hot-pressing at 1750 °C, for 30 min using a pressure of 20 MPa. Compressive creep tests were carried out in air, between 1250 and 1300 °C, for 60 h, under stresses of 200-300 MPa. The stress exponent under all conditions was determined to be about unity. The apparent activation energy obtained was around 460 kJ mol⁻¹, corresponding to the heat of solution of the Si₃N₄ in the glassy phase. Both the stress exponent n and apparent activation energy Q are within the range of values reported in other studies of the compressive creep of Y₂O₃-Al₂O₃-doped-Si₃N₄ ceramics. X-ray diffraction (XRD) characterization shows a global reorientation of the β-Si₃N₄ grains and SEM observations detected no grain growth after the creep tests. These results indicate that grain-boundary sliding controlled by viscous flow is the dominant creep mechanism observed in the present study. The creep resistance presented of this samples indicates that this additive CRE₂O₃ can be a cheap alternative in the fabrication of Si₃N₄ ceramics, resulting in promising mechanical properties.     

Effects of oxidation on intergranular phases in silicon nitride ceramics

The effects of oxidation on changes in the secondary phases of two Si₃N₄ ceramics were investigated by transmission electron microscopy. The Si₃N₄ materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb₂O₃+0.5 vol% Al₂O₃, containing a Yb₂Si₂O₇ crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si₃N₄ material, sintered with 5 wt% Y₂O₃+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y₂Si₂O₇ accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.     

Sintering of Si3N4 in the presence of additives from Y2O3-SiO2-Al2O3 system

The sintering process of Si₃N₄ in the presence of a liquid phase from the Y₂O₃-SiO₂-Al₂O₃ system was investigated. The starting composition of liquid phase was varied according to data in the phase diagram of the Y₂O₃-SiO₂-Al₂O₃ system, in order to lower the temperature of liquid formation because it might exhibit an influence on the sintering behaviour of Si₃N₄. Densification as well as phase analysis were followed as a function of composition and the amount of liquid phase, both in the sintered and in hot pressed samples.     

Formation of SiC whiskers from silicon nitride

Attempts have been made to produce SiC whiskers through vacuum pyrolysis of Si₃N₄ without any addition of extraneous carbon. Vacuum pyrolysis of Si₃N₄ granules and powder compacts, has been carried out at 1550 and 1700°C using a graphite resistance furnace. The products of pyrolysis have been identified through XRD and SEM as SiC whiskers and particles. Small amounts of elemental silicon at 1550°C and free carbon at 1700°C have been detected through X-ray diffraction. Detection of elemental silicon through X-ray diffraction and solidified silicon droplets at the whisker tips in the SEM provide important clues regarding the mechanism of SiC_w formation, as the one involving the reaction 2Si(l) + CO(g) SiC(s) + SiO(g) Silicon carbide whiskers, 3–4 mm long, have been grown from Si₃N₄ compacts at 1550°C over a short period of 0.5 h. It has been shown in the present study that Si₃N₄ can be completely converted to SiC_w, when a loose bed of Si₃N₄ in the form of granules is pyrolysed in the presence of CO at about 1550°C.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.     

Stability of Si3N4-Al2O3-ZrO2 composites in oxygen environments

Oxidation of dense Si₃N₄-Al₂O₃-ZrO₂ and Si₃N₄-Al₂O₃ compacts, at 873–1773 K and 98 KPa air atmosphere, results in two different parabolic oxidation regimes. Oxygen diffusion is likely to be the governing step at low temperature (T<1623 K (H= 100kJ mol^–1)), whereas at T> 1623 K (H = 800kJ mol^–1) metal cation diffusion through the grain boundary phase appears limiting. The excellent stability in oxygen environments of the Si₃N₄-Al₂O₃-ZrO₂ composites compared to other ZrO₂-Si₃N₄ materials derives from (i) absence of easy-to-oxidize Zr-O-N phases; (ii) reduced amount of grain boundary phase, and possibly (iii) decreased solubilization rate of the nitride phases in the high viscous oxide film.     

Preparation and properties ofX-sialon

X-sialon has been produced by hot pressing Si₃N₄-Al₂O₃-SiO₂ powder mixtures and Si₃N₄ Powder-mullite gel mixtures at 1650 °C. The formation mechanism ofX-sialon has been studied and is correlated with the processing technique for the two preparation routes. Microprobe analysis of the obtainedX-sialon phase, combined with previous observed compositions and literature formulae suggests that, at 1650 °C,X-sialon exists as a narrow solid solubility region on the Si₃N₄-mullite line in the Si₃N₄-SiO₂-Al₂O₃-AIN phase diagram. The physical, mechanical and chemical properties ofX-sialon have been evaluated.X-sialon has a modest hardness of 1280 kg mm^–2, a fracture toughness of 1.7 Mpa m^1/2, and an elastic modulus of 213 GPa.X-sialon exhibits excellent chemical stability in contact with iron-based alloys at 1200 °C.     

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Abstract

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K^?1 upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m^?1 K^?1) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.     

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.     

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO₂ content on the oxidation of sintered bodies from the conventional Si₃N₄-Y₂O₃-Al₂O₃-AlN system was investigated. Sintered specimens composed of Si₃N₄, Y₂O₃, Al₂O₃, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO₂ in the range of 0 to 5 wt% to Si₃N₄, were fabricated at 1775 °C for 4 h at 0.5 MPa of N₂. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO₂ content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y₃Al₅O₁₂ and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y₂O₃·2TiO₂ (Y2T) appeared in their place and increased with increasing TiO₂ content. In those oxidized at 1400 °C, -cristobalite was dominant and very small amounts of Y₂O₃·2SiO₂ and Y2T were contained. There was a tendency for more -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO₂ formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO₂ decreases oxidation resistance.