Extra-Large Grains in the Silicon Nitride Ceramics Doped with Yttria and Hafnia

Microstructural development of silicon nitride doped with Y₂O₃ and HfO₂ was investigated to determine how extra-large grains develop during gas pressure sintering. Grains as long as 200 µm and a few tens of micrometers wide developed at high temperatures (>2173 K) in a fine-grained matrix containing a limited amount of liquid which had a poor propensity to spread. A small number of grains could grow quite large before their growth was halted by neighboring grains of comparable size.     

Control of Electrical Resistivity in Silicon Carbide Ceramics Sintered with Aluminum Nitride and Yttria

The electrical properties of β‐SiC ceramics were found to be adjustable through appropriate AlN–Y₂O₃ codoping. Polycrystalline β‐SiC specimens were obtained by hot pressing silicon carbide (SiC) powder mixtures containing AlN and Y₂O₃ as sintering additives in a nitrogen atmosphere. The electrical resistivity of the SiC specimens, which exhibited n‐type character, increased with AlN doping and decreased with Y₂O₃ doping. The increase in resistivity is attributed to Al‐derived acceptors trapping carriers excited from the N‐derived donors. The results suggest that the electrical resistivity of the β‐SiC ceramics may be varied in the 10⁴–10^?3 Ω·cm range by manipulating the compensation of the two impurity states. The photoluminescence (PL) spectrum of the specimens was found to evolve with the addition of dopants. The presence of N‐donor and Al‐acceptor states within the band gap of 3C–SiC could be identified by analyzing the PL data.     

Role of the Powder Bed in the Densification of Silicon Carbide Sintered with Yttria and Alumina Additives

The effect of a powder bed on the densification behavior of a compact made from a silicon carbide powder and containing yttria and alumina additives has been studied. It has been found that where the powder bed is made of just silicon carbide grit, relative densities of up to 0.92 of the theoretical density can be obtained. Additions of alumina to the powder pack increased the apparent relative density to 0.98. Further experiments have shown that the pack cannot act as a physical barrier to the diffusion of volatile components of the sintering aids. The alumina additions to the pack increase the relative density by allowing the diffusion of alumina species into the sample, rather than by affecting its shrinkage behavior. It is suggested that the predominant effect of the pack is to ensure that the partial pressure of silicon-containing species, such as SiO and Si, within the sample is sufficient to allow reaction between them and the volatile sub-oxides of yttrium and aluminum, forming a liquid phase.     

Microstructural Design of Silicon Nitride with Improved Fracture Toughness: II, Effects of Yttria and Alumina Additives

Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated ß-Si₃N₄ seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R -curve and extensive interfacial debonding between the elongated ß-Si₃N₄ grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the ß´-SiAlON growth layer formed on the elongated ß-Si₃N₄ grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the ß´-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct ß´-SiAlON growth layer forms. It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed.     

Microstructural Evolution during Sintering of Aluminum Nitride Ceramics Doped with Alumina and Yttria

The microstructural evolution of AlN sintered at >1950°C was studied in a specimen doped with 10 wt% Al₂O₃ and 5 wt% Y₂O₃. The constituent phases of the specimen were AlN, YAG, γ-AlON, and AlON polytypoids (compositional polytypes). Transmission and scanning electron microscopy revealed the microstructural characters: platelike 7AlN·Al₂O₃ first crystallized with concurrent formation of a residual liquid, then spherical AlN crystals formed. The liquid itself changed composition with the progress of the crystallization and reached the eutectic composition in the pseudobinary system AlN–YAG, and crystallized to an aggregate of AlN and YAG during cooling. As a product of the reaction of 7AlN·Al₂O₃, γ-AlON was formed.     

Indentation Crack Length Anisotropy in Silicon Nitride with Aligned Reinforcing Grains

Vickers indentation was performed on surfaces of silicon nitride with an aligned microstructure in order to study the interaction between cracks and the microstructure. Although there was not much evidence of crack bridging, the transverse radial cracks were very short, resulting in high fracture toughness values. The longitudinal radial cracks tended to propagate along the grain boundary of the reinforcements and were much longer than the transverse cracks. As the sintering temperature increased, the lateral cracks on the casting surface led to spalling and consumed more energy for the crack formation, making the longitudinal cracks shorter. On the surface normal to the alignment direction, there was no spalling and the indentation cracks became longer as the sintering temperature increased.     

Si3N4陶瓷烧结中烧结助剂的研究进展

研究了多种烧结助剂对氮化硅烧结性能和烧结过程的影响.研究表明,多组分助剂比单一组分助剂对氮化硅的助烧效果好,其中稀土氧化物和MgO-Al2O3-SiO2体系比较受重视.     

Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina

The microstructure of two hot-pressed silicon nitrides containing Y₂O₃ and Al₂O₃ was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y₂O₃+2 wt% Al₂O₃, as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y₂O₃+5 wt% Al₂O₃ was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si₃N₄ grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y₂O₃/Al₂O₃ fluxing.     

Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride: (II) Effects of Magnesium Compound and Yttria Additives

The effects of the magnesium compound and yttria additives on the processing, microstructure, and thermal conductivity of sintered reaction-bonded silicon (Si) nitride (SRBSN) were investigated using two additive compositions of Y₂O₃–MgO and Y₂O₃–MgSiN₂, and a high-purity coarse Si powder as the starting powder. The replacement of MgO by MgSiN₂ leads to the different characteristics in RBSN after complete nitridation at 1400°C for 8 h, such as a higher β-Si₃N₄ content but finer β-Si₃N₄ grains with a rod-like shape, different crystalline secondary phases, lower nitrided density, and coarser porous structure. The densification, α→β phase transformation, crystalline secondary phase, and microstructure during the post-sintering were investigated in detail. For both cases, the similar microstructure observed suggests that the β-Si₃N₄ nuclei in RBSN may play a dominant role in the microstructural evolution of SRBSN rather than the intergranular glassy chemistry during post-sintering. It is found that the SRBSN materials exhibit an increase in the thermal conductivity from ∼110 to ∼133 (Wm·K)⁻¹ for both cases with the increased time from 6 to 24 h at 1900°C, but there is almost no difference in the thermal conductivity between them, which can be explained by the similar microstructure. The present investigation reveals that as second additives, the MgO is as effective as the MgSiN₂ for enhancing the thermal conductivity of SRBSN.     

Push-Pull Fatigue of Sintered Silicon Nitride Ceramics at Elevated Temperatures

The fatigue tests under push-pull completely reversed loading and pulsating loading were performed for silicon nitride ceramics at elevated temperatures. Then the effects of stress wave form, stress rate, and cyclic understressing on fatigue strength, and cyclic straining behavior, were examined. The cycle-number-based fatigue life is found to be shorter under trapezoidal stress wave loading than under triangular stress wave loading, and to become shorter with increasing hold time under the trapezoidal stress wave loading. Meanwhile, the equivalent time-based life curve, which is estimated from the concept of slow crack growth, almost agrees with the static fatigue life curve in the short and intermediate life regions, showing the small cyclic stress effect and the dominant stress-imposing period effect on cyclic fatigue life. The fatigue strength increased in stepwise stress amplitude increasing test, where stress amplitude is increased stepwise every given number of stress cycles, at 1100° and 1200°C. Occurrence of cyclic strengthening was proved through a gradual decrease in strain amplitude during a pulsating loading test at 1200°C in this material, corresponding to the above cyclic understressing effect on fatigue strength.     

Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot-Pressed with Alumina and Yttria

Commercial silicon nitride powder with A1₂O₃ and Y₂O₃ additives was hot-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α-Si₃N₄. The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was β-sialon (Si_6−zAl_zO_zN_8−z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

Microhardness Load Size Effect in Individual Grains of a Gas Pressure Sintered Silicon Nitride

Vickers microhardness as a function of the indentation load and of grain orientation was studied in individual grains of a gas pressure sintered (GPS) polycrystalline silicon nitride using indentation loads in the range from 1 to 50 g. The microhardness values are lower than the microhardness of ß-Si₃N₄ single crystals, which is probably caused by the softening of the silicon nitride lattice by the presence of oxygen and aluminum atoms. The indentation load size effect (ISE) was more evident in the prismatic planes of the grains. The threshold load for indent formation is below 1 g for both basal and prismatic planes of Si₃N₄ grains in the studied GPS silicon nitride.     

Dispersion Methods of Yttria in Silicon Nitride by Comminution

Sintering additives were incorporated into Si₃N₄ by attrition and ball milling using both Si₃N₄ and Al₂O₃ media. Dispersion of Y₂O₃ was observed by backscattered electron imaging. Attrition milling for only 15 min using an Si₃N₄ medium, was equivalent to 24 h of ball milling. Minimal contamination by the Si₃N₄ was encountered. [Key words: silicon nitride, yttria, comminution, sintering, dispersion.     

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y₂O₃ addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 10¹ Ω·cm) that is larger by a factor of ~10² compared to that (1.3 × 10^?1 Ω·cm) of a baseline specimen sintered with Y₂O₃ only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10^?1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10^?2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.     

Oxidation of Silicon Nitride Sintered with Rare-Earth Oxide Additions

The effects of rare-earth oxide additions on the oxidation of sintered Si³N⁴ were examined. Insignificant oxidation occurred at 700o and 1000oC, with no evidence of phase instability. At 1372oC, the oxidation rate was lowest for Y203 and increased for additions of La₂O₃, Sm₂O₃, and CeO₂, in that order. Data obtained from X-ray diffraction, electron microprobe analysis, and scanning electron microscopy indicate that oxidation occurs via diffusion of cationic species from Si₃N₄ grain boundaries.     

Silicon Oxynitride Ceramics Prepared by Plasma Activated Sintering of Nanosized Amorphous Silicon Nitride Powder without Additives

Si₂N₂O ceramics were prepared by plasma activated sintering using nanosized amorphous Si₃N₄ powder without sintering additives within a temperature range of 1400°C–1600°C in vacuum. A mixed Si–N_4?n–O_n (n = 0, 1…4) amorphous structure was formed in the process of sintering, and Si₂N₂O crystals were nucleated where the local structure was similar with Si₂N₂O. After sintering at 1600°C, the Si₂N₂O ceramic was composed of elongated plate‐like Si₂N₂O grains and amorphous phase. The Si₂N₂O grains showed a width of less than 100 nm and a very high aspect ratio.     

Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina

The fracture-toughness-determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness.     

Sintering Silicon Nitride Ceramics in Air

Silicon nitride (Si₃N₄) ceramics, prepared with Y₂O₃ and Al₂O₃ sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi₂ element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y₂O₃ and 3 wt% Al₂O₃. Densification is accompanied by a small weight gain (typically <1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si₃N₄ transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si₂N₂O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y₂Si₂O₇ being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si₃N₄ elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si₃N₄-based ceramics.     

Creep Behavior of a Sintered Silicon Nitride

A commercial sintered silicon nitride has been crept in bending and compression at temperatures of 1100°C to 1400°C. In the as-sintered condition the material contains an amorphous intergranular phase. This phase undergoes partial devitrification as a result of high temperature exposure. Preannealing the material to a stable microstructure has very little effect on the creep properties. Deformation behavior compares well with that predicted from a model for creep due to viscous flow of a non-Newtonian grain boundary phase. In bending, the model predicts an initial constant strain rate at low strains as the intergranular phase is squeezed out from between grains under compression. Samples crept in compression are not expected to have this same initial constant strain rate regime. The model also predicts a strong initial strain rate dependence (in bending) on the initial thickness of the amorphous grain boundary layer. Experimentally this strain rate is not affected by partial grain boundary crystallization, suggesting that partial devitrification does not alter the intergranular film thickness or viscosity. This is supported by transmission electron microscopy, which has shown that crystallization of the intergranular phase occurs largely in the pockets between grains, leaving amorphous films between grains.