Properties Of Silicon Nitride Ceramics Research Articles

High-Temperature Properties of Silicon Nitride with Lu-Si-O-N Grain Boundary Phases

High-temperature properties of silicon nitride ceramics with Lu-silicon-oxynitride grain boundary phases were investigated. Si3N4 powder with 1.2 mol% (SN12) and 4.8 mol% (SN48) of Lu2O3 were gas-pressure hot-pressed at 1950°C for 2 h under 20 MPa in 1 MPa N2. SN12 consisted of elongated β-Si3N4 and a secondary phase, Lu4Si2O7N2, whilst SN48 consisted of elongated β-Si3N4 and Lu4Si2O7N2 + Lu2SiO5. At 1500°C and 1600°C, the stress-strain curve of SN48 was nonlinear, whilst that of SN12 was linear, indicating that SN12 broke as a brittle fracture at these temperatures. SN12 had excellent oxidation resistance and weight gain during the oxidation at 1500°C for 1000 h was 4 g/m2. Creep lifetime of SN12 at 1500°C under tensile stress of 137 MPa exceeded 1678.5 h.

Influence of Planetary High‐Energy Ball Milling on Microstructure and Mechanical Properties of Silicon Nitride Ceramics

The influence of ball‐milling methods on microstructure and mechanical properties of silicon nitride (Si3N4) ceramics produced by pressureless sintering for a sintering additive from MgO–Al2O3–SiO2 system was investigated. For planetary high‐energy ball milling, the mechanical properties of Si3N4 ceramics were evidently improved and a homogeneous microstructure developed. In contrast, some exaggerated elongated grains were developed due to the local enrichment of sintering additives in the specimen prepared by general ball milling. For Si3N4 ceramics produced by planetary ball milling, flexure strength of 1.06 GPa, Vickers hardness of 14.2 GPa, and fracture toughness of 6.6 MPa·m0.5 were achieved. The differences in the mechanical properties of Si3N4 ceramics produced by different processing seem to arise mainly from the changes in microstructural homogenization and sinterability. The planetary high‐energy ball‐milling process provides a good route to mix starting powders for developing ceramics with uniform microstructure and promising mechanical properties.

Influence of Additives and Hot-Press Sintering on Mechanical and Lipophilic Properties of Silicon Nitride Ceramics

Fe3O4 and Mo and were added to hot-press sintered Si3N4 ceramics to improve their lipophilic and mechanical properties. The bending strength, relative density, hardness and fracture toughness of Si3N4 ceramics with added Fe3O4 and Mo were improved by hot-press sintering compared with samples prepared with the pressureless process. In particular, the bending strength and relative density were improved by about 200 and 104%, respectively. Both the macro- and micro-lipophilicity of Fe3O4- and Mo-added ceramics were improved when prepared with the hot-press process, compared with those prepared with the pressureless process. This can be attributed to the addition of Fe3O4, and the formation of MoO3 and Si2N2O during hot-press sintering. The lower friction coefficient and high wear resistance of Fe3O4- and Mo-added ceramics have been achieved by applying the hot press process. The high wear resistance is considered to be due to the improvement of hardness and fracture toughness, which decreased the loss of particles during the wear test.

Microstructure and mechanical properties of silicon nitride ceramics prepared by pressureless sintering with MgO–Al 2O 3–SiO 2 as sintering additive

The microstructure and mechanical properties of pressureless sintered Si 3N 4 ceramics prepared both by conventional ball-milling and planetary high-energy ball-milling for a sintering additive from the MgO–Al 2O 3–SiO 2 system were investigated by XRD, SEM, TEM, HRTEM, and EDS. For pressureless sintered Si 3N 4 ceramics prepared by planetary ball-milling, a flexure strength of 1.06 GPa, Vickers hardness of 14.2 GPa, and fracture toughness of 6.4 MPa·m 0.5 were achieved. The microstructure of the sintered materials consists of elongated grains with almost identical size and aspect ratio that uniformly distributed throughout the body. The improved mixing efficiency by planetary ball-milling has simply reduced the particle size and improved the distribution of sintering additives, leading to a slightly improved densification and microstructural homogenization that are responsible for the promising properties of Si 3N 4 ceramics. In comparison with traditional ball-milling route, the planetary high-energy ball-milling is an efficient processing to produce high homogeneous microstructure and promising mechanical properties.

Tribological properties of silicon nitride ceramics modified by ion implantation

Surface modification by ion implantation has been carried out in order to improve the tribological properties of gas-pressure sintered silicon nitride ceramics. B +, N +, Si + or Ti + ions were implanted into the silicon nitride ceramics with a fluence of 2×10 17 ions/cm 2 at an energy of 200 keV. To evaluate the tribological properties of Si 3N 4, Block-on-Ring wear tests were conducted without lubricant, using the ion implanted Si 3N 4 as block specimens and commercially supplied Si 3N 4 as ring specimens. The specific wear rate of the ion-implanted Si 3N 4 could be drastically reduced to a value of 1.6×10 −9 mm 2/N, accompanied by a decrease in the friction coefficient in the initial stage. According to surface analyses it was considered that the high wear resistance and low friction coefficient are attributed to the amorphization and the increase of surface hardness of the ion implanted layer.

Hardness and Tribological Properties of Silicon Nitride Ceramics Implanted by Titanium Ions with MEVVA Sources

Hardness and Tribological Properties of Silicon Nitride Ceramics Implanted by Titanium Ions with MEVVA Sources

Preparation, Microstructure and Properties of Silicon Nitride Ceramics with Aligned Whisker Seeds

Preparation, Microstructure and Properties of Silicon Nitride Ceramics with Aligned Whisker Seeds

OS08W0211 A Study on the mechanical properties of silicon nitride ceramics sintered by 28 GHz millimeter-wave heating method

We study the application of the millimeter wave heating method to the silicon nitride, which includes Ytterbium oxide (Yb_2O_3) and alumina (Al_2O_3) as additives. Millimeter wave heating method has capability in enhancing densification of sintered bodies. As a result, we successfully obtained high-density sintered bodies with 5-wt% Yb_2O_3 and 3-wt% Al_2O_3. The sintering temperature was 1650℃, which was 200℃ lower compared with the conventional external heating method. Moreover, the annealing at 1220℃ with millimeter wave heating method after the sintering was proved to be effective. The sintered bodies showed excellent mechanical properties at room and high temperatures. The Vickers hardness was 7.5 GPa at 1400℃ in a vacuum, 17.4 GPa at room temperature in the air, and fracture toughness K_<IC> was 9.3 MPa m^<1/2> at room temperature in the air. These experimental results showed that the application of the millimeter wave heating method to the silicon nitride is very effective.

Microstructure and Mechanical Properties of Silicon Nitride Ceramics with Controlled Porosity

Porous silicon nitride ceramic with a porosity from 0–0.3 was fabricated by partial hot‐pressing of a powder mixture of α‐Si3N4 and 5 wt% Yb2O3 as sintering additive. Irrespective of the porosity, the samples exhibited almost the same microstructural features including grain size, grain aspect ratio, and pore size. Porosity dependences of Young's modulus, flexural strength, and fracture toughness (KIC) were investigated. All these properties decreased with increasing porosity. However, because of the fibrous microstructure, the decreases of flexural strength and fracture toughness were moderate compared with the much greater decrease of Young's modulus. Thus, the strain tolerance (fracture strength/Young's modulus) increased with increasing porosity. The critical energy release rate also increased slightly with an increasing volume fraction of porosity to 0.166 and remained at the same level with that of the dense sample when the porosity was 0.233. They decreased as porosity increased further.

Effects of α/β ratio in starting powder on microstructure and mechanical properties of silicon nitride ceramics

The effects of α/β ratio in the starting powder on microstructure development and mechanical properties of sintered silicon nitride ceramics were investigated. With increasing β-phase content, the microstructure became finer, the finest-grained microstructure was developed from the powder with pure β-phase, whereas a duplex microstructure was developed from high α-phase content powder. The results show that the microstructure of silicon nitride ceramics can be tailored by changing the α/β ratio in the starting powder. The mechanical properties have an optimum region where the α/β ratio equals 1. Both flexural strength and fracture toughness can be improved by controlling size, number, aspect ratio and size distribution of the rod-like β-Si 3N 4 grains

Effect of α/β phase ratio on microstructure and mechanical properties of silicon nitride ceramics

Highly densed silicon nitride ceramics with various α/β phase ratios were produced by pulse electric current sintering process. The β-phase content of Si3N4 in sintered materials varied from 20 to 100 wt% depending on the sintering condition. The microstructure was observed by scanning electron microscopy and investigated by image analysis. Young's modulus, hardness, fracture toughness, and strength were strongly dependent on the α/β phase ratio. The fracture toughness increased from 4.6 MPa m1/2 for 20-wt% b-phase content to 8.2 MPa m1/2 for 95-wt% β-phase content, and the fracture strength showed a maximum value of about 1.6 GPa at 60-to-80-wt% β-phase content.

Photothermal and photoacoustic measurement of thermal and thermoelastic properties of ceramics with residual stresses

The influence of residual stress on thermal, thermoelastic, and elastic properties of silicon nitride ceramics is analyzed. The results of the multimode experimental investigations based on photodeflection, photoreflectance, and photoacoustic piezoelectric measurements of Vickers indented ceramics are presented. It is shown that residual stress does not strongly influence thermal properties of silicon nitride ceramics. The modified Murnaghan nonlinear thermoelastic model is used for estimation of the nonlinear thermoelastic and elastic properties of silicon nitride ceramics.

234 28GHzミリ波加熱法により作製された窒化ケイ素セラミックスの機械的特性について(OS6-8 材料評価)(OS6 実験力学の新展開)

234 28GHzミリ波加熱法により作製された窒化ケイ素セラミックスの機械的特性について(OS6-8 材料評価)(OS6 実験力学の新展開)

“Special Boundaries” in Silicon Nitride With High Thermal Conductivty

Significant improvements in the fracture resistance, fracture toughness and thermal properties of silicon nitride ceramics are obtained by tailoring the microstructure. Combined use of seeding and tape casting techniques allowed the production of highly anisotropic microstructures. The seeded silicon nitrides exhibited a distinct bimodal microstructure, with large elongated β-Si3N4 grains, grown from seeds, dispersed within a fine-grained matrix. These large grains in the seeded silicon nitrides lie in the casting planes and self-align along the casting direction during tape forming process. It is here, when due to the high degree of alignment that “special boundaries” without the, otherwise, ubiquitous amorphous phase occurs. These “special” boundaries, hardly seen in three dimensionally random microstructures, are the object of the present study.Silicon nitride with high thermal conductivity of up to 120 W/mK (ref. 3) is produced by hot-pressing at 1800 °C for 2 h. powders with the following nominal composition: α-Si3N4 ;5 wt% Y203; 5 vol.% (β-Si3N4 seeds.

Effects of MgAl2O4-ZrO2 Addition on Sintering Behaviors and Mechanical Properties of Silicon Nitride Ceramics.

The influences of the sintering additive content of MgAl2O4-ZrO2 (total 5-15mass% with a MgAl2O4/ZrO2 ratio 1:1) on sintering behaviors, microstructure and mechanical properties of Si3N4 ceramics were investigated. Sintering was carried out in the temperature range of 1200 to 1900°C. Densification was enhanced with increasing sintering additive content due to an improved particle rearrangement; however, phase transformation and grain growth were significantly suppressed. The phase transformation and grain growth occurred through a solution-reprecipitation mechanism controlled by a diffusion process in this additive system. With the addition of a small amount of the above-mentioned additive mixture (5mass%), both high flexural strength and high fracture toughness could be obtained.

Influence of the Interfacial Properties on the Microstructural Development and Properties of Silicon Nitride Ceramics

ABSTRACTThe influence of the sintering additive system on the grain growth anisotropy of silicon nitride has been studied in the two supersaturated oxynitride glass systems M-Si-Al-O-N (where M = Yb, Gd, Nd, La, and Y) and M-Si-Mg-O-N (where M = Lu, Sm, La, Sc and Y). The glasses contained a low volume fraction of grains and offer the possibility to study grain growth without impingement. Both systems show an increase of the aspect ratio with an increasing cation radius of the rare earth elements. The aspect ratios for the Sc- and Y-based glasses are higher than expected from the ionic radius.Analytical transmission electron microscopy of the bulk glass and grain boundary film between flocculated particles reveals a difference in chemical composition. The larger rare earth cations are enriched in the film and the smaller ones are depleted compared to the bulk composition. The enrichment of the larger cations can be considered as an adsorption layer which reduces the growth rate of the prism planes. Glass systems with smaller cations (Yb, Gd) reveal a pronounced growth of the prism planes and sialon is formed in the outer region of the grains. The interfacial strength which determines the fracture toughness increases with a decreasing cation radius, but the effect is mainly attributed to the sialon formation rather than by the cation itself.

Fabrication and Mechanical Properties of Silicon Nitride Ceramics with Unidirectionally Oriented Rodlike Grains.

Silicon nitride with unidirectionally oriented elongated grains was fabricated by the combination of seeding and extrusion process. Development of such highly anisotropic microstructure is due to the high shear stress exerted during the extrusion process, which allowed the unidirectional alignment of rod-like seed particles. Small matrix grains were almost consumed by selective grain growth into larger grains grown from seed particles, consequently, large elongated grains with high aspect ratio were dominant in the microstructure of this material. Such microstructure development is related to the fact that the unidirectional alignment of seeds along a precise direction inhibits mutual impingement of elongated grains during growth along their length direction. Both high strength of about 1.4GPa and high fracture toughness of about 14MPa·m1/2 could be achieved by testing in the direction parallel to grain alignment.

Mechanical properties of oxidized silicon nitride ceramics

The effect of oxidation on the room-temperature mechanical properties of silicon nitride ceramics with four different compositions is studied. Room temperature values of bending strength and elastic modulus of the whole oxidized material and of hardness, the fracture toughness and elastic modulus of the internal region (without the strongly oxidized surface layer) are determined for the materials oxidized at five different temperatures up to 1400°C. The strength decreases with increasing oxidation temperature for all the materials investigated. The fracture toughness of the internal region decreases upon oxidation at 1000°C. A reduction of the elastic modulus of the materials oxidized at 1400°C is also observed. The oxidation-induced changes in the mechanical properties are partially explained by the changes in the phase composition.

Tribological properties of silicon nitride ceramics modified by titanium and subsequent oxygen implantation

A silicon nitride-based ceramic was implanted with 150 keV titanium to a fluence of 10 17 ion cm −2. Subsequently, the samples were implanted with oxygen using plasma immersion ion implantation (PIII) and conventional beam-line implantation to stimulate the formation of lubricating oxides in the surface. The wear is reduced for all implanted samples. The friction coefficient is slightly higher than in the nonimplanted case for all implantation conditions investigated, although oxide formation in the wear track is suggested by detection of a relatively high amount of oxygen in the wear track as indicated by energy-dispersive X-ray spectroscopy (EDX). We discuss the possible mechanisms that lead to the observed tribological behavior

High temperature creep behaviour of [formula omitted] ceramics

The improvement in high temperature mechanical properties of silicon nitride ceramics has been explored by fabrication of a β′-Si 3N 4 α-YSiAlON ceramic densified, without additives and without HIP treatment, by sintering of a mixture of α-Si 3N 4 and α- YSiAlON powders under a nitrogen pressure of 5 MPa. The high temperature creep behaviour was investigated in the ranges 100–400 MPa and 1280–1400 ° C by compressive tests. The creep performances were very impressive. The activation energy (Q = 770 kJ/mol) and the stress exponent ( n = 1) are consistent with a deformation mechanism resulting from grain boundary sliding accommodated by volume diffusion. Microstructure was not affected by the deformation conditions, in particular no cavity formation was observed after creep at 1400 °C and 300 MPa. This behaviour is related to the probable absence of a thick boundary film which may result from crystallization during cooling of small glass pockets and from internal stresses caused by the volume change of crystallizing pockets.