Abstract: Silicon nitride samples without and with 3 wt% of the aligned b-silicon nitride whisker seeds were prepared with 8.2 wt% Er2O3 and 1.9 wt% AlN. After sintering at 2148 K for 4h, the samples exhibited densities higher than 99.5% TD. The microstructures and properties of the samples were compared with those of the samples sintered with 4.8 wt% Y2O3 and 2.2 wt% Al2O3 at 2273 K for
4h. For samples without the whiskers, the sample with 4.8 wt% Y2O3 + 2.2 wt% Al2O3 had coarser microstructures than those with with 8.2 wt% Er2O3 + 1.9 wt% AlN. However, the samples with the whisker seeds, the former sample appeared to have only slightly larger grains than the latter sample in spite of the significant difference in the sintering temperatures. For the samples without
the whisker seeds, the room temperature flexural strength was higher for the sample with Er2O3 + AlN. However, for the samples with the aligned whisker seeds, the sample with Y2O3 + Al2O3 exhibited higher room temperature flexural strength than that with Er2O3 + AlN although the average grain width of the former sample was larger than that of the latter sample. In case of the high temperature flexural strength at 1673 K, the flexural strengths of the samples with the whisker seeds were higher than double the strengths of the samples without the whisker seeds. For
samples without the whisker seeds, the sample with Er2O3 + AlN exhibited better mechanical properties than that with Y2O3 + Al2O3. However, for the samples with the aligned whisker seeds, the sample with Y2O3 + Al2O3 exhibited better mechanical properties than those with Er2O3 + AlN. The results were explained in terms of the microstructures of the samples.
Abstract: Silicon nitride (Si3N4) has been researched intensively because of superior mechanical properties up to high temperature. The mechanical properties of Si3N4 are strongly related to microstructure. The microstructure control of silicon nitride is well known to be a key issue for tailoring the mechanical properties of structural ceramics. This work was performed to reveal the effect of microstructure on dielectric properties at microwave frequency. Three starting powders were
used fine, course a-Si3N4 and b-Si3N4. Sintering additives, 5 wt.% Y2O3, 2 wt.% Al2O3 and 1 wt.% MgO were mixed with each starting powder. Si3N4 ceramic with different b/a phase specimen were obtained by hot pressing. The post-resonator method was used for the measurement of dielectric properties, dielectric constant (e′) and dielectric loss (tand), at microwave frequency range. Silicon
nitride ceramics show dielectric constant of 8.1 – 8.6 and dielectric loss 1.1 x 10-3 – 5.6 x 10-3. The effect of grain size and the role of phase on microwave dielectric properties are discussed.
Abstract: We investigated grain boundary crystallization of gas-pressure-sintered silicon nitride with zirconia and magnesia as sintering aids. Cation compositions were mostly uniform throughout the specimen however, ZrO2 was crystallized in the surface region, while ZrN in the inside. When the specimen was heat-treated at 1 atm nitrogen atmosphere, ZrO2 in the surface region transformed to ZrN. The transformation, however, was suppressed when alumina was incorporated as an additional sintering aid. Based on these results, we propose a model describing
the reaction among Si3N4, SiO2, ZrO2, ZrN and N2. Observed microstructures and measured mechanical properties were consistent with the model.
Abstract: M-Si-Al-O-N glasses (where M = Y or rare earth cation) are intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of these oxynitride glass phases determine the properties of the material, in particular, high temperature mechanical behaviour. Investigations on oxynitride glass formation and properties have shown that nitrogen increases the glass transition and softening
temperatures, viscosity, elastic modulus and hardness. By changing the cation ratios or the type of rare earth cation incorporated, properties such as viscosity can be increased further. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity. These effects have important implications for silicon nitride based ceramics where amorphous intergranular films control high temperature properties such as creep resistance.
Abstract: Sintered reaction bonded silicon nitride with aligned whisker seeds was prepared by tape casting silicon slurry with 5 wt% b-Si3N4 whisker seeds followed by nitridation and sintering. Three different sintering additives were used for the samples; 7 wt% Y2O3, 6 wt% Y2O3 + 1 wt% Al2O3 and 5 wt% Y2O3 + 2 wt% Al2O3. The sample with 5 wt% Y2O3 + 2 wt% Al2O3 showed the fastest a to b phase transformation after nitridation and the highest fracture toughness and flexural strength after gas pressure sintering among the samples. It also had finer microstructure than the
other samples after sintering at 2248 K and at 2273 K. The finer microstructure was related to the faster phase transformation after nitridation, which resulted in the higher flexural strength.
Abstract: Porous silicon nitride was prepared by extrusion of silicon followed by nitridation at 1723 K. PMMA spheres with 20 µm in diameter were employed as the pore-forming precursors. b-silicon nitride whiskers were added to the dough for extrusion and their effect on the properties of porous RBSN were examined. The nitridation rate that was obtained from the weight change of the sample due to the nitridation process was between 75% and 80%. However, XRD patterns of the samples after nitridation had no Si peak. That means the actual nitridation rate of the sample was higher than that obtained from the weight change. Porosity of the sample was between 45% and 55%. The XRD patterns from the surfaces of the samples with the silicon nitride whiskers parallel and perpendicular to the extrusion direction showed a slight anisotropy. The pore size distributions of the samples showed a highly populated pores smaller than 3 micrometer, especially for the
samples with the whiskers. The room temperature flexural strengths of the samples were between 25 MPa and 35 MPa, the sample with 5 wt% whiskers showing the highest value. The microstructures of the samples contained pores with about 100 micrometer in diameter as well as fine pores with a few micrometer in diameter. Closer observation of the fracture surface of the samples revealed that fine whiskers were inside the pores. A small honeycomb was fabricated by
reaction bonding of silicon.
Abstract: Silicon nitride based ceramics have been investigated since more than forty years.
Nevertheless, a good understanding of the microstructure-properties relationships on the level of small samples is not enough to bring ceramic parts into the industrial market. More convincing is a demonstration of the excellent potential in specific applications where the requirements of a more complex system are considered. In this work, sialons are investigated for a use in lubricated wear applications. The target system is a fuel injection pump for pressures up to 30 MPa. It requires very
small friction coefficients and wear rates. Isooctane was used as lubricating liquid. A friction coefficient of 0.3 has been achieved with a piston-on-plate configuration for two sialon ceramics with different a/b-ratios. The sample surfaces were in a ground condition. For comparison, tribological testing was also performed with commercial alumina and silicon nitride materials which exhibited higher friction coefficients. In order to be closer to the real conditions a test rig has been designed and constructed with a piston running in a cylinder that actually pumps isooctane at the desired pressures of up to 30 MPa. With it, the high potential of sialon ceramics for lubricated sliding applications was proven. In the fields of microstructural design neodymia and ytterbia containing sialons with varying a/b-sialon ratios and different amounts of additives have been investigated. These parameters have a strong effect on the achieved aspect ratios of the sialon grains which can be correlated to the amount of liquid phase during sintering, kinetic considerations and
the additives' cationic radii. Finally, the effect of these varying grain shapes on the mechanical properties hardness and fracture toughness has been determined.
Abstract: Y-SiAlON glasses of composition 36.5 Y: 42.3 Si: 21.2 Al with different amounts of N
(0, 5, 8, 15 and 22 in e/o) were produced by melting appropriate mixtures of powders under flowing nitrogen at 1715°C. This composition is known to give B-phase (Y2SiAlO5N) on crystallisation at temperatures below 1050°C. In this work, the effect of nitrogen in the starting glass composition on the crystalline phases formed is discussed. High temperature in-situ XRD analysis was performed on powdered glass samples up to 1150°C by using a Philips X’pert PRO MPD (Multi Purpose Diffractometer) with a HTK1200 Oven Camera (Anton Paar, Austria). As expected, the results show that different nitrogen contents affect the crystalline phases formed. In all glasses, yttrium apatite silicate forms first, followed by crystallisation of B-phase. The phase transformation from B-phase to Iw-phase (Y3Si2Al[O,N10] i.e. 10 e/o N) takes place at relatively low temperatures (1050°C) for the lower nitrogen containing samples (5 and 8 e/o), whereas, the transformation does not take place for the glasses with higher nitrogen contents even at the maximum temperature studied (1150°C). This work also confirms that there is a correlation between the temperature where the first crystals appear and the amount of nitrogen in the starting glass.
Abstract: The effect of glassy-phase, using AlN and Lu2O3 as sintering additives, on the
microstructure and mechanical properties of liquid-phase-sintered, and subsequently annealed SiC ceramics was investigated. The microstructure was strongly influenced by the sintering additive composition, which determines the intergranular phase (IGP). The average thickness of SiC grains
increased with increasing the Lu2O3 /(AlN + Lu2O3) ratio, whereas the average aspect ratio decreased with increasing the molar ratio. The homophase and heterophase boundaries of the SiC ceramics were completely crystalline in all specimens. The room temperature (RT) strength decreased with increasing the molar ratio whereas the RT toughness showed a minimum at the molar ratio of 0.6. The best results at RT were obtained when the molar ratio was 0.2. The flexural strength and fracture toughness of the ceramics were >700 MPa and ~6 MPa.m1/2 at RT. The high temperature strength was critically affected by the chemistry, especially the content of Al in the IGP. The best strength at temperatures ³ 1500oC was obtained when the molar ratio was 0.5. Flexural strengths of the ceramics
at 1500oC and 1600oC were 610 ± 80 MPa and 540 ± 30 MPa, respectively. The beneficial effect of the new additive compositions (Lu2O3-AlN) on high-temperature strength of SiC ceramics was attributed to the crystallization or removal of IGP and introduction of Al into SiC, i.e., removal or reduction of Al content from the IGP, resulting in an improved refractoriness of the IGP.