Abstract: Aqueous dispersions of SiAlON forming powders (Si3N4, AlN and Y2O3) were obtained.
Dispersibility of powders was found to be improved by addition of sodium tripolyphosphate. Stable
aqueous suspensions of SiAlON forming powders were prepared by using a phosphate ester,
sodium tripolyphosphate (STPP, Na5P3O10). Effect of the STPP on rheological properties of the
powder mixture was investigated by zeta potential measurements, sedimentation and milling
studies. It has been shown that addition of the STPP to the aqueous SiAlON forming powder
mixture improves the dispersion behavior significantly. These results suggest that STPP can be
utilized as a dispersant in such ceramic systems.
Abstract: In this study, textured α-SiAlON ceramic was produced by templated grain growth
process. In order to enhance the alignment of the seeds, gated tape casting arrangement was utilized.
Y-Ce codoped seeds were synthesized to induce the preferred orientation. For the production of the
seeds, Ce3+ was used as a sintering additive due to its well-known effect on the formation of
elongated α-SiAlON grains. In addition, Y3+ was used to stabilize Ce3+ in α-SiAlON structure.
Gated tape casting arrangement was also used to enhance the alignment of the seeds in the green
tape. After the lamination and sintering, preferentially aligned α-SiAlON grains were observed by
the microstructural investigation. In addition, XRD results obtained from the planes parallel and
perpendicular to the casting direction indicated the alignment of grains in α-SiAlON ceramics.
Hardness measurements also confirmed that the alignment of the grains results in 8% increase in
hardness value of the plane perpendicular to the casting direction with respect to that of the parallel
Abstract: In this study, the production of β-Si6-zAlzOzN8-z (z =3) powders by the carbothermal
reduction and nitridation of kaolin (Al2O3.2SiO2.2H2O) of Turkish origin (Can-Canakkale) was
carried out with different processing parameters. It was found that the morphology of the produced
SiAlON powder was mixture of irregular and whisker like grains. Therefore this morphology is
suitable for liquid metal infiltration since the powders are already high porous. The kaolin powder
containing stoichiometric rate carbon black and 30 % charcoal were pressed and reacted under
nitrogen flow (2 lt/min) between 1400-1475°C for 4 h. After a carbothermal reduction and
nitridation (CRN) process, porous β- SiAlON ceramic was produced from natural kaolin. Residual
carbon and charcoal in the produced ß-SiAlON ceramic were fired at 1000°C giving extra porosity.
The porous ß-SiAlON ceramics were sintered under N2 atmosphere at 1550°C for 2 h to make
preform for infiltration process. SEM image analyses were carried out to determine preform and
pore morphology and XRD analysis were performed for phase transformation.
Abstract: At temperature above 1200°C, the thermal stability of α-SiAlON phases has been
debated since 1992; however, it has been discussed if any α-SiAlON phase can be formed in Ce,
La, Eu and Sr-doped SiAlON systems. In our previous studies it was shown that the use of Mg-Ce
and Mg-Sr elements as dopants SiAlON compositions, in which all elements just have very low or
no stability in the α-SiAlON structure, would promote the stability of Mg-Ce elements in the α-
SiAlON phase [1, 2]. However, in Mg-Sr systems, it was obtained that Mg2+ is predominantly
incorporated in α-SiAlON structure whereas Sr2+ mainly remains in the grain boundaries . In this
study, by applying spark plasma sintering (SPS) (at 1400-1700°C) and post-sintering thermal heat
treatment (at 1500°C for 5 hrs and 1700°C for 2hrs) Mg or Mg-Sr doped SiAlON (50:50 mole
ratios) ceramics were prepared. The results were compared with GPS sintered samples data. The
effect of sintering temperature on densification process, phase transformation, microstructure and
mechanical properties of samples were investigated. The results showed that by using SPS, Sr-Mg
doped samples can be sintered at lower temperature (at 1600°C) than at GPS (at 1800°C) and it has
no Sr-doped grain boundary phases.
Abstract: In this study, effect of seed addition on the microstructural development of Y-Sm
α-SiAlON system has been investigated. Besides using pre-produced rod-like α-SiAlON seeds,
growth in the glassy phase and harvested by chemical treatment, powdered Y-Sm/α-SiAlON was
also used as nuclei. Microstructural results of both systems were compared with un-seeded samples.
It was found that the powdered α-SiAlON addition has also caused the acicular α-SiAlON grain
formation, which indicates the dominant effect of thermodynamic stability on the preferential
growth of α-SiAlON grains in the c direction.
Abstract: Silicon nitride samples were pressureless sintered with up to 5 w/o MgO to give densities
in the range 98-99% of theoretical. After pressureless sintering, selected samples were placed in a
vacuum heat treatment furnace surrounded by a carbon bed in a carbon crucible at a pressure of less
than 4x10-4 mbar, and vacuum heat treated at different temperatures and times to remove grainboundary
glass. The results showed that this was substantially achieved at 1575oC for 3h and that
increasing the time to 5 hours gave still further improvement. SEM images, EDX analysis and
oxidation tests provided additional evidence for the removal of Mg from the samples.
Abstract: The intergranular microstructure in a liquid phase sintered silicon nitride based ceramic
may be viewed as an oxynitride glass-ceramic. This work is concerned with the incorporation of
yttrium B-phase, which is a five-component phase, into the intergranular regions of silicon nitride
ceramics. The silicon nitride materials were fabricated with the addition of a powdered B-phase
parent glass with composition (e/o) 35Y:45Si:20Al:83O:17N, or the addition of a mixture of Y2O3,
SiO2 and Al2O3 with cation composition (e/o) 35Y:45Si:20Al. The starting powder mixtures
contained 10 wt% of sintering additives. Sintering for 2 h at 1800°C was followed by a two-step
post-densification heat treatment in order to promote nucleation and growth of yttrium B-phase.
Detailed imaging and elemental analysis of the intergranular regions was carried out by EDX in a
Abstract: 25α:75β SiAlON composition was designed with different cations and at different
molar ratios. Effect of the type of cations both on the composition and the type of intergranular
phase investigated after gas pressure sintering and further post sintering heat treatment.
Abstract: Multiwall carbon nanotube reinforced silicon nitride composites have been prepared by
hot isostatic pressing. A manufacturing process has been worked out to avoid the damage of
nanotubes during sintering. This method provides their preservation even in severe circumstances at
temperature 1700°C and gas pressure 20 MPa. As shown by scanning and transmission electron
microscopy after low and high pressure processing, carbon nanotubes have good adherence to the
silicon nitride grains. Moreover, carbon nanotubes have been found to be located not only at grain
surfaces, but in several cases they are well integrated with the silicon nitride grains. Composites
with higher strengths can be obtained by increasing the nitrogen gas pressure.