Abstract: The influence of two measuring conditions, the elapsed time after indentation and the
condition of edge of an indenter, on the indentation fracture toughness of silicon nitrides was assessed.
No slow crack-growth after unloading was confirmed by optical microscopic observation of a crack
tip induced by the indentation, which led to the negligible difference in fracture toughness measured
at 1 and 30 min after the indentation. Measurements with relatively new and used indenters gave
almost the same fracture toughness data, indicating that the crack lengths were hardly affected by the
slight damage of the corner of the indenter. It was suggested that the large scattering of the
indentation fracture toughness reported by the round-robin tests such as VAMAS was not originated
from these factors.
Abstract: SiC fabric including SiC (60 vol%)-Al2O3 (40 vol%) mixed powders and SiC (60
vol%)-Al2O3 (40 vol%) sheet of 60 μm thick were alternatively laminated and hot-pressed to 79 -
83 % of theoretical density under a pressure of 39 MPa in an Ar atmosphere at 1600 °C. Four cracks
were formed on the polished laminates with 30 ± 3 and 43 ± 2 vol% SiC fabric along the center line
perpendicular to the direction of length by Vickers indentor at the load of 98 N and healed in air at
1100 °C for 24 h. The strength (253 MPa) of the as-hot-pressed laminate without SiC fabric
decreased to 119 MPa after the introduction of the cracks but was recovered to 336 MPa after the
healing. The crack size decreased because of the oxidation of SiC particles during the healing.
However, the laminates with SiC fabric had a high damage tolerance and gave no change in the
strength (~100 MPa) after the introduction of the cracks. The SiC fabric prevented the propagation
of the cracks. After the healing, the porosity of the laminates decreased owing to the volume
increase during the oxidation of SiC particles and SiC fabric. The strength of the healed laminates
was comparable to that of as-hot-pressed laminates.
Abstract: The mechanical properties of brittle coating structures were characterized by various
indentation techniques. The adhesion properties of the coatings were evaluated by in situ scratch and
sphere indentation method. Physical vapor deposited TiN coatings on transparent substrates,
sapphire, were scratched by diamond cone indenter and in situ observed through the transparent
substrate. In situ scratch results reveal that the failure of coating is originated from the damage of the
substrate and the plastic deformation of substrate is a primary factor for determining the adhesion
breakage. The unique characterization technique for the strength measurement of brittle thin coating
has been developed. The strength of the thin coating was evaluated by the sphere indentation on the
trilayer structure. The CVD SiC coatings on graphite were characterized by the technique. It is
concluded that the microstructure of SiC coatings influences the strength. In this paper, the various
indentation technique were applied to evaluate the mechanical properties of TiN and SiC coatings and
the effect of microstructure on the reliability of the brittle coating system was discussed.
Abstract: In recent years, it has been increasing in demand for high performance CMCs for high
temperature application. CMCs are most promising materials for high temperature structural
materials of gas turbines for aerospace and power generation. Mechanical performances of CMCs
are highly dependent on properties of the reinforcement.
The oxygen free SiC fiber (Hi-Nicalon) has been commercially produced by an electron beam
curing process. And then the SiC fiber (Hi-Nicalon Type S) having stoichiometric SiC composition
and high crystallinity was developed. Hi-Nicalon fiber has higher elastic modulus and thermal
stability than Nicalon fiber. The Type S fiber has the highest elastic modulus and thermal stability
and excellent creep resistance in three types of Nicalon family fibers.
Recently, the Type S fibers as industrial products were developed and put on the market. The
Type S (industrial version) fibers had a high tensile strength of 2.8 GPa, a high elastic modulus of
390 GPa. The Type S fiber retained a tensile strength of 2.2 GPa and an elastic modulus of 390 GPa
after exposure at 1873 K. Moreover, the Type S fiber had outstanding creep resistance; the Type S
fiber showed higher stress relaxation ratio than many other ceramic fibers after thermal exposure
over 1673 K. The Type S fibers could be the best candidate for the reinforcement of CMCs. The
fibers can be supplied about 30 kg per a month at present.
Now, the Type S fiber/BN/SiC composites are being developed as the components of gas turbine
for aerospace and land based power generation such as shrouds and combustors.
Abstract: The atomic scale structure of amorphous Si-C-O ceramics fibers produced from the
pyrolysis of a polycarbosilane precursor has been investigated by X-ray diffraction using high-energy
synchrotron radiation at SPring-8. First peak in the total correlation function T(r) of the amorphous
and the heat-treated fibers is analyzed to consist of two contributions: Si-C (1.89 Å) and Si-O (1.61 Å)
bonds. The coordination number of C and/or O around Si is about four. This suggests that the Si-C-O
fibers basically have a network structure that consists of two tetrahedral units: SiC4 and SiO4. The
local chemical and structural orders vary continuously in the materials from the disordered network
structure of SiC4 and SiO4 tetrahedra (mixture of amorphous SiC and SiO2) to nanocrystals of SiC
and SiO2, through the ternary Si-C-O solid solution which is believed to have an intermediate
structure between the amorphous and crystalline states.
Abstract: Silicon carbide base ceramic tubes were synthesized from blend polymers of
polycarbosilane (PCS) and methylhydrogen silicon oil (H-oil) by polymer precursor method. This
precursor method consisted of melt spinning, thermal oxidation curing and pyrolysis processes. Pore
structure observed at cross sections of obtained tubes depended on H-oil content, melt-spinning
temperature and oxidation curing conditions. At 578K for melt-spinning, however, a considerable
amount of H-oil was decomposed during the spinning process. The resulting H-oil contents were
usually lower than the starting H-oil contents. In the case of the 578K melt spinning, however, unique
single pore structures were often observed in the tubes by adjusting the curing conditions. At 40% of
the H-oil content, large pores with thin walls were observed at the cross-section, while such structures
were difficult to be controlled. By reducing the melt-spinning temperature to 543K, the starting H-oil
contents could be maintained during the spinning process. The cross sections of the tubes often
showed multi pores in this case.
Abstract: A bimodal powder system of 800 nm SiC (75 vol%) - 30 nm SiC (25 vol%) was dispersed
at 20 vol% solid in a 0.3 M Y(NO3)3 solution containing 0.2 μm Al2O3 and 1.0 mg/m2 polyacrylic
acid (PAA: dispersant). The SiC (97.6 vol%)-Al2O3 (1.2 vol%)-Y2O3 (1.2 vol%)-PAA system
suspension was consolidated by casting in a gysum mold. Polytitanocarbosilane (PTC) of 3 vol%
was infiltrated into the SiC compact calcined at 800 °C to increase the mechanical properties and
Weibull modulus. Both the calcined powder compacts with and without PTC were hot-pressed to
relative density above 97 % at 1950 °C. The hot-pressed SiC with or without PTC provided the
following excellent mechanical properties: average four-point flexural strength of 911 and 812 MPa,
fracture toughness of 5.2 and 6.0 MPa·m1/2, and Weibull modulus 11.3 and 5.8 for PTC addition and
no addition, respectively. The PTC addition was effective to decrease the shape factor of flaw and
increased the strength and Weibull modulus.
Abstract: In this study, Tyranno SA fiber cloth was coated with carbon black and SiC powder
containing sintering aids by means of electrophoretic deposition method, and SiC/SiC composites
with three different fiber volume fractions were fabricated using the Tyranno SA cloth by
hot-pressing at 1700oC. The sufficient formation of the SiC matrix between each fiber could be
observed. The composite fractured in non-brittle manner, and bending strength decreased with
increasing fiber volume fraction. The crack propagation and fracture behavior depended on the fiber
volume fraction. These differences in bending strength and fracture behavior would be caused by
the difference in the interfacial bonding between fiber cloth and the matrix.
Abstract: In this paper, the dispersing abilities of three polyelectrolytes, poly (acrylic acid), poly
(acrylic acid-co-acrylate), and a four-member copolymer containing various anionic functional
groups were tested for aqueous Al2O3 suspensions. The influence of the dispersants on the surface
charge of the powder was evaluated by measuring the zeta potential in dilute Al2O3 suspensions. It
was found that all the three dispersants shifted the isoelectric point to a lower pH value. Rheological
measurements showed that the four-member dispersant decreased the viscosity in the widest pH
range, which should be ascribed to the synergistic effect of different functional groups. The 58vol%
concentrated Al2O3 slurry using the four-member dispersant was further consolidated using gel
casting and was fully sintered at 1600oC.
Abstract: Tungstate-based inorganic−organic hybrid nanobelts/nanotubes were synthesized in a
system of H2W2O7·xH2O/n-octylamine/heptane (n-octylamine:H2W2O7·xH2O molar ratio: 30), and
the effects of the volume ratios of heptane to n-octylamine and the amounts of interlayer water in
H2W2O7·xH2O on the formation behavior of the hybrids were investigated. The belt/tubelike hybrids
obtained were 10–20 +m in length and 200–500 nm in apparent diameter. Large volume ratios of
heptane to n-octylamine not only enhanced the degree of the long-range order of the lamellar
structures in the hybrids, but they also improved the morphologic uniformity of the hybrids. The
existence of interlayer water in H2W2O7·xH2O was indispensable to the formation of tungstate-based
inorganic−organic hybrid nanobelts/nanotubes. The amounts of interlayer water in H2W2O7·xH2O
varied over a wide range (x, from 0.85 to 4.1), had a neglectable effect on the morphology of the
tungstate-based nanophase hybrids, but exerted a remarkable influence on the rate of the reaction of
H2W2O7·xH2O with n-octylamine in the heptane solvent. The larger the amount of interlayer water,
the more rapid the reaction rate.