Abstract: Alumina/zirconia nano-composite coating was fabricated by plasma spraying using
agglomerated feedstock from fine powders of about 100 nm. The coating was consisted of fine
γ-alumina and zirconia crystals with size of several nano meter and some amorphous boundary layers.
The amorphous phase was crystallized and disappeared after heat treatment at 930°C. However, the
crystallite size was kept under 50 nm even after 1500°C-100hr heating, so the alumina-zirconia
nano-composite showed good thermal stability against the grain growth.
Abstract: We have been trying to find new oxide compounds with large thermal
expansion coefficients and low thermal conductivities by means of a material
calculation technique. Among thousands of compounds in the databases, we found that
there were some materials with low thermal conductivities and large thermal expansion
coefficients in the group of strontium-niobium oxides. For example, Sr4Nb2O9 has a
thermal expansion coefficient of 14.510-6 / and thermal conductivity of 1.0 W/mK,
although a slight amount of other phases appear during long-term annealing. These
thermal properties are better than those of yttria-stabilized zirconia, which is the
standard material for thermal barrier coatings. To prevent the precipitation of other
phases, we prepared the solid solutions, Sr4Nb2-xMxO9. In this study, the thermal
conductivities and thermal expansion coefficients of these solid solutions were
measured, and their thermal stabilities were evaluated by long-term annealing.
Abstract: Thermal conductivity of pure and Y2O3-doped ZrO2 was calculated using a perturbed
molecular dynamics method in order to analyze phonon scattering mechanism which is responsible
for the reduction of thermal conductivity. Although absolute values of thermal conductivity were
overestimated due to a simple model used in this study, relative values were in good agreement with
experiment, which indicates that phonon scattering due to Y2O3 addition is reproduced well. It is
found from quantitative analysis of the phonon scattering using the mean field theory that decrease
of the thermal conductivity upon Y2O3 addition is attributed not only to the introduction of O2-
vacancies but also to substitution of Y3+ ions for Zr4+ ions.
Abstract: The effects of thermal fatigue conditions on the mechanical and contact damage behavior
in thermal barrier coatings (TBCs) are investigated as functions of the bond coat thickness and the
preparation method. Three kinds of TBCs with different thickness in the bond coat are prepared by
two different methods of APS and HVOF. The static and cyclic thermal fatigues for the TBCs are
conducted at temperatures of 950 and 1100 with different dwell times of 10 and 100 hr and 10
cycles at each temperature. Mechanical properties, hardness H and modulus E, in each condition and
component are measured by nano-indentation. The contact damage behaviors are investigated by
Hertzian indentation, including the cyclic fatigue behavior on the surface of the TBCs. The TGO
formation is dependent on both temperature tested and time exposed, showing a similar effect with
the cyclic thermal fatigues. The mechanical properties of the TBCs are increased due to the
re-sintering of the top coating and the composition change of the bond coat during the thermal
fatigues. The contact damage behaviors are affected by the thermal fatigue conditions and the
fabrication method, independent of the bond coat thickness.
Abstract: Nanostructured and conventional Al2O3, ZrO2, and TiO2 were deposited using an
atmospheric plasma spraying (APS). The size of commercial nano-ceramic powders was varied
from 5nm up to 150nm. The microstructure and phase composition of the plasma sprayed coatings
on metallic substrate were characterized by X-ray diffraction (XRD), scanning electron microscopy
(SEM), and transmission electron microscopy (TEM). It was found that nano-sized ceramic
powders enhanced the deposition efficiency on the metallic substrate rather than the micro-sized
conventional commercial powders. Density and mechanical property such as microhardness were
better in the case of the nano-sized ceramic powders than that of the conventional micro-sized
ceramic powders, which are associated with the fine surface roughness and less size in pores of the
coating layers. The wear rate of the nanostructured coating was lower than that of the conventional
coating. The results were explained in terms of their microstructure of the coatings layers. Also,
photocatalytic characterization of the plasma sprayed coatings, using nanocrystalline size TiO2 as
feedstock with various powder sizes and shapes as well as adding with different photocatalytic
oxides, was performed. The photocatalytic reactivity using plasma sprayed coating layers can be
utilized into various applications.
Abstract: HVOF thermal spraying has been developed to deposit dense Al2O3-coatings for improved
protective properties. As compared to generally used plasma sprayed coatings HVOF coatings can be
prepared much denser and thus are better suited for applications where protective properties of the
coating are needed. In this paper we describe the development of HVOF spraying technologies for
nanocrystalline Al2O3- and Al2O3-Ni-coatings. The microstructure and the mechanical properties of
these novel coatings are reported and compared to a conventionally processed Al2O3-coating.
Abstract: Special mechanical properties have widely been demonstrated with bulk nanocrystalline
materials. An increasing effort has been made to transfer such improvements also into thermal
sprayed ceramic coatings. This paper focuses on such efforts in alumina-based ceramic coatings. The
optimization of process conditions and effect of different process parameters on the mechanical
performance of high velocity oxy-fuel (HVOF) sprayed ceramic coatings is discussed.
Abstract: SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of the most promising
materials for high temperature structural applications such as power generation and propulsion
systems. SiC/SiC composites are, however, susceptible to accelerated attacks in water vapor
environments through oxidation and volatilization reaction. For protection from such attacks,
Environmental Barrier Coatings (EBCs) are indispensable.
We have investigated some oxides and rare-earth silicates as topcoat candidate materials for
EBCs. Topcoat materials must be stable in the high-water-vapor pressurized environments at high
temperatures. Also, it is important that the thermal expansion coefficient of topcoat materials is
similar to that of the SiC/SiC composites.
In this study, first, zirconium oxides, lutetium silicates and yttrium silicates were selected as
topcoat candidate materials. They were exposed in a water-containing atmosphere at a temperature of
1673 K for 100 h under a total pressure 0.96 MPa. Mass changes, structure of crystals and
microstructures were investigated after the exposure experiments in order to evaluate the thermal
stability of these materials. After their estimation, lutetium silicates were considered to be promising
for topcoat materials.
Then, lutetium silicates were coated as the topcoat of an EBC system on SiC/SiC composites, and
their fracture toughness and microstructures were investigated after exposure to an oxidizing
atmosphere. The evaluation results of the topcoat materials are reported in this paper.
Abstract: There is an increasing reliability concern of thermal stress-induced failures in multilevel
coatings in recent years. This work reports investigations of cracking of NiCr coatings due to thermal
cycling. The temperature cycling in accelerated testing was performed in three temperature range of
150, 175 and 200°C. The NiCr coatings were considered to have failed when the sheet resistance
changed by 30% relative to an initial value. As the cyclic repetition of thermal shock increased, the
sheet resistance of NiCr coatings increased. The Coffin-Manson equation was applied to the failure
mechanism of cracking of NiCr coatings and the SEM observation of cracks and delamination in NiCr
coatings due to thermal cycling agreed well with the failure mechanism.