Yutaka Ishiwata

Influence of high-temperature protective coatings on the mechanical properties of nickel-based superalloys

The low-pressure plasma spray coating process has been established in the field of gas turbines and is used for hot parts, such as turbine blades and duct segments, which are exposed to corrosive gases at high temperatures. Overlay coatings based on the MCrAlY alloy system (M is Ni, CO or both) are commonly employed as oxidation- and corrosion-resistant coatings. Mechanical properties, such as creep and fatigue lives, of various MCrAlY coating systems were investigated at high temperature as compared with the uncoated substrates, such as eqiaxis IN738LC, directional solidified CM247LC and single-crystal CMSX-2. It was clear that the MCrAlY coatings had no significant influence on the creep lives of substrates for the sake of superior ductility of MCrAlY coatings at high temperature. The low-cycle fatigue lives of MCrAlY coated superalloys at high temperature showed only a little superior performance in comparison with the uncoated results. However, the high-cycle fatigue lives of MCrAlY coated superalloys at high temperature showed inferior performance in comparison with the uncoated results. It was because that the low-cycle fatigue cracks initiated at casting cavities inside the substrate in both the coated and the noncoated cases. However, the high-cycle fatigue cracks initiated at interface defects, such as small pores and grit residues, between the MCrAlY coating and the substrate and grew into the MCrAlY coating, and then into the substrate.

Strain Tolerance and Microstructure of Thermal Barrier Coatings Produced by Electron Beam Physical Vapor Deposition Process

Several kinds of thermal barrier coatings (TBCs) deposited by electron beam physical vapor deposition (EB-PVD) were produced as a function of electron beam power in order to evaluate their strain tolerance. The deposition temperatures were changed from 1210 K to 1303 K depending on EB power. In order to evaluate strain tolerances of the EB-PVD/TBCs, a uniaxial compressive spallation test was newly proposed in this study. In addition, the microstructures of the layers were observed with SEM and Young’s moduli were measured by a nanoindentation test. The strain tolerance in as-deposited samples decreased with an increase in deposition temperature. In the sample deposited at 1210 and 1268 K, high-temperature aging treatment at 1273 K for 10 h remarkably promoted the reduction of the strain tolerance. The growth of thermally grown oxide (TGO) layer generated at the interface between topcoat and bondcoat layers was the principal reason for this strain tolerance reduction. We observed TGO-layer growth even in the as-deposited sample. Although the thickness of the initial TGO layer in the sample deposited at high temperature was thicker, the growth rate during aging treatment was smaller than those of the other specimens. This result suggests that we can improve the oxidation resistance of TBC systems by controlling the processing parameters in the EB-PVD process.

Transmission Electron Microscopy Study of Thermal Barrier Coatings Fabricated by Electron Beam-Physical Vapor Deposition

Yttria stabilized zirconia (YSZ) film was deposited on to a stationary metal substrate by electron beam-physical vapor deposition (EB-PVD) at 850 C ° . The film was characterized using transmission electron microscopy and scanning electron microscopy. The film was composed of inverted triangular-based pyramidal grains (T-grains) and inverted diamond-based pyramidal grains (D-grains). The T-grains were aligned in the <111> direction and D-grains in <110> perpendicularly to the substrate. Furthermore, striated lines of nano-pores, which should be strongly related to the thermal conductivity of the film, were observed in each YSZ grain. The pores were oriented in the <110> in the (111) plane and distributed across { 4 11 } planes in the YSZ grains aligned in the <111> (T-grains). In the YSZ grain aligned in the <110> (D-grains), the pores were oriented in the [ 10 1 ] and the [001] directions in the (110) and distributed across the (001) and ( 10 1 ).

A Study for Life Properties of High-Temperature Protective Coated Systems (Report 1). Low-Cycle Fatigue Life Properties of MCrAlY Alloy Coated Systems.

The low-pressure plasma spray coating process has been established in the field of gas turbines and is used for parts (turbine blades, duct segments, etc.) which are exposed to corrosive gases at high temperatures. Overlay coatings based on the MCrAlY alloy system (M is Ni, Co or Fe) are commonly employed as oxidation- and corrosion- resistant coatings. Mechanical properties, such as low-cycle fatigue lives, of CoCrAlY, NiCrAlY and CoNiCrAlY coated systems were investigated at high-temperature as compared with the uncoated conventional IN738LC, unidirectional solidified CM247LC and single crystal CMSX-2.Low-cycle fatigue properties of the MCrAlY coated superalloys at high-temperature (IN738LC : 1123 K, CM247LC, UMSX-2: 1173 K) showed only a little superior performance in comparison with the uncoated results. It was because that the low-cycle fatigue cracks initiated at casting cavities inside the substrate in any cases and the low-cycle fatigue crack initiation due to coatings could not be observed. It seems to be possible to evaluate the low-cycle fatigue lives of the MCrAlY coated superalloys at high-temperature by using the uncoated results.

Creep Life Properties of MCrAlY alloy coated Systems. A study for Life Properties of High-Temperature Protective Coated Systems (Report 3).

The low-pressure plasma spray coating process has been established in gas turbine and is used for some parts, such as turbine blades and duct segments, which are exposed to corrosive gases at high temperatures. Overlay coatings based on the MCrA1Y alloy system (M is Ni and/or Co) are commonly employed as oxidation- and corrosion-resistant coatings. Mechanical properties, such as short-time tensile strength and creep-rupture lives, of CoCrA1Y- and CoNiCrAlY-coated systems were investigated at high temperature and compared with the uncoated substrates, such as equiaxis IN738LC, directional solidified CM247LC and single-crystal CMSX-2. As a result, it was clarified that MCrAIY coatings had no significant influence on the tensile strength and creep lives at high temperature. However, the tensile strength at room temperature was reduced for the low-ductility of MCrAlY coatings.

Strength Properties of Yttrium-Oxide-Dispersed Tungsten Alloy.

The microstructure of an yttrium-oxide-dispersed tungsten alloy, newly developed, was investigated by examining the effect of sintering temperature on the particle size of yttrium oxide and the crystal size of tungsten. Also, it was confirmed that the bending strength of yttrium-oxide-dispersed tungsten alloy was affected by the sintering temperature in comparison with a sintered tungsten sample. The residual stress, induced by the coefficient of thermal expansion mismatch, is analyzed for the tungsten matrix composite with a particle of yttrium oxide using the finite element method. Because of the high residual stress, particles of yttrium oxide become crack initiation sites under the fabrication process. Finally, it is also shown that the bending strength of yttrium-oxide-dispersed tungsten alloy can be estimated simply by the fracture mechanics approach, based on the assumption of a flaw introduction effect by the yttrium oxide dispersed.

Study of Gas Reaction by Electron Beam Irradiation.

In a batch system with an electron beam irradiation, experiments have been carried out for accelerating chemical gas reactions. Carbon dioxide (CO2) is a low free energy compound, and either a high energy input of electron beams or a use of other high free energy reactants is needed for radiation chemical reactions. In this paper, the effects of content of added gas elements and an exposure dose of the electron beam irradiation on the CO2 gas reactions were investigated by monitoring the variations of gas temperature and gas pressure. As a result, there was a tendency that the CO2 gas reactions with an added acetylene gas (C2H2) increased with increasing the content of C2H2, the exposure does of electron beams and the gas temperature. Also, in this experiment, it was confirmed that carbon monoxide, methanol and some solid polymer could be produced under the condition of room temperature and atomospheric pressure.

Hot Isostatic Pressing and Sintering Behavior of Yttrium Oxide Dispersed Tungsten

Densification of tungsten with various yttrium oxide additions was investigated and the effect of capsule-free and encapsuled HIP treatment on density and bending strength at room and elevated temperature was examined. The sintered density of tungsten was greatly improved by addition of the yttrium oxide and a relative density above 99% was obtained by even in the case of capsule-free HIP treatment. Also, the bending strength was much improved and improved homogeneity could be obtained by HIP treatment. Significant difference were not found between capsule-free and encapsuled HIP treated tungsten with yttrium oxide. It was clear that dense and homogeneous tungsten components can be produced by HIP treatment without encapsulation through the addition of small amount of yttrium oxide.

Residual Stress Characteristics of Ceramic Coatings and Their Cracking Behavior

The residual stress of metal-to-ceramic laminates (stabilized ZrO2/Nb and Al2O3/Nb) produced by solid state bonding was studied using finite element method (FEM) analysis. The results indicated that the dimensionless residual stress at the ceramic coatings decreased with increasing coating thickness ratio, (hc/hs), and elastic modulus ratio, (1-μs)Ec/[(1-μc)Es].