Papers by Author: Kazuhiro Ogawa

Abstract: Cold spray (CS) is a solid-state deposition technique of micron-sized metallic powder in an ultra-high velocity gas using a de Laval nozzle. CS is a unique deposition technique due to its use of relatively lower gas temperatures in comparison to other thermal processes. Consequently, high-temperature oxidation and phase transformations of deposited powders are largely restricted while the operating cost of CS is much lower than that of other thermal processes. Generally, the low pressure cold spray (LPCS) technique is used for the deposition of metallic powders on metallic substrates, while only a few studies of metallic particle deposition on ceramic substrates have been conducted, and it was found that the deposition of metallic powders on ceramic substrates was quite difficult. In this study, improved LPCS deposition of copper coatings on zirconia substrates was investigated. It is known that deposition of a metallic powder on a ceramic substrate is difficult due to the differences in material bonding and several properties of the two materials. These difficulties in LPCS deposition were solved using three different approaches, namely 1) use of copper and aluminum composite powders and 2) laser pre-treatment and 3) laser texturing of zirconia substrates. It was found that pure copper powder coatings on the as-received and various treated substrates were delaminated in the interface as expected. However, the deposition was improved for all substrates by using the copper and aluminum composite powder. While the laser pre-treated substrate was not effective for the deposition of the copper and aluminum composite powder, thick coatings were obtained for the deposition on the laser pre-treated with heat treatment substrate and the laser-textured substrate.

Abstract: Semi-crystalline polymers such as fluoropolymers are extremely difficult to coat with using solid-state deposition technique such as cold spray (CS) due to its high viscoelastic-viscoplastic behavior. Generally, fluoropolymers solid-state deposition using CS are characterized by a relatively low deposition efficiency (DE) as compared to metallic materials using this deposition technique. In this article, the study on the effect of hydrophobic fumed nanosilica (FNS) and fumed nanoalumina (FNA) in Perfluoroalkoxy (PFA) solid-state deposition using CS has been studied. This study incorporated certain parameters related to the CS process. In addition, powder modification technique using hydrophobic FNS and FNA as additive to feedstock has been studied as well. The results show a number of parameters affected the DE; particle size, traverse speed, gas temperature, and addition of hydrophobic fumed nanoparticles, indicated a better DE. Moreover, PFA coating produced in this manner, retained its hydrophobicity.

Abstract: This paper presents the results of a study of the microstructure and oxidation behavior of thermal barrier coating (TBC) with air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited using two different spraying techniques, low pressure plasma spray (LPPS) and cold spray (CS). The objective is to investigate the thermally grown oxide (TGO) thickness and oxide scale composition of TBC subjected to isothermal oxidation and creep tests at 900 °C by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDX) analyses in order to evaluate the reliability of the CS technique. It was found that the TGO thicknesses for TBC with CS bond coats were smaller and the TGO was composed of mainly alumina with little or no mixed oxides. TGO growth rate was also affected by the applied stress. Smaller TGO thicknesses were observed for the non-creep TBC for both CS and LPPS bond coats. Overall findings indicate that the oxidation behavior of the TBC with CS bond coat is superior compared to that of the TBC with LPPS bond coat.

Abstract: Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.

Abstract: Hot section parts of combined cycle gas turbines are susceptible to degradation due to high temperature creep, crack formation by thermal stress, and high temperature oxidation, etc. Thus, regularly repairing or replacing the hot section parts such as gas turbine blades is inevitable. For this purpose, revolutionary and advanced repair technologies for gas turbines have been developed to enhance reliability of the repaired parts and reduce the maintenance cost of the gas turbines. The cold spraying process, which has been studied as not only a new coating technology but also as a process for obtaining a thick deposition layer, is proposed as a potential repairing solution. The process results in little or no oxidation of the spray materials, so the surfaces stay clean, which in turn enables superior bonding. Since the operating temperature is relatively low, the particles do not melt and the shrinkage on cooling is very low. In this study, the cold spraying conditions were optimized by taking into account the particle kinetic energy and the rebound energy for application in repairing gas turbine blades. A high quality cold-sprayed layer is that which has lowest porosity; thus the spraying parameters were optimized to achieve low-porosity layer, which was verified by scanning electron microscopy (SEM).

Abstract: In order to assure the reliability of advanced gas turbine systems, it is very important to evaluate the damage of high temperature materials such as Ni-based superalloys under creep and fatigue conditions quantitatively. The refractive index of the gamma-prime phase is found to be smaller than that of the gamma phase in the Ni-based superalloy, when the wavelength of an irradiated laser beam is shorter than 500 nm. Therefore, it is possible to evaluate the creep damage of this material quantitatively and non-destructively by observing the change of the micro texture in a grain (rafting) using a scanning laser microscope.

Abstract: The yoke type magnetization coil and cross type magnetizer as a magnetic source, which induce a static magnetic field on a specimen, have been used to detect cracks on a ferromagnetic material. However, the cracks on paramagnetic materials such as aluminum alloy and stainless steel cannot be detected by using a static magnetic source. This paper proposes a magnetic source, which uses the induced sheet type current, for the detection of cracks on a paramagnetic material. The sheet type AC current can be induced by using a primary coil and core. And the copper film, which includes a sheet type current, is positioned on the specimen. Eddy currents are induced around a crack on the specimen because the secondary sheet type current can be induced on the specimen surface. The signal processing electrical circuits, which measure a Hall sensor peak output with the eddy current frequency, are developed and discussed. Also the validity of the proposed signal processing circuit is verified by detection of slit type cracks and a fatigue crack, which are introduced on aluminum alloy.

Abstract: Nondestructive testing using magnetic field is useful for detection of a crack on ferromagnetic material. The magnetic field distribution has to be obtained for quantitative evaluation of crack direction, size, and shape. Also, a crack can be evaluated by using the inverse problem analysis. However, an analysis method using a dipole model can be used to analyze the magnetic field distribution around a crack at a higher speed than the finite element method (FEM). Therefore, a dipole model simulation can provide useful information which can be used for the inverse problem analysis. However, the magnetic charge per unit area, m, and the permeability, μ, has been treated as constants. Therefore, analyzed results have been different from experimental results in most cases. This paper proposes the improved dipole model simulation method, which assumes that the magnetic charges per unit area exist at the section areas, edge lines and summits of a crack. Also, the magnetic charges per unit area were assumed to depend on the square of the crack depth. The improved method is validated by comparing its results with the experiment results obtained with the use of the magnetic camera.

Abstract: The subcommittee on superalloys and coatings, The Society for Materials and Science, Japan (JSMS) was established in 1997. As second stage of the committee, we investigated a fundamental study on thermal plasma sprayed thermal barrier coatings (TBCs). Especially, relationships between spray conditions and coating properties are discussed. It was used 4 spraying conditions such as 1) standard condition, 2) larger particle used and lower velocity, 3) normal particle used and lower velocity, and 4) standard condition and heating up the substrate to 473K. As a round robin test, porosity ratio, residual stress, and bonding strength were evaluated by several techniques for the specimens, which were sprayed by above 4 conditions. As a result, Young’s modulus, and tensile strength increased with decreasing porosity ratio. In case of tests for residual stress evaluation, as-sprayed specimens have small tensile residual stresses. After thermal exposure, residual stresses shifted to compressive.

Abstract: The mechanism of the degradation process of nickel based superalloy CM247LC under creep loading was analyzed by considering the microscopic chemical composition of the material such as phase boundary of γ and γ’ phases. In this study, a Directionally Solidified (DS) nickel-based superalloys CM247LC was used as test specimens. The creep test was performed at 900oC under an uni-axial stress of 216 MPa. Each specimen was creep ruptured and interrupted at different hours and then analyzed. The change of chemical composition around the interface analyzed precisely by used Auger Electron Spectroscopy (AES). It was found that the local enrichment or lack of cobalt and chromium was found at the interface. This tendency was not found in the stress free area. Since chromium is the well-known element that dominates the degradation of this material, such an enrichment or lack of cobalt and chromium may play an important role for forming a crack propagation path near the interface. This local segregation should be analyzed further to make clear degradation mechanism of this material.