Cold spray deposition of polycarbonate on the various substrates has been investigated. The polycarbonate particles are sieved and accelerated at elevated temperature in air through a DeLaval type nozzle, and are deposited on the metallic and ceramic substrates. The influences of the particle size, the gas temperature, the thermal conductivity and surface roughness of substrate on the deposition process are studied. As a result, the continuous deposits are formed on the metallic substrate. The powder sieved below 300 μm shows better deposition efficiency. Thin film of melted polycarbonate has been formed on the surface of substrate to act as a bonding layer, and its crystalline structure is changed to be amorphous, which is the more stable state for the polycarbonate. The coating seems to be better when the thermal conductivity of metallic substrate is low. For the ceramic substrates, there is no deposition whatever was the thermal conductivity.

Thermal-sprayed MCrAlY coatings are widely used for land-based gas turbine applications. The cold spray may increase the coating density owing to the high-velocity particle impacts during spraying. Many researchers have considered critical velocity to be the most important factor of the deposition mechanism of cold-sprayed coatings. However, this dominant parameter of critical deposition condition has not been completely understood. In order to understand the mechanism, two approaches were used in this study. One is the transmission electron microscope (TEM) observation of the interface between the coating and the substrate, and the other is the cross-sectional observation of the deposited particle by using the focused ion beam (FIB) cutting technique. From the TEM observations, there are no evidences of melting at the interface, and it is found that the actual bonding occurred at the nascent surfaces. Generally, there is a native oxide on the surface of the particles and substrate. After the plastic deformation of the particles and substrate, the native oxide breaks down; subsequently, a nascent surface can be created and direct contact initiates deposition. From the results of these investigations, it is thought that the dominant factor for deposition is the plastic deformation of the particles and substrates.

A kinetic metallization technique, which is one of the cold spraying systems, has been studied as a new coating system for metallic bond coats of thermal barrier coatings for components used in hot section of advanced gas turbines. In this study, in-situ residual stresses in atmospheric plasma sprayed yttria-stabilized zirconia (YSZ) top coating with two different bond coat spraying systems, deposited by a low pressure plasma spraying and a cold spraying, were evaluated and compared by thermal cycle tests. From the results of 1st thermal cycle, in the case of the plasma sprayed bond coat, a tensile residual stress was observed at the elevated temperature up to 400°C. Relaxation of the residual stress was started beyond 400°C. On the other hand, the gradual increase of tensile residual stress was observed up to 1000 °C in the case of cold sprayed bond coat. In addition, transition behaviors of residual stress between plasma sprayed and cold sprayed coatings were varied in 3-thermal cycles.

Thermally grown oxide (TGO) grows at the top / bond coating interface of the thermal barrier coating (TBC) in service. It is supposed that the failures of the TBC occur due to thermal stress and the decrease of adhesive strength caused by the TGO growth. Recently, large local stress has been found to change both the diffusion constant of oxygen through an existing oxide and the rate of chemical reaction at the oxide / oxidized material interface. Since high thermal stress occurs in the TBC, the volume expansion of the newly grown oxide, and centrifugal force, the growth rate of the TGO may change depending on not only temperature but also the stress. The aim of this study is to make clear the influence of stress on the growth rate of the TGO quantitatively. As a result, the thickness of the TGO clearly increases with increase of the amplitude of the applied stress and temperature. The increase rate of the TGO thickness is approximately 23% when the applied stress is increased from 0 to 205 MPa at 900 °C, and approximately 29 % when the stress is increased from 0 to 150 MPa at 950 °C.

Aluminium alloys are widely used for transportation facilities, because of light weight and high corrosion-resistance. If there are some cracks in transportation, sometimes they repair by welding. However, it is difficult to weld aluminium materials. Because, Aluminium has high specific thermal conductivity and high coefficient of thermal expansion compared with that of steel. The cold spray technique is known as a new technique not only for coating but also for thick depositions. It has many advantages, i.e. dense coating, high deposition rate and low oxidation. Therefore, it has a possibility to apply the cold spray technique instead of welding to repair the cracks. What seems to be lacking, however, is deposition mechanisms and mechanical properties of deposition produced by low pressure type cold spraying. This is a very important issue for applying the cold spray to repair some structures. In this study, elucidation of deposition mechanisms and evaluation of mechanical properties for the low pressure type cold sprayed aluminium depositions were investigated. As a result of elucidation of deposition mechanisms, it can be clear that the particle deposition needs to activate the surface by several impingements. Furthermore, as a result of evaluation of mechanical properties, the cold sprayed specimen showed higher strength than the monolithic specimen in the case of compressive loading to the coating.

The effect of particle size range on oxidation behavior was investigated according to exposure time in isothermal oxidation condition. Emphasis was placed upon oxygen content, porosity, and oxide scale formation. Commercially available CoNi- and CoCrAlY powders of several different particle size ranges were vacuum-plasma sprayed on a nickel alloy substrate. The results show that the isothermal degradation of coatings is considerably influenced by the particle size distribution. It can be clearly observed that a remarkable increase in the oxygen content in the as-sprayed coating occurred with a decrease in the mean particle size. But after thermal exposure, the difference of the oxygen contents between the smaller and larger particle coatings is decreased. The distribution of particle size plays the important role of porosity than only the mean particle size. The powder which has the widest range and sample variance leads to make good porosity inside coatings during the deposition process.

Thermal-sprayed (i.e. LPPS or HVOF) MCrAlY coatings are widely used for land-based gas turbine applications against high-temperature oxidation and hot corrosion. However, due to requirement for further improvement of turbine efficiency, dense and stable coatings are necessary. The cold spray (also referred to as cold gas dynamic spray) makes it possible to increase coating density, due to high velocity particle impact during spraying. However, deposition mechanisms of cold spraying have not been elucidated yet. In this study, we investigated the deposition mechanisms focused on the behavior of interface between a coating and a substrate. The mechanisms were evaluated by the spray impact phenomena simulation tests, namely laser shock flier impact tests, and STEM-EDX elemental analyses at the interface between the substrate and the cold sprayed coating. From the results of STEM-EDX for as-sprayed coating and of SEM-EDX of the flier specimen, the bonding between the CoNiCrAlY coating and the substrate occurred at the only particular phase combination.

The integrated model of thermofluid, splat formation and coating formation for a cold spray process has been established. The in-flight behavior of micro and submicron particles, the interaction between shock wave and particles in a supersonic jet impinging onto the substrate are clarified by this integrated model in detail. Then, the effect of electrostatic force on the particle acceleration is discussed in detail by carrying out a real-time computational simulation. It is shown that coating can be formed with the assist of electrostatic acceleration even though there is a lack of particle acceleration over critical velocity only through momentum transfer from airflow. Thus, the utilization of electrostatic acceleration enhances the performance of cold spray coating and contributes the extension of application range of a cold spray process.

In thermal barrier coating (TBC) system, thermally grown oxide (TGO) forms at the interface between the top-coat and bond-coat during service. Delamination or spallation at the interface can be occurred by the TGO formation and growth. Therefore, Modifications of the bond-coat materials are one means to inhibit the TGO formation and to improve the bonding strength of TBCs. In this study, morphologies of TGO were controlled by using Ce and Si addition to conventional CoNiCrAlY bond-coat material. As a result, when the TBCs with Ce added bond-coat materials were aged at 1373K for 100 hours, morphologies of TGO were changed drastically. It is expected that the morphologies can improve bonding strength of TBCs. We carried out to evaluate the bonding strength by using four-point bending tests. As a result, TBC coated with Ce added bond-coat materials indicated excellent bonding strength.

In waste-to-energy power generation plants, increasing the combustion temperature improves plant efficiency. However, due to problems caused by molten chloride corrosion, the combustion temperature cannot be increased. Consequently, in order to increase the temperature, it is first necessary to protect components from molten chloride corrosion. In this study, CoNiCrAlY coating is proposed as a protective film against molten chloride corrosion. Three kinds of specimens were prepared. One was standard coating made from conventional CoNiCrAlY. The others included the addition of Mo to the CoNiCrAlY by two different techniques. One technique is mechanical alloying (MA), and the other is a gas-atomizing technique. The mechanic-chemical reaction that occurs during the mechanical alloying process can be expected to create new functionality for the material. The effect of Mo content was evaluated for corrosion resistance. These specimens were coated by low pressure plasma spraying (LPPS). The specimens were exposed to NaCl-KCl for the molten chloride corrosion test. The results of the corrosion tests show that corrosion resistance improved in only MA CoNiCrAlY coatings. These results reveal that mechanically alloyed CoNiCrAlY-Mo coating has excellent corrosion resistance, and its corrosion resistance behavior is different from that of gas-atomized CoNiCrAlY-Mo.

In order to improve the efficiency of gas turbines, thermal barrier coatings (TBCs) have been applied to components in the hot sections of advanced gas turbines. During service, thermally grown oxide (TGO), which consists of an Al 2 O 3 layer and a mixed oxide layer, forms at the interface between the top coating and bond coating. It is supposed that the reason for failures of TBCs, such as cracking, delamination or spalling, is due to decreased bond strength caused by TGO growth or due to the formation of stress concentration sites caused by porosities in the mixed oxide. In this study, to inhibit the growth of TGO, plasma sprayed CoNiCrAlY bond coating was remelted with a YAG laser prior to spraying the top coating. A thin Al 2 O 3 layer formed at the top coating/bond coating interface, and the formation of porous mixed oxide during thermal aging tests was inhibited. Four-point bending tests showed that the bond strength of TBC with remelted CoNiCrAlY was superior to standard TBC.

Recently, thermal barrier coatings (TBCs) have been used in advanced gas turbine plants for improved performance. Usually, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating. According to several studies, the failure of the TBC is induced by thermal stresses due to the formation of thermally grown oxide (TGO) at the interface between the TBC and MCrAlY. Therefore, it is important to investigate the high temperature oxidation behavior of the interface. In this work, the TGO is characterized in detail. In particular, in order to clarify the role of the TBC top coating in regard to initiation and growth of TGO at the interface, a specimen with TBC and one without TBC were compared. In both specimens, the TGO had two different contrasting layers. One was alumina, and the other was a combination of chromium oxide, nickel oxide, cobalt oxide, and spinels (hereafter call mixed oxide). The TGO thickness of the specimen with TBC was thicker than that obtained without TBC. These specimens had different oxidation behaviors. It is thought that the reason for the difference in TGO thicknesses of both specimens is due to a difference in oxygen potential, as the oxide compositions in the mixed oxides were different. In case of the specimen with TBC, the mixed oxide consists of chromium oxide, nickel oxide, and cobalt oxide, separately. On the other hand, in case of the specimen without TBC, the mixed oxide consists mainly of spinels such as (Ni, Co)(Cr, Al) 2 O 4 .

This study addresses a potential problem with thermal barrier coatings used on turbine blades and other superalloy components. High operating temperatures stimulate the growth of oxides at the interface between the thermal-barrier topcoat layer of the TBC and the metallic bond coat that anchors it to the substrate. Past studies have shown that cracks readily develop in these thermally grown oxide layers, reducing adhesion and giving rise to stress concentrations. To improve adhesion, a modified powder for the bond coat was developed by adding Ce and Si to the MCrAlY mixture. Zirconia-based TBCs sprayed using conventional and modified bond coat powders are examined in this present work in order to evaluate the effect of Ce and Si additions on oxide formation and adhesion strength at the bond coat interface. Paper includes a German-language abstract.

In recent years, thermal barrier coatings (TBC) have been used in advanced gas turbine plants for improved durability and performance. Typically, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating (8wt% yttria stabilized zirconia (YSZ)). According to several studies, the failure of coating is induced by thermal stress due to formation of oxides at the interface between YSZ and MCrAlY. Therefore, it is important to investigate kinetics of oxidation at the interface. In this work, the interface between YSZ and MCrAlY is studied and characterized. TBC specimens are thermally aged at 1000 deg. C to simulate the surface temperature of first rotating blades. After aging, thermally grown oxide (TGO) is formed at the interface. The TGO has two layers of different contrasts. One layer is black, and is closer to MCrAlY; the other layer is gray, and is closer to YSZ. The black layer is identified as Al 2 O 3 by energy dispersive X-ray spectroscopy (EDX) and electron probe micro analyzer (EPMA). The EDX and EPMA spectra of the gray layer contain various peaks of Al, Cr, Co, Ni, and O, which suggests that the layer can be mixed oxide which is a combination of NiO, CoO, Cr 2 O 3 , and Ni(Cr, Al) 2 O 4 . However, the outer layer of mixed oxide contains only Cr and O. Accordingly, Cr 2 O 3 forms at the outer layer of mixed oxide, and the other oxides are distributed at the inner layer of mixed oxide. The thickness of the two oxide layers increases with aging time. While the formation of mixed oxide layer obeys a parabolic law, the formation of an alumina layer cannot be expressed in terms of a parabolic law. Due to the formation of protective mixed oxide on the alumina layer, the oxidation rate of alumina decreases as the thickness of mixed oxide increases.