Plenary Lecture

Plenary Session I:

“New Concepts of Thermal Barrier Coatings”

Prof. Kazuhiro OGAWA
Fracture and Reliability Research Institute, Tohoku University, Japan (E-mail : kogawa@rift.mech.tohoku.ac.jp)

In gas turbine plants, increased turbine inlet temperature is required to improve the thermal efficiency of the plant and reduce the emission of greenhouse gases, such as carbon dioxide. However, at high temperatures, the Ni-based superalloy gas turbine blades can experience damage and degradation such as burning out, creep, and high-temperature oxidation. To prevent these damages, the surfaces of gas turbine blades are protected by thermal barrier coatings (TBCs). To obtain a low thermal conductivity, TBCs are generally applied to a superalloy substrate and contain a metallic bond coat and a refractory ceramic top coat. In general, the fabrication of the TBC, involves the application of a MCrAlY (M = Co, Ni, or both) bond coat to a Ni-based superalloy substrate. A top coat of yttria-stabilized zirconia (YSZ) follows. However, failure of these conventional TBC systems has been reported to mainly result from the thermal expansion mismatch between the ceramic and metal coating layers of the systems.
In this study, to overcome the failure, new concepts of TBC are introduced. Firstly, new bond coat materials with excellent interfacial strength for thermal barrier coatings were developed. By using Ce and CeO2 added bond coat material, wedge-like TGO was form at the interface between a top coat and a bond coat. As a result, interfacial strength was improved by the Ce added bond coat materials. Secondly, functionally graded TBCs (FG-TBCs) were developed by cold spray technique. In this study, cold spray technique was used to fabricate FG-TBCs for one processing fabrication. However, the cold spray system has limitations for fabrication of FG-TBCs with ceramic materials. Therefore, the cermet powder is used to solve the limitations of cold spray technique. As a result, it is possible to make FG-TBCs by cold spray technique.

“Research advances on splat formation during plasma spraying: from morphology to interface bonding: towards the control of coating microstructure”

Prof. Chang-Jiu Li
State Key Laboratory for Mechanical Behavior of Materials, Schoolof Materials Science and Engineering, Xi’an Jiaotong University, China (E-mail: licj@mail.xjtu.edu.cn)

Plasma spraying is one of major thermal spray coating processes. The most important feature of plasma spraying is to deposit coatings of any material from metal alloy to ceramics. The splat formation is a fundamental process to determine coating microstructure and properties. In last two decades, many studies were dedicated to morphologial variations of splats. Many phenomena have become clear through understanding the mechanisms of splashing during splat formation of plasma spraying, while morphology variation of splats deposited on a rough surface, and effect of thermal interaction of molten droplet/substrate on spreading process and the splat interface bonding formation are still subjects to need investigations. In this report, several splashing mechanisms associating with splat morphology variations including evaporative adsorbates desorption induced splashing, local substrate surface melting induced splashing, spreading front rapid solidification induced splashing and surface roughness induced splashing will be presented along with the involved physics based on the role of dominant factors involving molten droplet splatting. It will be found that the deposition temperature, molten droplet parameters and substrate surface morphology are key factors to determine splat shape. Besides morphology variation, moreover, the recent progress on the interface bonding formation during splatting through splat formation investigation assisted by FIB sanpling will be reviewed. It will be shown that the deposition temperature is also key factor to determine the bonding formation at the interface between splat and substrate. By controlling deposition temperature and molten droplet temperature, the interface bonding at the splat/substrate interface during splatting can be controlled. Accordingly, it will be illustrated that the coating microstructure in terms of intersplat can be controlled for development of high performance.
 


 

Plenary Session II:

“LOW THERMAL CONDUCTIVITY OF YSZ AND ZIRCONATES IN PLASMA-SPRAYED THERMAL BARRIER COATINGS”

Prof. Kyeong-Ho Baik
Department of Materials Science and Engineering, Chungnam National University, KOREA. (E-mail: khbaik@cnu.ac.kr)

Y2O3-stabilized ZrO2 (YSZ) is widely employed as thermal barrier coatings (TBCs) because of an unusual combination of low thermal conductivity, relatively high thermal expansion, and good environmental stability. Typical plasma-sprayed YSZ coatings exhibit thermal conductivities in the rage of 1.0 to 1.5 W/mK which are mainly affected by the amounts of micro-sized defects including unfilled pores, splat boundary pores and microcracks. Further reduction in thermal conductivity in TBCs can be achieved by either highly incorporating micro-sized defects or employing novel low-conductive ceramics. In this study, we described the relationship between micro-sized defects and thermal conductivity in YSZ coatings, and substantial improvements in thermal insulation capability in TBCs using pyrochlore zirconates. The plasma-sprayed YSZ coatings manufactured from different feedstock powders at different processing conditions exhibited a density range of 4.0 to 5.6 g/cm3, and a low-density YSZ coating contained a greater amounts of micro-sized defects, in particular nano-sized pores. Thermal conductivity of YSZ coatings in this study was in the range of 0.7-1.5 W/mK at room temperature. Low-conductive TBCs were manufactured using pyrochlore zirconates including Gd2Zr2O7, La2Zr2O7 and Ce2Zr2O7. These zirconate coatings exhibited relatively lower thermal conductivities of 0.65-0.71 W/mK, even if they contained low levels of micro-sized defects. All of pyrochlore-zirconate TBCs suffered from a low thermal cycling/shock resistance, mainly due to a relatively larger CTE mismatch between TBCs and underlying Ni-based superalloy. Strong interfacial bonding strength and/or multi-layer TBCs resulted in substantial improvements in thermal cycling/shock performance.

“Science of Cold Spraying: From Discovery to Applications”

Prof. Frank Gaertner
University of the Federal Armed Forces Hamburg, Faculty of Mechanical Engineering, GERMANY (E-mail: Frank.Gaertner@hsu-hh.de)

The presentation summarizes the major achievements to explore cold spraying for making the method suitable for applications. Being invented in the late 1980ies at the Institute of Theoretical and Applied Mechanics of Russian Academy of Science, Novosibirsk, Russia, particularly within the last two decades, cold spraying developed from laboratory scale to a reliable industrial process. As a powder spray technique dealing with solid impacts, cold spraying results in coatings of high purity and unique properties, not attainable by other spray methods.
Already in the early works, exceeding a material dependent critical velocity was identified as key parameter for successful coating build-up. Supported by modelling, reaching and exceeding the critical velocity could be associated with the occurrence of adiabatic shear instabilities, means thermal softening over-compensating strain and strain rate hardening, at particle-substrate and particle-particle interfaces. Properties of the deposit improve with increasing the interface areas that reach adiabatic shear instabilities under enhanced process conditions, in detail the ratio between individual particle impact velocity and critical velocity at attained impact temperature. Looking into more detail, also the spread of heat by deformation and spread of hardening effects were identified to be more prominent for smaller dimensions. Thus, impact conditions governed by acceleration, heating and cooling have to trade off with particle size dependent critical velocities. This has to be considered for the optimization of needed powder size cuts. Apart from that, also locations of shear instabilities have to be tuned for in ideal case equal spread at particle and deposit sites. For coating formation that can be adjusted over effective surface temperatures by heating the part or fine tuning of kinematics as travers line speed and spray distance. For cold spraying of composites, the situation is getting more difficult, demanding either for already well-bonded feedstock or in case of blends tuning area of shear instabilities of the soft constituents to occur at dissimilar interfaces.
First applications of cold spraying were on the market in 2003, benefitting from the high thermal conductivity between electronic parts and heat sinks. Later applications aimed for maximizing the mechanical strength of deposits for example to serve as repair method for aerospace parts. Nowadays, cold spraying is explored for high strength materials, either as repair method but also for additive manufacturing techniques.
As compared to thermal spraying, the description of fluid mechanics and bonding of cold spraying is straighter forward and easier to handle. Nevertheless, more work is needed to explore its full potential for enhancing applications.