Harbin Institute of Technology

About: Harbin Institute of Technology is a education organization based out in Harbin, China. It is known for research contribution in the topics: Microstructure & Control theory. The organization has 88259 authors who have published 109297 publications receiving 1603393 citations. The organization is also known as: HIT.

Effects of Platinum on the Interdiffusion and Oxidation Behavior of Ni-Al-Based Alloys

Abstract: Thermal barrier coating (TBC) systems, needed for higher thrust with increased efficiency in gas turbines, typically consist of an alumina-scale forming metallic bond coat and a ceramic topcoat. The durability and reliability of TBC systems are critically linked to the oxidation behavior of the bond coat. Ideally, the bond coat should oxidize to form a slow-growing, non-porous and adherent thermally grown oxide (TGO) scale layer of α-Al2O3. The ability to promote such ideal TGO formation depends critically on the composition and microstructure of the bond coat, together with the presence of minor elements (metal and non-metal) that with time diffuse into the coating from the substrate during service. An experimental program was undertaken to attain a more detailed fundamental understanding of the phase equilibria in the Ni-Al-Pt system and the influences of alloy composition on the formation, growth and spallation behavior of the resulting TGO scales formed during isothermal and thermal cycling tests at 1150°C. Additional studies were conducted to determine the influence of platinum on interdiffusion behavior in the Ni-Al system, and how this influence would impact coating/substrate interdiffusion. It will be shown that platinum has a profound effect on the oxidation and interdiffusion behaviors, to the extent that novel advanced coating systems can be developed. Introduction The demand for improved performance in high-temperature mechanical systems has led to increasingly severe operating environments, particularly for the components in advanced gas-turbine engines. Future improvements in gas-turbine performance will require even higher operating efficiencies, longer operating lifetimes, reduced emissions and, therefore, higher turbine operating temperatures. Advanced cooling schemes coupled with thermal barrier coatings (TBCs) can enable the current families of nickel-base superalloys to meet the materials needs for the engines of tomorrow. Thermal barrier coating systems currently provide average metal temperature reductions of about 80°C, while potential benefits are estimated to be greater than 170°C. However, lack of reliability, more than any other design factor, limits the general use of TBC systems for gas turbines. Commercial advanced TBC systems are typically two-layered, consisting of a ceramic topcoat and an underlying metallic bond coat. The properties of the ceramic topcoat are such that it has a low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion. The topcoat is also made “strain tolerant” by depositing a structure that contains numerous pores and/or pathways. The consequently high oxygen permeability of the topcoat imposes the constraint that the metallic bond coat must be resistant to oxidation attack. The bond coat should therefore be sufficiently rich in aluminum to form a protective, thermally grown oxide (TGO) scale of α-Al2O3. In addition to imparting oxidation resistance, the TGO serves to bond the ceramic topcoat to the substrate/bond coat system. Notwithstanding, it is generally found that spallation and/or cracking of the growing TGO scale is the ultimate failure mechanism of Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 213-222 doi:10.4028/www.scientific.net/MSF.461-464.213 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-11/03/20,15:01:32) commercial TBCs, particularly EB-PVD TBCs [1-3]. Thus, improving the adhesion and integrity of the interfacial TGO scale is critical to the development of more reliable TBCs. Current bond coats are typically either an MCrAlY overlay (where M = Ni,Co, or both) or a platinum-modified diffusion aluminide (β-NiAl-Pt). The composition and phase constitutions of such bond coats vary. Both the MCrAlY and NiAl-Pt types of coating were originally developed to enhance long-term oxidation and corrosion protection of turbine components rather than specifically as bond coats. The oxidation resistance provided by these coatings allowed alloy developers to maximize the high-temperature mechanical properties of nickel-base superalloys. The original coatings required a high aluminum content in order to ensure re-healing of the Al2O3 scale after repeated cracking and spalling during service. In the case of TBC systems, however, Al2O3 healing after scale spallation is not an important requirement for ceramic topcoat adhesion. This is because the adhesion, and therefore the reliability, of a TBC system is dictated primarily by the first spallation event of the TGO scale. As a consequence, the currently used bond coats do not necessarily possess optimum compositions and/or structures for prime reliant TBC systems. In assessing a given bond coat system, it is also important to realize that its composition and structure change with time in service due to both TGO-scale growth and interdiffusion with the substrate. The occurrence of coating/substrate interdiffusion decreases the concentration of aluminum in the coating, thereby reducing the ability of the coating to sustain exclusive Al2O3-scale growth, particularly in the event of localized detachment and/or microcracking, and introduces unwanted elements (e.g., sulfur and titanium) which can promote oxide-scale spallation [4]. A further consequence of coating/substrate interdiffusion, particularly for the next generation of superalloys with up to 6 wt.% rhenium, is the formation of topologically close-packed (TCP) phases in the region of the original coating/substrate interface, which can be deleterious to the mechanical properties of the superalloy substrate. The results presented in this paper are from a larger US Office of Naval Research sponsored study that is concerned with gaining a more detailed fundamental understanding of the influences of alloy composition and microstructure on the adhesion/spallation resistance of TGO scales for the purpose of improving the reliability and durability of advanced TBC systems. An experimental approach was taken initially, from which the important chemical and microstructural factors governing TGO growth and adhesion were determined. The starting point for this study was a determination of the (partial) Ni-Al-Pt phase diagram, which to date has only been shown to be speculative [5] (see Fig.1). Oxidation and interdiffusion experiments were then conducted on the various Ni-Al-Pt bulk alloys processed to determine the potential effects of phase constitution and Pt content on overall coating performance. Experimental Procedures Ni-Al-Pt alloys were prepared by argon arc melting high-purity pieces of the constituents. To ensure homogenization and equilibration, all alloys were annealed at 1100 ̊C or 1150 ̊C for at least one week in a flowing argon atmosphere and then quenched in water to retain their high-temperature structure. The alloys were then cut into coupon samples and polished to a 600-grit finish for the further testing. The equilibrated alloy samples were first analyzed using X-ray diffraction (XRD) for phase identification and then prepared for metallographic analyses by cold mounting them in an epoxy resin followed by polishing to a 0.5 μm finish. Microstructure observations were initially carried out on etched samples using an optical microscope. Concentration profiles were obtained from unetched (i.e., re-polished) samples by either energy (EDS) or wavelength (WDS) dispersive spectrometry, with the former utilizing a scanning electron microscope (SEM) and the latter an 214 High Temperature Corrosion and Protection of Materials 6

Robust extended dissipative control for sampled-data Markov jump systems

Abstract: This paper investigates the problem of the sampled-data extended dissipative control for uncertain Markov jump systems. The systems considered are transformed into Markov jump systems with polytopic uncertainties and sawtooth delays by using an input delay approach. The focus is on the design of a mode-independent sampled-data controller such that the resulting closed-loop system is mean-square exponentially stable with a given decay rate and extended dissipative. A novel exponential stability criterion and an extended dissipativty condition are established by proposing a new integral inequality. The reduced conservatism of the criteria is demonstrated by two numerical examples. Furthermore, a sufficient condition for the existence of a desired mode-independent sampled-data controller is obtained by solving a convex optimisation problem. Finally, a resistance, inductance and capacitance (RLC) series circuit is employed to illustrate the effectiveness of the proposed approach.