Advances in the oxidation resistance of high‐temperature turbine materials

Abstract

This overview highlights some of the key factors in understanding the protective ability of aluminum oxide scales for current gas turbine engine alloys and coatings. Dense, phase-pure, slow-growing and adherent scales are required. In most cases, solutions have been found through educated empiricism, with more complete understanding often lagging by decades. Nevertheless, the relentless application of new techniques to answer old questions has often provided radically different viewpoints and pointed the way to new engineering developments. Equally important, this understanding prevents prior misconceptions from taking too strong a hold and maintaining a limited perspective. The microstructure, kinetics, shortcircuit diffusion, phase transformation, growth and residual stresses, interfacial segregation and adhesion provide a richness of topics for this singular and effective system. Copyright INTRODUCTIONThe successful application of many materials requires excellent high-temperature properties. For example, the thermal materials used in electric furnaces, waste incinerators, boilers, heat exchangers and heat engines all require long-term mechanical and chemical stability at high temperatures. The surface stability of the candidate material is necessary to prevent rapid material consumption at high temperatures. In general, this dictates that the exposed material is able to form a slow-growing, stable, adherent surface scale that serves to protect the underlying substrate material. The most important and effective scales are oxides; they are easily produced by reaction with the oxygen in air or in other oxygen-containing H 2 O, CO 2 or CO gaseous environments. And the most sought-after oxide scales are Al 2 O 3 , Cr 2 O 3 , and SiO 2 because of their outstanding protective qualities. Many of the high-temperature components used in aircraft and ground power gas turbines rely on these scales for protection.The purpose of the present review is to highlight some of the recent advances that have been made to high-temperature, oxidation-resistant turbine materials. It is intended to be neither comprehensive nor critical, but rather to give a general, albeit parochial, synopsis of a number of key issues seen to rise to prominence from the perspective of this author. In light of this venue, an attempt will be made to give special emphasis to advances in understanding and performance that were associated with and facilitated by various characterization and surface analysis techniques. Oxidation primerA few points regarding the basic tenents of oxidation theory relevant to all systems are worth reviewing before dealing with specific issues. (The cardinal reference text on the subject, elegantly covering all aspects in great detail, is that by Kofstad. 1 ) Very briefly, the pertinent control issues are the thermodynamics of the possible gas-solid reactions and the diffusion rates of the oxide scales in the presence of a large oxygen activity (P O 2 ) gradient. The free energy of formation of binary oxides ranges from abou...

In-Situ observations of oxidation and phase stability in cast nickel-based intermetallic alloys

“…(1) In both the Ni 3 Al and IC221M alloys under slow heating and cooling, the ZrO 2 formation occurs before the Al 2 O 3 formation is detected. (2) With rapid heating and cooling, as expected, the Ni 3 Al alloy did not show extensive oxidation and, in contrast, the IC221M alloy showed rapid oxidation. (3) In the IC221M alloy, an abrupt change in the lattice parameter of ␥Ј was observed, in conjunction with the initiation of ZrO 2 formation.…”

“…[2] The present work is part of the ongoing efforts to extend this knowledge to high-volume-fraction ␥Ј alloys close to the Ni 3 Al composition. Previous work on Ni 3 Al alloys has indicated that the oxidation resistance of these alloys is generally good compared to other nickel-based alloys.…”

Lifetime of environmental/thermal barrier coatings deposited on a niobium silicide composite with boron containing M7Si6‐based bond coat

“…Furthermore, attention has been focused on the failure of the E/TBC system on the front side of the samples far from the edges. Under these aspects, the results obtained in this study are rated by comparing with data reported for furnace cycling tests of TBCs on Ni-based superalloys [38], albeit the spallation lifetime substantially depends also on sample geometry, thermal gradients during heating and cooling periods and layer thickness. The thermal cycling behaviour of the latter TBC systems depended upon the bond coats, their pre-treatments and the substrates.…”

Improved oxidation life of segmented plasma sprayed 8YSZ thermal barrier coatings

“…[5] Alternatively, scale adhesion may be improved by removing S, either by hydrogen (H 2 ) annealing or melt fluxing. A quantitative correlation between scale adhesion and S and a literature survey can be found in recent reviews, [6,7] with specific attention to low S superalloys. [8][9][10] Many of the above precepts may be applied to the cyclic oxidation behavior of TBCs and their delamination failure.…”