Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)1405-1419 www.elsevier.comlocate/jeurceramsoc The influence of oxides on the performance of advanced gas turbines A G. Evans D R. Clarke. C G. Lev Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106-5050, USA Available online 28 January 2008 Zirconia and alumina have been successfully incorporated into turbines used for propulsion and power generation. They exert a crucial influence on the fuel efficiency. The roles of these oxides within the overall system are described, relative to those for the other constituents, and their most important properties are outlined. The mechanisms that govern their properties are presented ar hes for adjusting them in desirable directions are discussed. Opportunities for new materials with potential for superior performance are 2007 Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings; Alumina; Zirconia; Interfaces: Thermal properties 1. The motivation excess of 1200C)within an oxidizing environment. A single material would be incapable of satisfying these requir emen Oxides are present in turbines used for propulsion and power The viable solution is an oxide/metal multilayer(Fig. 2). The generation. Their benefits are manifest in a substantial outer oxide imparts thermal protection: while the metallic layer in the longevity of various hot section components (bond coat) affords oxidation protection through the formation technology demonstrates how oxides can be used to of a second oxide, as well as plastic accommodation of strain. -8 structural members that experience environmental extremes At the technology inception, the preferred insulating oxide Documenting the principles that underlie this success facilitates was determined to be yttria-stabilized zirconia (YSZ), chosen dissemination to other systems. The technology involves choices because of its low, temperature-invariant, thermal conductivity of materials and spatial configurations, as well as survivability(Fig 3a). The most desirable phase was ascertained by conduct upon extreme temperature cycling without loss of functionality. ing laboratory-based thermal cycle tests to seek the composition durability: that is, the lar ber of l.I. Materials and configurations cycles before the coating spalls(Fig. 3b). The outcome w 7wt. o yttria-stabilized zirconia(7-YSZ). This composition The following considerations have motivated the choice of still used. &e 1-20 It remains the material of choice because e the discovery of lower thermal conductiv materials and their spatial configurations. The thermal require- Ity opt ig.1).By directing air through other properties(especially toughness2-2)are also crucial channels, the structural alloy is internally cooled: with heat On rotating components, the layer thickness is important. It i transfer coefficient determined by the flow rate and the chan a compromise between having sufficient thickness to achieve nel geometry. Subject to a combustion temperature, Tgas, and the desired temperature drop, yet thin enough to avert exces- an external heat transfer coefficient, superposing an external sive inertial loads, due to the extra mass. The outcome is insulting oxide allows Tgas to be raised while retaining the thickness in the range 100= Htbc 3250 umOn stationary com- alloy at an allowable maximum temperature. Remarkably, insu- ponents, such as shrouds and combustors, the mass is less critical lating oxides deposited onto geometrically complex structural and much thicker layers can be used. The choice is typically, components, such as airfoils, remain attached for extended peri- 500um sTbc I mm. ods despite cycling through an enormous temperature range (in gov materials for oxidation protec tion are straightforwar with nuanced implementation (i)A thermally grown GO) forms at the bond coat sur- orresponding author. face by reaction with the combustion gas. The preferred TGO E-mail address: agevans @engineering. ucsb.edu(A G. Evans) should have the lowest possible oxygen ingress at the temper 0955-2219/S-see front matter o 2007 Elsevier Ltd. All rights reserved. doi: 10.1016/j-jeurceramsoc 2007 12.023Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 1405–1419 The influence of oxides on the performance of advanced gas turbines A.G. Evans ∗, D.R. Clarke, C.G. Levi Materials Department, University of California, Santa Barbara, Santa Barbara, CA 93106-5050, USA Available online 28 January 2008 Abstract Zirconia and alumina have been successfully incorporated into turbines used for propulsion and power generation. They exert a crucial influence on the fuel efficiency. The roles of these oxides within the overall system are described, relative to those for the other constituents, and their most important properties are outlined. The mechanisms that govern their properties are presented and approaches for adjusting them in desirable directions are discussed. Opportunities for new materials with potential for superior performance are assessed. © 2007 Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings; Alumina; Zirconia; Interfaces; Thermal properties 1. The motivation Oxides are present in turbines used for propulsion and power generation. Their benefits are manifest in a substantial increase in the longevity of various hot section components.1–8 The technology demonstrates how oxides can be used to protect structural members that experience environmental extremes. Documenting the principles that underlie this success facilitates dissemination to other systems. The technology involves choices of materials and spatial configurations, as well as survivability upon extreme temperature cycling without loss of functionality. 1.1. Materials and configurations The following considerations have motivated the choice of materials and their spatial configurations. The thermal require￾ments are straightforward (Fig. 1). By directing air through channels, the structural alloy is internally cooled: with heat transfer coefficient determined by the flow rate and the chan￾nel geometry. Subject to a combustion temperature, Tgas, and an external heat transfer coefficient, superposing an external insulting oxide allows Tgas to be raised while retaining the alloy at an allowable maximum temperature. Remarkably, insu￾lating oxides deposited onto geometrically complex structural components, such as airfoils, remain attached for extended peri￾ods despite cycling through an enormous temperature range (in ∗ Corresponding author. E-mail address: agevans@engineering.ucsb.edu (A.G. Evans). excess of 1200 ◦C) within an oxidizing environment. A single material would be incapable of satisfying these requirements. The viable solution is an oxide/metal multilayer (Fig. 2). The outer oxide imparts thermal protection: while the metallic layer (bond coat) affords oxidation protection through the formation of a second oxide, as well as plastic accommodation of strain.1–8 At the technology inception, the preferred insulating oxide was determined to be yttria-stabilized zirconia (YSZ), chosen because of its low, temperature-invariant, thermal conductivity9 (Fig. 3a). The most desirable phase was ascertained by conduct￾ing laboratory-based thermal cycle tests to seek the composition affording greatest durability: that is, the largest number of cycles before the coating spalls (Fig. 3b).16 The outcome was 7 wt.% yttria-stabilized zirconia (7-YSZ). This composition is still used, despite the discovery of lower thermal conductiv￾ity options.4,17–20 It remains the material of choice because other properties (especially toughness21–25) are also crucial. On rotating components, the layer thickness is important. It is a compromise between having sufficient thickness to achieve the desired temperature drop, yet thin enough to avert exces￾sive inertial loads, due to the extra mass. The outcome is thickness in the range 100 ≤ Htbc ≤ 250m. On stationary com￾ponents, such as shrouds and combustors, the mass is less critical and much thicker layers can be used. The choice is typically, 500m ≤ Htbc ≤ 1 mm. The principles governing the materials for oxidation protec￾tion are straightforward: albeit with nuanced implementation. (i) A thermally grown oxide (TGO) forms at the bond coat sur￾face by reaction with the combustion gas. The preferred TGO should have the lowest possible oxygen ingress at the temper- 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.12.023