Oxidation of Al 2 O 3 Scale-Forming MAX Phases in Turbine Environments
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Oxidation of Al2O3 Scale-Forming MAX Phases in Turbine Environments JAMES L. SMIALEK High temperature oxidation of alumina-forming MAX phases, Ti2AlC and Cr2AlC, were examined under turbine engine environments and coating configurations. Thermogravimetric furnace tests of Ti2AlC showed a rapid initial transient due to non-protective TiO2 growth. Subsequent well-behaved cubic kinetics for alumina scale growth were shown from 1273 K to 1673 K (1000 °C to 1400 °C). These possessed an activation energy of 335 kJ/mol, consistent with estimates of grain boundary diffusivity of oxygen (375 kJ/mol). The durability of Ti2AlC under combustion conditions was demonstrated by high pressure burner rig testing at 1373 K to 1573 K (1100 °C to 1300 °C). Here good stability and cubic kinetics also applied, but produced lower weight gains due to volatile TiO(OH)2 formation in water vapor combustion gas. Excellent thermal stability was also shown for yttria-stabilized zirconia thermal barrier coatings deposited on Ti2AlC substrates in 2500-hour furnace tests at 1373 K to 1573 K (1100 °C to 1300 °C). These sustained a record 35 µm of scale as compared to 7 μm observed at failure for typical superalloy systems. In contrast, scale and TBC spallation became prevalent on Cr2AlC substrates above 1423 K (1150 °C). Cr2AlC diffusion couples with superalloys exhibited good long-term mechanical/oxidative stability at 1073 K (800 °C), as would be needed for corrosionresistant coatings. However, diffusion zones containing a NiAl-Cr7C3 matrix with MC and M3B2 particulates were commonly formed and became extensive at 1423 K (1150 °C). DOI: 10.1007/s11661-017-4346-9 © The Minerals, Metals & Materials Society and ASM International (outside the USA) 2017
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INTRODUCTION
IT is well known that turbine engine efficiency and performance increase with operating temperature as the lean fuel-to-air ratio approaches chemical stoichiometry. Current turbine engines rely heavily on Ni-base superalloys for high temperature components because of their robust mechanical properties, even up to ∼85 pct of their melting temperatures [1673 K (1400 °C)]. This is well below the maximum gas temperature so further benefits could be accrued using more heat-resistant materials. The most widely adopted material approach is that of thermal barrier coatings (TBC), where an insulative yttria-stabilized zirconia (YSZ) layer is deposited on the superalloy component. This allows a temperature gradient of ∼100 °C for back-side-cooled articles, giving a YSZ maximum temperature in the vicinity of 1523 K (1250 °C). Higher temperatures induce cyclic failure from thermal expansion mismatch stresses. Another approach is that of SiC ceramic matrix composites (CMCs), which could, theoretically, operate near 1673 K (1400 °C). While each approach has proven advantages, they must make some compromises to JAMES L. SMIALEK is with the NASA Glenn Research Center, Cleveland, OH 44135. Contact e-mail: [email protected] Manuscript submitted July 7, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
