Oxygen diffusivity in alumina scales grown on Al-MAX phases

Abstract

a b s t r a c t Ti 3 AlC 2 , Ti 2 AlC, and Cr 2 AlC are oxidation resistant MAX phase compounds distinguished by the formation of protective Al 2 O 3 scales with well controlled kinetics. A modified Wagner treatment was used to calculate interfacial grain boundary diffusivity from scale growth rates and corresponding interfacial grain size, based on the pressure dependence of oxygen vacancies and diffusivity. MAX phase data from the literature yielded grain boundary diffusivity nearly coincident with that for Zr-doped FeCrAl (and many other FeCrAl alloys), suggesting similar oxidation mechanisms. The consolidated body of diffusivity data was consistent with an activation energy of 375 ± 25 kJ/mol.Published by Elsevier Ltd.

Oxidative durability of TBCs on Ti2AlC MAX phase substrates

“…A number of in-depth oxidation studies (as reviewed and analyzed by Tallman et al [2]) have established slow growing alumina kinetics similar to oxidation resistant metallic FeCrAl-type heater alloys. Due to grain growth in the scale, the kinetics follow a cubic rate law, except for an initial transient due to rapid TiO 2 growth on the order of just 5 min [2,7,9,10]. Furthermore, growth models produce grain boundary diffusion rates in the scale in accord with those obtained for oxidation resistant alumina-forming FeCrAl alloys [3,9,10,11].…”

Compiled furnace cyclic lives of EB-PVD thermal barrier coatings

“…In that regard, the 382 kJ/mol value obtained in Eq. (1) for the Pt-modified aluminide coating and that reported for Pt-Al bulk alloys, 356 kJ/mol (85 kcal/mol) [48] are essentially equivalent to the 375 kJ/mol estimated for grain boundary diffusion of oxygen in alumina scales [49]. (This consistency only holds if there is little grain growth in the scales formed on these coatings.)…”

Environmental resistance of a Ti2AlC-type MAX phase in a high pressure burner rig

“…The value of 279 kJ/mol was measured for another Ti 2 AlC material in air (and 261 kJ/mol in 100% H 2 O) by intermittent oxidation testing [27]. By comparison, the value estimated for grain boundary diffusion of oxygen in alumina scales was ∼375 kJ/mol [28]. An example of Zr-doped FeCrAl oxidation data used for grain boundary diffusivity is also shown in Fig.…”

“…This behavior has been widely studied in the literature and related to grain growth in the scale, with rate control by oxygen grain boundary diffusion. [24,25,17,23,27,28] Little differences are noted between Ti 2 AlC or Ti 3 AlC 2 substrates [17]. Static furnace oxidation in water vapor has not been shown to produce significant changes in the kinetic law or rate constants.…”

“…An example of Zr-doped FeCrAl oxidation data used for grain boundary diffusivity is also shown in Fig. 5 (right axis) [23,28,29], where it was suggested that Al 2 O 3 scale growth was controlled by similar oxygen grain boundary diffusivity for Ti 3 AlC 2 , Ti 2 AlC, Cr 2 AlC, and FeCrAl. These activation energies are offered for completeness and some noted similarities.…”