Ordered and twinned multidomain structure in highly Al-rich mullite
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The structure of a region of a highly Al-rich mullite (Al4+2xSi2–2xO10–x with x ∼ 0.82; 47Al2O3-6SiO2; Al/Si ∼16) in a chemically vapor-deposited, functionally graded mullite coating was studied by high-resolution transmission electron microscopy. The region consisted of a fully ordered and twinned multidomain structure. Within each domain, well-ordered oxygen vacancies resulted in antiphase boundaries (APBs) characterized by a shift vector of [001]. The APBs were oriented parallel to (601) and (6¯01). Along [001], the structure consisted of alternating layers of (601)- and (6¯01)-domains with a spacing of 17d601, separated by (001) twinning planes. The (001) twinning did not generate any additional spots in the [010] mullite diffraction pattern. The lattice parameters of this ultrahigh Al-rich mullite phase were calculated from selected-area electron-diffraction patterns to be: a ⳱ 0.762 nm; b ⳱ 0.754 nm; and c ⳱ 0.291 nm.
I. INTRODUCTION
Silicon-based ceramics, such as SiC and Si3N4, are subject to hot-corrosion 1 and recession 2 in hightemperature combustion environments in gas turbines. To overcome these problems, mullite-based environmental barrier coatings have been extensively studied,3–6 especially due to their close coefficient of thermal expansion (CTE) match with SiC. Recently, functionally graded mullite coatings have been deposited on SiC by chemical vapor deposition (CVD), in which the composition was graded from close to 3/2 mullite (3Al2O32SiO2) at the coating–substrate interface for CTE match, to highly Al-rich compositions at the surface for hotcorrosion and recession resistance.7 It is well known that the mullite can exist in compositions of a general formula Al4+2xSi2–2xO10–x5–8 where the x represents the number of oxygen vacancies in the unit cell. An oxygen vacancy results when two Al atoms substitute for two Si atoms in the mullite structure due to the charge equilibrium. Accompanying the increase in the value of x (or Al/Si atomic ratio) is a modification of the orthorhombic structure (space group Pbam) of mullite, with an increase in the lattice constants a and c and a decrease in b.9,10 The atomic structures of mullite have been studied extensively by high-resolution electron microscopy a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0400 3210 J. Mater. Res., Vol. 22, No. 11, Nov 2007 http://journals.cambridge.org Downloaded: 03 Apr 2015
(HREM) and selected-area electron-diffraction (SAED) by several research groups. 11–17 Epicier and colleagues11,12 studied 3/2 mullite (Al/Si ∼3.0) by HREM imaging along [001] and image simulations, and reported that the oxygen vacancies were randomly distributed. Schryvers et al.13 studied the structure of mullite with an Al/Si ratio of ∼3.87 and also reported a disordered oxygen vacancy structure. Rahman and Weichert14 studied the structure of 2/1 mullite (2Al2O3-SiO2) with an Al/Si ratio of ∼4.0, and reported that the (001) projection was the best crystallographic orientation for a direct observation of vacanc
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