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H.Z. Wang Division of Materials Science and Engineering, Boston University, Brookline, Massachusetts 02446; and Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467

S.N. Basu and V.K. Sarin Division of Materials Science and Engineering, Boston University, Brookline, Massachusetts 02446; and Department of Mechanical Engineering, Boston University, Boston, Massachusetts 02215 (Received 2 September 2008; accepted 29 October 2008)

Dense, crystalline mullite (3Al2O3
2SiO2) coatings have been deposited by chemical vapor deposition on Si-based substrates using the AlCl3–SiCl4–CO2–H2 system. A graded coating composition has been achieved in the coatings, with the Al/Si ratio being stoichiometric (3) at the coating/substrate interface, and increasing monotonically toward the outer coating surface. The highest reported Al-rich mullite has been deposited in the process. At high Al/Si ratios, the mullite structure breaks down and an aluminosilicate phase similar to the metastable d* Al2O3 is nucleated. Experimental evidence is presented in this study that this phase has some Si-incorporation in it and has been called d*(Si)Al2O3. Like the other known aluminosilicates, d*(Si)Al2O3 converts to mullite on heating at elevated temperatures. I. INTRODUCTION

The Al2O3
SiO2 system has three known polymorphs, namely sillimanite, kyanite, and andalusite that were all discovered in the early nineteenth century.1 Bowen and co-workers2 have proved that the only solid solution stable phase within this system was a composition of a 3:2 (Al2O3/SiO2) ratio instead of 1:1 (Al2O3/SiO2). Mullite has been the subject of numerous studies because of its outstanding electrical, mechanical, and thermal properties. The compositional variations and corresponding ranges in the crystal structure of mullite have been the subject of intense characterization throughout the last 50 years. The Al/Si ratio, which is 3 in stoichiometric mullite, depends on various factors including precursors and processing methods used, and the latter constitutes sinter-mullite, fused-mullite, and chemical vapor deposited (CVD) mullite. In this study, CVD has been used to deposit mullite coatings with potential promise to protect Si-based ceramics for high temperature applications.3–6 The composition of these functionally graded mullite coatings was varied from silica-rich close to the coating/substrate (SiC) interface to alumina-rich toward the outer surface of the coating, with the aim of achieving silica-free mullite hypothesized as i-alumina by Cameron.7 It is a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0062

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http://journals.cambridge.org

J. Mater. Res., Vol. 24, No. 2, Feb 2009 Downloaded: 11 Apr 2015

well known that it is possible to change x from 0 to 1 in the general mullite formula of Al4+2x Si2–2x O10–x.8,9 At one end of the spectrum is sillimanite at x = 0, and at the other end is i-alumina at x = 1, which has not been synthesized to date. The highest Al-rich mullite that has been reporte