FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings

Soumendra Basu, Boston University, DMR 0233952

Growth, Structure and Corrosion Properties

Functionally graded mullite coatings have been deposited by CVD on SiC substrates to achieve some of the highest alumina-rich mullite to date.

The real structure of mullite (Al4+2xSi2-2xO10-x), with x ~ 0.82; which corresponds to 47Al2O3•6SiO2, or Al/Si ~ 16, which is one of the most alumina-rich mullite reported to-date has been examined by HRTEM and the nature of the domain structure and oxygen vacancy distribution in these mullite compositions has been reported for the first

time. The functionally graded mullite coatings were tested with uncoated SiC samples under very severe corrosive exposures to molten Na2SO4 at 1200 °C for 100 hours. The weight of the coated samples did not change after corrosion and the coating structure remained the same. The uncoated sample, on the other hand, had localized regions that underwent severe corrosion accompanied by the formation of a glassy phase. The performance of these functionally graded mullite coating at 1200°C under these severe hot-corrosion conditions is highly promising.

Coated and uncoated SiC samples after 100 hour exposure to molten Na2SO4 at 1200°C.

Mechanical PropertiesA laser-based ultrasonic system (LBU) is used to measure the mechanical properties and thickness of mullite environmental barrier coatings deposited on SiC substrates. Narrow-band surface acoustic waves (SAWs) are generated with an amplitude modulated laser source, and a photorefractive crystal based interferometer is used to detect the resulting surface displacement. The elastic properties and thickness of coatings found using this non-contact approach agree well with those measured using optical microscopy and nanoindentation. A spatial averaging approach has been developed to overcome error in property measurement due to substrate porosity. The LBU system is potentially suitable for in-situ process monitoring.

FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings

Soumendra Basu, Boston University, DMR 0233952

Elastic modulus profile in a typical mullite coating measured using nanoindentation (left) and a processed optical microscopy image used to determine the mean coating thickness (right).

(b)

Surface wave velocity measurements as a function of frequency (left) ; spatial averaging is used to minimize the influence of substrate porosity and theoretical fit of the dispersion curve (right). The mean modulus and thickness values show excellent agreement with those found using nanoindentation and optical microscopy.

Education and OutreachPh.D. student Tushar Kulkarni has been involved in the growth of the functionally graded mullite coatings and the study of their corrosion resistance. Ph.D. student Thomas Steen has been involved in the study of the mechanical behavior of the coatings. Post doctoral researcher Dr. Hengzhi Wang has been studying the real structure of the coatings by HRTEM. The research team has presented papers at several international conferences.

The PI’s organized the “Learning Experiences for New Scientists (LENS) camp for the second time at the end of July 2007. Activities included composite materials fabrication and testing, an introduction to optics through pinhole photography and holography, using ultrasound for medical imaging and the nondestructive evaluation of materials, and an introduction to nanotechnology. The camp lasts for one week and seeks to promote interest and excitement in science and technology in 7th and 8th grade students. (for more details, see http://www.bu.edu /lernet/lens/index.html).

FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings

Soumendra Basu, Boston University, DMR 0233952

Middle school students participating in the LENS program in July 2007