FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings

1. FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings PI Soumendra Basu, Boston University, DMR-0233952 Structure and Growth Functionally graded coatings have been grown by changing the gas phase composition during growth. The highest alumina-rich mullite (Al/Si ratio of 33) reported to date has been grown by this technique. At such high Al/Si ratios, there is a very large concentration of oxygen vacancies in the structure. A high-resolution TEM micrograph of a region of such composition shows the formation of domains formed due to the ordering of these vacancies, leading to the formation of anti-phase boundaries (APBs) aligned along the (601) planes in the structure. HREM micrograph of high Al/Si mullite, showing the formation of domains separated by APBs aligned along (601) planes due to vacancy ordering. The kinetics of coating deposition as a function of deposition parameters has also been studied. It was found that increasing the reactor temperature, reactor total pressure and total metal chloride partial pressure led to increased deposition rates and grain sizes, before gas-phase nucleation occurred leading to cauliflower like growths on the coating surface accompanied by a decrease in the deposition rates.

2. FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings PI Soumendra Basu, Boston University, DMR-0233952 Mechanical Properties A laser based ultrasonic system has been developed which allows for thickness and elastic property measurements on as-grown coatings with optically rough surfaces. The laser source generates surface acoustic waves which propagate over the substrate/coating system. The dispersion characteristics of these waves are related to the coating properties. The laser ultrasonic results for film modulus show good agreement with local coating measurements made through the coating thickness using nanoindentation. Surface acoustic wave dispersion measurements on a mullite coating (left) and a comparison of mean elastic modulus values obtained over 3 regions of a coating using nanoindentation and laser based ultrasonics (right). An array of nano-indentations across a polished mullite coating cross-section. High frequency laser ultrasonic system used for coating inspection.

3. FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings PI Soumendra Basu, Boston University, DMR-0233952 Outreach and Education Three graduate students and one post-doc have contributed to this work. In addition two REU students have been supported this summer, working on fluid flow and thermodynamic simulations as well as assisting in CVD growth and characterization. The research team presented papers at several International Conferences. The PI’s organized two events for local high school and junior high students. The first was an “AME Day” in which students came to Boston University to learn about engineering careers. The event consisted of an introductory lecture, a career panel, and lab tours. The second event was the LENS (Learning Experiences for New Scientists) program. Activities included composite materials fabrication and testing, an introduction to optics through pinhole photography and holography, and using ultrasound for medical imaging (for more details, go to http://www.bu.edu/lernet/programs/index.html ).