Chapter 37. Thermal Stability of Yttrium Aluminates
- John B. Wachtman Jr.
Published Online: 26 MAR 2008
Copyright © 1992 The American Ceramics Society
Proceedings of the 16th Annual Conference on Composites and Advanced Ceramic Materials, Part 2 of 2: Ceramic Engineering and Science Proceedings, Volume 13, Issue 9/10
How to Cite
Mah, T., Keller, K. A. and Kerans, R. J. (1994) Thermal Stability of Yttrium Aluminates, in Proceedings of the 16th Annual Conference on Composites and Advanced Ceramic Materials, Part 2 of 2: Ceramic Engineering and Science Proceedings, Volume 13, Issue 9/10 (ed J. B. Wachtman), John Wiley & Sons, Inc., Hoboken, NJ, USA. doi: 10.1002/9780470313978.ch37
- Published Online: 26 MAR 2008
- Published Print: 1 JAN 1994
Print ISBN: 9780470375198
Online ISBN: 9780470313978
A series of publications have appeared in the literature regarding the thermal instability of the compounds in the Al2O3—Y2O3 system. The objective of this research was to resolve the confusion concerning this thermal instability issue, since the claimed thermal instabilities were actually caused by impurities. Another objective of this research was to identify this impurity-induced-degradation mechanism of the yttrium aluminates. The thermal stability of yttrium aluminates (Y3A15O12, YAlO3, Y4Al2O9) was studied in various environments. All of the yttrium aluminates were stable in vacuum or in air to at least 1650°C for prolonged periods of time. The yttrium aluminates react extensively with silicon bearing compounds (SiO2, SiO, SiC, Si3N4 etc.) in contact as well as through vapor phases. An experiment was carried out, involving the exposure of yttrium aluminates adjacent to hot-pressed SiC (NC-203), at 1650°C for 1/2 hr in 10–5 torr. After the heat-treatment, the specimens were characterized using XRD, SEM and EPMA. The XRD analysis revealed the formation of yttrium silicates on the surface. The cross-sectional SEM and EPMA analyses, using Y3Al5O12 as an example, showed the formation of yttrium silicates at the outermost scale followed by Y2O3, Y4Al2O9, and YAlO3 in the degradation zone and bulk Y3Al5O12. The probable degradation mechanism of yttrium aluminates was based upon the redox reaction and the reaction sequence was presented using Y3Al5O12 as an example.