Advanced High Temperature Coating Systems Beyond Current State of the Art Systems.
Final technical rept. 1 Jan 80-31 Dec 84,
PITTSBURGH UNIV PA DEPT OF MATERIALS SCIENCE AND ENGINEERING
Pagination or Media Count:
Alpha-Al2O3 and the SiO2 are the two most effective reaction product barriers that can be used to protect alloys from oxidation at temperatures above 1000 C. In the present work the techniques to improve the adherence of Al2O3 scales on MCrA1 coating alloys are studied and the characteristics of SiO2 scales formed on Ni-Si alloys are investigated. The theoretical aspects of oxide-metal adhesion are examined and the factors which affect the adherence of oxide scales in general are clearly defined. A critical review of the previously proposed mechanisms of the improved scale adherence by active element additions is presented. Experimentally, the effects of small additions of yttrium and hafnium on the isothermal and cyclic oxidation behavior of MCrAl type alloys are studied. The base alloys used in the study of Al2O3 scale adherence were Ni-20wtCr-1OwtAl and Co-20wtCr-10wtAl. Both the isothermal and the cyclic oxidation tests were carried out at 1100 C for varying durations. Emphasis is given to the in situ study of the oxide scale failure process. An acoustic emission AE technique was employed for this purpose. The above study indicates that the Al2O3 scale failure processes are quite different in the doped i.e. containing oxygen active elements and updoped alloys. Although both yttrium and hafnium can be very effective in improving the adherence of Al2O3 scales, there are some basic differences in their oxidation behavior in alloys. Therefore, the optimum conditions for their addition also differ. Alloys in the Ni-Si system 5 to 22.5wt Si have been oxidized in the temperature range 900 to 1100 C.
- Coatings, Colorants and Finishes