The failure of protective oxides on plasma-sprayed NiCrAlY overlay coatings

The oxidation behavior in air of air-plasma sprayed (APS) overlay coatings of Ni-25Cr-6Al-Y have been studied at 1,100 C. A protective alumina scale developed after 5- to 10-hr exposure with, initially, parabolic growth kinetics. With protracted exposures (>100 hr), subparabolic behavior develope...

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Bibliographic Details
Published inOxidation of metals Vol. 53; no. 3-4; pp. 241 - 258
Main Authors NIRANATLUMPONG, P, PONTON, C. B, EVANS, H. E
Format Journal Article
LanguageEnglish
Published Heidelberg Springer 01.04.2000
Subjects
ALUMINIUM ALLOYS
ALUMINIUM OXIDES
Applied sciences
CHROMIUM ALLOYS
COATINGS
Corrosion
Corrosion environments
Exact sciences and technology
FAILURES
KINETICS
MATERIALS SCIENCE
Metals. Metallurgy
NICKEL ALLOYS
OXIDATION
TEMPERATURE RANGE 1000-4000 K
YTTRIUM ALLOYS
Aluminium alloy
Experimental study
Coatings
Surface treatment
Oxidation rate
Multi-element alloys
Chromium alloy
Oxidation
Spallation
Kinetics
Yttrium alloy
Overlay
Protective coatings
Nickel alloy
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Summary:The oxidation behavior in air of air-plasma sprayed (APS) overlay coatings of Ni-25Cr-6Al-Y have been studied at 1,100 C. A protective alumina scale developed after 5- to 10-hr exposure with, initially, parabolic growth kinetics. With protracted exposures (>100 hr), subparabolic behavior developed, associated with aluminum depletion within the coating caused, principally, by internal oxidation of the low-density APS structure. This depletion caused intrinsic chemical failure, manifested by the formation of a layer of Cr,Al,Ni-rich oxide beneath the residual alumina layer. Associated with this process of chemical failure was the formation of a layer of porous Ni,Cr-rich oxide above the alumina layer. Oxide spallation occurred by delamination within this layer during cooling; the spallation sites tended to lie above protuberances in the underlying coating. Initial spallation occurred at a critical temperature drop, which decreased rapidly with increasing exposure time. A nonrigorous model of this spallation process has been developed which envisages that delamination occurs by the propagation of an oxide void under the action of out-of-plane tensile stresses developed during cooling. Agreement with the spallation data is encouraging and shows that the deterioration of spallation resistance with exposure time arises not only because oxide thickness increases but also because the maximum void size within the porous oxide layer increases.
ISSN:0030-770X
1573-4889
DOI:10.1023/a:1004549219013