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Relationships between oxide morphology and coating structure
Thin SoBd Films, 84 (1981)73
METALLURGICALAND PROTECTIVECOATINGS
73
RELATIONSHIPS BETWEEN OXIDE MORPHOLOGY AND COATING STRUCTURE* D. P. WHITTLE,S. SCHAFFERAND D. H. BOONE MMRD, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 (U.S.A.)
The performance of almost all protective coating systems is dependent on the formation of a protective adherent oxide, usually A120 3. However, there is little real understanding of the critical factors affecting its initial formation, growth, adherence and long-term stability. Coating composition, structure, method of processing, minor or active element addition and underlying substrate are all known to exert considerable influence on the overall performance of coatings. As part of an overall program addressing these needs, the oxidation behavior of a number of Co-Cr-A1 coatings containing active element additions such as yttrium and hafnium has been studied. All the coatings form an A1z 0 3 scale on oxidation. The adherence of the scale to the coating, however, depends on the presence of the active elements which encourage the formation of oxide pegs which grow inwards, penetrating into the coating and keying the scale to the surface. The growth of these pegs is best studied by a newly devised deep-etching technique in which a B r - C H 3 O H solution is used to remove sufficient metal so that the underside of the oxide can be examined using scanning electron microscopy. In this way the peg development can be closely correlated with the coating structure. The morphology of the pegs has been found to be critical in maximizing the scale adhesion, and this is markedly dependent on the structure of the coating. In particular, pegs appear to grow preferentially down the ~-13 interfaces in the coating. With electron beam physically vapor-deposited coatings, which have a highly oriented structure normal to the surface, this means that the pegs can act as effective anchors, pinning the scale to the coating. With plasma-sprayed coatings, the ~/13 structure does not have such a favorable orientation, and keying of the scale is not effective. The coating composition is also important: increasing the aluminum concentration increases the total amount of the 13phase and consequently decreases the amount of interface favorable for peg formation. This work was supported by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (Contract W-7405-ENG-48). *Abstract of a paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A.,April 6-10, 1981. 0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedin The Netherlands
METALLURGICALAND PROTECTIVECOATINGS
73
RELATIONSHIPS BETWEEN OXIDE MORPHOLOGY AND COATING STRUCTURE* D. P. WHITTLE,S. SCHAFFERAND D. H. BOONE MMRD, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 (U.S.A.)
The performance of almost all protective coating systems is dependent on the formation of a protective adherent oxide, usually A120 3. However, there is little real understanding of the critical factors affecting its initial formation, growth, adherence and long-term stability. Coating composition, structure, method of processing, minor or active element addition and underlying substrate are all known to exert considerable influence on the overall performance of coatings. As part of an overall program addressing these needs, the oxidation behavior of a number of Co-Cr-A1 coatings containing active element additions such as yttrium and hafnium has been studied. All the coatings form an A1z 0 3 scale on oxidation. The adherence of the scale to the coating, however, depends on the presence of the active elements which encourage the formation of oxide pegs which grow inwards, penetrating into the coating and keying the scale to the surface. The growth of these pegs is best studied by a newly devised deep-etching technique in which a B r - C H 3 O H solution is used to remove sufficient metal so that the underside of the oxide can be examined using scanning electron microscopy. In this way the peg development can be closely correlated with the coating structure. The morphology of the pegs has been found to be critical in maximizing the scale adhesion, and this is markedly dependent on the structure of the coating. In particular, pegs appear to grow preferentially down the ~-13 interfaces in the coating. With electron beam physically vapor-deposited coatings, which have a highly oriented structure normal to the surface, this means that the pegs can act as effective anchors, pinning the scale to the coating. With plasma-sprayed coatings, the ~/13 structure does not have such a favorable orientation, and keying of the scale is not effective. The coating composition is also important: increasing the aluminum concentration increases the total amount of the 13phase and consequently decreases the amount of interface favorable for peg formation. This work was supported by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (Contract W-7405-ENG-48). *Abstract of a paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A.,April 6-10, 1981. 0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedin The Netherlands