Spalling of Al₂O₃ Coating Film on Alloys Doped with Different Oxide or Rare Earth Metal

Abstract

Our previous works show that S promotes the spalling of Al₂O₃ coating film, and that Y₂O₃ dispersion, as well as rare earth metal (REM) addition, suppresses the spalling through trapping S. In this work comparison was made between dispersion effect of Y₂O₃ and other oxides. Surface segregation of S was measured in an Auger electron spectroscopy system on several alloys. Then they were coated with Al₂O₃ and cyclically heated at 1100 K. The spalling promotion effect of S was little on Fe-20Cr ferritic alloys with any oxide dispersion because of small difference in expansion coefficient between Al₂O₃ and the alloy. In accord with previous works the S effect was very extensive on type 316 austenitic stainless steel by casting without oxide dispersion. But the spalling was moderate on mechanically alloyed 316 steels with Y₂O₃ or Al₂O₃ dispersion although any oxide could not trap S. This result suggests that mechanical alloying method gives the steels some effect to suppress the spalling. Thus the difference between Y₂O₃ and other oxides dispersion was scarcely observed on these 2 series of alloys. On the other hand, the defference between Y₂O₃ and REM was very clear. Y₂O₃ suppressed surface segregation of P but REM did not. The S trapping effect by REM was reconfirmed.