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

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.