Oxidation behavior of Hf-modified aluminide coatings on Haynes-188 at 1050 °C

Cyclic oxidation tests were carried out on simple beta-(CoNi)Al, Hafnium (Hf)-modified beta-(CoNi)Al, Platinum (Pt)-modified beta-(CoNi)Al+xi-PtAl2, and Hf-Pt-modified beta-(CoNi)l coatings on Co-based Haynes-188 substrates at 1050 degrees C in air for up to 4670 h to identify the effects of Hf doping. The results showed that the beta-(CoNi)Al coating reached 0.55 mg/cm(2) weight gain at 1960 h and became unstable, resulting in massive spallation at the sample edges; the Hf-(CoNi)Al coating exhibited stable weight gain up to 4520 h followed by a rapid weight increase, corresponding to the formation of Cr- and Mn-containing spinel phases. Both Pt-(CoNi)Al+xi-PtAl2 and Hf-Pt-(CoNi)Al coatings showed stable weight gain for the duration of the test. Examination of the surface and cross-section microstructure by Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) indicated that the original coating was completely transformed into a gamma-(CoNiCr...) phase in the (CoNi)Al and Hf-(CoNi)Al coatings, leading to the formation of Cr2O3 scales or spinel phases. The beta-(CoNiPt)Al phase persisted to the end of the test in the Pt-(CoNi)Al and Hf-Pt-(CoNi)Al coatings, showing dense, adherent Al2O3 scales. The oxidation resistance from the best to the worst can be ranked as: Hf-Pt-(CoNi)Al or Pt-(CoNi)Al>Hf-(CoNi)Al>(CoNi)Al. (C) 2012 Elsevier B.V. All rights reserved.