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

Cyclic oxidation tests were carried out on simple β-(CoNi)Al, Hafnium (Hf)-modified β-(CoNi)Al, Platinum (Pt)-modified β-(CoNi)Al + ξ-PtAl2, and Hf–Pt-modified β-(CoNi)Al coatings on Co-based Haynes-188 substrates at 1050 °C in air for up to 4670 h to identify the effects of Hf doping. The results showed that the β-(CoNi)Al coating reached 0.55 mg/cm2 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 + ξ-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 γ-(CoNiCr…) phase in the (CoNi)Al and Hf–(CoNi)Al coatings, leading to the formation of Cr2O3 scales or spinel phases. The β-(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.