Cyclic oxidation behavior and microstructure evolution of aluminized, Pt-aluminized high velocity oxygen fuel sprayed CoNiCrAlY coatings

Abstract

In this study, the Hastelloy-X superalloy samples were firstly overlaid by a CoNiCrAlY bond coating utilizing a high pressure, high velocity oxygen fuel (HVOF) spray process. Then platinum thin film approx. 7.5 µm thick was introduced to selected test samples of CoNiCrAlY coatings by a magnetron sputtering deposition process. Then the HVOF sprayed superalloy coupons, with and without Pt coating were pack aluminized for 4 h at 850 °C to produce (Co,Ni)Al and PtAl 2 aluminide phases on their surfaces, respectively. All specimens were subjected to a thermal cycling test at 1100 °C. Then the aluminizing and Pt-aluminizing effects relative to cyclical oxidation behavior and microstructure evolutions of the coatings were evaluated. Scanning electron microscopy (SEM), X-ray diffractometry (XRD) and electron probe microanalyzer (EPMA) were used to identify crystalline phases and microstructures of each coating. Results clearly indicated that the surface roughness of the HVOF sprayed CoNiCrAlY coatings were unchanged after aluminizing or the Pt-aluminizing process. The oxide scales spalled after 50 h and 100 h cyclic oxidation for the HVOF sprayed sample and aluminized sample respectively, while the oxide scale attached successfully to the substrate for the Pt-aluminized sample after testing for 150 h. It is obvious that the Pt-aluminizing process significantly improves the oxidation resistance of HVOF sprayed coatings, while the isolated aluminizing process demonstrated negligible effect.