Microstructure and oxidation resistance of a NiCrAlY/Al2O3-sprayed coating on Ti-19Al-10Nb-V alloy

A Ti3Al (α2) alloy (Ti-19Al-10Nb-3V, wt%) coated by air plasma spraying (APS) with Ni22Cr10AlY and Ni22Cr10AlY/Al2O3 has been tested against cyclic oxidation in air at 850 °C for 10 cycles of 10 h each. Effects of pre-oxidation and cyclic oxidation treatments, both in air, on the microstructure of the single α2/Ni22Cr10AlY and double coated α2/Ni22Cr10AlY/Al2O3 specimens were investigated by X-ray diffraction (XRD), SEM-EDS and Raman spectroscopy. Several distinct results appear through the cyclic oxidization of the pre-oxidized α2/Ni22Cr10AlY, namely, elemental diffusion where Ti and Al diffuses from one side and Ni and Cr from the other side, oxygen is found to be present at the support-bond coat interface and a partial reaction of the superficial NiO with α-Al2O3 to form the non-protective NiAl2O4 takes place. Conversely, pre-oxidation treatment of the double coated α2/Ni22Cr10AlY/Al2O3 specimens causes transient alumina polymorphs obtained by APS to convert to α-Al2O3 and hence to ensure the lowest cyclic oxidation rate seen by a kp value of 2.62×10−12 g2 cm−4 s−1. Good adhesion to support of all the coated specimens is recorded following the cyclic oxidization treatment.