Evaluation of Thermal Cycling Performance of Plasma Sprayed Alumina Coating on Inconel 600 with Different Bond Coats

Plasma sprayed alumina coatings find numerous applications owing to their high electrical resistivity and wear resistance. However, the high-temperature thermal cycling performance of such coatings is a concern in many applications. Inconel 600 collars used in the high-temperature fission chamber neutron detectors need to have a ceramic coating with high electrical resistivity and thermal cycling performance. Alumina is coated on Inconel 600 collars by plasma spray process with an intermediate Ni-50Cr bond coat deposited by HVOF technique. The coated collars exhibited superior thermal cycling performance and insulation resistance of 2 × 108 Ω at 600 V DC after 100 thermal cycles at 923 K. The satisfactory performance of coated Inconel 600 collars suggests its potential application as fast reactor neutron detector. Further, the durability of alumina coated samples with Ni-50Cr and NiCrAlY bond coats deposited by plasma spray coating were also studied. Inconel 600 samples coated with Ni-50Cr bond coat with and without alumina top coat subjected to isothermal aging at 923 K for 1000 h showed a higher weight gain than those coated with NiCrAlY bond coat. The isothermally aged Ni-50Cr bond coat showed undesirable epitaxial growth with protruding Cr2O3 crystals (13.5 µm) while the NiCrAlY bond coat showed uniform thermally grown oxide (TGO) of 1 µm with fine disc-shaped pegs. The results indicated higher long-term durability of plasma sprayed alumina coated samples with NiCrAlY bond coat and the mechanisms of formation of TGO with Ni-50Cr and NiCrAlY bond coats at the service temperature are elucidated.