Thermal shock cycling behavior of NiCoCrAlYSiB coatings on Ni-base superalloys: I. Accelerated oxidation attack

NiCoCrAlYSiB coatings were obtained using an arc ion plating technique on a directionally solidified Ni-base superalloy. Thermal shock resistance of the bare and coated specimens were investigated by cycling between high temperature (900, 1000 and 1050 °C) and room temperature (forced water quenching). The experienced attack during thermal shock cycling was studied by gravimetry, XRD, SEM and EDS. The results showed that bare alloys and coated specimens experienced aggravating oxidation under thermal shock cycling conditions. The coated specimens exhibited excellent thermal shock resistance and provided enough protective effect on the superalloys under thermal shock cycling conditions. The accelerated oxidation mechanism and crack formation and propagation in the coatings were also investigated. The accelerated oxidation can be attributed to the accelerating oxide formation and breakdown due to compressive stresses accumulation in the oxides due to oxide growth at high temperature and thermal mismatch during cooling. The formation and propagation of cracks formed in coatings can be attributed to tensile stresses in oxides generated by irreversible cyclic creep accumulation in the metallic coatings and the “wedging” effect of oxide protruded into the coatings.