Thermal cyclic performance of NiAl/alumina-stabilized zirconia thermal barrier coatings deposited using a hybrid arc and magnetron sputtering system

In the present work, a hybrid arc/sputter deposition system is used to deposit alumina-stabilized zirconia (ASZ) thermal barrier coatings. An initial NiAl bond coat is deposited on the nickel super alloy substrates, in the same coating system, by arc ion plating alone. The as-deposited ASZ coatings have a dense columnar structure. From X-ray diffractometry (XRD), it is found that the as-deposited ASZ films are almost entirely tetragonal. This is believed to result from the relatively high concentration of aluminum (10 at.%) in the ASZ films. Thermal testing is carried out over numerous cycles, each consisting of 30 and 10 min at 1100 °C and ambient, respectively. During thermal testing, a significant amount of the tetragonal phase is transformed to monoclinic. Furthermore a significant number of cracks, both parallel and perpendicular to the substrate surface, are found to occur in the ASZ layer as a result of tensile and compressive thermal stresses. Although a thermally grown oxide (TGO) layer between the top and bond coat, and Kirkendall voids in the interfacial region between substrate and bond coat occur in this coating system, the failure occurs principally as a result of parallel cracks which form just above the TGO. After 200 thermal cycles, a great portion of the topcoat has failed. The formation of cracks, and the subsequent coating failure, is likely to result from the dense coatings being unable to accommodate the thermal stresses parallel to the surface during the cycling tests.