Microstructural Characterization and Correlation with Mechanical Properties of Plasma-Sprayed Zirconia Coatings

NiAl bond coats were deposited on a number of AISI 310 specimens using HVOF spray technique. Furthermore, these specimens were coated, to form a duplex coating, with ZrO₂–10% Y₂O₃–18% TiO₂ using plasma spray technique. Duplex coatings were then characterized using optical microscope, Scanning Electron Microscopy (SEM) coupled with energy dispersive X-ray analysis (EDS), and X-ray Diffraction (XRD). The fracture toughness of these coatings was evaluated using micro-indentation techniques; additionally the abrasion wear resistance was measured employing the standard rubber wheel abrasion test. The results show that as sprayed microstructure is typically porous with partially melted particles and splats, which was directly related to anisotropic results of indentation toughness. Cracks were found to initiate and propagate easily in direction parallel to the coating growth directions than along the splat boundaries. EDS analyses revealed that oxidation of aluminum, in the NiAl alloy, occur in the high-temperature plasma-spray stream during deposition. Abrasive wear traces of the as-sprayed coatings were very rough and observable with the bare eye, which indicates that the material was removed in large pieces with the size of a lamella.