Effect of APS Arc Current and Hydrogen Flow Rate on the Structure and Mechanical Properties of Gd2Zr2O7 Topcoats in a Multilayer YSZ/GZO Thermal Barrier System

Thermal barrier coatings protect metallic components operating under severe high-temperature conditions, but conventional yttria-stabilized zirconia (YSZ) coatings face limitations at elevated temperatures. Gadolinium zirconate (Gd2Zr2O7, GZO) is a promising alternative topcoat material; however, controlling its porosity and mechanical integrity through atmospheric plasma spraying (APS) remains a key challenge. A hybrid multilayer thermal barrier coating system consisting of an Inconel 718 substrate, an atmospheric plasma-sprayed (APS) NiCrAlY bond coat, a high-velocity oxy-fuel (HVOF)-deposited 8YSZ intermediate layer, and an APS-deposited Gd2Zr2O7 (GZO) topcoat was investigated. The study focused on establishing the relationship between APS processing parameters and the structural and mechanical characteristics of the GZO layer. Four deposition regimes were examined by varying the arc current between 550 and 600 A and the H2 flow rate between 1.1 and 1.8 L/min, while maintaining an Ar flow rate of 40 L/min and a spraying distance of 100 mm. The coatings were characterized by SEM/EDS, XRD, image-based porosity analysis, surface profilometry, instrumented indentation, and pull-off testing. XRD revealed Gd2Zr2O7 together with cubic an<3d tetragonal ZrO2-related phases for all investigated conditions, whereas the most pronounced effect of the APS parameters was observed in coating porosity and mechanical response. The GZO1 condition (550 A, 1.5 L/min H2) produced the lowest porosity of 1.0% and the highest indentation resistance. A single-specimen pull-off test yielded a strength of 27.66 MPa; however, the failure location was not determined. Increasing the arc current to 600 A increased porosity to 16.55% and reduced the pull-off strength to 17.92 MPa. Surface roughness varied only slightly, within Ra = 5.25–5.80 µm. The results demonstrate a non-monotonic process–structure–property relationship and show that maximizing plasma-energy input does not necessarily improve GZO coating quality. Among the investigated conditions, 550 A and 1.5 L/min H2 yielded the lowest porosity and highest indentation resistance; however, thermal conductivity and thermal-cycling performance were not evaluated, and the thermal-barrier performance of this condition remains to be established.