Effects of Spray Passes and Yttrium Content on ZrO ₂ –Al ₂ O ₃ Coatings Deposited by APS on Metal and FRP Substrates

ABSTRACT This study investigates the effects of spray pass number and yttrium content on the microstructure and mechanical behavior of ZrO ₂ –Al ₂ O ₃ coatings deposited by atmospheric plasma spraying (APS) on metal and fiber‐reinforced polymer (FRP) substrates. The research gap lies in the limited understanding of sub‐3 mol% yttria compositions applied to thermally sensitive substrates. Coatings with 2 and 2.5 mol% Y ₂ O ₃ were deposited using 1, 3, and 5 spray passes. Surface roughness ( R _a ) decreased from ∼13.7 µm (single pass) to ∼9.2–9.7 µm (five passes) on metal substrates, indicating improved uniformity. Microstructural observations revealed progressive densification and lamellar stacking with increasing passes. Nanoindentation showed enhanced hardness up to ∼622 HV and improved elastic recovery with higher yttrium content and deposition cycles, while FRP substrates exhibited significantly lower hardness (11–46 HV) due to thermal sensitivity. Although adhesion strength was not quantitatively measured, interfacial morphology suggests stronger bonding on metal substrates. These findings establish a process–structure–property relationship for reduced‐yttria APS coatings. Future work will focus on quantitative adhesion testing, porosity analysis, and optimization of spray parameters for polymer‐based substrates.