Atmospheric plasma-sprayed (APS) TiO₂ coating is an effective surface strengthening technique for titanium alloys. However, their broader application is constrained by limited bonding strength and fracture toughness. In this study, TiO₂ coating with a heterogeneous microstructure was fabricated by APS using nano-agglomerated TiO₂ (n-TiO₂) feedstock. The resulting coating comprised partially melted nanograins, dynamically recrystallized grains originating from partially melted particles (PM-DRX), and columnar grains. Micron-agglomerated TiO₂ (m-TiO₂) and fused-and-crushed TiO₂ (f-TiO₂) powders were also employed as comparative feedstocks. The phase composition, microstructure, and mechanical properties of the optimized n-TiO₂, m-TiO₂, and f-TiO₂ coatings were systematically investigated. Owing to the synergistic strengthening effect of its distinctive heterogeneous microstructure, the n-TiO₂ coating exhibited markedly superior mechanical performance, including a microhardness of 1030 HV_0.3, a fracture toughness of 3.23 MPa·m^1/2, and, most notably, a bonding strength of 46.47 MPa. The bonding strength was approximately 45.18% and 33.85% higher than those of the m-TiO₂ and f-TiO₂ coatings, respectively. These findings demonstrate that the unique heterogeneous microstructure developed in the APS-deposited n-TiO₂ coating significantly enhances its overall mechanical properties.
 

Atmospheric plasma spraying, TiO₂ coatings, Nano-agglomerated powder, Heterogeneous microstructure, Dynamic recrystallization.