Influence of Atmospheric Plasma Spray Parameters on Particle Dynamics, Adhesion, and Microstructure of Titanium Coatings Deposited on PEEK

Abstract Polyetheretherketone (PEEK) is a leading high-performance polymer for orthopedic applications due to its favorable biomechanical properties; however, its inherent bioinertness and low surface energy often lead to poor osseointegration. While titanium coatings are used to overcome these limitations, the existing literature lacks comprehensive data on the direct correlation between plasma parameters, in-flight particle dynamics, and the resulting coating properties on thermally sensitive PEEK. This study addresses this gap by investigating the influence of atmospheric plasma spray (APS) parameters on in-flight particle behavior and coating properties. To isolate the effects of thermal and kinetic energy, coatings were deposited by varying plasma current (425-475 A) and stand-off distance (300-350 mm) under constant plasma gas conditions, with particle temperature and velocity measured directly at the substrate plane. Statistical analysis via one-way ANOVA revealed that while increasing plasma current significantly enhanced surface roughness and thickness, it concurrently reduced tensile adhesion strength ( p < 0.05). Fractographic and microstructural analysis identified that intensified thermal flux at higher currents is associated with localized substrate outgassing and plume turbulence, leading to macroscopic defects and a transition to mixed adhesive-cohesive failure modes. The maximum adhesion strength of 14.64 ± 1.21 MPa was achieved at 425 A and 300 mm, yielding a surface roughness of Ra = 11.8 μm, which is well suited for promoting primary stability and long-term osseointegration. By establishing a quantitative process–structure–property framework, this work defines a viable operational regime that balances mechanical robustness with thermal safety, providing reproducible guidance for engineering titanium coatings on polymer-based substrates.