Double-bipolar-pulse HiPIMS enables Cr deposition on tube inner walls

Depositing high-performance coatings on tubular inner walls is constrained by the coupled challenge of sustaining a dense plasma while delivering ions into deep cavities. High-Power Impulse Magnetron Sputtering (HiPIMS) produces highly ionized fluxes, but deep-cavity deposition can be limited by, plasma decay, and metal-ion return, which reduce ion availability and degrade coating quality toward the tube interior. Here, we propose a Double-Bipolar-Pulse HiPIMS (DBP-HiPIMS) mode that decouples plasma generation from directional transport. By employing a −/+/−/+ pulse sequence (negative sputtering pulses followed by reverse-bias extraction pulses), DBP-HiPIMS sustains a high-density plasma reservoir at the onset of extraction and enhances ion transport into the cavity. In a model tube (L/D ≈ 3), DBP-HiPIMS increased the integrated substrate charge at the tube tail by approximately 42% and increased Cr coating thickness in the deep interior by about 21% relative to unipolar HiPIMS. The enhanced ion bombardment promotes microstructural consolidation, producing compact Cr coatings with hardness up to 9.3 GPa. These results show that mitigating deep-cavity shadowing requires not only electrostatic acceleration but also synchronizing the extraction field with peak plasma density, offering a general waveform-engineering route for functionalizing complex internal geometries.