High-temperature oxidation and thermal-shock behavior of chromium coatings on tube interiors by bipolar pulsed magnetron sputtering

The inner walls of tubes usually demand protective coatings capable of resisting oxidation and thermal fatigue under extreme conditions. However, depositing dense and adherent coatings in confined geometries remains challenging. Cr coatings were deposited inside 40 mm-diameter steel tubes via bipolar pulse magnetron sputtering. Increasing the positive pulse voltage from 50 V to 300 V enhanced ion bombardment energy, resulting in a microstructural evolution from open and columnar to fully dense, with a concomitant increase in compressive residual stress. The 100 V coating achieved the optimal balance between density and residual stress, featuring a low surface roughness of 2.4 nm, a fully dense columnar microstructure, and a moderate compressive residual stress of approximately 1626 MPa. This balanced stress state effectively accommodated thermal expansion mismatches during oxidation and thermal cycling, thereby significantly enhancing resistance to crackling and spallation. In the course of isothermal oxidation at 900 °C, elevating the positive pulse voltage enhanced the growth of a dense and protective Cr₂O₃ scale, significantly retarding the oxidative failure of the Cr coating. Nevertheless, the excessive compressive stress induced at 300 V triggers critical stress mismatch during oxidation, leading to pronounced buckling and spallation of the oxide scale. Under thermal-shock cycling, the 100 V coating endured more than 50 cycles before failure, significantly outperforming porous coatings and over-dense coatings. These findings establish positive-pulse voltage as a critical parameter for tailoring microstructure and optimizing the high-temperature performance of Cr coatings in tubular applications.