Internal cylindrical cathodic-arc deposited Cr coatings on slender tubes: Uniformity, toughness, and oxidation resistance

Facing the challenges of confined geometries, axial uniformity, and inner wall cleaning, the deposition of Cr coatings on the inner wall of slender tubes is considerably challenging. Hence, based on the limitations of conventional arc ion plating techniques, we propose an innovative internal cylindrical cathodic-arc ion plating methodology that integrates electric field-assisted deposition with dynamic cleaning mechanisms, enabling simultaneous inner wall cleaning and coating deposition within a single processing cycle. Notably, despite the remarkably narrow target-to-substrate distance of merely 7.5 mm, the developed technique demonstrates exceptional deposition uniformity along the axial direction, achieving a remarkably relative thickness deviation as low as 0.95%. Building upon this achievement, we systematically investigate the thickness-dependent (5.6–18.4 μm) evolution of structural characteristics and functional properties of the Cr coatings by adjusting the deposition time. The comprehensive investigation encompasses three critical aspects of the deposited coatings: (1) microstructural characteristics, (2) mechanical properties, and (3) high-temperature oxidation resistance. The experimental results demonstrate a strong correlation between coating thickness and mechanical performance enhancement, revealing that increased thickness substantially improves cracking resistance, toughness, and critical load capacity (Lc3 > 100 N). Regarding oxidation resistance, microstructural analysis indicates that thicker coatings develop a characteristic network of fine mesh surface cracks, which effectively inhibits crack propagation toward the substrate interface, thereby significantly improving protective capabilities. These findings highlight the crucial role of coating thickness optimization in enhancing the functional performance of Cr coatings on internal tubular surfaces, offering valuable insights for the development of durable, high-performance tubular components operating under extreme service conditions.