Enhancing Hardness and Adhesion Strength of Cr Thin Coatings for Large-Aspect-Ratio Tube Interiors via Bias-Voltage-Tuned Microstructure

The surface treatment of inner walls in large-aspect-ratio tubes plays a pivotal role in ensuring long-term stable operation of equipment. Although traditional methods like electroplating are widely used, they often suffer from limited protection, poor adhesion, and environmental drawbacks. Magnetron sputtering (MS) represents a promising alternative; however, few studies have successfully developed coatings that balance high hardness with strong adhesion on the inner walls of tubes featuring large length-to-diameter ratios. In this study, we deposited Cr coating on the inner wall of a tube with an aspect ratio of 13.3 using MS technology, and regulated the process to enhance the hardness and adhesion strength of the coating. By systematically varying the bias voltage, the microstructure and properties of the coating were effectively tailored. Results show that a bias voltage of −200 V promotes pronounced (110) preferred orientation in the Cr coating, yielding an optimal combination of high hardness (~11.54 GPa) and a low coefficient of friction (~0.4). Furthermore, scratch testing revealed robust coating–substrate adhesion strength, with initial failure loads exceeding 50 N under various bias conditions. These findings demonstrate that optimizing the bias voltage during MS deposition is an effective approach for fabricating high-performance Cr coatings, providing a viable strategy to improve the durability and reliability of high-end equipment.