Title: Study on surface strengthening of titanium alloy by hollow cathode plasma source nitriding

 

Professor Yang Li at the School of Nuclear Equipment and Engineering, Yantai University, and Deputy Director General of the Shandong Provincial Heat Treatment Professional Committee. His research interests include ion chemical heat treatment, coatings and films, tribology, and other fields. Hosted projects such as the National Natural Science Foundation of China and the China Postdoctoral Fund, participated in two 973 projects, published over 60 SCI academic papers, and has been granted 18 invention patents.

 

With high specific strength, toughness, corrosion resistance, high temperature resistance and good biocompatibility, titanium and titanium alloys are the basic materials for aerospace manufacturing, biomedical implants, marine engineering equipment and automotive products. In order to meet the use scenario of titanium alloy, titanium alloy workpieces will develop in the direction of large, integral, miniaturized and complex, so 3D printed titanium alloy applied in medical and health care, aerospace, marine engineering and other industries is getting more and more attention. However, their poor tribological properties often limit titanium and titanium alloys. In order to improve the defects of titanium and titanium alloys, several surface strengthening techniques have been attempted to improve the overall performance of titanium alloys. This study investigates the surface modification of cast TC4 and EBMTC4 samples using Hollow Cathode Plasma Source Nitriding (HCPSN) technology. The relationship between the microstructure of the modified layer and its corrosion resistance is analyzed, along with the friction and corrosion behaviors of the TC4 titanium alloy after nitriding at various temperatures. The results show the following key findings: (1) After nitriding, the surface of the samples consists of a mixture of TiN, Ti2N, and α-Ti(N) phases. The compound layer, from the surface to the core matrix, is composed of a nanocrystalline TiN top layer, a crystalline TiN sub-layer, a Ti2N interlayer, and a solid solution α-Ti(N) bottom layer. (2) The surface hardness of the samples increased by 2 to 5 times after nitriding, the surface roughness increased slightly, and the effective nitrided layer thickness reached 50–110 μm. (3) Compared with untreated TC4 alloy, the TiN-based nitrided layer exhibited a distinct passive region, significantly reduced corrosion rate, and enhanced pitting resistance. However, the corrosion resistance of the nitrided layer containing the Ti2N phase was relatively degraded. The passive film on the TiN nitrided layer surface showed higher triple-layer resistance and lower impedance capacitance, effectively hindering the penetration and migration of active ions, thereby significantly improving corrosion resistance. (4) The average friction coefficient of the TC4 titanium alloy substrate was 0.41, and the wear mechanism of the TC4 substrate was primarily plastic deformation and severe adhesive wear. After nitriding, the samples exhibited higher resistance to plastic deformation, improved tribological performance, and the wear mechanisms shifted to mild adhesive wear and abrasive wear, with reductions in both the friction coefficient and wear depth. (5) After nitriding, the EBM samples showed more stable open-circuit potential, and the corrosion current density decreased to one-tenth of that of the substrate. (6) During the corrosion-friction process of the EBMTC4 substrate, the wear mechanism was a combination of corrosion wear and adhesive wear, with slight abrasive wear. The wear mechanism of the nitrided samples after HCPSN treatment was mild abrasive wear and corrosion wear, and the corrosion-wear resistance of the EBM nitrided samples was significantly improved.