Comparative Study of Ultrasonic Nanocrystalline Surface Modification, Ion-Plasma Nitriding, and Their Combined Treatment on the Tribological Properties of AISI 321 Stainless Steel

This study presents a comparative investigation of the effects of ultrasonic nanocrystalline surface modification (UNSM), ion-plasma nitriding (IPN), and their sequential combination on the mechanical and tribological properties of 12X18H10T austenitic stainless steel. To evaluate the effectiveness of the treatments, the phase composition, hardness, elastic modulus, wear track morphology, coefficient of friction, and wear rate were investigated. It was found that IPN provides the most significant increase in surface hardness, from 260 to 1239 HV, and a reduction in wear rate to approximately 2 × 10−5 mm3/(N·m). At the same time, the average width of the wear track decreases to ≈405 μm compared to ≈972 μm for the initial specimen. After UNSM, the hardness increases to 511 HV, the coefficient of friction decreases to ≈0.62, and the wear rate is ≈2.8 × 10−4 mm3/(N·m). The combined UNSM+IPN treatment yields a minimum coefficient of friction of ≈0.55 and a maximum elastic modulus of 251.5 GPa, with hardness reaching 689 HV. SEM and X-ray phase analysis results showed that differences in tribological behavior are associated with the characteristics of structural hardening, phase formation, and the resistance of the modified surface layer to plastic deformation and local failure. Importantly, the comparative analysis revealed that the coefficient of friction, surface hardness, and wear rate do not exhibit a direct proportional relationship. In particular, IPN produced the highest hardness and the lowest wear rate despite a relatively high coefficient of friction, whereas the combined UNSM+IPN treatment resulted in the lowest coefficient of friction but did not provide the minimum wear rate. This decoupling demonstrates that friction and wear behavior are governed by different aspects of the structural and mechanical state of the modified surface. Therefore, the effectiveness of surface treatments should be evaluated using a combination of mechanical, structural, and tribological characteristics rather than a single parameter.