Performance-based selection of cathodic-arc PVD TiAlN and AlCrN coatings for SR34 piston-ring steel: Tribological, tribocorrosion and thermal assessment

Piston rings operate under severe sliding conditions, high temperatures, and corrosive environments, making surface degradation a critical factor affecting engine durability. This study presents a comparative evaluation of cathodic-arc physical vapor deposition (CA-PVD) TiAlN and AlCrN coatings on SR34 martensitic stainless-steel piston-ring material using structural, mechanical, tribological, thermal, and tribocorrosion assessments. FTIR, XRD, XPS, and SEM–EDAX analyses indicated that the TiAlN and AlCrN coatings had thicknesses of approximately 3.98 µm and 3.78 µm, respectively, and were continuous. AlCrN exhibited the lowest thermal-mismatch stress (12.47 MPa), a thermal expansion coefficient that closely matched that of SR34, and the smoothest surface (Ra = 0.0783 µm). In contrast, TiAlN exhibited superior mechanical properties, with an average Vickers hardness of 2626.46 HV and a nanoindentation hardness of 28.02 GPa. During lubricated sliding tests under 10 kg and 15 kg loads, TiAlN displayed the lowest COF, weight loss, and specific wear rate. Additionally, the critical failure loads determined through scratch testing were higher for TiAlN. Tribocorrosion tests revealed that TiAlN possessed the noblest corrosion potential, the lowest corrosion current density, the highest charge-transfer resistance, and the most stable friction response. Overall, TiAlN is recommended for applications where wear and tribocorrosion resistance are paramount. In contrast, AlCrN is preferable for applications prioritizing surface smoothness and thermal compatibility in piston-ring design and selection.