Effect of Normal Load on Wear Behavior of Low-Temperature Plasma-Nitrided TC4 Titanium Alloy

TC4 titanium alloy suffers from inferior wear resistance, which severely restricts its service in tribological key components, including aero-engine compressor blades and aircraft landing gear. Although plasma nitriding can strengthen alloy surfaces, conventional high-temperature nitriding frequently induces matrix grain coarsening, deteriorated mechanical properties, and workpiece dimensional distortion. In this study, low-temperature plasma nitriding was performed on TC4 alloys at 700 °C and 750 °C, and reciprocating wear behaviors under 5 N, 10 N, and 15 N normal loads were systematically investigated. A TiN/Ti2N compound layer was fabricated on the alloy surface after nitriding. When the nitriding temperature rose from 700 °C to 750 °C, the nitride layer thickness increased from 2.07 µm to 2.75 µm, and surface hardness increased from 507.67 HV to 592.37 HV, far exceeding the 309.70 HV of the original substrate. All nitrided specimens exhibited significantly lower wear depths and specific wear rates than the untreated TC4 alloy, which was associated with the increased surface hardness and the formation of the nitrided layer. The specimen nitrided at 750 °C exhibited shallower wear scars and lower specific wear rates under all investigated loads, demonstrating better load adaptability and wear resistance. Plasma nitriding fundamentally switches the primary wear mechanism of TC4 alloy from severe adhesive-abrasive wear to mild abrasive wear with minor oxidation and micro-spallation.