Scratch resistance analysis of plasma-nitrided Ti-6Al-4V alloy

A modified plasma nitriding treatment developed for Ti-6Al-4V alloy at a low temperature (600 °C) and in a dilute nitriding atmosphere (3% N2) has shown to improve the alloy tribological properties and resistance to surface crack initiation and propagation. This modification formed a thin compound layer (CL) (1.9 μm) on the surface of Ti-6Al-4V followed by an α-Case (1.8 μm) and a deep nitrogen diffusion zone (DZ) (44.4 μm). Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses revealed that the compound layer consisted of Ti2N, TiN, and TiN0.3 nitrides. The failure mechanisms of the nitrided samples subjected to micro-scratch testing under both constant and progressive loading were investigated using scanning and transmission electron microscopy, focused ion beam milling (FIB), and optical surface profilometry. The onset of failure events was observed as the applied load reached 0.75 N (mean Hertzian pressure of 4.4 GPa) with the appearance of angular cracks on scratch path rims and subsurface microcracks within the CL likely at grain boundaries. Normal loads higher than 8 N (9.7 GPa) indicated the advent of tensile surface cracks perpendicular to the scratch direction and subsurface cracks which propagated beyond the CL. As the normal load exceeded 15.7 N (12.1 GPa), angular and tensile cracks joined and formed semi-circular cracks. FIB observations indicated that these cracks penetrated into the diffusion zone but were effectively stopped by ductile β particles at α grain boundaries. The plasma-nitriding also led to a significant reduction in COF without any signs of compound layer spallation and delamination during micro-scratch testing.