Investigating the impact of plasma nitriding on Ti6Al4V surface, structural, and mechanical properties and their simultaneous evaluation via laser opto-ultrasonic dual detection (LOUD) approach

Titanium alloys possess exceptional properties, but due to their poor surface characteristics, several engineering processes have been developed to modify surface properties. Surface modification methodologies such as plasma nitriding exhibit a favourable pathway to improve these properties. This study investigates the impact of plasma nitriding on surface properties and their evaluation via the laser opto-ultrasonic dual detection (LOUD) technique. The scanning electron microscope, X-ray diffraction, and tensile testing results showed that with the increase in nitride plasma power density from 1.42 to 10.0 × 102 W/cm2, a substantial variation in microstructure and phase transformation occurs, eventually refining the grain size, increasing the hardness from 310.83-HV5.0 to 745.50-HV5.0 and elastic modulus (E) from 115.26 to 128.35 GPa, respectively. Furthermore, these characteristics were assessed concurrently through ultrasonic and optical signal processing for LOUD detection. The results of G.S and E from the acoustic attenuation coefficient and longitudinal and shear wave velocities are reliable with the results of conventional optical microscopy and tensile testing with (R2 = 0.996). Meanwhile, the optical spectral data were analyzed to determine the hardness. The results showed that the calibration curve of the intensity ratios (Ti-II/Ti-I) and plasma electron temperature exhibited a linear relationship with hardness (R2 = 0.989), which showed a good approximation.