Preparation of Nitrided Titanium Alloys using Bipolar-Pulse Plasma

Abstract

In this study, the Ti-6Al-4V alloy, widely employed in biomedical implant applications, underwent plasma nitriding treatment. One investigation focused on varying the frequency of the power supply in bipolar pulse plasma, while the other examined the effect of the H2/N2 gas ratio on the surface properties of plasma-nitrided Ti-6Al-4V alloy. The nitriding process utilized an N2-H2 plasma, with hydrogen flow rates ranging from 100 (duty cycle of 20%) to 500 sccm at a duty cycle of 10%, and a nitriding temperature of 650±5 ºC for 4 hours. Bipolar pulse frequencies ranged from 25 to 200 kHz. Samples subjected to a 20% duty cycle exhibited a matte surface, possibly due to disparities in ion bombardment energy levels. Grazing incidence X-ray diffraction spectrometry (GI-XRD) analysis revealed the presence of δ-TiN and ε-Ti2N phases in all nitrided samples, with increased ε-Ti2N formation observed at a 20% duty cycle. Results indicated a surface hardness approximately three times greater than that of the unnitrided sample, with maximum hardness observed in samples subjected to a 20% duty cycle. Glow discharge emission spectroscopy (GD-OES) confirmed higher surface nitrogen content in samples with a 20% duty cycle, suggesting deeper nitrogen penetration. Post-plasma nitriding, surface roughness slightly increased, particularly under the 20% duty cycle condition, resulting in elevated water contact angles and reduced work of adhesion. The specific wear rate of all nitrided samples decreased, notably at a bipolar pulse frequency of 50 kHz, aligning with stable coefficients of friction after 6000 sliding cycles. Additionally, samples nitrided at 50 kHz exhibited the lowest corrosion current density in artificial saliva, as determined by the Tafel potential polarization method.

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