Tribocorrosion behaviors of nc-TiC/a-C:H nanocomposite coatings: In-situ electrochemical response

Abstract

Tribocorrosion is a complex process coupled with wear and corrosion behaviors in aggressive solutions, which is closely related with in-situ electrochemical response in terms of the transient variation coefficient of friction (COF) and open circuit potential (OCP). Hence, in this work, nc-TiC/a-C:H nanocomposite coatings were deposited using filtered cathodic vacuum arc at various C2H2 flow rates (fC2H2). With an increase of fC2H2 from 20 to 70 sccm, the coatings exhibit the structure evolution from (111)-texture to random orientation along with the decreasing crystallinity degree. Dense microstructure and smooth morphology are obtained in nc-TiC/a-C:H nanocomposite coatings. The sp2/sp3 ratios show an initial increase from 2.2 to 2.8, then followed by a decrease to 2.5. Accordingly, the decrease of hardness(H), hardness and effective Young's modulus (E*) ratio (H/E*), and H3/E*2 are obtained from 45.8 GPa, 0.12 and 0.67 at 20 sccm to 30.7 GPa, 0.11 and 0.38 at 70 sccm, respectively. Moreover, in potentiodynamic polarization tests, the coatings deposited at 70 sccm show the highest corrosion potential of 0.14 V and corrosion current density of 4.67 × 10−8 Acm−1 , thanks to the contribution from amorphous carbon phase. During tribocorrosion tests in 3.5 wt.% NaCl aqueous solution, however, the coatings at 20 and 70 sccm exhibit mild abrasive wear with similar values of COF of 0.2, OCP of -0.025 V and specific wear rate of 1.03 × 10−6 mm−3 N−1 m−1. The enhanced tribocorrosion properties of the coating at 20 sccm are attributed to the high hardness, H/E* and H3/E*2. The amorphous carbon phase in the coating at 70 sccm makes up for the deficiency of hardness, H/E* and H3/E*2. Moreover, in cycling tribocorrosion tests, nc-TiC/a-C:H nanocomposite coatings reveal that passive films formed on sliding contact surface possess a strong ability of regeneration and self-repairation.