Tribological behaviour of DLC-coated Ti6Al4V under dry and submerged sliding contact: Role of PA-CVD process gas mixture

Ti6Al4V is widely used in biomedical implants due to its corrosion resistance and biocompatibility. However, it suffers from poor tribological performance and early failures. To enhance its surface properties, Diamond-like carbon (DLC) coatings are applied and, when deposited by plasma-assisted chemical vapour deposition (PA-CVD), the addition of different gas precursors might improve their properties. In this study, four DLC coatings (a-C:H, a-C:H:Si, a-C:H:N, and a-C:H deposited with supplemental H₂ gas) were deposited onto Ti6Al4V using PA-CVD. Their structure, thickness, mechanical properties and adhesion were characterized. Tribological behaviour was evaluated by Pin-On-Disk tests using an alumina ball under dry conditions or submerged in either 5 % NaCl or Ringer's solution. Friction coefficients were measured, and wear tracks were analysed using optical, confocal and electron microscopy. Additionally, time-to-failure tests were conducted under Ringer's solution until coating failure. The silicon-doped DLC exhibited the highest mechanical properties, thickness and adhesion. Nevertheless, it showed the highest wear volume under dry conditions, 8 × 10⁻⁶ mm³ N⁻¹ m⁻¹, likely due to the formation of hard and abrasive particles. For the other coatings, wear rates were two orders of magnitude lower, around 9 × 10⁻⁸ mm³ N⁻¹ m⁻¹ in dry conditions. In submerged tests, the Si-doped DLC failed early, whereas the a-C:H deposited with supplemental H₂ gas demonstrated the best wear resistance at 1 × 10⁻⁷ mm³ N⁻¹ m⁻¹ and a friction coefficient of about 0.08. In time-to-failure tests, nitrogen-doped coating displayed an outstanding resistance, breaking after 420,000 cycles, likely due to higher residual compressive stresses.