Designing hydrogen-free diamond like multilayer carbon coatings for superior mechanical and tribological performance

Diamond like carbon (DLC) coatings are extensively employed for their outstanding mechanical and tribological properties. To overcome the inherent high residual stress, therefore brittleness, DLC coatings with multilayer architecture were developed in the form of stacking alternate hard and soft layers to avoid premature failure under severe loading conditions. The current study was designed to investigate the impact of bilayer thickness (hard & soft) on wear of multilayer DLC particularly at high contact stress (2.0 GPa, 2.7 GPa, 3.0 GPa, 3.4 GPa). Seven DLC multilayer (1:1 bilayer ratio) samples with 1, 2, 5, 10, 20, 40, 80 bilayers were deposited on 440 C steel and the overall coating thickness was mainatined at ~1 µm. Moreover, the bilayer thickness effect was determined on mechanical, scratch adhesion and structural properties. Transmission electron microscope (TEM) was used to visualize discrete hard and soft layers in 80 bilayers (~6 nm thin layer). G peak suppression and ID/IG increment were observed with reduction in bilayer thickness. Hardness, modulus, elastic strain to failure (H/E), plastic deformation resistance (H³/E²) and residual stresses showed an inverse relation with bilayer thickness. Furthermore, micro scratch adhesion decreases with layer thickness reduction. Only 100 nm and above bilayer thickness samples (1, 2, 5, 10 bilayers) could survive under high contact stress while the other three (20, 40, 80 bilayers) showed brittle failure using pin-on-disc tribometer. Additionally, 10 bilayers (100 nm thickness) produced the minimum wear (~7.9 ×10^-8 mm³/Nm) at 80 N.