Optimizing mechanical properties in single-layered and multi-layered amorphous carbon coatings

Hydrogenated amorphous carbon coatings improve mechanical performance characteristics on a variety of substrates, resulting in widespread industry applications requiring a combination of high hardness, low coefficient of friction, wear resistance, elasticity, and adhesion. However, such a combination of properties is difficult to achieve. We investigated the structure, mechanical properties and scratch behavior in single-layered and multilayered amorphous carbon thin films made by plasma immersion ion implantation-based plasma enhanced chemical vapor deposition. The elastic modulus and hardness in single-layered films were improved up to 149 GPa and 17 GPa respectively by varying the applied bias voltage and pulse width. We observed a non-equivalent impact on mechanical properties and coating structure based on whether pulse width or frequency were varied in the duty cycle range of 1-12.5%. Mechanical properties were independent of pulse frequency within experimental uncertainty. All the single-layered films showed exceptional elastic recovery of ~70%. Film microstructures as measured by Raman microscopy showed that the maxima in modulus/hardness as a function of bias voltage correlated to films with the highest sp(3) bonding fraction, highest sp(2) disorder, and overall lowest disorder. The maxima in hardness and elastic modulus as a function of pulse width correlated with intermediate sp(3) bonding fraction and highest overall disorder, but the sp(2) disorder was nearly unchanged. Based on single-layer results, multilayers were developed. Nanoindentation and scratch testing showed that while multilayering preserved high hardness and improved friction behavior irrespective of the recipe, the scratch volume was strongly correlated to the hardness.