Comparison of mechanical and tribological properties of diamond-like carbon coatings doped with Europium and Gadolinium produced by HiPIMS

Abstract

Hydrogen-free diamond-like carbon (DLC) thin films can be employed for numerous applications, including engine components, once they exhibit good properties for that. They have been used as coatings due to their attractive properties, including high hardness, high wear resistance, and low CoF even under high load/pressure. One of the major limitations of hard hydrogen-free ta-C coatings applications is related to the low reactivity with oil additives used nowadays. The Ionic liquids (ILs) emerged as a novel class of lubricants that can be used in future lubricated systems due to possessing unique physical properties, like high thermal stability and conductivity. To improve the lubrication performance of DLC with ILs, the films were doped with rare earth metals such as Gd and Eu. These non-carbide-forming elements can be introduced in the DLC matrix, incorporated as single atoms, and improve the surface adsorption and reactivity of phosphorus-based IL, and consequently, the lubricating properties of DLC/ILs sliding contacts. However, many demanding applications require good performance across the different regimes (from boundary to hydrodynamic regime). Across all regimes, the boundary is the most challenging, where metal-to-metal contact occurs. Therefore, it is essential to study the influence of the doping elements on the doped-DLCs performance under the boundary regime. For comparison, a pure-DLC was used. In this study, Eu and Gd-DLC tribological systems are characterized by pin-on-disk test in dry conditions and by scratch test. Also, mechanical properties are studied through nano-indentation. Results show that samples doped with a low atomic concentration of Eu or Gd (1%–3%), despite having a CoF higher than un-doped DLC films, show typical values for pure-DLC coatings, like low specific wear rate (<10⁻¹⁶ m³/Nm) and high hardness (23 GPa), permitting, in the future, the combination of novel nanostructured alloyed-DLCs and ILs needed to achieve the optimal lubrication performance.