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.

Effect of Europium and Gadolinium Alloying Elements on the Tribological Response of Low Hydrogen Content Amorphous Carbon

“…While most published studies focused on doping and/or alloying a-C coatings with metals or metalloids, doping a-C films with rare-earth elements − and rare-earth oxides , has only recently been explored. As one example, Omiya et al, Sadeghi et al, and Shaikh et al evaluated the tribological properties of gadolinium (Gd)- and europium (Eu)-alloyed a-C films deposited by high power impulse magnetron sputtering (HiPIMS) in the presence of synthetic oil containing a class of promising additives for transmission fluids, namely phosphonium phosphate ionic liquids. , The results of tribological experiments and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) measurements indicated that only the introduction of Gd in the a-C matrix favors the formation of a transfer layer on the steel counterbody that enhances the generation and adhesion of a lubricious and antiwear boundary layer (i.e., tribofilm) from the lubricant additives .…”

Tribological Behavior of Doped DLC Coatings in the Presence of Ionic Liquid Additive under Different Lubrication Regimes

“…They include chemical substances that can adhere to and/or interact with solid surfaces to form protective, low-shear-strength reaction layers (referred to as "tribofilms"), which are intended to lower wear and friction [2]. Furthermore, diamond-like carbon (DLC) thin films have been shown to have high hardness and wear resistance and a low friction coefficient (CoF) [3,4], and to be biocompatible. DLC thin films, one of the most promising categories of surface engineering solutions, have been the subject of many investigations, specifically in the automotive industry.…”

“…The tribological behaviors of DLC films depend on external factors such as temperature, which results in various degrees of wear rates for the same film [5,6]. Several research projects have investigated the doping of DLC films with various metallic elements to further enhance their qualities and, primarily, to adjust some undesired traits, even beyond the good mechanical and tribological capabilities of currently available DLC coatings [4]. Usually, two categories of alloying materials are used to adjust the characteristics of DLC coatings: carbides (for instance, Ti, Cr, F, and W) and non-carbide formers (for example, Cu and Ag) [7][8][9][10][11][12][13].…”

“…Ti, Cr, W, Nb, Mo, and Zr as well as the non-carbide formers Ag, Cu, and Al have been investigated as doping choices to enable a reduction in the high internal residual stress and to enhance adhesion, thermal stability, corrosion resistance, and biocompatibility [2,9,11,[14][15][16][17]. Tungsten is the most researched metallic element among those chosen for doping as a result of its potential to modify the tribological behavior of DLC under high temperatures [4,13].…”

“…Lanthanides (which include the Eu and Gd elements) are believed to have a positive effect on tribolayer formation when ionic liquids are used as additives because they show a high affinity for ILs [18,19]. On the one hand, DLC thin films that are doped with Eu or Gd at an atomic concentration between 1% and 3% exhibit the usual traits of un-doped DLC coatings, including a low specific wear rate (10 −16 m 3 /Nm) and high hardness (23 GPa) [4]. Nonetheless, if the doping elements possess a high atomic concentration in the doped DLC coating, the consequence can be detrimental to the film's characteristics [4].…”