Amorphous W–S–N thin films: The atomic structure behind ultra-low friction

Abstract

Amorphous W-S-N in the form of thin films has been identified experimentally as an ultra-low friction material, enabling easy sliding by the formation of a WS 2 tribofilm. However, the atomic-level structure and bonding arrangements in amorphous W-S-N, which give such optimum conditions for WS 2 formation and ultra-low friction, are not known. In this study, amorphous thin films with up to 37 at.% N are deposited, and experimental as well as state-of-the-art ab initio techniques are employed to reveal the complex structure of W-S-N at the atomic level. Excellent agreement between experimental and calculated coordination numbers and bond distances is demonstrated. Furthermore, the simulated structures are found to contain N bonded in molecular form, i.e. N 2 , which is experimentally confirmed by near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy analysis. Such N 2 units are located in cages in the material, where they are coordinated mainly by S atoms. Thus this ultra-low friction material is shown to be a complex amorphous network of W, S and N atoms, with easy access to W and S for continuous formation of WS 2 in the contact region, and with the possibility of swift removal of excess nitrogen present as N 2 molecules.

CrCuAgN PVD nanocomposite coatings: Effects of annealing on coating morphology and nanostructure

“…In recent years, self-lubricating PVD tribological thin films have been intensively investigated [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Among these, PVD nanocomposite coatings containing soft metals (e.g.…”

Influence of composition, structure and testing atmosphere on the tribological performance of W–S–N coatings

“…Later studies have demonstrated extraordinarily low friction and wear, particularly under high load, as the ultra-low friction reported by Gustavsson et al [8] for an amorphous W-S-N coating. The structure of amorphous W-S-N was more closely described in a recent study by Isaeva et al [12], as a completely amorphous network without any WS 2 layers. W-S-N coatings thus exhibit beneficial mechanical properties, being denser and harder than pure W-S, as well as low friction and wear due to the formation of WS 2 tribofilms.…”

“…The broad features observed for the W-S-N RT and some of the HT coatings are not possible to assign to any specific phase. It can be noted that the nearest neighbour distances for the atoms in amorphous W-S-N are similar to the plane distances corresponding to 2θ angles of 30°-40° [12], so an increased background in this area can be expected. For the HT-47 coating, which has the highest N content, clear peaks from another crystalline phase are observed.…”

“…The N1s region of the spectra (not shown) displays a main contribution at around 397.0-397.5 eV (with the lower binding energies for higher N contents), which is similar to N bonded to W [23]. There are also smaller contributions at 400 eV corresponding to N-N bonds [12] and 402 eVcorresponding to oxidised species on the surface, where the latter is removed by sputter etching. Overall, the X-ray photoelectron spectra display gradual changes in the chemical bonding with the addition of N. Similar values and trends are observed for the HT and RT series, indicating that the local bonding conditions are similar, even though the long-range structure as reflected by XRD is different for HT and RT.…”

“…The pure W-S coating deposited at room temperature, RT-0, showed broad peaks indicating the presence of a poorly crystalline WS x phase, while the N-containing RT coatings were X-ray amorphous, with no distinguishable peaks. The same coatings were studied in [12], where theoretical calculations showed that a high N content lead to the formation of a completely amorphous network, and TEM studies of a coating of similar composition in reference [8] showed it to be amorphous. For the HT-0 coating, clear WS x peaks appeared in the diffractogram.…”