Self-Lubricity of WSe_x Nanocomposite Coatings

Abstract

Transition metal chalcogenides with lamellar structure are known for their use in tribological applications although limited to vacuum due to their easy degradation in presence of oxygen and/or moisture. Here we present a tailored WSe x coating with low friction (0.07) and low wear rates (3×10 -7 mm 3 Nm -1 ) even in ambient air. To understand the low friction behaviour and lower chemical reactivity a tribological study is carried out in a high vacuum tribometer under variable pressures (atmospheric pressure to 10 -8 mbar). A detailed investigation of the film nanostructure and composition by advanced transmission electron microscopy techniques with nanoscale resolution determined that the topmost layer is formed by nanocrystals of WSe 2 embedded in an amorphous matrix richer in W, a-W(Se). After the friction test, an increased crystalline order and orientation of WSe 2 lamellas along the sliding direction 2 have been observed in the interfacial region. Based on high angle annular dark field, scanning transmission electron microscopy and energy dispersive X-ray analysis, the release of W atoms from the interstitial basal planes of the a-W(Se) phase is proposed.These W atoms reaching the surface, play a sacrificial role preventing the lubricant WSe 2 phase from oxidation. The increase of the WSe 2 crystalline order and the buffer effect of W capturing oxygen atoms would explain the enhanced chemical and tribological response of this designed nanocomposite material.

Effect of Humidity on Friction and Wear—A Critical Review

“…Making composites with PTFE [118] and WC [119] were also claimed to improve the tribological properties of the coating. Meanwhile, it was also reported that if the crystalline order of the coating is increased, the humid resistance could be improved [120][121][122]. This phenomenon is also suggestive that the edge sites play an important role in the humidity dependence.…”

Friction Force Microscopy Analysis of Self-Adaptive W–S–C Coatings: Nanoscale Friction and Wear

“…1−4 Low friction stems from the ability to form an oriented film on the sliding surface, with the basal planes aligned parallel to the surface, due to low adhesion and shear strength between the planes. 5 At the nanoscale, a rotational disorder is expected between the planes in order to decrease dissipated energy and thus friction (incommensurate sliding). 4 One of the main drawbacks of pure TMD coatings (typically prepared by magnetron sputtering) is their low load-bearing capacity and limited wear resistance.…”

Preparation of Low Friction MoSex/nc-Mo Coatings Containing Spherical Mo Nanoparticles

“…Domínguez-Meister et al [22] have obtained the low friction coatings containing a composition of TMD nanocrystals with metal-based amorphous phase. This nanocomposition of the coatings allowed implementing low friction (0.07) and low wearing rate (3 10 -7 mm 3 N -1 m -1 ) even in ambient air.…”