Nanoparticle based and sputtered WS2 low-friction coatings — Differences and similarities with respect to friction mechanisms and tribofilm formation

• Published in: Surface & coatings technology Vol. 232; pp. 616 - 626
• Main Authors: Gustavsson, Fredrik, Svahn, Fredrik, Bexell, Ulf, Jacobson, Staffan
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.10.2013; Elsevier
• Subjects: Applied sciences; Coatings; Cross-disciplinary physics: materials science; rheology; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Tribo Materials; Exact sciences and technology; Exact solutions; Formations; Friction; Inorganic fullerene-like nanoparticles; Low-friction coating; Materials science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Molybdenum disulfide; Nanoparticles; Nanostructure; Oxidation resistance; Physics; Production techniques; Steel Forming and Surface Engineering; Stålformning och ytteknik; Surface treatment; Surface treatments; Teknisk fysik med inriktning mot materialvetenskap; Teknisk fysik med inriktning mot tribomaterial; TEM; Tribofilm; Tungsten disulfide; Inorganic fullerene-like nanoparticles; Tribofilm; TEM; Tungsten disulfide; Low-friction coating; Mechanical properties; Surface treatments; Mechanism; Sputtering; Nanoparticles; Transmission electron microscopy; Friction; Sputtered coatings
• ISSN: 0257-8972; 1879-3347; 1879-3347
• DOI: 10.1016/j.surfcoat.2013.06.045

MoS2 and WS2 are widely known intrinsic low-friction materials that have been extensively used and thoroughly investigated in literature. They are commonly produced in the form of sputtered coatings and show extremely low friction coefficients in non-humid environments, but rapidly degrade in humid conditions. Close nested fullerene-like nanoparticles of these materials have been proposed to have better oxidation resistance due to their closed form with the absence of dangling bonds. In the present study, an electrochemically deposited coating consisting of fullerene-like nanoparticles of WS2 embedded in a Ni–P matrix is tested under various loads and humidity conditions and compared with a sputtered WS2 coating with respect to their tribological behavior. The formation of a tribofilm on both surfaces is known to be crucial for the low-friction mechanism of WS2 and the different mechanisms behind this formation for the two types of coatings are investigated. It is shown that despite the completely different transformation processes, the resulting tribofilms are very similar. This is analyzed thoroughly using SEM, AES and TEM. The friction coefficient is known to be lower at higher normal loads for these materials and in the present study the mechanical and chemical responses of the tribofilm to higher normal loads during sliding are investigated. It was observed that the basal planes become aligned more parallel to the surface at higher loads, and that the tribofilm is less oxidized. It is suggested that these mechanisms are connected and are crucial keys to the wear life of these materials. •A coating with WS2 nanoparticles in a Ni-matrix was compared to PVD sputtered WS2.•Friction and wear behaviors from different environments and conditions are analyzed.•Effect of humidity was investigated for both structures.•Differences and similarities in formation mechanisms of the tribofilms are shown.•TEM investigations display improved properties of the tribofilm at higher load.