Effect of Argon Flow Rate on the Tribological Performance of Self-lubricating WS2/a-C Sputtered Coating

Layered transition metal dichalcogenides (TMD) such as WS2 are well-known materials for their solid lubricating properties [1]. However, the lubricating performance degrades through oxidation or moisture and it is also limited by its low load-bearing capacity. In contrast amorphous diamond-like carbon (DLC) films are reported to have several features which contribute to excellent tribological characteristics, such as high hardness, wear resistance with both low friction coefficient and low wear rate[2]. The coating processing parameters such as argon pressure may have a strong influence on the structure and properties of the TMD coating[3]. The present research aims at depositing WS2/a-C nanocomposite coatings on silicon wafers by magnetron co-sputtering method. The effect of argon flow rate on the composition, microstructure and mechanical performance of the nanocomposite coating was investigated. The WS2/a-C nanocomposite tribocoating was scrutinized by various microscopy techniques (AFM, HRSEM, and HRTEM) and mechanical testing. Results indicate that with increasing argon flow from 10 sccm to 25 sccm, the S/W stoichiometric ratio and deposition rate of the coating increase whereas the hardness and elastic modulus decrease. These findings indicate that the coating becomes looser and in fact it turns out becoming more columnar-like, as was also confirmed by the enhanced roughness.
Ball-on-disk tribotests show that the coefficient of friction against 100 Cr6 steel ball as counterpart can be as low as 0.017 in a dry environment (5% relative humidity). It reaches around a value of 0.15 in a high humidity surrounding and remains rather stable within 20000 sliding cycles. Transmission electron microscopy reveals feathery WS2 platelets distributed in the amorphous carbon matrix, which account for the ultralow friction.