Tribological behaviour of laser-treated C-alloyed TMD coatings in rubber contact

Abstract

Os dicalcogenetos de metais de transição (TMDs) com uma estrutura hexagonal oferecem atritos muito baixos e são usados como revestimentos sólidos auto-lubrificantes. Os revestimentos de TMD puro são macios devido às suas morfologias muito abertas e são usualmente ligados com carbono para combinar uma boa dureza com um baixo coeficiente de atrito. A adição de carbono usando técnicas de pulverização catódica dá origem a uma estrutura amorfa tornando difícil o caracter auto-lubrificante, em comparação com o WS2 puro, especialmente durante o contato em deslizamento com borracha. O objetivo deste estudo é melhorar a auto-lubrificação de revestimentos do tipo W-S-C fazendo uso de tratamentos laser. A influência do tratamento laser foi investigada comparando o desempenho tribológico entre um aço não revestido e aços revestidos com WS2 puro, e revestimentos W-S-C (50% at. de C) antes e após serem tratados com laser. A composição química, a morfologia e a estrutura foram analisadas por SEM/WDS, XRD, and espectroscopia Raman enquanto que a dureza foi avaliada por nanoindentação. Para compreender o comportamento tribológico, foi utilizado um tribómetro do tipo pino-disco e os testes foram realizados à temperatura ambiente. As pistas de desgaste foram analisadas com um perfilómetro 3D e observadas por SEM/EDS. A principal conclusão deste estudo foi que o tratamento com uma baixa potência de laser provocou a cristalização do WS2 a partir do revestimento W-S-C e, consequentemente, permitiu melhorar as propriedades tribológicas.

Transition metal dichalcogenides (TMDs) in their hexagonal crystal structure offer very low friction and are used as solid, self-lubricating coatings. Pure TMD coatings are soft because of their open morphology and are usually doped with carbon to combine low friction with good hardness. Alloying of W-S films with C using sputtering techniques can result in an amorphous structure and decline the self-lubricious property in comparison with the pure WS2 film, especially during sliding against rubber. The present study aims to improve the self-lubricity of a W-S-C coating using a laser treatment method. In fact, the influence of laser treatment was investigated by comparing the tribological behaviour among uncoated steel, pure WS2, and untreated and treated WSC (50% at. C) coating. The chemical composition, morphology, and structure of the coatings were investigated using SEM/WDS, XRD, and Raman spectroscopy whereas hardness was evaluated by nano-indentation. To understand the tribological behaviour, a uni-direction pin on disc tribometer was used at room temperature. The wear tracks were investigated under a 3-D profilometer and SEM/EDS. Main conclusion was that the treatment using low laser power can cause WS2 crystallization in the W-S-C coatings and, consequently, can improve their tribological properties. The work presented in this thesis is focused on the effect of laser treatment on the tribological behaviour of C-alloyed WS2 coating in rubber contact. Thin films of WS2 and WSC were deposited by magnetron sputtering. Four different samples were prepared which are uncoated steel polished up to 3μm, WS2, untreated, and UV laser-treated WSC (50% at. C).SEM images showed improved porosity and compactness in the coating as a result of C-alloying. Raman spectroscopy presented the typical crystalline peaks for the unalloyed WS2 coating while for the W-S-C coating it revealed the amorphous nature of the carbon phase. For the laser treated sample it revealed crystallization of WS2 in the treated areas, with signs of graphitization for the areas that are slightly overexposed. XRD diffractogram showed broad amorphous carbon peak in the region, 2θ≈30°-50°, and showed no structural difference in WSC coating when treated by laser as XRD technique is not suitable to detect small-sized WS2 crystals.Alloying of C in WS2 coatings resulted in an increase in hardness from ~0.23 GPa to ~7 GPa. The hardness of the WSC coating in treated zones was reduced to ~3 GPa from ~7 GPa. The intended mechanism of low friction for pure and alloyed WS2-based coatings is the formation of WS2 tribofilms on the outermost surface. The reason for easy shearing and ease of sliding along the sliding direction is the horizontal alignment of WS2 tribofilms. Thus, having crystalline and horizontally aligned WS2 in the coating are necessary conditions. Pure WS2 coatings have standing platelets and the basal planes realign in the direction of sliding, while for secondary coatings (amorphous or poorly crystalline), there is a rearrangement of the planes in the contact. However, WS2 tribofilms formation is true in both cases by reorientation of the basal planes during the running-in period in order to realign them in the direction of sliding.In conclusion, laser treatment of the coatings mainly influenced the running-in and the early steady-state tribological behaviour of the coatings.