Growth, structure and friction behavior of the nanocomposite hard coatings WS2-Ti
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Keywords
Superlubrication, WS2-Ti, Raman, Pin on disk.
Abstract
Tungsten disulphide (WS2) and titanium doped tungsten disulphide (Ti-WS2) nanocomposites were deposited on silicon substrates, varying substrate temperature and target power, using magnetron co–sputtering in order to obtain different Ti contents in the nanocomposite. The films were analyzed using X-ray difraction (XRD), high resolution scanning electron micrscopy (HRSEM/EDS) and high resolution transmision electron microscopy (HRTEM) to observe the crystallinity and morphology behavior respect the induced Ti percentage and the substrate temperature variation. The inclusion of Ti on the co–sputtering process prevents the WS2 crystallization forming dispersed amorphous nanocrystals (1-3 nm). The friction tests performed in a Pin on Disk (POD) at low temperatures, shows that the room temperature and low Ti concentrations films (between 5 and 14%at) possesses higher life times that pure WS2 films but no significant changes in friction coefficients (COF) were observed. The same effect is determined in high temperature POD tests (500◦C) with higher changes in COF. To study solid lubricant mechanisms, samples prepared by focus ion bean (FIB), were analyzed by Raman spectroscopy, determining surface deformation and tribo-chemical compounds formation in the wear track. The formation of WO3 in the surface during wear (tribo–oxidation) and transfer to the counterface (third body generation) was observed. Ti doping, producing a nanocomposite, is a procedure that improves tribological properties of the material in humid and high temperature environments. Obtaining these improvements by means of element doping has been poorly studied in detail and a reaction mechanism is presented allowing the phenomena explanation using advanced novel analysis techniques like FIB.
PACS: 31.15.E, 81.15.-z
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