Synthesis and characterization of super hard, self-lubricating Ti–Si–C–N nanocomposite coatings

Abstract

Quaternary super hard Ti–Si–C–N coatings with different carbon contents were deposited on high-speed steel substrates by pulsed direct current plasma-enhanced chemical vapor deposition (PECVD) technology, using a gaseous mixture of TiCl 4/SiCl 4/N 2/H 2/CH 4/Ar. A variety of technologies have been employed to characterize the coatings, including X-ray diffraction, scanning and transmission electron microcopies, X-ray photoelectron spectroscopy, energy dispersive X-ray analysis, automated load–depth sensing and pin-on-disc. The super hard Ti–Si–C–N coatings were found to have unique nanocomposite structures composed of nanocrystallite and amorphous nc-Ti(C,N)/a-Si 3N 4/a-C and/or nc-Ti(C,N)/nc-TiSi 2/nc-Si/a-Si 3N 4/a-C, depending on the carbon contents in the coatings. The friction coefficient of the Ti–Si–C–N coatings with a higher carbon content (nc-Ti(C,N)/a-Si 3N 4/a-C nanocomposites) were found to be much lower than those of the Ti–Si–N coatings both at room and elevated temperatures, suggesting the formation of a graphite-like lubricious phase of amorphous carbon. However, they are still super hard (32–48 GPa) in spite of the carbon incorporation. This is due to a strong, thermodynamically driven and diffusion-rate-controlled (spinodal) phase segregation that leads to the formation of a stable nanostructure by self-organization. The energy difference between the grain boundary and the crystallite/amorphous phase interface hinders grain boundary mobility, leading to a gradual decrease in the grain size of the nanocrystallites. As a result, nanocomposite Ti–Si–C–N coatings with high hardness and a low friction coefficient can be produced. The coatings are foreseen to have high potential in dry and high-speed cutting tool applications, thus providing for cleaner, healthier and more pleasant machining conditions.