Relationship of chemical and structural properties with the tribological behavior of sputtered SiCN films

Abstract

SiCN films have been produced along the tie line SiC–Si3N4, SiC–C3N4 and Si3N4–C3N4 by means of reactive magnetron sputtering of a silicon target in an Argon/Nitrogen/Acetylene atmosphere. The mechanical, chemical and structural properties have been thoroughly investigated by means of indentation hardness testing, pin on disk wear testing in reciprocating sliding motion, GDOES, FT-IR, Raman spectroscopy, XPS and STA.The main aim of this investigation was to establish the relationship between deposition conditions, resulting mechanical, chemical and structural properties and the respective wear properties. Depending on their position in the Si–C–N phase diagram, the hardness of the films varies over a broad range, with maximum values of around 30 GPa, while Young's modulus remains in a narrow range around 200 GPa. XPS spectra showed the main component to be Si–C but Si–N and to a lesser extent C–C bonds were also detected. Further, IR spectra suggested the presence of the carbodiimide group. Raman Spectra show a varying ratio of sp3 to sp2 carbon, depending on deposition condition.The hardest films were found along the SiC–Si3N4 tie line. These films exhibited the highest load carrying capacity in lubricated sliding as well as the best adhesion. In dry sliding their brittleness coupled with a high coefficient of friction led to premature coating failure. Carbon rich films have a very low coefficient of friction leading to good wear behaviour in dry conditions, but their ability to withstand high Hertzian pressures is reduced. The low coefficient of friction is attributed the more graphitic structure of the free carbon in the films. Good wear properties in dry reciprocating sliding as well as high load carrying capacity in lubricated sliding were achieved by producing gradient films from silicon rich to carbon rich at the surface.