Mechanical and tribological properties of cBN films on silicon and tungsten carbide substrates

Cubic boron nitride (cBN) films with a thickness of more than 2 µm on silicon and up to 0.8 µm on tungsten carbide substrates were prepared by reactive rf sputtering, in an Ar/N2 discharge, using an electrically conducting boron carbide (B4C) target. The increase in cBN film thickness was reached with a process including: a boron carbide interlayer with a variable thickness, and a subsequent gradient layer consisting of B, C and N. First, the boron carbide layer was deposited in a pure Ar discharge followed by a graded interlayer, which was conducted by continuously replacing Ar by N2 gas. Structure and composition of the films were investigated by: IR spectroscopy; TEM and SEM; as well as by SIMS. Tribological and mechanical properties of relatively thick (~ 1 µm) and long time stable films were investigated in detail. Quantities like: friction coefficients against different materials; abrasive wear rates; as well as hardness; Youngs's modulus; or surface tension data of cBN films were determined. The hardness measured with a nano-indentation technique was approximately 65 GPa and corresponds to the known cBN bulk value. Abrasive wear rates were clearly lower than for hard coating materials like TiN. Furthermore, results concerning process transfer to technically relevant substrates like tungsten carbide (WC-Co) will be presented.