Deposition of well adhering cBN films up to 2 μm thickness by B–C–N gradient layer system

Deposition of cBN films was carried out in a R.F. diode sputtering apparatus with an auxiliary magnetic field using a boron carbide (B 4C) target. Adhesion of the cBN films was vastly improved by a compositional gradient layer which contains B, C and N. By employing this interlayer, the deposition of thick (up to ∼2 μm) and stable cBN films were enabled. The SIMS depth profile measurements revealed that the gradient layer system started with a boron carbide layer, which is the first layer on the substrate, and had a composition approximately that of B 4C. At the interface between the B 4C and the cBN layer, a step-like or continuous increase of N was clearly visible while B and C decreased, respectively. From the IR spectra of each gradient layer step, the B–C bonding was observed for the B 4C layer and was gradually replaced by sp 2 B–N as the N concentration was increased. The gradient layer system exhibited relatively high plastic hardnesses ranging from 24 to 30 GPa. The IR spectra of thick films showed that these films contain a high amount of cBN phase. The X-ray diffraction confirmed that the film was predominately cBN and was [110] textured. The hardness measurements carried out by nano-indentation showed that these cBN layers have a hardness of up to 60 GPa. The surface morphology was observed by AFM and the surface roughness increased with increasing coating thickness. Based on the results of structural and mechanical property analysis, a primary mechanism for adhesion improvement will be proposed.