Structural features of thick c-boron nitride coatings deposited via a graded B–C–N interlayer

Thick c-BN films (up to 2.7 μm) were deposited onto Si substrates by an r.f. diode apparatus using boron carbide (B4C) targets. The c-BN films were deposited on a compositionally graded interlayer, which consisted of B, C and N. A thin B4C layer (∼200 nm) had been initially deposited onto Si substrate in a pure Ar gas discharge. The following formation of the graded interlayer was conducted by step-like or smoothly replacing Ar with N2 gas. Depending on the method implemented, the secondary ion mass spectroscopy (SIMS) depth profile showed relatively smooth or step-like changes in the elemental concentration of B, C and N. The primary analysis on the chemical bond of the graded interlayer was conducted by measuring the chemical shift of B1s, C1s and N1s spectra by X-ray photoelectron spectrometry (XPS). It is shown that the BC bond, which was a major bonding component in the B4C layer, was gradually replaced by a mixture of BN and CC bond as the N2 fraction was increased. Transmission electron microscopy (TEM) images of the gradient layer showed that (0002) oriented turbostratic BN (t-BN) structure started to appear after the N2 concentration was increased by more than 2%. It was also observed that the c-BN phase nucleated non-uniformly in the gradient layer at 10% of N2 fraction.