Phase separation and formation of the self-organised nanostructure in C/Cr coatings in conditions of high ion irradiation

The paper discusses the effect of ion irradiation on a new type of nanoscale multilayer structure in C/Cr PVD coatings formed by a self-organisation mechanism. C/Cr coatings were deposited by the combined steered cathodic arc/unbalanced magnetron sputtering technique, at wide range of bias voltages, Ub from − 65 to − 550 V. Plasma diagnostics carried out by electrostatic probe measurements revealed that C/Cr films grow under conditions of intensive ion bombardment with ion-to-neutral ratio exceeding Ji/Jn = 6. Under these conditions the high diffusion mobility and the reactivity of the C leads to distinct changes in the coatings microstructure and phase composition.

Raman spectroscopy and XPS (X-ray photoelectron spectrometer) analysis of the chemical bonding in the films showed that the phase composition of the films gradually transforms from more graphite like (sp2 C–C bonded) to more Me-carbon (Cr–C bonded), where the content of the carbide phase increases with increase of the bias voltage to Ub = − 350 V and higher. In parallel HRTEM (high-resolution transmission electron microscopy) employing HAADF (high-angle annular dark field) imaging revealed that the microstructure evolved from columnar with carbon accumulated at the column boundaries (Ub = − 65 V, − 95 V) to a structure dominated by onion like C–Cr clusters (Ub = − 120 V), which than converts to a distinct nanoscale layered structure (Ub = − 350 V, − 450 V), finally transforming to a uniform fine grain structure at Ub = − 550 V. The new nanoscale layered structure forms via ion irradiation induced self-organisation mechanism. It is characterized by an abnormally large values for the bi-layer thickness of 20 nm and 25 nm, which are not related to substrate rotation, for films grown at Ub = − 350 V and Ub = − 450 V, respectively. ADF (annular dark field) STEM (scanning TEM) imaging and quantitative EELS (electron-energy loss spectroscopy) analysis showed that the nanoscale multilayer structure comprises of alternating layers of Me-carbide phase (48%C, 52%Cr) and almost pure C (91.34%C), where the bias voltage defines the bi-layer thickness. A coating growth model is proposed accounting for the irradiation-induced ion mixing, re-sputtering, condensation surface temperature effects, nucleation and kinetic segregation process, as well as the diffusivity of the coating elements to explain the phase separation and formation of the self-organised layered nanostructure observed in C/Cr coatings.