Combined FIB technique with acoustic microscopy to detect steel-DLC interface defects

In this work, focused ion beam (FIB) technique was employed to characterize subsurface defects in chromium-containing DLC (Cr-DLC) coatings. Subsurface defects as small as one micron were successfully detected in a flat Cr-DLC coated steel coupon by scanning acoustic microscopy (SAM). The nature of the subsurface defects in the coating was investigated by repeated FIB milling and a scanning electron microscopy (SEM) imaging technique where element analysis was done by Energy Dispersive X-ray spectroscopy (EDX). It has been found that defects are located mostly at the DLC steel interface. A model for the bump-like defects is proposed based on the SAM, EDX and FIB measurements. Model is that of a delamination/void between Cr layer and steel substrate. The existence of these defects at the interface leads to the defects visible on the surface of the DLC coating; bumps or pits which are larger in diameter compared to the subsurface defects.