Clarification of Peeling off Mechanism of DLC Film during Heating/Cooling Process

Abstract

In this work, we tried to clarify the peeling-off mechanism of diamond-like carbon (DLC) during heating/cooling process. DLC film was deposited on a stainless-steel substrate (SUS 304, JIS) by ion beam sputtering of carbon target. Then, the DLC film was heated up to 250 °C, and then kept at the temperature for 60 min, and finally cooled down to the room temperature. The optical microscope observation revealed that the DLC started to peel off at around 70 °C during cooling process. The residual stress measurement with Storney's method revealed that the residual stress of DLC changes with increasing and decreasing temperature; however, the residual stresses at the same temperature during heating and cooling are almost the same, indicating the there was no irreversible change in the residual stress during heating/cooling process but carry out reversible change caused by the change of thermal stress. The scratching test revealed that the critical shear stress of DLC films decreased during heating/cooling process. It can be concluded that the peeling-off of the DLC is caused mainly by the decrease of the interface strength, or critical shear stress between DLC and substrate, which was estimated to decrease monotonically with time during heating/cooling process.