On the unique effect of hexamethyldisiloxane on thick a-C:H films deposited at elevated temperatures

Doping of hard amorphous carbon films with silicon typically leads to lower internal stresses and a moderate reduction in hardness and Young's modulus, allowing for the growth of thicker coatings without the risk of spalling. Even though the effect of silicon doping on harder DLC films is known, its role in softer films grown at higher deposition temperatures remains unclear. This work now aims to elucidate the effects of silicon incorporation into soft and thick (>27 μm) a-C:H coatings grown at elevated temperatures using a pulsed direct current plasma assisted chemical vapor deposition (DC PACVD) system. The a-C:H:Si:O films were deposited using C2H2 at 450 °C and 550 °C with an additional hexamethyldisiloxane (HMDSO) flow of 0–25 sccm. Remarkably, the effect of silicon modification on the films was significantly different from other reports in literature. Instead of reducing the residual stresses, an increase in HMDSO flow led to a marked jump in stress, accompanied by an increase in density and load-bearing capacity. In addition, Young's modulus and hardness increased by more than three times compared to the undoped films. The relationship between hardness and silicon concentration followed a linear trend. A deviation from this linear relationship was observed at the highest HMDSO flow. Elastic recoil detection analysis (ERDA) revealed that this was likely related to a saturation of the positive effects of Si combined with the incorporation of hydrogen and oxygen into the coating's structure.