Controlling adhesion and thermal stability of tetrahedral amorphous carbon coatings with intermediate sp3C fraction

Tetrahedral amorphous carbon (ta-C) coatings with an intermediate sp3C fraction were deposited on single-crystal silicon wafer substrates via intermittent filtered cathode vacuum arc (FCVA) deposition. The ta-C layer thickness and sp3C fraction in the coatings were adjusted using the deposition duration and bias voltage applied to the substrate, respectively. Some of the coated specimens were annealed at 300 and 600 degrees C for 4 h in a 5-Pa vacuum. The morphologies and microstructures of the specimens were characterized using scanning electron microscopy and transmission electron microscopy. The hybridization of carbon atoms was analyzed using X-ray photoelectron spectroscopy and micro-Raman spectroscopy. The mechanical characteristics of the coatings, including the critical adhesion load between the coatings and substrates, residual stress in the coatings, and hardness, were characterized. For the specimens deposited by the same bias voltage, even their top ta-C layer thickness ranged from approximately 500 nm to 1500 nm, their residual compressive stress in the coatings was maintained at a low value by the intermittent deposition of FCVA, which varied slightly with the coating thickness. The adhesion strength was improved by increasing the thickness of the as-deposited coating from approximately 12 to 40 N. The residual compressive stress of the post-annealing specimens with sp3C fraction of 45-53.8 % exhibited monotonously increasing or V-shaped variation with increasing annealing temperature. Herein, the underlying mechanisms are discussed in detail.