Mechanism of enhanced adhesion between hydrogenated amorphous carbon films and tungsten preimplanted steel substrates

Hydrogenated amorphous carbon $(a-C:H)$ films have good mechanical properties but poor adhesion on substrates such as tool steels thereby limiting their applications. Film adhesion can be improved by conducting tungsten preimplantation into the steel substrates before deposition of the carbon film. The enhancement mechanism is investigated in this study by depositing the films on W plasma-preimplanted (⁠$20kV$⁠, $5×1017ionscm−2$⁠) and untreated steel substrates using mixed acetylene and argon plasmas with different flow rate ratios by means of plasma immersion ion implantation and deposition. Preimplantation creates a graded WC interface that mitigates diffusion of the dissociated carbon atoms on the surface. Compared to the films deposited on untreated steel substrates, the ones deposited on the W-implanted steel substrates exhibit improved adhesion strength, especially for the lower acetylene to argon flow rate ratios. Tungsten preimplantation is critical to the successful fabrication of $a-C:H$ films on steel substrates. Furthermore, the flow rate ratio also has a large impact on the film properties due to the variations in the deposition rate causing different internal stress in the film. A nucleation model is proposed to explain the enhancement of the adhesion properties.