Effect of Si-DLC intermediate layer on duplex process of plasma nitriding and diamond-like carbon-coating deposition

In recent years, physical vapor deposition and chemical vapor deposition (CVD) methods have exhibited remarkable advances owing to the increasing demand for surface modification technologies. In this study, diamond-like carbon (DLC) is deposited as a hard thin film by the plasma-enhanced CVD method. DLC has properties such as high hardness, low friction, wear resistance, and chemical stability. However, low adhesion caused by residual stress and the difference in hardness between the film and the substrate material is a drawback. Therefore, studies are being conducted to improve adhesion by introducing a DLC intermediate layer containing metallic elements to reduce residual stress or by applying treatments to harden the substrate material, such as nitriding or carburizing. Active screen plasma nitriding (ASPN) is a nitriding method that eliminates an edge effect to electrically insulate the sample during the process. However, during nitriding, deposits cover the sample and slow down the nitriding rate. Therefore, a nitriding method, “direct-current plasma nitriding with screen (S-DCPN),” was also reported in which a voltage is applied to the sample and screen during ASPN to remove the deposits by sputtering action and increase the nitriding rate. Although the duplex process of ASPN and DLC-coating deposition has been studied thus far, only few reports on the duplex process with S-DCPN exist. In this study, the effect of the composition of the intermediate layer on mechanical properties was investigated by forming a nitrided layer on the surface of SUS304 by S-DCPN treatment, depositing a Si-DLC intermediate layer with a different composition, and depositing a DLC film on the top surface. The results of this study demonstrated that the lower the Si ratio in the Si-DLC intermediate layer, the better the wear resistance. Furthermore, it was revealed that the wear resistance and adhesion were improved compared to the samples without S-DCPN treatment.