Investigating the effect of bias voltage on the microstructural thermal stability and tribological performance of N-doped hydrogenated diamond-like carbon coatings

Abstract

Here, N-doped hydrogenated diamond-like carbon (a-C:H:N) coatings were deposited onto 316 L stainless steel and silicon wafer substrates by applying plasma-enhanced chemical vapor deposition at different bias voltages (Vb). The coatings were annealed at 25–590 °C for 4 h. The structure and chemical bonding of the coatings were characterized using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The residual stress, surface hardness, and tribological performance were investigated by using the Stoney method, a nanoindenter, and sliding against alumina. The fraction of the constituent bonds in the coating was controlled depending on Vb. The evolution of these bonds during annealing determined the thermal and mechanical performance of the a-C:H:N coatings, particularly the CHn group, sp2C bonds, and CO bonds (CO and CO). As Vb increased, the fraction of CHn groups, CO bonds, and CN bonds decreased, whereas that of sp2C increased. When annealed at above 430 °C, the breakage of the CHn groups and CN bonds accelerated the sp3 → sp2 transformation. The coating prepared at −500 V exhibited the highest hardness of 22.5 GPa and low residual stress of −0.57 GPa; after annealing at 590 °C for 4 h, a hardness of 15 GPa was maintained. The wear resistance was positively related to the surface hardness of the coatings, whereas the coefficient of friction was responsible for the fraction of sp3CHn, CO bonds, and sp2C bonds. The wear resistance of the coating deposited at −500 V was double that of the coating deposited at −740 V.