Effect of nitrogen doping on the microstructure and thermal stability of diamond-like carbon coatings containing silicon and oxygen

Using hexamethyldisiloxane, acetylene, and N2 as the precursors, binary Si/0 and ternary Si/O/N co-doped diamond-like carbon (DLC) coatings were deposited on 316 L stainless steel and single-crystal Si wafer substrates via high-bias voltage plasma-enhanced chemical deposition. The coated specimen was annealed in the temperature range 200-650 degrees C in a chamber maintained at 5 Pa vacuum. The microstructure and chemical bonds were characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The mechanical properties were investigated using a residual stress tester and a nano-indenter. It was found that approximately 50% of the Si in the binary Si/O co-doped coating were present as Si-O bonds. Notably, N doping reduced the sp C-C fraction, hindered the formation of 0-containing bonds, and enhanced the Si-C bonds in the as-deposited ternary Si/O/N co-doped coatings; however, the dominant chemical state of the N atom remained in the form of C=N bonds. Furthermore, N doping was beneficial toward the stabilization of the Si-C and Si-O bonds and hindered graphitization during annealing. The thermal stability of the microstructure and mechanical performance of the ternary co-doped DLC coatings were superior to those of the binary Si/O-co-doped DLC coatings. After annealing at 500 degrees C, the ternary coatings retained a hardness of 16.5 GPa.