TaC-based wear-resistant coatings obtained by magnetron sputtering and electro-spark deposition for wedge gate valve protection

Abstract

Ta–Zr–Si–B–C coatings were deposited by magnetron sputtering (MS) of a TaSi2–Ta3B4–(Ta, Zr)B2 multi-component target in an Ar + C2H4 gas mixture. TaC–Cr–Mo–Ni based coatings were obtained by electro-spark deposition (ESD) using TaC–Cr–Mo–Ni electrode. The composition and structure of the coatings were studied using scanning electron microscopy, energy-dispersive spectroscopy, glow discharge optical emission spectroscopy and X-ray diffraction. Mechanical and tribological properties of coatings were determined using nanoindentation and pin-on-disk tests. The study showed that the coatings have a homogeneous and defect-free structure, with the main structural component being the fcc-TaC phase. The MS coating exhibited a 30 % higher concentration of the TaC phase compared to the ESD coating. The TaC crystallite sizes for the MS and ESD coatings were 3 and 30 nm, respectively. The presence of a high fraction of the carbide phase and small crystallite size for the MS coating resulted in superior hardness (H = 28 GPa) compared to the ESD sample (H = 10 GPa). Both coatings exhibited similar values of the friction coefficient (about 0.15) and demonstrated reduced wear rates (<10–7 mm3/(N·m)). The deposition of coatings on a steel substrate led to a decrease in the friction coefficient by five times and the wear rate by four orders of magnitude. Pilot tests were conducted on coatings applied to wedge gate valve of shut-off devices used in the oil and gas industry for pumping liquids. The results indicated that the service life of the steel wedge gate valve increased by 25 and 70 % with deposited MS and ESD coatings, respectively.