Comparative investigation of single-layer and multilayer Nb-doped TiC coatings deposited by pulsed vacuum deposition techniques

Abstract

Hard-yet-tough coatings are required to protect structural materials, cutting, stamping, and forging tools operating under harsh conditions that combine wear, corrosion, and elevated temperatures. To reveal the high potential of niobium (Nb) as an additive to binary Ti-based coatings, single-layer and multilayer Nb-doped TiC coatings were obtained by pulsed arc evaporation (PAE), electro-spark deposition (ESD) in vacuum, and combination of these methods. Structure, elemental, and phase compositions were studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The coatings were characterized in terms of their hardness, elastic modulus, tribological properties, and electrochemical behavior in 3.5% NaCl solution. Tribological tests were carried out under various conditions, such as applied load (5 and 10 N), medium (air and 3.5% NaCl solution), and counterpart material (Al2O3 and 440C steel). Thin PAE coating (1.8 μm thick) consisted of 2–5 × 50–100 nm (Ti,Nb)C crystallites, elongated in direction of the coating growth, embedded in an amorphous matrix (75% sp2 + 25% sp3-hybridized carbon). Thick ESD coating (20 μm) contained TiC grains uniformly distributed in an eutectic Fe(Co)-Fe2(Ti,Nb) matrix with a minor content of Ti-Ni and γ-Fe phases. The single-layer PAE coating demonstrated superior tribological performance both in air and 3.5% NaCl solution, as well as high oxidation resistance during tribocorrosion tests. As a top layer in the PAE/ESD coating, it provided wear protection of less wear-resistant ESD sublayer. It is assumed that the main benefit of the ESD sublayer in the PAE/ESD coating may be that it provides enhanced toughness and high thickness, which prevents a soft substrate from high stress-induced plastic deformation.