Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct Functionally graded Ti(C,N) coatings and their production on titanium using solid-state carburization associated with induction heat treatment Aleksandr Fomin ⁎ , Aleksey Voyko, Marina Fomina, Sergey Mokrousov, Vladimir Koshuro Yuri Gagarin State Technical University of Saratov, 77 Politechnicheskaya str., Saratov 410054, Russia ARTICLE INFO Keywords: Titanium Carbonitride coating Induction thermochemical treatment Hardness Functionally graded structure ABSTRACT On the titanium surface functionally graded layers in the form of carbonitride TiC 0.45–0.72 N 0.28–0.47 coatings were obtained by solid-state carburization associated with induction thermochemical (ITC) treatment at temperatures from 1050–1100 to 1300–1400 °C and exposure time within 8 min. A homogeneous porous structure of coatings with the thickness of 2.9–5.9 μm was formed in the low (1050–1100 °C) and medium (1250–1300 °C) tem- perature ranges. The average nanograin size varied from 63–68 to 100–121 nm and the size of the holes (pores) ranged from 45–58 to 64–84 nm. The hardness of coatings varied from 20.8 to 49.7 GPa and elasticity modulus – from 235 to 525 GPa. However, a high hardness value H = 47.6 ± 12.9 GPa, a low elasticity modulus E = 290 ± 73 GPa combined with microhardness H 0.49 = 12.0 ± 0.9 GPa and calculated plasticity index H/ E = 0.16 and brittle fracture resistance H 3 /E 2 = 1.28 GPa were provided by ITC treatment of titanium at a temperature T = 1250–1300 °C and exposure duration t = 4 min. High hardness at various indentation loads of the carbonitride TiC 0.58 N 0.42 coating was also explained by the formation of an underlying modified layer with an α′-Ti structure. 1. Introduction Metal products are subjected to various methods of structure mod- ification and strengthening in order to improve their quality, in parti- cular thermal (quenching, tempering, annealing, precipitation hard- ening) [1] and chemical-thermal treatment (carburizing, nitriding, nitrocarburizing, etc.) [2,3], deposition of functional films and coatings [4]. This enables changing the surface morphology (in particular, creation of nanoscale relief elements), increasing the chemical stability (oxidative stability under the influence of chemical and temperature factors), improving mechanical properties (strength, hardness, etc.), which is necessary for metal products in harsh operational conditions, e.g. in tribo- and bioengineering systems. It is known that to increase the hardness and wear resistance of steel products, e.g. gears from 40Cr steel, titanium carbide (TiC) is deposited on their surfaces [5]. To improve the adhesive strength of the wear- resistant coating during laser cladding, metal powders (titanium cp-Ti, 12CrNi2 steel) and graphite are also added to the composition of the powders. As a result of this treatment, a hard (about 1000 HV 0.2 ) and rather thick (about 0.5 mm) TiC layer is formed, which also contains brittle intermetallic compounds (FeTi and Fe 2 Ti). Ceramic composite coatings with different mass fractions of TiC, TiN and B 4 C are prepared on the surface of C45 steel by laser cladding [6]. The binder for this composite coating is a nickel-chromium Ni204 powder, which is taken in an amount of not < 70 wt%. The resulting coating has a hardness of about 770 HV 0.5 , which is almost 4 times higher than that of a steel substrate. The maximum values of hardness combined with a minimum coefficient of friction equal to 0.47–0.48 correspond to the same amount of additive (10 wt%) of each of the reinforcing components. TiN, TiCN, and Ti-DLC (diamond-like carbon) films are formed on 316L stainless steel by PVD (physical vapor deposition) method [7].A maximum hardness of about 37 GPa (at 10 mN) is observed for TiCN film, the thickness of which is about 1.5 μm. This material is synthe- sized by spraying titanium in a gas mixture (N 2 +C 2 H 2 ). When me- thane (CH 4 ) is used, a Ti-DLC film with a lower hardness of about 27 GPa is formed; however, the minimum friction coefficient is ob- served in the range of 0.02–0.03 (an order of magnitude less than that for 316L stainless steel). Nitride and carbonitride coatings (TiN, TiCN, CrN, etc.) with high hardness (about 16–23 GPa) are obtained on 9Cr18 steel by reactive magnetron sputtering of titanium and chromium tar- gets in Ar, N 2 and C 2 H 2 mixed atmosphere [8]. The minimum wear rate is observed for TiN/TiCN two-layer coatings. The friction coefficient for this coating is 0.25, which is almost 3 times lower than that for a single component TiN coating. TiC/Ti(CN)/TiN multilayer coatings are prepared on the surface of high-strength steels, e.g. 35CrMo, 42CrMo, and 40CrNiMo, by chemical https://doi.org/10.1016/j.compstruct.2020.112393 Received 27 February 2020; Received in revised form 4 April 2020; Accepted 16 April 2020 ⁎ Corresponding author. E-mail address: afominalex@rambler.ru (A. Fomin). Composite Structures 245 (2020) 112393 Available online 20 April 2020 0263-8223/ © 2020 Elsevier Ltd. All rights reserved. T