Contents lists available at ScienceDirect Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec The effect of heat treatment on the microstructural changes in electrodeposited Ni-Mo coatings Agnieszka Bigos a, ⁎ , Marta Janusz-Skuza a , Maciej Jakub Szczerba a , Marcin Kot b , Sławomir Zimowski b , Adam Dębski a , Ewa Beltowska-Lehman a a Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta St., 30-059, Krakow, Poland b Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology, 30 Mickiewicza Av., 30-059, Krakow, Poland ARTICLE INFO Associate Editor: Yip-Wah Chung Keywords: Ni-Mo coatings Electrodeposition Heat treatment Microstructure Functional properties ABSTRACT Nanocrystalline Ni-Mo (16 wt% of Mo) coatings electrodeposited on steel substrates from citrate electrolyte solution were annealed in argon atmosphere, in the temperature range from 300 to 800 ˚C. The effect of the heat treatment on the coatings’ microstructure (surface morphology, phase composition and grain size), as well as their mechanical, tribological and corrosion properties was determined. It was found that, up to 500 °C, the average crystallite size remains almost unchanged, but relaxation of grain boundaries leads to significant in- crease of the coatings’ microhardness (up to 1190 HV). For Ni-Mo alloys annealed at 500 °C, the wear index and friction coefficient increase significantly from 33·10 −6 mm 3 /Nm and 0.4 (for as-deposited samples) to 89·10 −6 mm 3 /Nm and 0.68. Above 500 °C, thermally-induced gradual fluctuations of chemical composition in the Ni-Mo alloy, and interdiffusion of iron (from the substrate) and nickel (from the coating), were observed. The segregation of elements promotes an increase of grain size (from 10 nm to 33 nm for as-deposited and annealed at 800 °C Ni-Mo coatings, respectively) resulting in decreased microhardness to 487 HV. However, the coatings become less stiff (E IT up to 205 GPa) and are characterised by enhancement of wear (Wv about 2.2–7.8⋅10 −6 mm 3 /Nm) and friction resistance (about 0.4–0.5). No significant influence of the heat treatment temperature on the corrosion parameters of the coatings in the sulphate environment was found. The corrosion resistance of the Ni-Mo coating annealed at 800 °C decreased slightly (I corr =5.9 μA/cm 2 ,R p = 1530 Ω) in comparison to the as-deposited alloy (I corr =4.8 μA/cm 2 , R p = 2430 Ω) and that annealed at 500 °C (I corr = 4.7 μA/cm 2 ,R p = 2220 Ω). 1. Introduction Electrodeposited nanocrystalline nickel-based alloys containing re- fractory metal such as molybdenum are under constant evaluation due to their unique physicochemical properties such as high hardness, and resistance to wear and corrosion. Production of homogenous coatings characterised by enhanced functional properties is possible due to ap- propriate control of the electrolyte solution and the electrolysis para- meters. Plating baths containing polycarboxylic acid salts such as ci- trates are commonly used in the manufacture of Ni-Mo coatings. As described by Chassaing et al (2004), it is possible to obtain nanocrys- talline Ni-Mo alloys with different amounts of Mo (analysed in the range from 5 to 40 wt%) from citrate-sulphate solutions of relatively simple chemical composition and by the selection of electrolysis current conditions. Coatings containing from 15 to 30 wt% of Mo exhibited the best corrosion resistance in the de-aerated 1 N HCl solution, a result much higher than commercially used Hastelloy B alloys. Moreover, alloys containing 17 wt% of Mo are characterised by high microhard- ness (about 800 HV). Similar results, enhanced by tribological in- vestigations, were presented by Beltowska-Lehman et al. (2012). The best tribological properties, with relatively high microhardness values (about 700 HV), were obtained at current densities above 3.5 A/dm 2 . Further investigation conducted by Bigos et al. (2017) and Bigos et al. (2017a) complemented previous studies by presenting the optimal condition of the plating bath pH (pH above 7), temperature (in the range of 20 °C to 40 °C), and hydrodynamic conditions (referred to ro- tating disc electrode speed in the range of 260 to 640 rpm, or to the presence of an ultrasonic field). Under these conditions, coatings con- taining more than 16 wt% of Mo can be deposited. These deposits are characterised by excellent adhesion to the steel substrate, wear re- sistance and the highest microhardness (above 635 HV). However, in order to produce advanced Ni-Mo coatings meeting the https://doi.org/10.1016/j.jmatprotec.2019.116397 Received 11 April 2019; Received in revised form 30 July 2019; Accepted 9 September 2019 ⁎ Corresponding author. E-mail address: a.bigos@imim.pl (A. Bigos). Journal of Materials Processing Tech. 276 (2020) 116397 Available online 11 September 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved. T