Mechanical and tribological properties of sputtered Mo–O–N coatings J. Musil a, ⁎, P. Novák a , M. Hromádka a , R. Čerstvý a , Z. Soukup a , J. Savková b a Department of Physics, Faculty of Applied Sciences, University of West Bohemia, Univerzitní 22, CZ-30614 Plzeň, Czech Republic b New Technologies - Research Centre, University of West Bohemia, Univerzitní 8, CZ-30614 Plzeň, Czech Republic abstract article info Available online 5 November 2012 Keywords: Mo–O–N coatings Structure Mechanical properties Friction Wear Magnetron sputtering The article reports on mechanical and tribological properties of Mo–O–N coatings prepared by reactive magne- tron sputtering using a continuous and pulsed inlet of oxygen and nitrogen. Main attention was devoted to a de- tailed investigation of correlations between the coating structure and its mechanical and tribological properties. The structure of coatings was characterized using X-ray diffraction (XRD), the hardness H, the effective Young's modulus E ⁎ and the elastic recovery W e were measured by a microhardness tester, and the coefficient of friction μ and the coefficient of wear k were determined using a pin-on-disk tribometer. It was found that (1) μ of the Mo–O–N coating correlates quite well with their hardness H and effective Young's modulus E ⁎ and depends on its structure, (2) k of the Mo–O–N coatings is almost independent on the value of E ⁎ and its value is very low of about ~0.15×10 -6 mm 3 /Nm, (3) the incorporation of oxygen in the Mo–N coating increases its coefficient of friction μ at room temperature (RT), and (4) the coefficient of friction μ of both Mo–N and Mo–O–N coatings at first increases with increasing temperature and above ~350 °C decreases to the value of μ of the as-deposited coating (μ ≈0.4 and 0.5 for the Mo–N and Mo–O–N coatings, respectively). © 2012 Elsevier B.V. All rights reserved. 1. Introduction The tribological properties of nitrides and oxides of transition metal coatings deposited by reactive magnetron sputtering have been inten- sively studied already for three decades. These coatings exhibit relatively high hardness and excellent wear resistance. However, a surface oxida- tion of some of these coatings results in a reduction of the friction at high temperatures above ~250 °C in the air [1,2]. Many papers report on the formation of lubricious surface oxides on (i) nitride coatings, e.g. TiN [3,4], CrN [3], VN [3], WN [5], MoN [6–9], and MoN/Me [4,10–12], and (ii) oxide coatings, e.g. W–O [2,5] and V–O [2], which reduce the friction at elevated temperatures. Molybdenum nitride (MoN) has interesting properties which pre- determine it to be used as a good material for protective coatings [6,7,10,11]. Therefore, a lot of investigations on Mo–N and Mo–N based coatings have been carried out. The molybdenum nitride coating can exhibit two phases with different crystal structures: hexagonal δ-MoN and cubic γ-Mo 2 N [8,12]. Two-phase nanocomposite coating of the type nc-Mo 2 C/(a-C+ a-Mo 2 N) exhibits a high hardness (up to ~50 GPa) [13]. The oxidation resistance of the Mo–N coating is low mainly due to a loss of its protective ability and the volatility of oxides already at slightly increased temperatures; oxidation starts between 350 and 400 °C [9]. Considerable attention is also devoted to alloying of the MoN with selective metals such as Ag and Cu [4,10–12]. These coatings were investigated at elevated temperatures up to 400 °C. It was found that the coefficient of friction μ oscillates around 0.4 at room temperature (RT) in ambient air. This low value of μ is typical for nitride coatings and is explained by a water lubrication mechanism. At temperatures above 100 °C, the water desorbs from the coating surface and the water lubrication mechanism fails. At temperatures ranging from 100 °C to 200 °C the coefficient of friction μ increases to ~ 0.8. At higher temperatures ranging from 200 °C to 400 °C the coefficient of friction μ decreases from 0.8 to 0.5 due to surface oxidation [10–12]. Therefore, it could be interesting to incorporate the oxygen directly in the Mo–N coating and to investigate tribological properties of the Mo–O–N coatings. However, such study was not carried out so far. Papers devoted to the MoN x O y coatings report on their structure, hardness H, Young' modulus E, and the effects of (i) the oxygen on amorphization of nitride phases [14] and (ii) thermal annealing on the coating structure [15]. At present, there is no paper describing tribological properties of MoN x O y coatings. Therefore, this paper is devoted to a detailed investigation of the structure, mechanical and tribological properties of the MoN x O y coatings. The aim of our study is to find correlations between the mechanical and tribological properties of the MoN x O y coating and to investigate the effect of annealing temperature T a on its friction and wear. 2. Experimental The MoN coatings were reactively sputtered using a round unbal- anced magnetron equipped with a molybdenum target (Ø=100 mm, 99.95% purity and fixed to the cathode by a stainless steel ring with Surface & Coatings Technology 215 (2013) 386–392 ⁎ Corresponding author at: University of West Bohemia, Univerzitní 22, CZ-30614 Plzen, Czech Republic. Tel.: +420 37763 2225; fax: +420 37763 2202. E-mail address: musil@kfy.zcu.cz (J. Musil). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.06.090 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat