Tribological properties and oxidation resistance of tungsten and tungsten nitride films at temperatures up to 500 °C
Introduction
Tungsten (W) [1] exhibits the highest melting point (3422 °C) amongst pure metals and is one of the hardest transition metals (3.9 GPa [2] as a bulk material and up to 24.5 GPa [3] as a nanocrystalline film). These superior properties of the tungsten are due to its relatively covalent character caused by half-filled d orbital ([Xe] 4f145d56s1). Thanks to these properties, tungsten is very attractive for combinations with other elements (B, C, N, S, etc.) in binary (W-N [4,5], W-C [[6], [7], [8]], W-S [9]) and ternary (W-S-C [[10], [11], [12]], W-S-N [[13], [14], [15]], W-B-N [16], W-C-N [17,18]) systems which exhibit very interesting mechanical and tribological properties such as a high hardness, low friction coefficient (μ), etc. Tungsten is also widely used as a doping element of DLC coatings, improving their hardness and high-temperature tribological properties [[19], [20], [21], [22]] at preserved low μ and high wear resistance.
Tungsten nitride (WNx) belongs to a class of refractory metal nitrides and exhibits excellent combination of mechanical properties (hardness over 40 GPa and high elasticity expressed by high ratio H/E∗ over 0.1 [4,5,23]), good chemical stability, excellent adhesion on steel substrates [[23], [24], [25]] and good tribological properties (wear resistance) [4,5,26]. Therefore, WNx constitutes a potential candidate for protective films of cutting tools in dry and high-speed machining, automotive engine parts and structural components, etc. However, the development of stable harsh-environment protective films requires the investigation of the film oxidation and its effect on the tribological properties. There are several studies [5,12,16,22,27,28] dealing with tribological properties of W-based films at high temperatures (T). Fig. 1 shows a review on the corresponding μ values obtained in a range of testing conditions including relative humidity of 20–50%, loads of 3–5 N, sliding distance of 37–188 m, sliding velocity of 2.1–12 cm/s, etc. The figure shows that hard (H ≥ 20 GPa) W, WNx and WNx-based films exhibit qualitatively similar concave μ(T) dependencies, at significantly higher μ values compared to those achievable for soft (H ≤ 10 GPa) WCx and WCx-based films with dominant sp2 (graphite-like) bonds.
The main goal of this study is to investigate the oxidation and temperature-dependent tribological properties of WNx films measured using long test time (333 min) and sliding distance (1000 m) in a wide range of compositions (N content from 0 up to 60 at.%, i.e. x = [N]/[W] up to 1.5) and temperatures (up to 500 °C). This is contrary to the previous studies which deal with either only the room temperature [4] or only a limited range of N contents (x ≥ 0.4) [5,26], let alone short sliding distances up to 200 m. In order to explain the evolution of the tribological properties (especially the oxidation wear at high temperatures) of x ≤ 0.20 and x ≥ 0.27 WNx films, the oxidation of these films is investigated in detail. In particular, we focus on ellipsometric characterization of the corresponding WO3 scale formed on the film surface. The effect of the structure, microstructure and hardness (H) on the μ, and the effect of the hardness to effective Young's modulus ratio (H/E∗; E∗ = E/(1-ν2) where E is the Young's modulus and ν is the Poisson's ratio) on the wear rate (k) are investigated as well.
Section snippets
Experimental details
The WNx films (2.7 ± 0.3 μm thick) were reactively sputter deposited in a wide range of Ar + N2 gas mixtures using an unbalanced magnetron (Ø 100 mm). The magnetron was equipped with a W target, which was sputtered using a pulsed power supply (DORA Power System, Poland [29]) which combines (i) macro-pulses, i.e. pulsing of wave packets with a repetition frequency fr = 1/Tp = 1 kHz at a duty cycle τon/Tp = 0.15, with (ii) micro-pulses, i.e. unipolar medium-frequency AC pulsing at a frequency fac
Structure, microstructure and mechanical properties
Table 2 shows that increasing nitrogen partial pressure from 0 to 0.5 Pa leads to increasing nitrogen content in the films from 0 to 60 at.% (increasing x from 0 to 1.5). The oxygen content of 1–13 at.%, resulting from the residual atmosphere and from the high formation enthalpy of tungsten oxides (ΔHWO2 = −571 to −595 kJ/mol and ΔHWO2.7-3 = −743 to −853 kJ/mol [1], compared to e.g. ΔHβ-W2N = −22 kJ/mol [1,30] or −71 kJ/mol [31] and ΔHδ-WN = −15 kJ/mol [1,30]), was neglected when calculating
Conclusions
The detailed investigation of tribological properties and oxidation resistance of magnetron sputtered W and WNx films tested at temperatures T ranging from RT to 500 °C in ambient air can be summarized as follows:
- 1.
Tribological properties of W and WNx films are strongly influenced by (i) the relative humidity of ambient air, (ii) the WO3 surface scale created at T > 150 °C and (iii) mechanical properties of their dominating phase.
- 2.
The friction coefficient μ and wear rate k of the W and WNx films
Acknowledgements
This work was supported by the Czech Science Foundation under Project No. GA16-18183S.
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