Wear and friction of TiAlN/VN coatings against Al2O3 in air at room and elevated temperatures
Introduction
Titanium nitride (TiN) with the B1 NaCl structure has been widely used as a hard wear-protective coating since the 1980s. Subsequently, TiAlN coatings were developed that provide much improved high-temperature oxidation resistance, up to 750–900 °C, and have consequently been used extensively for high-temperature cutting operations with minimum use of lubricant or dry machining [1]. Both the friction and wear performance of TiAlN can be improved through the quaternary addition of V, as monolithic Ti–Al–V–N coatings or as TiAlN/VN multilayers. TiAlN/VN multilayer coatings have alternating nanoscale dimensions of TiAlN and VN layers (typical period 3–5 nm). They have exhibited superior hardness and sliding wear resistance (wear rate 1.26 × 10−17 m3 N−1 m−1) with a lower friction coefficient (μ = 0.5 ± 0.1, Al2O3 ball counterpart, sliding speed 0.1 m s−1 and 5 N load) after ball-on-disc test at room temperature in comparison to other wear resistant coatings [2], e.g. TiAlN, TiAlN/CrN (specific wear rate 2.38 × 10−16 m3 N−1 m−1, μ = 0.7–0.9 under similar test conditions). Laboratory dry sliding wear tests of TiAlN/VN coatings under ambient conditions (μ = 0.54, Al2O3 ball counterpart, sliding speed 0.1 m s−1 and 5 N load) yielded wear debris containing V2O5 detected by Raman spectroscopy [3]. The V2O5 has a melting point 674 °C [4] and inherently low friction, leading to suggestions that formation of this oxide is the key to low frictional behaviour of these coatings. High-temperature wear tests of TiAlN/VN multilayer coatings (V∼50 at.%) against Al2O3 showed increase of friction coefficient from 0.55 at 25 °C to 0.96 at 500 °C, followed by a sharp decrease to 0.18 at 700 °C [5]. Similarly, wear tests of Ti–Al–V–N monolithic films (2.40 at.% ⩽ V ⩽ 11.10 at.%) against an Al2O3 ball exhibited friction coefficients in the range of 0.6–0.85 at 25 °C, which increased to 1.1 at 500 °C and dropped to the range of 0.8–0.2 at 700 °C (friction coefficient value depended on the concentration of V) [6]. Wear studies of VN coatings by Gassner et al. [7] and Fateh et al. [8] confirmed the reduced friction coefficient associated with the formation of lubricious vanadium oxides, i.e. Magnéli phases at elevated temperatures. In particular, Fateh et al. compared the behaviour of TiN and VN under identical conditions. The TiN exhibited a high friction coefficient (0.5–0.7) at all temperatures, while the VN exhibited a decrease in friction with temperatures higher than 400 °C, showing a value as low as 0.25 at 700 °C. Using XRD and Raman spectroscopy, they observed a range of vanadium-based oxides on the worn surface of the VN, namely V2O5, VO2 and V6O13. These agree well with static oxidation of VN [9] and TiAlN/VN [10] which suggested that oxidation started at 400 °C and 550 °C respectively and V2O5 was dominant on TiAlN/VN higher than 638 °C.
The friction behaviour is dependent on the exact composition of the coating, not just the presence of V, as shown by the differences in behaviour of VN and TiAlVN, for example. The friction coefficient observed for TiAlN/VN (V∼50 at.%) and Ti–Al–V–N (2.40 at.% ⩽ V ⩽ 11.10 at.%) exhibited large differences, as described above, suggesting that the presence of Al and/or Ti increases friction at 500 °C. Increasing V from 2 at.% to 10 at.% in Ti–Al–N led to a decrease in friction from 0.85 to 0.73 at room temperature and 1.10–0.80 at 500 °C [6]. Further increasing V up to 50 at.% in TiAlN/VN showed a friction coefficient between 0.40 and 0.55 at room temperature, and 0.95 at 500 °C [5]. The effect is particularly marked at 700 °C when molten V2O5 is produced (μ = 0.8 with 2 at.%, μ = 0.25 with 25 at.%, and μ = 0.18 with 50 at.%).
The role of surface films as a direct result of frictional contact (so-called tribofilms [11]) remains a topic of some debate, largely because the structure of such films is not fully understood, but is known to vary from film to film. The tribofilm is most likely a compositional mixture of the two base materials and oxidation products caused by friction heating. A thin tribofilm is believed to be associated with low wear rates (so-called mild wear) and is believed to be a controlling factor in the observed friction coefficient. Equally, the observation of roll-like wear debris on worn surfaces is now relatively common and is believed to be associated with systems operating at low temperature that exhibit low wear rates. However, the structure of such roll-like wear debris has not been well studied and therefore the origin remains unknown.
The current work presents a detailed examination of the worn surface structure in order to understand the role of V in the coating in promoting low friction at high temperatures and the origin of high friction at lower temperatures. The same TiAlN/VN multilayer coating (V 55.2 at.%, Ti 28.5 at.% and Al 16.3 at.%) was used in our published oxidation study [10]. Focused ion beam (FIB) microscopy was used to produce site-specific transmission electron microscopy (TEM) cross-sections to study the structure of the worn surface at the region containing roll-like wear debris. Extensive energy loss spectroscopy (EELS) and electron diffraction was used to determine the local structure and bonding state in the surface tribofilm as a function of temperature. Fourier transform infrared (FTIR) and Raman spectroscopy was further used to investigate the structure of these surface layers. These techniques were used to address the following specific questions: (1) Can the surface tribofilms explain the friction behaviour as a function of temperature for multilayer TiAlN/VN? and (2) What is the true structure of the tribofilm and therefore how are V, Al, Ti and O involved in its formation?
Section snippets
Coating
TiAlN/VN multilayer coatings were deposited using the combined steered cathodic arc/unbalanced magnetron (UBM) sputtering (ABS™: arc-bond sputtering) technique using an industrial Hauzer HTC 1000-4 PVD coater. Deposition was undertaken in an Ar + N2 atmosphere, with two TiAl targets (50:50 at.%, purity 99.99%) and two V (purity 99.8%) targets, and stainless steel substrates (AISI304). The coating process comprised three steps: (i) V+ etching of the substrates in cathodic arc mode by applying a
Specific wear rates and friction coefficient of TiAlN/VN
Table 1 lists the test parameters, friction and wear results. Fig. 1 shows the friction curves as a function of sliding distance. Steady state wear was obtained after a short sliding distance of <50 m for all temperatures, with the average friction coefficients then being 0.53 at 25 °C, sharply increasing to 1.03 at 300 °C and dropping to 0.46 at 635 °C. The specific wear rate (K) of the coating was low at 25 °C (2.3 × 10−15 m3 N−1 m−1) and doubled at 300 °C. A sharp increase in wear rate was observed at
Friction behaviour at 25 °C and 300 °C
At both 25 °C and 300 °C, the outer worn surface was covered in a tribofilm. It is generally believed that the friction is strongly dependent on the presence and properties of a tribofilm, as the interfacial shear strength of the film will be a strong contributor to the resistance to motion. In many systems the formation of such tribofilms is engineered, for example through the use of additives to oil-based lubricants (e.g. zinc dialkyl-dithio-phosphate (ZDDP)). In the present work the marked
Conclusions
- (1)
The friction coefficient exhibited by the coating sliding against alumina was 0.53 at 25 °C, increased to 1.03 at 300 °C, and decreased to 0.46 at 635 °C.
- (2)
The low friction 635 °C was attributed to the formation of a V2O5 “glaze layer” on the oxide surface. This layer comprised a nanoscale polycrystalline structure, with no obvious preferred crystallographic orientation. There was no strong evidence of other vanadium-based oxides in the layer, and the Ti and Al bearing oxides were present below the V2
Acknowledgements
The authors would like to acknowledge the funding of this work by the UK Engineering and Physical Sciences Research Council (EPSRC).
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