Vacuum 53 (1999) 123 — 126
Tribological investigation of TiAlCrN and TiAlN/CrN coatings grown by combined steered-arc/unbalanced magnetron deposition Q. Luo *, W.M. Rainforth , L.A. Donohue, I. Wadsworth, W-D. Mu¨nz Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK Materials Research Institute, Sheffield Hallam University, Pond Street, Sheffield S1 1WB, UK
Abstract The dry sliding wear of monolayer TiAlCrN and multilayer TiAlN/CrN coatings has been investigated against a BM2 tool steel counterface. The coatings were deposited on a BM2 tool steel substrate by combined steered-arc/unbalanced-magnetron deposition. Increasing either contact load or sliding speed led to a reduction in friction coefficient, typically from 1.1 to 0.2. Increasing load resulted in an increase in wear rate for both TiAlCrN and TiAlN/CrN (e.g. from 7;10\ mm/m at 22 N to 4;10\ mm/m at 189 N for the TiAlCrN monolayer coating, and from 7;10\ mm/m at 22 N to 2.5;10\ mm/m at 189 N for TiAlN/CrN multilayer). The wear rate for all coatings was at least an order of magnitude lower than the uncoated BM2 steel. The wear rate of the TiAlCrN coating tended to decrease with an increase in sliding speed (from 7.4;10\ mm/m at 0.2 m/s to 1.3;10\ mm/m at 1.1m/s) while the wear rate of the TiAlN/CrN was approximately constant as a function of sliding speed (&1.5;10\ mm/m). 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Wear resistant PVD nitride coatings were developed in three generations , e.g. TiN  (first generation), TiAlN and TiAlCrN [1,3] (second generation) and multilayered TiAlN/CrN  (third generation). TiN has been extensively exploited commercially because of its greater hardness to high speed steel at temperatures below 500°C [3,5]. TiAlN [3,5,6] offers superior hardness, corrosion and wear resistance to TiN. A further improvement in coating properties was achieved by Cr ion etching applied to the nitride/substrate interface . The multilayered film structure , the key feature of the third generation coatings, brought about significant advances in coating design, and imparts ultra-high hardness . TiAlN/CrN is one example of a multilayer coating, and exhibits substantially higher hardness and oxidation resistance than TiAlN. With the improved properties, better tribological performance is expected. In this paper, comparative tribological experiments were carried out on TiAlN/CrN multilayer and TiAlCrN
*Corresponding author. Tel.: 0114 222 5929; fax: 0114 275 4325; e-mail: [email protected]
monolayer coatings to reveal their friction and wear behaviour as a function of load and sliding speed.
2. Experimental details The coatings were grown on a BM2 tool steel in the hardened condition. Following machining, the tool steel surface was prepared to a high metallographic finish. A Hauzer HTC1000-4 coating machine was used to grow both TiAlN/CrN and TiAlCrN coatings using the combined cathodic arc etching and unbalanced magnetron sputtering process [1,3,4,6]. Four targets are employed, three being cast intermetallic alloy (50 at% Ti and 50 at% Al) and the other being pure chromium. For the TiAlN/CrN multilayer coating, the process started with Cr metal ion etching in the steered arc mode, followed by the unbalanced magnetron sputtering deposition for 30 min of a base layer of TiAlN (the three TiAl targets, each 50 : 50 at% Ti : Al, were operated at 8 kW). The multilayer coating was then deposited with the additional Cr target operated at 5 kW and the TiAl targets at 8 kW for 210 min. The deposition parameters for TiAlCrN monolayer coating was the same as the TiAlN/CrN multilayer coating except that the Cr target
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 4 0 6 - 0
Q. Luo et al. / Vacuum 53 (1999) 123—126
was operated continuously at 0.5 kW in the coating mode. The deposited coatings were characterised by Knoop indentation, scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and transmission electron microscopy of cross sections of the coating (XTEM). The cross-section foil preparation followed a procedure reported elsewhere . A block-on-ring tribometer (Cameron—Plint multi purpose friction and wear testing machine) was used for dry sliding test under ambient conditions. The coated block was a cylinder 12 mm in both height and diameter, run against a hardened BM2 ring 60 mm in diameter, resulting in a concentrated linear contact. The effect of load (applied by dead weight) in the range 22—189 N was investigated for a constant sliding speed of 0.4 m/s. The effect of sliding speed in a range 0.2—1.1 m/s was investigated at a constant load of 91 N. A total sliding distance of 1000 m was used in each case. The tangential force was continuously recorded by computer, which provided a measure of the friction coefficient. The wear scar (contained entirely within the coating) was measured using a two-dimensional micrometer so that the volume of the wear scar could be calculated. Worn surfaces and wear debris were investigated by SEM and EDS analysis. Fig. 1. XTEM micrograph of TiAlCrN-BM2, showing columnar polycrystalline microstrucutre of TiAlCrN and the Cr-etched interface.
3. Results and discussion 3.1. Characterisation of coatings
3.2. Friction and wear properties
The properties of the two coatings are given in Table 1. TiAlCrN and TiAlN/CrN had similar thickness and hardness. TiAlN/CrN exhibited strong (1 1 1) texture while TiAlCrN exhibited weak texture. XTEM micrographs show the columnar grains of TiAlCrN (Fig. 1) and TiAlN/CrN (Fig. 2). Both were polycrystalline and TiAlCrN exhibited finer grains than TiAlN/CrN. No additional phase was observed at the nitride/steel interface where Cr ion etching was applied. The contrast between TiAlN/CrN and the pre-deposited TiAlN is visible. The TiAlN/CrN multilayer feature is resolved at higher magnification.
The friction and wear properties of the coatings and uncoated tool steel BM2 are summarised in Table 2. For all test samples, the friction coefficients decreased with the increase of either normal load or sliding speed. The lowest value for TiAlN/CrN (0.23) and TiAlCrN (0.31) were observed at the highest speed. The friction coefficients of TiAlCrN and TiAlN/CrN were comparable under most test conditions and were similar to the uncoated tool steel. However, the TiAlN/CrN exhibited a higher coefficient (0.93) at 0.2 m/s, 91 N, compared with that of TiAlCrN (0.61). In addition, the friction coefficients of TiAlCrN (1.10) and TiAlN/CrN (1.1) were both higher than that of BM2 (0.93) at 0.4 m/s, 22 N. The wear rates of uncoated BM2 were substantially higher that those of the nitride coatings. Interestingly, the increase in wear rate with load (Table 2) was greater for the BM2 tool steel and was least for the TiAlN/CrN multilayer. The wear rates of TiAlCrN and TiAlN/CrN were comparable at the lower loads (22 and 44 N) but the multilayer exhibited a lower wear rate at the higher loads (91 and 189 N).
Table 1 Characteristics of PVD coatings Coatings
HK (GPa) Thickness (lm) Texture (I :I :I )
29.7$6.1 3.13$0.05 9.4 : 1 : 7.9
28.1$4.7 3.18$0.09 1 : 1.4 : 1.7
Q. Luo et al. / Vacuum 53 (1999) 123—126
For a constant load of 91 N, the wear rate of TiAlCrN decreased progressively from 7.42;10\ to 1.25; 10\ mm/m as the sliding speed was increased from 0.2 to 1.1 m/s. For TiAlN/CrN, wear rate was high at 0.2 m/s, (1.04;10\ mm/m), but was much lower for 0.4—1.1m/s (in the range 1.57—2.58;10\ mm/m). Figs. 3 and 4 show the nature of worn surface and wear debris. Transferred steel and oxides (bright in BEI image) are present. The cracking of the coatings was extensive,
Fig. 2. XTEM micrographs of TiAlN/CrN-BM2, showing columnar polycrystalline microstructure of TiAlN base layer and TiAlN/CrN coating, with a higher magnification insert, indicating the multilayer structure.
especially at the higher loads. However, the adhesion of the coating to the substrate was excellent, as shown by the retention of the coating even where it was extensively cracked. The wear debris consisted a mixture of steel particles and iron oxide powder (the proportion of coating was too small to detect). The proportion of iron oxide increased with both load and speed. Similar phenomena have been reported in TiN-steel and TiCN-steel sliding pairs [8,9]. The wear mechanism of the coatings comprised several modes such as polishing, cohesive spalling and adhesive spalling due to tribo-chemical wear and mechanical wear [8,10—12]. At low load, it is believed that the tribo-chemical behaviour  of the nitride determines its wear response, and therefore the wear behaviour of the monolayer and multilayer coatings was similar. However, at the higher loads, the mechanical degradation of the coating dominates its wear behaviour, and in this regime the multilayer provides superior wear resistance to the monolayer. A detailed analysis of the wear mechanisms of the multilayer and monolayer coatings is provided elsewhere .
Fig. 3. Back-scattered electron image (BEI) of the worn TiAlCrN tested at 91 N and 1.1 m/s, indicating material transfer and cracks perpendicular to the sliding direction.
Table 2 Summary of the friction and wear properties Sliding speed (m/s)
0.4 0.4 0.4 0.4 0.2 0.4 0.8 1.1
Applied load (N)
22 42 91 189 91 91 91 91
Wear rate (;10\ mm/m)
1.05 0.84 0.77 0.59 0.93 0.77 0.48 0.23
1.12 0.73 0.76 0.60 0.61 0.76 0.44 0.31
0.93 — 1.02 — — 1.02 0.49 0.52
6.9 12.6 25.8 24.8 204.0 25.8 15.7 21.0
6.7 7.4 39.8 37.3 74.2 39.8 40.2 12.5
163.0 — 608.0 — — 608.0 138.0 132.0
Q. Luo et al. / Vacuum 53 (1999) 123—126
4. The nitride coatings exhibited comparable friction behaviour to each other and to the uncoated steel. Their friction coefficient decreased with the increase of either applied load or sliding speed.
Acknowledgements QL acknowledges the financial support from a joint studentship between the University of Sheffield and Sheffield Hallam University.
References Fig. 4. Secondary electron image of wear debris produced from the test in Fig. 3, which is predominantly iron oxide powder with a few steel particles.
4. Conclusions 1. The wear rate increased with applied load for TiAlCrN and TiAlN/CrN coatings. The rate of increase was least for the multilayer TiAlN/CrN and worst for the uncoated BM2 steel. 2. The wear rate of TiAlN/CrN was stable and low in the sliding speed range 0.4—1.1 m/s while that of TiAlCrN was sensitive to change in speed in this range. 3. The nitride coatings increased the wear resistance of tool steel by up to tens times.
 Donohue LA, Mu¨nz W-D, Lewis DB, Cawley J, Hurkmans T, Trinh T, Petrov I, Greene JE. Surf Coat Technol 1997;93:69.  Sproul WD. Surf Coat Technol 1987;33:133.  Donohue LA, Smith IJ, Mu¨nz W-D, Petrov I, Greene JE. Surf Coat Technol 1997;94/95:226.  Wadsworth I, Smith IJ, Donohue LA, Mu¨nz W-D. Surf Coat Technol 1997;94—95:315.  Ichimura H, Kawana A. J Mater Res 1993;93:1093.  Smith IJ, Gillibrand D, Brooks JS, Mu¨nz W-D, Harvey S, Goodwin R. Surf Coat Technol 1997;90:164.  Sproul WD. J Vac Sci Technol 1994;A12:1595.  Wilson S, Alpas AT. Surf Coat Technol 1997;94/95:53.  Zhao X, Liu J. J Mater Sci 1997;32:2963.  Kennedy FG, Tang L. In: Dowson D, editor. Tribo ser 17: mechanics of coatings. Amsterdam. Elsevier, 1990:409.  Combadieve L, Machet J. Surf Coat Technol 1996;88:17.  Scholl M. Wear 1997;203—204:57.  Luo Q, Rainforth WM, Mu¨nz W-D. Wear, submitted.