Evaluation of TiB2 coatings in sliding contact against aluminium

Surface and Coatings Technology 149 (2002) 14–20

Evaluation of TiB2 coatings in sliding contact against aluminium M. Berger*, S. Hogmark ˚ ¨ Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden The Tribomaterials Group, The Angstrom Received 27 November 2000; accepted in revised form 4 July 2001

Abstract A major problem in machining aluminium alloys is abrasive wear of the cutting tool due to the presence of oxides and other hard particles in the metal. The strong tendency of aluminium to adhere to the tool and form built-up edges is another problem, which affects the finish of the cut surface. Bulk titanium diboride TiB2 is hard, and known to have a high chemical resistance when exposed to aluminium. These characteristics make this material an interesting candidate to be applied as a coating on tools aimed at machining aluminium alloys. Polycrystalline diamond (PCD) tools are often used today in operations involving both shaping (removal of large volumes) and surface finishing. However, they tend to produce too rough surfaces. In this paper, the possibility of magnetron sputtered TiB2 coatings reducing the ability of aluminium to be transferred to the tool surface has been experimentally evaluated by sliding against an aluminium alloy (AA7075). It was found that TiB2 coatings with relatively low residual compressive residual stress (y0.5 GPa) performed better than a reference TiN coating and uncoated cemented carbide as to the tendency of aluminium pick up. In addition, this TiB2 coating was more resistant to detachment and chemical wear as compared to highly stressed TiB2 (y6.1 GPa). 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: TiB2; Aluminium; Sliding contact; Metal cutting

1. Introduction A problem in machining and extrusion of aluminium (Al) and its alloys is the transfer and adhesion of work material to the tool. Titanium diboride (TiB2) has proven to have a very low solubility in liquid Al w1–3x, as well as being a very good diffusion barrier for Al w4x, which indicates that the tendency of work material sticking should be relatively low. TiB2 also has a high thermal conductivity w5x and a high hot hardness w6x, which would guarantee a low mechanical wear. Since Al is a relatively soft metal, the wear of the hard coatings cannot be abrasive as long as the alloy does not contain and hard phases. However, if the work material sticks strongly to the coating it could result in * Corresponding author. Department of Materials Science, Uppsala University, Box 534, 75121 Uppsala, Sweden. Tel.: q46-18-4717237; fax: q46-18-471-3572. E-mail address: [email protected] (M. Berger).

severe cohesive delamination or detachment of the coating. Another wear mechanism that has to be considered is the chemical wear (i.e. dissolution of the coating material into the work material). Of course, this type of wear is more aggressive if the temperature is high. In this work TiB2 coated cemented carbide (CC) cylinders have been evaluated in sliding contact against an Al cylinder. Uncoated and titanium nitride (TiN) coated CC were used as references. A specially designed test with crossed cylinders geometry, see Fig. 1, was used to ensure resemblance with actual metal cutting w7 x . 2. Experimental 2.1. Materials Cemented carbide cylinders (⭋ 50 mm) uncoated and coated with TiN as well as TiB2 were used as test materials.

0257-8972/01/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 6 1 - 5

M. Berger, S. Hogmark / Surface and Coatings Technology 149 (2002) 14–20

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2.2. Experimental setup

Fig. 1. Schematic illustration of the experimental setup. The vertical test pin is pressed by the normal force FN against a large rotating cylinder. In order to simulate actual metal cutting, where the tool constantly meets new work material, the test pin is slowly moved laterally as the test proceeds.

An Al-alloy AA7075 was used to simulate a typical work-piece material. This high strength Al-alloy has a nominal chemical composition (in wt.%) of 5.6 Zn, 2.5 Mg, 1.6 Cu and 0.3 Cr. Applications of this material are found in e.g. aircrafts and hydraulic fittings. The nominal chemical composition (wt.%) of the CC substrate is 8 Co and 92 tungsten carbide (WC). The average WC grains size is approximately 0.8 mm, and the microhardness is 1730 HV50. All coatings were deposited in a PVD unit (Balzers BAI 640R). The substrate cylinders were held in position by magnets. The TiN coating was deposited using reactive evaporation w8x and the TiB2 coatings by using d.c. magnetron sputtering of a TiB2 target w9x. The mechanical properties of these coatings are listed in Table 1. Before fixtured in the PVD coater, the substrates were cleaned in an ultrasonic bath of alkali wash and alcohol. Prior to deposition, the substrates were resistively heated to approximately 4508C for 60 min, and thereafter argon-ion sputter etched during 30 min with a substrate bias Vs of y200 V.

In order to simulate the actual conditions of work materialytool contact in metal cutting, a crossed cylin¨ ders test developed by Soderberg et al. w7x was utilised, see Fig. 1. In this test, a long cylinder (⭋ 255 mm) of the work material is rotated with constant speed. The tool, represented by a small cylindrical test pin, is then pressed against the work material with a constant normal force FN. There is also a continuous lateral movement of the test pin in order to have a ‘fresh’ contact during the whole test. The sliding speed was kept low (0.1 m ys), in order not to exaggerate the aluminium transfer in the contact spot between the crossed cylinders. Initial experiments showed that the transfer of aluminium onto the uncoated CC cylinders was too extensive when the sliding speed exceeded this value. Three different values of FN were used, 4.5, 9 and 27 N. During the test, the friction coefficient was recorded by a load-cell placed below the test pin. The sampling rate for this measurement was 5 Hz. Before each experiment, the aluminium

Fig. 2. Typical friction recording. (TiB2 0 V, FNs9 N).

Table 1 Bias voltage and mechanical properties of test coatings (CC for comparison) Coating

Coating designation

Substrate bias (Vs)

Residual stress (GPa)a

Hardness (GPa)b

Young’s modulus (GPa)b

Critical load in scratch test (N)

TiB2

TiB2 0 V

y4.7"0.5

48"4

579"45

33"1

TiB2 TiB2 TiN CC

TiB2q10 V TiB2q50 V TiN y

Floating potential q10 q50 y110 –

y0.5"0.2 y0.5"0.2 y4.1"0.4 –

48"4 54"9 28"3 17"2

580"70 600"85 420"20 620"30

45"1 42"1 90"1 -

a b

Measured using the beam deflection technique w10x. Measured using nano indentation w11x.

M. Berger, S. Hogmark / Surface and Coatings Technology 149 (2002) 14–20

16 Table 2 Friction values of test materials Normal load wN x

TiB2 0 V

TiB2 q10 V

TiB2 q50 V

TiN

CC

4.5 9 27

1.7"0.3 1.4"0.3 1.1"0.4

1.6"0.3 1.4"0.3 1.1"0.2

1.8"0.4 1.4"0.3 1.0"0.4

2.0"0.4 1.7"0.3 1.2"0.4

1.9"0.4 1.6"0.4 1.2"0.4

Fig. 4. SEM micrograph of the wear path on the work material after sliding against uncoated CC (FNs27 N, sliding direction of the test pin upwards).

cylinder was finished by turning in order to guarantee identical surface conditions of the counter material. Post-test examination of cross-sections of the contact spots were done by scanning electron microscopy (SEM) using a LEO 1550 equipped with a field emission gun. In order to examine the test surface underneath adhered Al, the latter was removed by treating the pins in a hot, 30 wt.% NaOH solution for 5 min. The topography of the sliding path on the Al cylinder was studied by SEM on replicas made with silicon based rubber material normally used to model teeth implantants (OPTOSIL䉸-XANTOPRE䉸 ). 3. Results

Fig. 3. SEM micrographs of the contact region of (a) TiB2q50 V, (b) TiN and (c) uncoated CC (FNs4.5 N).

The friction coefficient was high and oscillated typically between 0.5 and 2.5 during each test and for all tested surfaces, see Fig. 2. However, the average friction coefficient was somewhat lower for the TiB2 coatings as compared to TiN and uncoated CC, see Table 2. It was seen that the friction was reduced when the load was increased. Studies in the SEM showed that Al material had transferred to all test pins. However, the tendency of transfer to the elliptical contact areas was somewhat smaller for the TiB2 coatings as compared to that of TiN and uncoated CC, cp. Fig. 3. The contact path on the Al-cylinder correspondingly revealed frequent lumps of piled up Al-material, see Fig. 4. Cross-sectional examination of the contact regions on the test pins showed that the transferred Al material had a lamellae structure, cp. Fig. 5. For the highest normal load (27 N) coating failures were observed for the TiB2 0 V coating as well as the TiN coating, but no

M. Berger, S. Hogmark / Surface and Coatings Technology 149 (2002) 14–20

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Fig. 5. Typical SEM-micrographs of cross-sections of adhered patches of Al-material, c.p Fig. 4. (a) TiB2 q10 V, (b) close up of (a), (c) TiN, (d) close up of (c), (e) uncoated CC and (f) close up of (e). (FNs4.5 N.)

failures were observed for the TiB2q10 V and q50 V coatings at this load, see Figs. 6 and 7. For the lower loads (4.5 and 9 N) failures were not observed on any coating. 4. Discussion Aluminium has a strong tendency to adhere to almost any counter material in dry sliding. In this test, there

was a relatively cyclic formation of patches in the sliding path of the Al cylinder, cp. Fig. 4, initiated by adhesion between the test pin surface and the surface of the Al cylinder. The subsequent velocity accommodation is managed by successive shear in the Al material, probably at a depth corresponding to the thickness of the work hardened layer produced during finishing. When an adhered patch of aluminium has reached a

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M. Berger, S. Hogmark / Surface and Coatings Technology 149 (2002) 14–20

Fig. 6. Typical SEM cross-sections of the test pin contact areas with transferred Al material after sliding at FN s27 N. (a) TiB2 0 V, (b) TiB2q50 V and (c) TiN.

critical thickness, it is left in the sliding path of the Al cylinder. However, some of them are deposited to the test pin, cp. Figs. 5 and 6. A more detailed description of a similar mechanism found in self mated sliding of steel has been described by Hogmark and Vingsbo w12x. Once a patch has been released, the adhesive process starts from the beginning. From Fig. 4 we can see that this roughly happens every 0.5 s. However, the average distance between the patches in the sliding path is approximately 5 mm, while the sampling frequency of 5 Hz corresponds to 20 mm. There is also an unknown inertia in the test device. Instead of being directly related to the deposition of Al patches on the Al cylinder, the recorded variations in friction are probably due to a more irregular adhesion and exchange of adhered Al on the test pin. Features of the first and third sliding path seen from the right in Fig. 4 may reveal the mechanism behind the friction oscillation. The top surface of the patch in the sliding track is rough and so is the topography of the path immediately above it. Consequently, the test pin surface is also rough due to adhered Al. The corresponding topography of patch and path in the third path are smooth, indicating a smooth test pin surface, free from adhered Al. The latter probably accounts for the low friction level and the former for the high. The observation that friction is slightly lower for TiB2 compared to TiN and CC is probably a result of reduced Al adhesion. The strong oscillating friction force puts high demands on the adhesion and cohesion of the tested materials. In addition, the temperature rise associated to the friction is relatively high since it is concentrated to the aluminium patches, which are approximately 3–5 times smaller than the pin contact areas, cp. Figs. 3, 4 and 7. Thus the test coatings are subject to both mechanical fatigue and chemical attack from the Al material. The TiN coating suffers both chemical degeneration and mechanical decohesion and detachment, as revealed by the combination of smooth and rough areas in the test surface, see Fig. 6c, Fig. 7d,e. Chemical reactions between TiN and Al has been shown by other investigations w13,14x. Contrary to TiN, no chemical dissolution could be detected in the TiB2 test surfaces, cp. Figs. 6 and 7 and references w1–4x. However, the TiB2 with high residual stress (y6.1 GPa) detached from the CC substrate at the highest normal force (27 N), which from the size of the Al patches can be estimated to approximately 100 MPa. There is a possibility of Al diffusing through pores, cracks and other defects of the PVD coatings weakening the interface, but this has not been confirmed in the present investigation. At this moment, the difference in behaviour between the TiB2 coatings is thought to originate from their different stress

M. Berger, S. Hogmark / Surface and Coatings Technology 149 (2002) 14–20

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Fig. 7. Representative SEM micrographs of the test pin contact area after testing with FN s27 N. Any transferred Al-material has been removed by NaOH etching. (a) TiB2 0 V, (b) close up of (a), (c) TiB2q50 V, (d) TiN, and (e) close up of (d).

states, which obviously yield a better result for the low stressed TiB2 coatings in the scratch test, see Table 1.

against Al (Al7075). The main conclusions are the following:

5. Conclusions

● No wear, adhesive detachment or chemical dissolution, was evident for the low stressed TiB2 coatings, the reference TiN coating suffered from both these mechanisms;

In this work PVD TiB2 coatings were evaluated and compared to TiN and uncoated CC in dry sliding contact

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M. Berger, S. Hogmark / Surface and Coatings Technology 149 (2002) 14–20

● the strong sticking of Al demands very good cohesion and adhesion of the coating. Therefore, a high residual stress level in the PVD coating may be detrimental for these contact situations; and ● the TiB2 coating also has a reduced tendency to pick up Al from the counter surface compared to TiN and uncoated CC, which correlates to their slightly lower friction coefficient. Acknowledgements This work has been supported by SECO Tools AB and the Swedish Research Council for Engineering Sciences (TFR). References w1x M. Kornmann, R. Funk, Aluminium 53 (1977) 249–252. w2x J.D. Rigney, J.J. Lewandowski, J. Mater, Science 28 (1993) 3911–3922.

w3x A.V. Smith, D.D.L. Chung, J. Mater, Science 31 (1996) 5961– 5973. w4x D.J. Peterman, J. Vac. Sci. Technol. A 14 (3) (1996) 768–770. w5x R.K. Williams, R.S. Graves, F.J. Weaver, J. Appl. Phys. 59 (5) (1986) 1552–1556. w6x J.R. Ramberg, W.S. Williams, J. Mater. Sci. 22 (1987) 1815– 1826. w7x M. Olsson, S. Soderberg, ¨ S. Jacobson, S. Hogmark, Int. J. Mach. Tools Manufact. 29 (3) (1989) 377–390. w8x M. Larsson, M. Bromark, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 76y77 (1995) 202–205. w9x M. Berger, M. Larsson, S. Hogmark, Surf. Coat. Technol. 124 (2000) 253–261. w10x P.M. Ramsey, H.W. Chandler, T.F. Page, Surf. Coat. Technol. 43y44 (1990) 223–233. w11x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. w12x S. Hogmark, O. Vingsbo, Wear 38 (1976) 341–359. w13x H.-J. Lee, R. Sinclair, P. Li, B. Roberts, J. Appl. Phys. 86 (6) (1999) 3096–3103. w14x T. Bjork, ¨ R. Westergard, ¨ P. Hedenqv˚ S. Hogmark, J. Bergstrom, ist, Wear 225–229 (1999) 1123–1130.