TiN coatings with high amounts of phase boundaries

Surface and Coatings Technology, 36 (1988) 707 - 714

707

PREPARATION AND BEHAVIOUR OF WEAR-RESISTANT TiC/TiB2, TiN/TiB2 AND TiC/TiN COATINGS WITh HIGH AMOUNTS OF PHASE BOUNDARIES* H. HOLLECK and H. SCHULZt Kern forschungszentrum Karisruhe, Institut für Material- und Festkorperforschung, Postfach 3640, D-7500 Karlsruhe 1 (F.R.G.) (Received April 11, 1988)

Summary The toughness of hard coatings is one important parameter influencing flank wear in metal cutting. Large amounts of phase boundaries can enhance the toughness of hard coatings. A method is described for producing coatings with large amounts of phase boundaries in the system TiC! TiN/TIE2 by magnetron sputtering. The composition, properties and flank wear of these sequential coatings were examined during continuous and interrupted cutting. For the determination of the toughness of the hard coatings the crack propagation from Vickers’ indentations and hertzian indentations was used. Coatings with 100 250 layers, with a total layer thickness of 5 pm, show optimum properties and performance. -

1. Introduction The resistance against abrasive wear is normally related to the hardness of the wear parts [1]. In the case where the abrasive material is softer than the abraded material it can be shown that the toughness also plays an important role in the wear rate [2, 3]. The toughness of bulk hard materials can be increased by introducing large amounts of phase boundaries, and it can be shown that such materials have lower flank wear rates in metal cutting than single-phase hard materials [4J. This can be explained by the role of the phase boundaries in dissipating strain energy [4]. The fundamental relations between composition and properties of bulk hard materials are also valid for hard coatings [5 7]. Therefore the abrasive wear resistance of hard coatings should also be increased by introducing large amounts of -

*Paper presented at the 15th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 11 - 15, 1988. tPresent address: Baizers Limited, 9496 Balzers, Liechtenstein. 0257-8972/88/$3.50

© Elsevier Sequoia/Printed in The Netherlands

708

phase boundaries. Because abrasive wear is the main wear mechanism of flank wear in metal cutting [8], these coatings should also have an increased resistance against flank wear. 2. Experimental details Coatings with a large amount of phase boundaries can be prepared using several sputtering techniques [61. Figure 1 shows the sequential sputtering technique. In this technique the substrates are moved between the cathodes on a rotating table. Sequential coatings with the combinations TiC/TiB2, TiB2/TiN and TiC/TiN were prepared with 10, 100, 250, 500 and 1000 layers to a final thickness of 5 pm (argon pressure, 0.2 Pa; d.c. power, 1000 W for both magnetron cathodes; cathode diameter, 150 mm; substrates, several grades of cemented carbide).

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Fig. 1. Technique of sequential sputtering with two cathodes.

3. Results and discussion 3.1. Composition of the phase boundaries in sequential coatings A knowledge of the composition of the phase boundaries is important for the interpretation of the properties and the wear behaviour of sequential coatings. The composition of the phase boundaries in these coatings was examined using Auger electron spectroscopy (AES) and X-ray analysis. Details concerning these measurements and the results are described in ref. 9. When sputtering TiC onto TiN or TiN onto TiC a mixed phase of Ti(C, N) is formed. The width of this cubic mixed phase was determined to be 3 4 nm. The X-ray diffraction pattern of a sequential TiC/TiN coating with 1000 layers and a layer thickness of 5 nm therefore shows strong Ti(C, N) peaks and only small peaks of TiC and TiN (see Fig. 2). The phase boundary obtained by sputtering TiC or TiN onto TiB2 is characterized as a Ti(B, C) or Ti(B, N) mixed phase and an epitaxial growth -

709 28 42

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Fig. 2. X-ray diffraction pattern of a 1000-layer TiC/TiN coating.

of TiC or TiN on these mixed phases. The widths of the Ti(B, C) and Ti(B, N) phases were determined by AES to be approximately 2 nm and 1 nm respectively. The atomic positions of titanium in the mixed phase are the same as in the (001) plane of hexagonal TiB2. With the sputtering conditions of these investigations TiE2 grows in the (001) plane. Epitaxial growth of cubic TiC and TiN is observed on the mixed phase. Many possibilities exist for coherence between TiB2 and TiC or TiN [10]. In sputtered coatings only the coherence between the (001) plane of TiB2 and the (111) plane of TiC or TiN was observed. These planes are the close packed planes of these two structures. Epitaxial growth of TiC (to about 20 nm) occurs on the hexagonal Ti(B, C) mixed phase. At a layer thickness of around 20 nm the number of crystal defects increases and the crystal orientation changes to the (100) plane, which is the standard orientation we observed in TiC monolayers with our sputter conditions. The change from the (200) texture to the (111) texture in the TiC phase in 10 and 250 layer sequential TiC/TiB2 coatings is shown in Fig. 3. TiN does not crystallize on the hexagonal Ti(B, N) mixed phase in the [1111 direction as observed for TiC. TiN coatings normally show columnar crystals. The orientation of some nuclei, however, is influenced by the Ti(B, N) mixed phase. Figure 4 shows schematically the structure of the sputtered phase boundaries between TiE2 and TiC or TiN. When sputtering TiB2 onto TiC or TiN an intermixed boundary phase is also formed. This phase probably has a strong lattice distortion, because TiE2 grows to 5 nm on the Ti(C, B) phase and to 10 nm on the Ti(N, B) phase in an amorphous state. Figure 5 shows the X-ray pattern of two different 250-layer sequential TiC/TiE2 coatings. When both phases are deposited with a single-layer thickness of 20 nm the TIE2 layers are partially crystalline and the TiC layers grow epitaxially in the [1111 direction. When the thickness of the TiB2 layers is reduced to 5 nm, the TiC layers grow in the [200] direction, because the amorphous TiE2 layers do not influence the nucleation of the TiC layers. The different compositions of the phase transitions in sequential coatings are also reflected in the microstructure. Coatings with large amounts of amorphous zones, for instance TiC/TIE2 and TiB2/TiN coatings with 1000

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monolayers, show a smooth fracture surface. The completely crystalline 1000-layer sequential TiC/TiN coating has a columnar microstructure (see Fig. 6). 3.2. Properties of sequential coatings The hardness of sequential coatings is hardly influenced by the amount of phase boundaries (see Fig. 7). The decrease in hardness of the 1000-layer TiC/TiN coatings is due to the grain boundaries of the columnar crystals. The small decrease in hardness in the 1000-layer TiC/TiB2 and TiB2/TiN coatings is due to the increasing volume of amorphous zones. Simultaneously sputtered, amorphous TiC—TIE2 coatings, i.e. coatings sputtered from a two-

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phase TiC—TiE2 target, also show a lower hardness than monolayer TiC or TiB2 coatings [11]. No standard testing methods exist for the determination of the toughness of hard coatings. In our work the coating toughness was investigated by crack formation and crack propagation from Vickers’ indentations (Palmqvist toughness) [12] and hertzian indentations [13]. Both methods are suitable for the determination of the toughness of hard coatings [9]. Here, only some results of the Vickers indentation methods are reported.

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The crack resistance, which is the reciprocal of the slope of the plot of load vs. crack length, was taken as a toughness value. When the Palmqvist method was applied to cemented carbides coated using physical vapour deposition cracks were generated in the coating and in the substrate. Therefore an absolute toughness value for a coating could not be evaluated. A relative comparison of the toughness of various coatings is possible, if coatings with the same thickness on identical substrates are examined. Sequential coatings show a strong relationship between the crack resistance and the amount of phase boundaries (see Fig. 8). T1C/TiB2 and TiB2/TiN coatings have the highest crack resistance with 100 layers. This can be explained by a dissipation of crack energy and crack deflection by the phase boundaries. The decreasing crack resistance from 100 to 1000 layers is due to the increasing volume of the amorphous TiB2 phase. The crack resistance of TiC/TiN coatings slowly increases from 10 to 250 layers. The decrease up

713

to 1000 layers can be explained by the change in microstructure. The crack resistance of the sequential and monolayer coatings can only be compared by consideration of the internal stresses [9]. 3.3. Wear tests Cutting tests with coated inserts were performed under continuous and interrupted cutting of unalloyed low carbon steel (AISI 1042). The flank wear of monolayer and sequential TiC/TiB2 coatings under continuous cutting conditions is shown in Fig. 9. Coatings with 100 phase boundaries have the lowest flank wear. This can be explained by the optimized toughness properties of these coatings. Flank wear is an abrasive process, which occurs on hard materials by a propagation of microcracks and by tearing off particles from the coating. Therefore with increasing crack resistance, higher flank wear resistance is observed.

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Under severe cutting conditions the advantage of sequential coatings in éomparison with monolayer physically vapour deposited (PVD) and chemically vapour deposited (CVD) coatings can be shown. Only the CVD samples with a monolayer TiN coating could withstand the severe conditions of the bar turning test (interrupted cutting conditions). Commercial CVD samples with TiC, TiC/TiN and TiC/A1203 coatings failed after a few impacts. The sequential TiC/TiB2 and TiC/TiN coatings with 100 layers showed the lowest flank wear (Fig. 10). The non-uniform flank wear of the sputtered coatings is due to the separated coating run for the rake and the clearance face. With a special coating process, where the samples can be coated in one step uniformly over the cutting edge, a uniform flank wear should be possible.

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4. Conclusions It was shown that the toughness of hard coatings is influenced by introducing large amounts of phase boundaries. Coatings with optimized toughness properties have a high resistance against flank wear. The dissipation of crack energy and crack deflection by phase boundaries is important in the toughness properties of hard coatings. Another factor is the change in microstructure by the deposition of different phases. For a better understanding of the effect of phase boundaries in hard coatings further research is necessary, especially with different materials and with additional analytical methods such as high resolution transmission electron microscopy. References 1 K. H. Habig, Verschleiss und Hàrte von Werkstoffen, Carl Hanser Verlag, MünchenWien, 1980. 2 W. Dawihl and U. Dworak, Arch. Eisenhuettenwes., 47(1976) 757 - 762. 3 K. H. Zum Gahr, in K. H. Zum Gahr (ed.), Reibung und Verschleiss bei metallischen und nichtmetallischen Werkstoffen, DGM-Verlag, Oberursel, 1986, p. 39. 4 H. Holleck and C. Kühl, in H. Bildstein and H. M. Ortner (eds.), Proc. 11th Plansee Seminar, Reutte/Tirol, Austria, 1985, Vol. 1, pp. 913 - 926. 5 H. Hoileck, Z. Werkstofftech., 17 (1986) 334 - 341. 6 H. Holleck, J. Vac. Sci. Technol. A, 4 (1986) 2661 - 2669. 7 H. Holleck, C. Kühl and H. Schulz, J. Vac. Sci. Technol. A, 3 (1985) 2445 - 2449. 8 B. M. Kramer and P. K. Judd, J. Vac. Sci. TechnoL A, 3 (1985) 2439 - 2444. 9 H. Schulz, KIK-Rep. 4306 1987 (Kerforschungszentrum Karlsruhe). 10 H. Holleck, H. Leiste and W. Schneider, in P. Vincenzini (ed.), High Tech Ceramics, Elsevier, Amsterdam, 1987, p. 2609. 11 H. Schulz and H. Holleck, Proc. 2nd Physical Vapour Deposition Conf. Darmstadt, Technische Hochschule Darmstadt, Schriftenr. Wiss. Tech., 30 (1986) 145 - 155. 12 S. Palmqvist, Arch. Eisenhuettenwes., 33 (1962)1 -6. 13 R. Warren and H. J. Matzke, in R. K. Viswanadham, D. J. Rowclifte and J. Gurland (eds.), Proc. mt. Conf. on The Science of Hard Materials, Jackson, 1981, Plenum, New York, 1983, pp. 563 - 582.