Application of the indentation method for cracking resistance evaluation of hard coatings on tool steels/Tooriistateraste ohukeste kovapinnete vastupanu hindamine pragunemisele indenteerimismeetodi abil.

Sivitski, Alina ; Gregor, Andre ; Saarna, Mart 等

1. INTRODUCTION

The functional properties of hard coatings have gained increasing importance as their application for sheet metal cold forming tools has grown recently. Fatigue and cracking resistance are the most important properties in such long-term applications where alternating loads are applied. Investigations of the coating crack development intensity and fatigue are commonly carried out using the indentation method for its ability to measure mechanical properties in the microscopic range [1-5].

The main elements of damage evolution and fatigue response in coated brittle systems are mechanical properties like the Young modulus/hardness ratio (E/H ratio) [4,6], microstructure and deposition technique. Two major crack types were observed in brittle materials during indentation tests: cone and radial (Palmgvist) cracks [1,4,5]. The elastic brittle hard coatings do not show plastic deformation up to tensile fracture (normal stress) and may not suffer fatigue damage. Thus the crack propagation of the coated system may be attributed to the ratcheting deformation of the substrate under cyclic indentation. Two damage modes of coated systems: the tensile-driven cone cracking ("brittle" mode) and shear-driven microdamage accumulation ("quasi-plastic" mode) were determined [5].

This paper investigates cracking resistance of hard ceramic coatings subjected to cyclic Vickers indentation. The dependence of the crack types and the intensity of crack development on the coating type, deposition technique and substrate material are determined.

2. EXPERIMENTAL PROCEDURE

2.1. Materials

A spray formed (SF) cold work tool steel--Weartec[TM] and powder metallurgical (PM) high-speed steel (HSS) Vanadis 6, produced by Uddeholm, were used as substrates for coating deposition (Table 1). According to the manufacturer, PM steels have smaller carbide size and more homogeneous structure than SF steels resulting in better chipping/cracking resistance.

Five different PVD coatings, among them monolayer TiN and multilayer gradient TiCN, nanocomposite nc-(AlTi)N/a-[Si.sub.3][N.sub.4] (nACo[R]), TiAIN and AlTiN, were studied. TiAlN and AlTiN coatings were deposited only on Weartec[TM] substrate to reduce the number of experiments. Cross-sections of coated specimens, observed on the scanning electron microscope (SEM) are presented in Fig. 1. Mechanical properties of the coatings (Table 2) were obtained by MTS Nano Indenter XP[R] in a depth mode with a target depth of 150 nm and average indentation force of 12 mN. The thicknesses of the coatings, measured by Kalotester kaloMax[R], was about 2.3 gym. The results presented in Table 2 are in good correlation with those available in the literature. In [7] the TiN coating with a thickness of 2.1 [micro]m on HSS had the nanohardness of 27 GPa and Young's modulus about 305 GPa. Fouvry et al. [8] investigated a TiCN coating with a thickness of 2.5 [micro]m on HSS which had Young's modulus of 550 GPa. The substrate material and coating surface roughness were measured by a surface roughness measuring instrument of Taylor Hobson Ltd. Surtronic 3+ (using CR filter) with an accuracy of 2% (Tables 1 and 2).

[FIGURE 1 OMITTED]

2.2. Coating deposition procedure

Heat-treated samples (plates) of the size 20 x 20 x 5 mm of two different substrate materials were grinded, diamond polished (powder grain size 1 gm) and degreased ultrasonically in the phosphate-alkali solution before deposition. Coatings were deposited using an arc ion plating PVD technique, on R-80 Platit equipment. The parameters of the deposition process varied from coating to coating and are presented in Table 3.

2.3. Cyclic indentation test procedure

A servo hydraulic fatigue test system INSTRON 8800 and Vickers diamond pyramid indenter were used in the indentation experiments. The total indenting load of 500 N (mean compressive load of 275 N, alternating load of 225 N), stress ratio of 0.1 with a sinusoidal loading pattern and loading frequency of 0.5 ... 15 Hz were applied. The optical microscope Axiovert 25 (ZEISS) with 500x magnification and Buehler Omnimet Image Analysis System 5.40, including the package for crack length measurement (Palmgvist method [9]), was used. The qualitative evaluation criteria of the cracking (0... VI) from crack formation, propagation to delamination (Table 4) were considered.

[TABLE 4 OMITTED]

2.4. Comparative adhesion testing

The adhesion test on the Zwick/ZHR 8150 Rockwell hardness tester at an indentation load of 1471 N (150 kgf) was performed to asses the quality of the coatings. The test procedure followed the VDI 3198 (1992) standard [10].

3. RESULTS AND DISCUSSION

3.1. Adhesion testing

The results of the indentation test are presented by micrographs in Fig. 2. The type and the volume of a failure zone indicate to film adhesion and its brittleness, which correspond to the microstructure and the mechanical properties of the coatings. Coatings of higher E/H ratio withstand the load without nucleation of long radial and conical cracks (TiN and TiCN). However, considerable amount of long radial cracks of 10...50 gm were generated, causing the exfoliation of coating layers that is a typical behaviour of TiN and TiCN coatings under loading [11]. The longest radial cracks are present in the TiN coating and with numerous enfolds and larger exfoliations of the coating layer (Fig. 2a). It is obvious that radial cracks predispose that kind of the coating failure. The same features are seen in the case of the TiAIN coating with only difference--the conical cracks are also present (Fig. 2c). It seems that short radial cracks accelerate chipping on the bordering area of the coating-indenter contact. Emerged chips tend to make connections with the closest of the sides forming the ring or conical crack. The structural defects, presented on the surface, such as pores and non-metallic inclusions, simplify this action. Finally, the most drastically fractured case is the nACo[R] coating (Fig. 2e) with numerous short radial and closed conical cracks. First, the very brittle structure collapses around the indenter and then starts to take up (absorb) fracture energy by the formation of radial cracks. Eventually those are blunted by the perpendicularly formed conical cracks and do not reach the last "ring".

Among all the tested coatings, the TiCN seems to be the most endurable. The mixed failure modes are characteristic of the studied coated system. Most widely presented are cohesive (chipping, caused by the normal component of the stress tensor) and the delamination with buckling and fracture mode (decohesion of a coating with the formation of microcracks, caused by a combination of shear and normal stresses).

[FIGURE 2 OMITTED]

3.2. Coated system behaviour under cyclic loading

The results of cyclic indentation are given in Table 5 and in Fig. 3. The fracture analysis of the TiCN coating revealed that the "quasi-plastic" damage mode with the shortest radial cracks about 20 gm after 10 000 cycles prevailed when the Vickers diamond pyramid was used in the indentation test. In the contrast to the spherical indenters, a sharp indenter penetrates easily into the coating surface and cone cracking formation is suppressed by the radial cracks nucleation (Fig. 3). Radial cracks also become the dominant mode of the indentation fracture of the TiAlN and AlTiN coatings and lead to delamination and accelerated cracking resistance degradation (Fig. 4b). Within multilayer coatings with the "quasi-plastic" damage mode (TiCN, TiAlN, AlTiN) the cracking resistance increased with E/H ratio growth after 10 000 cycles (Fig. 5). Apart from other coatings, in nACo[R] with the lowest E/H ratio, "brittle" damage mode occurred with the formation of cone cracks, driven by tensile stresses in addition to "quasi-plastic" damage mode (Fig. 4c). On the other hand, the obscured subsurface damage and the formation of radial cracks could be found below cone cracks. The multilayer coatings are preferable to the monolayer TiN coating with "quasi-plastic" damage mode, one of the lowest cracking resistance and the formation of strong radial cracks, despite the high E/H ratio of TiN (Figs. 4a, 5a).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The observations of both substrates, coated with TiCN, exposed the advantages of PM steel Vanadis 6 over Weartec[TM]. Vanadis 6 has higher Young's modulus, smaller carbide size and uniform carbides distribution that hampers the propagation of cracks, and crystallographic similarity with the coating, resulting in good adhesion (Fig. 5a). However, Vanadis 6 as a substrate for TiN and nACo[R] did not show benefits over Weartec[TM].

4. CONCLUSIONS

Indentation cyclic tests of coated tool steels were carried out and the following conclusions can be drawn.

1. The "quasi-plastic" damage mode with the formation of radial cracks prevailed in indentation cycling testing and is typical for high E/H values of coatings. The increase of the number of the indentation cycles leads to the radial crack growth. "Brittle" damage with the formation of cone cracks along with "quasi-plastic" damage mode is characteristic of the cyclically loaded PVD coatings with the lowest E/H ratio.

2. For multilayered coatings with the "quasi-plastic" damage mode (TiCN, TiAlN, AlTiN) the cracking resistance increases with the coating E/H ratio growth. TiCN on Vanadis 6 has the best cracking resistance (II criteria) after 10 000 cycles.

3. Multilayer coatings had higher fatigue resistance than monolayer coatings with higher E/H ratio.

doi: 10.3176/eng.2009.4.08

ACKNOWLEDGEMENTS

This work was supported by the Estonian Ministry of Education and Research (project SF0140091s08) and the Estonian Science Foundation (grants Nos. 7227, 7442 and 7889).

Received 3 September 2009, in revised form 6 October 2009

REFERENCES

[1.] Richter, J. Application of Vickers indentation for assessment of PVD TiN coated new nonledeburitic high-speed steels. Surf. Coat. Tech., 2003, 162, 119-130.

[2.] Zoestbergen, E. and De Hosson, J. Th. M. Crack resistance of PVD coatings: Influence of surface treatment prior to deposition. Surf. Eng., 2002, 18, 283-288.

[3.] Tarres, E., Ramirez, G., Gaillard, Y., Jimenez-Pique, E. and Llanes, L. Contact fatigue behavior of PVD-coated hardmetals. Int. J. Refract. Met. Hard Mater., 2009, 27, 323-331.

[4.] Cook, R. F. and Pharr, G. M. Direct observation and analysis of indentation cracking in glasses and ceramics. J. Am. Ceram. Soc., 1990, 73, 787-817.

[5.] Kim, D. K., Jung, Y.-G., Peterson, I. M. and Lawn, B. R. Cyclic contact fatigue of intrinsically brittle ceramics in contact with spheres. Acta Mater, 1999, 47, 4711-4725.

[6.] Chiang, S. S., Marshall, D. B. and Evans, A. G. The response of solids to elastic/plastic indentation. II. Fracture initiation. J. Appl. Phys., 1982, 53, 312-317.

[7.] Holmberg, K., Ronkainen, H., Laukkanen, A., Wallin, K., Hogmark, S., Jacobson, S., Wiklund, U., Souza, R. M. and St5hle, P. Residual stresses in TiN, DLC and MOSZ coated surfaces with regard to their tribological fracture behaviour. In Proc. 13th Nordic Symposium on Tribology NORDTRIB 2008. Tampere, Finland, 2008, 37 p.

[8.] Fouvry, S., Liskiewicz, T. and Paulin, C. A global-local wear approach to quantify the contact endurance under reciprocating-fretting sliding conditions. Wear, 2007, 263, 518-531.

[9.] Palmqvist, S. The work for the formation of a crack during Vickers indentation as a measure of the toughness of hard metals. Arch. Eisenhuttenwes., 1962, 33, 629-634.

[10.] VDI3198:1992-08, Coating (CVD, PVD) of Cold Forging Tools. VDI-Verlag, Dusseldorf, 1991.

[11.] Bouzakis, K.-D., Siganos, A., Leyendecker, T. and Erkens, G. Thin hard coatings fracture propagation during the impact test. Thin Solid Films, 2004, 460, 181-189.

Alina Sivitski (a), Andre Gregor (b), Mart Saarna (b), Priit Kulu (b) and Fjodor Sergejev (b)

(a) Department of Mechatronics, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; alina.sivitski@ttu.ee

(b) Department of Materials Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; andre.gregor@ttu.ee

Table 1. Chemical compositions, mechanical and surface properties of
substrate materials

               Chemical composition, wt%

Substrate       C    Si    Mn    Cr    Mo     V

Weartec [TM]   2.8   0.8   0.7   7.0   2.3   8.9
Vanadis 6      2.1   1.0   0.4   6.8   1.5   5.4

               Hardness   Young's         Surface
Substrate      HRC/HV30   modulus        roughness
                          E, GPa    [R.sub.a], [micro]m

Weartec [TM]    64/843      199     0.51 [+ or -] 0.10
Vanadis 6       64/843      210     0.51 [+ or -] 0.10

Table 2. Mechanical and surface properties of coatings

Coating/        Type            Young's
Substrate                       modulus        Nano-hardness,
                                E, GPa               GPa

TiN         Monolayer       438 [+ or -] 80   28.5 [+ or -] 0.6
TiCN        Multilayer      500 [+ or -] 90   26.6 [+ or -] 1.4
nACo[R]     Nanocomposite   323 [+ or -] 13   29.0 [+ or -] 1.5
TiAIN       Multilayer      301 [+ or -] 90   19.9 [+ or -] 1.2
AlTiN       Multilayer      336 [+ or -] 13   23.8 [+ or -] 1.0

Coating/     E/H          Surface
Substrate   ratio        roughness
                    [R.sub.a], [micro]m

TiN         15.4    0.08 [+ or -] 0.01
TiCN        18.8    0.10 [+ or -] 0.04
nACo[R]     11.1    0.10 [+ or -] 0.04
TiAIN       15.2    0.10 [+ or -] 0.04
AlTiN       14.1    0.05 [+ or -] 0.01

Table 3. Deposition parameters

Coating     Bias voltage,                 Pressure,
                  V                          mbar

TiN         -75 ... -120               8 x [10.sup.-3]
TiCN        -60 ... -120           (5 ... 7) x [10.sup.-3]
nACo        -75 ... -150    9 x [10.sup.-3] ... 1.2 x [10.sup.-2]
TiAlN       -60 ... -150    8 x [10.sup.-3] ... 1.5 x [10.sup.-2]
AlTiN       -60 ... -150    4 x [10.sup.-3] ... 1.2 x [10.sup.-2]

Coating        Ti-Al/AlSi cathode       Temperature,   Ar
                  arc current,           [degrees]C
                        A

TiN              (100 ... 125)/-            450        6
TiCN             (120 ... 130)/-            450        6
nACo        (82 ... 125)/(65 ... 100)   435 ... 475    6
TiAlN       (85 ... 125)/(65 ... 115)       475        6
AlTiN       (60 ... 125)/(52 ... 130)   430 ... 450    6

Coating     [N.sub.2] flow,   [C.sub.2][H.sub.2]
                 seem

TiN               200                 --
TiCN          165 ... 180          7 ... 39
nACo              200                 --
TiAlN             200                 --
AlTiN         150 ... 200             --

Table 5. Crack types and crack evaluation criteria

                                  Cycles

                       1      10               50

TiN/Weartec[TM]        I      I                I
TiN/Vanadis 6          I      I                I
TiCN/Weartec[TM]       0, I   0, I             0, I
TiCN/Vanadis 6         0      0                0
nACo[R]/Weartec[TM]    I      I, II            II
                                Starting         Delamination
                                delamination

nACo[R]/Vanadis 6      0      I                V
                                Starting       Cone crack
                                delamination
TiAlN/Weartec[TM]      0, I   I                I
AlTiN/Weartec[TM]      I      I                I

                                    Cycles

                       100              1000         10 000

TiN/Weartec[TM]        I, II            II           IV
TiN/Vanadis 6          I                II           IV
TiCN/Weartec[TM]       I                I            III
TiCN/Vanadis 6         0, 1             0, I         III
nACo[R]/Weartec[TM]    II               V            V
                         Delamination   Cone crack   Cone crack
nACo[R]/Vanadis 6      V                V            V
                       Cone crack       Cone crack   Cone crack
TiAlN/Weartec[TM]      III              II, III      VI
AlTiN/Weartec[TM]      I                II           VI