STUDY OF FATIGUE BEHAVIOUR OF HARD COATINGS OBTAINED BY PVD TECHNIQUE USING SPECIFIC TEST RIGS Y. GACHON HEF, Surface Mechanics and Tribology Department, ZI Sud rue Benoît Fourneyron F42166 Andrézieux Bouthéon Cedex, FRANCE; e-mail: ygachon@hef.fr C. LANGLADE-BOMBA, A. B. VANNES Ecole Centrale de Lyon, IFoS (UMR CNRS 5621) 36 avenue Guy de Collongue, BP 163, F69131 Ecully Cedex, FRANCE; e-mail: cecile.langlade-bomba@ec-lyon.fr SUMMARY A major challenge faced by elaborators of PVD coatings is to characterise their mechanical properties to widespread industrial applications of these films, especially among automotive and forming tool industry. The present study focuses on several films (TiN, TiBN) elaborated by PVD (magnetron sputtering) or PACVD technique on cutting tool substrate. The adherence of the films was studied by scratch testing. AE monitoring was used to detect film failure. Pin on disc tests were performed on each film. The above mentioned test is suited for testing a solicitation including sliding. Other applications like gear teeth, forming tools or moulds encounter more shocks during service life. Therefore a specific test bench performing repetitive impacts at the same place to simulate surface fatigue was used.

Keywords: PVD coatings, fatigue, acoustic emission, impact wear

1 INTRODUCTION During the past ten years mechanical applications of PVD (Physical Vapour Deposition) coatings have become numerous. If these films are now widely used for tool parts, they also exhibit promising properties (abrasion resistance, hardness, high Young's modulus) for other sectors like cold forming or automotive industry. In these cases there are still some difficulties to use them in replacement of traditional surface treatments. There is indeed a great difference between a superficially modified material (with a treatment depth around 100 - 500 µm) and a very hard film which thickness does not exceed 5 µm. Besides mechanical parts that could be treated by such methods are usually submitted to surface fatigue solicitations that require to be reproduced by specific tests.

Several experimental techniques were used. In addition to repetitive impact test, the friction behaviour of the coatings was tested with a pin on disc tribometer equipped with AE detection. To optimise the interpretation of AE signals, scratch testing with AE detection was also performed. This mechanical test is very short so it is a good way to choose the right operating parameters to detect AE signals (they radically differ from one coating to another). When examining the scratch a correspondence may be easily established between several damage mechanisms (cracking, spalling, coating delamination) and the characteristics of resulting AE signals. 2 EXPERIMENTAL TECHNIQUES

2.1 Scratch testing Scratch test were performed on a CSEM RevetestTM apparatus. The applied load varied from 0 to 70 N with

a loading range of 100 N/min and a translation speed of 10 mm/min.

The acoustic emission (AE) activity was monitored by a transducer mounted on the arm of the apparatus. AE were counted during 1 s every 2 s. This corresponds to an increase of the load of 5 N from one measure to another. The installation is represented in figure 1. AE activity was found to generally occur slightly be-fore critical load corresponding to spallation of the coatin [1]. This can be explained by the appearance of cracks beneath the surface of the coating. This phenomenon can been revealed by the use of acoustic microscopy [2, 3]

2.2 Friction tests Tests were performed on a ball on disc tribometer operated by computer. The test rig is represented in figure 1. A 3 mm diameter carbide ball (the same material as the stylus of surface fatigue test) was used for the tests. Rotational speed was set at a value of 60 rpm for all tests. The corresponding sliding speed is comparable to the vertical speed of the stylus of surface fatigue apparatus. A 1 N load was applied. During testing, the tangential force was constantly measured and recorded via computer. From these records, the evolution of maximum, minimum and average values for the coefficient of friction per cycle were drawn. As long as damage to the coating remains restricted to cracking or localised spallation, there is very little to observe in the friction curve. However, these events may be detected by use of acoustic emission (AE) [4]. Some operating precautions must however be observed according to the work of Baranov et al [5]. Among them the roughness has to be the same for all sample to avoid amplitude scattering of the signals. The acoustic emission transducer was set on the opposite end of the indenter. The system was isolated from the metallic arm

of the tribometer by a PTFE (Polytetrafluoroethylene) ring. Number of counts and RMS value were plotted during tests. After the test, the wear of the ball is observed. Generally there is no noticeable wear (which means that AE signals are related to degradation of the coating on the disk). in some other cases the mechanism of degradation of the ball is fine abrasion. It results in continuous AE emission that can be separated from sudden damage appearance.

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2.3 Surface fatigue test A test with pin on disc configuration is suited for testing a solicitation including sliding. Other applications like gear teeth, forming tools or moulds encounter more shocks during service life. Therefore a specific test bench performing repetitive impacts at the same place was used. The principle of the test is described in figure 2. A stylus the displacement of which is controlled with an electromagnetic system, hits the surface at given places. Translation of the stylus body can be programmed in both horizontal directions which enables to hit samples at given places and allows spacing control of the impacts. It is also possible to perform several impacts at the same place. In the present work we performed lines of successive marks. Each mark is the result of N successive impacts (N varying from 1 to 500). The space between two

neighbouring impacts was chosen large enough to avoid possible interactions between cracks networks. The impact force can be varied in a wide range and the contact pressure may be adjusted by modifying the contact geometry. This test was found to be representative of a large number of solicitations encountered by hard coatings (erosion [6], cutting tools [7]). This impact test also reveals structural transformations that may happen under mechanical efforts [8, 9] (such transformations are now well investigated in fretting). These phenomena are more likely to happen in non stoechiometric films which structure is metastable.

Tests were performed with a 55 N impact force. If the coating exhibited no damage, the force was increased to 115 N. The stylus printed lines with respectively 1, 5, 10, 20, 50, 100, 200 cycles. The diameter of the stylus tip was 1 mm.

StylusThe stylus hits the film N1 times at each place

Line with N0 cyclesLine with N1 cycles

Sample

Figure 2: Principle of surface fatigue test 3 SAMPLES TESTED Coatings were deposited on X85WmoCrV 06 05 04 02 tool steel substrate with a hardness of 63 HRC. Tin and TiBN films were elaborated by PVD process (DC magnetron sputtering) in a TSD800 unit. Prior to film deposition the substrate was mirror polished to an average roughness Ra of 0.02 µm.

The main characteristics of samples tested are summarised in table 1. Film thickness was measured by ball cratering method. Hardness and modulus were determined using a Fischerscope H100 apparatus.

Sample Nature of the film Thickness (µm)

Young’s Modulus (GPa)

Hardness (HV 50 mN)

Critical Load (N)

1 TiN 2.5 320 2500 52 2 TiN 3 320 2450 46 3 TiBN 4 390 3400 24.5

Table 1: Characteristics of the coatings

4 EXPERIMENTAL RESULTS

4.1 Scratch testing For samples 1 and 3, two scratch tests were performed with AE recording. For each film, observed degradations were similar from one test to another. On TiN coating (nr1) the following degradations occurred: fine spalling in the middle of the track (25 N/26 N), semi circular cracks in the track (55 N/50 N), removal of the coating (65 N/60 N). TiBN coating (sample nr3) exhibited following degradations : numerous cracks in the track (19.5 N/17 N). cohesive spalling along

the track (24 N/20 N), coating removal in the track (42 N/42 N).

The curves of AE records on TiN and TiBN films are given respectively on figure 3 and 4 with optical photographs of the scratch at load values corresponding to characteristic types of damage. Observation of the curves shows that the nature of acoustic activity varies with the nature of damage. AE occurs before cracks or flakes appear on the surface of the films. Coating removal generates more energetic signals than small flakes or fine cracking. On both type of coatings, we can notice a peak on the counts curve (the same trend is also

visible on the RMS curve when the phenomenon is energetic enough) just before a given type of damage appears. Then there is a decrease of the curve. This can be explained by the accumulated deformation energy stored in the coating that is suddenly released when a new type of damage occurs.

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22 and 27 N (b) between 38 and 43 N (width of the photos: 500 µm)

4.2 Friction tests Friction curves of tests performed on TiN and TiBN coatings are respectively plotted on figures 5 and 6. the observation of the wear tracks on TiN samples after the tests shows only oxidation of the TiN film and partial transfer of the tungsten carbide from the ball. This was confirmed by an EDX analysis on the centre of the wear track. At the beginning of the friction curve, there is some scattering in the extreme values of the coefficient of friction within a cycle. This phenomenon corresponds to an irregular flow of debris in the contact (removal of material from the ball does not conduct to a stable three

bodies sliding regime). After 2000 cycles, the friction coefficient becomes more stable but increases to 0.65. the observation of both the ball and the coated disk after the tests exhibit fine wear debris.

The TiBN coating exhibits a lower friction coefficient. The stable friction regime is reached earlier than o TiN, after only 500 cycles.

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Figure 5: Friction test on TiN coating with corresponding AE activity. Photographs (a) of the ball(width 650 µm) (b) of the track before cleaning

showing debris on the edges (c) after alcohol cleaning. (width of photos b and c 325 µm)

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Figure 6: Friction test on TiBN coating with corresponding AE activity Photographs (a) of the ball after the test (width 650 µm) (b) of the track after the

test (width of the photo 650 µm)

4.3 Surface fatigue test The fact that the apparatus has made lines of several impacts with a given number of cycles conveys a statistical value to the observations.

The behaviour of TiN and TiBN films were rather different. Cohesive flakes appeared on the edges on the marks on both TiN films (figure 7a, 7b). On the thicker one (nr2), flakes appeared at the lowest impact force after only 5 cycles. After 200 cycles (figure 7a), the substrate is clearly visible. This emphasises that increasing the thickness of the film does not always lead to better mechanical resistance. This can be explained by an evolution of the structure of the layer during the growth process. This phenomenon has already been observed for other applications of PVD coatings [10, 11].

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Figure 7: Observation of surface fatigue test. The width of photographs a, b, c is 650 µm. (a) TiN film nr2,force 55 N, 100cycles (b) TiN film nr1, force 115 N 50 cycles (c) TiBN film force 55 N 200 cycles (d) TiBN film force

115 N 200 cycles (SEM observation)

No flake was seen on TiBN films even with an impact force of 115 N. Observation of the marks in Scanning Electron Microscope (SEM) did not reveal cracks on the edges (figures 7c, 7d). 5 CONCLUSIONS This study has revealed different behaviours between TiN and TiBN coatings. Observations of scratch tests showed that none of the films had adherence problems. Comparison between scratches conveys the feeling that TiBN is more fragile because cracks and flakes appear at lower load values than on TiN films. (moreover the critical load is lower for TiBN than for TiN) The opposite trend can be observed after surface fatigue tests. TiBN resisted far better than TiN to repetitive impact. This emphasises that scratch testing is not

sufficient to characterise the mechanical behaviour of thin films.

A real mechanical part will never encounter a solicitation as severe as on scratch test, where the coating is submitted to an important deformation. For example a forming tool will undergo repetitive compression at its extremity like on surface fatigue test. The flanks will undergo shearing stress which severity directly depends on the coefficient of friction between the coating and the material. This can be evaluated by friction test.

The combination of friction tests and surface fatigue tests are more favourable to TiBN coatings for tool applications whereas the only observation of scratch test would conduct to prefer TiN. 6 AKNOWLEDGEMENTS The authors would like to thank Sara Maiez for technical help for performing surface fatigue tests. 7 REFERENCES [1] J. Sekler, P. A. Steinmann, H. E. Hintermann, Surface and Coatings Technology 36 (1988) 519-529. [2] Robert L. , Brunet N. , Flaherty T. , Randles T. , Matthaei-Schulz E. , Vetters H. , Rats D. , Von Stebut J Surface & coatings technology, 116-19 (1999), 327-334. [3] Von Stebut J. , Lapostolle F. , Bucsa M. , Vallen H Surface & coatings technology, 116-19, 160-171. [4] A. A. Pollock, Acoustic emission inspection, in Metal Handbook 9th edition, ASM international (1989) vol 17, p278-294. [5] V. M. Baranov, E. M. Kudryavstev, G. A. Sarychev, Wear 202 (1997)125-133. [6] Y. Gachon, A. B.Vannes, M. C. Sainte Catherine, I. Caron, G. Inglebert, Study of sand particle erosion of magnetron sputtered multilayer coatings, Wear 233-235 (1999),263-274. [7] Bouzakis K.-D. , Koutoupas G. , Siganos A. , Leyendecker T. , Erkens G. , Papapanagiotou A. , Nikolakakis P, Surface & Coatings Technology 133-34 (2000) 548-554 [8] D. Kaczorowski, Y. Gachon, A. B. Vannes, D. Hertz Characterisation of surface treatment with a controlled energy impact system, Proceedings of Euromat 2000 Conference (Tours, France). [9] A. Sekkal, PhD Thesis, Ecole Centrale de Lyon (Ecully, France) nr 2000-31(2000). [10] Gachon Y. , Ienny P. , Forner A. , Farges G. , Sainte Catherine M. C. , Vannes A. B, Surface & Coatings Technology 113 (1999), 140-148. [11] Harry E. , Ignat M. , Pauleau Y. , Rouzaud A. , Juliet P. Surface & Coatings Technology, 125 (2000) 185-189.