Powder Metallurgy Progress, Vol.11 (2011), No 1-2 165

THE USE OF INDENTATION TESTS FOR EVALUATION OF THIN PVD COATINGS OF (Ti,Al) N TYPE

D. Jakubéczyová, P. Hvizdoš, M. Hagarová, M. Kočík

Abstract The work deals with the evaluation of thin PVD layer type (Ti, Al) N, applied to a steel produced by powder metallurgy methods, by indentation - scratch test and nanohardness. The adhesive properties of the applied coatings were studied by the scratch test, to be the basis on which their quality, in terms of possible applications, was to be assessed. Nanohardness was tested by the Berkovich indenter during controlled loading to the limit value and the subsequent unloading down to zero. From the indentation curves determined were hardness and elastic modulus of layers and of the base material, depending on the depth of penetration. Microscopic examination was used as a complementary evaluation of the tested locations. Analysis of the employed coating systems indicates their possible applications on selected type of powder metallurgy steel and a possibility of increase in the lifetime in working conditions. Keywords: layer, scratch test, adhesion, nanohardness, elastic modulus

INTRODUCTION In the past decade was recorded a considerable progress in the field of technology

of depositing thin coatings on metal machine parts and tools for the purpose of extending their service life under operation conditions. The development advanced from depositing mono- to multi-layer coatings, based on various metal and non-metal elements, combined according to their individual properties and resulted in consistent improvement of surface properties of materials. Coatings of the (Ti, Al) N type exhibit, not only excellent resistance to oxidation and high temperatures in comparison with TiN coatings, but are associated also with superb behaviour of coated high-speed cutting tools and interesting tribological properties [1,2,3,4]. However, there are physical limits to production of hard cubic structure of AlN, and thus the systems with ratios Ti : Al ≤ 35 : 65, suitable for a narrower field of applications, became less stabile [5]. Detailed knowledge of surface properties is often the limiting factor for selection of reliable application of materials for particular processes of wear. The high demands on production of protective PVD coatings require adequate methods for evaluation of quality. Development of new types of protective coatings places high demands on evaluation of their mechanical properties and the indentation methods are among those serving this purpose. Although technical capabilities of modern diagnostic instruments underwent substantial improvement concerning their reliability and sensitivity of measurement, different approaches have been used

Dagmar Jakubéczyová, Pavol Hvizdoš, Marek Kočík, Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovak Republic

Mária Hagarová, Department of Materials Science, Faculty of Metallurgy, Technical University of Košice, Košice, Slovak Republic

Powder Metallurgy Progress, Vol.11 (2011), No 1-2 166 for evaluation of physical-mechanical, tribological and other properties of the system thin coating/substrate [6].

The present study used indentation tests of the type of scratch test and nanoindentation with dynamic loading. The scratch test is a practical method providing numerical value corresponding to adhesion of hard PVD coatings. Usually the normal load is maintained constant or increases progressively along the scratch-track. Progressively increasing load reaches finally the critical level which results in damage to the coating, development of cracks (in case of brittle coatings) or delamination of the coating [7]. This test determines the critical load Ls which causes exposure or even detachment of the coating from the substrate and indicates the degree of adhesion of the respective coating. With regard to the common degree of adhesion, the load force is in the range of 20 - 120 N [8,9,10,11]. Because of increasing requirements of evaluation of material surfaces and coatings with decreasing thickness, it appears necessary to switch from evaluation by means of conventional microhardness to lower loads that eliminate the influence of the substrate on measurements. In that case, due to small dimensions and thus unreadability of the impression, it is necessary to evaluate the indentation curve constructed from the nanoindenter record. Nanoindenters allow one to perform measurements at very low loads, i.e. several tenths of mN, and provide the flexible technique for characterisation of thin coatings’ microhardness. The instrument records and analyses response of the material to the respective way of loading and the corresponding record enables one to calculate, not only microhardness, but also the ratio of elastic and plastic deformation in the course of the loading cycle and additional characteristics of the material system [12,13]. In the process of evaluation one should not underrate the surface roughness which is closely related to the concentration and size of macro- and microparticles that are an inseparable part of the coating deposited by cathode arc evaporation technologies in the vacuum [14,15]. Surface roughness affects the size of the contact area at the onset of indentation when the indenter is not in contact with the surface of the entire presumed area. Surface roughness also affects determination of the zero-point of the indenter. The respective standard [16] specifies condition for the depth of the impression as h > 20 Ra (Ra – mean arithmetic profile deviation).

EXPERIMENTAL PROGRAMME Ledeburitic tool steel Böhler K190 Microclean, produced by powder metallurgy

(PM) served as a substrate for depositing the coatings. It has the highly homogenous, fine-grained structure and is suitable for depositing hard coatings by PVD and PE CVD processes. Before depositing the coatings, the specimens were prepared by common metallographic methods and polished to roughness Ra ∼ 5 nm. Onto this way prepared surface we deposited TiN and AlTiN coatings of thickness max. 3000 nm by PVD (cathode arc evaporation) technology. The roughness of Ra coatings analysed by AFM reached 33 – 46 nm. The specimens were evaluated by the scratch test with incremental (1 N.mm-1) loading within the range of 0 - 80 N. With increasing load L the character of the scratch-track changes and according to the quality of the coating 4 principal locations are observed: the first failure of the coating (development of cracks) = Lc1; the first adhesion failure (smaller spalling) = Lc2; the first adhesion failure of the bigger extent = Lc3 and complete exposure of the substrate = Ls. Adhesion value bigger than 60 N ensures that no detachment or spalling will occur for common applications [17]. The track generated by the tip was evaluated by light microscopy to determine locations of the mentioned respective failures.

Powder Metallurgy Progress, Vol.11 (2011), No 1-2 167

Nanohardness of coating TiN and AlTiN was measured on the substrate K190 onto which these coatings were deposited. The tests were carried out employing in instrument TTX-NHT (CSM Instruments, Switzerland). We used a Berkovich indenter (three-sided pyramid) operated in dynamic sinusoidal mode with maximum load of 400 mN and loading rate 800 mN.min-1. The results obtained were processed by the method of Oliver and Pharr [18] which provided values of indentation hardness (H) and Young’s modulus of elasticity (E). The dynamic mode enabled us to measure these values continuously during loading and record them as a function of the relevant penetration depth. This method is suitable for characterisation of coatings and thin layers as it allows one, under certain conditions, to separate the values corresponding to layers and the substrate. Each specimen was subjected to minimally 20 indentations and the results were processed statistically.

RESULTS AND DISCUSSION Results of both tests carried out on relevant specimens were evaluated,

documented by light microscopy and processed graphically. The critical forces at respective failure locations are presented in Table 1.

Tab.1 Values of critical forces of systems coating/substrate

Specimen Lc1 [N] Lc2 [N] Lc3 [N] Ls [N] TiN / K190 37.5 76 - -

AlTiN / K190 22.8 58 64 75 The specimen TiN/K190 showed excellent adhesion (Fig.1) with only fine

cohesion cracks at scratch-track edges at the load of Lc1 = 37.5 N (Fig. 2a) which did not affect integrity of the coating. The load of Lc2 = 76 N, Fig. 2b, resulted in small detachment of the coating. Neither failure to a bigger extent nor complete exposure of the substrate occurred within the predetermined range of normal load.

Fig.1. Total view of the scratch-track of the system TiN / K190.

a) Lc1 = 37.5 N b) Lc2 = 76 N

Fig.2. Details of failure of the system TiN / K190 at a) Lc1 and b) Lc2.

Powder Metallurgy Progress, Vol.11 (2011), No 1-2 168

Fig. 3 shows morphology of the scratch-track for the specimen AlTiN / K190. Application of the load Lc2 = 58 N caused the first adhesion failure – spalling of the upper AlTiN layer and exposure of the second TiN layer and the load of Ls = 75 N resulted in traces of exposed substrate. Character of this scratch-track was affected by the bigger proportion of macroparticles on the surface (see Fig.5c). Generally both layers showed good adhesion as the substrate was not exposed or its exposure occurred only at loads exceeding 60 N.

Fig.3. Total view of the scratch-track of the system AlTiN / K190.

a) Lc2 = 58 N b) Ls = 75 N

Fig.4. Details of failure of the system AlTiN / K190 at a) Lc2 and b) Ls.

Figure 5 shows nanoindentation impressions on the surface of K190 and in the layers TiN and AlTiN. The impressions are regular without marked failures or deformations and their dimensions differ, which is related to the differences in hardness. There is a considerable number of Al-based macroparticles on the surface of the AlTiN / K190 system as a result of the deposition process.

a) b) c)

Fig.5. Nanoindentation impressions a) on substrate K190, b) in TiN layer, c) in AlTiN layer.

Relationship between hardness and indentation depth for the three experimental systems is shown in Fig.6. This figure clearly reflects the effect of deposited hard coatings in comparison with behaviour of the base substrate. The initial values in the figure are

Powder Metallurgy Progress, Vol.11 (2011), No 1-2 169 related to the contact of the indenter with the substrate and stabilisation of its zero position. After stabilisation of the hardness value in the case of coated specimens, they correspond to the values of deposited coatings. These values are stable to the depth of approximately 300 nm, which corresponds to about 1/10 of the total coating thickness. This is in agreement with the international standard for determination of coating's hardness [19]. After exceeding this depth, we can observe manifestations of the influence of the substrate because the stress field of indentation penetrates to approximately 10-fold depth of the real depth of indenter penetration [20] and thus starts to penetrate into the substrate. According to Fig. 6, after exceeding this depth, the hardness decreases uniformly with increasing indenter load for both types of layers; in the layer AlTiN from 30 to 21 GPa and in the layer TiN from 28 to 16.5 GPa. This is a zone in which the influence of substrate with lower hardness gradually increases. Contrary to that, the course of hardness of the substrate material was uniform without fluctuations, which is indicative of high quality of the surface before deposition and the hardness reached approximately 12.5 GPa. Deposition of AlTiN resulted in higher hardness compared to TiN layer, which was attributed to the existence of an AlN component. This was indicated by the character of the scratch-track of the respective system shown in Fig.4.

0

5

10

15

20

25

30

35

40Average K190-AlTiN

Average K190-TiN

0 500 1000 1500

Displacement Into Surface [nm]

Har

dnes

s H

V [G

Pa]

Average K190

Fig.6. Relationship between hardness and indenter impression depth for dynamic loadi .

Besides determination of the hardness, one can use the release sections of indentati

ng

on curves to calculate the modulus of elasticity E, because in these sections the applied load is related to the elastic response of the material. In this way one can map elasticity of individual components of material systems at the micrometric level. Relationship between the modulus of elasticity of the experimental materials and the indenter impression depth for dynamic loadings is illustrated by curves in Fig.7. Their course was related to hardness, but the relationship between E and the total response was more complicated and was affected presumably by the substrate material. According to the figure, the value of E in the substrate was according to expectations constant within the measurement error and corresponded to the value of the steel used (230 - 250 GPa) [21]. In the coated materials it decreased with increasing load and was almost parallel for both

Powder Metallurgy Progress, Vol.11 (2011), No 1-2 170 layers. In this case, according to the standard [19], the E value of coatings is determined by extrapolation down to zero depth which in our case corresponded to values ~ 450 GPa for TiN and ~ 420 GPa for AlTiN, which complied with information presented in references [22]. Figure 7 also shows that for a depth bigger than approximately 1500 nm, the value of E was almost the same for all three materials, i.e. for depths bigger than this, the influence of elastic response of coatings on load is negligible.

0

50

100

150

200

250

300

350

400

450

0 500 1000 1500

Displacement Into Surface [nm]

Inde

ntat

ion

Mod

ulus

of E

last

icity

E [G

Pa]

Average K190-TiNAverage K190-AlTiNAverage K190

Fig.7. Relationship between the modulus of elasticity and indenter impression depth fo

CONCLUSIONS sents analytical methods that enabled us to determine in greater

detail th

d nanohard

the following conclusi

of hardness of the substrate material was uniform without fluctuations which

• lTiN layer resulted in higher hardness compared to that reached by the TiN layer which is attributed to the presence of an AlN component;

rdynamic loading.

The paper pree properties of deposited coatings. When studying the coating/substrate systems we

focused on surface roughness and hardness, adhesion properties, nanoindentation at specific loads and evaluation of elastic modulus from indentation curves. Machine steel K190, produced by powder metallurgy (PM), was used as a substrate for depositing TiN and AlTiN layers. AFM method was used to determine roughness of the substrate which reached Ra ∼ 5 nm. After depositing the TiN and AlTiN layers of thickness max. 3000 nm by PVD technology the roughness (Ra), analysed by AFM, reached 33 nm on layer TiN and 46 nm on layer AlTiN. The higher roughness is caused by the existence of nanoparticles.

The specimens were tested by indentation methods - scratch test anness. The character of scratch-tracks indicated good adhesion of both layers as the

substrate was not exposed or exposure occurred at loads exceeding 60 N. The nanoindentation evaluation of hardness allowed us to draw

ons: • The course

was indicative of high quality of surface preparation before deposition. The hardness reached 12.5 GPa; Deposition of the A

Powder Metallurgy Progress, Vol.11 (2011), No 1-2 171

• In addition to hardness, the release sections of indentation curves served for the determination of the dependence of the modulus of elasticity E on the indenter

• ed to values ~ 450 GPa for TiN and ~ 420 GPa for AlTiN;

co h have excellent adhesion to the selected substrate. The depositio

Authors are grateful for the support of experimental works by national projects VEGA 2/0120/10.

[1] Nakonechna, O., Cselle, T., Morstein, M., Karimi, A.: Thin Solid Films, vol. 447–448,

[3] .: Wear, vol. 259, 2005, p. 1168 d coatings

onference. Atlanta, November 2002

ol. 4, March 2007 (in Czech)

[7] . 214 [8] hesive

hesis. ZCU, 2007

[10] o. 3 [ Innovation transfer), vol. 15, 2009, p. 64 (in

[13] M., Štěpánek, I. In: Metal 2005 [online] 24.-26.5.2005 [cit. 2010-04-30] (in

ěm, ČR, CD ROM l. 45,

impression depth for dynamic loading. Their course was related to hardness, but the relationship between E and the total response was more complex and was probably affected by the substrate material. The value of E of the substrate was according to expectations constant within the measurement error and corresponded to the value of the steel used (230 - 250 GPa); The value of E of the coatings was determined by extrapolation down to zero depth which in our case correspond

• For depths larger than approximately 1500 nm, the value of E was about the same for all three materials, i.e. for depths bigger than this, the influence of elastic response ofcoatings on load is negligible.

We can conclude that steels prepared by powder metallurgy are suitable for ating by thin hard layers whic

n resulted in increase of hardness in comparison with uncoated substrate. This fact allows us to assume that the mentioned coatings will ensure increase of lifetime of the studied steel in practical use, as stated in work [23].

Acknowledgement

VEGA 2/0060/11 and

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