TiAlN multilayer coatings

Surface & Coatings Technology 206 (2011) 1886–1892

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Influence of bilayer period and thickness ratio on the mechanical and tribological properties of CrSiN/TiAlN multilayer coatings Meng-Ko Wu a, Jyh-Wei Lee b, c,⁎, Yu-Chen Chan d, Hsien-Wei Chen d, Jenq-Gong Duh d a

Department of Mechanical Engineering, Tungnan University, Taipei, Taiwan Department of Materials Engineering, Ming Chi University of Technology, Taipei, Taiwan Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taipei, Taiwan d Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan b c

a r t i c l e

i n f o

Available online 17 August 2011 Keywords: Pulsed DC reactive magnetron sputtering CrSiN/TiAlN multilayer coating Bilayer period Fracture toughness

a b s t r a c t Nanostructured CrSiN/TiAlN multilayer coatings were deposited by a bipolar asymmetric reactive pulsed DC magnetron sputtering system. The thickness ratio of CrSiN to TiAlN layers was fixed at 1:1. The bilayer periods of the coatings were controlled to be from 6 to 40 nm. Furthermore, two CrSiN/TiAlN multilayer coatings with the same bilayer period (20 nm) but different CrSiN/TiAlN thickness ratios (2:8 and 8:2) were also deposited to explore the influence of thickness ratio on the mechanical properties of the multilayer coatings. The crystalline structures of the coatings were determined by a glancing angle X-ray diffractometer. The microstructures of thin films were examined by a scanning electron microscopy and a transmission electron microscopy, respectively. A nanoindenter, a micro Vickers hardness tester, and a pin-on-disk wear tester were used to evaluate the hardness, the toughness and the tribological properties of the thin films, respectively. The maximum hardness of the multilayers was obtained when the bilayer period was at 10 nm for the coating with the same thickness ratio of CrSiN to TiAlN layers (1:1). Meanwhile, the thickness ratio of CrSiN to TiAlN layer had great influence on the hardness and the toughness properties of the multilayer coatings. The hardness and the toughness of the CrSiN/TiAlN multilayer coatings increased as the individual TiAlN layer thickness increased. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Hard coatings are widely used to enhance the performance and the lifetime of cutting tools in industrial applications. High hardness, good wear resistance, low friction, chemical inertness and high temperature stability are required for these coatings. Consequently, the structure designs of nanocomposite and nanoscale multilayered thin films have been developed to fulfill the specific demands [1–6]. Pronounced strength enhancement, optimal hardness/toughness ratios and excellent wear resistance can be obtained through a proper critical bilayer thickness design for nanoscale multilayered coatings [2–4]. Among the transition metal nitride coating systems, the multilayered TiAlN/CrN coatings have drawn lots of attentions due to their high hardness, good oxidation resistance and successful protection coating applications [7–9]. In the previous work, a series of TiAlN/CrSiN multilayered thin films with various bilayer periods, Λ, ranging from 5 to 40 nm were prepared by a bipolar asymmetric reactive pulsed DC magnetron sputtering system [10]. The thickness ratio of CrSiN to TiAlN layer ⁎ Corresponding author at: Department of Materials Engineering, Ming Chi University of Technology, Taipei, Taiwan. Tel.: + 886 2 29089899x4437; fax: + 886 2 29084901. E-mail address: jeffl[email protected] (J.-W. Lee). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.07.045

(‘CrSiN :‘TiAlN ) of each CrSiN/TiAlN multilayered thin film was fixed at 2:8. It was reported that an average hardness of 35 GPa, elastic modulus of 334 GPa and low friction coefficient of 0.43 have been achieved for the multilayered coating at 12 nm bilayer period. However, the dependence of relative layer thickness, i.e. the thickness ratio of CrSiN to TiAlN layer, (‘CrSiN :‘TiAlN ), on the mechanical and toughness properties of TiAlN/CrSiN multilayered coatings has not yet been studied in the previous work [10]. In this study, CrSiN/TiAlN multilayered thin films with different Λ values and different relative thickness ratios of CrSiN to TiAlN, (‘CrSiN :‘TiAlN ), were synthesized by a reactive pulsed DC magnetron sputtering system. The effects of bilayer periods, Λ, and different ‘CrSiN :‘TiAlN ratios on the microstructures, mechanical, toughness and tribological properties of coatings are discussed in this paper. The possible strengthening mechanism due to the thickness difference between the high and low modulus TiAlN and CrSiN layers is also investigated. Suitable microstructure design to an optimal mechanical properties and tribological performance is also proposed. 2. Experimental procedure Two series of CrSiN/TiAlN multilayered thin films were deposited on p-type (100) silicon wafer substrates by a bipolar asymmetry

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Table 1 Typical deposition conditions, sample designation, calculated bilayer period and surface roughness for L series CrSiN/TiAlN multilayered coatings. Sample designation

L6

Designed bilayer period (nm) Plasma holding time (s) TiAl target power (W) CrSi target power (W) Base pressure (Pa) Plasma etching Interlayer Working pressure (Pa) Ar:N2 ratio Substrate heating Pulsed substrate bias Calculated bilayer period (nm) Surface roughness (nm)

6 8 10 20 26 33 200 100 1.6 × 10–3 Ar plasma for 10 min at 1.2 Pa under substrate bias −500 V 100 nm thick TiAl interlayer 4.0 × 10–1 1:1 250 °C −100 V under 2 kHz pulsed frequency 5.74 7.75 10.85 2.53 ± 0.01 1.99 ± 0.14 1.37 ± 0.14

L8

L10

pulsed DC reactive magnetron sputtering system. The first series (denoted as L series) included seven multilayered coatings with the same thickness ratios of CrSiN:TiAlN layers (‘CrSiN :‘TiAlN ) = 1:1 and different bilayer periods. The second series (denoted as T series) were two multilayered coatings fabricated with the same Λ value, 20 nm and different ‘CrSiN :‘TiAlN ratios, which were 2:8 and 8:2, respectively. Detailed description of the sputtering method has been reported elsewhere [10]. In this work, the 90Cr–10Si and 64Ti–36Al (both in wt.%) alloy targets were in opposite positions and substrates were mounted on two sides of a rotating barrel between two targets. Multilayers were deposited by alternately rotating the substrates between the plasma of 90Cr–10Si and 64Ti–36Al targets. Various bilayer periods were achieved by controlling the holding time of substrates in the plasma stream from 90Cr–10Si or 64Ti–36Al target. The deposition time of each coating was controlled to achieve a fixed thickness around 1 μm. The starting nitride layer and the uppermost layer were TiAlN and CrSiN coatings, respectively, for all specimens. Sample designation and typical deposition conditions for the L and T series coatings are listed in Tables 1 and 2, respectively. Chemical compositions of coatings were analyzed with a field emission electron probe microanalyzer (FE-EPMA, JXA-8500F, JEOL, Japan) with a ZAF-corrected program. The surface morphology and surface roughness of each coating were investigated by an AFM (DI 3100, Bruker, USA). The cross-sectional morphologies of coatings were examined with a field emission FE-SEM (JSM-6701F, JEOL, Japan) and TEM (JSM-2010, JEOL, Japan). A glancing angle X-ray diffractometer (XRD-6000, Shimadzu, Japan) with an incidence angle of 2° was utilized to study the crystal structure of the coatings. The nanohardness and the elastic modulus of the multilayered thin films were investigated by means of a nanoindenter (TI-900, TriboIndenter, Hysitron, USA) using a Berkovich 142.3° diamond probe at a maximum applied load of 5 mN. A Vickers micro hardness

Table 2 Typical deposition conditions and sample designation for T series CrSiN/TiAlN multilayered coatings. Sample designation

T2

Designed bilayer period (nm) CrSiN:TiAlN thickness ratio TiAl target power (W) CrSi target power (W) Base pressure (Pa) Plasma etching

20 8:2 5:5(1:1) 2:8 50 200 200 101 100 25 1.6 ×∗ 10−3 Ar plasma for 10 min at 1.2 Pa under substrate bias −500 V 4.0 ×∗ 10−1 1:1 250 °C −100 V under 2 kHz pulsed frequency

Working pressure (Pa) Ar:N2 ratio Substrate heating Pulsed substrate bias

T5

T8

L12

L20

L30

L40

12 40

20 66

30 100

40 133

12.40 2.15 ± 0.05

19.38 1.74 ± 0.14

28.47 1.7 ± 0.05

41.08 2.56 ± 0.60

tester was used to further evaluate the fracture toughness, KIC, of coatings based on the following equation [11]:

KIC = δð

P

c

rffiffiffiffi E H

Þ 3=2

ð1Þ

where P is the applied indentation load and δ is an indenter geometry constant, equal to 4 N and 0.016, respectively, for a Vickers diamond pyramid indenter. E, H and c are elastic modulus, hardness and radial crack length of the coating, respectively. The radial crack length was evaluated using a SEM. A pin-on-disk wear method was used to investigate the wear resistance of the coatings. A cemented tungsten carbide (WC + 6 wt.% Co) ball, 5 mm in diameter was adopted as the stationary pin. A normal load of 1 N was applied. The sliding speed was 27.2 mm/s with a wear track diameter of 8 mm. The test temperature was 20 °C, and the relative humidity was kept at 60%. The wear time was 40 min for each test. 3. Results and discussion 3.1. Composition and microstructure of CrSiN/TiAlN multilayers The chemical compositions for all the coatings obtained by the FE-EPMA analysis are listed in Table 3. For L series coatings, very similar chemical composition can be found with the following mean values: 14.8% Al–12.7% Ti–19.8% Cr–3.1% Si–48.0% N–1.3% O (in at.%). The average atomic ratios of Al:Ti and Cr:Si of each coating are around 54:46 and 86:14, respectively, which are different from previous work, 55:45 and 75:25, possibly due to an oxygen contamination in this study [10]. On the other hand, the chemical compositions varied for T coatings due to different target power applied during sputtering. It is noticed that the sample L20 is the same as sample T5. Fig. 1(a) and (b) illustrate the glancing angle X-ray diffraction patterns of L and T series multilayer coatings. For L series coatings

Table 3 Chemical compositions for L and T series CrSiN/TiAlN multilayered coatings. Chemical composition (at.%) Sample designation

L6

L8

L10

L12

L20

L30

L40

T2

T8

Ti Al Cr Si N O Ar

12.5 14.7 19.4 2.9 48.6 1.4 0.3

12.4 14.4 19.5 3.1 49.2 1.2 0.3

12.8 15.1 19.5 3.2 47.7 1.3 0.4

12.8 15.0 20.1 3.3 47.3 1.2 0.2

12.7 14.9 20.0 3.2 48.0 1.1 0.2

12.7 14.6 20.4 3.2 47.3 1.4 0.3

12.6 15.0 20.0 3.1 47.7 1.2 0.4

3.4 4.3 37.6 5.4 46.1 1.7 1.6

20.0 24.4 2.2 0.8 52.0 0.3 0.4

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(a)

(b)

TiN(111) TiN(200)

TiN(220) CrN(220) CrN(311) TiN(311)

3.2. Mechanical properties evaluation of CrSiN/TiAlN multilayered thin films

Cr 2N(200)

Intensity(arb.units)

CrN(111)CrN(200)

=20nm(T2)

Through the AFM analysis, a typical fine granular structure was observed on the surface of each thin film. The average surface roughness values, Ra, of L series coatings are listed in Table 1. In general, the dependence of bilayer period on surface roughness is not obvious. The lower magnification cross-sectional SEM morphologies of L10 and L40 coatings are depicted in Fig. 2(a) and (b), respectively. On the other hand, the cross-sectional SEM morphologies of L12 and L30 coatings at higher magnification are shown in Fig. 3(a) and (b), respectively. Very compact and laminated microstructure can be found for each coating indicating a good multilayered structure control was achieved in this work. On the other hand, the cross-sectional micrographs of T2 and T8 coatings are also illustrated in Fig. 4(a) and (b), respectively. Obvious nanolayered feature and more clearly columnar structures can be found. Meanwhile, the TEM technique was adopted to further explore the detailed microstructures of multilayered coatings with less than 12 nm bilayer periods. The low and high magnification cross-sectional TEM micrographs of L6 coating are depicted in Fig. 5(a) and (b), respectively. Long columnar structures around several tens of nm in width can be found throughout the whole multilayer coating. Clear laminated and columnar structures are also observed for L6 coating. The gray color and dark color regions illustrated in Fig. 5(b) are the TiAlN and CrSiN layers (indicated by arrows), respectively. After observing the cross-sectional FE-SEM and TEM micrographs of each coating, the average bilayer period of L series coating was measured and listed in Table 1. It is found that the deviation between measured thickness and designed value is less than 10%.

Si

The hardness and elastic modulus of L series multilayered coating as a function of bilayer period is presented in Fig. 6(a). The hardness values of monolayer TiAlN and CrSiN coatings, both around 29 GPa,

=20nm(T5)

=20nm(T8)

20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 Diffraction Angle(2θ) Fig. 1. X-ray diffraction patterns of (a) L series and (b) T series CrSiN/TiAlN multilayered coatings.

(‘CrSiN :‘TiAlN = 1:1) with different Λ values, a preferred orientation of NaCl-type structure TiN (200) reflection, around 42.7°, can be found for each coating, which is different from the data revealed in previous work [10]. It is suggested that the starting nitride layer was TiAlN, which made the following CrSiN layer atoms grow on the lattice of TiAlN layer and became a superlattice structure due to their low lattice mismatch. In addition, the XRD peaks of Si substrate were also observed due to a high incidence angle, 2°, of X-ray used to penetrate the CrSiN/TiAlN multilayered coating. For T series coating (fixed Λ = 20 nm), different phase evolution can be found in Fig. 1(b). The preferred orientation changed from TiN(111) to TiN(200) and then shifted to CrN(200) as the thickness ratios of CrSiN:TiAlN (‘CrSiN :‘TiAlN ) changed from 2:8 to 1:1 and 8:2, respectively. Apparently, the CrN reflection intensities become stronger and more obvious as the thickness of CrSiN increases. On the other hand, a reflection, around 40.6°, corresponding to Cr2N (200) can be found for T2 coating. The formation of lamellae Cr2N phase in the TiAlN/CrNx multilayered coating was revealed by Panjan et al. [12] due to a rather high deposition rate and constant nitrogen partial pressure. It is believed that the similar situation can be found in this work.

Fig. 2. The cross-sectional FE-SEM images of (a) L10 (Λ = 10 nm) and (b) L40 (Λ = 40 nm), CrSiN/TiAlN multilayered coatings.

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Fig. 3. The cross-sectional FE-SEM images of (a) L12 (Λ = 12 nm) and (b) L30 (Λ= 30 nm) CrSiN/TiAlN multilayered coatings.

are also inserted. Apparently, the hardness values of multilayered coatings, except L6, are all higher than that of monolayer TiAlN or CrSiN. It is also found that the hardness of the multilayers reaches a

Fig. 4. The cross-sectional FE-SEM images of (a) T2 and (b) T8 coatings.

Fig. 5. The cross-sectional TEM micrographs of L6 (Λ = 6 nm) coating at (a) lower and (b) higher magnifications.

maximum value at Λ = 10 nm and levels off at Λ = 12–40 nm and, followed by a drastic decrease when the bilayer periods decreases to less than 10 nm. A maximum hardness, 31 GPa, was observed for the L10 coating with a 10 nm bilayer period, whereas a minimum hardness, 28 GPa, was found for the L6 coating. As compared with the hardness results reported in previous work, which was a serious of TiAlN/CrSiN multilayered coatings with ‘CrSiN :‘TiAlN = 8:2 [10], a decrease of maximum hardness around 4 GPa is observed in this study. It is suggested that the lower hardness enhancement is primary attributed to the same CrSiN to TiAlN thickness ratio (‘CrSiN :‘TiAlN = 1:1) in this work. The possible mechanism will be explained later. On the other hand, the elastic modulus of each multilayered coating is between the values of TiAlN and CrSiN coatings, which is also different from previous report [10] due to the same thickness ratio (‘CrSiN :‘TiAlN = 1:1) produced in this study. On the contrary, the maximum elastic modulus around 339 GPa is found for the coating at Λ = 40 nm and decreases with decreasing bilayer period and then reaches to a higher value at Λ = 6 nm. Since Leyland and Matthews [13] discovered that the H/E ratio is an important factor to describe the resistance of materials against elastic strain to failure, the H/E ratio of each coating is also plotted in Fig. 6. A different tendency of H/E ratio versus bilayer period is found. The H/E ratio of the multilayers reaches a maximum value, 0.101, at Λ = 8 nm and decreases as Λ value increases and followed by a drastic decrease when the bilayer periods decreases to less than 8 nm. The lowest value, 0.077, is found for the monolayer TiAlN. Again, the resistance against elastic strain to failure of each multilayered coating was higher than that of single layer TiAlN and was more or less equal to CrSiN coating. The fracture toughness versus the bilayer period of L series coatings is shown in Fig. 6(b). A little bit different tendency is observed as compared with other mechanical properties of multilayered coatings. In general, the fracture toughness increases with decreasing bilayer

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Knoop hardness of HK0.025 3500 was reported by Lewis et al. [7] when the thickness of the individual layers was equal. On the other hand, a hardness decrease was reported by Nordin and Larsson [16] for the TiN/CrN multilayered coatings when the layer thickness of CrN was higher than that of TiN. Similarly, Nodrin et al. [17] also concluded that the individual CrN layer thickness should be kept thin in order to obtain the good mechanical and tribological properties of the TiN/CrN multilayered coating. In this work, the strengthening was observed for the CrSiN/TiAlN multilayered coating with Λ = 20 nm as the individual CrSiN layer thickness, ‘CrSiN decreased, although the hardness of TiAlN or CrSiN monolayer is equal to 29 GPa. It can be explained based on the literature reported by Chu and Barnett [3]. The overall superlattice strength σtot of the CrSiN/TiAlN multilayered nitride thin film is given in the following forms [3]: σtot = σ0 +

0

σ0 = σTiAlN

     2αb cosθ 0 ‘ ‘ 0 GCrSiN ln CrSiN + GTiAliN ln TiAlN mΛ b cosθ b cosθ

    ‘TiAlN ‘ 0 + σCrSiN CrSiN Λ Λ

ð3Þ

HTiAlN H 0 0 ≈σTiAlN ≈ CrSiN ≈σCrSiN 3 3

Bilayer period=20nm 0.12 0.11 0.1 0.09 0.08 0.07

CrSiN TiAlN

400 TiAlN

360 320 280

CrSiN

40

240

36 32

TiAlN CrSiN

Elastic Modulus (GPa)

H/E

(a)

Hardness(GPa)

28 24 T2(8:2)

T5(5:5)

T8(2:8)

CrSiN:TiAlN thickness ratio

(b)

Bilayer period=20nm 0.12 0.11 0.1 0.09 0.08 36 0.07

CrSiN TiAlN 2.5 2 1.5 1 0.5 0

H/E

pffiffiffiffiffi periods, except the L12 coating. A maximum value, 1.7 MPa m, is obtained for L6 coating. On the other hand, the hardness, elastic modulus, H/E and fracture toughness for T series coatings are shown in Fig. 7. Apparently, the hardness, elastic modulus and H/E ratio of T series coating increase with increasing thickness of TiAlN layer. The maximum hardness of T series coating reaches to 35 GPa as the CrSiN:TiAlN thickness ratio is 2:8. On the other hand, the fracture toughness for T2 coating (CrSiN:TiAlN thickness ratio is 8:2) was not obtained because either no cracking was found under 4 N loads or film was broken under higher indentation load. Nevertheless, an increasing the coating toughness with respect to the increasing thickness ratio of TiAlN to CrSiN is also observed. It is well known that the strengthening effect of nanoscale multilayered nitride coatings can be found when the bilayer periods reach a given point and then level off with increasing bilayer period and decrease with decreasing bilayer periods [1–4]. Dislocation blockage by interfaces, Koehler theory [14] and the alternating-stress strengthening theory [15] are often used to explain the hardness enhancement of nanoscale multilayered coatings. According to the calculation in previous study [10], a maximum hardness enhancement of 2 GPa compared with monolayer TiAlN can be found in this work. It is noticeable that a higher value, around 5 GPa enhancement was found in previous work for the coating with Λ = 12 nm and the CrSiN:TiAlN thickness ratio was 2:8. Research work on the dependence of individual layer thickness on the hardness enhancement effect of TiAlN/CrN multilayered coating was limited and showed contradictory or conflicting results [2,3,7,16,17]. According to the research work by Chu and Barnett [3], varying the individual layer thicknesses had relatively little effect on the strength enhancement. However, a peak hardness around 45 GPa was reported for the single-crystal TiN/VN superlattices coating when the Λ = 6.5 nm and the TiN layer thickness (lTiN) and bilayer period ratio, lTiN/Λ, was 0.3 [2]. Further increase or decrease the lTiN/Λ ratio will decrease the hardness of multilayered coating. Meanwhile, for the TiAlN/CrN multilayered coating, the maximum

ð4Þ

where b is the magnitude of the dislocation Burgers vector, α is π/4 for screw dislocation and π(1 − ν) / 4 for edge dislocation, m = 0.3, θ is the angle between dislocation and interface normal, G0TiAliN and G0CrSiN are shear modulus of single TiAlN and CrSiN coatings, which can be calculated to 140 GPa and 125 GPa [10], respectively. A yield stress

Fracture toughness (MPa*m^0.5)

Fig. 6. The relationships among (a) the hardness, elastic modulus, H/E ratio and bilayer period and (b) fracture toughness, H/E ratio and bilayer period of L series CrSiN/TiAlN multilayered coatings.

ð2Þ

CrSiN

TiAlN

T2(8:2)

T5(5:5)

T8(2:8)

CrSiN:TiAlN thickness ratio Fig. 7. The relationships among (a) the hardness, elastic modulus, H/E ratio and bilayer period and (b) fracture toughness, H/E ratio and bilayer period of T series CrSiN/TiAlN multilayered coatings.

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enhancement of the CrSiN/TiAlN multilayered nitride thin film can be expressed below [3]:    0 0 σtot −σ0 = σtot −σ0 Þ=ðGTiAliN + GCrSiN Þcosθ

ð5Þ

Since the hardness values of TiAlN and CrSiN coatings are almost the same, 29 GPa, as shown in Eq. (4), the σ0 in Eq. (5) becomes a constant and the yield stress enhancement,σtot ⁎ − σ0⁎, increases as the value of σtot increases. Apparently, the value of σtot increases when the thickness of high modulus layer, TiAlN layer, ‘TiAlN in Eq. (2) increases and the thickness of low modulus layer, CrSiN layer, ‘CrSiN decreases as well. According to the toughness evaluation results shown in Figs. 6(b) and 7(b), the toughness of coatings can be enhanced effectively by the introducing of multilayered features, which act as a crack inhibitor to achieve high hardness, adequate fracture resistance and high toughness values simultaneously [18,19]. The plots of coefficient of friction (COF) versus the wear length of L series thin films are shown in Fig. 8. The average COF of each coating is also inserted in Fig. 8. A minimum COF around 0.49 ± 0.05 was found for the L10 coating due to its highest hardness and the lowest surface roughness. The wear track for the L10 coating was around 66 μm wide, which is also the narrowest one. In this work, no delamination or film crack was observed for each multilayered coating. The wear resistance of multilayer coatings was proposed by Martinez et al. [20] that the anti-wear performance will be enhanced by the progressively worn out of very thin coating layers. The better wear resistance of nanoscale multilayered coatings can be attributed to the numerous interfaces between the CrSiN and TiAlN layers. Since each interface becomes an obstacle to the crack propagation, the wear resistance is most evident for the smooth and hard multilayered coating at Λ = 10 nm. On the other hand, the rather high COF values N1 are found on the multilayered coatings with bilayer periods larger than 30 nm. Fig. 9 depicts the backscattered electron image (BEI) of wear scar morphology of L8 coating, which exhibited a COF, 0.63 ± 0.02. A narrow wear track, around 80 μm wide, was found on the L8 coating. Again, no crack or delamination was observed adjacent to the wear track. Some white color wear debris as indicated by arrows can be observed in Fig. 9, which is tungsten-rich oxide wear debris. Luo et al.

Fig. 9. The backscattered electron image (BEI) of wear scar of the L8 (Λ= 8 nm) coating after the pin-on-disk wear test.

[21] and Kao et al. [10] also revealed the transferred particles and oxides from the counterpart embedded on the wear track of the multilayered coatings. Similar results were also found on all multilayered coatings in this work. This phenomenon of cemented tungsten carbide (WC/Co) ball debris trapped in the wear track due to tribo-oxidation [22] was discussed in previous work [10]. In this work, through a proper multilayer structure design, an optimal hardness/toughness/tribological properties combination was achieved for the CrSiN/TiAlN multilayered coating at Λ = 10 nm with equal individual CrSiN/TiAlN layer thickness ratio (‘CrSiN :‘TiAlN = 1:1). However, a higher hardness was obtained for a CrSiN/TiAlN multilayered coating at Λ = 20 nm with ‘CrSiN :‘TiAlN = 2:8, which was primary caused by the thicker high modulus TiAlN layer. 4. Conclusions Seven CrSiN/TiAlN multilayered thin films with equal CrSiN/TiAlN layer thickness ratio and bilayer periods ranging from 6 to 40 nm, and two CrSiN/TiAlN multilayered thin films at Λ = 20 nm with different individual CrSiN/TiAlN layer thickness ratios were fabricated successfully by the bipolar asymmetric reactive pulsed DC magnetron sputtering system. For coatings with equal individual layer thickness ratio, the hardness reached a maximum at Λ = 10 nm, then leveled off with increasing bilayer period and decreased with decreasing bilayer periods. For the multilayered coating at fixed Λ = 20 nm, the hardness, toughness and H/E values increased with increasing thickness ratio of TiAlN/CrSiN. It is concluded that for the multilayered coating at 10 nm bilayer period and equal individual layer thickness ratio, a combination of excellent mechanical properties, tribological performance and adequate toughness including an average hardness of 31 GPa, elastic modulus of 320 GPa and low COF of 0.49 have been achieved in this work. Acknowledgment The authors gratefully acknowledge the financial support of the National Science Council, Taiwan through contracts No. NSC 99-2221E-131-043. Reference

Fig. 8. The friction coefficients of L series CrSiN/TiAlN multilayered coatings against a WC-Co ball as a function of wear length.

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