Si3N4-Cu nanocomposite coatings

Applied Surface Science 320 (2014) 689–698

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Effects of Si content on microstructure and mechanical properties of TiAlN/Si3 N4 -Cu nanocomposite coatings Changjie Feng a , Shuilian Hu a , Yuanfei Jiang a,∗ , Namei Wu a , Mingsheng Li b , Li Xin c , Shenglong Zhu c , Fuhui Wang c a

School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang, 330063, China Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang, 330013, China c State Key Laboratory for Corrosion and Protection, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang, 110016, China b

a r t i c l e

i n f o

Article history: Received 1 June 2014 Received in revised form 5 September 2014 Accepted 5 September 2014 Available online 16 September 2014 Keywords: TiAlN/Si3 N4 -Cu nanocomposite coatings Si content Microstructure Mechanical property Wear resistance

a b s t r a c t TiAlN/Si3 N4 -Cu nanocomposite coatings of various Si content (0–5.09 at.%) were deposited on AISI-304 stainless steel by DC reactive magnetron sputtering technique. The chemical composition, microstructure, mechanical and tribological properties of these coatings were systematically investigated by means of X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), nanoindentation tester, a home-made indentation system, a scratch tester and a wear tester. Results indicated that with increasing Si content in these coatings, a reduction of grain size and surface roughness, a transformation of the (1 1 1) preferred orientation was detected by XRD and FESEM. Furthermore the hardness of these coatings increase from 9.672 GPa to 18.628 GPa, and the elastic modulus reveal the rising trend that increase from 224.654 GPa to 251.933 GPa. However, the elastic modulus of TiAlN/Si3 N4 -Cu coating containing 3.39 at.% Si content dropped rapidly and changed to about 180.775 GPa. The H3 /E2 ratio is proportional to the film resistance to plastic deformation. The H3 /E2 ratio of the TiAlN/Si3 N4 -Cu coating containing 3.39 at.% Si content possess of the maximum of 0.11 GPa, and the indentation test indicate that few and fine cracks were observed from its indentation morphologies. The growth pattern of cracks is mainly bending growing. The present results show that the best toughness is obtained for TiAlN/Si3 N4 -Cu nanocomposite coating containing 3.39 at.% Si content. In addition, the TiAlN/Si3 N4 -Cu coating containing 3.39 at.% Si content also has good adhesion property and superior wear resistance, and the wear mechanism is mainly adhesion wear. © 2014 Published by Elsevier B.V.

1. Introduction In recent years, due to the high hardness, excellent tribological properties and high temperature oxidation resistance, Ti-Al-N coatings are widely used on cutting tools to improve their performance and service lifetime [1–4]. However, it is necessary that the properties of Ti-Al-N coatings should be further improved to satisfy the ever-growing demands of modern development in industrial applications. In the early days, many researchers had paid attention to two broad types of nanocomposite coatings. One had been put forward by Veprek and Reiprich [5], which consists of nano-crystalline nitride and super hard amorphous phase. Such as, TiN/Si3 N4 , TiAlN/Si3 N4 , etc. Veprek et al. had deposited TiN/Si3 N4 nanocomposite coating and found that the hardness

∗ Corresponding author. Tel.: +86 15807006864. E-mail address: [email protected] (Y. Jiang). http://dx.doi.org/10.1016/j.apsusc.2014.09.041 0169-4332/© 2014 Published by Elsevier B.V.

of the coating was up to about 80–105 GPa. Besides, SiO2 layer formed by the oxidation of these nanocomposite coatings have the functions of anti-attrition and anti-oxidation. The other one had been put forward by Misina et al. [6] and is generally researched as one potential of this new kind of nanocomposite coatings. Its formulation can be expressed as nc-nitride/soft-metal (soft-metal: Ni, Cu, Y, Ag). Such as, TiN/Cu [7], CrN/Cu [8], TiN/Ag [9], etc. Hyun et al. [10] had deposited Ti-Cu-N nanocomposite films with various copper content using a reactive arc ion plating and magnetron sputtering hybrid system. The results shown that Ti-Cu-N films containing 1.5 at.% copper exhibited maximum hardness of 45 GPa and relatively low friction coefficient of 0.3. The further increase of copper content in the film resulted in a sharp decrease of hardness. Copper is immiscible with TiN, and their nitrides are unstable. Thus, when these materials are reactively co-deposited, the coatings tend to form a nanocomposite structure with nanoparticle of copper embedded in the transition metal nitride matrix. Recently, adding both soft and hard materials into nitride is one of research direction of super-hard nitride coatings. Musil et al. [11]

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C. Feng et al. / Applied Surface Science 320 (2014) 689–698

(a) P

(b) Indenter

300M ultrafort steel

Superhard TiAlN/Si3N4 Coating on surface layer The annular area of the deformation of film

Weight

The coating sample Fig. 1. Indenter (a) and tester (b) for toughness measurement of coatings.

have deposited Al-Si-Cu-N films with a micro-hardness H = 59 GPa containing 7 at.% Cu and 6 at.% Si by an unbalanced DC magnetron sputtering. There is considerable difference on mechanical properties for both the Si3 N4 and the copper phase. Though the further incorporation of soft metal can lead to decreasing of hardness, the toughness of the coating can be significantly enhanced. As a consequence, if the synergistic effect between Si3 N4 and Cu can be controlled, the nanocomposite coatings will have both high hardness and high toughness at the same time. Realization of this nanocomposite coating will cause enormous response in the high-end computer numerical control machine tools and cutting technology. In this study, TiAlN/Si3 N4 -Cu nanocomposite coatings with different Si content were prepared by DC reactive magnetron sputtering. This study is aimed at investigating the influence of Si content on the microstructure, hardness, toughness, adhesion properties and wear resistant properties of the TiAlN/Si3 N4 -Cu nanocomposite coatings. 2. Experimental TiAlN/Si3 N4 -Cu nanocomposite coatings were prepared on polished Si (1 0 0) wafer and AISI-304 stainless steel plates with a dimension of 15 mm × 10 mm × 2 mm by an MS-3 DC reactive magnetron sputtering system. Ti0.5 Al0.5 , Ti0.5 Al0.4 Si0.1 alloy targets and pure Cu target with a diameter of 100 mm and a thickness of 10 mm were used for sputtering target materials. In order to obtain coatings containing of various Si content, Ti0.5 Al0.4 Si0.1 and Ti0.5 Al0.5 targets were cut into five equal parts by a WEDM machine. Every part has the same width. Then the incised alloy targets were selectively used in different combination ways to obtain different Si content of the sputtering targets. Prior to deposition, the stainless steel substrates were ground by 400#, 800#, 1000# and 2000# abrasive papers in turn, polished and ultrasonically cleaned in absolute ethyl alcohol to remove surface contaminants. In the chamber, the substrate were mounted on a rotational substrate holder that lies between TiAl(Si) alloy target and Cu target after the preliminary treatments, and its revolving speed was controlled 20 rpm during the deposition. The distances between the substrate material and TiAl(Si) alloy target, Cu target were controlled about 3 cm and 13 cm, respectively. The base pressure of the sputtering chamber was evacuated below 1 × 10−4 Pa by an oil diffusion pump, backed by a mechanical pump. Parameters applied to deposition processes were: temperature 245–250 ◦ C, working time 200 min, N2 /Ar ratio 1:1 (both gases with 99.99% purity) and working pressure 0.6 Pa. The sputtering power of TiAl(Si) target and Cu target were controlled about 500 W and 11.25 W, respectively. The phase analysis of these coatings was examined by XRD equipped with CuK␣ X-ray source (XPERT-PRO-MRD) at a scanning

range of 15–85◦ . The morphologies and composition were analyzed by an environmental scanning electron microscope (quanta 200) and an energy dispersive spectrometer (INCA 250X-Max 50). The cross-sectional morphologies of coatings were observed using the field-emission scanning electron microscope (Nova Nano SEM450). Nanohardness and elastic modulus of coatings were measured using the nano-indenter (NANO G200, MTS, USA). The crackresistant of TiAlN/Si3 N4 -Cu nanocomposite coatings was tested using an HV-1000 Z Vickers diamond indenter at a load of 2 N for a loading time of 20 s. In order to evaluate the ability of crack-growth resistance, the indentation morphologies were observed by SEM (SU1510). In order to further verify the toughness of TiAlN/Si3 N4 -Cu coatings, these coatings were tested by a home-made indentation machine, as shown Fig. 1. This machine can produce about 200–3000 MPa pressure specific to the nitrides in the zone of about 0.95 mm2 . At this experiment, the pressure of 400 MPa was applied on the surface of TiAlN/Si3 N4 -Cu nanocomposite coatings. The adhesion properties of coatings were evaluated using a WS2005 automatic scratching tester fitted with a diamond stylus (120◦ cone with a 200 ␮m radius hemispherical tip). The parameters of scratch test were: scratch speed 0.06 mm/s, ultimate maximum load 40 N and scratch length 6 mm. The friction and wear properties of the coatings were tested using a HT-1000 wear testing machine at room temperature and at a relative humidity of 50–60% under dry sliding conditions. Stainless steel (GCr15) balls with a diameter of 6 mm were used as the counterparts, and all the sliding tests were conducted for 20 min at a load of 2.6 N and the sliding speed of 0.17 m/s. 3. Results and discussion 3.1. Microstructure and chemical composition of the coatings Table 1 shows the composition of TiAlN/Si3 N4 -Cu coatings on AISI-304 stainless steel analyzed by EDS. It can be found that the Si content varies from 0 to 5.09 at.% and the Al content gradually decreases from 26.77 at.% to 21.42 at.%, while the content of Ti and N maintains a relative stable value of 2 ± 1 at.% and 45 ± 3 at.%. Some studies [2,12] showed that the maximum solubility of Al in cubic TiN systems was between 60% and 70%. Since the content of Al is less Table 1 Compositions of the TiAlN/Si3 N4 -Cu coatings (at.%) analyzed by EDS. Samples

Ti

Al

Si

Cu

N

(a) (b) (c) (d)

26.78 26.16 27.51 24.96

26.77 24.84 22.94 21.42

0 1.44 3.39 5.09

1.10 1.73 2.80 1.36

45.35 45.83 43.36 47.17

C. Feng et al. / Applied Surface Science 320 (2014) 689–698

691

Fig. 2. Surface morphologies of the TiAlN/Si3 N4 -Cu coatings with different Si contents (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%.

than this value, Al should be dissolved in TiN structure and a solid solution of TiAlN phase should be formed. The researchers [13] consider that the Ti-Al-Si-Cu-N films are composed of nano-crystalline Ti-Al-N, amorphous Cu and amorphous Si3 N4 , of which the amorphous Si3 N4 matrix had segregated the TiAlN nanocrystals.

Fig. 2 shows the surface morphologies of the TiAlN/Si3 N4 -Cu coatings with different Si contents (0.0–5.09 at.%). These coatings were uniform and compact. With increasing the silicon content, the surface roughness of TiAlN/Si3 N4 -Cu coatings gradually diminished.

Fig. 3. Cross-sectional morphologies of the TiAlN/Si3 N4 -Cu coatings with different Si contents (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%.

C. Feng et al. / Applied Surface Science 320 (2014) 689–698

(d)

S

S

S:substrate N:nitride coating

S

60

N(222)

N(110) N(111)

692

50

Grain Size/ nm

40

Intensity

(c)

(b)

30

20

10

0

(a) 30

40

50

60

70

0.00at.%Si

80

2θ / ° Fig. 4. X-ray diffraction patterns of the TiAlN/Si3 N4 -Cu coatings with different Si contents (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%.

5.09at.%Si

when the mobility of ions or adatoms was enhanced. The lowest lattice parameter corresponds to a Ti-Si-Al-N phase where some of the Si and Al atoms are occupying Ti positions in the fcc TiN lattice. TiN peaks became broader as a result of the grain refinement, which is initiated by the deposition of two immiscible phases to form a nanocomposite structure. The average grain sizes were calculated from the FWHM of the diffraction peaks using the scherrer formula [22,23] given by: D = k/ˇcos  where D is the average grain size,  is the wavelength,  is the diffraction angle and ˇ is the FWHM of the same diffraction peak. The grain size values of TiAlN/Si3 N4 -Cu nanocomposite coatings with different Si content are displayed in Fig. 5. The average grain sizes of nanocomposite coatings with Si content of 0.0 at.%, 1.44 at.%, 3.39 at.% and 5.09 at.% are 55.44 nm, 42.63 nm, 5.46 nm and 4.03 nm, respectively. The curve of the TiAlN/Si3 N4 -Cu coatings demonstrates that the incorporation of Si element had hindered the grain growth, in agreement with other reported work on multivariate nitride films [24,25]. They also considered that additive element which it or its nitrides filled in the grain boundary regions played a role that stopped the grain growth and stimulated a renucleation of grains. 3.2. Mechanical properties Fig. 6 shows the hardness and elastic modulus of TiAlN/Si3 N4 Cu nanocomposite coatings with different Si content. The hardness and elastic modulus of each coating as a function of Si content had been investigated using nano-indentation technique. The hardness 22

20

Elastic modulus

260

Hardness

250 240

18

230 16 220 14

210 200

12

Elastic modulus/Gpa

The cross-sectional images of TiAlN/Si3 N4 -Cu coatings with different Si contents are shown in Fig. 3. The well-grown columnar morphology was explicitly found in TiAlN/Cu coating and TiAlN/Si3 N4 -Cu nanocomposite coating containing 1.44 at.% Si content, as shown in Fig. 3(a) and (b). However, the feature of its columnar structure became slightly blurred due to the incorporation of Si. When Si atoms unceasingly incorporated in the coatings, these coatings have changed to be dense and developed a noncolumnar structure owing to the amorphous-Si3 N4 hindered the grain growth and initiated the re-nucleation of grains [14,15]. Philippon et al. [16] investigated the effects of Si content on the microstructure and tribological properties of TiAlSiN films, and found that when the Si content increased from 7 at.% to 15 at.%, the open voids of surface morphology and the grain size decreased. Similar results are also reported in references [17] and [18]. Shi et al. [19] also studied microstructure and characteristics of nanocomposite Ti-Al-Si-Cu-N films with various Si content, and they found that the columnar structure of Ti-Al-Si-Cu-N films changed to be a fine globular structure with increase of the Si content in the films. The diffraction peaks detected in the XRD patterns for TiAlN/Si3 N4 -Cu nanocomposite coatings with different Si content are displayed in Fig. 4. All coatings peaks were found correspond to the cubic TiN with a NaCl-type structure and Fe, and any diffraction peaks for the Si3 N4 phase were not detected. TiAlN/Cu coating showed a strong intensity of TiN(1 1 1) peak and TiN(2 2 2) peak. However, when Si element was introduced into the coatings, the intensity of the peak (1 1 1) dropped sharply and the (2 2 2) diffraction peak eventually disappeared. Compared with TiAlN/Cu coating, the peak TiN(1 1 1) of TiAlN/Si3 N4 -Cu coating with 1.44 at.% Si content became broader. It is known that the XRD peak broadening is a result of small crystallite size in the growth direction of the coating, strains, stacking faults, dislocations or point defects. The broadening of TiN(1 1 1) peak and TiN(1 1 0) peak was attributed to the diminution of the grain size or the residual stress induced in the crystal lattice [20]. As the Si content reached about 3.39 at.%, the phenomenon that the preferred orientations of coating changed into (1 1 0) might result from the formation of Ti-Al-Si-N solid solution. Ti-Al-Si-N solid solution is the metastable phases, and its formation reason is the way of the pseudo crystal growth under the condition of high deposition rate, low bombarding energy and deposition temperature. Caralho et al. [21] had prepared (Ti,Al,Si)N nanocomposite coatings by reactive magnetron sputtering. They found that the lattice parameter changed from 0.418 to 0.429 nm

3.39at.%Si

1.44at.%Si

Fig. 5. Average grain size values of TiAlN/Si3 N4 -Cu nanocomposite coatings with different Si contents.

Hardness/Gpa

20

190 10

8

180 0.00at.%Si

1.44at.%Si

2.80at.%Si

3.39at.%Si

4.51at.%Si

5.09at.%Si

170

Fig. 6. Hardness and elastic modulus of TiAlN/Si3 N4 -Cu coatings with different Si contents.

C. Feng et al. / Applied Surface Science 320 (2014) 689–698

Fig. 7. The indentation morphologies of TiAlN/Si3 N4 -Cu coatings with different Si contents at a load of 2 N (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%.

Fig. 8. The indentation cross-sectional morphologies of TiAlN/Si3 N4 -Cu coatings under the 400 MPa load (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%.

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and elastic modulus of TiAlN-Cu coating is about 9.672 GPa and 224.654 GPa, respectively. With increasing the silicon content, the hardness was improved and the hardness of TiAlN/Si3 N4 -Cu nanocomposite coatings with 5.09 at.% Si reached maximum value of 18.628 GPa. The elastic modulus of TiAlN/Si3 N4 -Cu coatings has the rising tendency, but the value of TiAlN/Si3 N4 -Cu coatings containing 3.39 at.% Si sharply dropped to 180.775 GPa. Yu et al. [23] had prepared the TiAlSiN coatings with different Si content. The microhardness of TiAlSiN with different Si content (0.0–22.14 at.%) was determined. They found that the microhardness has a rapid increase from 25 GPa to 35 GPa at the range of Si content 0.0–10.0 at.%. The hardness of these nanostructured materials is considered to be driven by grain boundary enhancement [26]. The trend growth of hardness is similarities with above-mentioned results. A large number of voids within shaped crystallites were detected in the surface of the TiAlN/Cu coating, as shown in Fig. 2(a). The lower hardness in coating might be due to its columnar structure, which was normally porous with large number of voids within shaped crystallites because of low energetic ion bombardment and limited adatom mobility [19,27]. It is well known that hardness increases with grain size reduction, seen in Fig. 5, which can be explained by increase in grain boundary areas that hinder dislocation motions in the form of dislocation pile-ups (Hall–Petch effect) [28]. As a consequence, the hardness can gradually increase with augment of silicon content within certain limits. Veprek et al. [29] put forward a viewpoint that the highest hardness is achieved when TiN crystals of few nm in size are covered with one monolayer of Si3 N4 , i.e. when the interfacial area reaches a maximum. Shi et al. [19] studied the Si content of 1 at.%, 6 at.% and 10 at.% on nanocomposite Ti-Al-Si-Cu-N films deposited by cathodic vacuum arc ion plating, and with the increase of Si content from 1 at.% to 5.09 at.%, the hardness of their results showed the same rising tendency with ours. The highest resistance to plastic deformation of coating would be related with hindering dislocation formation or movement due to nano-sized TiAlN crystallites and the strong inhibiting of the crack propagation in amorphous Si3 N4 under high applied load. The elastic modulus of TiAlN/Si3 N4 -Cu nanocomposite coating is affinitive with Si content and Cu content. The reason that elastic modulus of TiAlN/Si3 N4 -Cu coating had increased may be affected by amorphous Si3 N4 and soft-metal which have the effect of grain refinement. TiAlN/Si3 N4 -Cu coating with 3.39 at.% Si content has 2.8 at.% Cu in the EDS result, as shown in Table 1, and its value is higher than that of other nanocomposite coatings. Wei et al. [30] had studied the influence of Cu content on the mechanical properties of the TiN-Cu films and found that the elastic modulus increases with the increasing of Cu content, and then decreases. The main reason for the decrease in elastic modulus is the interaction of nano-crystalline TiN and amorphous Cu in the grain boundary regions. The possible softening effect by grain boundary sliding is suppressed by the presence of amorphous Cu. Both the hardness and toughness of coatings are the vital parameters in the engineering. The toughness presents the ability of a material to absorb energy during deformation up to fracture. It can be evaluated from the resistance against crack formation and propagation. Fig. 7 shows the diamond-shaped indentation morphologies of TiAlN/Si3 N4 -Cu nanocomposite coatings with different Si content at a load of 2 N for a loading time of 20 s. The indentation morphology of TiAlN/Cu coating had found a lot of cracks in and around the diamond-shaped indentation. However, when the Si element was imported to TiAlN/Cu coating, the cracks had significantly reduced. The cracks only were observed at the boundary region of diamond-shaped indentation, especially for TiAlN/Si3 N4 -Cu coating containing 3.39 at.% Si content. It indicated

0.10

0.08

H3/E2/ GPa

694

0.06

0.04

0.02

0.00

0.00at.%Si

1.44at.%Si

3.39at.%Si

5.09at.%Si

Fig. 9. H3 /E2 ratio of TiAlN/Si3 N4 -Cu coatings with different Si contents.

that the toughness of TiAlN/Si3 N4 -Cu coatings is superior to TiAlN/Cu coating. High-load indentation images are shown in Fig. 8. The shapes have two kinds of extended modes on the surface: traverse cracks and reticular cracks. Generally, traverse cracks are all penetrative cracks (see Fig. 8(a)–(c)). The traverse crack has originated from the edge of coating and motion to the other side by the modalities of diffusion and penetration. TiAlN/Si3 N4 -Cu coating containing 5.09 at.% Cu appeared the fractures under the high-load. However, TiAlN/Si3 N4 Cu composite coating with 3.39 at.% Si content produce a lot of tiny micro-cracks and their growth condition is mainly bending growth. TiAlN/Si3 N4 -Cu composition coatings with 3.39 at.% Si content reveal good toughness under the load of 400 MPa. Many researchers [31] studied the ability for crack retardation of nanocomposite and nanomultilayers coatings. The possible reasons are that amorphous Si3 N4 and Cu mainly filled in grain boundary region. Dislocation is prone to traverse the lower-hardness amorphous Cu, but the highmodulus amorphous Si3 N4 phase plays a big role of hindering the dislocation motion and grain slippage, and makes crack deflection. Musil and Jirout [32] found that the resistance to cracking of thin films increases with increasing the ratio of H3 /E2 . The H3 /E2 ratio is proportional to the film resistance to plastic deformation, as shown in Fig. 9. The H3 /E2 ratio of TiAlN/Si3 N4 -Cu coatings with 3.39 at.% Si content is maximal and the quantity of micro-cracks is the least. Fig. 10 compares the acoustic signals of TiAlN/Si3 N4 -Cu coatings with different Si content during scratch tests, and the critical loads (Lc ) were 22.4 N, 12.3 N, 5.1 N and 0.7 N for Si content of 0.0 at.%, 1.44 at.%, 3.39 at.% and 5.09 at.%, correspondingly. TiAlN/Si3 N4 -Cu coating with 5.09 at.% Si shows consecutive acoustic signals during scratch tests, and this means that the coating had produced consecutive peelings under the destruction of the diamond stylus. The acoustic signal peaks whose intensity over 2500 a.u. can be found in the TiAlN/Cu coating and the TiAlN/Si3 N4 -Cu coatings with 1.44 at.% and 5.09 at.% Si content, while TiAlN/Si3 N4 -Cu coating with 3.39 at.% Si content was detected that the signal intensity is under 1500 a.u. This phenomenon may be ascribed to good toughness of the coating, as shown in Fig. 6(c). Critical loads can be used directly to quantify the “scratch toughness” of thin films [33]. TiAlN/Si3 N4 -Cu nanocomposite coating containing 3.39 at.% Si shows higher hardness and well resistance to plastic deformation at contact loads. This may avoid brittle fracture at high levels of loading, and also create the possibility to support high contact loads by local coating compliance and load distribution onto

C. Feng et al. / Applied Surface Science 320 (2014) 689–698 4000

695

5000

(a)

(b)

3500 4000

Lc

2500 2000

Signal intensity/a.u.

Signal intensity/a.u.

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Lc

1500 1000

3000

2000

1000

500 0

0

-500 0

10

20

30

40

0

10

Load/N 2800

20

30

40

30

40

Load/N

(c)

3000

2400

(d)

2500

Signal intensity/a.u.

Signal intensity/a.u.

2000 1600

Lc 1200 800

2000

Lc

1500 1000 500

400

0

0 0

10

20

30

40

Load/N

0

10

20

Load/N

Fig. 10. Typical acoustic signal diagrams of the TiAlN/Si3 N4 -Cu coatings during scratch tests (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%

larger areas. Apparently, the adhesion property between the nitride films and substrate reduces with the increase of Si content, but TiAlN/Si3 N4 -Cu coatings with 3.39 at.% Si has superior adhesion property between coating and substrate since its excellent toughness. The corresponding scratch morphologies of the TiAlN/Cu and TiAlN/Si3 N4 -Cu coatings are shown in Fig. 11. In Fig. 11(a), almost no peelings were found in the scratch track of the TiAlN/Cu coating. However, obvious delamination formed in the scratch track of TiAlN/Si3 N4 -Cu coating with 5.09 at.% Si. It can be seen that a lot of consecutive peelings were found in the TiAlN/Si3 N4 -Cu coatings. The rear part of the scratch track of the TiAlN/Si3 N4 -Cu coatings with 1.44 at.% Si content produced serious peelings, while TiAlN/Si3 N4 -Cu coating with 3.39 at.% had interrupted peelings. This may be related to the internal stress and toughness of coatings.

Fig. 11. The scratch tracks of TiAlN/Si3 N4 -Cu coatings with different Si contents (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%.

Many researchers [20,34] considered that the critical adhesion load increased with increasing of Si content and the toughness of the thin films increased by increasing the Si content. However, this relation is valid only for coatings that have the same of internal stress. XRD results indicated that broadening of the TiN peaks is observed. The most important contributions to XRD peak broadening originate from particle size and microstress-induced by alterations of lattice [20]. Adhesion of the coating to the substrate decreases because of the increasing of the residual stress. As a consequence, TiAlN/Si3 N4 Cu coatings can provide with superior adhesion property and better toughness when the silicon is controlled at a certain value. Fig. 12 shows the evolution of friction coefficient with sliding time of the TiAlN/Si3 N4 -Cu coatings against GCr15 steel ball under dry sliding condition at the load of 2.6 N. For the TiAlN/Cu coating, its friction coefficient remains the ascent stage for the initial 7 min, which may be related to its hardness and contact areas. The hardness of the TiAlN/Cu coating only was 9.67 GPa. During wearing, the contact regions of counterparts were immersing in the coatings. The friction coefficient is usually increased for the increasing of the friction areas. After sliding for about 8 min, friction coefficient fluctuates between 0.8 and 0.9, which indicates that intensive adhesive and abrasive wear occurred to the TiAlN/Cu coating, as shown in Fig. 13(a). These TiAlN/Si3 N4 Cu coatings showed stable friction coefficient with a very short running-in stage which did not exceed 1.5 min. The average friction coefficients of the TiAlN/Si3 N4 -Cu nanocomposite coatings with 1.44 at.%, 3.39 at.% and 5.09 at.% Si are about 0.9, 0.8 and 0.7, respectively. From Fig. 2, it can be observed that the roughness of the

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1.2

b c

Friction coefficient

1.0

0.8

0.6

d

0.4

a

0.2

0.0 0

5

10

15

20

Time/min Fig. 12. Friction coefficient of the TiAlN/Si3 N4 -Cu coatings with different Si content (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%.

coatings was reduced with the increasing content of Si, and this may lead to the gradually reduce of the friction coefficient values. Further evidence of wear mechanisms and tribofilm formation will be given below, which will help to elucidate their friction behavior. Due to the lower hardness, the TiAlN/Cu coating and TiAlN/Si3 N4 -Cu coating with 1.44 at.% Si trigger wear and oxidation by the sliding of the counterparts. It is clear to see that there is a lot of wear debris that occurred near the edge of the wear tracks (Fig. 13(a) and (b)). The debris is rich in Ti, Al and O element by EDS analysis. The wear mechanism is mainly abrasive wear.

With increasing the silicon content, the hardness was improved and the hardness of TiAlN/Si3 N4 -Cu coatings with 5.09 at.% Si reached to approximately 18.628 GPa, as can be seen in Fig. 6. Moreover, there is a lot of wear debris that has lubrication effect in the worn surfaces, as shown in Fig. 13(d). According to the result of the EDS analysis, the white area in this track showed that it was rich in Fe and O elements, while the darker area in this track was rich in Ti, Al and O elements. It can be seen from Fig. 14 that there are some peeling films in the worn surfaces. The adhesion property between TiAlN/Si3 N4 -Cu coating with 5.09 at.% Si and substrate is worse than other coatings, and this indicates that TiAlN/Si3 N4 -Cu coating with 5.09 at.% Si was tend to peeling during wear process. These peelings had played a role in lubrication. It can be explained that the coefficient curve of TiAlN/Si3 N4 -Cu coating with 5.09 at.% Si has decreasing trend. Many furrows and debris can be observed in the worn surface of coatings. Based on the morphologies and composition analysis, the wear mechanism of the TiAlN/Si3 N4 -Cu coating with 5.09 at.% Si includes abrasive wear, adhesive wear and oxidation wear. Wear mechanism of the TiAlN/Si3 N4 -Cu coatings with 3.39 at.% Si content is mainly adhesive wear according to the debris feature in the worn surface. The EDS analysis of the white area (Fig. 13(c)) in the wear tracks exhibited that in addition to the presence of Fe, it was also rich in Cr and O, indicating the occurrence of materials transfer from the GCr15 ball to the coating surface. The low friction coefficient might originate from the synthetic lubricant effects brought by the higher soft copper phase (Table 1) and iron oxide, and the decreased crystallite size of coating [23,35]. TiAlN/Si3 N4 -Cu coatings with 3.39 at.% Si content have higher hardness, excellent adhesion property and good toughness. As can be seen from the Fig. 15, the wear volumes of TiAlN/Si3 N4 -Cu coating with 3.39 at.%

Fig. 13. SEM images of the worn surfaces of TiAlN/Si3 N4 -Cu coatings with different Si contents (a): 0.0 at.% (b): 1.44 at.% (c): 3.39 at.% (d): 5.09 at.%

C. Feng et al. / Applied Surface Science 320 (2014) 689–698

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Fig. 14. An enlarged picture of worn surfaces of the TiAlN/Si3 N4 -Cu coatings with 5.09 at.%.

abrasive resistance and its wear mechanisms is mainly adhesive wear.

0.00016 0.00015

Acknowledgment

0.00013

The authors express their gratitude to the financial support of the National Natural Science Foundation of China (Grant Nos. 51005111 and 51365041).

Wear volume/mm

3

0.00014

0.00012 0.00011 0.00010

References

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Fig. 15. Wear volumes of TiAlN/Si3 N4 -Cu coatings with different Si contents.

Si was the smallest, indicating it had good abrasive resistance and excellent mechanical property. 4. Conclusions (1) TiAlN/Cu coating have well-grown columnar structure. With increasing of Si content form 0.00 at.% to 5.09 at.%, the columnar structure of the TiAlN/Si3 N4 -Cu coating became slightly blurred and the surface roughness of coatings gradually diminishes. The average grain sizes of the TiAlN/Si3 N4 -Cu coatings decreases from 55.44 nm to 4.032 nm. The change of microstructure is attributed to Si elements whose nitrides (amorphous Si3 N4 ) fill in the grain boundary regions play a role that stopped the grain growth and promoted the grain refinement. (2) The hardness and elastic modulus of TiAlN/Si3 N4 -Cu nanocomposite coatings increases with the increasing of Si content, but the elastic modulus value of TiAlN/Si3 N4 -Cu coatings with 3.39 at.% Si sharply fell to 180.775 GPa, due to the content of copper that was higher than the other coatings. The H3 /E2 ratio of TiAlN/Si3 N4 -Cu coating with 3.39 at.% Si content is maximal and shows the best toughness. The quantity of cracks is the least in the diamond-shaped indentation morphologies. The reducing of adhesion property between the nitride coatings and substrate is related with the content of Si and the ratio H3 /E2 . TiAlN/Si3 N4 -Cu coating with 3.39 at.% Si has good

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