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Microstructure, mechanical properties and tribological performance of TiSiN–WS2 hard-lubricant coatings
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Microstructure, mechanical properties and tribological performance of TiSiN–WS2 hard-lubricant coatings Shipeng Li a,b , Jianxin Deng a,b,∗ , Guangyuan Yan a,b , Kedong Zhang a,b , Guodong Zhang a,b a b
School of Mechanical Engineering, Shandong University, Jinan 250061, PR China Key Laboratory of High-efficiency and Clean Mechanical Manufacture, Shandong University, Ministry of Education, PR China
a r t i c l e
i n f o
Article history: Received 26 January 2014 Received in revised form 23 April 2014 Accepted 4 May 2014 Available online xxx Keywords: TiSiN coating WS2 coating Hard-soft coating Tribological performance
a b s t r a c t TiSiN–WS2 hard-lubricant coatings were deposited on WC/TiC/Co cemented carbides by arc ion plating and middle-frequency magnetron sputtering. The microstructure, mechanical properties and tribological performance were examined. Results showed that WS2 layer had (0 0 2) and (1 0 1) multi-orientation structure and TiN grains of TiSiN layer became bigger due to the annealing effect in deposition of WS2 layer. As the thickness of WS2 layer increased, hardness of TiSiN–WS2 coatings decreased largely while the adhesive strength increased, which is mainly beneficial from the release of residual stress of TiSiN layer in deposition process of WS2 layer and the low tensile stress caused by the lubricant effect of WS2 in scratch test. The deposition of WS2 layer on top of TiSiN layer significantly improved coatings’ tribological performance with lower friction coefficient, less adhesive materials and smaller wear rate of steel ball. Even after WS2 layer was worn out, the friction coefficient was still lower than TiSiN coating, which is attributed to that WS2 remaining in wear track still worked. © 2014 Elsevier B.V. All rights reserved.
1. Introduction For recent years, dry machining are more and more widely studied and applied because the absence of cooling fluids is economic and environmental [1]. However, due to higher pressure and temperate in tool-chip interface in dry machining, cutting tools should have better performance. Thus, tools coated with TiN, ZrN, TiAlN, ZrTiN, etc. have been applied because of their high hardness and wear resistance [2–4]. TiSiN coating, with ultra high hardness, better oxidation resistance and superior wear resistance [5,6], is supposed to be a promising candidate for tool application in dry machining. However, friction tests showed that TiSiN coating had a high friction coefficient to steels [7–9], which may lead adhesion wear and coating’s delamination under large friction force. That is terrible in machining process. MoS2 and WS2 coatings as solid lubricants have effective lubricant performance. But those coatings are so soft (with a hardness of 1.5–5 GPa) [10,11] that if they were used along in dry machining, substrates of tools would be exposed in a short period, which is not much helpful to improve the wear resistance of tools.
Kenneth Holmberg and Allan Matthews [12] suggested that coatings with a soft/lubricant layer on a hard layer can combine the lubricant effect of soft coatings and the high hardness and wear resistance of hard coatings. Some researches have investigated these coatings, like TiAlN–WC/C and TiN–MoS2 /Ti. Results showed that tools with these hard-lubricant coatings had much better performance [13–15]. But more information of these coatings need be provided so as to expand their industrial applications. In this study, WS2 coating as the soft layer was deposited on TiSiN hard coatings, which can combine advantages of these coatings and solve the high friction coefficient of TiSiN coating and low wear resistance of WS2 coating. Arc ion plating (AIP) and medium-frequency (MF) magnetron sputtering technology were used as the deposition method, and WC/TiC/Co cemented carbides were employed for the substrates. The influence of various thickness of WS2 layer on coating’s microstructure, hardness and adhesive force were examined. Sliding wear tests against hardened steel were carried out with a ball-on-disk tribometer to investigate TiSiN–WS2 coating’s tribological properties. Results can provide useful information for industrial application of this coating. 2. Experimental
∗ Corresponding author at: Department of Mechanical Engineering, Shandong University, No. 17923 Jingshi Road, 250061 Jinan, Shandong Province, PR China. Tel.: +86 531 88399769. E-mail addresses: [email protected] (S. Li), [email protected] (J. Deng).
2.1. Deposition Hard-lubricant coatings were deposited on WC/TiC/Co cemented carbides (fined grains) using the combination of AIP
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Table 1 Multilayer coatings with different thicknesses of WS2 layers. Coatings
TiSiN (m)
TW0.5 (m)
TW1 (m)
TW1.5 (m)
TW2 (m)
Thickness of WS2 layer Thickness of TiSiN layer
0
0.5
1.0 2
1.5
2.0
Dry reciprocating friction tests were carried out by a ball-ondisk tribometer (UMT-2, USA) in ambient air. The ball was made of 45 hardened steel with a diameter of 9.525 mm and a hardness of HRC 55–60. The specimen was fixed while the ball was reciprocating and sliding with a speed of 10 mm/s. The length of wear trace was 8 mm. Every test lasted for 60 min. A normal load of 25 N was applied in the tests. The friction coefficient was recorded by the tribometer’s computer. The morphology of worn regions was examined by WLI and SEM. 3. Results Fig. 1. The structure of TiSiN–WS2 hard-lubricant coating.
method and MF magnetron sputtering technology. The substrates were polished and cleaned ultrasonically in ethanol and acetone progressively, then placed vertically on the rotational sample holder. The base pressure of the chamber was below 7.0 × 10−3 Pa and the temperature was fixed at 200 ◦ C. Before deposition, the substrates were cleaned again by ion bombardment for 15 min with a bias voltage of −800 V under the Ar atmosphere of 1.5 Pa. Five kinds of coatings were prepared and named according to different thickness of WS2 layer, as shown as in Table 1. Fig. 1 shows the coating’s structure. Coatings with TiSiN and WS2 layers were deposited with two steps: firstly, TiSiN single layer was prepared on substrate using one Ti arc target and two Si magnetron sputtering targets; then WS2 layer was deposited on TiSiN coating with two WS2 magnetron sputtering targets. The detailed deposition parameters are listed in Table 2. By controlling the deposition time, coatings with various thicknesses of WS2 layer were obtained. 2.2. Characterization The surface morphology and cross-sectional micrographs of coatings were observed by a scanning electron microscope (SEM, Oxford INCA Penta FETX3). Chemical composition of coatings was analyzed by an energy dispersive X-ray detector (EPS). The microstructure was studied by X-ray diffraction (XRD) using a D8 ADVANCE (Bruker AXS) diffractometer with Cu K˛ radiation. White light interferometer (WLI, Wyko NT9300) was used to measure thickness of the coatings by preparing a small area on substrates where coatings would not be deposited. The microhardness measurement was conducted on an MH-6 microhardness tester using various loading: 10 g, 25 g, 50 g and 100 g. Scratch tests were conducted on a MFT-3000 device to evaluate adhesive strength of coatings. Table 2 Deposition parameters of different layers. Parameters
TiSiN layer
WS2 layer
Base pressure (Pa) Working pressure (Pa) Nitrogen flowrate (sccm) Ti arc target current (A) Si sputter targets current (A) WS2 sputtering targets current (A) Deposition temperature (◦ C) Bias voltage (V) Deposition time (min)
7.0 × 10−3 0.6 180 65 4.0 – 200 −80 150
7.0 × 10−3 0.5 – – – 1.0 200 −100 0, 35, 70,105, 140
3.1. Microstructures The chemical composition of TiSiN and WS2 layer analyzed by EDS was listed in Table 3. Si content of TiSiN layer was 8.0 at.%. For WS2 layer, the ratio of S/W was about 1.5, which was lower than the stoichiometric ratio of 2.0 in WS2 target. Two factors can induce the deficiency of S element, one was the different sputtering efficiency between S and W atoms, another one was that S element could be pumped out from the chamber after reacting with the residual oxygen or hydrogen [16,17]. The surface morphology and cross-section graphs of TW1.5 coating were shown in Fig. 2. It is shown that coating surface was uniform and no delamination occurred at the interface of TiSiN/substrate or WS2 /TiSiN. The thickness of TiSiN layer was about 2 m and WS2 layer was about 1.5 m. The thickness errors of TiSiN layer and WS2 layer were 0.1 m and 0.15 m. It also revealed that TiSiN layer was very dense while WS2 exhibited a loose structure. Fig. 3 shows the X-ray diffraction analysis of these coatings. It can be seen that WS2 layer had two crystal orientation planes of (0 0 2) and (1 0 1), and as the thickness of WS2 layer increased, the peak of (1 0 1) plane became stronger while no obvious change occurred for (0 0 2) plane. Only TiN (2 0 0) plane existed in single TiSiN coating except for diffraction peaks from the substrates. However, the deposition of WS2 layer made TiN (2 2 0) plane come out, and the thicker WS2 layer, the stronger the new diffraction peak, which is caused by the annealing effect in the deposition process of WS2 layer. In this study, increasing thickness of WS2 layer meant a longer deposition time of WS2 , so the annealing time for TiSiN layer increased accordingly, which can promote grains growing. As a result, the peak of TiN (2 2 0) became stronger with increasing thickness of WS2 layer. 3.2. Mechanical properties Hardness of coatings measured under various applied load were shown in Fig. 4. It reveals that the thickness of WS2 layer had a larger influence on coating’s hardness. When WS2 layer’s thickness increased from 0.5 m to 1 m, the hardness suffered a large Table 3 The chemical composition of TiSiN and WS2 layers. TiSiN layer
WS2 layer
Ti content (at.%)
Si content (at.%)
N content (at.%)
39.5
8.0
52.5
W content (at.%)
S content (at.%)
S/W
40.2
59.5
1.5
Please cite this article in press as: S. Li, et al., Microstructure, mechanical properties and tribological performance of TiSiN–WS2 hardlubricant coatings, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.012
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Fig. 2. The surface morphology and cross-section pictures of TW1.5 coating.
Fig. 3. X-ray diffraction analysis of coatings with variable thickness of WS2 layer.
Fig. 4. Hardness of TiSiN–WS2 coatings at different applied load with variable thickness of WS2 layer.
reduction. As WS2 layer’s thickness further increased, the hardness continued to decrease, but only slightly. Besides, all hard-lubricant coatings hardness increased with increase of applied load in hardness tests. It was because the indentation depth generated on WS2 layer in hardness test was larger than the thickness of WS2 layer. So the test value was affected by the bottom TiSiN layer that had a much larger hardness. As higher load applied, the effect would be enhanced. Thus, at high load the increment value became larger (Fig. 4). While for single TiSiN coating, the influence from the substrate was just the opposite due to the lower hardness of substrate compared to TiSiN coating. Adhesive strength of these coatings was measured by scratch tests. Fig. 5 shows the adhesive strength of coatings with variable thickness of WS2 layer. It can be observed that as thickness of WS2 layer increased to 1 m, the critical load increased from 35 N to 63 N, indicating that WS2 layer on TiSiN coating could largely increased the adhesion strength. With further thickening WS2 layer, no obvious enhancement appeared. The curves of friction force, coefficient and acoustical signal in scratch tests were shown in Fig. 6. For WS2 coating, the curves were smooth and steady, almost no acoustical signal appeared. For TiSiN coating, large fluctuation occurred when coating failure happened, which indicate that WS2 coating was scraped off gradually, while TiSiN coating flaked at the critical load. Fig. 6c illustrates that there
Fig. 5. Adhesive strength of TiSiN–WS2 coatings with variable thickness of WS2 layer.
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Fig. 6. Curves of friction coefficient, friction force and acoustical signal in scratch tests of (a) single WS2 coating, (b), single TiSiN coating, (c) hard-lubricant coating (TW1).
were two stages in scratch process of hard-lubricant coating: the failure of WS2 layer and the failure of TiSiN layer. However, the scratching process of TW1 coating was not just simple combination of these layers. The incorporation of WS2 layer can significantly prolong the period before total failure happened. The cross-sectional curve of the scratch of TW1 coating is shown in Fig. 7, which demonstrates the scratch process more clearly. When the indenter was touching WS2 layer at the beginning, severe plastic deformation happened, consequently the curve descended steeply. Then the stylus rubbed against WS2 layer until which was
Fig. 7. Cross-sectional profile curve of the scratch track of TW1 coating.
scraped off and TiSiN layer was exposed. Subsequently, the stylus directly scratched TiSiN layer for a relatively longer period. After that, the groove depth sharply dropped, indicating that TiSiN layer flaked. In the scratch process of TiSiN layer, the depth went through a gradual declination. It was caused by the wear of TiSiN layer and deformation of the substrate under the applied load. 3.3. Tribological properties 3.3.1. Friction coefficients Dry reciprocating friction tests were carried out to study coating’s friction properties with the load of 25 N and speed of 10 mm/s. For TiSiN coating, loud noise was generated in its friction process, while tests of hard-lubricant coatings were stable. The friction coefficient as a function of sliding time of these coatings was shown in Fig. 8. Obviously, single TiSiN coating had the highest friction coefficient with an average value of 0.71. For TiSiN–WS2 coatings, there were two stages in friction tests. At the beginning, their friction coefficients were ultra low, in the range of 0.06–0.08, which
Fig. 8. Friction coefficient of TiSiN–WS2 coatings with various thickness of WS2 layer at the speed of 10 mm/s and with the load of 25 N.
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Fig. 9. Wear surfaces of steel balls after 60 min friction tests sliding with coating of (a) TiSiN, (b) TW0.5, (c) TW1, (d) TW1.5, (e) TW2 at the speed of 10 mm/s and with the load of 25 N.
was ascribed to that WS2 layer with low shear elasticity can form a transferred layer between friction pairs. With prolonged time, WS2 layer was worn off, the coefficient started to increase and then it tended to be steady at a relatively high value. It is important to point out that after the failure of WS2 layer, the friction coefficient was still much lower than single TiSiN coating’s. With increasing thickness of WS2 layer, the duration of low friction coefficient was extended significantly. TW0.5 coating only lasted for ∼500 s. But when WS2 layer thickened to 1 m, the duration increased to ∼1750 s. The longest duration was about 2600 s for TW2 coating. However, the duration of WS2 layer was not proportional to its thickness. Its growth reduced as WS2 became thicker. It may be related to that large plastic deformation easily occurred on its surface under stress, which can make WS2 be ploughed out of the wear track by the friction counterpart so that part of WS2 layer would not participate in friction process to play the lubricant role. This effect could be enhanced by increasing thickness of WS2 or sharp friction counterpart. So thickening WS2 layer to prolong the low friction stage had limited effects.
3.3.2. Wear rate of steel ball The wear surfaces of steel balls after 60 min dry friction tests were shown in Fig. 9. It is obvious that there were lots of deep scratches on worn surface of steel ball sliding with TiSiN coating (Fig. 9a), but with hard-lubricant coating, the scratches were very slight. At the same time, as WS2 layer became thicker, the wear scar largely decreased. The diameters of wear scar on these balls sliding with coating of TiSiN, TW0.5, TW1, TW1.5 and TW2 were approximately 1.34 mm, 1.14 mm, 0.89 mm, 0.78 mm and 0.71 mm, respectively. And according to the geometrical relationship, the volume loss of steel ball can be calculated with the following
equation [18,19]:
D/2
V= √
D2 −d2 /2
2 D 2
−x
2
dx =
D3 2 D − d2 (2D2 + d2 ) − 12 24 (1)
where D is the diameter of steel ball, d is the diameter of wear scar. Then the wear rate can be obtained by the equation: W=
V FL
(2)
where W represents the wear rate, F and L are the applied load and sliding distance in friction tests. The results are shown in Fig. 10. It reveals that the addition of WS2 layer significantly reduced wear
Fig. 10. Wear rates of steel balls against different coatings at the speed of 10 mm/s and with the load of 25 N.
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Fig. 11. SEM images of wear track on coating of (a) TiSiN, (b) TW0.5, (c) TW1, (d) TW1.5, (e) TW2 after 60 min friction tests at the speed of 10 mm/s and with the load of 25 N.
Fig. 12. 3D WLI images of wear track on coating of (a) TiSiN, (b) TW0.5, (c) TW1, (d) TW1.5, (e) TW2 after 60 min friction tests at the speed of 10 mm/s and with the load of 25 N.
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Table 4 Properties of TiSiN, substrate and diamond stylus. Materials
Young’s modulus (GPa)
Poisson’s ratio
TiSiN Substrate Diamond stylus
457 510 1100
0.23 0.25 0.07
rates of these steel balls. And with the increase of WS2 layer’s thickness, the wear rate decreased. Compared to TiSiN coating, the wear rate of steel ball sliding with TW2 coating was lower by an order of magnitude. 3.3.3. Worn surfaces of coatings. SEM micrographs and 3D WLI images of wear tracks are shown in Figs. 11 and 12, respectively. The width of wear track was generally in agreement with the diameter of wear scar on steel ball (Fig. 9). Because WS2 layer had a low hardness and wear resistance, it was worn out after friction test, leading a deeper wear track for hard-lubricant coating than TiSiN coating (Fig. 12). From these pictures it can be seen that lots of plowing grooves appeared on the wear track of TiSiN coating, indicating that seriously abrasive wear occurred on TiSiN coating. While no obviously grooves appeared on hard-lubricant coating, but adhesion happened, proving that TiSiN layer was not worn. As WS2 layer thickened, adhesion materials decreased gradually. There was almost no Fe adhesion on the wear track of TW2 coating. 4. Discussion 4.1. Adhesive strength The deposition of WS2 layer on TiSiN coating largely increased the adhesive strength (Fig. 5). Figs. 6 and 7 prove that in the scratch process of TiSiN–WS2 coatings, two stages appeared: the stage of rubbing WS2 layer and the stage of scratching TiSiN layer. In other words, WS2 layer delayed the scratching effect for TiSiN layer by the stylus. As a result, the scratch process before the failure of TiSiN layer was prolonged, which can lead to a larger adhesive strength. In addition, in the deposition of WS2 layer, TiSiN layer experienced
Density (kg/m3 ) 3750 11,300 3500
Thickness/diameter (m) 2 100 200
an annealing process. That can reduce the residual stress in TiSiN layer and be beneficial for improving the adhesion between coating and substrate [20,21]. Furthermore, it can be found from Fig. 6b and c that the friction coefficient of TiSiN–WS2 coating in the stage when the stylus scratched on TiSiN layer was lower than single TiSiN coating. That is caused by the residual WS2 in the scratch track. In scratch test, fiction coefficient had a large effect on the stress generated on coatings, thus affecting the critical load [22,23]. To obtain quantitative analysis, the finite element method (FEM) was used to study the effect of friction coefficient on stress and its distribution in scratch tests. Materials properties were listed in Table 4. In order to reduce the calculation time, the model was simplified: a symmetric model was used; only TiSiN coating and cemented carbides comprised the sample (Fig. 13a); the applied load was kept at 35 N when the stylus moved according to the critical load of single TiSiN coating. On the basis of the friction coefficients of TiSiN and TW1 coating in scratch tests, the friction coefficients were set at 0.2 (TiSiN–WS2 coating) and 0.33 (single TiSiN coating). The model after meshing was shown in Fig. 13b. Around the contact area, the mesh was refined. As for the boundary condition for constraint, the bottom face was constrained in all directions. The maximum principle stress was often used to represent the stress distribution in coating’s scratching test [24]. The results of the maximum principle stress were contoured in Fig. 13c and d. It can be observed that the large tensile stress concentration was located behind the stylus. The maximum tensile stress of TiSiN–WS2 coating was about 41% lower than TiSiN coating’s. Reduction of tensile stress can decrease the generation of cracks and hinder coating’s delamination under friction force. Thus, the lubricant effect of residual WS2 of hard-lubricant coatings is helpful to increase the critical load in scratch tests.
Fig. 13. FEM model (a) before meshing, (b) after meshing; the max principal stress distribution of (c) single TiSiN coating, (d) hard-lubricant coating.
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Fig. 14. (a) SEM image of wear track on TW2 coating, (b) EDS analysis of A point marked in (a).
4.2. Tribological performance Compared to TiSiN coating, TiSiN–WS2 coating showed better tribological performance, such as low friction coefficient, less abrasive wear and small wear rate of steel ball. This mainly benefited from the lubricant function of WS2 . However, after WS2 layer was worn out, the friction coefficient was still lower than TiSiN coating (Fig. 6). By the EDS analysis of wear track of TW2 coating and steel ball (Figs. 9f and 11f), WS2 was not found in the middle of wear track but was detected near the edge. It means that WS2 layer still worked around the edge areas. Thus, because of lubricant effect of WS2 in edge area, the friction coefficient of TiSiN–WS2 coating was still lower than TiSiN coating after partial WS2 layer was worn out. In addition, it was found that there were some holes among wear track, as shown as in Fig. 14a In deposition of TiSiN layer, AIP method was employed, which can produce Ti droplets on coating surface. When the droplets fell off, macroholes generated. Fig. 14b shows EDS composition analysis in the hole of Fig. 14a. It can be seen that the holes were filled with WS2 , which would play an important role as lubrication after WS2 layer was worn out. This is another reason for the lower friction coefficient. Fabricating small holes or grooves or textures on TiSiN layer or substrates is an effective way to prolong the lubricant time of WS2 layer and beneficial for the industrial applications of these coatings, which need to be further studied. 5. Conclusions TiSiN–WS2 hard-lubricant coatings were deposited on WC/TiC/Co cemented carbides. The microstructure, mechanical properties and tribological performance were investigated. Through the results, the following conclusions can be obtained: (1) TiSiN–WS2 hard-lubricant coatings show higher adhesive strength in comparison with single TiSiN coating, and its tribological performance is largely improved, with no abrasive wear, lower friction coefficient and smaller wear rate of steel ball. As the thickness of WS2 layer increased, tribological performance was improved much more, but the enhancement in adhesive strength was limited that after the thickness WS2 layer exceeded 1 m, no obvious enhancement appeared. (2) Three reasons can explain the enhancement mechanism of adhesive strength: WS2 layer delayed the scratching effect by diamond stylus for TiSiN layer; residual stress in TiSiN layer
could be reduced in the deposition of WS2 ; WS2 remaining in scratch track reduced the friction coefficient and the tensile stress generated on coatings. (3) The improvement of tribological performance of TiSiN–WS2 coatings is mainly ascribed to the lubricant effect of WS2 layer. Even after WS2 was worn out, the friction coefficient was still lower than TiSiN coating. It is attributed to WS2 that remained near the edge of wear track and in the macroholes of TiSiN layer.
Acknowledgements This work was supported by “the National Natural Science Foundation of China (51375271)”, “the Fundamental Research Funds of Shandong University (2014JC039)”, “the Independent Innovation Foundation of Universities in Jinan (201401226)”.
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[19] W.L. Ma, J.J. Lu, Effect of surface texture on transfer layer formation and tribological behavior of copper–graphite composite, Wear 270 (2011) 218–229. [20] H.W. Choi, M.W. Moon, T.Y. Kim, K.R. Lee, K.H. Oh, The thermal annealing effect on the residual stress and interface adhesion in the compressive stressed DLC Film, MRS Proc. 795 (2003) 42.1–42.6. [21] B.K. Tay, X. Shi, E. Liu, S.P. Lau, L.K. Cheah, Z. Sun, J. Shi, Stress relief of tetrahedral amorphous carbon films by post-deposition thermal annealing, Surf. Coat. Technol. 120 (121) (1999) 448–450. [22] P.A. Steinmann, Y. Tardy, H.E. Hintermann, Adhesion testing by the scratch test method: the influence of intrinsic and extrinsic parameters on the critical load, Thin Solid Films 154 (1987) 333–349. [23] S.J. Bull, D.S. Rickerby, A. Matthews, A. Leyland, A.R. Pace, J. Valli, The use of scratch adhesion testing for the determination of interfacial adhesion: the importance of frictional drag, Surf. Coat. Technol. 36 (1988) 503–517. [24] J. Li, W. Beres, Three-dimensional finite element modelling of the scratch test for a TiN coated titanium alloy substrate, Wear 260 (2006) 1232–1242.
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Microstructure, mechanical properties and tribological performance of TiSiN–WS2 hard-lubricant coatings Shipeng Li a,b , Jianxin Deng a,b,∗ , Guangyuan Yan a,b , Kedong Zhang a,b , Guodong Zhang a,b a b
School of Mechanical Engineering, Shandong University, Jinan 250061, PR China Key Laboratory of High-efficiency and Clean Mechanical Manufacture, Shandong University, Ministry of Education, PR China
a r t i c l e
i n f o
Article history: Received 26 January 2014 Received in revised form 23 April 2014 Accepted 4 May 2014 Available online xxx Keywords: TiSiN coating WS2 coating Hard-soft coating Tribological performance
a b s t r a c t TiSiN–WS2 hard-lubricant coatings were deposited on WC/TiC/Co cemented carbides by arc ion plating and middle-frequency magnetron sputtering. The microstructure, mechanical properties and tribological performance were examined. Results showed that WS2 layer had (0 0 2) and (1 0 1) multi-orientation structure and TiN grains of TiSiN layer became bigger due to the annealing effect in deposition of WS2 layer. As the thickness of WS2 layer increased, hardness of TiSiN–WS2 coatings decreased largely while the adhesive strength increased, which is mainly beneficial from the release of residual stress of TiSiN layer in deposition process of WS2 layer and the low tensile stress caused by the lubricant effect of WS2 in scratch test. The deposition of WS2 layer on top of TiSiN layer significantly improved coatings’ tribological performance with lower friction coefficient, less adhesive materials and smaller wear rate of steel ball. Even after WS2 layer was worn out, the friction coefficient was still lower than TiSiN coating, which is attributed to that WS2 remaining in wear track still worked. © 2014 Elsevier B.V. All rights reserved.
1. Introduction For recent years, dry machining are more and more widely studied and applied because the absence of cooling fluids is economic and environmental [1]. However, due to higher pressure and temperate in tool-chip interface in dry machining, cutting tools should have better performance. Thus, tools coated with TiN, ZrN, TiAlN, ZrTiN, etc. have been applied because of their high hardness and wear resistance [2–4]. TiSiN coating, with ultra high hardness, better oxidation resistance and superior wear resistance [5,6], is supposed to be a promising candidate for tool application in dry machining. However, friction tests showed that TiSiN coating had a high friction coefficient to steels [7–9], which may lead adhesion wear and coating’s delamination under large friction force. That is terrible in machining process. MoS2 and WS2 coatings as solid lubricants have effective lubricant performance. But those coatings are so soft (with a hardness of 1.5–5 GPa) [10,11] that if they were used along in dry machining, substrates of tools would be exposed in a short period, which is not much helpful to improve the wear resistance of tools.
Kenneth Holmberg and Allan Matthews [12] suggested that coatings with a soft/lubricant layer on a hard layer can combine the lubricant effect of soft coatings and the high hardness and wear resistance of hard coatings. Some researches have investigated these coatings, like TiAlN–WC/C and TiN–MoS2 /Ti. Results showed that tools with these hard-lubricant coatings had much better performance [13–15]. But more information of these coatings need be provided so as to expand their industrial applications. In this study, WS2 coating as the soft layer was deposited on TiSiN hard coatings, which can combine advantages of these coatings and solve the high friction coefficient of TiSiN coating and low wear resistance of WS2 coating. Arc ion plating (AIP) and medium-frequency (MF) magnetron sputtering technology were used as the deposition method, and WC/TiC/Co cemented carbides were employed for the substrates. The influence of various thickness of WS2 layer on coating’s microstructure, hardness and adhesive force were examined. Sliding wear tests against hardened steel were carried out with a ball-on-disk tribometer to investigate TiSiN–WS2 coating’s tribological properties. Results can provide useful information for industrial application of this coating. 2. Experimental
∗ Corresponding author at: Department of Mechanical Engineering, Shandong University, No. 17923 Jingshi Road, 250061 Jinan, Shandong Province, PR China. Tel.: +86 531 88399769. E-mail addresses: [email protected] (S. Li), [email protected] (J. Deng).
2.1. Deposition Hard-lubricant coatings were deposited on WC/TiC/Co cemented carbides (fined grains) using the combination of AIP
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Table 1 Multilayer coatings with different thicknesses of WS2 layers. Coatings
TiSiN (m)
TW0.5 (m)
TW1 (m)
TW1.5 (m)
TW2 (m)
Thickness of WS2 layer Thickness of TiSiN layer
0
0.5
1.0 2
1.5
2.0
Dry reciprocating friction tests were carried out by a ball-ondisk tribometer (UMT-2, USA) in ambient air. The ball was made of 45 hardened steel with a diameter of 9.525 mm and a hardness of HRC 55–60. The specimen was fixed while the ball was reciprocating and sliding with a speed of 10 mm/s. The length of wear trace was 8 mm. Every test lasted for 60 min. A normal load of 25 N was applied in the tests. The friction coefficient was recorded by the tribometer’s computer. The morphology of worn regions was examined by WLI and SEM. 3. Results Fig. 1. The structure of TiSiN–WS2 hard-lubricant coating.
method and MF magnetron sputtering technology. The substrates were polished and cleaned ultrasonically in ethanol and acetone progressively, then placed vertically on the rotational sample holder. The base pressure of the chamber was below 7.0 × 10−3 Pa and the temperature was fixed at 200 ◦ C. Before deposition, the substrates were cleaned again by ion bombardment for 15 min with a bias voltage of −800 V under the Ar atmosphere of 1.5 Pa. Five kinds of coatings were prepared and named according to different thickness of WS2 layer, as shown as in Table 1. Fig. 1 shows the coating’s structure. Coatings with TiSiN and WS2 layers were deposited with two steps: firstly, TiSiN single layer was prepared on substrate using one Ti arc target and two Si magnetron sputtering targets; then WS2 layer was deposited on TiSiN coating with two WS2 magnetron sputtering targets. The detailed deposition parameters are listed in Table 2. By controlling the deposition time, coatings with various thicknesses of WS2 layer were obtained. 2.2. Characterization The surface morphology and cross-sectional micrographs of coatings were observed by a scanning electron microscope (SEM, Oxford INCA Penta FETX3). Chemical composition of coatings was analyzed by an energy dispersive X-ray detector (EPS). The microstructure was studied by X-ray diffraction (XRD) using a D8 ADVANCE (Bruker AXS) diffractometer with Cu K˛ radiation. White light interferometer (WLI, Wyko NT9300) was used to measure thickness of the coatings by preparing a small area on substrates where coatings would not be deposited. The microhardness measurement was conducted on an MH-6 microhardness tester using various loading: 10 g, 25 g, 50 g and 100 g. Scratch tests were conducted on a MFT-3000 device to evaluate adhesive strength of coatings. Table 2 Deposition parameters of different layers. Parameters
TiSiN layer
WS2 layer
Base pressure (Pa) Working pressure (Pa) Nitrogen flowrate (sccm) Ti arc target current (A) Si sputter targets current (A) WS2 sputtering targets current (A) Deposition temperature (◦ C) Bias voltage (V) Deposition time (min)
7.0 × 10−3 0.6 180 65 4.0 – 200 −80 150
7.0 × 10−3 0.5 – – – 1.0 200 −100 0, 35, 70,105, 140
3.1. Microstructures The chemical composition of TiSiN and WS2 layer analyzed by EDS was listed in Table 3. Si content of TiSiN layer was 8.0 at.%. For WS2 layer, the ratio of S/W was about 1.5, which was lower than the stoichiometric ratio of 2.0 in WS2 target. Two factors can induce the deficiency of S element, one was the different sputtering efficiency between S and W atoms, another one was that S element could be pumped out from the chamber after reacting with the residual oxygen or hydrogen [16,17]. The surface morphology and cross-section graphs of TW1.5 coating were shown in Fig. 2. It is shown that coating surface was uniform and no delamination occurred at the interface of TiSiN/substrate or WS2 /TiSiN. The thickness of TiSiN layer was about 2 m and WS2 layer was about 1.5 m. The thickness errors of TiSiN layer and WS2 layer were 0.1 m and 0.15 m. It also revealed that TiSiN layer was very dense while WS2 exhibited a loose structure. Fig. 3 shows the X-ray diffraction analysis of these coatings. It can be seen that WS2 layer had two crystal orientation planes of (0 0 2) and (1 0 1), and as the thickness of WS2 layer increased, the peak of (1 0 1) plane became stronger while no obvious change occurred for (0 0 2) plane. Only TiN (2 0 0) plane existed in single TiSiN coating except for diffraction peaks from the substrates. However, the deposition of WS2 layer made TiN (2 2 0) plane come out, and the thicker WS2 layer, the stronger the new diffraction peak, which is caused by the annealing effect in the deposition process of WS2 layer. In this study, increasing thickness of WS2 layer meant a longer deposition time of WS2 , so the annealing time for TiSiN layer increased accordingly, which can promote grains growing. As a result, the peak of TiN (2 2 0) became stronger with increasing thickness of WS2 layer. 3.2. Mechanical properties Hardness of coatings measured under various applied load were shown in Fig. 4. It reveals that the thickness of WS2 layer had a larger influence on coating’s hardness. When WS2 layer’s thickness increased from 0.5 m to 1 m, the hardness suffered a large Table 3 The chemical composition of TiSiN and WS2 layers. TiSiN layer
WS2 layer
Ti content (at.%)
Si content (at.%)
N content (at.%)
39.5
8.0
52.5
W content (at.%)
S content (at.%)
S/W
40.2
59.5
1.5
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Fig. 2. The surface morphology and cross-section pictures of TW1.5 coating.
Fig. 3. X-ray diffraction analysis of coatings with variable thickness of WS2 layer.
Fig. 4. Hardness of TiSiN–WS2 coatings at different applied load with variable thickness of WS2 layer.
reduction. As WS2 layer’s thickness further increased, the hardness continued to decrease, but only slightly. Besides, all hard-lubricant coatings hardness increased with increase of applied load in hardness tests. It was because the indentation depth generated on WS2 layer in hardness test was larger than the thickness of WS2 layer. So the test value was affected by the bottom TiSiN layer that had a much larger hardness. As higher load applied, the effect would be enhanced. Thus, at high load the increment value became larger (Fig. 4). While for single TiSiN coating, the influence from the substrate was just the opposite due to the lower hardness of substrate compared to TiSiN coating. Adhesive strength of these coatings was measured by scratch tests. Fig. 5 shows the adhesive strength of coatings with variable thickness of WS2 layer. It can be observed that as thickness of WS2 layer increased to 1 m, the critical load increased from 35 N to 63 N, indicating that WS2 layer on TiSiN coating could largely increased the adhesion strength. With further thickening WS2 layer, no obvious enhancement appeared. The curves of friction force, coefficient and acoustical signal in scratch tests were shown in Fig. 6. For WS2 coating, the curves were smooth and steady, almost no acoustical signal appeared. For TiSiN coating, large fluctuation occurred when coating failure happened, which indicate that WS2 coating was scraped off gradually, while TiSiN coating flaked at the critical load. Fig. 6c illustrates that there
Fig. 5. Adhesive strength of TiSiN–WS2 coatings with variable thickness of WS2 layer.
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Fig. 6. Curves of friction coefficient, friction force and acoustical signal in scratch tests of (a) single WS2 coating, (b), single TiSiN coating, (c) hard-lubricant coating (TW1).
were two stages in scratch process of hard-lubricant coating: the failure of WS2 layer and the failure of TiSiN layer. However, the scratching process of TW1 coating was not just simple combination of these layers. The incorporation of WS2 layer can significantly prolong the period before total failure happened. The cross-sectional curve of the scratch of TW1 coating is shown in Fig. 7, which demonstrates the scratch process more clearly. When the indenter was touching WS2 layer at the beginning, severe plastic deformation happened, consequently the curve descended steeply. Then the stylus rubbed against WS2 layer until which was
Fig. 7. Cross-sectional profile curve of the scratch track of TW1 coating.
scraped off and TiSiN layer was exposed. Subsequently, the stylus directly scratched TiSiN layer for a relatively longer period. After that, the groove depth sharply dropped, indicating that TiSiN layer flaked. In the scratch process of TiSiN layer, the depth went through a gradual declination. It was caused by the wear of TiSiN layer and deformation of the substrate under the applied load. 3.3. Tribological properties 3.3.1. Friction coefficients Dry reciprocating friction tests were carried out to study coating’s friction properties with the load of 25 N and speed of 10 mm/s. For TiSiN coating, loud noise was generated in its friction process, while tests of hard-lubricant coatings were stable. The friction coefficient as a function of sliding time of these coatings was shown in Fig. 8. Obviously, single TiSiN coating had the highest friction coefficient with an average value of 0.71. For TiSiN–WS2 coatings, there were two stages in friction tests. At the beginning, their friction coefficients were ultra low, in the range of 0.06–0.08, which
Fig. 8. Friction coefficient of TiSiN–WS2 coatings with various thickness of WS2 layer at the speed of 10 mm/s and with the load of 25 N.
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Fig. 9. Wear surfaces of steel balls after 60 min friction tests sliding with coating of (a) TiSiN, (b) TW0.5, (c) TW1, (d) TW1.5, (e) TW2 at the speed of 10 mm/s and with the load of 25 N.
was ascribed to that WS2 layer with low shear elasticity can form a transferred layer between friction pairs. With prolonged time, WS2 layer was worn off, the coefficient started to increase and then it tended to be steady at a relatively high value. It is important to point out that after the failure of WS2 layer, the friction coefficient was still much lower than single TiSiN coating’s. With increasing thickness of WS2 layer, the duration of low friction coefficient was extended significantly. TW0.5 coating only lasted for ∼500 s. But when WS2 layer thickened to 1 m, the duration increased to ∼1750 s. The longest duration was about 2600 s for TW2 coating. However, the duration of WS2 layer was not proportional to its thickness. Its growth reduced as WS2 became thicker. It may be related to that large plastic deformation easily occurred on its surface under stress, which can make WS2 be ploughed out of the wear track by the friction counterpart so that part of WS2 layer would not participate in friction process to play the lubricant role. This effect could be enhanced by increasing thickness of WS2 or sharp friction counterpart. So thickening WS2 layer to prolong the low friction stage had limited effects.
3.3.2. Wear rate of steel ball The wear surfaces of steel balls after 60 min dry friction tests were shown in Fig. 9. It is obvious that there were lots of deep scratches on worn surface of steel ball sliding with TiSiN coating (Fig. 9a), but with hard-lubricant coating, the scratches were very slight. At the same time, as WS2 layer became thicker, the wear scar largely decreased. The diameters of wear scar on these balls sliding with coating of TiSiN, TW0.5, TW1, TW1.5 and TW2 were approximately 1.34 mm, 1.14 mm, 0.89 mm, 0.78 mm and 0.71 mm, respectively. And according to the geometrical relationship, the volume loss of steel ball can be calculated with the following
equation [18,19]:
D/2
V= √
D2 −d2 /2
2 D 2
−x
2
dx =
D3 2 D − d2 (2D2 + d2 ) − 12 24 (1)
where D is the diameter of steel ball, d is the diameter of wear scar. Then the wear rate can be obtained by the equation: W=
V FL
(2)
where W represents the wear rate, F and L are the applied load and sliding distance in friction tests. The results are shown in Fig. 10. It reveals that the addition of WS2 layer significantly reduced wear
Fig. 10. Wear rates of steel balls against different coatings at the speed of 10 mm/s and with the load of 25 N.
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Fig. 11. SEM images of wear track on coating of (a) TiSiN, (b) TW0.5, (c) TW1, (d) TW1.5, (e) TW2 after 60 min friction tests at the speed of 10 mm/s and with the load of 25 N.
Fig. 12. 3D WLI images of wear track on coating of (a) TiSiN, (b) TW0.5, (c) TW1, (d) TW1.5, (e) TW2 after 60 min friction tests at the speed of 10 mm/s and with the load of 25 N.
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Table 4 Properties of TiSiN, substrate and diamond stylus. Materials
Young’s modulus (GPa)
Poisson’s ratio
TiSiN Substrate Diamond stylus
457 510 1100
0.23 0.25 0.07
rates of these steel balls. And with the increase of WS2 layer’s thickness, the wear rate decreased. Compared to TiSiN coating, the wear rate of steel ball sliding with TW2 coating was lower by an order of magnitude. 3.3.3. Worn surfaces of coatings. SEM micrographs and 3D WLI images of wear tracks are shown in Figs. 11 and 12, respectively. The width of wear track was generally in agreement with the diameter of wear scar on steel ball (Fig. 9). Because WS2 layer had a low hardness and wear resistance, it was worn out after friction test, leading a deeper wear track for hard-lubricant coating than TiSiN coating (Fig. 12). From these pictures it can be seen that lots of plowing grooves appeared on the wear track of TiSiN coating, indicating that seriously abrasive wear occurred on TiSiN coating. While no obviously grooves appeared on hard-lubricant coating, but adhesion happened, proving that TiSiN layer was not worn. As WS2 layer thickened, adhesion materials decreased gradually. There was almost no Fe adhesion on the wear track of TW2 coating. 4. Discussion 4.1. Adhesive strength The deposition of WS2 layer on TiSiN coating largely increased the adhesive strength (Fig. 5). Figs. 6 and 7 prove that in the scratch process of TiSiN–WS2 coatings, two stages appeared: the stage of rubbing WS2 layer and the stage of scratching TiSiN layer. In other words, WS2 layer delayed the scratching effect for TiSiN layer by the stylus. As a result, the scratch process before the failure of TiSiN layer was prolonged, which can lead to a larger adhesive strength. In addition, in the deposition of WS2 layer, TiSiN layer experienced
Density (kg/m3 ) 3750 11,300 3500
Thickness/diameter (m) 2 100 200
an annealing process. That can reduce the residual stress in TiSiN layer and be beneficial for improving the adhesion between coating and substrate [20,21]. Furthermore, it can be found from Fig. 6b and c that the friction coefficient of TiSiN–WS2 coating in the stage when the stylus scratched on TiSiN layer was lower than single TiSiN coating. That is caused by the residual WS2 in the scratch track. In scratch test, fiction coefficient had a large effect on the stress generated on coatings, thus affecting the critical load [22,23]. To obtain quantitative analysis, the finite element method (FEM) was used to study the effect of friction coefficient on stress and its distribution in scratch tests. Materials properties were listed in Table 4. In order to reduce the calculation time, the model was simplified: a symmetric model was used; only TiSiN coating and cemented carbides comprised the sample (Fig. 13a); the applied load was kept at 35 N when the stylus moved according to the critical load of single TiSiN coating. On the basis of the friction coefficients of TiSiN and TW1 coating in scratch tests, the friction coefficients were set at 0.2 (TiSiN–WS2 coating) and 0.33 (single TiSiN coating). The model after meshing was shown in Fig. 13b. Around the contact area, the mesh was refined. As for the boundary condition for constraint, the bottom face was constrained in all directions. The maximum principle stress was often used to represent the stress distribution in coating’s scratching test [24]. The results of the maximum principle stress were contoured in Fig. 13c and d. It can be observed that the large tensile stress concentration was located behind the stylus. The maximum tensile stress of TiSiN–WS2 coating was about 41% lower than TiSiN coating’s. Reduction of tensile stress can decrease the generation of cracks and hinder coating’s delamination under friction force. Thus, the lubricant effect of residual WS2 of hard-lubricant coatings is helpful to increase the critical load in scratch tests.
Fig. 13. FEM model (a) before meshing, (b) after meshing; the max principal stress distribution of (c) single TiSiN coating, (d) hard-lubricant coating.
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Fig. 14. (a) SEM image of wear track on TW2 coating, (b) EDS analysis of A point marked in (a).
4.2. Tribological performance Compared to TiSiN coating, TiSiN–WS2 coating showed better tribological performance, such as low friction coefficient, less abrasive wear and small wear rate of steel ball. This mainly benefited from the lubricant function of WS2 . However, after WS2 layer was worn out, the friction coefficient was still lower than TiSiN coating (Fig. 6). By the EDS analysis of wear track of TW2 coating and steel ball (Figs. 9f and 11f), WS2 was not found in the middle of wear track but was detected near the edge. It means that WS2 layer still worked around the edge areas. Thus, because of lubricant effect of WS2 in edge area, the friction coefficient of TiSiN–WS2 coating was still lower than TiSiN coating after partial WS2 layer was worn out. In addition, it was found that there were some holes among wear track, as shown as in Fig. 14a In deposition of TiSiN layer, AIP method was employed, which can produce Ti droplets on coating surface. When the droplets fell off, macroholes generated. Fig. 14b shows EDS composition analysis in the hole of Fig. 14a. It can be seen that the holes were filled with WS2 , which would play an important role as lubrication after WS2 layer was worn out. This is another reason for the lower friction coefficient. Fabricating small holes or grooves or textures on TiSiN layer or substrates is an effective way to prolong the lubricant time of WS2 layer and beneficial for the industrial applications of these coatings, which need to be further studied. 5. Conclusions TiSiN–WS2 hard-lubricant coatings were deposited on WC/TiC/Co cemented carbides. The microstructure, mechanical properties and tribological performance were investigated. Through the results, the following conclusions can be obtained: (1) TiSiN–WS2 hard-lubricant coatings show higher adhesive strength in comparison with single TiSiN coating, and its tribological performance is largely improved, with no abrasive wear, lower friction coefficient and smaller wear rate of steel ball. As the thickness of WS2 layer increased, tribological performance was improved much more, but the enhancement in adhesive strength was limited that after the thickness WS2 layer exceeded 1 m, no obvious enhancement appeared. (2) Three reasons can explain the enhancement mechanism of adhesive strength: WS2 layer delayed the scratching effect by diamond stylus for TiSiN layer; residual stress in TiSiN layer
could be reduced in the deposition of WS2 ; WS2 remaining in scratch track reduced the friction coefficient and the tensile stress generated on coatings. (3) The improvement of tribological performance of TiSiN–WS2 coatings is mainly ascribed to the lubricant effect of WS2 layer. Even after WS2 was worn out, the friction coefficient was still lower than TiSiN coating. It is attributed to WS2 that remained near the edge of wear track and in the macroholes of TiSiN layer.
Acknowledgements This work was supported by “the National Natural Science Foundation of China (51375271)”, “the Fundamental Research Funds of Shandong University (2014JC039)”, “the Independent Innovation Foundation of Universities in Jinan (201401226)”.
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