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Microstructure and tribological behavior of stripe patterned Ti0.6Al0.4N thin coatings prepared by masked deposition
SCT-20598; No of Pages 9 Surface & Coatings Technology xxx (2015) xxx–xxx
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Microstructure and tribological behavior of stripe patterned Ti0.6Al0.4N thin coatings prepared by masked deposition Zhe Geng, Tianmin Shao ⁎ State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 23 July 2015 Revised 19 September 2015 Accepted in revised form 24 September 2015 Available online xxxx Keywords: Patterned coating TiAlN Masked deposition Microstructure Friction and wear
a b s t r a c t Stripe patterned Ti0.6Al0.4N thin coatings were prepared by masked deposition. Surface morphology, microstructure and nanohardness of the coatings were characterized and their tribological behavior under the dry friction condition was studied. Effects of the surface patterning on the coating microstructure and tribological behavior were investigated. Results show that the surface patterning of coatings favored the spinodal decomposition of the metastable (Ti,Al)N phase into the Ti- and Al-rich (Ti,Al)N phases because the space between the coating stripes could relieve the coherency strain energy. And as a consequence, the preferred orientation was changed from (200) for the full coating to (111) for the patterned coatings. The grain size of the patterned coatings decreased, which might be due to the low deposition temperature caused by the reduced coating area which suffered the bombardment of the ions. The changes in microstructure resulted in the slight increase of the nanohardness of the patterned coatings. Both the frictional coefficient and wear volume of the patterned coatings were lower than those of the full coating and decreased as the distance between the coating stripes increased. The improved tribological behavior of the surface patterned coatings is related to the modification of coating microstructure and the effect of surface patterns. © 2015 Published by Elsevier B.V.
1. Introduction Ti–Al–N coatings have been widely used in industry as wear resistant coatings, especially for cutting and forming tools due to their high hardness, favorable oxidation resistance and good thermal stability [1–3]. The tribological behavior of Ti–Al–N coatings is closely related to the microstructure, residual stress and mechanical property of the coatings [4]. In recent years, surface patterning technology has emerged to be an effective way to improve tribological behavior of material surfaces by storing wear debris, changing contact conditions between sliding pairs, forming local hydrodynamic lubrication and acting as reservoirs to retain lubricants [5–9]. To take the advantages of hard coatings and the beneficial effects of the surface patterns, a technique called surface patterning of the thin coatings has been developed [10]. Many researches have been conducted to study the tribological properties of patterned coatings [11–16]. It has been found that the pattern parameters such as shape, size and distribution and the working conditions such as temperature, load, sliding speed, lubrication state and so on are important factors influencing the tribological behavior of patterned coatings. ⁎ Corresponding author. E-mail address: [email protected] (T. Shao).
The preparation method is another important factor which influences the performance of patterned coatings. Preparation methods for patterned coatings can be divided into three kinds according to the sequence of preparing the surface pattern and the coating: produce pattern on coating surface, deposit coating on patterned surface and both the coating and the pattern are produced simultaneously. Ding et al. [17] found that grooves on stainless steel substrate surface induce the enhancement of local electric field, resulting in the hard–soft gradient diamond-like carbon (DLC) film structures. This kind of film structure could terminate the cracks and suppress the enlargement of delamination during friction. Amanov et al. [18] found that high-energy laser produced nanocrystalline graphite on textured Si-DLC film, which increased the hardness, elastic modulus and thus improved the tribological characteristics of the film. Masked deposition is a simple patterning coating process which can deposit any material without prior patterning of the substrate or post direct interacting with coatings, and it does not cause damage to the coatings [16,19,20]. However, the effect of surface patterning of coatings prepared by masked deposition on the coating microstructure and tribological behavior is still unclear. Driven by this aspect, the present work aims to study the microstructure and tribological behavior of Ti–Al–N surface patterned thin coatings prepared by masked deposition and investigate the effect of
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Fig. 1. Schematic diagram of preparing patterned coating by masked deposition. Table 1 Deposition parameters of the Ti–Al–N coating by a multi-arc ion deposition system. Deposition process Pretreatment
Ti–Al–N coating deposition
Parameters
Values
Base pressure (Pa) Argon flow rate (sccm) Pulsed bias voltage (V) Time (min) Nitrogen flow rate (sccm) Argon flow rate (sccm) Pulsed bias voltage (V) Arc current (A) Substrate temperature (°C) Time (min)
3.8 × 10−4 180 800 20 180 5 100 60 200 120
surface patterning on the coating microstructure and tribological behavior. 2. Experiment 2.1. Coating deposition Stripe patterned Ti–Al–N coatings were deposited on AISI304 stainless steel plate substrates using a multi-arc ion deposition system. The size of the stainless steel plates is 20 mm × 20 mm × 2 mm and the surface roughness (Ra) is 0.05 μm. The stainless steel substrates were ultrasonically cleaned with acetone, absolute alcohol and deionized
Fig. 2. Surface morphology of the patterned coatings with different spatial distances: (a)W0.6D0.3; (b) W0.6D0.6; (c)W0.6D1.2 and (d) W0.6D1.8.
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water consecutively for 30 min. The substrates were then covered by the stainless steel masks with the hollow width of 0.6 mm and different interspaces of 0.3, 0.6, 1.2 and 1.8 mm [W0.6D0.3, W0.6D0.6, W0.6D1.2 and W0.6D1.8, respectively]. The preparation process for the patterned thin coating by masked deposition can be seen in Fig. 1. Full Ti–Al–N coating without surface pattern was also prepared for comparison. Prior to deposition, the substrates in vacuum chamber were bombarded by argon ions to remove the residual contaminants and to activate surface atoms. A Ti0.5Al0.5 target was used to prepare the Ti–Al–N coatings. The detailed deposition parameters are listed in Table 1.
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2.2. Coating characterization Surface morphology of the stripe patterned Ti–Al–N coatings was examined by a white light interferometer (MicroXAM, ADE Phase Shift) with a measuring area of 3.46 mm × 2.56 mm, and the thickness of the patterned coatings was measured to be approximately 0.6 μm. Micro surface morphology of the Ti–Al–N coatings was studied by an atomic force microscope (AFM, NanoMan VS, Veeco) in tapping mode with a measuring area of 1 μm × 1 μm, and root mean square (RMS) roughness was calculated. Nanohardness and elastic modulus of the
Fig. 3. AFM images of full and patterned coating surfaces: (a) full coating; (b)W0.6D0.3; (c) W0.6D0.6; (d)W0.6D1.2 and (e) W0.6D1.8.
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Table 2 Root mean square (RMS) roughness values of the full and surface patterned coatings. Ti0.6Al0.4N coatings
Full
RMS roughness values (nm) 7.99
W0.6D0.3 W0.6D0.6 W0.6D1.2 W0.6D1.8 4.52
2.20
5.84
3.18
coatings were measured by using a nano-indentation tester (NHT2, CSM Instruments SA). The applied load was 5.0 mN. 20 measurements were conducted and the results were averaged. X-ray diffraction (XRD, Rigaku) analyses with Cu Kα radiation were conducted to determine the coating structure. Two scanning modes were used: (1) grazing incidence mode with glancing incidence angle 1° and scanning speed 4°/min; (2) step-scanned with normal 2θ/θ mode with the step length 0.01° and dwell time 2 s. A transmission electron microscope (TEM, JEOL 2010F) operated at 200 kV was used for analyzing the microstructure of the coatings. TEM specimens for cross-sectional view observations were prepared by a focused ion beam (FIB, TESCAN LYRA3) system. The composition of coatings measured by energy dispersive X-ray spectrometer (EDAX) in the scanning electron microscope (SEM, TESCAN LYRA3) and TEM was ~35 ± 3 at.% for Ti, ~ 24 ± 1 at.% for Al and ~ 41 ± 3 at.% for N. The coating composition will therefore be referred to as Ti0.6Al0.4N.
2.3. Tribological tests Tribological behavior of the stripe patterned Ti0.6Al0.4N coatings was investigated on a UMT-2 tribometer (CETR, USA) at room temperature with a reciprocating cylinder-on-disk line contact mode under dry friction condition. The sliding direction was perpendicular to the stripes. The AISI-52100 stainless steel cylinder with a diameter of 8 mm and a length of 4 mm was employed as counterpart with a new cylinder being used for each test. The load applied on the disk was 1 N and the frequency of the reciprocating motion was 3 Hz with a reciprocating stroke of 3 mm for 10 min. Each tribological test was repeated three times. After each test, the volume loss of the coatings was measured by the white light interferometer. Five single-line traces were made across the worn track. From each trace, an area of material loss caused by wear could be calculated by integration. Then, the traces were integrated along the track midline to yield a measure of volume loss. Morphology and composition of the worn surfaces were investigated by SEM equipped with EDAX. Identification of tribochemical reactions and the chemical structure of the wear products were conducted by Raman spectroscopy using a Raman spectrometer (JY HR800, Horiba). The laser, with a wavelength of 514 nm, was focused on the sample through a 50× objective microscope.
3. Results 3.1. Characterization of the Ti0.6Al0.4N coatings Surface morphologies of the patterned coatings with different spatial distances are shown in Fig. 2. The bright area is the coating surface and the dark area is the steel surface. The coating surface approximately maintained the original macro surface morphology of stainless steel. The practical widths of the coating stripes were larger than the designed values due to the partial deposition in the shadow area between the mask and the substrate. AFM images of the full and the patterned coatings are shown in Fig. 3. Individual big particles ejected from the cathode source can be seen on the coating surfaces. The root mean square (RMS) roughness values of coatings are listed in Table 2. The surface roughness of the patterned coatings is lower than that of the full coating. X-ray diffractograms of the full and the patterned coatings are shown in Fig. 4. Fig. 4a shows that all coatings have a NaCl type face centered cubic (FCC) crystal structure and the full coating presents a preferred growth orientation along (200) while the patterned coatings exhibit a preferred growth orientation along (111). The XRD pattern by longer time acquisition in Fig. 4b shows that around 2θ = 37°, the full coating exhibits one single symmetric peak while the patterned coatings exhibit an asymmetric peak or even two distinct peaks (W0.6D0.3 coating), which could be interpreted as a sign of the spinodal decomposition of metastable (Ti,Al)N phase into the Ti- and Al-rich (Ti,Al)N. The surface patterning could decrease the residual stress of coating as compared with that of the full coating [3,21], which may favor the spinodal decomposition of Ti0.6Al0.4N coating. However, for the W0.6D0.6, W0.6D1.2 and W0.6D1.8 coatings, the two peak feature was not obvious. As the distance between the coating stripes increases, the FWHM (full width at half maximum) values of the (111) peak increases, which indicates that the grain size of the coating gradually decreases. The reason might be that as the distance between coating stripes increased, the local deposition condition such as temperature was changed, which led to the change in coating microstructure. To study the microstructures of the coatings in detail, the crosssectional view samples were prepared for TEM observation. Fig. 5a and b show that both the full coating and W0.6D1.8 coating have a dense and fine columnar structure. The full coating has a wide column width distribution with the average column width of 31.3 nm (Fig. 5c). The W0.6D1.8 coating has a narrow column width distribution with the average column width of 23.6 nm (Fig. 5d), indicating that the columnar grain size of the patterned coating is smaller and tends to be more consistent than the full coating. The selected area electron diffraction (SAED) patterns for the full (Fig. 5e) and the patterned (Fig. 5f)
Fig. 4. X-ray diffractograms of the full and the patterned coatings: (a) grazing incidence mode; (b) step-scanned with normal 2θ/θ mode.
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Fig. 5. TEM cross sections, statistical distribution of column width and the SAED patterns: (a, c and e) for the full coating, (b, d and f) for W0.6D1.8 coating. The numbered diffractions were indexed as seen in Table 3.
coatings show that both coatings have a FCC crystal structure. The diffraction rings of the W0.6D1.8 coating are more continuous than the full coating indicating that its grain size is smaller, which is consistent with the XRD result of Fig. 4. The inter-planar d-spacings of the W0.6D1.8 coating calculated from the diffraction patterns are slightly larger than those of the full coating (Table 3), which might be related to the phase transformation and the decreased compressive residual stress of the patterned coating.
3.2. Mechanical properties of the Ti0.6Al0.4N coatings Nanohardness and elastic modulus of the full and the patterned coatings are shown in Fig. 6. As compared with the full coating, nanohardness of W0.6D0.3 coating keeps stable while nanohardness of the patterned coatings with spatial distance between 0.6 and 1.8 mm is slightly higher. The reduction of the compressive residual stress generally corresponds to the decrease of coating hardness
Table 3 Indexing of the rings in diffraction patterns in Fig. 5e and f with the calculated inter-planar d-spacings. No. in Fig. 5 Full d (nm) W0.6D1.8 Indices of the plane
1
2
3
4
5
6
7
8
9
10
0.245 0.251 (111)
0.211 0.218 (200)
0.150 0.155 (220)
0.129 0.133 (311)
0.123 0.127 (222)
0.106 0.110 (400)
0.098 – (331)
0.095 0.098 (420)
0.087 0.090 (422)
0.081 – (511)
–: d-spacing could not be obtained due to the unclear diffraction ring.
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Fig. 6. Nanohardness and elastic modulus of the full and the patterned coatings.
[1,3,22]. However, the grain refinement, the (111) preferred orientation [23] and the products of spinodal decomposition [24] could lead to the increase of hardness. Eventually, a stable even higher hardness was achieved for the patterned coatings. 3.3. Frictional coefficient and wear volume of the Ti0.6Al0.4N coatings The frictional coefficient of the Ti0.6Al0.4N coatings and the pure stainless steel is shown in Fig. 7a. Under the dry friction condition, the frictional coefficient of both the full coating and the pure stainless steel are higher than that of the patterned coatings. Frictional coefficient of the patterned coatings demonstrated a tendency to decrease as the spatial distance increased between the coating stripes. The lowest frictional coefficient was achieved for the W0.6D1.8 coating. Fig. 7b shows the wear volume of the Ti0.6Al0.4N coatings and the pure stainless steel. The volume losses of the patterned Ti0.6Al0.4N coatings are much smaller than those of the full coating and the pure stainless steel. As the spatial distance between the coating stripes increased above 0.6 mm, the volume loss of the patterned coatings obviously decreased and achieved the lowest value for the W0.6D1.8 coating. 3.4. Morphology and phase of the worn surfaces The morphology of the worn surfaces of the full coating, the patterned coatings and the pure stainless steel were examined by SEM (Fig. 8). After the tribotest, wide and deep grooves, some craters and
much iron debris were seen on the worn surface of the full coating (Fig. 8a). The worn surface of the pure stainless steel exhibited severe plastic deformation and deep grooves (Fig. 8b). The material in the dark area was iron oxides, as validated by EDAX. Both the W0.6D0.3 coating and the substrate between coating stripes suffered plowing whereas it was much less severe in the full coating and the pure stainless steel (Fig. 8c and d). The grooves became fine but the quantity was large for the worn surface of W0.6D0.6 coating (Fig. 8e). No obvious grooves and only a little wear debris were seen on the worn surface of the substrate between the coating stripes of the W0.6D0.6 coating (Fig. 8f). The worn surface of the W0.6D1.2 coating exhibited some very fine and shallow grooves and the substrate suffered slight scrapes (Fig. 8g and h). The iron oxides started to present on the worn surface of coating and substrate which were much less for the W0.6D0.3 and W0.6D0.6 coatings. The worn surface of the W0.6D1.8 coating suffered little damage and was adhered to by many dark materials mainly composed of iron oxides (Fig. 8i). The substrate between the coating stripes accommodated much accumulated iron oxides (Fig. 8j). Raman spectroscopy was used to investigate the phase structure of wear products and the results are shown in Fig. 9. More iron oxides formed on the worn surface of the patterned coatings than on the full coating. The peak around 670 cm−1 of the W0.6D1.2 and W0.6D1.8 coatings becomes broader and the γ-Fe2O3 peak at 715 cm−1 is more pronounced, which indicates that the tribochemical reaction was more intense during the tribological process of the W0.6D1.2 and W0.6D1.8 coatings. 4. Discussions 4.1. Effect of surface patterning on the microstructure of the coatings For the Ti0.6Al0.4N coating, the metastable (Ti,Al)N phase tends to decompose into the Ti- and Al-rich (Ti,Al)N phases by the alleged spinodal decomposition process [25–27]. This process could cause the elastic coherency strain energy, which increases the free energy of the coating system [28]. The spinodal decomposition was inhibited in the full Ti0.6Al0.4N coating since the coating has relatively high residual stress and could hardly accommodate the extra lattice strain. However, the gaps between the coating stripes in the patterned coatings provided space for the release of the coherency strain energy, and the spinodal decomposition easily occurred (Fig. 4), which could increase the thermal stability of the coating [25,28]. As a consequence, the preferential orientation has changed from (200) for the full coating to (111) for the patterned coatings which has the lowest strain energy in NaCl type FCC crystal structure [29]. Furthermore, during the deposition of the patterned coatings, as the distance between the coating stripes
Fig. 7. Frictional coefficient (a) and wear volume (b) of the full, the patterned Ti0.6Al0.4N coatings and the pure stainless steel.
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Fig. 8. Morphology of the worn surfaces after tribotest: (a) full coating; (b) stainless steel; (c) W0.6D0.3 coating; (d) substrate between the stripes of W0.6D0.3 coating; (e) W0.6D0.6 coating; (f) substrate between the stripes of W0.6D0.6 coating; (g) W0.6D1.2 coating; (h) substrate between the stripes of W0.6D1.2 coating; (i) W0.6D1.8 coating; (j) substrate between the stripes of W0.6D1.8 coating.
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Fig. 9. Raman spectra of the adhered material on the worn surface after tribotest: (a) coated areas; (b) uncoated areas between coating stripes.
increased, the area of the coating which suffered the bombardment of the ions decreased, which led to the decrease of the deposition temperature. This might be the main reason for the smaller column grain size of the patterned coatings compared to that of the full coating (Figs. 4 and 5). 4.2. Effect of surface patterning on the tribological behavior of the coatings According to the test results, the relatively small column grain size, (111) preferred orientation and spinodal decomposition were achieved for the patterned coatings. The grain refinement could increase the coating hardness due to the classic Hall–Petch effect without decreasing the toughness of the coating. The (111) texture has more compact and perfect crystallites than the (200) texture and is beneficial to increase the coating hardness [23]. Moreover, the formation of the Ti- and Al-rich (Ti,Al)N phases could also have the hardening effects on the coating due to the formation of the coherent cubic-phase nanometer-size domains [24,28]. As a result, the strengthening of the coatings was achieved. This is one of the main reasons for the improved tribological behavior of the patterned coatings. Residual stress is another factor influencing the tribological behavior of the coatings. For the full coating, the residual stress is relatively high and could cause the partial spallation of the coating [30], leaving craters on the worn surface (Fig. 8a). For the patterned coatings, the clearance between the coating stripes provides space for the stress relaxation, resulting in the reduction of the residual stress [21,31], which decreased the occurrence of the cracking and peeling off of the coating during the tribological process (Fig. 8c, e, g and i). The space between the coating stripes is also important in influencing the tribological behavior of coatings. On the one hand, the existence of the space relieved the combined stress (residual stress, contact stress and friction induced stress), which was helpful to reduce the extent of coating damage. On the other hand, the space could accommodate the wear debris expelled from the contact interface (Fig. 8h and j), which avoided the abrasive wear. Thus, the surface patterned Ti0.6Al0.4N coating could effectively reduce friction and wear as compared with the full coating. 5. Conclusions (1) Surface patterning of Ti0.6Al0.4N coating favors the spinodal decomposition of the metastable (Ti,Al)N phase into the Ti- and Al-rich (Ti,Al)N phases since the space between the coating stripes could release the coherency strain energy. And as a consequence, the preferred orientation of the grains changed from (200) for the full coating to (111) for the patterned coatings.
(2) The column grain size of the patterned coatings tends to decrease as the distance between the coating stripes increases, which might be due to the low deposition temperature induced by the decreased coating area which suffered the bombardment of the ions. (3) Compared to the full Ti0.6Al0.4N coating, the patterned coatings demonstrate lower friction coefficient and wear.
Acknowledgments The authors thank the State Key Basic Research Program (Grant no. 2012CB934101) and the National Natural Science Foundation of China (Grant no. 51321092) for the financial support, Dr. Hongfei Shang for depositing coatings corporately, Ms. Huihua Zhou in Beijing Electron Microscope Center at Tsinghua University for TEM observation, and Ms. Rong Wang for TEM sample preparation. References [1] S.P. Pemmasani, K. Valleti, R.C. Gundakaram, K.V. Rajulapati, R. Mantripragada, S. Koppoju, S.V. Joshi, Effect of microstructure and phase constitution on mechanical properties of Ti1–xAlxN coatings, Appl. Surf. Sci. 313 (2014) 936–946. [2] D.K. Li, J.F. Chen, C.W. Zou, J.H. Ma, P.F. Li, Y. Li, Effects of Al concentrations on the microstructure and mechanical properties of Ti–Al–N films deposited by RF-ICPIS enhanced magnetron sputtering, J. Alloys Compd. 609 (2014) 239–243. [3] A.E. Santana, A. Karimi, V.H. Derflinger, A. Schutze, Thermal treatment effects on microstructure and mechanical properties of TiAlN thin films, Tribol. Lett. 17 (2004) 689–696. [4] P.H. Mayrhofer, C. Mitterer, L. Hultman, H. Clemens, Microstructural design of hard coatings, Prog. Mater. Sci. 51 (2006) 1032–1114. [5] S. Schreck, K.H. Zum Gahr, Laser-assisted structuring of ceramic and steel surfaces for improving tribological properties, Appl. Surf. Sci. 247 (2005) 616–622. [6] A. Borghi, E. Gualtieri, D. Marchetto, L. Moretti, S. Valeri, Tribological effects of surface texturing on nitriding steel for high-performance engine applications, Wear 265 (2008) 1046–1051. [7] I. Etsion, Improving tribological performance of mechanical components by laser surface texturing, Tribol. Lett. 17 (2004) 733–737. [8] I. Etsion, Modeling of surface texturing in hydrodynamic lubrication, Friction 1 (2013) 195–209. [9] A. Erdemir, Review of engineered tribological interfaces for improved boundary lubrication, Tribol. Int. 38 (2005) 249–256. [10] T.M. Shao, Z. Geng, Research progress in patterned thin solid film techniques and their tribological performance (in Chinese), China Surf. Eng. 28 (2015) 1–26. [11] B.S. Kim, W.Y. Chung, M.H. Rhee, S.Y. Lee, Studies on the application of laser surface texturing to improve the tribological performance of AlCrSiN-coated surfaces, Met. Mater. Int. 18 (2012) 1023–1027. [12] R. Bandorf, D.M. Paulkowski, K.I. Schiffmann, R.L.A. Kuster, Tribological improvement of moving microparts by application of thin films and micropatterning, J. Phys. Condens. Matter 20 (2008) 354018. [13] C. Chouquet, J. Gavillet, C. Ducros, F. Sanchette, Effect of DLC surface texturing on friction and wear during lubricated sliding, Mater. Chem. Phys. 123 (2010) 367–371. [14] J.V. Pimentel, M. Danek, T. Polcar, A. Cavaleiro, Effect of rough surface patterning on the tribology of W–S–C–Cr self-lubricant coatings, Tribol. Int. 69 (2014) 77–83.
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Microstructure and tribological behavior of stripe patterned Ti0.6Al0.4N thin coatings prepared by masked deposition Zhe Geng, Tianmin Shao ⁎ State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 23 July 2015 Revised 19 September 2015 Accepted in revised form 24 September 2015 Available online xxxx Keywords: Patterned coating TiAlN Masked deposition Microstructure Friction and wear
a b s t r a c t Stripe patterned Ti0.6Al0.4N thin coatings were prepared by masked deposition. Surface morphology, microstructure and nanohardness of the coatings were characterized and their tribological behavior under the dry friction condition was studied. Effects of the surface patterning on the coating microstructure and tribological behavior were investigated. Results show that the surface patterning of coatings favored the spinodal decomposition of the metastable (Ti,Al)N phase into the Ti- and Al-rich (Ti,Al)N phases because the space between the coating stripes could relieve the coherency strain energy. And as a consequence, the preferred orientation was changed from (200) for the full coating to (111) for the patterned coatings. The grain size of the patterned coatings decreased, which might be due to the low deposition temperature caused by the reduced coating area which suffered the bombardment of the ions. The changes in microstructure resulted in the slight increase of the nanohardness of the patterned coatings. Both the frictional coefficient and wear volume of the patterned coatings were lower than those of the full coating and decreased as the distance between the coating stripes increased. The improved tribological behavior of the surface patterned coatings is related to the modification of coating microstructure and the effect of surface patterns. © 2015 Published by Elsevier B.V.
1. Introduction Ti–Al–N coatings have been widely used in industry as wear resistant coatings, especially for cutting and forming tools due to their high hardness, favorable oxidation resistance and good thermal stability [1–3]. The tribological behavior of Ti–Al–N coatings is closely related to the microstructure, residual stress and mechanical property of the coatings [4]. In recent years, surface patterning technology has emerged to be an effective way to improve tribological behavior of material surfaces by storing wear debris, changing contact conditions between sliding pairs, forming local hydrodynamic lubrication and acting as reservoirs to retain lubricants [5–9]. To take the advantages of hard coatings and the beneficial effects of the surface patterns, a technique called surface patterning of the thin coatings has been developed [10]. Many researches have been conducted to study the tribological properties of patterned coatings [11–16]. It has been found that the pattern parameters such as shape, size and distribution and the working conditions such as temperature, load, sliding speed, lubrication state and so on are important factors influencing the tribological behavior of patterned coatings. ⁎ Corresponding author. E-mail address: [email protected] (T. Shao).
The preparation method is another important factor which influences the performance of patterned coatings. Preparation methods for patterned coatings can be divided into three kinds according to the sequence of preparing the surface pattern and the coating: produce pattern on coating surface, deposit coating on patterned surface and both the coating and the pattern are produced simultaneously. Ding et al. [17] found that grooves on stainless steel substrate surface induce the enhancement of local electric field, resulting in the hard–soft gradient diamond-like carbon (DLC) film structures. This kind of film structure could terminate the cracks and suppress the enlargement of delamination during friction. Amanov et al. [18] found that high-energy laser produced nanocrystalline graphite on textured Si-DLC film, which increased the hardness, elastic modulus and thus improved the tribological characteristics of the film. Masked deposition is a simple patterning coating process which can deposit any material without prior patterning of the substrate or post direct interacting with coatings, and it does not cause damage to the coatings [16,19,20]. However, the effect of surface patterning of coatings prepared by masked deposition on the coating microstructure and tribological behavior is still unclear. Driven by this aspect, the present work aims to study the microstructure and tribological behavior of Ti–Al–N surface patterned thin coatings prepared by masked deposition and investigate the effect of
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Fig. 1. Schematic diagram of preparing patterned coating by masked deposition. Table 1 Deposition parameters of the Ti–Al–N coating by a multi-arc ion deposition system. Deposition process Pretreatment
Ti–Al–N coating deposition
Parameters
Values
Base pressure (Pa) Argon flow rate (sccm) Pulsed bias voltage (V) Time (min) Nitrogen flow rate (sccm) Argon flow rate (sccm) Pulsed bias voltage (V) Arc current (A) Substrate temperature (°C) Time (min)
3.8 × 10−4 180 800 20 180 5 100 60 200 120
surface patterning on the coating microstructure and tribological behavior. 2. Experiment 2.1. Coating deposition Stripe patterned Ti–Al–N coatings were deposited on AISI304 stainless steel plate substrates using a multi-arc ion deposition system. The size of the stainless steel plates is 20 mm × 20 mm × 2 mm and the surface roughness (Ra) is 0.05 μm. The stainless steel substrates were ultrasonically cleaned with acetone, absolute alcohol and deionized
Fig. 2. Surface morphology of the patterned coatings with different spatial distances: (a)W0.6D0.3; (b) W0.6D0.6; (c)W0.6D1.2 and (d) W0.6D1.8.
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water consecutively for 30 min. The substrates were then covered by the stainless steel masks with the hollow width of 0.6 mm and different interspaces of 0.3, 0.6, 1.2 and 1.8 mm [W0.6D0.3, W0.6D0.6, W0.6D1.2 and W0.6D1.8, respectively]. The preparation process for the patterned thin coating by masked deposition can be seen in Fig. 1. Full Ti–Al–N coating without surface pattern was also prepared for comparison. Prior to deposition, the substrates in vacuum chamber were bombarded by argon ions to remove the residual contaminants and to activate surface atoms. A Ti0.5Al0.5 target was used to prepare the Ti–Al–N coatings. The detailed deposition parameters are listed in Table 1.
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2.2. Coating characterization Surface morphology of the stripe patterned Ti–Al–N coatings was examined by a white light interferometer (MicroXAM, ADE Phase Shift) with a measuring area of 3.46 mm × 2.56 mm, and the thickness of the patterned coatings was measured to be approximately 0.6 μm. Micro surface morphology of the Ti–Al–N coatings was studied by an atomic force microscope (AFM, NanoMan VS, Veeco) in tapping mode with a measuring area of 1 μm × 1 μm, and root mean square (RMS) roughness was calculated. Nanohardness and elastic modulus of the
Fig. 3. AFM images of full and patterned coating surfaces: (a) full coating; (b)W0.6D0.3; (c) W0.6D0.6; (d)W0.6D1.2 and (e) W0.6D1.8.
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Table 2 Root mean square (RMS) roughness values of the full and surface patterned coatings. Ti0.6Al0.4N coatings
Full
RMS roughness values (nm) 7.99
W0.6D0.3 W0.6D0.6 W0.6D1.2 W0.6D1.8 4.52
2.20
5.84
3.18
coatings were measured by using a nano-indentation tester (NHT2, CSM Instruments SA). The applied load was 5.0 mN. 20 measurements were conducted and the results were averaged. X-ray diffraction (XRD, Rigaku) analyses with Cu Kα radiation were conducted to determine the coating structure. Two scanning modes were used: (1) grazing incidence mode with glancing incidence angle 1° and scanning speed 4°/min; (2) step-scanned with normal 2θ/θ mode with the step length 0.01° and dwell time 2 s. A transmission electron microscope (TEM, JEOL 2010F) operated at 200 kV was used for analyzing the microstructure of the coatings. TEM specimens for cross-sectional view observations were prepared by a focused ion beam (FIB, TESCAN LYRA3) system. The composition of coatings measured by energy dispersive X-ray spectrometer (EDAX) in the scanning electron microscope (SEM, TESCAN LYRA3) and TEM was ~35 ± 3 at.% for Ti, ~ 24 ± 1 at.% for Al and ~ 41 ± 3 at.% for N. The coating composition will therefore be referred to as Ti0.6Al0.4N.
2.3. Tribological tests Tribological behavior of the stripe patterned Ti0.6Al0.4N coatings was investigated on a UMT-2 tribometer (CETR, USA) at room temperature with a reciprocating cylinder-on-disk line contact mode under dry friction condition. The sliding direction was perpendicular to the stripes. The AISI-52100 stainless steel cylinder with a diameter of 8 mm and a length of 4 mm was employed as counterpart with a new cylinder being used for each test. The load applied on the disk was 1 N and the frequency of the reciprocating motion was 3 Hz with a reciprocating stroke of 3 mm for 10 min. Each tribological test was repeated three times. After each test, the volume loss of the coatings was measured by the white light interferometer. Five single-line traces were made across the worn track. From each trace, an area of material loss caused by wear could be calculated by integration. Then, the traces were integrated along the track midline to yield a measure of volume loss. Morphology and composition of the worn surfaces were investigated by SEM equipped with EDAX. Identification of tribochemical reactions and the chemical structure of the wear products were conducted by Raman spectroscopy using a Raman spectrometer (JY HR800, Horiba). The laser, with a wavelength of 514 nm, was focused on the sample through a 50× objective microscope.
3. Results 3.1. Characterization of the Ti0.6Al0.4N coatings Surface morphologies of the patterned coatings with different spatial distances are shown in Fig. 2. The bright area is the coating surface and the dark area is the steel surface. The coating surface approximately maintained the original macro surface morphology of stainless steel. The practical widths of the coating stripes were larger than the designed values due to the partial deposition in the shadow area between the mask and the substrate. AFM images of the full and the patterned coatings are shown in Fig. 3. Individual big particles ejected from the cathode source can be seen on the coating surfaces. The root mean square (RMS) roughness values of coatings are listed in Table 2. The surface roughness of the patterned coatings is lower than that of the full coating. X-ray diffractograms of the full and the patterned coatings are shown in Fig. 4. Fig. 4a shows that all coatings have a NaCl type face centered cubic (FCC) crystal structure and the full coating presents a preferred growth orientation along (200) while the patterned coatings exhibit a preferred growth orientation along (111). The XRD pattern by longer time acquisition in Fig. 4b shows that around 2θ = 37°, the full coating exhibits one single symmetric peak while the patterned coatings exhibit an asymmetric peak or even two distinct peaks (W0.6D0.3 coating), which could be interpreted as a sign of the spinodal decomposition of metastable (Ti,Al)N phase into the Ti- and Al-rich (Ti,Al)N. The surface patterning could decrease the residual stress of coating as compared with that of the full coating [3,21], which may favor the spinodal decomposition of Ti0.6Al0.4N coating. However, for the W0.6D0.6, W0.6D1.2 and W0.6D1.8 coatings, the two peak feature was not obvious. As the distance between the coating stripes increases, the FWHM (full width at half maximum) values of the (111) peak increases, which indicates that the grain size of the coating gradually decreases. The reason might be that as the distance between coating stripes increased, the local deposition condition such as temperature was changed, which led to the change in coating microstructure. To study the microstructures of the coatings in detail, the crosssectional view samples were prepared for TEM observation. Fig. 5a and b show that both the full coating and W0.6D1.8 coating have a dense and fine columnar structure. The full coating has a wide column width distribution with the average column width of 31.3 nm (Fig. 5c). The W0.6D1.8 coating has a narrow column width distribution with the average column width of 23.6 nm (Fig. 5d), indicating that the columnar grain size of the patterned coating is smaller and tends to be more consistent than the full coating. The selected area electron diffraction (SAED) patterns for the full (Fig. 5e) and the patterned (Fig. 5f)
Fig. 4. X-ray diffractograms of the full and the patterned coatings: (a) grazing incidence mode; (b) step-scanned with normal 2θ/θ mode.
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Fig. 5. TEM cross sections, statistical distribution of column width and the SAED patterns: (a, c and e) for the full coating, (b, d and f) for W0.6D1.8 coating. The numbered diffractions were indexed as seen in Table 3.
coatings show that both coatings have a FCC crystal structure. The diffraction rings of the W0.6D1.8 coating are more continuous than the full coating indicating that its grain size is smaller, which is consistent with the XRD result of Fig. 4. The inter-planar d-spacings of the W0.6D1.8 coating calculated from the diffraction patterns are slightly larger than those of the full coating (Table 3), which might be related to the phase transformation and the decreased compressive residual stress of the patterned coating.
3.2. Mechanical properties of the Ti0.6Al0.4N coatings Nanohardness and elastic modulus of the full and the patterned coatings are shown in Fig. 6. As compared with the full coating, nanohardness of W0.6D0.3 coating keeps stable while nanohardness of the patterned coatings with spatial distance between 0.6 and 1.8 mm is slightly higher. The reduction of the compressive residual stress generally corresponds to the decrease of coating hardness
Table 3 Indexing of the rings in diffraction patterns in Fig. 5e and f with the calculated inter-planar d-spacings. No. in Fig. 5 Full d (nm) W0.6D1.8 Indices of the plane
1
2
3
4
5
6
7
8
9
10
0.245 0.251 (111)
0.211 0.218 (200)
0.150 0.155 (220)
0.129 0.133 (311)
0.123 0.127 (222)
0.106 0.110 (400)
0.098 – (331)
0.095 0.098 (420)
0.087 0.090 (422)
0.081 – (511)
–: d-spacing could not be obtained due to the unclear diffraction ring.
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Fig. 6. Nanohardness and elastic modulus of the full and the patterned coatings.
[1,3,22]. However, the grain refinement, the (111) preferred orientation [23] and the products of spinodal decomposition [24] could lead to the increase of hardness. Eventually, a stable even higher hardness was achieved for the patterned coatings. 3.3. Frictional coefficient and wear volume of the Ti0.6Al0.4N coatings The frictional coefficient of the Ti0.6Al0.4N coatings and the pure stainless steel is shown in Fig. 7a. Under the dry friction condition, the frictional coefficient of both the full coating and the pure stainless steel are higher than that of the patterned coatings. Frictional coefficient of the patterned coatings demonstrated a tendency to decrease as the spatial distance increased between the coating stripes. The lowest frictional coefficient was achieved for the W0.6D1.8 coating. Fig. 7b shows the wear volume of the Ti0.6Al0.4N coatings and the pure stainless steel. The volume losses of the patterned Ti0.6Al0.4N coatings are much smaller than those of the full coating and the pure stainless steel. As the spatial distance between the coating stripes increased above 0.6 mm, the volume loss of the patterned coatings obviously decreased and achieved the lowest value for the W0.6D1.8 coating. 3.4. Morphology and phase of the worn surfaces The morphology of the worn surfaces of the full coating, the patterned coatings and the pure stainless steel were examined by SEM (Fig. 8). After the tribotest, wide and deep grooves, some craters and
much iron debris were seen on the worn surface of the full coating (Fig. 8a). The worn surface of the pure stainless steel exhibited severe plastic deformation and deep grooves (Fig. 8b). The material in the dark area was iron oxides, as validated by EDAX. Both the W0.6D0.3 coating and the substrate between coating stripes suffered plowing whereas it was much less severe in the full coating and the pure stainless steel (Fig. 8c and d). The grooves became fine but the quantity was large for the worn surface of W0.6D0.6 coating (Fig. 8e). No obvious grooves and only a little wear debris were seen on the worn surface of the substrate between the coating stripes of the W0.6D0.6 coating (Fig. 8f). The worn surface of the W0.6D1.2 coating exhibited some very fine and shallow grooves and the substrate suffered slight scrapes (Fig. 8g and h). The iron oxides started to present on the worn surface of coating and substrate which were much less for the W0.6D0.3 and W0.6D0.6 coatings. The worn surface of the W0.6D1.8 coating suffered little damage and was adhered to by many dark materials mainly composed of iron oxides (Fig. 8i). The substrate between the coating stripes accommodated much accumulated iron oxides (Fig. 8j). Raman spectroscopy was used to investigate the phase structure of wear products and the results are shown in Fig. 9. More iron oxides formed on the worn surface of the patterned coatings than on the full coating. The peak around 670 cm−1 of the W0.6D1.2 and W0.6D1.8 coatings becomes broader and the γ-Fe2O3 peak at 715 cm−1 is more pronounced, which indicates that the tribochemical reaction was more intense during the tribological process of the W0.6D1.2 and W0.6D1.8 coatings. 4. Discussions 4.1. Effect of surface patterning on the microstructure of the coatings For the Ti0.6Al0.4N coating, the metastable (Ti,Al)N phase tends to decompose into the Ti- and Al-rich (Ti,Al)N phases by the alleged spinodal decomposition process [25–27]. This process could cause the elastic coherency strain energy, which increases the free energy of the coating system [28]. The spinodal decomposition was inhibited in the full Ti0.6Al0.4N coating since the coating has relatively high residual stress and could hardly accommodate the extra lattice strain. However, the gaps between the coating stripes in the patterned coatings provided space for the release of the coherency strain energy, and the spinodal decomposition easily occurred (Fig. 4), which could increase the thermal stability of the coating [25,28]. As a consequence, the preferential orientation has changed from (200) for the full coating to (111) for the patterned coatings which has the lowest strain energy in NaCl type FCC crystal structure [29]. Furthermore, during the deposition of the patterned coatings, as the distance between the coating stripes
Fig. 7. Frictional coefficient (a) and wear volume (b) of the full, the patterned Ti0.6Al0.4N coatings and the pure stainless steel.
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Fig. 8. Morphology of the worn surfaces after tribotest: (a) full coating; (b) stainless steel; (c) W0.6D0.3 coating; (d) substrate between the stripes of W0.6D0.3 coating; (e) W0.6D0.6 coating; (f) substrate between the stripes of W0.6D0.6 coating; (g) W0.6D1.2 coating; (h) substrate between the stripes of W0.6D1.2 coating; (i) W0.6D1.8 coating; (j) substrate between the stripes of W0.6D1.8 coating.
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Fig. 9. Raman spectra of the adhered material on the worn surface after tribotest: (a) coated areas; (b) uncoated areas between coating stripes.
increased, the area of the coating which suffered the bombardment of the ions decreased, which led to the decrease of the deposition temperature. This might be the main reason for the smaller column grain size of the patterned coatings compared to that of the full coating (Figs. 4 and 5). 4.2. Effect of surface patterning on the tribological behavior of the coatings According to the test results, the relatively small column grain size, (111) preferred orientation and spinodal decomposition were achieved for the patterned coatings. The grain refinement could increase the coating hardness due to the classic Hall–Petch effect without decreasing the toughness of the coating. The (111) texture has more compact and perfect crystallites than the (200) texture and is beneficial to increase the coating hardness [23]. Moreover, the formation of the Ti- and Al-rich (Ti,Al)N phases could also have the hardening effects on the coating due to the formation of the coherent cubic-phase nanometer-size domains [24,28]. As a result, the strengthening of the coatings was achieved. This is one of the main reasons for the improved tribological behavior of the patterned coatings. Residual stress is another factor influencing the tribological behavior of the coatings. For the full coating, the residual stress is relatively high and could cause the partial spallation of the coating [30], leaving craters on the worn surface (Fig. 8a). For the patterned coatings, the clearance between the coating stripes provides space for the stress relaxation, resulting in the reduction of the residual stress [21,31], which decreased the occurrence of the cracking and peeling off of the coating during the tribological process (Fig. 8c, e, g and i). The space between the coating stripes is also important in influencing the tribological behavior of coatings. On the one hand, the existence of the space relieved the combined stress (residual stress, contact stress and friction induced stress), which was helpful to reduce the extent of coating damage. On the other hand, the space could accommodate the wear debris expelled from the contact interface (Fig. 8h and j), which avoided the abrasive wear. Thus, the surface patterned Ti0.6Al0.4N coating could effectively reduce friction and wear as compared with the full coating. 5. Conclusions (1) Surface patterning of Ti0.6Al0.4N coating favors the spinodal decomposition of the metastable (Ti,Al)N phase into the Ti- and Al-rich (Ti,Al)N phases since the space between the coating stripes could release the coherency strain energy. And as a consequence, the preferred orientation of the grains changed from (200) for the full coating to (111) for the patterned coatings.
(2) The column grain size of the patterned coatings tends to decrease as the distance between the coating stripes increases, which might be due to the low deposition temperature induced by the decreased coating area which suffered the bombardment of the ions. (3) Compared to the full Ti0.6Al0.4N coating, the patterned coatings demonstrate lower friction coefficient and wear.
Acknowledgments The authors thank the State Key Basic Research Program (Grant no. 2012CB934101) and the National Natural Science Foundation of China (Grant no. 51321092) for the financial support, Dr. Hongfei Shang for depositing coatings corporately, Ms. Huihua Zhou in Beijing Electron Microscope Center at Tsinghua University for TEM observation, and Ms. Rong Wang for TEM sample preparation. References [1] S.P. Pemmasani, K. Valleti, R.C. Gundakaram, K.V. Rajulapati, R. Mantripragada, S. Koppoju, S.V. Joshi, Effect of microstructure and phase constitution on mechanical properties of Ti1–xAlxN coatings, Appl. Surf. Sci. 313 (2014) 936–946. [2] D.K. Li, J.F. Chen, C.W. Zou, J.H. Ma, P.F. Li, Y. Li, Effects of Al concentrations on the microstructure and mechanical properties of Ti–Al–N films deposited by RF-ICPIS enhanced magnetron sputtering, J. Alloys Compd. 609 (2014) 239–243. [3] A.E. Santana, A. Karimi, V.H. Derflinger, A. Schutze, Thermal treatment effects on microstructure and mechanical properties of TiAlN thin films, Tribol. Lett. 17 (2004) 689–696. [4] P.H. Mayrhofer, C. Mitterer, L. Hultman, H. Clemens, Microstructural design of hard coatings, Prog. Mater. Sci. 51 (2006) 1032–1114. [5] S. Schreck, K.H. Zum Gahr, Laser-assisted structuring of ceramic and steel surfaces for improving tribological properties, Appl. Surf. Sci. 247 (2005) 616–622. [6] A. Borghi, E. Gualtieri, D. Marchetto, L. Moretti, S. Valeri, Tribological effects of surface texturing on nitriding steel for high-performance engine applications, Wear 265 (2008) 1046–1051. [7] I. Etsion, Improving tribological performance of mechanical components by laser surface texturing, Tribol. Lett. 17 (2004) 733–737. [8] I. Etsion, Modeling of surface texturing in hydrodynamic lubrication, Friction 1 (2013) 195–209. [9] A. Erdemir, Review of engineered tribological interfaces for improved boundary lubrication, Tribol. Int. 38 (2005) 249–256. [10] T.M. Shao, Z. Geng, Research progress in patterned thin solid film techniques and their tribological performance (in Chinese), China Surf. Eng. 28 (2015) 1–26. [11] B.S. Kim, W.Y. Chung, M.H. Rhee, S.Y. Lee, Studies on the application of laser surface texturing to improve the tribological performance of AlCrSiN-coated surfaces, Met. Mater. Int. 18 (2012) 1023–1027. [12] R. Bandorf, D.M. Paulkowski, K.I. Schiffmann, R.L.A. Kuster, Tribological improvement of moving microparts by application of thin films and micropatterning, J. Phys. Condens. Matter 20 (2008) 354018. [13] C. Chouquet, J. Gavillet, C. Ducros, F. Sanchette, Effect of DLC surface texturing on friction and wear during lubricated sliding, Mater. Chem. Phys. 123 (2010) 367–371. [14] J.V. Pimentel, M. Danek, T. Polcar, A. Cavaleiro, Effect of rough surface patterning on the tribology of W–S–C–Cr self-lubricant coatings, Tribol. Int. 69 (2014) 77–83.
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Please cite this article as: Z. Geng, T. Shao, Surf. Coat. Technol. (2015), http://dx.doi.org/10.1016/j.surfcoat.2015.09.045