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Ti) coating adhesion mechanism on a Ti–13Nb–13Zr alloy
Applied Surface Science 355 (2015) 502–508
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mono- and multiple TiN(/Ti) coating adhesion mechanism on a Ti–13Nb–13Zr alloy Jianzhong Li a,b , Hua Zheng b , Theo Sinkovits b , Ay Ching Hee b , Yue Zhao b,∗ a
School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China School of Mechanical Materials and Electronic Engineering, Faculty of Engineering and Information Science, University of Wollongong, NSW 2522, Australia b
a r t i c l e
i n f o
Article history: Received 30 April 2015 Received in revised form 26 June 2015 Accepted 19 July 2015 Available online 21 July 2015 Keywords: Coating adhesion Heat treatment Monolithic TiN coating Multilayers TiN/Ti Hardness
a b s t r a c t Mono- and multiple TiN(/Ti) coatings deposited on Ti–13Nb–13Zr alloy substrates by the filtered arc deposition system were examined using scratch testing and depth-sensing indentation in terms of the relationship between the coating adhesion, deformation mechanism, and microstructure, and mechanical properties at the film/substrate interface. The results show that multilayer TiN/Ti coatings offer a greater resistance to cracking and delamination than monolithic TiN coatings under the same conditions on the Ti–13Nb–13Zr alloys substrates. And increasing the number of layers for TiN multilayer coating improves the coatings adhesion. In contrast, for the coatings on the Ti–13Nb–13Zr alloys substrates that were heattreated to a higher hardness, the limited deformation in the substrates improved remarkably the coating adhesion indiscriminately. The substrate mechanical properties play the major roles in controlling the coating adhesion, and increasing thickness and layers of the TiN multilayer have a limited improvement to the adhesion of coating. © 2015 Elsevier B.V. All rights reserved.
1. Introduction TiN coatings have been widely used for both mechanical and chemical protection of metallic materials in a range of engineering applications due to their improved wear resistance, hardness, and higher oxidation resistance. These attractive properties have been a driving force for the early introduction to develop TiNbased coatings with higher levels of properties [1–3]. Tilbrook et al. [4] reported micostructural effects on indentation failure mechanisms in TiN coatings by finite element simulations. Romankova et al. [5] studied the effect of annealing treatment on the structure and properties of the nanograined TiN coatings produced by an ultrasonic coating process. With an increasing demand for industrial practice, further investigation of multilayer coatings in the feature of the improved wear resistance, hardness, and higher oxidation resistance with modulation of different phases has attracted much attention in the past few years [6–8]. TiN/Ti multilayer coating is one of the most interesting multilayer coatings. Cheng et al. and Gong et al. [9,10] investigated the mechanical and tribological behavior of the TiN/Ti multilayer coatings deposited by filter cathodic arc. Carvalho et al. [11] and Xie et al. [12] researched
∗ Corresponding author. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.apsusc.2015.07.126 0169-4332/© 2015 Elsevier B.V. All rights reserved.
deformation mechanisms in TiN multilayers under depth-sensing indentation. Wang et al. [13,14] studied corrosion properties and contact resistance of the TiN multilayers in simulated proton exchange membrane fuel cell environments. It has been found that TiN/Ti multilayer coatings can significantly improve the mechanical and tribological properties, corrosion resistance, and fretting fatigue behavior of metal substrates. Despite much research efforts on the development of monoor multilayer coatings with superior properties, limited data have been published in order to justify the adhesion mechanisms of enhancing the properties of the coatings taking into consideration their deformation features. This paper describes the results of investigation to determine the relationship between the coating adhesion mechanisms, deformation mechanisms of the coatings subjected to nanoindentation, microstructure and the surface properties of the substrate with different heat treatment status. To this end, commercial Ti–13Nb–13Zr alloys were chosen as the substrate, which were heat-treated under different schedule in order to achieve different phase status and microstructure. Mono- and multilayer TiN coatings examined in this paper were deposited by means of the Filtered Arc Deposition System (FADS). In the present work, scratch tests were performed by scratching the resultant films with an indenter to characterize the critical load at which failure occurs using a Revetest Xpress Scratch Tester. Nanoindentation experiments have been utilized to obtain mechanical property
J. Li et al. / Applied Surface Science 355 (2015) 502–508
data and an insight into the fracture phenomena of the coating/substrate systems. Important aspects of the contact-induced fracture mechanisms of the coated systems have been revealed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 2. Experimental The coatings examined in this paper were deposited onto Ti–13Zr–13Nb alloy (compositions in wt%) disks of 25 mm in diameter, which are under the two status: a) with heat treatment; b) as received ( solution treated by manufacturer). The heat treatment schedule is as following: 25 min
10 min
60 min
20 min
25◦ C −→ 550◦ C −→ 760◦ C −→ 760◦ C −→ 550◦ C → Air cooled to room temperature In order to make sure the same surface condition for the deposition process, the Ti–13Zr–13Nb alloy substrates were polished using a 320 mesh SiC paper with water as a lubricant for 4 min, and then with a 15 m diamond abrasive with a petroleum-based lubricant for 10 min. The substrates were then fine-polished with a 0.05 m abrasive and a chemical solution consisting of 25 ml of OP–S containing a mixture of 2.5 ml of ammonia and 1.5 ml of hydrogen peroxide for 5 min. These substrates were ultrasonically cleaned in ethanol, distilled water, acetone and dried before being coated in a FADS System. And then coatings of TiN and TiN/Ti were deposited on polished Ti substrates using the FADS System. The deposition chamber was pumped down to a base vacuum of 5.0 × 10−6 Torr. The substrates were pre-heated under vacuum to 300 ◦ C, and then an etching process was carried out under high substrate bias of −850 V using pure titanium ion beam. The bias voltage was reduced to −100 V when the coating process began. Pure-Ti buffer layers were first deposited on the substrates, and then the reactive gas (nitrogen) was introduced into the chamber via a mass flow controller. The N2 mass flow rate was increased gradually from 5 sccm to a stable value of 40 sccm to reduce stress and improve adhesion between the film and the substrate. The chamber pressure went up to and stabilized at 2.9 × 10−3 Torr during deposition. A monolayer of TiN was formed after 2 h deposition. In order to obtain multilayer coating of TiN/Ti, the N2 gas flow was discontinued 9 times during the coating process. The total deposition time was 3.5 h. Scratch tests were performed by scratching the resultant films with a Rockwell C type diamond stylus (cone apex angle 120◦ , tip radius 200 m) to characterize the critical load at which failure occurs in a Revetest Xpress Scratch Tester. The instrument can simultaneously detect the acoustic emission and the friction force. In order to verify the reproducibility of the results, three scratch experiments were conducted on each tested samples. Nanoindentations on the monolayer TiN and multilayer TiN/Ti coatings were made using the Ultra-Micro Indentation System (UMIS). A Berkovich indenter with a 200 nm tip was employed to test the hardness of the coatings. The resolution was limited by thermal drift, which was found to be <1 nm/min. The tip was initially calibrated by the standard material of fused silica. Incremental controlled loading and unloading tests were performed with a range of the maximum loads from 3 mN to 9 mN to determine the hardness of the coatings. A spherical diamond indenter with a 5 m diameter was employed to test the fracture modes of the coatings. A spherical indenter utilized in the fracture study because it has been shown to produce a more uniform stress field beneath the contact area than pyramidal pointed indenters [15]. The tested loads were 360 mN. The loading-unloading rate is 200 N/s for all indentations.
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A FEI XT Nova Nanolab 200 workstation which combines a dual beam Focused Ion Beam (FIB) and a Field Emission Scanning Electron Microscope (FESEM) were used to prepare thin foil samples for TEM observation. Details of the FIB procedure have been given elsewhere [16]. TEM was performed on samples prepared by the FIB, using a Philips CM200 equipped with a Brucker energy dispersive X-ray spectroscopy (EDAX) system. The coating phase composition was determined by X-ray diffraction (XRD) using –2 scan. The measurements were performed with a scan speed of 2◦ /min using Cu K␣. Surface observation was conducted using an SEM equipped with an EDX spectrometer (a JEOL LV). 3. Results and discussion 3.1. Scratch behavior for the different coatings Coating adhesion to the substrate plays a major role in improving the resistance to wear and environmental degradation for tribological applications. Scratch tests were performed by applying a progressive load ramping from 1000 mN to 50,000 mN for a length of 10 mm. Both the acoustic emission (AE) and friction force were recorded during the tests. Fig. 1 Fig. 1 also shows the entire optical image of scratch tracks after the tests. From the monolithic TiN coating on the as received Ti–13Zr–13Nb alloys surface, it can be seen that the acoustic emissions undergo a sudden change at a lower load of 25,800 mN with the obviously increasing friction force, although the acoustic emissions are also found as the load of about 20,000 mN. From the corresponding to optical image, the coating is clearly broken at a load of 25,800 mN, indicating that the monolithic TiN coating appear to have failed. Scratch tests for the TiN/Ti multilayer coating on the as received Ti–13Zr–13Nb alloys surface are presented in Fig. 1(b). As can be seen in the figure, acoustic emission signals show an abrupt increase at 37,500 mN, greater than that of the monolithic TiN coating. The corresponding optical image indicates that the multilayer coating fails at this load. Under the same substrate conditions, the multilayer coating shows greater resistance to the scratch damage than the monolithic coating. For the monolithic TiN coatings deposited on the Ti–13Zr–13Nb alloys with the heat treatment, deposition conditions of which are the same to those on the as received substrate, the scratch tests are shown in Fig. 1(c). It is seen in Fig. 1(c) that the acoustic emission has a sudden change at 48,100 mN, which is the maximum value in all cases. This is because a heat treatment induces the surface microstructure change of Ti–13Zr–13Nb alloys (seen in the third part results in this paper). An abrupt increase in the acoustic emission indicates that the surface layer has been delaminated. This demonstrates that a heat treatment for the Ti alloys substrate is a more effective method for improving the coating adhesion in all the cases tested. 3.2. Deformation of the different coating under indentation In order to determine the relationship between the adhesion mechanisms and deformation of the coatings or substrate, crosssectional overviews of indentation-induced deformation in the monolithic TiN coating and the multilayer TiN coatings with Ti on the as received Ti–13Zr–13Nb alloys surface are shown in Fig. 2 Fig. 2(a)–(f), respectively, with similar images for a monolithic TiN coating on Ti–13Zr–13Nb alloys surface with heat treatment shown for comparison (Fig. 2(g) and (h)). It can be seen that, while intercolumnar shear sliding was significantly reduced for layered structures, different deformation processes appeared to occur to accommodate strain within the three types of TiN coatings. For monolithic TiN coating on the as received substrate, a large
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Fig. 1. Micrograph of failure after scratch tests (a) monolithic TiN coating on as received substrate; (b) multilayers TiN/Ti coatings on as received substrate (c) monolithic TiN coating on heat-treated substrate.
number of large cracks were observed; most cracks were held within the TiN layer adjacent to the substrate in the form of radial and edge cracks; radial cracks also extended into the outer layers; stair-like intergranular shear sliding were evidently observed to occur at the coating/substrate interface (Fig. 2(b)). As to TiN multilayers with titanium interlayers, limited cracking can be seen, within both the TiN layers and the Ti interlayers. Edge cracks appear
to initiate at the coatings/the Ti interlayer interface, most likely due to local stress concentrations. Edge cracks were observed to propagate on a larger scale once activated. Coating deformation appeared to be accommodated primarily by intergranular shear sliding and plastic flow of the titanium interlayers. An intercolumnar shear step can be seen at the Ti interlayer. But stair-like shear could be seen hardly at the substrate interface (Fig. 2(d)–(f)). In contrast,
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Fig. 2. Cross-sectional SEM and TEM images of a 360 mN indent on mono- or multiple TiN(/Ti) coating: (a) and (b) are cross-sectional SEM and TEM of monolithic TiN coating on the as received Ti–13Zr–13Nb alloys surface, respectively; (c), (d), (e) and (f) are cross-sectional SEM and TEM of multiple TiN/Ti coating on the as received Ti–13Zr–13Nb alloys surface, respectively; (g) and (h) are cross-sectional SEM and TEM of monolithic TiN coating on the Ti–13Zr–13Nb alloys surface with heat treatment, respectively.
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for monolithic TiN coating on the substrate with heat treatment, a few cracks were found at the coating/substrate interface; stair-like intergranular shear sliding were also observed to occur evidently at the coating/substrate interface, whose height ‘ht2 ’ is lower compared to ‘ht1 ’ of the coating/substrate interface under the condition of greater magnification in Fig. 2(b). Based upon the observations of deformation mechanisms of layer TiN or TiN/Ti coatings on Ti–13Zr–13Nb alloys with heat treatment or not, two structural factors to influence coatings adhesion, i.e. the multilayer structure and the interlayer, have been identified as playing major roles in controlling intercolumnar sliding processes. The sliding distance depended upon the thickness of the layer to absorb the indentation energy. The energy to indent a multilayer coating (without the presence of the interlayers), Wm-TiN , to a depth ht is therefore [12]: 4√ 3/2 Wm-TiN = 2Tht + T (i − 1)cht 3 4√ 3/2 =T 2ht + (i − 1)cht (1) 3 where is the shear sliding stress, R is the radius of the indenter, T is the total thickness of the coating, ht is the sliding distance of annular columnar crystal and 2c is the width of annular columnar crystal. Notably, the first item in Eq. (1) represents the energy absorption, Wm-TiN , equivalent to that taken by the monolayer TiN, and the second item represents an increase in the energy needed for deforming the coatings, Wshear , due to the increase in intergranular shear area in comparison to the monolayer. Therefore, assuming coating thickness and intergranular shear stress to be constant, an increase in the number of layers would result in an increase in the intergranular shear area, which in turn would result in an increase in the energy absorption during the deformation of the coating. For the interlayers within the coating, it acted as physical barriers resisting intergranular shear sliding. The increase in deformation energy was caused by the presence of interlayers. Now we consider another structural factor, i.e. the substrate mechanical properties, which have a pronounced effect on load–displacement behavior, influencing the effective compliance during both the loading and unloading stages. The plastic deformation energy of the substrate, Wsub , has been determined in a previous study as [12]: Wsub = ARh2t
(2)
where A is a material factor dependent on the mechanical properties of the substrate, about 4.85 × 109 Pa for Ti alloys as reported by Xie et al. [12]. Because there is negligible plastic compression of the TiN coatings, it may be assumed that the coating acts as stiff elastic radial extension of the spherical indenter. The indentation energy of the substrate may, therefore, be obtained via Eq. (2) with an indenter of radius equivalent to the indenter radius plus the coating thickness. So the total indentation energy, Wsystem , may be considered as a combination of the energy contributions from substrate, Wsub , and the coating, Wcoat , which includes the contribution from the multilayer structure, Wm-TiN (i.e. WTiN+ Wshear ), and interlayers, Winterlayer : Wsystem = WTiN + Wshear + Winterlayer + Wsub = Wcoat + Wsub
(3)
Both FESEM and TEM cross-sectional analyses suggested that there was no observable difference in the coating thickness under the indent compared with outside the indent area, implying that predominant plastic flow occurred in the substrates, not in the coatings. In addition, observed via Eqs. (1)–(3), for Wm-TiN , the calculated values of monolithic TiN coatings is lower obviously than that of TiN multilayers under the same conditions for the substrate. So increasing the number of layers for TiN multilayer coating
improves the coatings adhesion. However, T should be of very small value due to its the total thickness of the coating as micrometer orders, and is then fed into Eq. (1) to determine to the small Wm-TiN results. For Wsub , A should be very larger values as more than 109 Pa for Ti alloys, and is determined to Wsub . As a result, we obtain Wsub Wm-TiN . This explains why the limited deformation for the substrates could improve remarkably the coatings adhesion. Based on these results, the we believe that the mechanical properties of the substrate play a major role in controlling the coating adhesion, and increasing thickness and numner of layers of the TiN multilayer contribute limited improvement to the coating adhesion. 3.3. Physical properties of Ti alloys substrate and the coatings From the above nanoindentation results, no delamination was observed at the coating/substrate interface, suggesting good interfacial bonding. In addition, the better coating adhesion should depend also on the fine microstructure and the surface properties of a Ti–13Zr–13Nb alloy substrate. The microstructures of the Ti–13Zr–13Nb alloy substrate before and after heat treatment are shown in Fig. 3 Fig. 3. The Ti alloys substrate specimen with heat treatment exhibits a martensitic microstructure (␣) while the as received substrate specimen shows a basket weave type of microstructure. These structures are typical of  solution-treated titanium alloys. The XRD studies of these specimens (Fig. 3(c)) show the presence of ␣ phase for the heat-treated specimen with a very weak peak of the ␣” phase. The as received specimens reveal the presence ␣ phase with a very weak peak of the  phase. Two different types of martensitic structure i.e. ␣ (hexagonal) and ␣” (orthorhombic) are observed in titanium alloys depending upon the content of  alloying elements. Higher  alloying content favors the formation of ␣” martensitic over ␣ [17]. This suggests the heat treatment induces a transition of  phase to form ␣” phase, contributing to an increase in hardness of the titanium alloys substrate (see Fig. 4(a)). In Fig. 4, hardness values as a function of the displacement normalized to the coating thickness, as well as the substrate, calculated by depth-sensing indentation are shown for the multilayer TiN/Ti, monolithic TiN layer and the Ti–13Zr–13Nb alloy substrate with heat treatment and as received. For the Ti–13Zr–13Nb alloy substrates, the measured hardness decreases with indentation depth as the effects of the softer substrate become more influential. The observed increase in hardness at very low indentation depths is mainly attributable to a continuing increase in elastic strain induced by the spherical-tipped indenter. For penetration depths of the order of a few hundred nanometers the influence of the substrate on the composite hardness values is much reduced. Note that hardness values for the heat treatment substrate are appreciably higher than that of the no-heat treatment. The noticeable scatter between individual measurements should be related to the combination of at least two different factors [18]: (1) different hardness values of the ␣ and  phases, whose contribution would depend on the corresponding ␣-phase/-phase ratio exiting within the tested area in each alloy; and (2) a significant influence of the crystal orientation on the nanoscale-dependent factors. For the coatings in three cases, relatively larger scatter was also observed at very low indentation depths, primarily due to a combination of the surface roughness the coatings. The hardness of the multilayers at low displacements is slightly higher than that of the monolithic layer on the substrate with no-heat treatment. This effect might be related to the higher covalence of the bonds in the TiN layer or to a hardening effect caused by the large number of interface parallel to the substrate surface [11]. By assuming that at an indenter penetration of 200 nm the influence of both tip-ship irregularities and substrate (penetration depth less than onetenth of coating hardness) are negligible, it is possible to estimate
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Fig. 3. SEM and XRD images of a Ti–13Zr–13Nb alloy with heat treatment or not: (a) SEM of the alloy with heat treatment conditions; (b) SEM of the as received alloy; (c) XRD of the alloys (No.1 – as received sample, No.2 – heat treatment sample).
Fig. 4. Hardness as a function of the displacement normalized to the different alloys substrate and the different coatings: (a) a Ti–13Zr–13Nb alloy with different conditions; (b) the different coatings.
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the coating hardness for TiN and TiN/Ti to be of the order of 26 GPa and 31 GPa, respectively. But the hardness values are higher for monolithic TiN layer on the heat-treated substrate than for the two coatings on the as received substrates. This can be explained by that the substrate mechanical properties exert significant influence on the overall indentation deformation behavior. Harder substrates exhibit stronger resistance to ductile deformation, providing more support to the coatings and requiring greater applied loads to break the coatings. 4. Conclusions Using scratch testing and depth-sensing nano-indentation we obtained deepened understanding of coating-substrate adhesion and mechanical behavior for mono- and multiple TiN(/Ti) coatings deposited on a Ti–13Nb–13Zr alloy prepared by filtered arc deposition. The use of load–displacement data and specific procedures for preparation of cross-sections of nanoindentations at the load of 360 mN has been shown to be suitable for the assessment of the coating adhesion mechanism with the substrate microstructure and mechanical properties. On the as received Ti–13Nb–13Zr alloy substrates, multilayer TiN/Ti coatings offer a greater resistance to coating deformation than monolithic TiN coatings through suppression of intercolumnar sliding, inducing a stronger adhesion. The effect of the substrate compliance on the adhesion strength of the coating was investigated by comparing between the heattreated substrates and the as received substrates. The former, stiffer than the latter, leads to an increased elastic strain energy density in the TiN coating, and hence a lower compressive strain. Different deformation mechanisms of the coating and the substrate were examined to determine the total indentation energy (Wsystem ), which includes energy absorption of the coating and deformation energy of the substrate. On the relatively soft as-received substrate, the coating adhesion for the multilayer TiN/Ti coatings is clearly higher than that of monolithic TiN coatings. And increasing the layers for TiN multilayers improves the coatings adhesion. In contrast, for the coatings on the heat-treated hard substrate, the limited deformation by the substrate remarkably improved the coatings adhesion. It is observed that the mechanical properties of the substrate play a major role in determining the coating adhesion. The increase of hardness in the heat-treated Ti–13Nb–13Zr substrate is believed to be due to transition of  phase to form ␣” phase induced by the heat treatment. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51374053), Special Fund for Basic
Scientific Research of Central Colleges (N130502002) and the University of Wollongong. References [1] N.M. Lin, X.B. Huang, X.Y. Zhang, A.L. Fan, L. Qin, B. Tang, In vitro assessments on bacterial adhesion and corrosion performance of TiN coating on Ti6Al4V titanium alloy synthesized by multi-arc ion plating, Appl. Surf. Sci. 258 (2012) 7047–7051. [2] L. Jin, A.R. Riahi, K. Farokhzadeh, A. Edrisy, Investigation on interfacial adhesion of Ti–6Al–4V/nitride coatings, Surf. Coat. Technol. 260 (2014) 155–167. [3] V. Jayaram, S. Bhowmick, Z.-H. Xie, S. Math, M. Hoffman, S.K. Biswas, Contact deformation of TiN coatings on metallic substrates, Mater. Sci. Eng. A 423 (2006) 8–13. [4] M.T. Tilbrool, D.J. Paton, Z.H. Xie, M. Hoffman, Microstructural effects on indentation failure mechanisms in TiN coatings: Finite element simulations, Acta Mater. 55 (2007) 2489–2501. [5] S. Romankov, Y. Hayasaka, N. Hayashi, E. Kasai, S. Komarov, Effect of annealing treatment on the structure and properties of the nanograined TiN coatings produced by ultrasonic-based coating process, J. Alloys Compd. 495 (2010) 625–628. [6] C. Mendibide, P. Steyer, J. Fontaine, P. Goudeau, Improvement of the tribological behaviour of PVD nanostratified TiN/CrN coatings—an explanation, Surf. Coat. Technol. 201 (2006) 4119–4124. [7] K.Y. Liu, J.W. Lee, F.B. Wu, Fabrication and tribological behavior of sputtering TaN coatings, Surf. Coat. Technol. 259 (2014) 123–128. [8] Y. Wang, D.O. Northwood, An investigation of the electrochemical properties of PVDTiN-coated SS410 in simulated PEM fuel cell environments, Int. J. Hydrogen Energy 32 (2007) 895–902. [9] Y.H. Cheng, T. Browne, B. Heckerman, C. Bowman, V. Gorokhovsky, E.I. Meletis, Mechanical and tribological properties of TiN/Ti multilayer coating, Surf. Coat. Technol. 205 (2010) 146–151. [10] H.F. Gong, T.M. Shao, in: J. Luo, Y. Meng, T. Shao, Q. Zhao (Eds.), Influence of Heat Treatment on Hardness, Elastic Modulus and Bonding Strength of Ti/Ta/TiN/TaN Nanomultilayer Coatings, Advanced Tribology, Springer, Berlin Heidelberg, 2010, pp. 808–811. [11] N.J.M. Carvalho, J.Th.M. De Hosson, Deformation mechanisms in TiN/(Ti Al) N multilayers under depth-sensing indentation, Acta Mater. 54 (2006) 1857–1862. [12] Z.H. Xie, M. Hoffman, P. Munroe, A. Bendavid, P.J. Martin, Deformation mechanisms of TiN multilayer coatings alternated by ductile or stiff interlayers, Acta Mater. 56 (2008) 852–861. [13] L. Wang, D.O. Northwood, X. Nie, J. Housden, E. Spain, A. Leyland, A. Matthews, Corrosion properties and contact resistance of TiN, TiAlN and CrN coatings in simulated proton exchange membrane fuel cell environments, J. Power Sources 195 (2010) 3814–3821. [14] N. Madaoui, N. Saoula, B. Zaid, D. Saidi, A. Si Ahmed, Structural, mechanical and electrochemical comparison of TiN and TiCN coatings on XC48 steel substrates in NaCl 3.5% water solution, Appl. Surf. Sci. 312 (2014) 134–138. [15] H. Chai, Transverse fracture in thin-film coatings under spherical indentation, Acta Mater. 53 (2005) 487–498. [16] Q. Zhu, H.T. Zhu, A.K. Tieu, C. Kong, Three dimensional microstructure study of oxide scale formed on a high-speed steel by means of SEM, FIB and TEM, Corros. Sci. 53 (2011) 3603–3611. [17] M. Geetha, A.K. Singh, K. Muraleedharan, A.K. Gogia, R. Asokamani, Effect of thermomechanical processing on microstructure of a Ti–13Nb–13Zr alloy, J. Alloys Compd. 329 (2010) 264–271. [18] D. Cáceres, C. Munuera, C. Ocal, J.A. Jiménez, A. Gutiérrez, M.F. López, Nanomechanical properties of surface-modified titanium alloys for biomedical applications, Acta Biomater. 4 (2008) 1545–1552.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mono- and multiple TiN(/Ti) coating adhesion mechanism on a Ti–13Nb–13Zr alloy Jianzhong Li a,b , Hua Zheng b , Theo Sinkovits b , Ay Ching Hee b , Yue Zhao b,∗ a
School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China School of Mechanical Materials and Electronic Engineering, Faculty of Engineering and Information Science, University of Wollongong, NSW 2522, Australia b
a r t i c l e
i n f o
Article history: Received 30 April 2015 Received in revised form 26 June 2015 Accepted 19 July 2015 Available online 21 July 2015 Keywords: Coating adhesion Heat treatment Monolithic TiN coating Multilayers TiN/Ti Hardness
a b s t r a c t Mono- and multiple TiN(/Ti) coatings deposited on Ti–13Nb–13Zr alloy substrates by the filtered arc deposition system were examined using scratch testing and depth-sensing indentation in terms of the relationship between the coating adhesion, deformation mechanism, and microstructure, and mechanical properties at the film/substrate interface. The results show that multilayer TiN/Ti coatings offer a greater resistance to cracking and delamination than monolithic TiN coatings under the same conditions on the Ti–13Nb–13Zr alloys substrates. And increasing the number of layers for TiN multilayer coating improves the coatings adhesion. In contrast, for the coatings on the Ti–13Nb–13Zr alloys substrates that were heattreated to a higher hardness, the limited deformation in the substrates improved remarkably the coating adhesion indiscriminately. The substrate mechanical properties play the major roles in controlling the coating adhesion, and increasing thickness and layers of the TiN multilayer have a limited improvement to the adhesion of coating. © 2015 Elsevier B.V. All rights reserved.
1. Introduction TiN coatings have been widely used for both mechanical and chemical protection of metallic materials in a range of engineering applications due to their improved wear resistance, hardness, and higher oxidation resistance. These attractive properties have been a driving force for the early introduction to develop TiNbased coatings with higher levels of properties [1–3]. Tilbrook et al. [4] reported micostructural effects on indentation failure mechanisms in TiN coatings by finite element simulations. Romankova et al. [5] studied the effect of annealing treatment on the structure and properties of the nanograined TiN coatings produced by an ultrasonic coating process. With an increasing demand for industrial practice, further investigation of multilayer coatings in the feature of the improved wear resistance, hardness, and higher oxidation resistance with modulation of different phases has attracted much attention in the past few years [6–8]. TiN/Ti multilayer coating is one of the most interesting multilayer coatings. Cheng et al. and Gong et al. [9,10] investigated the mechanical and tribological behavior of the TiN/Ti multilayer coatings deposited by filter cathodic arc. Carvalho et al. [11] and Xie et al. [12] researched
∗ Corresponding author. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.apsusc.2015.07.126 0169-4332/© 2015 Elsevier B.V. All rights reserved.
deformation mechanisms in TiN multilayers under depth-sensing indentation. Wang et al. [13,14] studied corrosion properties and contact resistance of the TiN multilayers in simulated proton exchange membrane fuel cell environments. It has been found that TiN/Ti multilayer coatings can significantly improve the mechanical and tribological properties, corrosion resistance, and fretting fatigue behavior of metal substrates. Despite much research efforts on the development of monoor multilayer coatings with superior properties, limited data have been published in order to justify the adhesion mechanisms of enhancing the properties of the coatings taking into consideration their deformation features. This paper describes the results of investigation to determine the relationship between the coating adhesion mechanisms, deformation mechanisms of the coatings subjected to nanoindentation, microstructure and the surface properties of the substrate with different heat treatment status. To this end, commercial Ti–13Nb–13Zr alloys were chosen as the substrate, which were heat-treated under different schedule in order to achieve different phase status and microstructure. Mono- and multilayer TiN coatings examined in this paper were deposited by means of the Filtered Arc Deposition System (FADS). In the present work, scratch tests were performed by scratching the resultant films with an indenter to characterize the critical load at which failure occurs using a Revetest Xpress Scratch Tester. Nanoindentation experiments have been utilized to obtain mechanical property
J. Li et al. / Applied Surface Science 355 (2015) 502–508
data and an insight into the fracture phenomena of the coating/substrate systems. Important aspects of the contact-induced fracture mechanisms of the coated systems have been revealed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 2. Experimental The coatings examined in this paper were deposited onto Ti–13Zr–13Nb alloy (compositions in wt%) disks of 25 mm in diameter, which are under the two status: a) with heat treatment; b) as received ( solution treated by manufacturer). The heat treatment schedule is as following: 25 min
10 min
60 min
20 min
25◦ C −→ 550◦ C −→ 760◦ C −→ 760◦ C −→ 550◦ C → Air cooled to room temperature In order to make sure the same surface condition for the deposition process, the Ti–13Zr–13Nb alloy substrates were polished using a 320 mesh SiC paper with water as a lubricant for 4 min, and then with a 15 m diamond abrasive with a petroleum-based lubricant for 10 min. The substrates were then fine-polished with a 0.05 m abrasive and a chemical solution consisting of 25 ml of OP–S containing a mixture of 2.5 ml of ammonia and 1.5 ml of hydrogen peroxide for 5 min. These substrates were ultrasonically cleaned in ethanol, distilled water, acetone and dried before being coated in a FADS System. And then coatings of TiN and TiN/Ti were deposited on polished Ti substrates using the FADS System. The deposition chamber was pumped down to a base vacuum of 5.0 × 10−6 Torr. The substrates were pre-heated under vacuum to 300 ◦ C, and then an etching process was carried out under high substrate bias of −850 V using pure titanium ion beam. The bias voltage was reduced to −100 V when the coating process began. Pure-Ti buffer layers were first deposited on the substrates, and then the reactive gas (nitrogen) was introduced into the chamber via a mass flow controller. The N2 mass flow rate was increased gradually from 5 sccm to a stable value of 40 sccm to reduce stress and improve adhesion between the film and the substrate. The chamber pressure went up to and stabilized at 2.9 × 10−3 Torr during deposition. A monolayer of TiN was formed after 2 h deposition. In order to obtain multilayer coating of TiN/Ti, the N2 gas flow was discontinued 9 times during the coating process. The total deposition time was 3.5 h. Scratch tests were performed by scratching the resultant films with a Rockwell C type diamond stylus (cone apex angle 120◦ , tip radius 200 m) to characterize the critical load at which failure occurs in a Revetest Xpress Scratch Tester. The instrument can simultaneously detect the acoustic emission and the friction force. In order to verify the reproducibility of the results, three scratch experiments were conducted on each tested samples. Nanoindentations on the monolayer TiN and multilayer TiN/Ti coatings were made using the Ultra-Micro Indentation System (UMIS). A Berkovich indenter with a 200 nm tip was employed to test the hardness of the coatings. The resolution was limited by thermal drift, which was found to be <1 nm/min. The tip was initially calibrated by the standard material of fused silica. Incremental controlled loading and unloading tests were performed with a range of the maximum loads from 3 mN to 9 mN to determine the hardness of the coatings. A spherical diamond indenter with a 5 m diameter was employed to test the fracture modes of the coatings. A spherical indenter utilized in the fracture study because it has been shown to produce a more uniform stress field beneath the contact area than pyramidal pointed indenters [15]. The tested loads were 360 mN. The loading-unloading rate is 200 N/s for all indentations.
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A FEI XT Nova Nanolab 200 workstation which combines a dual beam Focused Ion Beam (FIB) and a Field Emission Scanning Electron Microscope (FESEM) were used to prepare thin foil samples for TEM observation. Details of the FIB procedure have been given elsewhere [16]. TEM was performed on samples prepared by the FIB, using a Philips CM200 equipped with a Brucker energy dispersive X-ray spectroscopy (EDAX) system. The coating phase composition was determined by X-ray diffraction (XRD) using –2 scan. The measurements were performed with a scan speed of 2◦ /min using Cu K␣. Surface observation was conducted using an SEM equipped with an EDX spectrometer (a JEOL LV). 3. Results and discussion 3.1. Scratch behavior for the different coatings Coating adhesion to the substrate plays a major role in improving the resistance to wear and environmental degradation for tribological applications. Scratch tests were performed by applying a progressive load ramping from 1000 mN to 50,000 mN for a length of 10 mm. Both the acoustic emission (AE) and friction force were recorded during the tests. Fig. 1 Fig. 1 also shows the entire optical image of scratch tracks after the tests. From the monolithic TiN coating on the as received Ti–13Zr–13Nb alloys surface, it can be seen that the acoustic emissions undergo a sudden change at a lower load of 25,800 mN with the obviously increasing friction force, although the acoustic emissions are also found as the load of about 20,000 mN. From the corresponding to optical image, the coating is clearly broken at a load of 25,800 mN, indicating that the monolithic TiN coating appear to have failed. Scratch tests for the TiN/Ti multilayer coating on the as received Ti–13Zr–13Nb alloys surface are presented in Fig. 1(b). As can be seen in the figure, acoustic emission signals show an abrupt increase at 37,500 mN, greater than that of the monolithic TiN coating. The corresponding optical image indicates that the multilayer coating fails at this load. Under the same substrate conditions, the multilayer coating shows greater resistance to the scratch damage than the monolithic coating. For the monolithic TiN coatings deposited on the Ti–13Zr–13Nb alloys with the heat treatment, deposition conditions of which are the same to those on the as received substrate, the scratch tests are shown in Fig. 1(c). It is seen in Fig. 1(c) that the acoustic emission has a sudden change at 48,100 mN, which is the maximum value in all cases. This is because a heat treatment induces the surface microstructure change of Ti–13Zr–13Nb alloys (seen in the third part results in this paper). An abrupt increase in the acoustic emission indicates that the surface layer has been delaminated. This demonstrates that a heat treatment for the Ti alloys substrate is a more effective method for improving the coating adhesion in all the cases tested. 3.2. Deformation of the different coating under indentation In order to determine the relationship between the adhesion mechanisms and deformation of the coatings or substrate, crosssectional overviews of indentation-induced deformation in the monolithic TiN coating and the multilayer TiN coatings with Ti on the as received Ti–13Zr–13Nb alloys surface are shown in Fig. 2 Fig. 2(a)–(f), respectively, with similar images for a monolithic TiN coating on Ti–13Zr–13Nb alloys surface with heat treatment shown for comparison (Fig. 2(g) and (h)). It can be seen that, while intercolumnar shear sliding was significantly reduced for layered structures, different deformation processes appeared to occur to accommodate strain within the three types of TiN coatings. For monolithic TiN coating on the as received substrate, a large
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Fig. 1. Micrograph of failure after scratch tests (a) monolithic TiN coating on as received substrate; (b) multilayers TiN/Ti coatings on as received substrate (c) monolithic TiN coating on heat-treated substrate.
number of large cracks were observed; most cracks were held within the TiN layer adjacent to the substrate in the form of radial and edge cracks; radial cracks also extended into the outer layers; stair-like intergranular shear sliding were evidently observed to occur at the coating/substrate interface (Fig. 2(b)). As to TiN multilayers with titanium interlayers, limited cracking can be seen, within both the TiN layers and the Ti interlayers. Edge cracks appear
to initiate at the coatings/the Ti interlayer interface, most likely due to local stress concentrations. Edge cracks were observed to propagate on a larger scale once activated. Coating deformation appeared to be accommodated primarily by intergranular shear sliding and plastic flow of the titanium interlayers. An intercolumnar shear step can be seen at the Ti interlayer. But stair-like shear could be seen hardly at the substrate interface (Fig. 2(d)–(f)). In contrast,
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Fig. 2. Cross-sectional SEM and TEM images of a 360 mN indent on mono- or multiple TiN(/Ti) coating: (a) and (b) are cross-sectional SEM and TEM of monolithic TiN coating on the as received Ti–13Zr–13Nb alloys surface, respectively; (c), (d), (e) and (f) are cross-sectional SEM and TEM of multiple TiN/Ti coating on the as received Ti–13Zr–13Nb alloys surface, respectively; (g) and (h) are cross-sectional SEM and TEM of monolithic TiN coating on the Ti–13Zr–13Nb alloys surface with heat treatment, respectively.
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for monolithic TiN coating on the substrate with heat treatment, a few cracks were found at the coating/substrate interface; stair-like intergranular shear sliding were also observed to occur evidently at the coating/substrate interface, whose height ‘ht2 ’ is lower compared to ‘ht1 ’ of the coating/substrate interface under the condition of greater magnification in Fig. 2(b). Based upon the observations of deformation mechanisms of layer TiN or TiN/Ti coatings on Ti–13Zr–13Nb alloys with heat treatment or not, two structural factors to influence coatings adhesion, i.e. the multilayer structure and the interlayer, have been identified as playing major roles in controlling intercolumnar sliding processes. The sliding distance depended upon the thickness of the layer to absorb the indentation energy. The energy to indent a multilayer coating (without the presence of the interlayers), Wm-TiN , to a depth ht is therefore [12]: 4√ 3/2 Wm-TiN = 2Tht + T (i − 1)cht 3 4√ 3/2 =T 2ht + (i − 1)cht (1) 3 where is the shear sliding stress, R is the radius of the indenter, T is the total thickness of the coating, ht is the sliding distance of annular columnar crystal and 2c is the width of annular columnar crystal. Notably, the first item in Eq. (1) represents the energy absorption, Wm-TiN , equivalent to that taken by the monolayer TiN, and the second item represents an increase in the energy needed for deforming the coatings, Wshear , due to the increase in intergranular shear area in comparison to the monolayer. Therefore, assuming coating thickness and intergranular shear stress to be constant, an increase in the number of layers would result in an increase in the intergranular shear area, which in turn would result in an increase in the energy absorption during the deformation of the coating. For the interlayers within the coating, it acted as physical barriers resisting intergranular shear sliding. The increase in deformation energy was caused by the presence of interlayers. Now we consider another structural factor, i.e. the substrate mechanical properties, which have a pronounced effect on load–displacement behavior, influencing the effective compliance during both the loading and unloading stages. The plastic deformation energy of the substrate, Wsub , has been determined in a previous study as [12]: Wsub = ARh2t
(2)
where A is a material factor dependent on the mechanical properties of the substrate, about 4.85 × 109 Pa for Ti alloys as reported by Xie et al. [12]. Because there is negligible plastic compression of the TiN coatings, it may be assumed that the coating acts as stiff elastic radial extension of the spherical indenter. The indentation energy of the substrate may, therefore, be obtained via Eq. (2) with an indenter of radius equivalent to the indenter radius plus the coating thickness. So the total indentation energy, Wsystem , may be considered as a combination of the energy contributions from substrate, Wsub , and the coating, Wcoat , which includes the contribution from the multilayer structure, Wm-TiN (i.e. WTiN+ Wshear ), and interlayers, Winterlayer : Wsystem = WTiN + Wshear + Winterlayer + Wsub = Wcoat + Wsub
(3)
Both FESEM and TEM cross-sectional analyses suggested that there was no observable difference in the coating thickness under the indent compared with outside the indent area, implying that predominant plastic flow occurred in the substrates, not in the coatings. In addition, observed via Eqs. (1)–(3), for Wm-TiN , the calculated values of monolithic TiN coatings is lower obviously than that of TiN multilayers under the same conditions for the substrate. So increasing the number of layers for TiN multilayer coating
improves the coatings adhesion. However, T should be of very small value due to its the total thickness of the coating as micrometer orders, and is then fed into Eq. (1) to determine to the small Wm-TiN results. For Wsub , A should be very larger values as more than 109 Pa for Ti alloys, and is determined to Wsub . As a result, we obtain Wsub Wm-TiN . This explains why the limited deformation for the substrates could improve remarkably the coatings adhesion. Based on these results, the we believe that the mechanical properties of the substrate play a major role in controlling the coating adhesion, and increasing thickness and numner of layers of the TiN multilayer contribute limited improvement to the coating adhesion. 3.3. Physical properties of Ti alloys substrate and the coatings From the above nanoindentation results, no delamination was observed at the coating/substrate interface, suggesting good interfacial bonding. In addition, the better coating adhesion should depend also on the fine microstructure and the surface properties of a Ti–13Zr–13Nb alloy substrate. The microstructures of the Ti–13Zr–13Nb alloy substrate before and after heat treatment are shown in Fig. 3 Fig. 3. The Ti alloys substrate specimen with heat treatment exhibits a martensitic microstructure (␣) while the as received substrate specimen shows a basket weave type of microstructure. These structures are typical of  solution-treated titanium alloys. The XRD studies of these specimens (Fig. 3(c)) show the presence of ␣ phase for the heat-treated specimen with a very weak peak of the ␣” phase. The as received specimens reveal the presence ␣ phase with a very weak peak of the  phase. Two different types of martensitic structure i.e. ␣ (hexagonal) and ␣” (orthorhombic) are observed in titanium alloys depending upon the content of  alloying elements. Higher  alloying content favors the formation of ␣” martensitic over ␣ [17]. This suggests the heat treatment induces a transition of  phase to form ␣” phase, contributing to an increase in hardness of the titanium alloys substrate (see Fig. 4(a)). In Fig. 4, hardness values as a function of the displacement normalized to the coating thickness, as well as the substrate, calculated by depth-sensing indentation are shown for the multilayer TiN/Ti, monolithic TiN layer and the Ti–13Zr–13Nb alloy substrate with heat treatment and as received. For the Ti–13Zr–13Nb alloy substrates, the measured hardness decreases with indentation depth as the effects of the softer substrate become more influential. The observed increase in hardness at very low indentation depths is mainly attributable to a continuing increase in elastic strain induced by the spherical-tipped indenter. For penetration depths of the order of a few hundred nanometers the influence of the substrate on the composite hardness values is much reduced. Note that hardness values for the heat treatment substrate are appreciably higher than that of the no-heat treatment. The noticeable scatter between individual measurements should be related to the combination of at least two different factors [18]: (1) different hardness values of the ␣ and  phases, whose contribution would depend on the corresponding ␣-phase/-phase ratio exiting within the tested area in each alloy; and (2) a significant influence of the crystal orientation on the nanoscale-dependent factors. For the coatings in three cases, relatively larger scatter was also observed at very low indentation depths, primarily due to a combination of the surface roughness the coatings. The hardness of the multilayers at low displacements is slightly higher than that of the monolithic layer on the substrate with no-heat treatment. This effect might be related to the higher covalence of the bonds in the TiN layer or to a hardening effect caused by the large number of interface parallel to the substrate surface [11]. By assuming that at an indenter penetration of 200 nm the influence of both tip-ship irregularities and substrate (penetration depth less than onetenth of coating hardness) are negligible, it is possible to estimate
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Fig. 3. SEM and XRD images of a Ti–13Zr–13Nb alloy with heat treatment or not: (a) SEM of the alloy with heat treatment conditions; (b) SEM of the as received alloy; (c) XRD of the alloys (No.1 – as received sample, No.2 – heat treatment sample).
Fig. 4. Hardness as a function of the displacement normalized to the different alloys substrate and the different coatings: (a) a Ti–13Zr–13Nb alloy with different conditions; (b) the different coatings.
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the coating hardness for TiN and TiN/Ti to be of the order of 26 GPa and 31 GPa, respectively. But the hardness values are higher for monolithic TiN layer on the heat-treated substrate than for the two coatings on the as received substrates. This can be explained by that the substrate mechanical properties exert significant influence on the overall indentation deformation behavior. Harder substrates exhibit stronger resistance to ductile deformation, providing more support to the coatings and requiring greater applied loads to break the coatings. 4. Conclusions Using scratch testing and depth-sensing nano-indentation we obtained deepened understanding of coating-substrate adhesion and mechanical behavior for mono- and multiple TiN(/Ti) coatings deposited on a Ti–13Nb–13Zr alloy prepared by filtered arc deposition. The use of load–displacement data and specific procedures for preparation of cross-sections of nanoindentations at the load of 360 mN has been shown to be suitable for the assessment of the coating adhesion mechanism with the substrate microstructure and mechanical properties. On the as received Ti–13Nb–13Zr alloy substrates, multilayer TiN/Ti coatings offer a greater resistance to coating deformation than monolithic TiN coatings through suppression of intercolumnar sliding, inducing a stronger adhesion. The effect of the substrate compliance on the adhesion strength of the coating was investigated by comparing between the heattreated substrates and the as received substrates. The former, stiffer than the latter, leads to an increased elastic strain energy density in the TiN coating, and hence a lower compressive strain. Different deformation mechanisms of the coating and the substrate were examined to determine the total indentation energy (Wsystem ), which includes energy absorption of the coating and deformation energy of the substrate. On the relatively soft as-received substrate, the coating adhesion for the multilayer TiN/Ti coatings is clearly higher than that of monolithic TiN coatings. And increasing the layers for TiN multilayers improves the coatings adhesion. In contrast, for the coatings on the heat-treated hard substrate, the limited deformation by the substrate remarkably improved the coatings adhesion. It is observed that the mechanical properties of the substrate play a major role in determining the coating adhesion. The increase of hardness in the heat-treated Ti–13Nb–13Zr substrate is believed to be due to transition of  phase to form ␣” phase induced by the heat treatment. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51374053), Special Fund for Basic
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