Rare Metals2019年第10期

Microstructure,mechanical and corrosion properties of TiN/Ni nanomultilayered films

Chun-Lin He Jin-Lin Zhang Lei-Peng Xie Guo-Feng Ma Zhao-Fu Du Jian-Ming Wang Dong-Liang Zhao

Liaoning Provincial Key Laboratory of Advanced Materials, Shenyang University

Research Institute of Functional Materials,Central Iron and Steel Research Institute

作者简介：*Chun-Lin He e-mail:ccllhhe@126.com;

收稿日期：20 June 2017

基金：financially supported by the National Natural Science Foundation of China (No.51171118);

Microstructure,mechanical and corrosion properties of TiN/Ni nanomultilayered films

Chun-Lin He Jin-Lin Zhang Lei-Peng Xie Guo-Feng Ma Zhao-Fu Du Jian-Ming Wang Dong-Liang Zhao

Liaoning Provincial Key Laboratory of Advanced Materials, Shenyang University

Research Institute of Functional Materials,Central Iron and Steel Research Institute

Abstract：

Nanomultilayered TiN/Ni thin films with different bilayer periods(57.8-99.7 nm) and Ni single-layer thickness(3.9-19.2 nm) were prepared by alternatively sputtering Ti and Ni targets in N₂ gas atmosphere.The micros tructure,mechanical and corrosion properties of the multilayer films were investigated by X-ray diffraction(XRD),field emission scanning electron microscopy(FESEM),X-ray photoelectron spectroscopy(XPS),nanoindenter and electrochemical technologies.The multilayer films are fine with a mean grain size of ～8-9 nm independent of the bilayer period.However,the smoothness and compactness seem to decrease with the bilayer period increasing.The hardness(H) and elastic modulus(E) of the multilayer films gradually decrease as the bilayer period increases,and the multilayer film with bilayer period of 57.8 nm exhibits higher H,ratios of(H/E^*and H³/E^*2)(E*is effective Young's modulus)than the monolithic TiN film and the other multilayers.The multilayer films exhibit an obvious passivation phenomenon in 10% H₂SO₄ solution,and the passive current and corrosion current densities decrease,whereas the corrosion potential increases when the bilayer period or Ni single-layer thickness decreases.It is found that the passivating behavior and corrosion potential of the multilayers are more sensitive to Ni single-layer thickness than the bilayer period.More corrosion pits and lamellar flaking could be found on the films with larger bilayer period or Ni single-layer thickness.

Keyword：

TiN/Ni; Multilayer film; Reactive sputtering; Microstructure; Mechanical property; Corrosion resistance;

Received： 20 June 2017

1 Introduction

Binary transition metal nitride films have been widely used to improve the performance and extend the life of cutting tools,dies and molds,etc.However,these films (e.g.,TiN)cannot meet the requirements in many applications due to relatively low hardness,insufficient thermal stability and corrosion resistance [ 1] .In order to improve these properties,various films have been developed such as multicomponent,multilayered and nanocomposite films [ 2, 3, 4] .In recent years,ceramic/metal multilayers have attracted intensive attentions because of their superior mechanical properties and chemical stability compared with the monolithic hard films.Layered deposition is expected to minimize the columnar structure of the physical vapor deposition (PVD) films,lower the grain size and prevent the growth of pores and defects.Various ceramic/metal multilayers have been studied such as TiN/Ti [ 5, 6, 7, 8, 9] ,CrN/Cr [ 10, 11, 12, 13] ,AlN/A1 [ 14] ,CrN/Cu [ 15, 16] ,TiSiN/Ni [ 17] ,VC/Ni [ 18] and TiN/Ni [ 19] ,and their properties are found to be dominated by the selection of constituent layer materials,bilayer period (A,i.e.,modulation wavelength)and the interface structure.Many researches have revealed that lower A can lead to an improvement in hardness [ 7, 10, 19, 20] ,wear resistance [ 8, 10, 20] and corrosion resistance [ 10, 13] ,attributed to the presence of a higher number of interfaces preventing the movement of dislocation and the propagation of microcrack,and the decrease in grain size and pore amount through re-nucleation at interfaces.According to the Hall-Petch formula,Λbelow100 nm is expected to be more efficient in improving hardness of multilayers.

So far,most researches on PVD ceramic/metal multilayers have focused on TiN/Ti [ 5, 6, 7, 8, 9] and CrN/Cr [ 10, 11, 12, 13] combinations;herein,the soft metal (Ti,Cr) is none other than the one bonding with N.Main considerations may involve in good properties of TiN and CrN as well as improved adhesion at interface [ 5, 7, 21] .In opposite to the fact that Ni is widely used to tough transition metal nitride to form nanocomposites [ 4, 22, 23] ,relative less information can be found about Ni used as the soft metal in ceramic/metal multilayers [ 17, 18, 19] .Because Ni is alow N affinity metal,Ni/ceramic sublayer adhesion mainly depends on mechanical interlock and different interatomic action forces,such interface is sharp not graded;basically,it might be weaker than the interfaces containing high N affinity metal multilayers (e.g.,TiN/Ti,CrN/Cr and AlN/Al).However,Vladescu et al. [ 17] and Ge et al. [ 18] reported that TiSiN/Ni(Λ=7-27 nm) and VC/Ni (Λ=3.3-53.5 nm) multilayers exhibited increasing wear resistance at lower A compared with the monolithic films.And in TiN/Ni multilayers(Λ=1.8-62.0 nm),the highest hardness appeared at very small bilayer period (Λ=2.2 nm) [ 19] .Except the bilayer period,the thickness of ceramic or/and metal single layer also affects the multilayer properties [ 5, 19] ,especially for corrosion performance due to the appearance of galvanic couple at metal/ceramic interfaces.However,relative information is very insufficient so far.

In the present study,Ni was chosen as soft metal due to its perfect protective properties against corrosion and oxidation.TiN/Ni multilayer films with different A(57.8-99.7 nm) and different Ni single-layer thicknesses(3.9-19.2 nm) were prepared by reactively magnetron sputtering,and the effects of A and Ni single-layer thickness on the microstructure,mechanical and corrosion properties were investigated in detail.

2 Materials and methods

The TiN/Ni multilayer films were deposited on Si (100) and AISI 304 stainless steel substrates with dimensions of25 mm×25 mm×0.5 or 2.0 mm,respectively,by alternatively sputtering Ti and Ni targets in N₂ gas atmosphere at room temperature using reactive magnetron sputtering.Ti(99.99%) and Ni (99.99%) targets with diameter of 60 mm and thickness of 4 mm were mounted about 70 mm below the substrates.The substrate was ultrasonically cleaned for20 min in acetone followed by 10 min ultrasonic cleaning in ethanol and finally 5 min ultrasonic cleaning in deionized water.The base pressure of the deposition chamber was pumped to 6.0×10^-4 Pa,and the process pressure was fixed at 0.5 Pa.High-purity Ar (99.999%) and N₂ (99.999%)gases were used,and their flow rates were fixed at 30 and4 ml·min^-1,respectively.The alternative deposition TiN and Ni single layers were carried out under bias voltage of-70 V,and the top layer was TiN layer.The deposition time of TiN single layer (excluding the thinnest TiN single layer,its deposition time was 500 s) was fixed at 725 s with changing time of Ni single layer in order to obtain differentΛ,Ni single-layer thicknesses and thickness ratios of Ni to TiN single layer.For comparison,the monolithic TiN film was deposited for 2 h under the same condition.The currents applied on Ti (balanced) and Ni (unbalanced) targets were0.2 and 0.1 A (direct current),respectively.

The microstructures of the TiN/Ni multilayer films were observed using field emission scanning electron microscopy (FESEM,S4800) operated at voltage of 5-10 kV.The compositions and chemical bonding states of the film were investigated by X-ray photoelectron spectroscopy(XPS,ESCALAB250,Thermo VG).A monochromated Al Kαradiation was used as the excitation X-ray source.The photon energy (hv) was 1486.6 eV.The accelerating voltage and the emission current of X-ray source were maintained at 15 kV and 10 mA,respectively.The pass energy of the semi-spherical photoelectron energy analyzer was set to 50 eV,and the energy step size was 0.05 eV.The crystal structure was analyzed by X-ray diffractometer(XRD,PANalytical B.V.X Pert Pro MPD),the Cu Kαline(0.15406 nm) was used,and the incident beam angle was fixed at 2°.The nanohardness and elastic modulus were evaluated by MTS XP nanoindenter system equipped with a Berkovich diamond indenter under peak load of 1 mN,which were an average value from five indentations per film.The indentation depth was kept below 10%of the film thickness to minimize the substrate effect.

Polarization curves were measured in 10%H₂SO₄ solution at room temperature by PARSTAT 2273 advanced electrochemical system.The reference electrode was a saturated calomel electrode,and the counter electrode was graphite.Before measurements,the samples were immersed in solution until a ste ady open circuit potential was recorded.The scanning rate was 0.332 mV·s^-1.The specimens were covered with paraffin for electrical insulating except the surface for electrochemical measurements.

3 Results and discussion

3.1 Micros tructure

Figure 1 shows FESEM surface morphologies of TiN/Ni multilayer films deposited on Si (100) substrate with different∧.The surface microstructure is fine,dense and homogeneous at lowΛ.However,when A increases,the surface gradually becomes rough due to more large clusters present on the surface.Figure 1 clearly shows that the large clusters are composed of several fine particles.

Fig.1 Surface FESEM images of TiN/Ni nanomultilayer films on Si (100) substrate with various bilayer periods:a 57.8 nm,b 83.9 nm,c 85.3 nm and d 99.7 nm

Figure 2 shows backscattered electron (BSE) images of the cross sections of the multilayer films on Si (100) substrate.TiN layers appear darker and thicker,and Ni layers are brighter and thinner.The films are obviouslymultilayered structure which consists of ten TiN layers and nine Ni layers.The flat and sharp interfaces are obtained whenΛis lower than 85.3 nm (Fig.2a-c),whereas for99.70-nm film,the interface is not flat and Ni single layer is interrupted at some structural voids (Fig.2d).The thickness of either Ni single-layer or the multilayer film increases withΛincreasing.The estimated thickness andΛof the TiN/Ni multilayer films are listed in Table 1.Both TiN and Ni single-layer thicknesses are within 100 nm,so the films belong to nanomultilayer films.Basically,PVD TiN films should have a columnar structure [ 24] ;however,the column-like features are unobvious (Fig.2).This phenomenon is also observed in VC/Ni multilayer at largerΛ(30.3 and 53.5 nm) [ 18] .For the multilayers with smallerΛor thinner Ni layer (Fig.2a-c),the micro structure is dense,fine and homogeneous,contributed to re-nucleation lowering the grain size and preventing the growth of pores and holes [ 25] .However,when∧increases to 99.7 nm,obvious voids and holes appear (Fig.2d);obviously,this has great damage on the properties of film.It seems that the number of voids is less at the substrate/film interface and increases with film growth.

Fig.2 BSE images of cross-sectional TiN/Ni multilayers with various bilayer periods:a 57.8 nm,b 83.9 nm,c 85.3 nm and d 99.7 nm

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Table 1 Thickness and bilayer periods of TiN/Ni multilayer films

Figure 3 presents XRD analysis results of TiN/Ni multilayer films deposited on AISI 304 stainless steel substrate.The diffraction peaks correspond to B1 NaCl-typed TiN structure and Ni.The TiN diffraction peaks can be assigned to (111),(200),(220),(311) and (222) reflections with a preferential orientation of (200) reflection independent ofΛ,showing that the surface energy is dominant in the total energy during the multilayer growth [ 26, 27] .On the contrary,the monolithic TiN film deposited under the same condition reported by our group presented a preferred orientation in the (111) plane [ 24] ,showing that the deformation energy is dominant [ 28, 29] .This means that the layered deposition in TiN/Ni multilayer benefits the release of residual stress.The Ni diffraction peaks hardly appear when A is too low;however,the diffraction intensity at 2θ=44.5°gradually becomes strong withΛincreasing although the films grow thick (Fig.2).This diffraction peak can be ascribed to fcc Ni (111) reflection (PDF No.04-0850).Poly crystal Ni is also found in sputtered VC/Ni multilayers [ 18] .Figure 3 also clearly shows that the intensities of all the TiN diffraction peaks almost do not change with A;this is because Ni single layer is too thin.

Fig.3 XRD patterns of TiN/Ni multilayers on AISI 304 substrate with various bilayer periods

The TiN average grain sizes estimated by Scherrer formula are 8.5,8.3,8.8 and 8.7 nm based on the (200)plane diffraction peak for differentΛ,respectively,which is smaller than 11.0 nm for the monolithic TiN [ 24] ,showing that multilayer structure benefits grain refinement [ 25] .And the grain size of Ni crystallites based on (111)plane diffraction peak is 13.1 nm for the film with A=99.7 nm.This demonstrates that both TiN and Ni crystallites are nanograins,and TiN average grain size is independent of A,this is because the thickness of TiN inpidual layer does not change a lot,and the change of A is not great (Table 1).Flores et al. [ 8] reported that the grain size of TiN increased with the TiN single-layer thickening in TiN/Ti multilayers,and at lower∧(3.3-53.5 nm) the grain would refine with A decreasing [ 18] .

XPS survey and high-resolution scans were carried out on the sample ofΛ=57.8 nm.XPS depth profiles are shown in Fig.4.It is clearly seen that O content in the film surface without Ar⁺etching reaches～33 at%,and it rapidly decreases and maintains 5 at%or so,showing that a small amount of O presents in the multilayer film.Ni content in the multilayer film without etching is very low;it increases after etch depth (D) exceeds 24.8 nm and reaches the maximum value at D=58.0 nm and then decreases again,showing that the first layer Ni has been etched off.Meanwhile,N and Ti contents are exactly the lowest at D=58.0 nm while they almost do not change during the other etching period.This proves the lamellar structure.Figure 4 shows that N content is little higher than Ti content,and N/Ti atom ratio is about 1.04,showing that TiN in the multilayer is near stoichiometric.

Fig.4 XPS depth profile of TiN/Ni multilayer with bilayer period of57.8 nm

Fig.5 XPS spectra of a Ni 2p versus Ar⁺etch depth,b Ti 2p and c N 1s energy regions for TiN/Ni multilayers with bilayer period of 57.8 nm

High-resolution XPS spectra were taken in the energy regions of Ni 2p,Ti 2p and N 1s in order to examine the chemical bonding states in the films (Fig.5).Figure 5a shows the variations in XPS spectra of Ni 2p energy region for the TiN/Ni film withΛ=57.8 nm with Ar⁺etch depth.Ni in TiN/Ni film presents 2p_3/2 and 2p_1/2 peaks at binding energies of 852.3 and 869.7 eV,respectively,with the highest peak intensity at D=58.0 nm,and the peak positions do not change with etch depth.This indicates that Ni in TiN/Ni multilayer is metallic [ 30] ,contributed to the low affinity of nickel to nitrogen.The Ti 2p and N 1s peaks(Fig.5b,c) for the multilayer film after D>58.0 nm are centered at bonding energies of 454.8 and 397.0 eV,respectively,which correspond to TiN [ 30] .The results above are exactly in accordance with that of XRD analysis(Fig.3).

3.2 Mechanical properties

Hardness and elastic moduli of TiN and TiN/Ni multilayer films deposited on stainless steel substrate with different A are shown in Fig.6.The hardness and modulus of the monolithic TiN film are 20.2 and 263.2 GPa,respectively.The harness values of TiN/Ni multilayer films with A=57.8,83.9,85.3 and 99.7 nm are 22.6,19.4,18.1 and16.4 GPa,and the corresponding moduli are 258.2,259.4,248.7 and 235.7 GPa,respectively.This shows that the hardness and elastic moduli gradually decrease with A increasing,which is consistent with the Hall-Petch behavior that strength decreases withΛincreasing,and may also be associated with the increase in thickness of Ni single layer [ 5] .The evolution of hardness versusΛin the present study is similar to that of CrN/Cr [ 10, 11] ,CrN/Cu [ 15] ,TiN/Ti [ 5] ,TiN/Ni [ 19] ,TiN/NbN [ 31] and TiN/Mo₂N [ 20] multilayers,but different from that of VC/Ni [ 18] and CrN/Cr [ 12] multilayers where the hardness increases with A increasing.

Fig.6 Hardness and elastic modulus versus bilayer periods of TiN/Ni multilayers

The properties of the multilayer films are dependent on their microstructure.Firstly,denser and less detective films generally have higher hardness [ 32] .As shown in Fig.2,the multilayer films are much denser at lower∧(e.g.,57.8and 83.9 nm) than that at largerΛ(99.7 nm);thus,a higher hardness is presented.Secondly,the multilayers with lowerΛpossess more interfaces (because of their thinner thickness) and much thinner TiN and Ni single layers.At low∧,the interfaces between sublayers act as pinning sites for dislocations.Since the thicknesses of Ni layers are too small (<10 nm),the dislocation generation may not occur inside the Ni layers [ 31, 33] .Although the dislocations can be generated in TiN single layers,they move within the single layers and propagate toward the interfaces.As the interfacial energies are quite high,further movement of the dislocations is prevented,and hence,pileup of the dislocations takes place near the interfaces.Furthermore,each interface serves as crack tip deflectors.This leads to a substantial increase in the system hardness relative to that of the homogeneous materials.At largerΛsuch as99.7 nm,softening appears as a result of dislocation generation and motions within both the TiN and Ni single layers.Thirdly,the thickness ratio of Ni to TiN single layer is also responsible for the multilayer enhancement.A larger fraction of hard TiN in the multilayers can lead to higher hardness according to the rule of mixture;similarly,a thicker Ni single layer can result in a lower hardness [ 5] .For example (Table 1),the multilayers almost have the similarΛ(83.9 and 85.3 nm) and TiN single-layer thickness (78.0 and 77.6 nm),but the difference in thickness of Ni single layer (5.9 and 7.7 nm) or in thickness ratio of Ni/TiN single layer (0.076 and 0.099) results in the 83.9-nm multilayer with higher modulus and hardness (Fig.6).Lastly,the hardness evolution may also be associated with the residual stress in the deposited films as reported by Zhang et al. [ 34] .Thick films can lead to lower compressive stress,which benefits the decrease in hardness.

The ratios,H/E^*and H³/E^*2,of hardness and elastic modulus can be used to characterize elastic strain to failure resistance and plastic deformation resistance of the films,respectively,where H is hardness,E^*=E/(1-v²) is the effective Young’s modulus and v is the Poisson’s ratio.H/E^*and H³/E^*2 can be used to predict the ability to resist mechanical degradation and failure of nanocrystalline films [ 3, 35, 36] .Figure 7 shows the variations in H/E^*and H³/E^*2 with A.The H/E^*and H³/E^*2 present a similar behavior to the hardness.The multilayer with the lowest∧presents the largest H/E^*and H³/E^*2 ratios among the monolithic TiN and the multilayers,indicating that such multilayer possesses a higher elastic strain to failure resistance and plastic deformation resistance,which can be contributed to its relative higher interfaces,smaller thickness ratio of Ni to TiN single layer and much more compact and homogeneous microstructure.

Fig.7 H/E^*and H³/E^*2 versus bilayer periods of TiN/Ni multilayers

Fig.8 Potentiodynamic polarization curves of TiN/Ni multilayers in10%H₂SO₄ solution

3.3 Corrosion resistance

Potentiodynamic polarization curves of TiN/Ni multilayers on AISI 304 stainless steel substrate with differentΛin10%H₂SO₄ solution are shown in Fig.8.The relative electrochemical parameters are listed in Table 2.It is clearly seen that the monolithic TiN and the TiN/Ni multilayers with lower A (57.8 and 83.9 nm) or thinner Ni single layer (3.9 and 5.9 nm) represent a spontaneous passivating ability with a much lower passive current density (I_p),whereas the larger∧(85.3 and 99.7 nm) or thicker Ni single layers (7.7 and 19.2 nm) lead to an active-pas si ve transition with a higherI_p compared to the multilayers with lower∧and the substrate.This implies that the films with larger∧have no propensity to selfpassivating in 10%H₂SO₄ medium,which is associated with increased structural voids and defects as well as active Ni single-layer thickness in these films.The active dissolving peaks increase with A,and they become very evident when∧≥85.3 nm (Fig.8).The I_p decreases with A decreasing;however,it is still bigger than that of the monolithic TiN film.The multilayer with the lowest∧almost has the same small I_p as the monolithic TiN film.This implies that the passive properties of the multilayers can be improved via reducing∧,which is associated with the improvement in compactness with A decreasing(Fig.2).Generally,a pore-free film gives the substrate greater passivity than a film with pores [ 37] .Meantime,higher compressive residual stress in the thinner multilayer films and the monolithic TiN film could improve the passivity and corrosion resistance.The distinctly different passivating behaviors for the multilayers with A=83.9and 85.3 nm are associated with the difference in Ni singlelayer thickness in these two films.The oscillations in corrosion current density are observed in the passivation region for the monolithic TiN film and multilayers with lowerΛ(57.8 and 83.9 nm),indicating the repairing of passive films on the coated surfaces.

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Table 2 Relative parameters obtained from polarization curves in Fig.8

As shown in Fig.8 and Table 2,the corrosion potential(E_corr)of the multilayers monotonously increases from-237to 239 mV as∧decreases from 99.7 to 57.8 nm,showing that the electrode nobleness greatly increases with A decreasing.Similar trend was found by Marulanda et al. [ 10] .The multilayer with the lowestΛhas the most positive E_corr,which is65 and 270 mV higher than that of the monolithic TiN and the substrate,respectively,demonstrating that this film has the lowest corrosion tendency [ 25] .The multilayers with a close A (83.9 and 85.3 nm) exhibit distinctly different E_corr,which can be contributed to their different Ni single-layer thicknesses (5.9 and 7.7 nm) or thickness ratios of Ni/TiN single layer (0.076 and 0.099).This shows that E_corr is more sensitive to Ni single-layer thickness or thickness ratio of Ni/TiN single layer than∧,as shown in Fig.9.

Figure 8 and Table 2 also show that with A increasing from 57.8 to 99.7 nm,the corrosion current density (I_corr)of the multilayer film increases from 0.201 to 14.3μA·cm^-2,exhibiting an increasing trend with A.However,I_corr changes very slowly when∧is below 85.3 nm.The I_corr at the lowest∧is as small as that of the monolithic TiN film,which is more than one order of magnitude lower than that of the substrate.Except for the multilayer with the largest∧,all the other multilayers possess much lowerI_corr than the substrate,implying that they can provide efficient protection for the substrate.The effects of Ni single-layer thickness and thickness ratio of Ni to TiN single layer on I_corr are shown in Fig.10.It is clearly seen that the variation inI_corr is very small when the thickness of Ni single layer is<8 nm or the thickness ratio of Ni to TiN single layer is<0.1,after these critical values I_corr rapidly increase,showing thatI_corr is much more sensitive to both thickness of Ni single layer and thickness ratio of Ni to TiN single layer for the TiN/Ni multilayers.This means that galvanic corrosion present at ceramic/metal interface plays a key role in the corrosion resistance of ceramic/metal multilayers.The corrosion of ceramic/metal multilayers initiates at the ceramic/metal interface after corrosion medium enters in from the pores atop the ceramic film surface.Then,the galvanic corrosion starts,resulting in the rapid corrosion of the metal layer near the pores.Consequently,the pores widen and extend.And the corrosion medium more easily penetrates to the next ceramic/metal interface,where another galvanic couple is set up,and a new corrosion process takes place.This process repeatedly proceeds,and in the end,a galvanic corrosion starts at the film/substrate interface,leading to the substrate corrosion and film flaking.It is important to decide the critical values of metal single-layer thickness and the thickness ratio of metal/ceramic single layer for protection PVD ceramic/metal multilayers.Additionally,Table 2 also presents that all the multilayers and the monolithic TiN film have much higher pitting potential (E_pit) that the substrate,showing that they possess lower pitting susceptibility.

Fig.9 Effects of a Ni single-layer thickness and b thickness ratio of Ni to TiN single layer on E_corr of TiN/Ni multilayers

Fig.10 Effects of a Ni single-layer thickness and b thickness ratio of Ni to TiN single layer onI_corr of TiN/Ni multilayer

Fig.11 Typical corrosion SEM images of TiN/Ni multilayers on AISI 304 substrate with bilayer periods of a,b 57.8 nm and c,d 99.7 nm

These results above demonstrate that the corrosion resistance of TiN/Ni multilayer films is strongly dependent on the Ni single-layer thickness or thickness ratio of Ni/TiN inpidual layer as well as∧,and the multilayer films with a thinner Ni single layer (3.9-7.7 nm) or lower∧(57.8-85.3 nm) are very effective to protect the 304stainless steel substrate when exposed to 10%H₂SO₄solution.It is believed that the pores and the active Ni layer thickness are responsible for corrosion resistance of TiN/Ni multilayers.

Visual inspection found that the surfaces of TiN/Ni nanomultilayers with different∧after polarization tests in10%H₂SO₄ solution still presents beautiful golden yellow except for the 99.7-nm multilayer film which has lost its burnish.This demonstrates that the latter is corroded.

Figure 11 presents the corrosion morphologies of samples with A=57.8 and 99.7 nm after polarization tests.As clearly seen,the pits on the former film are fewer,larger and round,whereas more,smaller and irregular long pits appear on the latter (Fig.11a,c).The magnification image(Fig.l1d) clearly shows that corrosion proceeds along Ni single layer,resulting from galvanic corrosion between Ni and TiN layers where the active Ni acts as the anode.The voids and defects in the multilayer can become direct pathways for the corrosion medium to enter,causing that the Ni single layer quickly reacts with H₂SO₄ to form NiSO₄ and H₂ gas.Consequently,a part of TiN layer not only loses the Ni layer support,but also suffers from H₂pressure,resulting in a piece of TiN film to crack and peel off.Obviously,the thicker the Ni single layer is,the more serious such flaking of the TiN single layer is.An obvious lamellar flaking is found on the largest bilayer period film(Fig.lld).Because most of the pores and defects through film are blocked as a result of successive deposition of the two single layers (Fig.2d) [ 25] ,the corrosion on the lowest bilayer period film only takes place at fewer places with large structure voids or weakness spots in the form of localized film flaking [ 38] ;contrarily,the lamellar flaking is not obvious (Fig.11b).Similar local flaking (not lamellar flaking) phenomena were reported on corroded PVD films such as monolayer TiN [ 24, 38] and TiAlN [ 39] ,and multilayer TiN/Ti [ 6] ,TiAlN/CrN [ 39] and TiN/NbN [ 25] .The results above reveal that it is of great significance for corrosion protection to decrease the large structural pores or defects and the thickness of Ni single layer or∧in the sputtered TiN/Ni multilayer films.

4 Conclusion

TiN/Ni nanomultilayer films with different bilayer periods and Ni single-layer thickness were prepared by alternatively sputtering Ti and Ni targets in N₂ gas atmosphere at room temperature through changing Ni layer depositing time.The multilayer films are fine with a mean grain size of～8-9 nm independent of the bilayer period,but the smoothness and compactness seem to slightly decrease with bilayer period increasing.The diffraction peaks of the multilayer films correspond to fcc TiN and Ni with a preferential orientation of TiN (200)plane.The hardness and elastic moduli of the TiN/Ni multilayer films gradually decrease when the bilayer period increases from 57.8 to 99.7 nm.And the multilayer film with the lowest bilayer period exhibits a higher hardness,a little lower modulus and thus higher H/E^*and H³/E^*2 ratios than the monolithic TiN film.The electrochemical results show that the multilayer films exhibit an obvious passivation phenomenon in 10%H₂SO₄ solution,and the passive current density and corrosion current density decrease,whereas the corrosion potential increases when the bilayer period or Ni singlelayer thickness decreases.It is found that the passivating behavior and corrosion potential of the multilayers are more sensitive to Ni single-layer thickness or thickness ratio of Ni/TiN single layer compared to the bilayer period.More corrosion pits and lamellar flaking could be found on the films with larger bilayer period or Ni singlelayer thickness.The present results show that it is of great significance for corrosion protection to decrease both the large structural voids and thickness of Ni single-layer orthe bilayer period in the sputtered TiN/Ni multilayerfilms.

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